Introduction and Objective. Some
Abstract. Objective. To estimate the effect of calcar collar separation on the likelihood of calcar collar contact during in vitro periprosthetic fracture. Methods. Three groups of six composite femurs were implanted with a collared cementless femoral stem. Neck resection was increased between groups (group 1 = normal, group 2 = 3mm additional, group 3 = 6mm additional), to simulate failure to obtain calcar collar contact. Prior to each trial, the distances between anterior (ACC) and posterior (PCC) collar and the calcar were measured. Periprosthetic fractures of the femur were simulated using a previously published technique. High speed video recording identified when collar to calcar contact (CCC) occurred. The ACC and PCC were compared between trials where the CCC was and was not achieved. Regression estimated the odds of failing to achieve CCC for a given ACC or PCC. Results. CCC was achieved prior to fracture in all cases in group one, 50% in group two and 0% in group three. The median (range) ACC for those trials where CCC was achieved was 0.40 (0.00, 3.37) mm versus 6.15 (3.06 to 6.88) mm, where CCC was not achieved (p <0.01). The median (range) PCC for those trials where CCC was achieved was 0.85 (0.00 to 3.71) mm versus 5.97 (2.23 to 7.46) mm, where CCC was not achieved (p <0.01). Binomial logistic regression estimated risk of failure to obtain CCC increased 3.8 fold (95% confidence interval 1.6 to 30.2, p <0.05) for each millimetre of PCC. Conclusions. Increased separation between collar and calcar reduced the likelihood of calcar collar contact during a simulated periprosthetic fracture of the femur. Surgeons should aim to achieve a calcar-collar distance of less than 1mm following implantation to ensure calcar collar contact during
SUMMARY. We report a prospective study of clinical data collected pre, intra and post operation to remove both cup and head components of 118 failed, current generation metal on metal (MOM) hips. Whilst component position was important, the majority were unexplained failures and of these the majority (63%) had cup inclination angles of less than 55 degrees. Poor biocompatibility of the wear debris may explain many of the failures. BACKGROUND. Morlock et al reported a retrospective analysis of 267 MOM hips but only 34 head and cup couples (ie most were femoral neck fractures) and without data necessary to define cause of failure. The commonest cause of failure in the National Joint Registry (NJR) is unexplained. METHODS. We categorised the cause of failure, as defined by the NJR, of all MOM hips received over an 18 month period that had a full set of pre, intra and post op data. A group of 40 patients with unilateral well functioning MOM hips was used for comparison. RESULTS. In the retrieval group, the median age was 61 years (25 to 87) and there were 80 females and 38 males. The median time between primary and revision operation was 35 months (4 to 121). Femoral head size was <50mm in 89 and >=50mm in 29. The causes of failure were: unexplained in 75; aseptic loosening (acetabular) in 12; aseptic loosening (femoral) in 7; dislocation/subluxation in 1; infection in 11;