Heterotopic ossification (HO) describes the abnormal formation of bone tissue in soft tissues where bone does not typically exist, outside of the skeleton. While it can develop in various bodily locations, the hip is a frequently affected and significant site. This condition involves the creation of mature bone within muscles, tendons, or ligaments surrounding the joint, which can impede normal movement. Understanding HO of the hip involves recognizing its development and management strategies.
Causes and Major Risk Factors
Heterotopic ossification most often develops as a complication following specific traumatic events or medical procedures. One common trigger is total hip replacement, also known as total hip arthroplasty, where bone formation can occur around the surgical site. Direct trauma to the hip, such as from severe accidents or fractures of the acetabulum (the hip socket), can also initiate this process. Neurological injuries, including spinal cord injuries or traumatic brain injuries, frequently lead to HO.
Several factors increase an individual’s susceptibility to developing HO of the hip. A prior history of HO in any joint significantly raises the likelihood of recurrence. Certain inflammatory conditions, such as ankylosing spondylitis, are also associated with an elevated risk. Male gender is recognized as a contributing factor, as is being in the 20-40 age range. Specific surgical approaches during hip arthroplasty, like anterolateral and lateral approaches, may also influence the incidence of HO formation.
Recognizing the Signs and Getting a Diagnosis
Recognizing the presence of heterotopic ossification often begins with observing changes in hip function. A common early indication is increasing stiffness or a noticeable reduction in the range of motion of the hip joint. Patients might also experience localized pain, swelling, warmth, and redness in the soft tissues around the affected joint. A fever may also be present, which can sometimes be mistaken for other conditions.
Confirming a diagnosis of HO typically involves a combination of physical examination and imaging studies. During a physical exam, a healthcare provider will assess the hip’s range of motion and may feel a firm, palpable mass in the affected area. X-rays are commonly used for confirmation, as they can show the developing bone formation, though it may take several weeks for these changes to become visible. Computed tomography (CT) scans offer more detailed, three-dimensional views of the bone formation, providing information on its size and precise location. While not specific to HO, blood tests like alkaline phosphatase may show elevated levels, suggesting increased bone activity.
Prophylaxis and Prevention
Preventive measures are important for managing heterotopic ossification, particularly for individuals undergoing high-risk procedures like hip surgery. Surgeons implement strategies to reduce HO development before or immediately after such events. These prophylactic approaches aim to interrupt abnormal bone formation.
One primary method involves the use of non-steroidal anti-inflammatory drugs (NSAIDs). Medications like indomethacin are commonly prescribed to inhibit inflammatory pathways that contribute to bone growth. These drugs are typically administered for a period ranging from three to six weeks following surgery.
Another established preventive treatment is external beam radiation therapy. A single, low dose of radiation is precisely targeted to the hip area. This treatment is typically given within 24 hours before or 72 hours after surgery. The radiation works by preventing cells from differentiating into bone-forming cells, thereby reducing the risk of HO.
Managing and Treating Formed Bone
When heterotopic ossification has developed, management strategies focus on addressing symptoms and restoring function. Treatment decisions are guided by the severity of symptoms and the extent to which joint movement is affected. This phase of care focuses on existing bone formations.
For milder cases or while the abnormal bone is still maturing, non-surgical approaches are often employed. Physical therapy plays an important role in maintaining hip range of motion. Pain management through various medications improves comfort.
When HO significantly compromises hip mobility or causes persistent pain, surgical excision of the abnormal bone is the definitive treatment. Surgeons generally wait for the HO to “mature,” which can take several months to years after the initial event, before performing surgery. This waiting period reduces the risk of recurrence. Following surgical removal, prophylactic measures are often reinstituted to minimize new bone formation.