Motion is Maintained at the Tibial and Polyethylene Component Interface in a Mobile-Bearing Total Ankle Arthroplasty at Mid and Long-Term Follow-Up

CATEGORY: Ankle Arthritis; Ankle; Hindfoot INTRODUCTION/PURPOSE: Normal biomechanics of the ankle joint includes sagittal motion, but also anterior-posterior (AP) translation (1mm) as well as axial rotation (5-8 degrees). Sagittal motion is well described in total ankle arthroplasty (TAA), however, current understanding of mobile-bearing motion at the tibial-polyethylene interface in TAA is limited to AP motion and short-term data on axial rotation. The purpose of our study was to determine if polyethylene components maintain mobility adjacent to the tibial component in mobile bearing TAA at mid and long-term follow-up in both the sagittal and axial planes in patients more than 48 months post-operatively. METHODS: Patients who were a minimum of 48 months postoperative from a third-generation mobile-bearing TAAwere identified. Fluoroscopic images perpendicular to the tibial component were saved at maximum internal and external rotation on AP, and the lateral images were saved in maximum plantarflexion and dorsiflexion. Sagittal range of motion and AP translation of the polyethylene component were measured from the lateral images. Axial rotation was determined by measuring the relative position of the two wires within the polyethylene component on AP internal and external rotation imaging. This relationship was compared to a table previously developed from fluoroscopic images taken at standardized degrees of axial rotation of a nonimplanted polyethylene with the associated length relationship of the two imbedded wires. Total axial motion was determined by the difference between angle value determined for maximum internal and external rotation. RESULTS: Eight patients were included in this investigation, 5 (62.5%) were female and average age was 72.6 (range, 59-83) years. Time from surgery averaged 94.2 (range, 54.7-118.6) months. Total sagittal range of motion between the talar and the polyethylene component averaged 20.3 (range, 10-30) degrees. All patients (100%) maintained motion at the tibia-polyethylene interface. Axial motion for total internal and external rotation of the polyethylene component articulation on the tibial component averaged 5.3 (range, 2-12) degrees. AP translation of the polyethylene component relative to the tibial component averaged 1.1 (range, 0.4-2.6) mm during plantarflexion and dorsiflexion. There was no relationship between axial rotation or AP translation of the polyethylene component and ankle joint range of motion (P >.05). CONCLUSION: To our knowledge, this is the first investigation to measure axial and sagittal motion of the polyethylene component in mobile bearing TAA at the tibial implant interface in patients at mid and long-term follow-up. Based on our results, motion is maintained an average of 8 years post-operatively and concludes that a mobile-bearing TAA has the potential to fall within the parameters of normal polyaxial ankle motion several years after implantation. Such maintenance of motion may have a positive influence on gait pattern, polyethylene wear, joint reaction forces and implant longevity.