PREDICTION OF WEAR OF CERAMIC-ON-CERAMIC BEARINGS FROM THE OCCURRENCE AND SEVERITY OF EDGE LOADING CONDITION IN A HIP JOINT SIMULATOR

Introduction and Aims

Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres.

Methods

The Leeds II hip joint simulator with a standard walking cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX^® delta, DePuy Synthes Joint Reconstruction, Leeds, UK.) were used. The study was in two parts. Part one: a biomechanical study where the dynamic separation, the maximum load during edge loading, and the duration of edge loading alongside the magnitude of forces under edge loading (severity of edge loading) were assessed. Part two; a wear study where the wear rates of the bearing surfaces were assessed under a series of input conditions. These input testing conditions included inclining the acetabular cups at 45° and 65° cup inclination angle (in-vivo equivalent), with 2, 3, and 4mm medial-lateral component mismatch between the centres of the head and the cup. This equated to six conditions being assessed, each with three repeats for the biomechanical test, and six repeats completed for the wear study.

The severity of edge loading was assessed as described in Equation 1.

Severity of Edge Loading = ∫^t_t0 F(x) dx + ∫^t_t0 F(y) dy … Equation 1,

where F(x) is the axial load, F(y) is the medial-lateral load and t-t0 is the duration of edge loading.

The wear of the ceramic bearings were determined using gravimetric analysis (XP205, Mettler Toledo, UK).

Results

The wear rates of ceramic-on-ceramic bearings increased as the magnitude of dynamic separation (Figure 1), the maximum load at the rim during edge loading (Figure 2), and the severity of edge loading (Figure 3) increased. The magnitude of dynamic separation was found to have the highest correlation to the wear rate under the conditions tested in this study (R²=0.94).

Conclusions

A preclinical testing approach has been developed to understand the occurrence and severity of edge loading associated with variation of component positioning. A good correlation was found between the wear rates obtained for ceramic-on-ceramic bearings and the magnitude of parameters obtained under edge loading during a short-term biomechanical study.

For figures/tables, please contact authors directly.