SUBSIDENCE OF THE LONG STEM EXETER HIP

Background: - The excellent survivorship of the Exeter stem is thought, in part, to be related to limited subsidence of the stem within the cement mantle due to it’s polished, double taper design. The subsidence within the cement mantel is seen as a necessary part of the optimum transmission of load into the cement and hence the bone. The long Exeter stems do not all mimic the same double taper geometry as the standard 150mm stem. There is no published evidence that the long stem Exeter implant exhibits the same subsidence within the cement mantel as the standard stem. Method: - Using digitised x-rays and a computer aided design package we measured the subsidence of 35 standard Exeter stems and 40 long stems. Measurements were taken from the initial postoperative radiograph and repeated at intervals up to 5 years. The long stem implants were all used in cemented revisions without the use of impaction grafting. None of the implants exhibited radiological loosening at the last follow-up x-ray available. Results: - The analysis of the standard length stems illustrated that the method was able to demonstrate subsidence within the cement mantel, at rates comparable to that in the published literature. However the long stem implants exhibited a significantly reduced subsidence rate (at 12 months P Conclusion: - This study suggests that the subsidence of the long stem Exeter implant does not mirror that of the standard length stem. The loss of the fully tapered geometry of the longer stem Exeter implants may account for the reduced subsidence of the implant at the prosthesis/cement interface. The longevity of the standard length stem is, to a certain extent, thought to arise from a limited subsidence at this interface. This raises concerns that the survivorship of the longer stem implants should not be extrapolated from the excellent results of its shorter length relation.