Abstract
SUMMARY
The relationship between component position, wear rate and edge loading was investigated for 115 explanted current generation Metal-on-Metal (MoM) hips. Edge wear was detected in: 63% of all hips; and 48% of those with cups positioned within Lewinnek's box.
BACKGROUND
The link between steeply inclined cups (>55 degrees) and edge loading is known for all common hip bearing couples. Edge loading is associated with high rates of wear, and has been linked to premature failure of hips.
METHODS
The wear of failed hip joints was measured using a Taylor Hobson Talyrond 365. Edge loading was identified when the depth of the wear scar was maximum at the rim of the cup. The position of the cups was measured from plain radiographs or 3D CT.
RESULTS
A total of 115 retrieved hips were available with position and wear analysis. The median age of patients was 58 years (25 – 87) and the median time before revision was 38 months (4 – 121). All hips were measured for inclination and 78 were measured for version.
The median (range) of cup inclination was 51 degrees (15 – 82), and cup version was 18.5 degrees (-47 – 61). 63% of cups were found to be edge loaded. The median wear rate of the edge loaded cups was 12 μm per year compared to 1.88 μm per year for non-edge loaded cups. Edge loading was found in all components with an inclination steeper than 60°. Edge loading was found at inclination angles as low as 30°.
23 cups were placed within Lewinnek's safe zone: inclination of 40 ± 10degrees, and version of 15 ± 10 degrees. However, 11 of these “well positioned” components were edge loaded. Of the edge loaded components in Lewinnek's box, none had an inclination less than 40 degrees.
CONCLUSIONS
Edge loading occurred at cup inclination angles lower than previously reported. It is believed that edge wear is related to the contact patch between head and cup, overlapping the edge of the cup, causing disruption to the lubrication regime and increased contact pressures at the edge of the cup. Work is progressing to calculate the size of contact patch for the explanted hips and position in the cup.
