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Heterotopic Ossification, a Common Pathophysiologic Process

Posted Apr 08 2011 12:00am

Pathologists tend to be fascinated by unusual pathophysiologic processes and I am no exception. I stumbled on a recent article about heterotopic ossification but discovered that it's more common than I thought (see: Heterotopic ossification treatment: new research ), Below is an excerpt from it:

Promising new research reveals a potentially highly effective treatment for heterotopic ossification (HO), a painful and often debilitating abnormal buildup of bone tissue. HO comes in two main forms—one that appears in children and is congenital, another that strikes wounded military personnel and surgery patients and is triggered by severe injuries and wounds. An animal study by developmental biologists shows that a drug that interrupts a signaling-nuclear protein pathway can prevent HO. The study appeared online today in Nature Medicine. “There are currently no effective treatments for this disease,” said [one of the study authors] “Surgeons can remove the abnormal bone masses, but surgery itself may trigger more of those growths.”....The exact mechanism by which HO occurs is not fully understood, but trauma, surgery or deep burns cause local inflammation, followed by the arrival of skeletal cells that develop into chondrocytes...and are then replaced by intrusive bone. Thus, 10 to 13 percent of orthopedic patients may develop HO, mostly without major symptoms, after knee replacement or other invasive surgeries. The incidence of HO is far higher in wounded soldiers—nearly 65 percent—because modern weapons cause extreme, wide and deep tissue damage. Although HO is not life-threatening, the bone growths can press against nerves and blood vessels, resulting in chronic pain, limited motion, problems fitting prosthetic limbs and other complications

I must say that I was surprised by the incidence of heterotopic ossification (HO) in wounded soldiers. However, on reflection, this makes sense. Modern military surgery is rescuing soldiers with massive tissue injury who might have died in the past. Assuming that the likelihood of HO increases with the amount of damaged tissue and inflammation, injured soldiers would be the most likely candidates to develop it. Although the study does suggest a way to chemically block HO development, it obviously needs to be tested clinically. The agent might also block the normal repair of bone fractures.

Here's more information from the Wikipedia , indicating a high incidence of the HO among patients with total hip replacement, a common procedure and one with extensive tissue and bone disruption:

Heterotopic ossifications will develop in 10% to 80% of cases with varying severity after surgery or trauma to the hip and lower legs. About every third patient who has to face heterotopic ossification following total hip arthoplasty (replacement) or severe fracture of the long bones of the lower extremities will develop pain and dysfunction resulting from extensive heterotopic ossification. Heterotopic ossification jeopardizes functional outcome, impairs rehabilitation and is costly because of secondary surgical procedures. Pain first arises a few days after surgery with calcified structures appearing as blurred contours on x-rays at 3 to 6 weeks postoperatively. Patients with heterotopic ossification after a previous hip arthoplasty are at greatest risk of developing additional heterotopic ossification, with incidence between 50% and 90%.

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