Kinematic analysis of the agility total ankle during gait

BACKGROUND: With the increasing use of total ankle prostheses, kinematic analysis of these implants is important to our understanding of their specific biomechanics. Fluoroscopic analysis as used in this study has distinct advantages over previous experimental approaches and allows kinematic determination in vivo of dynamic weightbearing motions. METHODS: Ten patients with unilateral Agility (Depuy, A Johnson & Johnson Company, Warsaw, IN) total ankle replacements were tested using video fluoroscopy in weightbearing dynamic gait conditions. Their prosthetic ankle and normal ankle kinematics were then analyzed by computer with two-dimensional and three-dimensional model-fitting techniques. RESULTS: All of the total ankle prostheses in this study demonstrated less than 3.5 mm of posterior-to-anterior translation from heel strike to toe-off. In comparison, more variability was seen in posterior-to-anterior motion of the normal contralateral ankles, with three ankles translating more than 6 mm. When inversion and eversion and internal and external rotation of the ankle were measured, wide variations were seen among patients. CONCLUSIONS: While the relative incongruence theoretically decreases the shear stresses transmitted to the bone-prosthesis interface, it does allow more inversion-eversion and rotational freedom that can lead to edge loading and higher contact stresses. Despite good medium-term results having been published for this prosthesis, polyethylene wear and osteolysis have been observed. Further studies looking at the effect of the inversion and eversion and rotational freedom on polyethylene wear and prosthesis survival will assist in our understanding of the factors leading to successful outcome of total ankle prostheses.