I’m considering having a two-level disc replacement surgery in my neck. I understand all the risks and benefits. The surgeon’s assistant has mentioned excess bone formation called heterotopic ossification. Why is this such a bad problem? It seems like the extra holding power would be a good thing. Please explain.

Artificial disc replacements were developed to avoid neck fusions and thus preserve neck motion. The idea came about when studies showed that cervical spine fusion often resulted in added wear and tear and degeneration of the adjacent (next level above or below) segment.

But there are concerns with the disc implants. They can migrate or move backwards in the spine. A position too far back can increase the risk of spinal cord or spinal nerve root compression. There is the possibility of subsidence (disc sinks down into the vertebral bone). Loosening of the implant and adjacent segment degeneration round out the list of possible problems or complications that can occur.

But it turns out the biggest problem is heterotopic ossification. With heterotopic ossification, there is additional bone formed outside the skeletal system. The extra bone usually forms in the surrounding soft tissues, especially the nearby muscles.

It’s a problem because the artificial disc replacement (ADR) is designed to maintain joint motion. The heterotopic ossification reduces soft tissue mobility and causes increased pain and stiffness and decreased neck motion.

There are several reasons why heterotopic ossification develops — especially after multiple-level cervical disc implantation. First, the disruption of soft tissues and damage to the muscles is greater when more than one level is involved.

Second, the risk of heterotopic ossification (HO) increases when patients are not given antiinflammatory medications (known to reduce the risk of HO). And third, removing the diseased disc and damaged bone involves a special milling process, which may contribute to the body’s response making more bone.

There aren’t a lot of long-term studies to show for sure how often the problem of HO develops. From what has been reported, it looks like almost half of all patients will develop HO after cervical disc replacements. More HO seems to develop as time goes by. Not only do more patients show evidence of HO in time, those who develop it early start to progress to more severe cases.