Abstract
Introduction
For many patients, total knee replacement (TKR) provides pain relief and restores motion for many years [1]. Some patients, however, experience early failures and require revision surgery. One of the suggested contributors to early failure has been excessive wear due to malalignment [2]. Previous work has shown that varus-valgus malalignment results in extreme condylar loading and could lead to high wear [3]. The purpose of this experiment, therefore, was to evaluate medial/lateral load sharing in an in vitro wear simulation.
Methods
Wear testing was conducted on midsized Attune and Sigma fixed bearing cruciate substituting TKR components (DePuy Synthes). The two systems differ in many aspects; notably, Attune employs antioxidant-stabilized moderately-crosslinked polyethylene and a gradually changing sagittal femoral curvature while Sigma uses remelted moderately-crosslinked polyethylene and a mulit-radius femoral design. Wear was evaluated across a wide range of medial/lateral (M/L) load splits: 10/90, 60/40, and 90/10 using an AMTI six-station knee simulator (Figure 1). Simulation was conducted for 3 million cycles using at 1 Hz using previously described methods [4] with ‘High Kinematic’ displacement controlled inputs in 25% bovine calf serum (Hyclone) at 37 ± 2°C supplemented with sodium azide and EDTA. Polyethylene wear was determined gravimetrically with load soak compensation every 0.5 Mcyc.
Results
The wear of Attune and Sigma components were consistently low across all load splits (Figure 2). The standard 60/40 load split was the lowest wearing condition for both designs, but the extreme lateralized and medialized loadings were not appreciably higher (Figure 3). No statistics are available due to the small sample size.
Conclusion
The medial/lateral load sharing did not affect wear for Attune and Sigma designs in this displacement controlled in vitro wear simulation. For each group, the inserts subjected to extreme load splits wore comparably to those with a standard 60/40 load split. It is important to note that all samples experienced similar motions with similar cross-shear and differed primarily in the condylar load sharing. With this in mind, the relationship of the results is supported by research in fundamental polyethylene behavior that suggests cross-shear may dominate loading effects [5]. Clinically, however, patients will likely experience altered motions and other effects due to malalignment that were not investigated in this study. While this study showed limited variation in wear across a wide range of condylar load ratios, surgeons and device manufacturers should continue to explore the full implications of malalignment.
