Relative contributions of design, alignment, and loading variability in knee replacement mechanics
Computational Biomechanics Lab, University of Denver, 2390 S. York Street, Denver, Colorado 80208. .Journal of Orthopaedic Research (Impact Factor: 2.99). 12/2012; 30(12):2015-24. DOI: 10.1002/jor.22169
Substantial variation in total knee replacement (TKR) outcomes exists within the patient population. Some of this variability is due to differences in the design of the implanted components and variation in surgical alignment, while other variability is due to differences in the applied forces and torques due to anatomic and physiological differences within a patient population. We evaluated the relative contributions of implant design, surgical alignment, and patient-specific loading variability to overall tibiofemoral joint mechanics to provide insight into which measures can be influenced through design and surgical decisions, and which are inherently dependent on variation within the patient population and should be considered in the robustness of the implant design and surgical procedure. Design, surgical, and loading parameters were assessed using probabilistic finite element methods during simulated stance-phase gait and squat activities. Patient-specific loading was found to be the primary contributor to joint loading and kinematics during low flexion, particularly under conditions of high external loads (for instance, the gait cycle with high internal-external torque), while design and surgical factors, particularly femoral posterior radius and posterior slope of the tibial insert became increasingly important in TKR performance in deeper flexion. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:2015-2024, 2012.
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- "The relationship between ligament constraint and TKA geometry is also an important component of TKA success, and knee models are an important factor in predicting kinematics from implant geometries. The accuracy of the model-based kinematics are dependent in part on the ligament parameters (e.g., reference strain and linear stiffness), and recent studies have shown the benefit of optimizing soft-tissue parameters to patient-specific data from laxity evaluations  . "
ABSTRACT: Understanding the contribution of soft-tissue to total joint constraint is important for predicting joint kinematics, developing surgical procedures, and increasing accuracy of computational models. Previous studies on the collateral ligaments have focused on quantifying strain and tension properties under discrete loads or kinematic paths; however, there has been little work to quantify collateral ligament contribution over a broad range of applied loads and range of motion in passive constraint. To accomplish this, passive envelopes were collected from nine cadaveric knees instrumented with implantable pressure transducers in the collateral ligaments. The contributions from medial and lateral collateral ligaments were quantified by the relative contribution of each structure at various flexion angles (0-120 degrees) and compound external loads (up to 10 N-m valgus-valgus, up to 8 N-m internal-external, up to 40 N anterior-posterior). Average MCL contributions were highest under external and valgus torques from 60 to 120 degrees flexion. Average LCL contributions were highest from 0 to 60 degrees flexion under external and varus torques, as well as internal torques from 60 to 110 degrees flexion. Statistically significant contributions for the LCL generally matched the highest regions of average contribution. The MCL showed significant contributions under external torques throughout the flexion range. Anterior and posterior loads generally reduced collateral contribution; however, posterior loads further reduced the MCL, while anterior loads further reduced the LCL. These results provide insight to the functional role of the collaterals over broad of passive constraint. Collateral mapping may be used to identify the effects of injury or surgical intervention on soft-tissue, and how collateral contributions to constraint correlate with activities of daily living.
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- "Therefore, rigid body constraints were applied in order to perform the finite element simulations of randomized geometries in a reasonable computational cost. (2) Although computational findings were in a good agreement with the available literature   , parts of the presented findings have not been reported elsewhere and further clinical investigations are required to test whether changes in the proposed dimensions can alleviate the competing effect of implant geometry on its performance metrics. Accordingly, various future directions from this study can be considered. "
ABSTRACT: Articular geometry of knee implant has a competing impact on kinematics and contact mechanics of total knee arthroplasty (TKA) such that geometry with lower contact pressure will impose more constraints on knee kinematics. The geometric parameters that may cause this competing effect have not been well understood. This study aimed to quantify the underlying relationships between implant geometry as input and its performance metrics as output. Parametric dimensions of a fixed-bearing cruciate retaining implant were randomized to generate a number of perturbed implant geometries. Performance metrics (i.e., maximum contact pressure, anterior-posterior range of motion [A-P ROM] and internal-external range of motion [I-E ROM]) of each randomized design were calculated using finite element analysis. The relative contributions of individual geometric variables to the performance metrics were then determined in terms of sensitivity indices (SI). The femoral and tibial distal or posterior radii and femoral frontal radius are the key parameters. In the sagittal plane, distal curvature of the femoral and tibial influenced both contact pressure, i.e., SI=0.57; SI=0.65, and A-P ROM, i.e., SI=0.58; SI=0.6, respectively. However, posterior curvature of the femoral and tibial implants had a smaller impact on the contact pressure, i.e., SI=0.31; SI=0.23 and a higher impact on the I-E ROM, i.e., SI=0.72; SI=0.58. It is noteworthy that in the frontal plane, frontal radius of the femoral implant impacted both contact pressure (SI=0.38) and I-E ROM (SI=0.35). Findings of this study highlighted how changes in the conformity of the femoral and tibial can impact the performance metrics. Copyright © 2015 Elsevier B.V. All rights reserved.
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- "The latter approach enables us to generate large probabilistic databases representing the inherent variability of a patient population or to model the complicated interactions between input variables and output metrics in terms of sensitivity indices. The aforementioned studies however have mostly attempted to investigate PS designs   or CR designs   . To best of our knowledge, no previous computational study has compared PS versus CR in a systematic approach. "
ABSTRACT: Commercially available fixed bearing knee prostheses are mainly divided into two groups: posterior stabilized (PS) versus cruciate retaining (CR). Despite the widespread comparative studies, the debate continues regarding the superiority of one type over the other. This study used a combined finite element (FE) simulation and principal component analysis (PCA) to evaluate "reliability" and "sensitivity" of two PS designs versus two CR designs over a patient population. Four fixed bearing implants were chosen: PFC (DePuy), PFC Sigma (DePuy), NexGen (Zimmer) and Genesis II (Smith & Nephew). Using PCA, a large probabilistic knee joint motion and loading database was generated based on the available experimental data from literature. The probabilistic knee joint data were applied to each implant in a FE simulation to calculate the potential envelopes of kinematics (i.e. anterior-posterior [AP] displacement and internal-external [IE] rotation) and contact mechanics. The performance envelopes were considered as an indicator of performance reliability. For each implant, PCA was used to highlight how much the implant performance was influenced by changes in each input parameter (sensitivity). Results showed that (1) conformity directly affected the reliability of the knee implant over a patient population such that lesser conformity designs (PS or CR), had higher kinematic variability and were more influenced by AP force and IE torque, (2) contact reliability did not differ noticeably among different designs and (3) CR or PS designs affected the relative rank of critical factors that influenced the reliability of each design. Such investigations enlighten the underlying biomechanics of various implant designs and can be utilized to estimate the potential performance of an implant design over a patient population. Copyright © 2015 IPEM. Published by Elsevier Ltd. All rights reserved.
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