I had an ankle replacement eight months ago and still have as much pain as I did before the surgery. I’ve done everything the surgeon and physical therapist have told me. I have a strong and stable ankle joint but it hurts like a son of a gun. No one seems to know why. What can you folks tell me (if anything)?

A study from Switzerlan may have the answers you are looking for. Surgeons at the Clinic of Orthopaedic Surgery in Liestal, Switzerland studied the results of 317 ankle joint replacements. They measured one factor in particular and looked at results in relation to this one variable.

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.

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.

Only 40 per cent of their group had a zero anteroposterior offset ratio. As their results 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 alignment and mimicking normal biomechanics as much as possible is likely to yield the kind of results patients are looking for.

This ratio may be something your surgeon can take a look at and see if it is part of the problem. There may be other (unknown) factors as well. More study is needed in this area to help the many patients like you who do not get the pain relief and improved motion they expected.