Influence of heterotopic ossification of the hip on bone densitometry : a study in spinal cord injured patients

To evaluate (1) the magnitude of falsely elevated bone density results caused by heterotopic ossification (HO) around the hip and (2) effect of age of patients when the measurement was taken, age of patients at injury, and age of injury (time since event) to the prevalence of HO. We blindly analyzed plain radiographs of the hip [(obtained within 1 month of dual energy X-ray absorptiometry (DEXA)] in 107 spinal cord injured (SCI) patients for HO and matched the result to the three regions of interest (ROI): the femoral neck, Ward's triangle, and the trochanter. The influence of HO on bone densitometric values was determined by the analysis of variance (ANOVA) and post-hoc analysis. Nineteen (18%) patients had HO; overlying the femoral neck (79%), trochanter (74%) and Ward's triangle (37%), respectively. Significant elevation of densitometric values (P < 0.05 or less) was observed in a various magnitude at each ROI, with the greatest elevation at Ward's triangle. The prevalence of HO was high when the patients were injured at age range of 20-39 years. HO around the hip can cause significantly elevated bone densitometry results at all ROIs, which can obscure underlying osteoporosis, leading to underestimation of fracture risk. Determination of bone density in this region with corresponding plain radiographs would be of help. In SCI patients, prevalence of HO was high when the age of patients at injury was 20-39 years.