I just came back from my six-month check-up after having artificial disc replacement surgery. They put the new disc in my neck at the C5-6 level. I guess I already have some bone growing where it shouldn’t be. I’m wondering if you can tell me why this is happening?

You may be experiencing a potential side effect of spine surgery called heterotopic ossification — bone that forms outside the skeletal system. Usually, streaks of calcification or bone develop in the nearby muscles and soft tissues.

This problem is common after hip and knee replacements. And with the increased number of artificial disc replacements in the neck and low back, surgeons are finding the same phenomenon. In fact, a recent study comparing three different types of implants found an overall rate of heterotopic ossification in 40 per cent of the patients.

In the case of excess bone formation after joint replacements, there is some evidence that tissue trauma during surgery, removal of bone dust, and the use of nonsteroidal antiinflammatories before and after surgery might be contributing factors. But studies have not been done to see if any of these factors increase the risk of heterotopic ossification with disc replacements.

From what we do know so far, it looks like the type of implant makes a significant difference. Implants that fit well up against the endplate seem to avoid the problem of bony overgrowth. The endplate is a circle of cartilage between the disc and the vertebral body.

As far as possible patient factors, scientists propose genetic traits might be at work. Specific patient characteristics (age, gender, lifestyle, body size, level of implant, diagnosis, type of surgery, and so on) as contributing risk factors have not been investigated.