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).

Achilles tendon moment arms are similar when computed using a single fixed axis versus a moving instantaneous helical axis.

Accurate location of the axis of ankle rotation is critical to in vivo estimates of Achilles tendon moment arm. Here we investigated how the plantarflexion moment arm of the Achilles tendon is affected by using an instantaneous helical axis that moves with ankle motion as opposed to a single fixed joint axis that approximates the average axis of rotation. Twenty young healthy adults performed a series of weightbearing cyclical plantar- and dorsi-flexion motions. Motion analysis tracked the motions of markers placed on the foot and shank and also tracked an ultrasound probe imaging the Achilles tendon. Differences in ATma between the methods were investigated using a two-way repeated-measures ANOVA with factors of joint angle (+5°, 0°, -5°, -10°, -15°) and method (instantaneous helical axes, fixed axis). Moment arms computed between the two methods were moderately to strongly correlated, especially in the mid-range of motion (for 0° to 10° plantarflexion, all r2 > 0.619 and all p < 0.004). The two methods produced Achilles tendon moment arms that were comparable and not significantly different except in the most dorsiflexed position, when they differed on average by 9.35 ± 3.23 mm (p = 0.001). Our results suggest that either approach for locating the axis of ankle rotation would be appropriate for the purpose of estimating ATma, but that a fixed axis may be preferable because it is applicable over a greater range of ankle motion.

Exploration of the validity of the two-dimensional sagittal plane assumption in modeling the standing long jump.

Most previous standing long jump studies have been based on the assumption of two-dimensional sagittal plane motion with bilateral symmetry. The purpose of this study was to investigate the validity of this assumption. Standing long jump trials were collected using six adult male participants. Each participant stood with a foot on each of two force plates and performed eight standing long jumps for maximal distance. Inverse dynamics analyses were performed for two-dimensional (2D) and three-dimensional (3D) models, and joint moments, powers, and work values were compared. The differences between these models with respect to the validity of the common planar jumping assumption were analyzed. Good agreement was observed between 2D and 3D methods for the lower body, with minimal differences in sagittal plane moments, power, and work for the ankle, knee, and lower back. There were significant, but relatively small differences in the sagittal plane kinematics and kinetics at the hip. For the upper body, the results contradicted the sagittal plane assumption in that significant moments and power were generated about the abduction/adduction axis of the shoulder and a similar amount of work was performed about both abduction/adduction and flexion/extension axes of the shoulder. The elbow also showed significant differences in power and work. These results indicate that an assumption of planar motion should be sufficient for many studies of the standing long jump that only examine lower body movement. However, for studies that include upper body motion, diagnosing injury risk, or investigating gender differences, a 3D model may be more appropriate.