All of the major joints can be replaced now: the shoulder, hip, knee, and ankle. Surgeons have the most practice with hips and knees. Ankles remain more difficult because of the complexity of the joint itself. Patients may get good pain relief with ankle joint replacement but they don’t always get better ankle motion.
The surgeons who conducted this study may have found the reason why improvement in ankle motion is small after joint replacement. They studied something called the anteroposterior offset ratio. They found that when this ratio is zero, patients had much better outcomes. They measured pain, ankle motion, and ankle function to compare results.
What is the anteroposterior offset ratio? It is a measure of the relationship between the two key components of the ankle joint implant. There is the flat piece that goes at the base of the tibia (shin bone) called the tibial plafond and the replacement for the talus (the bone at the top of the ankle that moves against the tibia).
When the center of the talus lines up with a vertical line drawn down the side of the tibia, the anteroposterior offset ratio is zero. That is a perfect line up of the two component parts. If these two points don’t line up, then there can be a positive ratio (measure greater than zero) or negative ratio (measure less than zero).
Putting the talus in just the right position to obtain a zero anteroposterior offset ratio is a challenge even for the most seasoned surgeon. The surgeon must contend with changes in the joint from degenerative arthritis as well as any other positional or alignment deformities. Skillful use of fluoroscopy (real-time X-rays) is required for this procedure.
The axis of rotation depends on perfect talar alignment but even in normal ankle joints, this axis changes during motion. Duplicating normal anatomic alignment and function is a complex and demanding step in the ankle joint replacement procedure.
But as these Swiss surgeons showed with 317 patients, this ratio represents component position. And any malposition of the talus has a direct effect on outcomes. Positive or negative ratios result in increased ankle pain, reduced ankle motion, and worse function compared with a zero ratio.
The surgeons who performed these procedures and reported on them in this study intend to follow these 317 patients over time. They hope to evaluate the effect of the anteroposterior offset ratio on long-term outcomes. There may be other reasons why patients’ results aren’t satisfactory. These factors will be explored and investigated as well.
And finally, with only 40 per cent of the group having a zero anteroposterior offset ratio, the authors hope to find ways to improve these results. As their study showed, small malalignments can cause stress on the soft tissues around the ankle and the other bones of the ankle and foot. Even minor increases of strain on the ligaments can cause pain and loss of motion. Improving the implant alignment and mimicking normal biomechanics as much as possible is likely to yield the kind of results patients are looking for.