Bone metabolism in heterotopic ossification is primarily influenced by the time after trauma or surgery, independent of anatomic location

Although several risk factors and prophylactic measures for heterotopic ossification (HO) have been identified in the past, knowledge about its growth and metabolic activity over time remains limited. This study aimed to identify specific characteristics of HO metabolism and time-dependent activity shared at two of the most prevalent sites of HO, namely HO after elbow trauma and total hip arthroplasty. Thirty-six HO specimens (elbow/hip: 11/25) were analyzed using histological analysis of central and peripheral regions, and quantitative backscattered electron imaging. All data were compared with iliac crest samples of healthy donors (n = 11) and analyzed based on the time after initial trauma or surgery. Both elbow and hip HO were characterized by signs of endochondral ossification and exhibited high proportions of woven bone and low matrix mineralization. Ongoing ossification was observed frequently along the HO rim, and high bone remodeling was evident, particularly in elbow HO. While elbow HO specimens were obtained at earlier stages, most characteristics were comparable to hip HO when matching for the time after trauma or surgery. Ongoing ossification did not change significantly with time after trauma, but the woven bone proportion and osteocyte density decreased, particularly in the first three years, while the calcium content increased inversely. Taken together, both elbow and hip HO are characterized by immature, woven bone that is metabolically highly active, especially within the first three years after trauma. Reduced metabolic activity and signs of bone maturation suggest that the risk of recurrence may be reduced if resection is performed at a later time point after trauma or surgery.