Heterotopic ossification (HO) is a condition where the body forms mature bone tissue in soft tissues like muscles, tendons, or ligaments where bone should not exist. This abnormal bone growth typically occurs as an unexpected complication following major physical trauma, such as severe fractures or burns, or after extensive orthopedic surgery, like a total hip replacement. Neurological events, including spinal cord injuries and traumatic brain injuries, are also common triggers for this condition. The central question for patients is whether this unwanted bone is a temporary reaction that will dissolve, or if it represents a permanent change.
Understanding the Formation Process
The formation of HO is essentially a misguided attempt by the body to repair damaged tissue, representing a change in the normal healing process. The process begins with an inflammatory response at the site of injury, which creates a highly stimulating environment. This initial, active phase is characterized by localized swelling, warmth, and pain, often weeks after the initial trauma.
Within this inflammatory environment, certain stem cells, known as mesenchymal progenitor cells, receive incorrect chemical signals. Instead of differentiating into their intended repair cells, they mistakenly turn into bone-forming cells, or osteoblasts. These osteoblasts then begin to produce osteoid, which is the unmineralized matrix of bone tissue.
This initial bone matrix eventually undergoes mineralization, which is the process of hardening into mature, calcified bone that is detectable on an X-ray. The process of maturation, where the bone stops growing and becomes stable, typically completes between six and eighteen months after the initiating injury. The distinction between the early, active phase and the later, mature phase is medically important because it determines when certain interventions can be safely considered.
Stability and Natural Regression
For established heterotopic ossification, the answer to whether it goes away is generally no; once the bone is mature, it is considered permanent tissue. The mature HO is structurally identical to normal skeletal bone, composed of lamellar bone, and the body does not have a mechanism to naturally dissolve or absorb this type of stable osseous structure. While the initial inflammatory symptoms of the active phase will eventually subside, the calcified bone itself remains in place.
Unlike temporary swelling or fluid accumulation, the mature bone mass will not shrink or disappear on its own over time. The presence of this stable, misplaced bone can limit joint movement, cause chronic pain, or compress surrounding nerves and blood vessels. Small or asymptomatic HO may be left alone without issue, but the established bone mass that causes functional problems will persist unless actively removed.
The only instances of potential natural “regression” are often limited to minor, very early-stage cases where the bone formation process might be aborted before full maturation. However, in the vast majority of symptomatic cases resulting from major trauma or neurological injury, the fully formed HO is a stable entity. Its stability is the reason medical strategies must focus on prevention during the active phase or surgical removal once it has matured.
Treatment Options for Established HO
Since established HO does not resolve spontaneously, the primary treatment for symptomatic cases that severely limit joint function or cause pain is surgical excision. This procedure involves carefully removing the mature, unwanted bone mass to restore joint mobility and alleviate pressure on surrounding soft tissues. The timing of this surgery is a crucial factor in preventing recurrence of the abnormal bone growth.
Surgeons typically wait until the HO is radiographically and clinically mature, often defined as a stable period of six to eighteen months after the initial injury. Performing the excision too early, while the tissue is still in the active, inflammatory phase, can re-stimulate the process and significantly increase the chance of the HO growing back. The mechanical trauma of the surgery itself can reactivate the mesenchymal stem cells, leading to a new cycle of bone formation.
To combat the high risk of recurrence following excision, post-operative prophylaxis is routinely used. This often includes a short course of non-steroidal anti-inflammatory drugs (NSAIDs), such as Indomethacin, which interfere with the chemical signals that drive bone formation. Another common strategy is a single, low dose of external beam radiation therapy, delivered to the surgical site shortly before or immediately after the excision. This radiation works to neutralize any remaining or newly introduced bone-forming cells, thereby reducing the likelihood that the heterotopic ossification will return.