112 - Heterotopic Ossification

Abstract

Heterotopic ossification (HO) is the process by which aberrant bone is formed in the soft connective tissues, joint capsule, or skeletal muscle outside of the normal confines of the skeleton. There are several etiologies for HO, which are generally grouped as neurologic, genetic, and traumatic, with orthopedic procedures included in the latter. This pathologic process most often occurs about the hip and is typically incited by a variety of surgical procedures, such as total hip arthroplasty (THA), repair of acetabular fractures, pelvic osteotomy, intramedullary nailing of femoral shaft fractures, and even hip arthroscopy with femoral or acetabuloplasty. HO also occurs about other joints, such as the shoulder, elbow, and knee. It can also appear in conjunction with other nonsurgical events, such as head injury, cerebrovascular accident, burns, and genetic disorders such as fibrodysplasia ossificans progressive (FOP). Most recently, related to armed conflicts and the use of improvised explosive devices (IEDs), HO has appeared in the residual limbs of up to two-thirds of combat-related amputees after blast injury. Nonetheless, it is clinically most often manifest as ectopic bone formation in the abductor musculature after THA. Factors intrinsic to the patient, injury to the abductor musculature, surgical approach, hemorrhage, and bone debris remaining in the soft tissues all contribute to the overall risk of HO in any individual patient. As a radiographic phenomenon without accompanying symptoms, HO has been reported to occur in as many as 90% of patients who have undergone THA. In this setting, it is manifest as small islands of bone in the soft tissues about the hip or as bony outgrowths from the margin of the acetabulum or the proximal femur, measuring less than 1 cm in length. However, in up to 10% of patients, ectopic bone formation after THA occurs to such an extent or in such a location that it results in clinical manifestations of pain soon after operation and late restriction of motion that are of consequence to the patient. In the most extreme cases, the resulting symptoms of pain and stiffness replace those that prompted the patient to seek surgery in the first place and may actually negate the beneficial effect of the arthroplasty.