Abstract
Introduction
Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions.
Materials and Methods
Ceramic-on-ceramic bearings where the ceramic liner was inserted into a titanium alloy cup (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes, UK) were tested on the six-station Leeds II hip simulator. The first test was run with the cups inclined at an angle equivalent, clinically, to 45° (n=6) and the second test was run with the cups inclined at an angle equivalent, clinically, to 65° (n=6). A standard gait cycle was run. A fixed surgical translational mal-positioning of 4mm between the centres of rotations of the head and the cup in the medial/lateral axis was applied on all stations. Both tests ran for three million cycles each. The lubricant used was 25% new-born calf serum. Wear was assessed gravimetrically using a microbalance (XP205, Mettler Toledo, UK) and geometrically using a coordinate measuring machine (CMM, Legex 322, Mitutoyo, UK). Statistical analysis was done using one way ANOVA with significance taken at p<0.05.
Results
The magnitude of dynamic microseparation was significantly (p<0.01) higher when the inclination angle of the cup was steeper (Figure 1) under the same level of translational mal-positioning of 4mm. This has resulted in significantly (p<0.01) higher wear rates of 1.01mm3/million cycles for the steep cup inclination group of 65° compared to 0.32mm3 /million cycles for the 45° inclined cups group (Figure 2). Furthermore, the penetration on the femoral heads was significantly (p<0.01) higher for the steep cup inclination angle group with a mean (±95% confidence limit) penetration of 33±6µm under the 65° cup inclination angle condition and 15±3µm under the 45° cup inclination angle condition (Figure 3).
Discussion and Conclusion
This study showed that cup inclination angle affects the magnitude of dynamic microseparation for a given surgical translational mal-position, thus leading to severe edge loading and increased wear rates with increased cup inclination angles. The occurrence and severity of the resulting edge loading causing increased wear in hip bearings will depend on the combinations of surgical variations, such as steep inclination angle, excessive version angle, medialised cups, head offset deficiencies, stem subsidence, and joint laxity. Future work will include studying the effect of these variables on the level of dynamic microseparation, severity of edge loading, the offset frictional torque and level of resulting wear.
