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Materials and Design Characteristics of Cervical Arthroplasty Devices
Abstract
Cervical disc arthroplasty has been developed in an attempt to address the concerns of adjacent level degeneration associated with cervical fusion. There are a wide variety of materials available for these devices and specific design characteristics that must be considered in selecting the most appropriate device.
... Total disk arthroplasty devices can be classified according to modular versus nonmodular design, fixation properties, articular design and composition (uniarticular, biarticular and nonarticular), and kinematics (constrained, semiconstrained and unconstrained) [15,16]. ...
The viscoelastic cervical disk prosthesis ESP is an innovative one-piece deformable but cohesive interbody spacer. It is an evolution of the LP ESP lumbar disk implanted since 2006. CP ESP provides six full degrees of freedom about the three axes including shock absorbtion. The prosthesis geometry allows limited rotation and translation with resistance to motion (elastic return property) aimed at avoiding overload of the posterior facets. The rotation center can vary freely during motion. The concept of the ESP prosthesis is fundamentally different from that of the devices currently used in the cervical spine. The originality of the concept of the ESP(®) prosthesis led to innovative and intense testing to validate the adhesion of the viscoelastic component of the disk on the titanium endplates and to assess the mechanical properties of the PCU cushion. The preliminary clinical and radiological results with 2-year follow-up are encouraging for pain, function and kinematic behavior (range of motion and evolution of the mean centers of rotation). In this series, we did not observe device-related specific complications, misalignment, instability or ossifications. Additional studies and longer patient follow-up are needed to assess long-term reliability of this innovative implant.
... In orthopedic surgery several self-mating metal couples are used for different applications such as screws, bone plates (Vadiraj and Kamaraj, 2006;Weinstein et al., 1973), total disc and hip arthroplasty (THA) (Bono and Garfin, 2004;Jacobs et al., 1994;Scales and Lowe, 1972;Stanton and Eck, 2010;Valdevit and Errico, 2004) as well as surface replacements (SR). For the hip, stainless steel couples were already tested 70 years ago by Wiles (Wiles, 1949;Wiles, 1958) and the Judet brothers (Sherk, 2003), however, these bearings failed for various reasons. ...
In orthopedic surgery, different self-mating metal couples are used for sliding wear applications. Despite the fact that in mechanical engineering, self-mating austenitic alloys often lead to adhesion and seizure in biomedical engineering, the different grades of Co-base alloys show good clinical results, e.g., as hip joints. The reason stems from the fact that they generate a so-called tribomaterial during articulation, which consists of a mixture of nanometer small metallic grains and organic substances from the interfacial medium, which act as a boundary lubricant. Even though stainless steel also generate such a tribomaterial, they were ruled out from the beginning already in the 1950s as "inappropriate". On the basis of materials with a clinical track record, this contribution shows that the cyclic creep characteristics within the shear zone underneath the tribomaterial are another important criterion for a sufficient wear behavior. By means of sliding wear and torsional fatigue tests followed by electron microscopy, it is shown that austenitic materials generate wear particles of either nano- or of microsize. The latter are produced by crack initiation and propagation within the shear fatigue zone which is related to the formation of subsurface dislocation cells and, therefore, by the fact that an Ni-containing CrNiMo solid solution allows for wavy-slip. In contrast to this, an Ni-free CrMnMo solid solution with further additions of C and N only shows planar slip. This leads to the formation of nanosize wear particles and distinctly improves the wear behavior. Still, the latter does not fully achieve that of CoCrMo, which also shows a solely planar-slip behavior. This explains why for metallurgical reasons the Ni-containing 316L-type of steels had to fail in such boundary lubricated sliding wear tribosystems.
Anterior cervical discectomy and fusion (ACDF) has a long track record of pain relief and positive patient-reported outcomes for patients with degenerative disc disease and spondylosis. However, there is a growing desire to preserve motion while relieving pain to prevent future adjacent segment degeneration and pseudoarthrosis. An FDA-approved treatment that has been growing in popularity recently is cervical total disc arthroplasty (TDA), which has proven positive long-term outcomes with strict adherence to indications and contraindications. Patients with single-level and two-level cervical disease should be assessed for infection, osteoporosis, axial neck pain, severe spondylosis or instability, facet arthropathy, less than 3 mm of disc space, and greater than 15° of kyphotic deformity since these are all contraindications to the procedure. In a few randomized controlled trials comparing ACDF and cervical TDA, arthroplasty has demonstrated superiority as far out as 10 years.
In contrast to cervical and lumbar fusion procedures, the principal aim of disk arthroplasty is to recapitulate the normal kinematics and biomechanics of the spinal segment affected. Following decompression of the neural elements, disk arthroplasty allows restoration of disk height and maintenance of spinal alignment. Based on clinical observations and biomechanical testing, the anticipated advantage of arthroplasty over standard arthrodesis techniques has been a proposed reduction in the development of symptomatic ALD. In this review of cervical and lumbar disk arthroplasty, we highlight the clinical results and experience with standard fusion techniques, incidence of ALD in the population of patients with surgical fusion, and indications for arthroplasty, as well as the biomechanical and clinical outcomes following arthroplasty. In addition, we introduce the devices currently available and provide a critical appraisal of the clinical evidence regarding arthroplasty procedures.
In a single-blind, randomised series of knee replacements in 116 patients, we used radiostereometric analysis (RSA) to measure micromotion in three types of tibial implant fixation for two years after knee replacement. We compared hydroxyapatite-augmented porous coating, porous coating, and cemented fixation of the same design of tibial component. At one to two years, porous-coated implants migrated at a statistically significantly higher rate than hydroxyapatite-augmented or cemented implants. There was no significant difference between hydroxyapatite-coated and cemented implants. We conclude that hydroxyapatite augmentation may offer a clinically relevant advantage over a simple porous coating for tibial component fixation, but is no better than cemented fixation.
We studied the incidence, prevalence, and radiographic progression of symptomatic adjacent-segment disease, which we defined as the development of new radiculopathy or myelopathy referable to a motion segment adjacent to the site of a previous anterior arthrodesis of the cervical spine.
A consecutive series of 374 patients who had a total of 409 anterior cervical arthrodeses for the treatment of cervical spondylosis with radiculopathy or myelopathy, or both, were followed for a maximum of twenty-one years after the operation. The annual incidence of symptomatic adjacent-segment disease was defined as the percentage of patients who had been disease-free at the start of a given year of follow-up in whom new disease developed during that year. The prevalence was defined as the percentage of all patients in whom symptomatic adjacent-segment disease developed within a given period of follow-up. The natural history of the disease was predicted with use of a Kaplan-Meier survivorship analysis. The hypothesis that new disease at an adjacent level is more likely to develop following a multilevel arthrodesis than it is following a single-level arthrodesis was tested with logistic regression.
Symptomatic adjacent-segment disease occurred at a relatively constant incidence of 2.9 percent per year (range, 0.0 to 4.8 percent per year) during the ten years after the operation. Survivorship analysis predicted that 25.6 percent of the patients (95 percent confidence interval, 20 to 32 percent) who had an anterior cervical arthrodesis would have new disease at an adjacent level within ten years after the operation. There were highly significant differences among the motion segments with regard to the likelihood of symptomatic adjacent-segment disease (p<0.0001); the greatest risk was at the interspaces between the fifth and sixth and between the sixth and seventh cervical vertebrae. Contrary to our hypothesis, we found that the risk of new disease at an adjacent level was significantly lower following a multilevel arthrodesis than it was following a single-level arthrodesis (p<0.001). More than two-thirds of all patients in whom the new disease developed had failure of nonoperative management and needed additional operative procedures.
Symptomatic adjacent-segment disease may affect more than one-fourth of all patients within ten years after an anterior cervical arthrodesis. A single-level arthrodesis involving the fifth or sixth cervical vertebra and preexisting radiographic evidence of degeneration at adjacent levels appear to be the greatest risk factors for new disease. Therefore, we believe that all degenerated segments causing radiculopathy or myelopathy should be included in an anterior cervical arthrodesis. Although our findings suggest that symptomatic adjacent-segment disease is the result of progressive spondylosis, patients should be informed of the substantial possibility that new disease will develop at an adjacent level over the long term.
Discectomy, decompression, and fusion are traditionally used to manage cervical disc disease accompanied by neural element compression that is refractory to conservative management. Concerns regarding stress at levels adjacent to fusion and possible adjacent-level degeneration as well as a desire to maintain a more normal biomechanical environment have led to investigation of cervical disc replacement as an alternative to fusion procedures. Cervical disc prostheses currently under investigation are constructed of predominantly metal-on-polyethylene or metal-on-metal bearing surfaces, and use roughened titanium surfaces and osteoconductive coatings to facilitate fixation. The unique anatomy and biomechanics of the cervical spine must be considered when extrapolating from the experience of appendicular arthroplasty and lumbar disc replacement.
Both total hip and knee arthroplasty have demonstrated outstanding clinical results. The functional spinal unit composed of the intervertebral disc and facet joints is at least as complex. The intricacies of the coupled motions of the functional spinal unit have made development of an artificial disc a challenge. There have been several failed attempts to create a disc replacement that recapitulates normal motion while providing significant longevity and a low incidence of complications.
Better understanding of the biomechanics of the intervertebral disc complex and improvements in implant material have made successful intervertebral disc replacement a likely reality, now that several artificial discs have completed Food and Drug Administration clinical trials. In this manuscript the authors detail the biomaterials used in disc arthroplasty and discuss joint wear and the host response to wear debris.
In a single-blind, randomised series of knee replacements in 116 patients, we used radiostereometric analysis (RSA) to measure micromotion in three types of tibial implant fixation for two years after knee replacement. We compared hydroxyapatite-augmented porous coating, porous coating, and cemented fixation of the same design of tibial component.
At one to two years, porous-coated implants migrated at a statistically significantly higher rate than hydroxyapatite-augmented or cemented implants. There was no significant difference between hydroxyapatite-coated and cemented implants.
We conclude that hydroxyapatite augmentation may offer a clinically relevant advantage over a simple porous coating for tibial component fixation, but is no better than cemented fixation.
Moldings have been prepared from carbon-PEEK prepreg (APC2) in which the fibers were unidirectional. Their transverse mechanical properties were compared with those of moldings made from unreinforced PEEK. In addition, the water absorption at 23°C, 60°C, and 90°C of the polymer and composite were measured, together with the mechanical properties after various durations of immersion in 90°C water. The results showed that the interface was not affected by the immersion, at least up to 8000 h. There was a slight loss of polymer strength (20%) and failure strain (25%), which was reflected in the composite properties. Water absorption was determined almost entirely by the amount of polymer present in the composite. However, there was a small additional absorption, probably due to the relief of residual stresses. The results with respect to the interface were far more clear cut than those obtained from single fiber tests, supporting the contention that single fiber tests should be phased out as soon as possible.
The objective of our study was to test alternative polymer-on-polymer articulations for cervical total disc arthroplasty with favourable biotribological properties and the benefit of radiolucency in comparison to the clinically well established metal-on-polyethylene coupling. In vitro wear simulation was performed according to ISO 18192-1:2008 (E) with the clinically introduced activ C cervical artificial disc (Aesculap AG Tuttlingen, Germany) made of UHMWPE/CoCr29Mo6 in a direct comparison to experimental disc articulations made of PEEK, CFR-PEEK and PEK. Each material combination was tested for 10 million cycles with a customised 6 station spinal wear simulator (EndoLab Thansau, Germany). Gravimetric and geometric wear assessment, optical surface characterisation and an estimation of particle size and morphology were performed. The gravimetric wear rate of the clinical reference polyethylene-on-cobalt-chromium was 1.0+/-0.1 mg/million cycles, compared to 1.4+/-0.4 mg/million cycles for PEEK, to 0.02+/-0.02 mg/million cycles for CFR-PEEK and 0.8+/-0.1 mg/million cycles for PEK. In conclusion, a number of different candidate materials for total cervical disc arthroplasty were compared using the same disc design. Whereas the polymer-on-polymer articulation of PEK showed no substantial benefit in comparison to polyethylene-on-cobalt-chromium and whereas natural PEEK tends towards pitting and delamination, the carbon fibre reinforced PEEK demonstrated an excellent wear behaviour with a reduction in order of a magnitude. Therefore, the CFR-PEEK based polymer-on-polymer articulations may be an alternative to polyethylene-on-metal and have a high potential for next generation disc replacements.
Wear, wear particle induced inflammation, and osteolysis following total disc arthroplasty were, until recently, not thought to be present due to limited intervertebral motion and the lack of a synovial membrane between the lower lumbar vertebrae. The purpose of this study was to evaluate the periprosthetic tissue reactions associated with total disc arthroplasty revision surgery. Periprosthetic samples of fibrous tissue were collected in all patients during revision surgery of SB Charité III disc prostheses. Revision was indicated for intractable pain after an average of 8 years. Histological evaluation was performed in tissue samples of 16 patients using light microscopy and polarized light microscopy with a magnification of 100x. Polyethylene particles were detected in 15 of 16 patients. The smallest particles were the most numerate. A positive correlation was present between the number of particles per mm(2) and the extent of the chronic inflammatory reaction in the periprosthetic fibrous tissue. Osteolysis was observed in one patient. In the tissue samples containing polyethylene particles, TNF-alpha and IL-6 were determined by immunohistochemistry. TNF-alpha and IL-6 were co-expressed as a subset of mononuclear macrophages and giant cells.
There has been extensive research and development into the use of motion-preserving technology to treat degenerative disc disease in the hope that it will provide a more functional range of motion and decrease adjacent-level disease associated with arthrodesis surgery. Recent reports with short-term follow-up have demonstrated both equivalent and favorable clinical results of total disc arthroplasty in comparison with lumbar arthrodesis surgery1-3. Despite the current use in patients, there is still a paucity of data on long-term outcome associated with the use of these devices. The few existing studies that have had a longer duration of follow-up have provided conflicting information, with some demonstrating overwhelming success4 and others demonstrating multiple complications and a less-than-desirable outcome5,6.
Experimental disc designs such as the AcroFlex (DePuy Spine, Raynham, Massachusetts) have demonstrated the difficulty of transitioning from in vitro to in vivo success. A pilot study that was published in 1993 detailed the results for six patients who had been managed with an AcroFlex disc7. In 1990, the United States Food and Drug Administration (FDA) expressed concern regarding the potential carcinogenicity of the benzene-based solvents used to vulcanize the rubber core8. In the present report, we describe the nineteen-year follow-up results for one of the original six patients who had received a first-generation AcroFlex disc replacement7. To our knowledge, the present report represents the longest-term follow-up on the results of total disc arthroplasty in the literature. This case is of particular interest because the short-term outcome for this patient was rated as “good” in the original 1993 publication7. The patient was informed that data concerning the case would be submitted for publication, and he consented.
A fifty-year-old man presented to the orthopaedic spine clinic in March 2007 …
Fifty-one consecutive patients with cervical radiculopathy or spondylosis were treated with single or multilevel anterior discectomy and fusion using a modified Smith-Robinson procedure. There were 33 single-level fusions, 16 two-level fusions, and 2 three-level fusions. The three modifications included: 1) the endplates at the fusion level were completely removed with a high-speed bur to exposed bleeding cancellous bone in parallel planes; 2) the Caspar distractor (Aesculap, Burlingame, CA) was used to increase distraction and improve visualization; 3) the tricortical autologous iliac crest bone graft was placed in reverse position, that is, with the cortical cross-section facing posteriorly, creating a stabilizing strut in the middle column. With an average follow-up of 1 year, the fusion rate was 94% (67 of 71 levels). The single-level fusion rate was 97%, the two-level fusion rate was 94%, and the three-level fusion rate was 83%. Of the four nonunions, only two were symptomatic. Results by clinical examination revealed 36 (71%) excellent, 11 (21%) good, 3 (6%) satisfactory, and 1 (2%) poor outcomes. There were no significant disc collapses or extrusions. One patient had an increase in kyphotic deformity of > 5 degrees, none with > 10 degrees kyphosis. There were no wound infections or neurologic complications. The modified Smith-Robinson procedure for anterior cervical discectomy and fusion has led to the successful treatment of cervical radiculopathy and spondylosis with improved results and few complications.
This is a case report of a Cloutier total knee arthroplasty that was removed from an obese, large, 66-year-old man three years after implantation because of aseptic loosening. At surgery there was a thickened black synovium and black cystic areas in the exposed bone. The articulating surfaces of the tibial component, which is made from Poly Two (a carbon polyethylene composite), were grossly abraded, and the supporting metal tray was broken in two. The femoral component showed signs of abnormal wear at the places where it was articulating with the displaced tibial component. The tissues showed a granulomatous reaction with marrow fibrosis and cystic destruction of bone. It is postulated that aseptic loosening was accelerated by a granulomatous response to overload, abrasion, and local dissemination of particles.
Properties of injection-molded high-density polyethylene reinforced with carbon fibers were investgiated. With 20% by weight carbon fibers tensile strength of polyethylene was almost doubled. Further increase of volume fraction of fibers did not produce as dramatic results. With 40% by weight carbon fibers, elastic modulus increased one order of magnitude: from 0.225 × 106 psi to 2.48 × 106 psi. This value is close to the value of the elastic modulus of bone, hence the material may substitute for bone where the same stiffness is required. The total strain at fracture was reduced two orders of magnitude, but still stayed comparable to total strain at in bone failures (1–2%).
No appreciable changes were noted in shear strength, impact strength and wear resistance. The work to fracture increased from 0.9 to 2.9 kg cm/cm2 with 40% of carbon fibers.
The most significant improvement of polyethylene with addition of carbon fibers was demonstrated in measurements of viscoelastic properties of the material. Creep characteristics were dramatically improved.
In summary, it can be stated that injection-molded reinforced polyethylene has much superior properties to the nonreinforced P.E. which can be further improved if better adhesion between P.E. matrix and reinforcing fibers can be secured.
The relative fatigue crack propagation resistance of plain and carbon fiber-reinforced ultrahigh molecular weight polyethylene (UHMWPE) was determined from cyclic loading tests performed on compact tension specimens machined from the tibial components of total knee prostheses. Both materials were characterized by dynamic mechanical spectroscopy, X-ray diffraction, and differential scanning calorimetry. The cyclic tests used loading in laboratory air at 5 Hz using a sinusoidal wave form. Dynamic mechanical spectroscopy showed that the reinforced UHMWPE had a higher elastic storage modulus than the plain UHMWPE, whereas X-ray diffraction and differential scanning calorimetry showed that the percent crystallinity and degree of order in the crystalline regions were similar for the two materials. Fatigue crack propagation in both materials proved to be very sensitive to small changes in the applied cyclic stress intensity range. A 10% increase in stress intensity resulted in approximately an order of magnitude increase in fatigue crack growth rate. The fatigue crack propagation resistance of the reinforced UHMWPE was found to be significantly worse than that of the plain UHMWPE. This result was attributed to poor bonding between the carbon fibers and the UHMWPE matrix and the ductile nature of the matrix itself.
Fiber/matrix interfacial bond strength significantly influences the mechanical behavior of fiber-reinforced polymer (FRP) composites. Interfacial bond strength durability is therefore particularly important in the development of FRP composites for implant applications where diffused moisture may potentially weaken the material over time. In this study, the long-term durability of interfacial bonding in carbon fiber/380 grade polyetheretherketone (C/PEEK) and carbon fiber/polysulfone (C/PSF) composites was investigated after exposure to hygrothermal environments. A single fiber pull-out test was used to quantitatively determine the ultimate bond strength (UBS) of the samples following exposure. Samples were tested at three temperatures (37, 65, and 95 degrees C) for six time periods (0-5000 h) and in two environments (dry and physiologic saline-immersed). A mathematical model based on nth order chemical reaction kinetics was applied to describe the long-term durability of the interface. The results of this study indicate that interfacial bond strengths in C/PSF and C/PEEK (380 grade) composites are significantly decreased by exposure to physiologic saline and are functions of both time and temperature. For each material, the kinetics of degradation analysis predicts further bond strength losses following initial saturation, which then stabilizes at temperature-dependent equilibrium bond strength levels.
The authors report a case of massive wear of carbon fiber-reinforced polyethylene used as an insert in a knee prosthesis. The finding, which was studied under a scanning electron microscope, in agreement with mechanical resistance and laboratory testing reported in the literature, confirms that this composite material (poly-two) is not more advantageous to use that ultra-high molecular weight polyethylene in terms of wear, despite favorable experimental premises.
Bone loss induced by the inflammatory response to wear particles is a major cause of long-term failure of total joint replacement. This review describes the cellular reaction occurring in response to these particles and what is currently known about the inflammatory mechanisms contributing to bone resorption.
A retrospective review of all patients surgically treated with a two-level anterior cervical discectomy and fusion with and without anterior plate fixation by a single surgeon.
To compare the clinical and radiographic success of two-level discectomy and the effect of anterior cervical plate fixation.
Prior studies of multisegment fusions have shown decreased fusion rates correlating with the number of increased levels. The use of anterior plates for single-level cervical fusions is controversial. However, their use in multilevel fusions may be warranted because of the increased pseudarthrosis rates.
Over a 6-year period, 60 patients were treated surgically with a two-level anterior cervical discectomy and fusion by the senior author. Thirty-two patients had cervical plates, and 28 underwent fusions without plates. These patients were followed for an average of 2.7 years. Clinical and radiographic follow-up evaluations were performed.
Of the 60 patients, 7 had a pseudarthrosis. The pseudarthrosis rates were 0% for patients with plating and 25% for those with no plating. This difference was statistically significant (P = 0.003). No correlation of pseudarthrosis with gender, age, level of surgery, history of tobacco use, or the presence of prior anterior surgery was found. There was significantly less graft collapse (P = 0.0001) in the patients without plates in whom pseudarthrosis developed (1.4 mm) than in those who had fusions with plates (0.3 mm). The amount of kyphotic deformity of the fused segment was 0.4 degree in patients with plating compared with 4.9 degrees in those without plating who developed a pseudarthrosis (P = 0.0001).
The addition of plate fixation for two-level anterior cervical discectomy and fusion is a safe procedure with no significant increase in complication rates. The pseudarthrosis rates are significantly higher in patients treated without plate fixation. No nonunions occurred in the patients treated with plate fixation. There was significantly less disc space collapse and kyphotic deformity with the plated fusions than with the nonplated fusions, in which a pseudarthrosis developed. The complication rates for plated fusions are extremely low and do not differ from those for nonplated fusions.
The vast majority of total hip prostheses currently implanted consist of a hard metal or ceramic femoral head articulating against an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup. Over the last 10 years, evidence has accumulated to show that these prostheses are prone to failure due to late aseptic loosening and few survive beyond 25 years. With an increasing need to implant hip prostheses in the younger, more active patient the need to understand the mechanisms of failure and to develop artificial hip joints using alternative materials have become major issues in the orthopaedic community. This review focuses initially on our current understanding of the biological reactions to UHMWPE prosthetic wear debris in vivo and in vitro since this is believed to be the main cause of late aseptic loosening. While the precise mechanisms of osteolysis induced by UHMWPE wear debris have not been elucidated, the major message to emerge is that it is not the wear volume that determines the biological response to the debris, but the concentration of the wear volume that is within the critical size range (0.2-0.8 micron) for macrophage activation. The review then considers whether the problem of wear-debris-induced osteolysis may be overcome with the use of new generation metal-on-metal or ceramic-on-ceramic prostheses. For metal-on-metal prostheses, the prospects for increasing the osteolysis free life of the implant are good but additional biological problems associated with the nanometre size and reactivity of the wear particles in vivo may emerge. For the ceramic-on-ceramic prostheses, although initial prospects are encouraging, more data are needed on the characteristics of the wear particles generated in vivo before predictions can be made. It is concluded that the pre-clinical testing of any new materials for joint replacement must include an analysis of the wear particle characteristics and their biological reactivity in addition to the usual assessment of wear.
Unlabelled:
This review constitutes the first of four reviews that systematically address contemporary knowledge about the mechanical behavior of the cervical vertebrae and the soft-tissues of the cervical spine, under normal conditions and under conditions that result in minor or major injuries. This first review considers the normal kinematics of the cervical spine, which predicates the appreciation of the biomechanics of cervical spine injury. It summarizes the cardinal anatomical features of the cervical spine that determine how the cervical vertebrae and their joints behave. The results are collated of multiple studies that have measured the range of motion of individual joints of the cervical spine. However, modern studies are highlighted that reveal that, even under normal conditions, range of motion is not consistent either in time or according to the direction of motion. As well, detailed studies are summarized that reveal the order of movement of individual vertebrae as the cervical spine flexes or extends. The review concludes with an account of the location of instantaneous centres of rotation and their biological basis.
Relevance:
The fact and precepts covered in this review underlie many observations that are critical to comprehending how the cervical spine behaves under adverse conditions, and how it might be injured. Forthcoming reviews draw on this information to explain how injuries might occur in situations where hitherto it was believed that no injury was possible, or that no evidence of injury could be detected.
Three different popular composite resin systems for dental restorative applications are microfilled, minifilled and midifilled types in which deformation under stress and recovery after stress removal may be strongly influenced by filler parameters and resin formulation. The purpose of this in vitro study was to evaluate these differences in selected composite resins.
Three composites, Clearfil (midifill), Charisma (minifill) and Durafill (microfill), were evaluated. Elastic, viscoelastic and viscous deformation of the composites under a constant stress and the subsequent recovery on removal of stress were measured. Dynamical mechanical properties of these materials were also characterized.
The mean values (and SD) of elastic, viscoelastic and viscous components of overall deformation (mm), respectively, under a constant stress of 45(5)KPa were as follows: Clearfil, 0.17(0.03), 0.06(0.02), 0.03(0.05); Charisma, 0.34(0.1), 0.09(0.04), 0.14(0.03); Durafill, 0.68(0.06), 0.14(0.03), 0.23(0.03) The results show that there are significant differences (P<0.0001) between the different composite resins in the elastic, viscoelastic and viscous segments of deformation. The microfilled system deformed significantly more than the minifilled, and the minifilled system more than the midifilled. Storage modulus values evaluated by dynamic mechanical analysis were also significantly different (P<0.0001). The observed differences were readily explained by the differences in the type and volume fraction of the filler content in the composites.
Differences in time dependent deformation and recovery as well as dynamic mechanical properties of dental composites were strongly influenced by filler parameter differences. The clinical performance of restorations may be strongly influenced by the observed differences.
The basic principles of cervical spine surgery continue to include adequate decompression, provision of a structurally competent, biologically functional bone graft, and creation of a stable construct to allow for solid fusion. In recent years, the options to achieve these goals have expanded significantly. Bone banking and bone graft substitutes yield increasingly viable alternatives to autogenous bone graft. New prosthetic implants and cages are currently under investigation. The science of bone growth factors seems to be promising and is expected to revolutionize the approach to spinal arthrodesis. Various plating systems are available to provide internal stability to cervical spine constructs. It is important to understand the biomechanics of plating systems so that the optimal system may be used in a given situation. Long constructs place significant loads on SGs and anterior plates. It is important to consider the use of additional fixation, such as posterior segmental fixation in long constructs, which may be prone to failure using only anterior plate fixation. Anterior cervical plating for single-level ACDF remains controversial, whereas plating has been shown to improve the results of multilevel ACDF. Plating may provide a useful salvage option for a cervical nonunion, especially if deformity or neurologic compression dictates an anterior approach. Hardware failures may occur with anterior cervical plating, but most remain asymptomatic and do not require operative intervention.
Lumbar disc prostheses have been used in treating symptomatic degenerative disc diseases. A few prostheses of the ball-socket design are currently available for clinical use, the joint mechanism being materialized either with a hard polymer core or a metal-to-metal couple. Other prostheses of "shock absorber" design were not available at the time of the study. The objective of this work was to establish whether there was a difference in the shock absorption capacity between a device having an ultra-high-molecular-weight polyethylene center core and a device having a metal-on-metal bearing. Vibration and shock loading were applied to two lumbar total disc prostheses: PRODISC, manufactured by Spine Solutions, and MAVERICK Total Disc Replacement, manufactured by Medtronic Sofamor Danek. The shock absorption capacity of the device was evaluated by comparing the input and the output force measurements. The disc prosthesis was mounted onto a test apparatus. Each side of the device was equipped with a force sensor. The input shock load and the output resulting forces were simultaneously measured and recorded. The loading force pattern included 1). a static preload of 350 N plus an oscillating vibration of 100 N with frequency sweeping from 0 to 100 Hz and 2). a sudden shock load of 250 N applied over a 0.1-second interval. Both input and output signal data were processed and were transformed into their frequency spectrums. The vibration and shock transmissibility of the device, defined as the ratio of the output spectrum over the input spectrum, were calculated in sweeping the frequency from 0 to 100 Hz. The phase deviation was calculated to characterize the shock absorber effects. For both tested devices under vibration and shock loading, the phase angle displacement between the input and the output signals was 10 degrees. Under oscillating vibration loading, both tested devices had a transmission ratio higher than 99.8%. Over the frequency interval 1-100 Hz, the difference in transmission ratio between the two devices was <0.3%. Under sudden shock loading, both tested devices had a transmission ratio higher than 98%. The difference between the two devices was <0.8%. Both tested devices have identical vibration and shock transmissibility.
Advances in cross-linking have led to the development of wear resistant ultrahigh molecular weight polyethylene for total joint replacement. This study compared wear reduction by two different cross-linking methods as measured in a hip wear simulator. One highly cross-linked polyethylene was treated with 7.5 Mrad gamma irradiation with post-irradiation annealing and a sterilization dose of 2.5 Mrad (10 Gamma), while the other used 9.5 Mrad warm irradiation with 10 MeV electron-beam (9.5 EB). Liners of the same design, made from nominally cross-linked (gamma sterilized) polyethylene were also tested. Gravimetric wear analysis was performed every 500,000 cycles for 5,000,000 cycles. After correcting for weight gain due to water absorption, the nominally cross-linked liners demonstrated mean wear rates of 15.7 (+/-1.7) and 12.5 (+/-1.0) mg/million cycles. Both highly cross-linked polyethylene liners demonstrated significantly less wear than their respective controls (with mean wear rates of 1.5 (+/-1.2) and -1.4 (+/-1.5) mg/million cycles). The 9.5 EB liners gained weight presumably due to increased fluid absorption, in addition to that measured in loaded-soaked control implants. Any wear occurring was therefore assumed to have been more than offset by weight gain. Highly cross-linked polyethylene was significantly more wear resistant than non- or nominally cross-linked polyethylene. The differences in wear rates between the two highly cross-linked polyethylene designs (9.5 EB or 10 Gamma) are probably too small to be clinically significant.
Knowledge gained through the clinical history of total joint replacement materials combined with the current promise of new biomaterials provides improved guidelines for biomaterial selection in total disc arthroplasty.
The following will detail: 1) current biomaterials technology; 2) how current designs of total disc arthroplasty seek to optimize implant performance through judicious biomaterial selection; and 3) what technical obstacles and clinical concerns remain.
Metals and polymers remain the central material components of state-of-the-art total joint arthroplasties. Polymers provide low friction surfaces for articulating bearings and some degree of shock absorption. Metals provide appropriate material properties such as high strength, ductility, fracture toughness, hardness, corrosion resistance, formability, and biocompatibility necessary for use in load-bearing roles required total disc replacement. There are three principal metal alloys used in orthopaedics and particularly in total joint replacement: 1) titanium based alloys; 2) cobalt based alloys; and 3) stainless steel alloys. Alloy specific differences in strength, ductility, and hardness generally determine which of these three alloys is used for a particular application or implant component.
Current designs. Two examples of current lumbar (Charitè and Prodisc) and cervical (Bryan and Prestige) disc replacements are compared. The similarities and differences in the biomaterials used for each demonstrate prevailing consensus and some idea of how to best optimize implant performance through biomaterial selection.
The primary factors governing total disc arthroplasty biomaterials are similar to those of all total joint arthroplasties: generation of wear debris is the primary source of implant degradation, and the subsequent tissue reaction to such debris is the primary factor limiting the longevity of joint replacement prostheses. Particulate debris generated by wear, fretting, or fragmentation induces the formation of an inflammatory reaction, which at a certain point promotes a foreign-body granulation tissue response that has the ability to invade the bone-implant interface. This commonly results in progressive, local bone loss that threatens the fixation of both cemented and cementless devices alike. All metal alloy implants corrode in vivo. When severe, the degradative process may reduce structural integrity of the implant, and the release of corrosion products is potentially toxic to the host. The corrosion resistance of implant alloys is primarily due to the formation of passive oxide films to prevent significant electrochemical dissolution from taking place. The result of this knowledge is a consensus of opinion as to which materials are best suited for use in current total disc arthroplasty designs, where most total disc replacement designs incorporate cobalt-chromium-molybdenum alloy endplates articulating internally on a relatively soft polymeric core and externally coated with titanium or titanium alloy for enhanced bone fixation.
UHMWPE wear particles have been implicated in osteolysis, implant loosening, and long-term failure of total hip arthroplasties in vivo. This study examined four carbon-based composite materials as alternatives for UHMWPE in joint bearings. These materials were HMU-CVD, SMS-CVD, P25-CVD, and CFR-PEEK. New bearing materials should satisfy certain criteria: they should have good wear properties that at least match UHMWPE, and produce wear particles with low levels of biological activity. Of the four materials tested in multidirectional pin-on-plate tribological tests, SMS-CVD, P25-CVD, and CFR-PEEK showed lower volumetric wear factors than UHMWPE. P25-CVD had the lowest wear factor of 0.54 +/- 0.34 x 10(-7) mm(3)/Nm. Analysis of P25-CVD wear particles by transmission electron microscopy showed that the debris was very small, with the vast majority of particles being under 100 nm in size, which was similar in size to metal wear particles. The P25-CVD particles were isolated and cultured with L929 fibroblasts and U937 monocytic cells to assess their effect on cell viability. P25-CVD particles were significantly less cytotoxic (p < 0.01, ANOVA) to both cell types than CoCr metal wear particles. This work suggests that carbon-carbon composite materials may have potential for use in total hip replacement bearings. Of the materials tested P25-CVD had the lowest wear factor, and produced very small wear debris that had minimal cytotoxic effect on L929 and U937 cells in vitro. Therefore carbon-carbon composites, such as P25-CVD, may be important in the development of next-generation implants with lower wear rates and reduced cytotoxic potential.
Higher levels of UHMWPE crosslinking currently are being advocated for improved wear resistance of acetabular cups. Pioneering Japanese studies, begun in 1971, have achieved good clinical results with UHMWPE irradiated to 1000 kGy for use with a cemented-cup design. The objective of our study was to use contemporary simulator techniques to determine the in vitro wear performance of such high-dose irradiated cups. Extruded UHMWPE cups were processed with 500, 1000, and 1500 kGy of gamma-radiation doses under vacuum, annealed, and machined to shape. The cups were mated with 26-mm alumina heads and run in a multidirectional simulator with bovine serum. Over a 6-million cycle (Mc) study, the weight loss of the nonirradiated control cups averaged 52.8 mg/Mc + 1.4% (wear = 57.2 mm(3)/Mc). In contrast, the irradiated wear cups had a consistent weight gain. Thus cups with irradiation of 500-1500 kGy had no detectable wear in this study. The original machining marks still were partially evident in the wear zones, along with some macrofissures in the 1000- and 1500-kGy cups. Areas adjacent to the fissures showed delaminating plaques of 100-300 microm in size. It also was noted that the wear cups systematically gained more weight than their corresponding soak controls. Each 200-kGy radiation gain increased the fluid sorption ratio by 10%. The increased fluid sorption and evidence of some surface deterioration may indicate that such high-dose irradiated cups are more susceptible to mechanical damage. This indicates that we should take care to ensure that our desire to reduce the wear debris to a zero amount does not result in a modified UHMWPE that lacks the necessary mechanical properties for contemporary metal-backed cup designs.
Cervical total disc replacement (TDR) is an attractive alternate to arthrodesis for management of disc degeneration and herniation in the cervical spine. Theoretic advantages of TDR include preservation of normal motion and biomechanics in the cervical spine and reduction of adjacent-segment degeneration. Other potential advantages include faster return to normal activity and elimination of the need for bone graft and associated donor site morbidity. This article introduces the rationale and various implant types available for cervical TDR. Part 2 of this series reviews the results and complications of specific implant designs.
Cervical arthroplasty after anterior decompression with insertion of a prosthetic total disc replacement has been suggested as an alternate to anterior cervical fusion. Currently there are four cervical arthroplasty devices available on the market whose results in clinical use have been reported. Each device varies in terms of materials, range of motion, insertion technique and constraint. It is not known which device is ideal. Early studies suggest that in the short term, the complication rate and efficacy is no worse than fusion surgery. Long-term results have not yet been reported. This review examines the current prostheses available on the market as well as discussing issues regarding indications and technique. Pitfalls are discussed and early experiences reviewed. In time, it is hoped that a refinement of cervical arthroplasty occurs in terms of both materials and design as well as in terms of indications and clinical outcomes as spinal surgeons enter a new era of the management of cervical spine disease.
A clinical case series of 4 patients undergoing anterior lumbar revision due to failure of total disc replacement surgery.
To assess the clinical significance of polyethylene wear debris in salvage surgery after initial total disc replacement, the pattern and the mechanisms of polyethylene wear in the retrieved cores, and the extent of polyethylene debris in the periprosthetic tissues obtained from 4 patients.
Previous in vitro wear tests have demonstrated low wear rates for lumbar artificial discs, suggesting that implant wear may not be a clinically relevant issue with total disc replacement. However, only long-term clinical investigations with analysis of retrieved implants and periprosthetic tissue can ultimately establish the significance of polyethylene wear debris for total disc arthroplasty.
Starting in 2004, we began routinely performing salvage procedures in patients with failed total disc replacements. We report on the short-term outcomes of 4 patients at our institution who were revised with a Charité prosthesis (DePuy Spine, Raynham, MA). Wear analysis of the retrieved prosthesis and histologic examination of the periprosthetic tissue were also performed.
All of the retrieved polyethylene cores showed evidence of wear, but the extent and severity varied among the 4 patients. Wear and fracture of the core were associated with osteolysis of the underlying sacrum in 1 patient. Histologic examination of the periprosthetic tissues confirmed the presence of wear debris lying in inflammatory fibrous tissue. In 3 of the 4 patients, implant wear was associated with an unfavorable biomechanical environment (e.g., subsidence, migration, undersizing, and adjacent fusion). The mechanisms of wear included adhesive/abrasive wear of the central domed region of the polyethylene core, as well as chronic rim impingement, resulting in rim fatigue and fracture.
This study demonstrates the clinical significance of polyethylene wear debris and the potential for osteolysis with total disc replacements. The authors recommend that patients undergoing lumbar disc arthroplasty receive long-term follow-up to monitor the wear and functional status of their implants.
Cross-sectional study of 10 patients to measure the serum levels of cobalt and chromium after TDA.
To investigate the release of cobalt and chromium ions into the serum following implantation of the metal-on-metal Maverick-type artificial lumbar disc.
In total hip endoprosthetics and consequently for TDA (total disc arthroplasty), metal-on-metal combinations are used with the aim of reducing wear debris. In metal-on-metal TDA the release of metal ions has until now been secondary to the main discussion.
We investigated the serum cobalt and chromium concentration following implantation of 15 Maverick TDAs (monosegmental L5-S1, n = 5; bisegmental L4-L5 and L5-S1, n = 5; average age, 36.5 years). Five healthy subjects (no metal implants) acted as a control group. The measurements of the metals were carried out using the HITACHI Z-8200 AAS polarized Zeeman atomic absorption spectrometer after an average of 14.8 months.
The concentrations of cobalt and chromium ions in the serum amounted on average to 4.75 microg/L (SD, 2.71) for cobalt and 1.10 microg/L (SD, 1.24) for chromium. Compared with control group, both the chromium and cobalt levels in the serum showed significant increases (Mann-Whitney U test, P = 0.0120). At follow-up,the Oswestry Disability Score was on average significantly decreased by 24.4 points (L5-S1) (t test, P < 0.05) and by 26.8 points (L4-S1) (t test, P < 0.05). The improved clinical situation is also represented by a significant decrease of the Visual Analog Pain Scale of 42.2 points after the follow-up (t test, P < 0.05).
Significant systemic release of Cr/Co was proven in the serum compared with the control group. The concentrations of Cr/Co measured in the serum are similar in terms of their level to the values measured in THA metal-on-metal combinations or exceed these values given in the literature. Long-term implication of this metal exposure is unknown and should be studied further.
Study design:
Prospective, randomized, controlled and double-blinded study on imaging of artificial discs.
Objective:
The purpose of this study is to compare postoperative imaging characteristics of the 4 currently available cervical arthroplasty devices at the level of implantation and at adjacent levels.
Summary of background data:
Cervical arthroplasty is being performed increasingly frequently for degenerative disc disease and, in most cases, with frank neural compression. Unlike lumbar arthroplasty, performed mainly for axial back pain, decompression of neural elements may need to be confirmed with postoperative imaging after cervical arthroplasty.
Methods:
Preoperative and postoperative magnetic resonance imaging scans of 20 patients who had undergone cervical arthroplasty were assessed for imaging quality. Five cases each of the Bryan (Medtronic Sofamor Danek, Memphis, TN), Prodisc-C (Synthes Spine, Paoli, PA), Prestige LP (Medtronic Sofamor Danek), and PCM devices (Cervitech, Rockaway, NJ) were analyzed. Six blinded spinal surgeons scored twice sagittal and axial T2-weighted images using the Jarvik 4-point scale. Statistical analysis was performed comparing quality before surgery and after disc implantation at the operated and adjacent levels and between implant types. RESULTS.: Moderate intraobserver and interobserver reliability was noted. Preoperative images of patients in all implant groups had high-quality images at operative and adjacent levels. The Bryan and Prestige LP devices allowed satisfactory visualization of the canal, exit foramina, cord, and adjacent levels after arthroplasty. Visualization was significantly impaired in all PCM and Prodisc-C cases at the operated level in both the spinal canal and neural foramina. At the adjacent levels, image quality was statistically poorer in the PCM and Prodisc-C than those of Prestige LP or Bryan. CONCLUSIONS.: Postoperative visualization of neural structures and adjacent levels after cervical arthroplasty is variable among current available devices. Devices containing nontitanium metals (cobalt-chrome-molybdenum alloys in the PCM and Prodisc-C) prevent accurate postoperative assessment with magnetic resonance imaging at the surgical and adjacent levels. Titanium devices, with or without polyethylene (Bryan disc or Prestige LP), allow for satisfactory monitoring of the adjacent and operated levels. This information is crucial for any surgeon who wishes to assess adequacy of neural decompression and where monitoring of adjacent levels is desired.
Symptomatic multisegment disease is most common at the C5-6 and C6-7 levels, and two-level anterior cervical discectomy and fusion (ACDF) is performed most often at these levels. Therefore, it may be clinically important to know whether a C5-7 fusion affects the superior C4-5 segment. A biomechanical study was carried out using cadaveric cervical spine specimens to determine the effect of lower two-level anterior cervical fusion on intradiscal pressure and segmental motion at the superior adjacent vertebral level.
Five cadaveric cervical spine specimens were used in this study. The specimens were stabilized at T-1 and loaded at C-3 to 15 degrees flexion, 10 degrees extension, and 10 degrees lateral bending before and after simulated two-level ACDF with plate placement at C5-7. Intradiscal pressure was recorded at the C4-5 level, and segmental motion was recorded from C-4 through C-7. Differences in mean intradiscal pressures were calculated and analyzed using a paired Student t-test. When the maximum calibrated intradiscal pressures were exceeded ("overshot") during measurements, data from the specimens involved were analyzed using the motion data with a Student t-test. Values for pressure and motion obtained before and after simulated ACDF were compared.
During flexion, the mean intradiscal pressure changes (+/- standard deviations) in the pre- and post-ACDF measurements were 1275 (+/- 225) mm Hg and 2475 (+/- 75) mm Hg, respectively (p < 0.05). When the results of pre-ACDF testing were compared with post-ACDF results, no significant difference was found in the mean changes in the intradiscal pressure during extension and lateral bending. The maximum calibrated intradiscal pressures were exceeded during the post-ACDF testing in four specimens in extension, three in flexion, and two in lateral bending. Comparison of pre- and post-ACDF data for all five specimens revealed significant differences in motion and intradiscal pressure (p < 0.05) during flexion, significant differences in motion (p < 0.05) but not in intradiscal pressure during extension, and significant differences in intradiscal pressure changes (p < 0.05) but not in motion during lateral bending.
Simulated C5-7 ACDF caused a significant increase in intradiscal pressure and segmental motion in the superior adjacent C4-5 level during physiological motion. The increased pressure and hypermobility might accelerate normal degenerative changes in the vertebral levels adjacent to the anterior cervical fusion.
Biomechanical study on the effect of cervical spine fusion on adjacent-level intradiscal pressure and segmental motion
- Eck
Delayed hyper-reactivity to metal ions after cervical disc arthroplasty: a case report and literature review
- Cavanaugh
Water absorption and fibre-matrix interface durability in carbon-PEEK
- Zhang