In Switzerland, orthopedic surgeons have been working for years to develop and perfect a total ankle replacement. In this article, long-term results of the Scandinavian Total Ankle Replacement (STAR) are reported. This is the second report from a group of patients who received the implant 10 to 15 years ago.
The first reported results for these 72 patients came after the initial two to four years. At that time, the short-term survivorship of the implant was very high. But over time, there were more and more implants that developed problems. The need for a second (revision) surgery increased after 10 years and continued to increase as time went by.
The main reason for implant failure was loosening. Degenerative changes where the implant and bone connected (called the prosthesis-implant interface) was observed. The cause of this degenerative process may be the coating used on some of the component parts of the implant. A single layer of hydroxyapatite was used. Hydroxyapatite can be found in teeth and bones and is commonly used as a coating to promote bone ingrowth into prosthetic implants. It seems the body tends to resorb the coating so that over time, it weakens and then comes off the implant.
Other problems observed in these patients included the formation of bone within the surrounding soft tissues (a condition known as heterotopic ossification), bone cysts, residual pain, and loss of motion.
Three-fourths of the patients in this later follow-up complained of ongoing pain and ankle motion was limited to about 24 degrees (less than one-third of normal). But despite these low functional outcomes, patient satisfaction remained high (reported as “satisfied” or “very satisfied”). The authors did not speculate as to why this may occur. They noted that the same level of patient satisfaction has been reported in other studies where function was progressively worse over time.
Most of the patients needed the ankle replacement because of severe end-stage osteoarthritis from a previous traumatic injury. A few patients had primary osteoarthritis, rheumatoid arthritis, or in one case, hemophilia (bleeding disorder). The diagnosis did not seem to be linked with implant failure. Instead, younger age at the time of the surgery was the biggest risk factor. This is not surprising since younger people tend to be more active and survive longer with the implant in place.
One other risk factor for implant failure identified in other studies is malalignment. Implants that are not anatomically correct can cause increased contact pressures. The result is a wearing away of the polyethylene (plastic) insert and eventual fracture of the insert. There were no cases of this type of problem in the patients in this study.
The authors concluded that the long-term results of the Scandinavian Total Ankle Replacement (STAR) are not nearly as good as the short-term results. They suggest the single-coating of hydroxyapatite may be the cause and should be investigated further. For those patients who did not need revision surgery (i.e., they kept the original implant), results were reported as “generally good.”