Abstract
Introduction
Patella femoral joint bearings in total knee replacements have shown low wear (3.1 mm3/MC) under standard gait simulator conditions1. However, the wear in retrieval studies have shown large variations between 1.3 to 45.2 mm3/year2. Previous in vitro studies on the tibial femoral joint have shown wear is dependent on design, materials and kinematics3.
The aim of this study was to investigate the influence of the design (geometry) and shape on the wear rate of patella femoral joints in total knee replacements.
Materials and Methods
The Leeds/Prosim knee simulator was used to investigate the wear of two types of commercially available patellae. The PFC Sigma cobalt chrome femoral component was coupled with 2 types of patellae buttons: round and oval dome. The UHMWPE was the same for the both types – GUR1020 GVF (gamma irradiated in vacuum and foiled packed). 25% bovine serum was used as the lubricant. The test were carried out at three conditions – high medial lateral (ML) rotations (<4°) and uncontrolled ML displacement (<4 mm), low ML rotation (<1°) and uncontrolled ML displacement (<4 mm); the physiological gait cycle; and low ML rotation and controlled ML displacement (<1.5 mm). In this abstract the two designs were tested in physiological gait condition (Figure 1). Patella ML displacement and tilt were passively controlled and measured after every 300,000 cycles. A ligament resisting force equivalent to 10 N4 was applied on the lateral side of the patella to avoid patella slip.
Five samples of each design were tested for 3 million cycles at a cycle rate of 1 Hz. The wear volume was obtained gravimetrically every million cycles and presented with 95% confidence limits. Statistical significance was taken at p<0.05.
Results and Discussions
The wear rate of PFC sigma round dome patella was higher (8.63 ± 3.44 mm3/million cycles) compared to the PFC sigma oval dome patella (6.28 ± 3.89 mm3/million cycles) (Figure 2). However, no significant difference in the wear rates was found between the two shapes (P=0.2). The low area of contact of oval dome patella (31.2% of total surface area) as compared to round dome patella (39.9%) led to low wear5.
The wear volume and the patellar tilt were positively correlated for oval dome and round dome patella, R2 > 0.9 (Figure 3). Increase in the patellar tilt resulted increase in sliding distance in perpendicular direction of polymer orientation. This led to higher wear volume.
Conclusions
Wear rate decreased with change of patella shape from round dome to oval dome although not significantly. Increase in the patellar tilt resulted in increase in the wear volume.
Acknowledgement
This research was supported in part by the DePuy and EPSRC. In addition, it was partially funded through WELMEC, a Centre of Excellence in Medical Engineering funded by the Wellcome Trust, under grant number WT 088908/Z/09/Z and additionally supported by the NIHR (National Institute for Health Research) as part of collaboration with the LMBRU (Leeds Musculoskeletal Biomedical Research Unit).
