The effect of valgus/varus malalignment on load distribution in total knee replacements
Introduction
Coronal malpositioning of the tibial component of total knee replacements may result in severe wear of the polyethylene component, component loosening, and ultimately revision surgery (Hungerford, 1995; D’Lima et al., 2001). Classically, the bone cut for the tibial component is perpendicular to the long axis of the tibia. Malpositioning of the tibial component can occur with a slightly angled cut such that too much bone is removed laterally and too little is removed medially or conversely. Variations in tibial coronal alignment of 3–5° can occur clinically (Maestro et al., 1998; Hungerford et al., 1989; Ritter et al., 1994) and therefore warrant an investigation into its effect on implant loading mechanics.
The purpose of this study was to determine in vitro how variations in tibial component angulation might affect (1) the joint pressure distribution, (2) the medial to lateral tibial component force distribution, and (3) the implant kinematics during simulated walking. We further wanted to compare the pressure and loading distributions when measured in a simulated surgical environment to those obtained using a physiological gait simulator.
Section snippets
Materials and methods
In each of seven left cadaver legs, a posterior cruciate sparing total knee replacement was implanted using standard surgical technique and alignment jigs. Symmetric and balanced flexion and extension gaps of 10 mm were surgically created.
Pressure measurements were made using a 2-mm thick sensor fabricated from a stainless steel plate and conductive ink pressure sensors (Tekscan, Inc., Boston, MA). The sensor consisted of 242 pressure-measuring sensals in both the medial and lateral
Results
Pressure data was obtained for all loading conditions for all inserts in six of the seven knees. Pressure data from the first knee was not included since it was not tested at all combinations of the static load cases. Using the knee simulator, in a static knee stance position near full extension, the neutrally aligned tibial insert distributed pressure evenly between the medial and lateral compartments of the tibial component (Fig. 5). Integration of the pressure data in each compartment
Discussion
This study examined several biomechanical consequences of poor coronal tibial component alignment during surgical trial reduction and simulated gait loading. This variation in coronal tibial alignment is related to, but different from, variations in the relative varus/valgus alignment between the tibia and femur which has been shown by others (Ritter et al., 1994; Jeffery et al., 1991) to be a factor in implant loosening.
The results suggest that under static loading, a tibial malposition of 3°
Acknowledgements
Supported in part by Johnson & Johnson, Professional, Inc.
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