Within one year of surgery, 8 out of 56 hips with a conventional cemented cup had dislocated. Considering the known high dislocation rate after THA for femoral neck fracture [1] it might be considered either not to use a posterior approach or select an implant less prone to dislocate. With the DAC we had no dislocations. As with DAC in our material, it has been reported that bipolar hip prostheses significantly reduce dislocation rates as compared to conventional THA [5], and they are widely used for treatment of femoral neck fractures [7]. However, bipolar implants have been associated with migration into the acetabulum and acetabular osteolysis [8].
In our study, all the THA were performed by one of 4 experienced orthopedic surgeons. Although 4 different THA systems were used in the conventional group and two different head sizes, these implants had been in use for a long period before the study was started, thus they were familiar to the surgeons, and our dislocation rate was similar to that reported in the literature [1]. Furthermore, no dislocations occurred after the introduction of DAC in spite of a possible "learning curve" with the introduction of a new implant.
That both 28 mm and 32 mm heads were used in the conventional THA group may be regarded as a weakness of the paper. Larger head size in THA has been associated with decreasing dislocation rates [9] which is in concordance with our material in which the dislocation for 28 mm femoral heads was 3 times higher than for 32 mm heads. However, some surgeons may be reluctant to opt for larger head sizes considering that they have been associated with higher polyethylene wear and increased risk of aseptic loosening [10–12].
The economy when choosing an implant is of great importance. The Dual articulation Avantage cup is approximately three times more expensive than a conventional cemented cup. In 56 femoral neck fracture patients we had 14 episodes of prosthetic dislocation in 8 patients within one year after surgery with associated additional costs, and there were 3 revisions due to recurrent dislocations. Adding to the cost of surgery there were additional costs for prolonged hospital stay as well as postoperative rehabilitation. Consequently the initial savings using a conventional implant, at least one with 28 mm head, may be questionable not considering the pain and suffering of the patients involved.