Heterotopic ossification is the abnormal growth of bone in soft tissues like muscles, tendons, or other connective tissues. This extraskeletal bone can grow much faster than normal bone, resulting in isolated formations in areas that should remain pliable. While it can occur anywhere, it most commonly affects major joints like the hips, knees, elbows, and shoulders.
Causes and Risk Factors
Significant physical trauma frequently triggers heterotopic ossification, such as severe bone fractures, joint dislocations, and major muscle injuries. The mechanism involves intense inflammation following an injury, which sends incorrect signals to local stem cells. These signals cause them to differentiate into osteoblasts, the cells that build bone, instead of repairing the soft tissue.
Surgical procedures, especially joint replacements like total hip arthroplasty, are a common cause. Many patients develop some extraskeletal bone growth post-surgery. The new bone often forms near the surgical site and can interfere with the new joint’s function.
Neurological injuries are another high-risk factor. Patients with a traumatic brain injury (TBI) or a spinal cord injury (SCI) have a higher incidence of this condition, with bone growth occurring below the level of the injury. Severe burns are also a risk factor.
While most cases are acquired, rare genetic disorders can also cause heterotopic ossification. Fibrodysplasia Ossificans Progressiva (FOP) is an inherited condition where the body replaces muscle and connective tissue with bone, leading to progressive loss of movement.
Signs and Symptoms
Initial signs of heterotopic ossification often appear 3 to 12 weeks after an injury or surgery. A common first symptom is a noticeable decrease in the range of motion of an affected joint. This stiffness can progress, sometimes leading to the joint becoming fixed in a bent position.
As the condition develops, localized inflammation becomes more apparent. The area around the affected joint may become swollen, warm to the touch, and painful. In some instances, a person might experience a low-grade fever, which can be more pronounced at night.
With time, a distinct, hard lump may become palpable under the skin within the muscle. Unlike a soft tissue mass, this lump will not be easily movable. Pushing on this area can be sore, as the new bone can be jagged or irregularly shaped.
The Diagnostic Process
Diagnosis begins with a physical examination, where a physician assesses symptoms like joint stiffness and feels for hard masses in the soft tissue. The initial inflammation can mimic other conditions like cellulitis or a deep vein thrombosis, so a careful evaluation is needed.
Blood tests may be ordered to check for serum alkaline phosphatase (ALP). ALP levels are often elevated during active bone formation, but this test is an indicator and not a definitive confirmation of the condition.
Imaging studies are the definitive method for confirming a diagnosis. Plain X-rays can identify mature bone but may not detect the condition in its early stages. A computed tomography (CT) scan is often used for a more detailed view of the bone’s size, shape, and location.
For very early detection, a three-phase bone scan is a highly sensitive tool that can identify new bone formation before it is visible on an X-ray. An ultrasound may also be used to visualize the early stages of soft tissue changes and bone development.
Treatment and Management
Preventative measures are often used for high-risk patients to stop abnormal bone formation before it starts. Two primary methods are non-steroidal anti-inflammatory drugs (NSAIDs) and low-dose radiation therapy. NSAIDs reduce the inflammatory response, while targeted radiation inhibits local cells from becoming bone-forming cells.
Once heterotopic ossification has formed, non-surgical management focuses on preserving function and reducing discomfort. Physical and occupational therapy use exercises and gentle stretching to maintain as much range of motion as possible. These therapies help prevent joint contractures and can alleviate pain from stiffness.
For severe cases where the extra bone limits movement, causes chronic pain, or interferes with daily function, surgical excision is the primary treatment. Surgeons delay this intervention until the bone is considered “mature,” meaning it has stopped actively growing.
Bone maturity is determined by monitoring it with serial X-rays until its size has stabilized, which can take 12 to 18 months. Performing surgery on immature bone carries a high risk of recurrence. Post-operative physical therapy plays a large part in regaining function after the bone is removed.