Axial Rotational Alignment of Mobile-Bearing Total Ankle Arthroplasty

Abstract

Category: Ankle Arthritis Introduction/Purpose: In total ankle arthroplasty (TAA), component malpositioning is a major cause of implant failure, possibly due to the altered force patterns caused by the malpositioning which are then transmitted to the bone-implant interface or neighboring joints during physiological loading. Mobile-bearing TAA with their second interface, may allow the talus to adapt its position based upon the individual anatomy. However, no data exist on the change in component positioning after implantation. It is unclear whether it is the result of initial positioning during implantation or secondary adjustments such as possible soft tissue adaptions. We aimed to determine the relative axial rotation between the talar and tibial components at the end of surgery and after a minimum of 3 years follow-up. Methods: The relative rotation between the tibial and talar components was measured in two groups. First, intraoperatively before wound closure, in a consecutive series of 58 patients (60 ankles; age 61.8 [31 to 86] years; females 25, males, 35) who underwent TAA between February and November 2018. A K-wire inserted along the medial border of the tibial component and a rectangular marker positioned at the anterior surface of PE insert were used to determine the angle of rotation. Second, in 48 patients (48 ankles; age 60.2 [31 - 82] years; females, 24; males, 24) out of 1411 patients who underwent TAA between January 2003 and December 2015, and in whom a weight-bearing CT scan was taken for evaluation at 6.3 (range, 3.0 -16.3) years. The medial border of the tibial component and a perpendicular line to the anterior surface of the PE insert were used to determine the angle of rotation. Results: The angle of rotation, thus the relative position of the talar component compared to the tibial component, did not differ between the two groups (p = 0.2). While the talus was internally rotated by 1.7 (range, -14.0 - 14.0) degrees at the end of surgery, it was internally rotated by 1.5 (range, -13.0 – 19.5) degrees after a minimum follow-up of 3 years (Figure 1). Conclusion: Although there was no significant difference in average axial position measured intraoperatively compared to a 3- years follow-up, there was a wide range of rotational measurements. The possibility of the talar component to find its position as given by individual anatomy may be crucial in TAA to avoid non-physiological joint loads and shear forces which may otherwise result in increased PE wear. Due to the wide range of measurements, our data suggests that axial talar rotation cannot be predicted preoperatively or intraoperatively by surgical techniques that reference the transtibial axis, tibial tuberosity and transmalleolar axis as guidance for tibial component positioning.