NEW APPROACH FOR PRECLINICAL TESTING OF HIP JOINT ARTHROPLASTY BEARINGS

Introduction and Aims

There are many surgical, implant design and patient factors that should be considered in preclinical testing of hip replacement which are not being considered in current standards. The aim of this study was to develop a preclinical testing method that consider surgical positioning, implant design and patient factors and predict the occurrence and severity of edge loading under the combination of such conditions. Then, assess the safety and reliability of the implant by predicting the wear, deformation and damage of the implant bearings under worst case conditions.

Methods

Ceramic-on-ceramic (CoC, 36mm, BIOLOX^® delta, Pinnacle^®, DePuy Synthes, UK) and metal-on polyethylene (MoP, 36mm, Marathon®, Pinnacle^®, DePuy Synthes, UK) bearings were used for this study on multi-station multi-axis hip joint simulators. Two factors were varied, cup inclination angles (45° and 65°) and translational mismatch between the femoral head and acetabular cup (0, 2, 3 and 4 (mm)). Under each condition for both CoC and MoP bearings, three million cycles of gait cycle testing were completed with wear, deformation and/or damage measurements completed at one million cycle intervals. Other outputs of the study were the level of dynamic separation between the femoral head and acetabular cup during gait, the maximum force at the rim during edge loading when the head was sliding back to the cup confinement. Means and 95% confidence limits were determined and statistical analysis were done using one way ANOVA with significance taken at p<0.05.

Results

As the level of mismatch and the cup inclination angle increased, the magnitude of dynamic separation and the force at the rim increased. The level of dynamic separation and the force on the rim correlated with the wear of CoC bearings (R= 0.96). For polyethylene, steeper inclination angle did not significantly increase the wear (p>0.05) however, edge loading under 4mm translational mismatch and steep cup inclination angle did (p<0.01). The combined wear and deformation of the polyethylene liners at the rim increased under larger levels of dynamic separation.

Conclusions

The magnitude of dynamic separation and force at the rim were predictive of the severity of edge loading. These parameters can be measured using short term testing (500 cycles). This will determine the effect of variations in surgical positioning, implant design and patient factors on the occurrence and severity of edge loading. Then, the wear, deformation and/or damage on hip replacement bearings can be determined using longer term simulator testing under selected conditions. The short term tests do not only help identify worst case scenarios but may identify the boundary of surgical position under which the implants performance may be considered acceptable.

A new approach for preclinical testing of hip replacement was developed:

Stage 1: Short biomechanical tests: assess the occurrence and severity of edge loading conditions where the outputs are:

1. Magnitude of medial-lateral dynamic separation

2. Maximum force under edge loading

Stage 2: Wear assessment: assess the tribological performance of hip replacement under selected conditions where the outputs are:

1. Wear rates

2. Deformation and/or damage on the bearing surface