What is Heterotopic Ossification?
Heterotopic ossification (HO) is the presence of bone in soft tissue such as muscle, tendons, or ligaments where bone typically does not exist (extraskeletal bone). Lesions can range from small clinically insignificant foci of ossification to large deposits of bone. HO often occurs after an injury or as a side effect of a surgical complication. But they may also occur for unknown reasons. The most common location of HO is between the muscle and joint capsule. For most individuals, extraskeletal bone pieces are small and do not cause many symptoms. However, large bone fragments may limit movement and cause serious problems in areas such as the hip or groin.
Types of Heterotopic Ossification
Heterotopic ossification can be genetic or nongenetic. Genetic types are far less common and usually affect the spine, hands, or feet. Nongenetic or acquired HO is the more common type and can affect individuals of all ages. They may occur following a localized trauma or a neurological injury or as a postoperative complication.
Causes of Heterotopic Ossification
The exact cause of heterotopic ossification is unknown but most thesis describes a cellular response to local tissue injury which results in the release of chemical mediators that stimulate exaggerated bone proliferation. Muscle trauma appears to be a significant triggering event. Heterotopic ossification is histologically similar to normal cortical and cancellous bone. Typically, the common acquired heterotopic ossification can be secondary to:
- Spinal cord injury
- Musculoskeletal trauma
- Traumatic brain injury
- Largely seen after joint arthroplasty or joint replacement (an estimated 90% of hip arthroplasty patients may have heterotopic ossification)
- Often forms close to metal hardware
Rare genetic disorders can cause heterotopic ossification such as fibrodysplasia ossificans progressiva (FOP).
Signs and Symptoms of Heterotopic Ossification
Some of the signs and symptoms of heterotopic ossification include:
- Joint tenderness
- Soft tissue swelling
- Increased muscle spasms
- Erythema or skin redness
- Poor mobility of the affected joint
- Bump/lump under the skin of the affected area
Diagnosis of Heterotopic Ossification
To diagnose heterotopic ossification, your physician may utilize physical examination and imaging tests such as a:
- CT scan
- Positron emission tomography (PET) scan
If you have developed heterotopic ossification after a hip arthroplasty surgery, your physician may utilize grading scales to evaluate how much HO has advanced. One of the most commonly used grading scales is the Brooker classification. This scale rates heterotopic ossification as:
- Grade 1: Small fragments of bone in the tissue surrounding the hip
- Grade 2: Bone spurs (irregular bone growths) around the femur (thigh bone) or pelvis, spaced at least 1 centimeter apart
- Grade 3: Bone spurs around the femur or pelvis spaced less than 1 centimeter apart
- Grade 4: Bone fusion (ankylosis) or rigidity in the hip joint
Treatment for Heterotopic Ossification
As prophylaxis or treatment for mild cases of heterotopic ossification, physical therapy may be recommended to increase the range of motion and reduce pain. A nonsteroidal anti-inflammatory drug (such as indomethacin), a diphosphonate (such as ethane-1-hydroxy-1,1-diphosphate), or other medications may be used to alleviate symptoms. Local radiation therapy may be recommended if medications or physical therapy have failed to mitigate symptoms. Prior to beginning therapy, bone scanning may be recommended to confirm the diagnosis of heterotopic ossification. In addition, surgical resection of heterotopic ossification may be utilized to preserve joint mobility. However, heterotopic ossification is likely to recur and possibly advance if resection is undertaken before the lesion has become mature. In order to avert the risk of HO recurrence and other surgical complications, a series of quantitative bone scans are utilized as an assistance to time surgical intervention.