Introduction: The objective of our study was to analyse the incidence of subsidence of an uncemented, hydroxyapatite coated, collarless prosthesis (Orthodynamics) and its impact on postoperative mobility in patients with fractured neck of femur.

Methods: 22 sets of case notes and X Rays were available for patients treated with the Orthodynamics hemiarthroplasty between February and November 2005. The mean age was 78 years (range 61 – 91 years). Analysis of standardised anteroposterior radiographs in the immediate postoperative period and at a mean interval of 14 weeks (ranging from 6 weeks to 64 weeks) was undertaken. Subsidence was calculated by measuring the reduction in the distance between the tip of lesser trochanter and a fixed point on the prosthesis.

Results: 18 of the 22 (81.8 %) cases showed subsidence of the prosthesis. The mean subsidence calculated was 9.1 millimetres (range 1 to 25 millimetres) and it showed no relationship to the postoperative interval at which it was calculated. Impact of subsidence on mobility state was assessed. Post operative mobility status was available for sixteen patients. Nine patients showed no change in the mobility at follow up. This group had a mean subsidence of 4.3 mm. Seven patients had a worsening of their mobility state. The mean subsidence in this group was 10.1 mm.

Discussion: Our study shows significant subsidence of an uncemented, collarless, hydroxyapatite coated prosthesis used for hip hemiarthroplasty and an associated worsening of mobility. We suggest that further review of this prosthesis is done and other proven prostheses used for hip hemiarthroplasty after a fractured neck of femur.