Heterotopic ossification (HO) is a frequent complication of hip surgery. In this study the incidence of HO is analyzed in high risk patients who received radiation therapy (RT) after total hip replacement (THA) with regular and miniposterolateral hip approach. Two hundred and thirty five high risk patients received a single dose of 700 rad after THA. The incidence of HO was 15.7%. The incidence of HO in the high risk subgroup with the miniincision was lower (5.7%) but not significantly different ( ). Hypertrophic osteoarthritis was demonstrated to be the consistent predisposing factor for HO formation ( ). 1. Introduction Heterotopic ossification (HO) is bone formation in the soft tissues which develops after hip replacement surgery without a well-defined precipitating event (Figure 1). HO represents one of the most frequent complications following THA with reported rates of HO after total hip arthroplasty (THA) ranging from 5% to 90% depending on the risk factors [1]. HO is approximately twice as prevalent in men as in women [2]. Patients with a history of HO after previous hip surgery are at increased risk for recurrent HO formation [3]. Men with hypertrophic osteoarthritis have markedly higher levels of HO after THA [4]. Hips with markedly diminished motion and hypertrophic ossification before surgery were statistically more likely to have HO [2, 4]. Surgical approach and trauma may be a risk factor for the development of HO after THA. In several studies anterior and lateral approaches increase the possibility of HO [5, 6]. The posterior approach for THA is associated with the lowest incidence of HO formation [5, 6]. HO has been found more after an epidural anesthesia than after general anesthesia [7]. Extraction of the femoral head in a fragmented state from the acetabulum has been linked with the occurrence of HO [8]. Patients who engaged in more physical activity before surgery developed HO more frequently than patients with minimal physical activity [7]. Bone formation has been found in patients with postoperative dislocations in the first week [9]. Also postoperative fever for more than five days, superficial wound infection, and postoperative hematoma have also been associated with HO [7, 8]. Figure 1: Bilateral HO formation 9 months after surgery. There has been no reported association between type of femoral fixation and HO [10, 11]. Severe ossification was more common in men than in women and in patients operated by relatively inexperienced surgeons [12]. Prophylactic measures include diphosphonates, indomethacin, and radiation therapy with
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