Optimal surgical component alignment minimizes TKR wear - An in silico study with nine alignment parameters.

Currently, preclinical mechanical wear testing of total knee replacements (TKRs) is done using ideally aligned components using standardized TKR level walking under either force or displacement-control regimes. To understand the influence of implant alignment and testing control regime, we studied the effect of nine component alignment parameters on TKR volumetric wear in silico. We used a computational framework combining Latin Hypercube sampling design of experiments, finite element analysis, and a numerical model of polyethylene wear, to create a predictive model of how component alignment affects wear rate for each control regime. Nine component alignment parameters were investigated, five femoral variables and four tibial variables. To investigate perturbations of the nine implant alignment variables, two separate 300-point designs were executed, one for each control regime. The results were then used to generate surrogate statistical models using stepwise multiple linear regression. Wear at the neutral position was 4.5mm3/million cycle and 8.6mm3/million cycle for displacement and force-control, respectively. Stepwise multiple linear regression surrogate models were highly significant for each control regime, but force-control generated a stronger predictive model, with a higher R2, more included terms, and a lower RMSE. Both models predicted transverse plane rotational mismatch can lead to large changes in predicted wear; a transverse plane alignment mismatch of 15° can elicit a change in wear of up to 5mm3/million cycle, almost double that of neutral alignment. Therefore, transverse plane alignment is particularly important when considering failure of the implant due to wear.

The effect of surgical alignment and soft tissue conditions on the kinematics and wear of a fixed bearing total knee replacement.

As life expectancy and activity levels of patients increase so does the demand on total knee replacements (TKRs). Abnormal mechanics and wear of TKRs can lead to implant loosening and revision. Component alignment after surgery varies due to the presurgical alignment, the accuracy of the surgical instrumentation and due to patient factors, such as the soft tissue balance. This study experimentally investigated the effect of variation in component alignment and the soft tissue conditions on the kinematics and wear of a fixed bearing TKR. DePuy Sigma fixed bearing TKRs with moderately cross-linked UHMWPE were used. Different alignment conditions were simulated in the coronal, sagittal and transverse planes in an ISO force-controlled simulation system. Three different soft tissue conditions were simulated using virtual springs to represent a stiff knee, a preserved PCL and a resected PCL. Four different alignment conditions were studied; ideal alignment, 4° tibial and femoral varus joint line, 14° rotational mismatch and 10° posterior tibial slope. The varus joint line alignment resulted in similar kinematics and lower wear rate compared to ideal alignment. The rotational mismatch alignment resulted in significantly higher tibial rotation and abduction-adduction as well as a significantly higher wear rate than ideal alignment. The posterior tibial slope alignment resulted in significantly higher wear than the ideal alignment and dislocated under the lower tension soft tissue conditions. Component alignment and the soft tissue conditions had a significant effect on the kinematics and wear of the TKR investigated in this study. The surgical alignment of the TKR is an important factor in the clinical outcome of the joint as factors such as increased tibial rotation can lead to anterior knee pain and instability and increased wear can lead to aseptic loosening and early failure resulting in revision.

Challenges of pre-clinical testing in orthopedic implant development.

The market for orthopedic implants is growing rapidly with the increasing prevalence of orthopedic diseases in an aging society. Different designs and materials have been developed over the years and have, in general, shown excellent results in pre-clinical testing. However, there have been incidences of serious complications when novel implants or materials are put into clinical use, with some well-known cases being metallosis in patients implanted with metal-on-metal hip replacements and osteolysis from polyethylene wear debris generated in hip and knee joint replacements. Unforeseen factors related to new designs, materials and surgical techniques can lead to different outcomes for pre-clinical testing and clinical use. While often an excellent indicator of a device's performance in clinical settings, pre-clinical testing does sometime fail to predict critical flaws in implant development. This article aims to explore the gaps in the current approach to testing. The ISO international standard of pre-clinical testing should be modified to more adequately capture actual clinical use of the implant and simulate daily activities. This article will also introduce modern methods for implant development, such as FEM, 3D printing and computer-aided orthopedic surgery, which can be widely applied to improve pre-clinical testing procedures and reduce the incidence of surgical malalignment by analyzing biomechanical performance, planning surgical procedure and providing surgical guide.