WEAR OF METAL-ON-METAL TOTAL HIP REPLACEMENTS UNDER SEVERE IN-VITRO TEST CONDITIONS

Introduction

Hip wear simulation is a widely used technique for the pre-clinical evaluation of new bearing designs. However, wear rates seen in vitro can often be significantly different to those seen clinically. This can be attributed to the difference between the optimal conditions in a simulator and wide ranging conditions in real patients.

This study aimed to develop more clinically relevant simulator tests, looking specifically at the effects of cup inclination angle (in vivo) and stop-dwell-start (SDS) protocols on a clinically available product.

Method

Five tests using a Paul type walking cycle (ISO 14242) were carried out on two ProSim hip simulators:

1. 28mm MoM, standard walking, cup inclination 45°, (n = 5)

2. 36mm MoM, standard walking, cup 45°, (n = 4)

3. 36mm MOM, SDS: 10 walking cycles and pause of 5s with stance load of 1250N cup 45°, (n = 5)

4. 36mm MOM, SDS: 10 walking cycles and pause of 30s with stance load of 1250N, (n = 5) cup 45°

5. 36mm MOM, standard walking, cup 55°(n = 5), and 65°(n = 5).

All samples had matched clearances, measured using a CMM (Prismo Navigator, Zeiss, Germany). Wear was measured gravimetrically (Sartorius ME235S: 0.01mg).

Results and Discussion

Metal-on-metal hip bearings are known to exhibit a bi-phasic wear pattern in-vitro (1), with the majority of wear occurring over the first 1–1.5mc, followed by lower steady state wear as the components conform to each other. Therefore bedding in wear over the first 1 million cycles was compared for each of the sample groups in this study. Bedding in wear for 36mm bearings under standard angle (45 inclination) and walking conditions was 0.16±0.15mm3/mc, while for 28mm bearings, wear was 2.67±2.06mm3/mc. Positioning the acetabular cup at an increased angle led to increased wear. At 55bedding in wear was .25±0.29mm3/mc, while at 65wear was 0.65±0.32mm3/mc.

When SDS conditions were applied, no significant increase in wear was observed under either the 5s or 30s pause conditions, with bedding in wear of 0.19±0.31mm3/mc and 0.14±0.13mm3/mc respectively.

Conclusion

This study illustrates the robust performance of the 36mm MoM bearing under extreme test conditions when compared with 28mm MoM. No effect was observed with the application of stop-dwell-start conditions, and only a small increase in wear was observed when the cup was inclined to a greater angle.

Despite the application of extreme test conditions, the 36mm MoM still demonstrates lower wear than 28mm under all conditions. In all but the 65 inclination conditions, this difference is statistically significant.

This study replicates some of the more frequently observed extreme conditions measured in real patients with THRs (2,3). Even under these more aggressive scenarios, wear of the 36mm bearing is still relatively low when compared with a commercially available 28mm THR.