Problems that develop after surgery for joint replacement can include heterotopic ossification (bone forms in soft tissue where it doesn't belong). This complication is not uncommon after hip and knee replacements. Now, we know the same holds true for ankle joint replacement.
In this study from South Korea, 25 per cent of the 80 adult patients followed after total ankle replacement (TAR) surgery developed heterotopic ossification (HO). In 10 per cent of the cases, it was painful and reduced ankle motion. Stiffness and difficulty with movement were severe enough to require surgery in a small number of patients.
Why does this problem develop, who is at risk, and what can be done to prevent it? These are three good questions with limited answers at this time. No one knows for sure why or how mature bone forms in the muscles and tendons around the joint. Some experts suggest both local and systemic factors.
Why do some people end up with heterotopic ossification (HO) after surgery and others do not is also something of a mystery. There are known risk factors but not all patients with those risk factors develop HO.
Some of the risk factors that have been identified so far include: male sex, older age, and infection. Bone trauma, extensive soft tissue dissection during surgery, and the formation of any hematomas (pocket of blood) during surgery are additional risk factors.
One more question -- how come only 10 per cent of the patients who develop heterotopic ossification have any symptoms? Again, no one knows for sure. If it weren't for studies like this one, we wouldn't even know the prevalence rate (25 per cent) is so high.
The authors of this study take things one step further and developed a classification system for heterotopic ossification following primary (the first) total ankle replacement. Having criteria for severity of the condition is helpful for surgeons following these patients.
The current proposed classification system is as follows:
- Class 0 No heterotopic ossification
- Class I Pieces or fragments of bone called islands within the soft tissue around the ankle
- Class II Bone spurs form inside the joint; up to half the joint space is reduced by the presence of the extra bone
- Class III Bone spurs reduce the joint space by 50 per cent or more
- Class IV Bone forms a bridge between two major bones of the ankle (tibia and talus)
As you can see, the classification model is based on severity (amount) of new bone formation. X-rays were used to examine each of the 80 patients when rating the severity of their heterotopic ossification (HO). There was a fairly equal number of patients in each of the classes from I to IV. The appearance of early signs of HO was seen in the first months after surgery for some and not until a year later in others.
Looking back and comparing the patients in this study who developed HO with those who did not, there were several variables that might have made the difference. The first was the condition of the soft tissues before surgery.
Nine of the 20 ankles with heterotopic ossification had plenty of scar tissue around the ankle (from previous injuries or surgery). These patients had more severe osteoarthritis from trauma as opposed to primary osteoarthritis (without trauma).
Reference: Keun-Bae Lee, MD, PhD, et al. Heterotopic Ossification After Primary Total Ankle Arthroplasty. In The Journal of Bone and Joint Surgery. April 2011. Vol. 93. No. 8. Pp. 751-758.