Abstract
Heterotopic ossification (HO) is the formation of bone at extra-skeletal sites. Genetic diseases, traumatic injuries, or severe burns can induce this pathological condition and can lead to severe immobility. While the mechanisms by which the bony lesions arise are not completely understood, intense inflammation associated with musculoskeletal injury and/or highly invasive orthopaedic surgery is thought to induce HO. The incidence of HO has been reported between 3% and 90% following total hip arthroplasty. While the vast majority of these cases are asymptomatic, some patients will present decreased range of motion and painful swelling around the affected joints leading to severe immobility. In severe cases, ectopic bone formation may be involved in implant failure, leading to costly and painful revision surgery. The effects of surgical-related intraoperative risk factors for the formation of HO can also play a role.
Prophylactic radiation therapy, and anti-inflammatory and biphosphonates agents have shown some promise in preventing HO, but their effects are mild to moderate at best and can be complicated with adverse effects. Irradiation around surgery could decrease the incidence of HO. However, high costs and the risk of soft tissue sarcoma inhibit the use of irradiation. Increased trials have demonstrated that nonsteroidal anti-inflammatory drugs (NSAID) are effective for the prevention of HO. However, the risk of gastrointestinal side effects caused by NSAID has drawn the attention of surgeons. The effect of the selective COX-2 inhibitor, celecoxib, is associated with a significant reduction in the incidence of HO in patients undergoing THA. Bone morphogenetic proteins (BMP) such as BMP2 identified another novel druggable target, i.e., the remote application of apyrase (ATP hydrolyzing agent) in the burn site decreased HO formation and mitigated functional impairment later. The question is if apyrase can be safely administered through other, such as systematical, routes. While the systemic treatments have shown general efficacy and are used clinically, there may be great benefit obtained from more localised treatment or from more targeted inhibitors of osteogenesis or chondrogenesis.
In the surgical setting, prophylaxis for HO is regularly indicated due to the considerable risk of functional impairment. Heterotopic ossification is a well-known complication of total hip arthroplasty, especially when the direct lateral approach is used. Possible intraoperative risks are the size of incision, approach, duration of surgery and gender that can be associated with higher rates of HO or increase of the severity of HO. Like inflammation and tissue damage/ischemia are likely to be the key in the formation of HO, kindness to the soft tissues, tissue preserving surgery, pulse lavage to remove bone inducing factors and avoiding damage to all tissues should be erased as a comorbidity. Incision length, tissue dissection and subsequent localised trauma and ischemia, blood loss, anesthetic type and length of surgery may all contribute to the local inflammatory response. Data suggest that the surgeon may control the extent and nature of HO formation by limiting the incision length and if possible the length of the operation.
Currently resection of HO is generally suggested after complete maturation (between 14–18 months), since earlier intervention is thought to predispose to recurrence. Reliable indicators of maturation of HO are diminishing activity on serial bone scans and/or decreasing levels of alkaline phosphatase. Although usually asymptomatic, heterotopic bone formation can cause major disability consisting of pain and a decreased range of motion in up to 7% of patients undergoing THA. Patients benefit from early resection of the heterotopic ossification with a proper and reliable postoperative strategy to prevent recurrence of HO with clinical implications.
