Total knee replacements: design and pre-clinical testing methods

Total knee replacement (TKR) is a common and successful treatment for severe osteoarthritis of the knee. However, a large minority of people remain dissatisfied after the operation, despite adequate pain relief. Over 50 designs of TKR are used in the UK each year, but differentiating between these devices in terms of patient function and making the right choice for each patient remains challenging. The aim of this research was to characterise designs of TKR in the laboratory, using pre-clinical testing methods, in order to better understand TKR function, and make suggestions for improved implant design and testing. Conventional, medial-pivot, guided-motion and bicruciate retaining (BCR) TKRs were tested. Standard ASTM test methods used for CE-marking purposes were demonstrated to differentiate between devices, but did not produce enough information to adequately understand how a new device will behave clinically, or what the potential benefits of a new device would be to patients. Guided-motion devices are meant to replicate normal knee motion, but there has been concern that they might cause too much rotation of the knee, leading to anterolateral knee pain. Results from cadaveric testing suggest that they do not adequately mimic normal knee motion and small design changes may have little impact on performance. A BCR TKR, designed to improve stability in the replaced knee joint, was also tested. Knee kinematics were measured for three design phases and surgical feasibility was also assessed for this more complicated procedure. BCR TKR was shown to lead to more normal levels of anteroposterior tibiofemoral laxity, compared to a conventional, anterior-cruciate-ligament-sacrificing TKR. Inherent variability between people’s anatomy and osteoarthritis pathology suggests there will never be a single, perfect, TKR, but more comprehensive pre-clinical testing could improve the regulatory approval process and inform better device selection, leading to improved patient outcomes.Open Acces

On the biomechanics of ligaments and muscles throughout the range of hip motion

At the limits of the range of hip motion, impingement, subluxation and edge loading can cause osteoarthritis in natural hips or early failure hip replacements. The aim of this PhD was to investigate the role of hip joint soft tissues throughout the range of hip motion to better understand their role in preventing (or perhaps even causing) these problematic load cases. A musculoskeletal model was used to investigate the muscular contribution to edge loading and found that in the mid-range of hip motion, the lines of action of hip muscles pointed inward from the acetabular rim and thus would stabilise the hip. However, in deep hip flexion with adduction, nearly half the muscles had unfavourable lines of action which could encourage edge loading. Conversely, in-vitro tests on nine cadaveric hips found that the hip capsular ligaments were slack in the mid-range of hip motion but tightened to restrain excessive hip rotation in positions close to the limits of hip motion. This passive restraint prevented the hip from moving into positions where the muscle lines of action were found to be unfavourable and thus could help protect the hip from edge loading. The ligaments were also found to protect the hip against impingement and dislocation. Out of the labrum, the ligamentum teres and the three capsular ligaments, it was found that the iliofemoral and ischiofemoral ligaments were primary restraints to hip rotation. These two capsular ligaments should be prioritised for protection/repair during hip surgery to maintain normal hip passive restraint. Whilst this can be technically demanding, failing to preserve/restore their function may increase the risk of osteoarthritic degeneration or hip replacement failure.Open Acces

PRIMARY FLEXION AXIS SELECTION IN TOTAL KNEE REPLACEMENTS USING COMPUTATIONAL ANALYSIS

Total knee replacements (TKR) are one of the most frequently implanted medical devices, with over 600,000 procedures performed in the United States in 2012. In order to ensure TKR longevity, wear tests are frequently conducted on these implants prior to patient implantation. Variations in implant geometry, material, and surface treatments are all tested, however, TKR alignment may also play a role in the long-term success of the knee implant. When testing knee designs with complex tibial and femoral geometries it is essential that the implant be aligned as the implant manufacturers intended so as to best represent the function of the implant system. Although critical, a key alignment variable that is largely overlooked is femoral axis selection. Currently, femoral axis alignment is simply selected so as to minimize its effect on implant mechanics during walking simulation; a result that might completely misrepresent the implant designer\u27s intent. The purpose of this study was to create a computational model to determine the effect of femoral axis selection on contact-point bearing migration prior to simulator fixation and examine trends in femoral axis selection based on implant geometry. Using 3D optical scans of seven femurs, 3Matic STL for model remeshing, and COMSOL Multiphysics for simulation this study recreated the single-axis rotation of each femoral component in a wear simulator. The lowest femoral contact point was then tracked between 0º and 120º flexion over four hundred possible femoral axes alignment options. The computational model was verified statistically and calculated the location of the ideal axes of rotation for all seven femurs. Reduction of P/D lowest contact-point translation during simulator flexion was found to be dependent on the range of flexion. Single-axis knee designs were found to exhibit a lower tolerance to varied femoral axes of rotation, but still maintained lower mean P/D displacements. Anterior/posterior translation patterns during simulator flexion were found to vary significantly with femoral axis selection. Interestingly, A/P translation patterns were more consistent between varying flexion axes in implants with multiple axes of curvature compared to single-axis designs. TKR alignment in single-axis simulators clearly affects proximal/distal and anterior/posterior lowest contact-point migration and thus possibly implant mechanics during functional testing. An implant that incorporates a geometry that is minimally affected by malalignment should enhance clinical outcomes and provide more consistent functional measures during simulation and use

Preliminary study of a customised total knee implant with musculoskeletal and dynamic squatting simulation

Customised total knee replacement could be the future therapy for knee joint osteoarthritis. A preliminary design of a customised total knee implant based on knee anatomy was studied in this article. To evaluate its biomechanical performance, a dynamic finite element model based on the Oxford knee rig was created to simulate a squatting motion. Unlike previous research, this dynamic model was simulated with patient-specific muscle and joint loads that were calculated from an OpenSim musculoskeletal model. The dynamic response of the customised total knee implant was simulated under three cruciate ligament scenarios: both cruciate ligaments retained, only anterior cruciate ligament removed and both cruciate ligaments removed. In addition, an off-the-shelf symmetric total knee implant with retained cruciate ligaments was simulated for comparison analysis. The customised total knee implant with both cruciate ligaments retained showed larger ranges of femoral external rotation and posterior translation than the symmetric total knee implant. The motion of the customised total knee implant was also in good agreement with a healthy knee. There were no big differences in the tibiofemoral compressive forces in the customised total knee implant model under the three scenarios. These forces were generally consistent with other experimental and simulation results. However, the customised total knee implant design resulted in larger tibiofemoral compressive force than the symmetric total knee implant after 50° knee flexion, which was caused by the larger tibiofemoral relative motion

Hip Mechanics of Unilateral Drop Landings

Increased hip forces are a proposed factor for osteoarthritis and femoroacetabular impingement. These forces can be estimated through musculoskeletal modeling using measured kinematics and kinetics. An understanding of hip joint loading during landing in a asymptomatic population will begin to elucidate what, if any, sex differences exist and how changes in landing condition alter hip mechanics. The overall purpose of this dissertation was to explore how sex and landing condition effect landing mechanics. Landing mechanics were quantified using ground reaction forces (GRF), hip joint forces (HJF), and lower extremity kinematics during unilateral drop landings from 30-cm, 40-cm, and 50-cm, as well as, a 40-cm land-and-cut task. The relationships between sex and limb side, sex and landing task, and sex and landing height on landing mechanics were assessed using three sub-studies. Eighty-three, recreationally active, adult volunteers completed landing tasks (40 participants completed the land-and-cut task). For sex-limb side, bilateral differences (right versus left) were examined at 40-cm. No bilateral differences were identified. For sex-landing task, 40-cm drop landings were compared to land-and-cuts. Higher peak GRF (pGRF) and pGRF loading rates were identified for landing-only. Landing-only tasks were performed with less ankle dorsiflexion range of motion for landing (ROML) and impact (ROMI) phases. Landing-only tasks demonstrated more hip adduction ROML and more hip flexion ROMI. For sex-landing height, landings were compared between 30-cm and 50-cm. Increasing landing height resulted in increased pGRF, pHJF, pGRF loading rate, and pHJF loading rate. With increased height, larger 3-D hip and knee flexion ROMI and ROML were identified, as well as increased ankle dorsiflexion ROML. There were no interaction effects between sex and landing condition. Sex differences across sub-studies demonstrated consistent trends. In all studies, females incurred larger pGRF compared to males, yet only the landing height analysis demonstrated increased pHJF. Females exhibited larger hip adduction and reduced hip rotation ROML. Females exhibited larger hip flexion, hip adduction, and knee flexion ROMI. The landing task analysis identified increased female ankle dorsiflexion ROMI. Sex differences were identified between landing conditions, yet the lack of sex-landing condition interaction indicates both sexes may utilize similar modifications in response to changing landing conditions