Abstract
Previous studies have confirmed a very strong correlation between penetration depth and the incidence of cup loosening due to impingement of the neck of the stem on the rim of the cup. The Charnley stem has been consistently manufactured in stainless steel – originally in EN58J then 316L. Introduction of the high nitrogen content stainless steel (ORTRON) allowed the reduction of the diameter of the neck from 12.5mm to 10mm. A prospective study was set up to establish the value of the reduced diameter neck in the context of the long-term survivorship of the cup revisions for wear and aseptic loosening. The benefit of the reduced diameter neck was clear from theoretical considerations – our objective was to establish the level of that benefit.
We compared the results where the 12.5mm diameter neck had been used with patients where the reduced 10mm diameter neck was used. We have analysed the correlation between the depth of cup penetration (mm) and the incidence of radiograph cup loosening as well as revision for aseptic cup loosening. The two groups comprised 1047 and 387 hips with a mean follow-up of 16.6 years (1–36) and 12.7 years (1–20) respectively.
When there was no measurable cup penetration, not only were there no revisions for aseptic cup loosening, but none of the cups were radiologically loose. At 1mm the decrease is 5% (from 9.4% to 4.4%), at 5mm the decrease is nearly 35% (from 56.7% to 22.2%). The effect on the reduction of the revision rate was nearly 3% at 1mm to nearly 8% at 5mm. The overall “gain” in the UHMWPE thickness is about 1mm which does suggest 10 years extra at a mean of 0.1mm penetration per year.
The abstracts were prepared by Mr Peter Kay, Editorial Secretary. Correspondence should be addressed to British Hip Society, The Hip Centre, Wrightington Hospital, Appley Bridge, Wigan, Lancashire WN6 9EP.
