Heterotopic ossification

What is heterotopic ossification?

Heterotopic ossification (HO) is a condition where bone forms in soft tissue, such as muscle, where it doesn't normally grow. It usually develops in soft tissue close to a joint, most often after an injury or surgery.

What are the symptoms of heterotopic ossification?

Symptoms vary depending on how much bone has formed and where. Around 1 in 3 people who have a hip replacement or a serious break in a long leg bone will develop some heterotopic ossification, though in most, it doesn't cause any symptoms. Symptoms include:

• pain, swelling and tenderness in the affected area
• a firm lump under the skin, which can grow quickly and may be tender to touch
• warmth and redness over the area
• reduced movement in a nearby joint, which can become increasingly stiff over time
• occasionally, a mild fever.

Symptoms usually appear 3 to 12 weeks after the injury or operation that triggered it, though it can sometimes take up to 6 months to show.

What are the causes of heterotopic ossification and who is at risk?

Heterotopic ossification usually happens after an injury or major surgery, though doctors don't fully understand why. It's thought that inflammation and tissue damage cause certain cells to develop into bone-forming cells in the wrong place.

You're at higher risk if you've had a bone fracture, a burn, a spinal cord injury, a traumatic brain injury, or a joint replacement, especially a hip replacement.

Nongenetic HO is especially common after some of these: for example, it can develop in around 2 to 3 in 10 people with a spinal cord injury, 1 to 2 in 10 people with a serious head injury, and more than 9 in 10 people who have an amputation after a traumatic injury, though it doesn't always cause symptoms.

Other risk factors include a long period of immobility, a joint infection, spasticity (muscle stiffness caused by a nerve condition), and pressure sores. Heterotopic ossification is somewhat more common in men, though it can affect anyone.

Very rarely, it's caused by an inherited genetic condition, such as fibrodysplasia ossificans progressiva. These genetic forms usually appear early in life and tend to be far more severe than the more common type that follows an injury.

Are there different types of heterotopic ossification?

Heterotopic ossification is mainly grouped into nongenetic and genetic types:

• Nongenetic HO: By far the most common, develops in response to an injury or surgery and can affect people of any age
• Genetic HO: Is much rarer and tends to be more severe, sometimes causing changes to the spine, hands or feet from birth.

If heterotopic ossification develops after a hip replacement, doctors often use a grading system, most commonly the Brooker classification, to describe how much bone has formed:

• Grade 1: Small islands of bone in the tissue around the hip
• Grade 2: Bone spurs from the pelvis or thigh bone, with at least 1cm between the opposing bone surfaces
• Grade 3: Bone spurs with less than 1cm between the opposing surfaces
• Grade 4: The bones have fused, leaving the hip joint completely rigid.

How is heterotopic ossification diagnosed?

Your doctor may use imaging tests such as an X-ray, ultrasound, CT or MRI scan to check for heterotopic ossification. Early on, a bone scan is often the most sensitive test, as it can sometimes detect changes before they show up on a standard X-ray.

Blood tests, including one that measures an enzyme called alkaline phosphatase, can support a diagnosis, particularly after a hip replacement, though these tests aren't specific to heterotopic ossification alone.

How is heterotopic ossification treated?

Treatment depends on your symptoms, the type of heterotopic ossification you have, and how far it has progressed.

Non-surgical options

• Physiotherapy: To help maintain movement and reduce pain
• Anti-inflammatory medicines: Which may also be used to help prevent heterotopic ossification forming after some operations
• Steroid medicines: Particularly for flare-ups in people with genetic forms of heterotopic ossification
• Radiotherapy: Most often used to help prevent heterotopic ossification forming after high-risk surgery, such as hip replacement, or to reduce the chance of it coming back after surgical removal.

If you're offered radiotherapy to prevent heterotopic ossification, it's usually given as a single, low dose a few hours before your surgery. It's similar to having an X-ray, and involves lying still for a few minutes while the treatment is delivered. Side effects are usually mild and settle within a week, and can include tiredness, skin redness, joint swelling, or loose stools.

Surgery

Surgery may be used to remove heterotopic ossification that's causing severe pain or restricting your movement. It's usually a last resort, partly because there's a chance the bone can form again, and it's normally delayed until the new bone has fully matured, which can take many months, to reduce the risk of it coming back.

When should I see a doctor?

See a doctor if you notice new pain, swelling, warmth or stiffness in a joint or muscle in the weeks or months after an injury, surgery, or a condition such as a spinal cord injury or brain injury.

If you're due to have radiotherapy for heterotopic ossification and you're pregnant, think you might be pregnant, or are planning to have children, tell your treatment team, as this can affect your care.