Abstract
Introduction
Current pre-clinical testing is performed using knee wear simulators with standardized walking profiles. Differences in generated damage patterns to those observed on retrieved liners have been explained with the absence of activities other than walking, less severe loading conditions, and a discrepancy in the simulator's tibiofemoral contact mechanics and in vivo knee excursion. While it has been recognized that rotational alignment of the knee may also drive the location and shape of wear scars, to the best of our knowledge this parameter has not been investigated in knee simulator studies.
Methods
Here, we use patient specific gait as input to the simulation to approximate the patient specific contact mechanics. Kinematic and kinetic input data was obtained from gait analysis of a patient with a MGII (Zimmer Inc.) prosthesis at 11 years post-op using the point cluster technique for tibiofemoral kinematics, and a mathematical model for internal force calculations. Using the identical type of prosthesis on the simulator, wear tests were conducted in displacement mode on a closed-loop controlled station. Because x-rays of the patient suggested an internal rotation of the tibial tray, it was varied form 0–10° and the effect on location and wear scar dimension was assessed. Results were compared with the retrieved liner (obtained after 13 years in vivo).
Results
The simulator inputs generated from the gait data were compared with ISO 14243–3 (Figure 1). The first contact force peak of the patient was significantly lower, while second contact force peak similar to ISO. There were minimal differences in the flexion/extension profiles. For stance phase, the anterior/posterior translation and internal/external rotation kinematics did not show similar patterns, but they did fall within similar ranges from zero. There was little similarity for the swing phase.
The total wear scar area of the retrieval was measured to be 919.8 mm2. The average total wear scar of the tested components was 853.0 ± 59.8 mm2. (p= 26.28%) The outcome values of the tested components compared to the retrieval are shown in Figure 2. All offsets produced a smaller wear scar than the retrieval, but the 7° offset produced the closest area which was within 1 mm2 of the retrieval. The 7° offset also had the closed centroid offset angle, which was within 0.2° of the retrieval (Figure 3). On the retrieval, a small wear scar was observed on the anterior- medial aspect of the intracondylar eminence (not shown). Among the tested components, the 7° and 10° offsets recreated damage at this location.
Discussion
Rotational alignment affected the wear scar size by as much as 15% in this study. Only, the 7° offset produced outcome values very similar to the retrieval, highlighting the importance of rotational mismatch for wear. It should be noted that ± 10° of rotational mismatch is clinically well tolerated [5] and therefore may occur frequently. All tested components had smaller wear scar areas than the retrieved liner. This suggests that other activities other than walking may have contributed to wear in vivo.
