Abstract
Introduction
Component and limb alignment (especially varus >3°) have been associated with soft-tissue imbalance, increased polyethylene wear, and tibial tray subsidence. However, not all clinical outcome studies have found significant correlation between tibial varus and revision surgery. While the link between limb alignment 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. In this study, we analyzed the effect of limb alignment and tibial tray alignment on the risk for bone damage and subsequent risk for tray loosening.
Methods
A finite element model of knee arthroplasty previously validated with in vitro cadaver testing was used. Models of four subjects were constructed with tibial resections simulating a 0°, 3°, 5°, and 7° varus alignment with respect to the mechanical axis of the tibia and the tray implanted at the corresponding angles. Tibial tray orientation was simulated without change in limb alignment (i.e. maintaining the mechanical axis of the knee at 0°) and with limb alignment ranging from 3° valgus to 7° varus (Fig 1).
A static load equivalent to three times the bodyweight of the subject was applied in line with the mechanical knee axis. Relative motion between the tibial tray and tibial bone was calculated. Elements with an equivalent von Mises strain >0.4% were selected and assigned an elastic modulus of 5 MPa to reflect damaged bone. Simulation was repeated and after-damage micromotion recorded.
Results
At neutral limb alignment, average tray micromotion was <10 μm and did not increase significantly with increasing tray varus (Fig 2). The after-damage micromotion also did not increase significantly. However, limb alignment had a more substantial effect on before- and after-damage micromotion (Fig 3). The magnitude of micromotion increased with increasing varus limb alignment.
Discussion
We did not find significant increase in micromotion with increased tray varus (of up to 7°) as long as neutral limb mechanical axis was maintained by compensating for tibial varus with femoral valgus. The volume of bone at risk also did not increase significantly with increasing tray varus. Removing the damaged bone did little to affect after-damage micromotion. This suggests that the “damaged” bone was not an important factor and likely did not contribute to the stability of the tray under the loading conditions analyzed in this report.
Changes in limb alignment significantly offset the net axial load vector resulting in damage in a greater volume of elements due to overloading. This is due to the shift in Mechanical axis and load vector with subsequent increase in moment applied to the model. The micromotion was also substantially increased after the damage indicating that the damaged bone was providing structural support to the tray. This emphasizes the effects of increasing the static coronal loading in this model. Consequently, it identifies the benefit of neutral limb alignment in this loading scenario. This model is an extremely valuable tool in studying the effect of surgical alignment, loading, and activity on damage to proximal bone.
