Introduction: Pseudotumours (soft-tissue masses relating to the hip joint) following metal-on-metal hip resurfacing arthroplasty (MoMHRA) have been associated with elevated serum and hip aspirate metal ion levels, suggesting that pseudotumours occur when there is increased wear. This study aimed to quantify in vivo wear of implants revised for pseudotumours and a control group of implants revised for other reasons of failure.

Methods: A total of 30 contemporary MoMHRA implants in two groups were investigated in this Institutional Review Board approved study:

8 MoMHRA implants revised due to pseudotumour;

The linear wear of retrieved implants was measured using a Taylor-Hobson Roundness machine. The average linear wear rate was defined as the maximum linear wear depth divided by the duration of the implant in vivo.

Results: In comparison with the non-pseudotumour implant group, the pseudotumour implant group was associated with:

significantly higher median linear wear rate of the femoral component: 8.1um/year (range 2.75–25.4um/year) vs. 1.79um/year (range 0.82–4.15um/year), p=0.002; and

Similarly, differences were also measured in absolute wear values. The median absolute linear wear was significantly higher in the pseudotumour implant group:

21.05um (range 2.74–164.80um) vs. 4.44um (range 1.50–8.80um) for the femoral component, p=0.005; and

Wear on the acetabular cup components in the pseudotumour group always involved the edge, indicating edge-loading of the bearing. In contrast, edge-loading was observed in only one acetabular component in the non-pseudotumour group of implants. The deepest wear was observed well within the bearing surface for the rest of the non-pseudotumour group. The difference in the incidence of edge-loading between the two groups was statistically significant (Fisher’s exact test, p=0.03).

Discussion: Significantly greater linear wear rates of the MoMHRA implants revised due to pseudotumour support the in vivo elevated metal ion concentrations in patients with pseudotumours. This study provides the first direct evidence to confirm that pseudotumour is associated with increased wear at the MoM articulation. Furthermore, edge-loading with the loss of fluid film lubrication may be the dominant wear generation mechanism in patients with pseudotumour.

Introduction: To develop a more physiologically relevant hip simulator test protocol and study the effect of microstructure on the wear performance of as-cast (AC) and double heat treated (DHT) devices under the new protocol.

Methods: Three pairs of AC and four pairs of DHT 50 mm CoCr metal-on-metal (MoM) devices were tested. The lubricant used was bovine serum. Stop-start motion was implemented between the two sets of kinetics and kinematics that alternated every 100 cycles throughout the test. Condition one: The flexion/extension was 30° and 15° respectively. The internal/external rotation was ±10°. The force was Paul type stance phase loading with a maximum load of 3 kN and a standard ISO swing phase load of 0.3 kN. Condition two: The flexion/extension was ±22°. The internal/external rotation was ±8°. The force was a maximum stance phase load of 2.2 kN and a swing phase load of 0.24 kN at 0.5 Hz frequency. Wear was assessed gravimetrically.

Result: The masking effect of 1 Hz speed and uninter-rupted motion, in providing exaggerated lubrication regime, was exposed under more physiologically relevant test conditions. The AC devices have significantly reduced wear when compared to the DHT devices. It can also be seen that from 0.5 to 2 Mc the divergence in wear has increased.

Conclusion: A more physiologically relevant hip simulator test protocol was successfully developed and implemented, in showing the effect of microstructure on wear as seen in vivo, where high wear of DHT devices has been observed. 295

Introduction: The purpose of this study was to determine the effects of edge loading on in vivo wear of hip resurfacings from retrievals.

Methods: The wear of retrieved BHR heads and cups was assessed using a Taylor-Hobson Talyrond 290 roundness machine. The maximum deviation of the profile from an ideal circle was taken as the maximum linear wear. Edge loaded devices (Figure 1a) were classified as cups which showed the maximum area of wear crossing over the edge of the cup. For all non-edge loaded pairs (Figure 1b), the wear area on the cup was within the sphere of the cup. In this study 50 pairs (diameter size 38 mm to 54 mm) were analysed.

Results: 28 pairs were classified as edge loaded, and 22 were not. Edge loaded pairs display greater linear wear than non-edge loaded components (Table 1). Edge loaded components showed no correlation between time in vivo and linear wear.

Discussion: Edge loaded pairs have a far greater range of linear wear which may be due to the variation of the angles of the components in vivo. Edge loading may be caused by an open cup, impingement and/or high combined anteversion angle of both the head and cup. The success of a hip resurfacing depends strongly upon articulation occurring within the sphere of the cup, which is reliant upon good component orientation.