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The Effects of Conformity and Load in Total Knee Replacement

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The Effects of Conformity and Load in Total Knee Replacement

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Abstract

The effects of different conformity ratios and loads on the ultrahigh molecular weight polyethylene stress levels acting on knee implants were examined using a nonlinear, finite element analysis. The contact condition between a rigid cylinder with a radius of 30 mm and a polyethylene plate was modeled. Nonlinear behavior of polyethylene was assumed. The polyethylene plate was constructed with varying radii, with a minimal thickness of 6 mm and with a width of 40 mm. The ratio of the cylinder radius to the radius of the polyethylene plate was defined as the conformity ratio; a conformity ratio of 0 represented a flat tibial inlay, whereas the highest ratio modeled of 0.99 was nearly conforming. The conformity ratios modeled were 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, 0.95, and 0.99. The loads applied were 1000 N, 2000 N, 3000 N, 4000 N, 5000 N, and 6000 N. The effects of different conformity ratios and loads on the contact area (mm2), the compressive surface stress (MPa), the shear stress (MPa), and the von Mises stress (MPa) were investigated. It was found that all of these parameters were affected by changes to the conformity ratio and to a lesser extent by load changes. That is, increasing the load from 3000 N to 6000 N resulted in a surface and shear stress increase lower than the increase in stress caused by the small change of the conformity ratio from 0.99 to 0.95. The effect of an increasing conformity ratio on the reduction in stress was more pronounced for conformity ratios above 0.8. In addition, the effect of a load increase for a flat tibial inlay was two times greater than for one with near full conformity.

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... In knee implants, this has been related to material properties, polyethylene thickness, component design, and alignment; contact stresses and load; and clinical variables such as implantation time. 4,[7][8][9][10] Not only is increasing femorotibial conformity associated with UHMWPE contact stress reduction in the knee, 10 but the effect of a load increase is higher in nonconforming joints. Correspondingly, retrieval studies 11,12 have shown less delamination in more conforming femorotibial pairs, whereas less conforming implants were associated with more surface delamination, 2 where compression stresses concentrate and shear stresses increase. ...
... In knee implants, this has been related to material properties, polyethylene thickness, component design, and alignment; contact stresses and load; and clinical variables such as implantation time. 4,[7][8][9][10] Not only is increasing femorotibial conformity associated with UHMWPE contact stress reduction in the knee, 10 but the effect of a load increase is higher in nonconforming joints. Correspondingly, retrieval studies 11,12 have shown less delamination in more conforming femorotibial pairs, whereas less conforming implants were associated with more surface delamination, 2 where compression stresses concentrate and shear stresses increase. ...
... These authors pointed out that the reduction in compressive stress was larger for conformity ratios >0.8. Instead of using constants loads of 1000-6000 N, as in referenced work, 10 we investigated the time-varying loads on the knee during the stance phase of walking and obtained the pressure in the synovial fl uid to investigate the generated stresses. Upon this simulation, lubrication facilitated the reduction of 47 MPa with CI = 0 to 1.4 MPa with CI = 1. ...
... To address this, developments in knee replacement design have included increases in conformity to reduce stress, and theoretically wear. This has been driven at least in part by finite element analysis that has shown reductions in stress due to increases in conformity and contact area [4][5][6]. Demand placed on TKR includes both durability and function or range of motion. This has created a paradox. ...
... This has created a paradox. Increased conformity for stress concerns can lead to increased constraint and increased stress on bone/implant fixation interfaces [4][5][6][7][8]. Higher constraint can also impair range of motion, as it is known that implant geometry influences kinematics [8,9]. ...
... Overall, the wear behavior seen here differs somewhat from the findings of others [5,6] that suggest increased sagittal conformity would be beneficial to stress and wear. Their statement is based on a computer model that applied a range of loads to contact models with varied conformity in the sagittal plane and constant (flat) contact in the coronal plane. ...
Article
Durability concerns for knee replacement design have caused increases in tibial–femoral component conformity to lower contact stress. This has lowered stress and improved stability, but may also increase constraint and reduce allowed range of motion. Since a large range of motion is desirable, this study considers the effect of tibial sagittal radius on constraint and wear. The posterior sagittal radius of a given design was relaxed with the purpose of investigating the impact this change has on wear, stress and constraint when subject to the same motions and loading as the original design.
... [125] A number of FEM models have used classical von Mises failure theory to simulate the true stress-strain behavior of UHMWPE. [15,[126][127][128][129][130][131][132] The von Mises criterion is based on the deviatoric stress tensor and is thereby independent of hydrostatic pressure and open in 3-D stress space. The total strain is a sum of elastic and plastic strain. ...
... The most dominant mode of failure of nonconformal geometries in knee implants is pitting and delamination. [124,132,141] Conformal geometries represented by hip implants exhibit polished surfaces. [124] Polishing (burnishing) is a hybrid of abrasion and fatigue wear mechanisms. ...
... Bartel et al. [139] A similar study was preformed on eight knee designs. [127] Contact pressures Kuster et al. [132] investigated the effect of conformity and load on stresses of polyethylene employing a parametric 2-D plane strain FEM model. Increasing loads increased stresses linearly for all conformity ratios. ...
... This specific contact geometry results in high contact stress above the yield point in flexion which can result in delamination and destruction of the inlay. [72] To recommend suitable physical activities after total knee replacement, it is important to consider both the load and the knee flexion angle of the peak load while for total hip replacement, which is a ball and socket joint, the flexion angle does not play an important role. Many total knee designs will show much smaller stress levels near extension than in flexion for the same load. ...
... Many total knee designs will show much smaller stress levels near extension than in flexion for the same load. [72] During activities such as hiking or jogging high joint loads occur between 40 and 60°of knee flexion where many knee designs are not conforming. [43] This will lead to very high polyethylene inlay stress. ...
Article
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This article presents a literature review of the current recommendations regarding sports after total joint replacement and also suggests scientifically based guidelines. Patients should be encouraged to remain physically active for general health and also for the quality of their bone. There is evidence that increased bone quality will improve prosthesis fixation and decrease the incidence of early loosening. To recommend a certain activity after total knee or hip replacement, factors such as wear, joint load, intensity and the type of prosthesis must be taken into account for each patient and sport. It has been shown that the reduction of wear is one of the main factors in improving long-term results after total joint replacement. Wear is dependent on the load, the number of steps and the material properties of total joint replacements. The most important question is, whether a specific activity is performed for exercise to obtain and maintain physical fitness or whether an activity is recreational only. To maintain physical fitness an endurance activity will be performed several times per week with high intensity. Since load will influence the amount of wear exponentially, only activities with low joint loads such as swimming, cycling or possibly power walking should be recommended. If an activity is carried out on a low intensity and therefore recreational base, activities with higher joint loads such as skiing or hiking can also be performed. It is unwise to start technically demanding activities after total joint replacement, as the joint loads and the risk for injuries are generally higher for these activities in unskilled individuals. Finally, it is important to distinguish between suitable activities following total knee and total hip replacement. To recommend suitable physical activities after total knee replacement, it is important to consider both the load and the knee flexion angle of the peak load, while for total hip replacement, which involves a ball and socket joint, the flexion angle does not play an important role. During activities such as hiking or jogging, high joint loads occur between 40 and 60 degrees of knee flexion where many knee designs are not conforming and high polyethylene inlay stress will occur. Regular jogging or hiking produces high inlay stress with the danger of delamination and polyethylene destruction for most current total knee prostheses. Based on these design differences between hip and knee replacements it is prudent to be more conservative after total knee arthroplasty than after total hip arthroplasty for activities that exhibit high joint loads in knee flexion.
... 1b) were varied to create the conformity level between femoral component and tibial insert dish. The conformity values of 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99 [5], and 1, for both in sagittal and frontal plane of TKR, were chosen. The conformity values were then randomized by using Microsoft Excel to give all frontal and sagittal conformities the equal chance to pair [6], as we can see in Table 1 (column two and three). ...
... 5) we can see that the contact pressure (CP) and the von Mises Stress (VMS) decreased by the increasing of average conformity (AC) ...
Article
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The excessive contact stresses have been proved by many investigations for their role in the tibial insert failure. One parameter in the total knee replacement (TKR) design that influences excessive contact stresses is the average conformity. Thus, the aim of this study was to investigate the effect of the average conformity on the contact stresses in the TKR tibial insert. Ten three dimensional-finite element models with different conformity level were created. The conformity values of 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, 0.95, 0.99, and 1 were applied to the sagittal and frontal plane of the TKR models. The results showed that the contact pressure and the von Mises stress were decreased if the average conformity were increased, and vice versa. And, the average conformity has a larger effect on the contact pressure than on the von Mises stress. Also, from these simulations, we were informed that the developing of the contact pressure and the von Mises stress in the TKR tibial insert was greatly influenced by the average conformity. And, the von Mises stress and the contact pressure had the same trend in response to the average conformity.
... 22,23 Conformity and load of PE plates were also assessed in total knee replacements. 24 However, relevant scientific data on all 4 polymers in their use in oncological craniofacial reconstruction is lacking. PEEK, PEKK, PPSU, and PE polymers can be produced patient-specific and are medically approved. ...
Article
Full-text available
Purpose Surgical reconstruction after segmental mandibulectomy in patients with oral cancer commonly requires implantation of titanium plates which cause metallic artifacts in CT examinations detrimental for image quality. Therefore, the aim of the present study was to evaluate CT artifacts and image quality of the polymer plate materials polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyphenylsulfone (PPSU) and polyethylene (PE) in mandible reconstruction. Methods Five plates (titanium, PEEK, PEKK, PPSU and PE) were successively surgically inserted in a human cadaveric specimen using eight 1) titanium and 2) zirconium oxide screws. Streak and blooming artifacts were assessed in 1 mm CT scans with and without metallic artifact reduction algorithms (MAR) in this cross-sectional study. The different plate materials were analyzed for their effect on Hounsfield unit (HU) values resulting in streak artifacts. Blooming artifacts were measured as virtual growth of plates compared to actual size. Subjective image quality was assessed by three independent observers. Statistics were conducted by multi-factorial ANOVA and Bonferroni’s post hoc test. Results Titanium (182.42 HU; SD ±3.09) was associated with significantly more streak and blooming artifacts compared to the polymers PEEK (86.41 HU; SD ±1.72; p<0.001), PEKK (85.50 HU; SD ±2.75; p<0.001), PPSU (85.51 HU; SD ±1.97; p<0.001) and PE (85.52 HU; SD ±3.33; p<0.001) implants. No significant differences could be seen between the screw types (titanium vs. zirconium oxide) (p=0.75) or the activation or inactivation of MAR (p=0.85). Image quality was best for PE and worst for titanium implants in combination with zirconium oxide screws. Conclusions Polymers plate materials PEEK, PEKK, PPSU and PE significantly reduce CT artifacts and improve image quality facilitating radiological interpretation of implant surrounding tissues. This effect is beneficial for radiological tumor follow-up and CT-based planning of adjuvant radiotherapy.
... 26 A recent finite-element analysis examined the effects of different conformity ratios and loads on polyethylene stress levels in total knee prostheses. 29 A ratio of 0 represented a flat-on-round design, and a ratio of 0.99 represented a fully conforming design. Polyethylene stresses were more sensitive to changes in conformity than to load changes. ...
Article
Full-text available
Mobile-bearing knee arthroplasty (MBKA) has potential advantages compared with conventional fixed-bearing total knee arthroplasty (TKA). By allowing unconstrained axial rotation, MBKA can offer greater articular conformity without an increased probability of loosening due to increased axial torque. Increased articular conformity minimizes polyethylene contact stresses, thereby reducing linear wear and subsurface fatigue failure. Axial rotation of the platform also enables self-correction of tibial component malrotation. Despite these advantages, the long-term clinical results obtained with current MBKA devices are similar to those obtained with well-designed fixed-bearing TKA prostheses, with no data suggesting their superiority. The disadvantages of MBKA include bearing dislocation and breakage, soft-tissue impingement, a steep technique learning curve, and concerns about volumetric wear. Hypothetically, longer-term follow-up of MBKA results may reveal a significant difference from fixed-bearing TKA results as the fatigue failure threshold of incongruent polyethylene is exceeded.
... Ainsi, les forces supportées par la hanche et le genou sont différentes pour la même activité sportive. En effet, le pic de force augmente au niveau du genou dès que le mouvement nécessite de la flexion [5,16,17]. Selon cet aspect mécanique, la pratique de la randonnée sera encouragée avec bâtons de ski et le port d'un sac à dos lourd sera à éviter. La pratique du ski alpin correspond aussi à d'importantes contraintes au niveau du genou de plus de dix fois le PdC selon le niveau de pratique. ...
Article
ObjectiveTo recommand sports activities after joint arthroplasty from the literature analysis, the French surgeon's opinion and wish patients.MethodFrom the Medline data base interrogation according to keywords: Sports, Arthroplasty, Athletics, Physical training, two different readers, an orthopedic surgeon and a Physical Medicine and Rehabilitation physician selected articles in French or English language according to the level of proofs of the french classification of the Accreditation and Health Evaluation National Agency (Anaes). Professional practices were estimated by the interrogation of 30 orthopedic surgeons members of the french West Orthopaedics Society (SOO). The demand of sports practice was studied with patients recently operated for a primary total knee arthroplasty (TKA) after gonarthrosis.ResultTwenty-two articles were selected from 305 articles obtained by the search according to keywords. Ten literature reviews are limited by the absence of prospective randomized study. A level II study and eleven level IV articles are reported.DiscussionAccording to the subjective orthropedic surgeon's opinion, the objective results based on the joint load studied and the percentage of arthroplasty revision, sport is beneficial for the individual health but perhaps not for the arthroplasty survey. However, aerobic and leisure activities are recommended (walking, swimming, cycling) in agreement with the demand of the patients recently operated with a TKA. TKA differs from Total Hip Arthroplasty for jogging because of knee joint constraints during the knee flexion. A single study reports sports possibilities after shoulder arthroplasty and ankle arthroplasty and no study reports results after elbow arthroplasty.
... Each of them supports a different testing philosophy on its own right, particularly when considering the existing variety of knee implant designs on the market. Highly conforming and congruent implants, for example, might experience unphysiologically high stress when tested in displacement control mode and being forced to follow the motion profiles [5][6][7][8]. Implants with flat tibial plateaus and a low degree of congruency, on the other hand, may undergo too much motion when tested in load control mode. Springs, acting as "soft bumpers" in the loading path of the simulator, limit implant motions. ...
Article
Two identical sets of posterior cruciate retaining total knee replacements were tested for wear on one contemporary knee joint simulator according to the current ISO standards. The first set of implants was tested in load control mode (ISO 14243-1), while the second set was tested in displacement control mode (ISO 14243-3). The type of control mode alludes to the feedback signal of anterior–posterior and internal–external degrees of freedom. Resulting wear scars and rates between the different control mode tests were compared. Significant differences in wear rates were found: the wear rates in displacement control were approximately half of the wear rates in load control mode. This result was mirrored in the wear scars, which were significantly different in size on the medial plateau. Comparing wear rate with wear scar size (area) yielded a positive correlation. Analyzing the motion data of the two tests revealed similar ranges of movement in anterior–posterior translation and internal–external rotation, however the time-scale of the rotational movements differed between the two control types, explaining the differences in wear. This study demonstrated that substantial variability in wear outcome is possible depending on the type of control mode. Future revisions of testing directions should incorporate more implant specific test parameters to produce clinically relevant wear results.
... 22 The results of TKA prostheses that retain or excise the posterior cruciate ligament are available, [24][25][26] as are polyethylene conformity studies that demonstrate how to improve TKA kinematics and minimize wear. 27 However, little information has been published on the mid-term outcomes of high-flexion, higher-conformity 28,29 polyethylene with patella-sparing designs. ...
Article
Full-text available
The major factors that determine a favorable long-term clinical and functional outcome after conventional total knee arthroplasty (TKA) include correct implant positioning and restoration of the mechanical axis with soft tissue balancing to reduce aseptic failure; hence, the need for further developmental strategies that improve the accuracy and reproducibility of the surgical technique remains paramount for contemporary navigation research. Not all navigation systems are the same. The literature published thus far on mid-term results of navigated TKA relies on software that has no step-by-step soft tissue balancing with the tibia-first technique. The results are equivalent to those of conventional TKA.Therefore, the current authors conducted a minimum 5-year follow-up of a soft tissue-based navigated TKA system with the goal of soft tissue balancing. They analyzed intraoperative alignment and range of motion measurements, functional outcomes, radiographic assessment, and survival rates of high-flexion, high-conformity unresurfaced patella TKAs. The results at 5 years revealed a component revision rate of 0% compared with other nonnavigated TKAs (2.8% revision rate). The authors achieved a well-balanced TKA with a 0°±2° mechanical axis and an improved range of motion from 95° preoperatively to 110° postoperatively.
... Ainsi, les forces supportées par la hanche et le genou sont différentes pour la même activité sportive. En effet, le pic de force augmente au niveau du genou dès que le mouvement nécessite de la flexion [5,16,17]. Selon cet aspect mécanique, la pratique de la randonnée sera encouragée avec bâtons de ski et le port d'un sac à dos lourd sera à éviter. La pratique du ski alpin correspond aussi à d'importantes contraintes au niveau du genou de plus de dix fois le PdC selon le niveau de pratique. ...
Article
To recommand sports activities after joint arthroplasty from the literature analysis, the French surgeon's opinion and wish patients. From the Medline data base interrogation according to keywords: Sports, Arthroplasty, Athletics, Physical training, two different readers, an orthopedic surgeon and a Physical Medicine and Rehabilitation physician selected articles in French or English language according to the level of proofs of the french classification of the Accreditation and Health Evaluation National Agency (Anaes). Professional practices were estimated by the interrogation of 30 orthopedic surgeons members of the french West Orthopaedics Society (SOO). The demand of sports practice was studied with patients recently operated for a primary total knee arthroplasty (TKA) after gonarthrosis. Twenty-two articles were selected from 305 articles obtained by the search according to keywords. Ten literature reviews are limited by the absence of prospective randomized study. A level II study and eleven level IV articles are reported. According to the subjective orthropedic surgeon's opinion, the objective results based on the joint load studied and the percentage of arthroplasty revision, sport is beneficial for the individual health but perhaps not for the arthroplasty survey. However, aerobic and leisure activities are recommended (walking, swimming, cycling) in agreement with the demand of the patients recently operated with a TKA. TKA differs from Total Hip Arthroplasty for jogging because of knee joint constraints during the knee flexion. A single study reports sports possibilities after shoulder arthroplasty and ankle arthroplasty and no study reports results after elbow arthroplasty.
... This minimum contact between the joint surfaces in flexion is responsible for a substantial increase in contact stress, which favors destructive wear and delamination of the polyethylene insert. 5,35,36 The peak force on the prosthesis increases when the activity involves high-amplitude knee flexion. In theory, the manner in which these movements occur with greater frequency have more detrimental effects on the knee prosthesis, in particular greater risk of loosening of the tibial component. ...
Article
The success of joint replacement surgery has been responsible for raising patients' expectations regarding the procedure. Many of these procedures are currently designed not only to relive the pain caused by arthrosis, but also to enable patients to achieve functional recovery and to engage in some degree of physical activity and sports. However, as physical exercise causes an increase in forces exercised through the articular prosthesis, it can be an important risk factor for its early failure. Scientific literature on sports after arthroplasty is limited to small-scale retrospective studies with short-term follow-up, which are mostly insufficient to evaluate articular prosthesis durability. This article presents a review of the literature on sports in the context of hip, knee, shoulder and intervertebral disc arthroplasty, and puts forward general recommendations based on the current scientific evidence. Systematic Review, Level of Evidence III.
... Many researchers have revealed the significant effect of conformity in developing stress and wear in TKR. Kuster et al. [3] have reported the serious effect of conformity on surface and shear stress. The conformity influence was greater than the load influence. ...
Article
Full-text available
Conformity of contact counterface is one of the important parameters in a total knee replacement (TKR) design. Understanding the effect of conformity on contact stress during static and dynamic loading is crucial. To date, many published reports discussing the effect of conformity were from the static analysis. Therefore, we were compelled to conduct an investigation on the effect of conformity using two-dimensional finite element analysis during dynamic or impact analysis. Five models taken from previous report have been made for this purpose. The model was composed of two parts: femoral indenter and tibial base which represented the femoral component and tibial insert, respectively. A 74 kg was taken as a body mass. The initial impact velocity of 0.6 m/s was applied. The comparative statistical analysis proved that the conformity of TKR significantly (p
Article
Bereits Ende der 70er Jahre existierten mehr als 300 verschiedene Knieprothesen auf dem Markt [23]. Trotz der Vielzahl an unterschiedlichen Designs war ein grundlegendes Problem bei der Konzipierung neuer Prothesen nicht gelöst. Das Rückwärtsrollen der Femurkondylen auf dem Tibiaplateau mit zunehmender Beugung ist essentiell, um ein Impingement der dorsalen Strukturen am Tibiaplateau zu vermeiden und damit die Beugefähigkeit zu optimieren. Ein weiterer Vorteil des physiologischen „Rollbacks“ der Femurkondylen ist, dass der Hebelarm der dorsalen Beugemuskulatur mit zunehmender Streckung erhöht wird und damit eine Hyperextension des Kniegelenkes effektiv verhindert Ähnliches gilt für den Hebelarm der Quadrizepsmuskulatur mit zunehmender Flexion. Im Rahmen der physiologischen Kniebeugung kommt es aufgrund der zunehmenden Innenrotation der Tibia während der Flexion zu einem wesentlich geringeren Rückwärtsrollen der medialen im Vergleich zur lateralen Femurkondyle. Anfang der 80er Jahre versuchte man durch möglichst flache Tibiaplateaus das natürliche Rückwärtsrollen zu erleichtern. Diese sogenannten Round-on-flat-Designs führen jedoch zu einer erheblichen Belastung des Polyethylens (Abb.1) [26]. Um den resultierenden Polyethylenabrieb zu verringern, war es notwendig, das Polyethylenplateau an die Form der Femurkondylen anzupassen (Abb.2) und so die Kontaktfläche zu vergrößern. Mit zunehmender Kontaktflche werden jedoch Tanslation und Rotation eingeschränkt. Abb.1 Inkongruente Oberflächen verursachen eine maximale Materialbelastung (Abbildung zur Verfügung gestellt von der Firma DePuy) Abb.2 Hohe Konformität zwischen Femurkomponente und mobilem Polyethyleneinsatz am Beispiel der LCS-prothese(Abbildung zur Verfügung gestellt von der Firma Depuy)
Article
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Wear remains a significant problem limiting the lifespan of total knee replacements (TKRs). Though increased conformity between TKR components has the potential to decrease wear, the optimal amount and planes of conformity have not been investigated. Furthermore, differing conformities in the medial and lateral compartments may provide designers the opportunity to address both wear and kinematic design goals simultaneously. This study used a computational model of a Stanmore knee simulator machine and a previously validated wear model to investigate this issue for simulated gait. TKR geometries with different amounts and planes of conformity on the medial and lateral sides were created and tested in two phases. The first phase utilized a wide range of sagittal and coronal conformity combinations to blanket a physically realistic design space. The second phase performed a focused investigation of the conformity conditions from the first phase to which predicted wear volume was sensitive. For the first phase, sagittal but not coronal conformity was found to have a significant effect on predicted wear volume. For the second phase, increased sagittal conformity was found to decrease predicted wear volume in a nonlinear fashion, with reductions gradually diminishing as conformity increased. These results suggest that TKR geometric design efforts aimed at minimizing wear should focus on sagittal rather than coronal conformity and that at least moderate sagittal conformity is desirable in both compartments.
Article
Wear of ultra-high-molecular-weight polyethylene (UHM-WPE) inlays is associated with aseptic loosening in total knee replacement (TKR). The aim of this study was to investigate the in vitro performance of a TKR system that combines several measures to decrease UHMWPE wear. Tests were carried out on a BPK-S Integration system (R&D, P. Brehm Chirurgie-Mechanik, Weisendorf, Germany) according to ISO 14,243-1 in a knee joint simulator. Calf serum with a high protein concentration of 30 g/l was chosen as the test lubricant. PE wear was measured gravimetrically. Particle analysis was performed by scanning electron microscopy, with measurement of particle size and shape. Low mean wear rates of 1.20 mg per million cycles were found for the fixed bearing type and 2.47 mg per million cycles for the rotating-platform bearing design. Anteroposterior deflection was low. The contact areas for both types of bearings were large and showed a constant pattern throughout the test. Backside wear was obvious on rotating platforms. Particle analysis revealed equally sized and round-shaped particles in both types of bearings (fixed, 0.35 microm; mobile, 0.32 microm). In conclusion, the combination of design features and surface modifications of the BPK-S integration TKR system leads to low gravimetric UHMWPE wear.
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The paper gives a bibliographical review of the finite element modeling and simulations in orthopedics from the theoretical as well as practical points of view. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1998 and 2005. At the end of this paper 1264 references are given dealing with subjects as: bones and their parts, special tissues; spine mechanics; movable joints and computational modeling.
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Ultra-high molecular weight polyethylene remains the gold standard as a bearing surface for total knee arthroplasty. But problems of oxidative degradation and wear have prompted new sterilization techniques and new forms of more highly cross-linked polyethylene. The value of these solutions can be shown initially from laboratory data, but in the end their usefulness will be confirmed through clinical performance. The significance of new polyethylenes cannot be established on the basis of a single type of test or even on a single goal such as improved wear resistance. Rather, a range of requirements dictated by design goals (function, fixation, and wear) must be considered. The requirements can be competing, so that matching one compromises matching another that, if not taken into account, can lead to failure in clinical use. Previous improvements to polyethylene ultimately failed because not all the requirements were met. The same concern exists with highly cross-linked polyethylenes; improvements in wear resistance are tempered by a decrease in toughness. A case can be made for and against these polyethylenes, but early clinical results in hip arthroplasty show significantly improved wear, raising the possibility of also improving wear performance in knee arthroplasties.
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In connection with technological advances in the manufacturing of medical ceramics, a newly developed ceramic femoral component was introduced in total knee arthroplasty (TKA). The motivation to consider ceramics in TKA is based on the allergological and tribological benefits as proven in total hip arthroplasty. Owing to the brittleness and reduced fracture toughness of ceramic materials, the biomechanical performance has to be examined intensely. Apart from standard testing, we calculated the implant performance under different worst case scenarios including malposition, bone defects and stumbling. A finite-element-model was developed to calculate the implant performance in situ. The worst case conditions revealed principal stresses 12.6 times higher during stumbling than during normal gait. Nevertheless, none of the calculated principal stress amounts were above the critical strength of the ceramic material used. The analysis of malposition showed the necessity of exact alignment of the implant components.
Article
A complication of total knee arthroplasty (TKA) is fatigue-type wear, which can destroy a tibial inlay in <10 years. This deleterious wear mechanism occurs during cyclic loading if the yield stress of polyethylene is exceeded. Because increased stress on and within the polyethylene inlay is associated with increased wear, it is important to reduce the inlay stress by either activity restrictions or conformity changes of design. All stress parameters are more sensitive to conformity changes (eg, design changes) than to load changes (eg, activity restrictions). However, the reduction of stress on and within the polyethylene through increased conformity will increase the stress at the tibial fixation interfaces. An attempt was made to solve this problem with the introduction of mobile-bearing designs. Many mobile-bearing designs exist with good long-term results. One important difference among the various designs is the amount of flexion range with full conformity between the femoral component and the tibial inlay. Although a single radius design reduces polyethylene stress throughout the flexion range, it may be disadvantageous for a revision design to intraoperatively adapt to different degrees of constraint. Aseptic loosening and osteolysis due to small abrasive and adhesive wear particles have also been reported as a cause of failure. The design and material parameters affecting polyethylene wear in TKAs, as well as the potential detrimental effects of wear particle size, are the key issues in defining the life of a TKA.
Article
Objective: To compare the clinical results of mobile-bearing and fixed-bearing prostheses in total knee arthroplasty (TKA) during 10 years follow-up so as to provide a reference for clinical selection of TKA prosthesis. Methods: Between January 2002 and December 2005, 113 patients with osteoarthritis of the knee joint underwent primary TKA, and the clinical data were retrospectively analyzed. Mobile-bearing prosthesis was used in 47 cases (group A) and fixed-bearing prosthesis in 66 cases (group B). There was no significant difference in age, gender, body mass index, varus and flexion deformity of the knee, range of motion (ROM) of the knee, knee society score (KSS), and Western Ontario and McMaster University Osteoarthritis Index (WOMAC) between 2 groups before operation ( P>0.05), so the data were comparable. Results: The operation time of groups A and B was (88.1±6.5) and (90.3±7.2) minutes respectively, showing no significant difference ( t=1.666, P=0.099). The wounds healed by first intention in all patients of both groups, and no postoperative early complications of incision infection and deep venous thrombosis occurred. The follow-up time was 10.2-12.3 years (mean, 10.8 years) in group A, and was 10.2-12.6 years (mean, 11.3 years) in group B. Revision was performed in 3 cases of group A and 4 cases of group B; the survival rates of prosthesis were 93.6% and 93.9% in groups A and B respectively, showing no significant difference ( χ2=0.005, P=0.944). The postoperative knee ROM, KSS score, and WOMAC score were significantly improved when compared with preoperative ones ( P<0.05). The knee ROM and KSS score of group B were significantly better than those of group A at 6 weeks after operation ( P<0.05), but no significant difference was found between 2 groups at 1, 3, and 10 years after operation ( P>0.05). The WOMAC score of group A was significantly better than that of group B at 10 years after operation ( t=2.086, P=0.037), but no significant difference was shown at 6 weeks, 1 year, and 3 years after operation ( P>0.05). At 10 years after operation, the excellent and good rate of KSS score was 87.2% in group A and was 84.8% in group B, showing no significant difference ( χ2=0.018, P=0.893). Conclusion: Good medium- and long-term clinical results can be achieved in TKA with both mobile-bearing and fixed-bearing prostheses. The TKA with fixed-bearing prosthesis is relatively simple with better early effectiveness of rehabilitation; and the TKA with mobile-bearing prosthesis could provide better long-term degree of satisfaction in WOMAC score, but a higher surgical skill and soft tissue balance techniques are needed.
Article
Aseptic loosening of knee implants is multifactorial. The purpose of this study was to examine the role of body mass index and tibial component size on loosening and migration of total knee arthroplasty (TKA) implants. Six thousand five hundred forty-eight primary TKAs (anatomic graduated components) were studied with aseptic loosening as the end point. A second cohort of 81 TKAs (anatomic graduated components) had radiostereometric analysis beads implanted, permitting implant migration assessment. Patient mass and tibial component surface area were used to calculate potential tibial stresses. We found that higher potential stresses were associated with increased rates of loosening in both metal-backed and all-polyethylene designs. Increased tibial implant migration, in both the varus-valgus and anterior-posterior directions, was associated with higher estimated stresses. We found an increased rate of tibial component loosening and implant migration in patients with a higher mass and smaller tibial component size.
Article
The influence of the total joint components' elastic deformation on lubrication is generally accepted, but little is known about the influence of joint conformity under hydrodynamic lubrication based on fluid film interposition. The aim of this study was to evaluate induced pressure and stresses in the knee under fluid film lubrication during the stance phase of walking under various joint conformity conditions. A theoretical two-dimensional (2D) geometric model of knee prosthesis contact, with Dirichlet boundary conditions at both edges, and with a conformity index (CI) of 0, 0.3, 0.5, 0.6, 0.7, 0.8, 0.9, 0.92, 0.94, 0.96, 0.98, 0.99, 0.995, and 1.0, was used to calculate the spatiotemporal lubricant flow on a synovial fluid rheological model. With the instantaneous load as a source term, the Reynolds lubrication equation was subsequently solved following a finite volume approach in two dimensions and three dimensions. Conformity strongly influenced the peak pressure, from 47 MPa with CI = 0 to 1.4 MPa with CI = 1, with a definite behavior change from CI = 0.96. The role of hydrodynamic lubrication was restricted to early steps of the stance phase. With CI < 0.96, there was a smooth maximum pressure decrease with increasing CI. In contrast, the maximum pressure fell abruptly with conformity > 0.96. The present model suggested the limited modifying effect of hydrodynamic lubrication in total knee replacement systems. However, its role during the early stance phase, coupled with high conformity, helps significantly to decrease compressive stresses on the polyethylene, fostering the beneficial effect of high conformity in a mixed lubrication regime. This beneficial effect may also be of great interest in total knee replacement systems based on materials with less deformation.
Article
The purpose of this randomized, single-blind clinical trial was to compare a rotating platform (RP) total knee arthroplasty to a fixed-bearing (FB) total knee arthroplasty. Ninety-five knees in 69 patients were implanted by 2 surgeons. There were no significant differences in the preoperative demographics. At a minimum of 2-year follow-up, clinical outcomes and complication rates were similar, with the exception that the RP group had significantly better stair-climbing scores (P = .04). Postoperative range of motion was equally good in both groups (FB knees, 1°-125°; RP knees, 1-126°). There were no bearing dislocations in the RP group. In conclusion, this RP design performs at least as well as the FB version, and the RP patients reported better stair-climbing ability. Enthusiasm for this finding should be tempered by the relatively small sample size.
Article
Patienten möchten sich nach einer Knieprothese häufig wieder sportlich betätigen. Die bisherigen Empfehlungen basieren jedoch auf dem Gefühl des Orthopäden und nicht auf wissenschaftlich fundierten Daten. Jede sportliche Aktivität bewirkt im Gelenk einen zusätzlichen Abrieb, was die Lebensdauer einer Prothese negativ beeinflussen kann. Um diesen Abrieb möglichst gering zu halten, sollten sportlichen Aktivitäten eine geringe Spannung auf dem Polyethylen Inlay hervorrufen. Die folgende Arbeit versucht anhand der Literatur sowie biomechanischer Überlegungen sportliche Aktivitäten mit möglichst geringen Belastungen des Inlays zu finden. Beim Gehen auf der Ebene können Kniegelenkskräfte von 3- bis 4-mal Körpergewicht bei 20 ° Knieflexion auftreten. Beim abwärts Gehen steigen die Gelenkkräfte aufs 8 fache des Körpergewichts bei 40 ° Knieflexion. Beim Fahrrad Fahren besteht eine Kniegelkenksbelastung von 1,2-mal Körpergewicht bei 80 ° und beim langsamen Joggen 8- bis 9-mal Körpergewicht bei 50 ° Knieflexion. Wegen der Geometrie der Femurkomponente spielt beim Kniegelenk, im Gegensatz zur Hüftprothese, auch der Flexionswinkel für die Kontaktfläche und die Inlay Spannung eine große Rolle. So kann eine Knieprothese extensionsnahe stärker belastet werden als in starker Flexion. Aktivitäten wie Joggen produzieren sehr hohe Inlayspannungen und sollten nach einer Knieprothese gemieden werden. Auch abwärts Gehen produziert wegen der grossen Gelenkkraft und des Flexionswinkels hohe Inlay Spannungen. Beim Wandern sollten sich die Patienten auf das aufwärts Gehen beschränken und abwärts die Bahn benutzen. Falls die Patienten dennoch abwärts gehen müssen, sollten unbedingt Stöcke zur Entlastung gebraucht werden. Dies bringt eine Reduktion der Kniegelenksbelastung bis zu 20 %. Weiter empfiehlt sich ein Verzicht auf Abkürzungen sowie langsames Gehen. Fahrradfahren oder Power-Walking scheinen geeignete Sportaktivitäten nach einer Knieprothese zu sein.
Article
Unicompartmental knee replacement offers an effective treatment for patients with single compartment knee disease and is becoming an increasingly popular alternative to total knee replacement. An important cause of failure in a unicompartmental knee replacement implant is polyethylene wear. Significant contributory factors to the amount of polyethylene wear are contact stress, bearing alignment, congruency and thickness. Four different unicompartmental knee replacement implant designs (Fully-Congruent; Partially-Congruent; Non-Congruent-metal-backed; Non-Congruent-all-polyethylene) were inserted into a validated finite element model of a proximal tibia. The effect that bearing congruency, alignment and thickness had on the polyethylene stresses during a simulated step-up activity for each design was investigated. Additionally, contact pressures were compared to those calculated from Hertz elastic theory. Only the Fully-Congruent bearing experienced peak von Mises and contact stresses below the lower fatigue limit for polyethylene during the step-up activity. The highest polyethylene contact stresses were observed for the Partially-Congruent and Non-Congruent-metal-backed designs, which experienced approximately three times the polyethylene lower fatigue limit. Increasing the bearing thickness from 3.5mm to 8.5mm of the Non-Congruent design decreased the contact stresses in the bearing; however they did not fall below the lower fatigue limit for polyethylene. Good agreement between finite element and Hertz contact pressures was found. Fully congruent unicompartmental knee replacement bearings can be markedly thinner without approaching the material failure limit, have a greater potential to preserve bone stock and are less likely to fail mechanically.
Article
This article presents a literature review of the current recommendations regarding sports after total joint replacement and also suggests scientifically based guidelines. Patients should be encouraged to remain physically active for general health and also for the quality of their bone. There is evidence that increased bone quality will improve prosthesis fixation and decrease the incidence of early loosening. To recommend a certain activity after total knee or hip replacement, factors such as wear, joint load, intensity and the type of prosthesis must be taken into account for each patient and sport. It has been shown that the reduction of wear is one of the main factors in improving long-term results after total joint replacement. Wear is dependent on the load, the number of steps and the material properties of total joint replacements. The most important question is, whether a specific activity is performed for exercise to obtain and maintain physical fitness or whether an activity is recreational only. To maintain physical fitness an endurance activity will be performed several times per week with high intensity. Since load will influence the amount of wear exponentially, only activities with low joint loads such as swimming, cycling or possibly power walking should be recommended. If an activity is carried out on a low intensity and therefore recreational base, activities with higher joint loads such as skiing or hiking can also be performed. It is unwise to start technically demanding activities after total joint replacement, as the joint loads and the risk for injuries are generally higher for these activities in unskilled individuals. Finally, it is important to distinguish between suitable activities following total knee and total hip replacement. To recommend suitable physical activities after total knee replacement, it is important to consider both the load and the knee flexion angle of the peak load, while for total hip replacement, which involves a ball and socket joint, the flexion angle does not play an important role. During activities such as hiking or jogging, high joint loads occur between 40 and 60° of knee flexion where many knee designs are not conforming and high polyethylene inlay stress will occur. Regular jogging or hiking produces high inlay stress with the danger of delamination and polyethylene destruction for most current total knee prostheses. Based on these design differences between hip and knee replacements it is prudent to be more conservative after total knee arthroplasty than after total hip arthroplasty for activities that exhibit high joint loads in knee flexion.
Chapter
Aseptic loosening after TKR is the most common cause of late failure and revision. The main culprit of aseptic loosening is increased wear of early generation polyethylene, especially in thin and low-conformity inserts, associated with some implant designs such as discontinuous porous-coating cementless components. Additionally, surgical factors such as malalignment, malposition, and uncorrected instability may contribute to aseptic loosening. It is crucial to maintain the integrity of the implant-bone interface to avoid debris penetration into the bone. Counseling of patients at risk, particularly young and active individuals with elevated body mass index, should play a role on the prevention of this condition. When aseptic loosening is the cause of an unsatisfactory outcome and warrants revision, a detailed history, physical exam, and additional test should be obtained thoroughly to avoid misdiagnosis and adverse revision results.
Article
Tegenwoordig zijn microfracture, mozaïekplastie en autologe chondrocyt implantatie herstelprocedures voor een kraakbeenlijden Graad III en IV. Het merendeel van de patiënten geeft aan beduidend beter te functioneren. De ACI geeft de meeste kans op kraakbeenregeneratie, weliswaar na een langdurige revalidatie. In tegenstelling tot MF en mozaïekplastie vertoont de ACI ook goede resultaten op lange termijn. Hoe de revalidatie optimaal dient te verlopen is nog onvoldoende onderzocht. De rationale voor het gebruik van bepaalde componenten zoals steunname, mobilisaties, bracing, continue passieve bewegingen, fietsen, roeien en andere ‘low impact’ activiteiten kunnen wel worden geduid.
Article
The purpose of this book is to bring together experts from the medical and engineering fields in which trauma acts as a fulcrum in understanding the engineering approach to medical cases. The emphasis of this book is on the retrospective study of medical scenarios as seen from the engineering perspective. An in-depth study is required to ensure the accuracy of both medical and engineering data. Where static, dynamic, temperature and impact loads and velocities/accelerations are unknown; they are evaluated using the material properties and fracture geometry of case studies. From the analytical techniques, a prospective study would assist in predicting the outcome of post-trauma damage. Generally, the book covers a wide spectrum of trauma case studies and could be used in medico-legal test cases. The medical opinion can be translated into the engineering analysis there by validating or invalidating the total medical decisions. Hence the engineering profession in many simple and complicate medical test cases would assist the medical profession in providing results, thereby, cementing the bond between them on a common platform. The book can be used by engineer and medical doctors actively involved in trauma and traumatic injuries, technologist, manufacturers, mathematicians and specialists in computer aided techniques and lawyers and experts in medico-legal cases involved in areas of validation and verifications. This book is also relevant to post graduate courses in trauma, biomechanics, medical equipments manufacture and for engineers, in particular, in the fields of impact, dynamics and medical management and trauma psychology.
Article
Background Rotating-platform TKA, although purported to have superior kinematics, has shown no clinical advantages over those of fixed-bearing TKA. Our design-matched retrieval study aimed to investigate if differences in bearing wear damage exist between fixed- and mobile-bearing TKAs with similar condylar geometry. Questions/purposes We asked whether (1) the rotating platform’s more conforming tibiofemoral articulation would be associated with less severe damage; (2) the location of damage and wear would be similar on the tibiofemoral or backside surfaces of two contemporary designs with similar condylar geometry; and (3) the combined damage and deformation measured as thickness would differ between the two designs. Methods We performed damage grading and damage mapping on 25 rotating-platform and 17 fixed-bearing inserts. The patient demographic data from each of these cohorts were comparable. Inserts were also laser-scanned from which we obtained thicknesses, and inferior surface three-dimensional scans, from which we determined dimensional changes. Results Rotating-platform and fixed-bearing inserts had similar tibiofemoral damage scores. However, the scores on the inferior surface of rotating platforms were greater, often as a result of third-body debris scratching observed on both damage mapping and three-dimensional scans. The extent of damage as a function of surface area was greater for rotating platforms, consistent with the greater tibiofemoral conformity. Dimensional changes on the inferior surfaces of the fixed bearing followed loading areas of the knee. However, no differences were seen in the thicknesses between fixed- and rotating-platform bearings. Conclusions The increased total damage score on the rotating platform, coupled with increased surface area damaged and the propensity for third-body debris, indicates no damage advantage to this mobile-bearing design.
Article
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Objectives The medially spherical GMK Sphere (Medacta International AG, Castel San Pietro, Switzerland) total knee arthroplasty (TKA) was previously shown to accommodate lateral rollback while pivoting around a stable medial compartment, aiming to replicate native knee kinematics in which some coronal laxity, especially laterally, is also present. We assess coronal plane kinematics of the GMK Sphere and explore the occurrence and pattern of articular separation during static and dynamic activities. Methods Using pulsed fluoroscopy and image matching, the coronal kinematics and articular surface separation of 16 well-functioning TKAs were studied during weight-bearing and non-weight-bearing, static, and dynamic activities. The closest distances between the modelled articular surfaces were examined with respect to knee position, and proportions of joint poses exhibiting separation were computed. Results Overall, 1717 joint poses were analyzed. At a 1.0 mm detection threshold, 37 instances of surface separation were observed in the lateral compartment and four medially (p < 0.001). Separation was activity-dependent, both laterally and medially (p < 0.001), occurring more commonly during static deep flexion in the lateral compartment, and during static rotation in the medial compartment. Lateral separation occurred more frequently than medial during kneeling (7/14 lateral vs 1/14 medial; p = 0.031) and stepping (20/1022 lateral vs 0/1022 medial; p < 0.001). Separation varied significantly between individuals during dynamic activities. Conclusion No consistent association between closest distances of the articular surfaces and knee position was found during any activity. Lift-off was infrequent and depended on the activity performed and the individual knee. Lateral separation was consistent with the design rationale. Medial lift-off was rare and mostly in non-weight-bearing activities. Cite this article: S. Key, G. Scott, J.G. Stammers, M. A. R. Freeman†, V. Pinskerova, R. E. Field, J. Skinner, S. A. Banks. Does lateral lift-off occur in static and dynamic activity in a medially spherical total knee arthroplasty? A pulsed-fluoroscopic investigation. Bone Joint Res 2019;8:207–215. DOI: 10.1302/2046-3758.85.BJR-2018-0237.R1.
Purpose: Total knee arthroplasty is one of the most successful procedures of modern orthopedics. Several implants have been proposed over the years with different designs, kinematics, and cementing techniques, with good results. The aim of the study was to assess the clinical and radiographic long-term follow-up of a series of patients undergoing total knee replacement that used a specific design of knee implant with cemented femoral and patellar components, and a hybrid fixation technique for tibial trays that used a cemented base plate and press-fit keels. Methods: A total of 145 implants in 135 patients were studied with clinical and radiologic evaluations. The mean follow-up was 17.1 years. Results: Seven failures for aseptic loosening and four failures for infection were registered. Twenty-nine implants showed nonprogressive radiolucencies, mostly at a single component, which did not need revision. The overall survivorship at the mean follow-up of 15 years considering aseptic loosening as the endpoint was 92.1%. Conclusions: The authors confirm the good rates of success and the long-term survival of this specific implant and the effectiveness of the tibial hybrid cementing technique, which is still debated among researchers.
Article
This study compared the damage scores and damage patterns in 19 tibial inserts from rotating hinge (RH) implants with 19 inserts from highly constrained (HC) implants. Each insert was divided into 16 damage zones and each zone was subjectively graded from a scale of 0-3 for seven different damage modes. The overall damage scores were comparable for the two groups (RH: 64.1 ± 15.4; HC: 66.1 ± 29.0; P = 0.59). The HC group, however, had greater post damage (compared to the post-hole of RH) while the RH group had greater backside damage. The pattern of damage was also different, with burnishing and cold flow being more common in HC group while pitting, scratching and embedded debris were more common in the RH group. Copyright © 2015. Published by Elsevier Inc.
Article
Background: A total knee arthroplasty (TKA) is still not able to reinstate the physiological kinematics of the knee. This results in a considerable number of unsatisfied patients, especially if they are younger and active. Recently developed TKA systems claim to consider these biomechanical properties. The current study investigates the outcome (range of motion, stability) of the new ATTUNE™ TKA system (DePuy, Warsaw, IN, USA) in in young patients. Material and methods: This was a prospective study design (55 patients, 55 knees, age < 70 years, 01/2012-07/2012). Patients received an ATTUNE™ TKA and were examined preoperatively and at 6 months postoperatively for range of motion and stability (Knee Society score). Results: Age: 63 ± 8 years; range of motion increased from 112.33 ± 13.6° to 123.60 ± 11,1° (p < 0.001). Coronal stability improved significantly (preoperatively vs. postoperatively, p < 0.001). Sagittal stability was not significantly different (preoperatively vs. postoperatively, p > 0.05). Conclusion: The data of this study show improved early functional results in younger patients in comparison to the current literature. However, until now no existing TKA system can be identified as ideal. But taking recent biomechanical knowledge into consideration, modern TKA designs have the potential to improve the functional outcome. Especially for younger patients with superior muscle-status and coordinative abilities this seems beneficial.
Article
Full-text available
Estimates of knee joint loadings were calculated for 12 normal subjects from kinematic and kinetic measures obtained during both level and downhill walking. The maximum tibiofemoral compressive force reached an average load of 3.9 times body-weight (BW) for level walking and 8 times BW for downhill walking, in each instance during the early stance phase. Muscle forces contributed 80% of the maximum bone-on-bone force during downhill walking and 70% during level walking whereas the ground reaction forces contributed only 20% and 30% respectively. Most total knee designs provide a tibiofemoral contact area of 100 to 300 mm ² . The yield point of these polyethylene inlays will therefore be exceeded with each step during downhill walking. Future evaluation of total knee designs should be based on a tibiofemoral joint load of 3.5 times BW at 20° knee flexion, 8 times BW at 40° and 6 times BW at 60°.
Article
Full-text available
Experiments were carried out to determine the optimum conformity between the femoral and tibial condyles in condylar replacement knee prostheses. Wear tests and observations from removed prostheses indicated that both high and low conformity produced characteristic abrasion and fatigue. Partly conforming condyles provided stability under load-bearing but allowed laxity to occur. Fixation to resist the various forces on the tibial components was enhanced by a short central intramedullary peg. Partial conformity is proposed as the optimum configuration between femoral and tibial components.
Article
Full-text available
Two women underwent a PCA total knee arthroplasty of the metal-backed type in which the tibial insert was only 4 mm thick in its thinnest portion. After 2 and 4 years, both women had pain on weight bearing due to polyethylene breakdown causing metal-to-metal contact. Such thin polyethylene components should not be used for knee replacements.
Article
Full-text available
A study of the walking activities of 243 individuals was carried out. The individuals came from four different occupations and had an age range of 17-83 years. The survey carried out in this investigation showed surprisingly little correlation between variables such as age, height, and weight of individuals and their speed of walking, length of stride, or distance walked. Correlation matrices were obtained for the whole sample and then for each sex, showing similar trends. The most significant correlation was between the height of an individual and the length of the stride, and there was a lesser correlation between age and the number of steps walked by an individual per day. Further statistical analyses showed that males in manual occupations walked most and those in sedentary occupations walked least. On the other hand, in the female groups housewives seemed to walk least and those in technical occupations walked most. The average number of steps walked per day by a male individual for the whole sample was 9537; that for females was 9839. The corresponding distances walked per day were 6.7 and 6.5 km. The differences were not statistically significant. Predictions of wear of prosthetic components made of ultrahigh molecular weight polyethylene were made on the basis of the above data and other variables affecting wear, such as the weight of the subject and the area available for contact during walking. Charts have been constructed of the penetration of the metallic component into the plastic one for both hip knee prostheses, thus enabling predictions of the wear of the plastic components of these two most widely used prostheses. Owing to the wide ranging values of the variables used in making the predictions of wear, these latter should be regarded only as 'safe' first estimates.
Article
Our aim was to analyse the influence of the size, shape and number of particles on the pathogenesis of osteolysis. We obtained peri-implant tissues from 18 patients having revision surgery for aseptically loosened Freeman total knee replacements (10), Charnley total hip replacements (3) and Imperial College/London Hospital double-cup surface hip replacements (5). The size and shape of the polyethylene particles were characterised using SEM and their concentration was calculated. The results were analysed with reference to the presence of radiological osteolysis. The concentration of polyethylene particles in 6 areas with osteolysis was significantly higher than that in 12 areas without osteolysis. There were no significant differences between the size and shape of the particles in these two groups. We conclude that the most critical factor in the pathogenesis of osteolysis is the concentration of polyethylene particles accumulated in the tissue.
Article
In patients with deficient bone in the proximal end of the tibia, the mechanical support of a conventional total knee replacement may be inadequate. We have developed a custom design for use in situations in which there is extensive deficiency of tibial cancellous bone. To do this, we examined conventional and custom prosthetic tibial components using finite-element analysis. Several loading configurations were tested, and the worst loading conditions were found to be those in which eccentric loads were placed on the margin of the tibial component. The results showed that the stresses on the cancellous bone beneath a conventional-design prosthesis may be lowered if a metal tray and metal peg are employed. A salvage-design concept for revision in the presence of deficient cancellous bone was tested analytically and used successfully. This concept requires that some portion of the applied load be transferred directly to the tibial cortical shell. Stresses in the remaining cancellous bone were lowered by the combination of a thickened metal tray and a metal support buttress. Clinical Relevance: This study demonstrates the advantage of metal trays for the tibial plateau and suggests that they should be used whenever the supporting tibial bone is insufficient. In knees in which there are large defects in the bone, direct transfer of the load to the cortical shell through the prosthesis, made possible by a custom design, appears to be necessary.
Article
Kinematic and clinical evaluation of a constant radius of curvature femoral component (Oxford knee) and one with a decreasing radius of curvature (LCS knee) favors the use of a femoral component with decreasing radius of curvature. Constant radius of curvature femoral components tend to move excessively in the posterior direction under load bearing and can increase the incidence of subluxation or dislocation of unconstrained meniscal bearings. Additionally, constant radius femoral components increase the stretch on collateral ligaments which inhibits flexion. They also create edge loading of the bearing at surface runout. Decreasing radius of curvature femoral components maintain more central positioning of meniscal bearings and allow natural collateral ligament tension to prevent bearing dislocations and allow normal flexion, even in the absence of the anterior cruciate. Properly designed decreasing radius of curvature femoral components have demonstrated exceptional wear and dislocation resistance over a 16-year interval.
Article
Objective: A new design concept has been developed to reduce the propensity for dislocation in total hip patients. The ability of this design to increase the stability of the hip joint is studied. Design: The new design involves a convex-curved acetabular lip, extending from the hemispherical articulating surface to the outer edge of the cup. The femoral component has a matching, reverse curve. Background: Dislocation is a continuing problem in total hip arthroplasty, a complication experienced by 2-11% of patients with primary surgeries, and much higher percentage of patients in revision series. Confounding factors and sources of variability in the clinical domain make it difficult to identify specific parameter influences. Methods: A three-dimensional nonlinear finite element model has been developed for the purpose of studying the dislocation event. We report the first use of this finite element model to analyze the potential for improving hip stability by a new total hip component design concept. Results: The results show that this new design achieves 28% more resisting moment build-up during dislocation, and has a higher range of motion from impingement to onset of subluxation. The new curved lip design also develops 50% less polyethylene von Mises stress in the impingement zone. Conclusions: This design has excellent potential for increasing the inherent stability of the total hip joint. Relevance: Recurrent dislocation is the second leading cause of total hip failure next to late loosening. This study shows the potential of a new total hip design to increase the stability of the artificial hip joint.
Article
The objective of this paper is to present a method to describe the three-dimensional variations of the geometry of the three portions forming the distal part of the human femur: the medial and lateral femoral condyles and the intercondylar fossa. The contours of equally spaced sagittal slices were digitized on the distal femur to determine its surface topography. Data collection was performed using a digitizer system which utilizes low-frequency, magnetic field technology to determine the position and orientation of a magnetic field sensor in relation to a specified reference frame. The generalized reduced gradient optimization method was used to reconstruct the profile of each slice utilizing two primitives: straight-line segments and circular arcs.
Article
We recovered 23 meniscal bearings from 18 failed bicompartmental Oxford knee prostheses. They had been implanted for one to nine years. The minimum thickness of the retrieved bearings was measured and compared with the thickness of 25 unused bearings. The mean penetration rate, calculated by two methods, was either 0.043 or 0.026 mm per annum. This compares with 0.19 mm per annum reported for the Charnley hip. The use of a fully congruous meniscal bearing prosthesis can reduce wear in knee arthroplasty to a very low rate.
Article
We examined 86 polyethylene inserts, retrieved from total and unicompartmental knee prostheses after an average of 39.5 months in situ, grading them from 0 to 3 for seven modes of polyethylene degradation. Severe wear, with delamination or deformation, was observed in 51% of the implants, and was associated with time in situ, lack of congruency, thin polyethylene, third-body wear debris, and heat-pressed polyethylene. Significant under-surface cold flow was identified in some areas of unsupported polyethylene, and was associated with delamination in the load-bearing areas of thin inserts above screw holes in the underlying metal tray. We recommend the use of thicker polyethylene inserts, particularly in young, active patients and in designs with screw holes in the tibial baseplate. Thin polyethylene inserts which are at risk for accelerated wear and premature failure should be monitored radiographically at annual intervals.
Article
Isolated wear of the polyethylene tibial component led to failure in five of a series of 108 uncemented porous-coated knee replacements. The clinical features included pain, effusion and instability with progressive varus deformity. In all cases there was extensive wear on the medial side of the polyethylene surface of the prosthesis. The mechanism of such wear is complex, being due in part to the unconstrained nature of the joint and the incongruity of its surfaces. Other design characteristics may have contributed.
Article
This study examines the histological effects of different sizes of polyethylene particles implanted into the rabbit tibia. Seventeen mature New Zealand white female rabbits were allocated into three groups. Group 1 (5 rabbits) received polyethylene particles averaging approximately 16 microns in diameter, implanted into the right proximal tibia through a drill hole. Group 2 (5 animals) received particles averaging 26 microns, and Group 3 (7 rabbits) received particles averaging 67 microns. The left tibia was drilled but not implanted. Animals were sacrificed after 16 weeks. Histological analysis disclosed decreased hematopoietic activity within the left tibial drill hole. In all groups, the right tibia demonstrated positively birefringent polyethylene particles surrounded by, and within (smaller particles), histiocytes and giant cells in a fibrous tissue stroma. Statistical analysis disclosed more fibrocytes and less marrow cells at the interface of Group 3 (largest particles) compared to Group 1 and 2. Larger polyethylene particles, being less readily phagocytosed, appear to produce more fibrous encapsulation, compared to particles of a smaller size. The histological reaction stimulated by the different sizes of polyethylene particles resembled the membrane surrounding loose joint arthroplasties in humans.
Article
We investigated in vitro a mechanism by which particulate debris may induce bone resorption and cause implant loosening. We first studied two standard particles: latex, which is considered to be inert, and zymosan, which is inflammatory. Macrophages that phagocytosed either particle became activated, and stimulated 15 times as much bone resorption as did control macrophages. For activation to occur, 100 times more latex than zymosan had to be phagocytosed. We also found that bone cement and polyethylene particles activated macrophages in a similar manner, and that the necessary amounts of these were intermediate between those of latex and zymosan. None of the particles were toxic. It was concluded that implant loosening may result from bone resorption stimulated by mediators released by macrophages that have phagocytosed particles of bone cement or polyethylene.
Article
Surfaces for condylar total knee replacement are designed using computergraphics techniques. An average anatomical femoral surface is represented mathematically. Mathematical equations are written to describe normal knee motion and normal laxity. Tibial surfaces are generated by placing the femur stepwise in multiple sequential positions, through a defined three-dimensional motion or laxity path. In addition, a flat tibial surface is defined, to represent the least amount of femoral-tibial conformity in currently-used knee replacements. Elasticity theory is used to calculate the maximum contact stresses at the femoral-tibial contact points. The least stresses are produced with a fixed axis cylindrical motion, while the highest are with a flat tibial surface. A surface based on laxity produces lower stresses than for normal knee motion, and is thought to be acceptable in terms of both freedom of motion and stability. Such a laxity surface is proposed as being suitable for total knee design.
Article
Debris resulting from damage to the surface of polyethylene components of total joint replacements has previously been shown to contribute to long-term problems such as loosening and infection. Surface damage has been associated with fatigue processes due to stresses arising from contact between the metal and polyethylene components in these prostheses. In the present study, we used elasticity and finite-element solutions to determine these stresses for total hip replacements with head diameters of twenty-two and twenty-eight millimeters and for a condylar total knee replacement. We also examined the effect on these stresses of using carbon-fiber-reinforced polyethylene instead of plain polyethylene. Stresses associated with surface damage in the tibial component of the total knee replacement were much larger than those in the hip replacements. The analysis of contact stress as a function of thickness of the polyethylene insert for tibial components showed that a thickness of more than eight to ten millimeters should be maintained when possible. The contact stress in the tibial components was reduced most when the articulating surfaces were more conforming in the medial-lateral direction. Contact stresses were much less sensitive to changes in geometry in the anterior-posterior direction. For the hip components, the stresses were lower in the acetabular component of the twenty-eight-millimeter hip replacement than in the twenty-two-millimeter replacement. The use of carbon-fiber-reinforced polyethylene resulted in stresses that were higher by as much as 40 per cent. Because the contact area between articulating surfaces moves during flexion, portions of the surface will be subjected to cyclic stresses. The contact area for the knee replacements in flexion was smaller than for the hip replacements, and the range of the maximum principal stress was larger. Consequently, the combination of the higher stress and the moving contact area is more likely to cause surface damage due to fatigue in tibial components than in acetabular components, which is consistent with clinical observations.
Article
A saggital plane biomechanical analysis of I I slow jogging trials yielded joint moments of force. power curves and positive and negative work at each of the joinrs of the lower limb. The following can be summarized: 1. The total moment of force pattern of the lower limb was primarily extensor during stance and flexor during swing. The hip had an extensor peak at 20"". the knee at 40Y, and the ankle at 60"" of stance. 2. The variability of the moment patterns across all trials was considerably less than that seen during natural walking. 3. Two power bursts wereseen at the ankle, absorption early in stance followed by adominant generation peak during late push-off. The average peak of power generation was 800 W with individual maximums exceeding 1500 W. 4. Power patterns for all trials showed the knee to have fivedistinct phases: an initial shock absorbing peak during weight acceptance. a small generation burst during early push-off, a major absorption pattern during late push-08continuing until maximum knee flexion, a third absorption peak decelerating the leg and foot prior to impact,and a final small positive burst as the knee flexors rotate the leg posteriorly to further reduce the forward velocity of the foot prior to heel contact. 5. Power patterns at the hip were neither large nor consistent indicating the dual role of hip flexors and extensors relative to the trunk and lower limb stability. 6. Positive work done by the ankle plantarflexors averaged three times that done by the knee extensors, and in some joggers the ankle muscles generated eight times that of the knee muscles. 7. Over the entire stride the knee muscles absorbed 3.6 times as much energy as they generated: the ankle muscles generated 2.9 times as much as they absorbed.
Article
Polyethylene stresses were examined in patellar components with convex-shaped articulating surfaces in contact with convex metallic surfaces. Two finite element models were used: 1 with geometry of a newly manufactured component and 1 with geometry of a retrieved patellar component that had worn and deformed so that the contact area was concave. The range of maximum principal stress and the maximum shear stress were more severe in the newly manufactured model than in the model of the retrieved component. These stresses were also more severe than determined previously for models of femorotibial contact. For both patellar models, the von Mises stress was at or near the polyethylene yield stress in most of the contact area, consistent with the large amount of permanent deformation observed on many retrieved patellar components. This suggests that deformation may continue, even when the component's surface has deformed and worn into a concave shape.
Article
A sagittal-plane model of the lower limb, which considered the possibility of antagonistic and synergistic muscle action and took account of the load-bearing roles of the cruciate ligaments, was applied to a dynamic analysis of level walking. It was hypothesized that: (1) the simple, one-sided constraints that intraarticular contact forces must be compressive and muscle and ligament forces tensile substantially reduce the redundancy of the load-transmitting structures of the lower limb, (2) many previously proposed optimization laws for muscle selection yield equivalent results, when they are applied to a finite set of admissible limiting solutions, and (3) the aforementioned optimization laws, when applied to a finite set of admissible limiting solutions, do not adequately predict the co-contraction of antagonistic muscles during gait. The problem of indeterminacy was resolved by considering all possible limiting solutions of the system unknowns on the dynamic equations.
Article
Various design criteria were examined in combination to find the ideal geometry for a condylar knee replacement. The criteria were the contact stresses on the plastic, femoral-tibial size interchangeability, patella lever arm, laxity and stability and the amount of bone resection required. The variables were the radii of curvature of the femoral and tibial bearing surfaces in the sagittal and frontal planes. Metal toroidal indentors were loaded onto dished surfaces of UHMWPE covering a range of radii and the contact areas measured. Using elasticity equations, the apparent elastic modulus of UHMWPE ranged from 400 to 600 MPa for less conforming to closely conforming surfaces. Using a value of 600 MPa, contact stresses were predicted for a complete spectrum of radii of curvature. Finite element analysis was used to determine the stresses beneath the contact patches when different femoral-tibial sizes were interchanged. A computergraphics program was written to analyse the effects of flexion, rotation and femoral roll-back on the contact point locations. An influential variable was the sagittal curvature of the femoral component, notably the point of transition between the posterior curve of small radius and the distal curve of larger radius. This affected the patella lever arm, the stability, and the bone resection. Interchangeability was primarily dependent upon the relative frontal radii. Contact stresses and contact locations depended upon the combination of sagittal and frontal radii. The most suitable geometrical combinations overall were discussed.
Article
We have studied 27 tibial prostheses retrieved from knee replacements after 1 to 9 years. In 22 the femoral components were of cobalt-chrome, in five polyacetal. The design of the components gave a nominal contact area of 320 mm2 on each condyle. The tibial component was of high-density polyethylene (HDP) at least 6 mm thick, and not heat-treated. In the metal/HDP prostheses the average wear rate was 0.025 mm/year. The relative wear on the medial and lateral sides was related to the leg axis. None of the retrieved prostheses showed any severe disruption of their surface. The polyacetal/HDP prostheses showed similar wear with a statistically insignificant trend towards slower penetration. We conclude that the rate of wear of HDP in a conforming tibiofemoral bearing with a fixed tibial component at least 6 mm thick and not heat-treated is slow enough to be safe in clinical practice.
Article
Wear debris particles have been associated with bone resorption and loosening of total joint implants. To characterize the wear particles around failed knee prostheses, tissues adjacent to 47 implants of 3 different designs were evaluated: 29 were posterior cruciate ligament retention type (Group I), 12 were posterior cruciate ligament sacrificing or substituting (Total Condylar, Insall-Burstein), (Group II), and 6 were mobile bearing (posterior cruciate ligament retaining LCS), (Group III). Wear particles were isolated and evaluated using electronic particle quantitation and scanning electron microscopy. The corresponding implants were also evaluated, and the surface damage quantified. The number of particles smaller than 10 microm ranged from 8.9 to 45.8 x 10(9) per gram of tissue (dry weight). Particle number directly correlated with duration of implantation. Group I implants had larger particles, and showed greater surface damage than the other 2 designs. Polyethylene surface damage significantly correlated with particle size more than 10 microm in length. Particle number showed a significant inverse correlation with the thickness of the polyethylene. These results support previous mechanical and retrieval studies suggesting that increased wear may be associated with thinner polyethylene, and there are differing degrees and patterns of wear based on implant design.
Article
The application of image analysis techniques to the characterization of wear particles from failed joint replacements has been described. Wear particles were extracted from periprosthetic tissues collected during revision surgery. Chemical digestive methods were used to separate the wear particles from the biological soft material. The particles isolated were examined by optical and scanning electron microscopy. Digitized particle images were analysed on a Macintosh computer by a specially developed software and by the image analysis program 'Prism'. The following numerical descriptors were used to characterize the particles: particle size, boundary fractal dimension and shape parameters such as form factor, roundness, convexity and aspect ratio. Elemental composition of the particles was determined by energy dispersive X-ray spectroscopy. Three selected types of wear particles were analysed and compared: titanium (Ti)-based and calcium (Ca)-based particles from a hip prosthesis and ultra-high molecular weight polyethylene (UHMWPE) particles from a knee prosthesis. The particles exhibited significantly different sizes and their shape numerical descriptors were also different. From the results obtained it appears that computer image analysis of wear particle morphology can be employed to determine the wear processes occurring in the joints. In some cases, the condition of the joint can also be assessed based on this analysis.