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Unwelcome Bone Formation after Joint Replacement

A small number of patients with a total hip or knee joint replacement can end up with less motion and more pain than before the operation. This is especially true if they develop a condition called heterotopic ossification (HO).

When this rare condition happens, the patient starts to form bone in the soft tissue. Without warning, bone cells start forming in the muscles. When it occurs, HO usually starts within 16 hours of the operation to replace a knee or hip joint. The result is a painful loss of motion. The patient can't sit, climb stairs, or put on shoes and socks. Signs of inflammation such as redness, swelling, and tenderness usually accompany HO.

Doctors don't know exactly what causes HO. There are some people who are more likely than others to develop this condition. Risk factors for HO include men with hypertrophic osteoarthritis in both hips or knees. This kind of arthritis causes tissue to increase in size. Anyone who's had HO before has an increased risk of getting it again. Some diseases, such as ankylosing spondylitis and Paget's disease, also put a patient at increased risk.

Other risk factors are less common but just as important. For example, certain methods of joint replacement have a higher rate of HO. They include using a cementless implant and replacing the hip joint from the front rather than the side or back. Anyone with a total knee replacement that has knee stiffness afterward and needs to have the knee manipulated by the doctor is at increased risk.

HO can be prevented. Patients at risk can use radiation or drugs, such as nonsteroidal antiinflammatories (NSAIDs). Radiation keeps fast growing cells such as bone cells from forming. These measures can be taken before the joint replacement. Some doctors watch patients at risk carefully and start treatment as soon as any sign of HO appears.

References: Richard Iorio, MD, and William L. Healy, MD. Heterotopic Ossification After Hip and Knee Arthroplasty: Risk Factors, Prevention, and Treatment. In Journal of the American Academy of Orthopaedic Surgeons. November/December 2002. Vol. 10. No. 6. Pp.409-416.

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