Does Total Ankle Arthroplasty Preserve Midfoot Function and Mitigate Excessive Adjacent Joint Loading?

Abstract

Category: Ankle Arthritis Introduction/Purpose: The progressive decline of ankle motion secondary to ankle arthritis increases midfoot joint loading during forward propulsive activities. Preservation or improvement of ankle motion following total ankle arthroplasty (TAA) is theorized to preserve adjacent joint function and potentially mitigate the cumulative effect of midfoot joint loading. If this theory were sound, the biomechanical profile of midfoot function would not change following TAA. Examination of this theory may inform TAA operative care and will directly impact postoperative rehabilitation paradigms. The purpose of this ongoing study was to be the first to prospectively evaluate the effect of TAA on midfoot function during walking, as measured by midfoot range of motion and power, at six month follow up. Methods: Nine adults [Mean (SD): Age 60.2 (3.5) years; BMI 30.4 (1.4) Kg/m2; 100% male] who received a TAA for end-stage ankle arthritis walked barefoot on level ground preoperatively and six months postoperatively. Three dimensional motion analysis was used to record three-segment foot motion (forefoot, rearfoot, tibia) and ground reaction forces. Midfoot (forefoot with respect to rearfoot) sagittal and transverse plane range of motion were measured. Inverse dynamic calculations were used to obtain midfoot negative (absorption) and positive (generation) peak power (joint torque x segmental velocity), since these metrics are biomechanical correlates of midfoot instability and stability, respectively. Accordingly, greater midfoot joint loading would be characterized by increased power absorption. Greater power generation would indicate an increased midfoot contribution to forward propulsion. Paired t-tests were used to evaluate differences across time in the involved limb and between limbs postoperatively for power variables. Results: Midfoot dorsiflexion / plantarflexion and abduction / adduction range of motion were not significantly different pre to postoperatively (mean differences < 0.4 degrees; both p > 0.7). Similarly, involved limb midfoot negative and positive power did not change (mean differences < 0.1 W/Kg; negative power p = 0.1; positive power p = 1.0). Postoperatively, negative power was minimally different between sides (mean difference 0.1 W/Kg; p = 0.4). However, involved limb midfoot positive power was 44% less than the uninvolved limb postoperatively (mean difference 0.3 W/Kg; p = 0.1) (Figure 1). Conclusion: Study findings provide preliminary substantiation to the theory that midfoot function is preserved following TAA. At six months, the biomechanical profile of individuals receiving TAA for end-stage ankle arthritis was not different from preoperative measures. Further, since postoperative power absorption was similar between limbs, the current TAA operative approach may help mitigate midfoot joint loading. However, the asymmetrical power generation (44% less on the involved limb) suggests the midfoot is unprepared to respond to anticipated ankle power improvements as patients advance to higher level activities. Accordingly, rehabilitation targeting midfoot stability through strengthening is recommended to complement current postoperative efforts.