Achilles tendon moment arm changes with total ankle arthroplasty

Prior research on total ankle arthroplasty (TAA) has focused on improvements in pain and function following the surgical treatment of ankle arthritis, but its effect on ankle joint mechanics has received relatively little attention. The plantarflexion moment arm of the Achilles tendon is a critical determinant of ankle function with the potential to be altered by TAA. Here we investigate the effect of TAA on Achilles tendon moment arm assessed using two methods. Standing sagittal‐plane radiographs were obtained for ten patients presurgery and postsurgery, from which anterior–posterior distance between the posterior calcaneus and the center of the talar dome was measured. Ultrasound imaging and three‐dimensional (3D) motion capture were used to obtain moment arm pre‐ and post‐TAA. The absolute changes in moment arm pre‐ to post‐TAA were significantly different from zero for both methods (9.6 mm from ultrasound and 4.6% of the calcaneus length from radiographs). Only 46% of the variance in postoperative 3D Achilles tendon moment arm was explained by the preoperative value (r² = 0.460; p = .031), while pre‐ and post‐TAA values from radiographs were not correlated (r² = 0.192, p = .206). While we did not find significant mean differences in Achilles tendon moment arm between pre‐ and post‐TAA, we did find absolute changes in 3D moment arm that were significantly different from zero and these changes were partially explained by a change in location of the talar dome as indicated by measurements from radiographs (r² = 0.497, p = .023).