Heterotopic ossification or myositis ossificans is a condition that is quite common in the spinal cord injured (SCI) and people who have suffered a severe brain injury. It is more commonly known as “boning”. It can be described as the laying down of bone outside of the bony skeleton – mainly in the muscle and tendons around the joint usually in the hips. However, it can also be found in patients without neurological disorders, especially after surgery, or after trauma to the hips and legs. Unfortunately, the mechanism that triggers this laying down of bone in unusual sites is unknown. The patient that has some sensation in his body might complain of a warm, tender, firm swelling in a muscle and a decreased range of motion in the joint served by the muscle involved. However, usually care givers or the patient will notice a decreased range of motion of the affected joint. Progressive boning can cause serious problems with seating, especially when it affects the range of motion of the hips and, or knees. In SCI, boning only affects the joints below the level of the injury. The event can also be triggered by the chronic infection of a pressure sore overlying a joint such as a hip.
The only definitive diagnostic test in the early, acute stage is a bone scan, which will show heterotopic ossification 7 to 10 days before an X-ray. However, an abnormality detected in the early phase may not progress to the formation of heterotopic bone. Later, it becomes clear on a normal X-ray film and certain blood tests (alkaline phosphatase) will also be done. When the initial sympotoms including swelling and high temperature in a leg appear, other potential causes such as fracture and thrombosis need to be excluded. It may be necessary to do both a bone scan and a venogram to differentiate between heterotopic ossification and venous thrombosis, as it is possible that both could be present simultaneously.
Treatment would focus on the early stages of mobilising the affected joint in order to prevent contracture. There are some drugs available that may be of value. For example, Didronel is a drug that is used to prevent calcium from being deposited in the bony matrix that has already been formed, but it won’t remove calcium that has already been deposited. It is a preventative drug, and has no effect on existing ossification. The drug also has no effect on the underlying process which produces the bony matrix. There is no set rule on how long Didronel should be continued. In most cases, there will be a brief flare-up of the heterotopic ossification following the discontinuation of Didronel and, in some cases, an increase in the amount of calcium deposited. There are no reliable tests to indicate that the heterotopic ossification is inactive and treatment can be discontinued safely. However, if the treatment is continued long enough this additional calcium deposition will be of minimal clinical significance. Unfortunately, the drug is very expensive. Other drugs that have been used in the treatment or retardation of boning are the non-steroidal anti-imflammatories such as Indocid. There is no hard scientific evidence that it is of value though.
Surgery and radiation therapy
In severe cases of boning, surgery may be required where the range of motion of the joint has been so affected that seating or positioning becomes problematic. However, the boning process should be dormant before surgery because if it is done whilst still active, flare-up is inevitable. Surgery is normally followed up by radiation therapy. Prophylactic radiation therapy for the prevention of heterotopic ossification has been used since the 1970s. Generally, radiation therapy should start as soon as possible after surgery. Single dose radiation therapy is well tolerated and cost effective.