Abstract
Introduction
Historical studies have reported incidences of taper corrosion in retrieved MoP hips of 20–50%. These studies relied on visual assessments, rather than using modern analytical techniques.
Patients/Materials and Methods
The Northern Retrieval Registry was initiated to routinely analyse all retrieved hips. The volumetric wear rates of retrieved Exeter head tapers were measured using a coordinate measuring machine using validated methods and compared to the available results obtained from an ongoing study of failed MoM prostheses. Power analysis suggested that we would need at least fifty Exeter head tapers to provide significant results. Non-parametric tests were used to assess differences. We have previously identified head diameter/offset/taper angle and taper surface roughness as variables associated with taper wear. The effect of bearing material combination remains unquantified. These design variables were entered into a multiple regression model following log normalisation of taper wear.
Results
56 consecutively retrieved Exeter head tapers underwent assessment. 36 were of size 28mm. Primary causes for revision were acetabular loosening/periprosthetic fractures. Mean duration in vivo was 87 (1 – 252) months. Median total volumetric wear was 0.29mm3 with a wear rate of 0.07mm3/year. There were 353 MoMs. Median taper wear rates for 28mm MoM was 0.07, for 36mm was 0.11, for 38–44mm was 0.31, for 45 – 48mm was 0.52, for 49–52 was 0.55mm and for >52mm was 1.36mm3/year. While Exeter taper wear was significantly lower than the MoM group as a whole (p<0.001), regression modelling indicated that bearing diameter had the greatest influence on taper wear.
Discussion
Contrary to previous literature reports, significant material loss at the taper junction of conventional arthroplasties is not commonly observed. Exeter hips appear to be protected to some extent by the smaller head size and lower taper roughness when compared to contemporary MoM devices. The effect of the bearing combination remains under investigation.
