PREDICTING BONE DAMAGE AND IMPLANT SUBSIDENCE AFTER TOTAL KNEE ARTHROPLASTY

Abstract

Introduction: Aligning the tibial tray is a critical step in total knee arthroplasty (TKA). Malalignment, (especially in varus) has been associated with failure and revision surgery. While the link between varus malalignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. We therefore constructed a finite element model of knee arthroplasty to test the hypothesis that varus malalignment of the tibial tray would increase the risk of tray subsidence.

Methods: Cadaver Testing: Fresh human knees (N = 4) were CT scanned and implanted with a TKA cruciate-retaining tibial tray (Triathlon CR. Stryker Orthopaedics). The specimens were subjected to ISO-recommended knee wear simulation loading for up to 100,000 cycles. Micromotion sensors were mounted between the tray and underlying bone to measure micromotion. In two of the specimens, the application of vertical load was shifted medially to generate a load distribution ratio of 55:45 (medial: lateral) to represent neutral varus-valgus alignment. In the remaining two specimens, a load distribution ratio of 75:25 was generated to represent varus alignment.

Finite element analysis: qCT scans of the tested knees were segmented using MIMICS (Materialise, Belgium). Material properties of bone were spatially assigned after converting bone density to elastic modulus. A finite element model of the tibia implanted with a tibial tray was constructed (Abaqus 6.8, Simulia, Dassault Systèmes). Boundary conditions were applied to simulate experimental mounting conditions and the tray was subjected to a single load cycle representing that applied during cadaver loading.

Results: The two cadaver specimens tested at 55:45 medial:lateral (M:L) force distribution survived the 100,000 cycle test, while both cadaver specimens tested at 75:25 M:L force distribution failed. The finite element model generated distinct differences in compressive strain distribution patterns in the proximal tibia. A threshold of 2000 microstrain was used for fatigue damage in bone under cyclic loading. Both specimens loaded under 75:25 M:L distribution demonstrated substantially larger cortical bone volumes in the proximal tibial cortex that were greater than this fatigue threshold.

Discussion and Conclusion: We validated a finite element model of tibial loading after TKA. Local compressive strains directly correlated with subsidence and failure in cadaver testing. A significantly greater volume of proximal tibial cortical bone was compressed to a strain greater than the fatigue threshold in the varus alignment group, indicating an increased risk for fatigue damage. This model is extremely valuable in studying the effect of surgical alignment, loading, and activity on damage to proximal bone.