Dependence of Total Ankle Tibial Baseplate Stability Upon Device Fixation Features

Abstract

Category: Ankle Arthritis Introduction/Purpose: Patient outcomes and construct longevity after total ankle replacement (TAR) are both linked to initial implant stability. Limited early micromotion between the implant and bone encourages stability. Tibial component design fixation features play a critical role in determining the initial stability, with retention of medial and lateral bone sidewalls and interference press-fit used to supplement fixation. The goal of this study was to characterize micromotion between the tibia and baseplate using finite element analysis (FEA) to determine how baseplate design features influence stability. Methods: Three commercially available TAR tibial component baseplate designs were virtually inserted into a computer model of the distal tibia of an end-stage ankle arthritis patient (Fig. 1a). Implant insertion was simulated using FEA for each tibial component with two different fixation cases: (1) no sidewalls+line-to-line fit and (2) sidewalls retained+50µm interference press-fit. Tibia models were derived from patient CT scans, with element-by-element material properties assigned based upon CT Hounsfield units. The tibial components were assigned titanium alloy properties. Following simulated implantation, FEA was performed using kinematic and kinetic profiles representing the stance phase of gait. Loads/moments scaled to patient body weight were applied to the implant (Fig. 1b, 1c), while the proximal tibia was held fixed. Micromotions were defined as the displacement difference between bone-implant closest nodal pairs, computed from FEA output using custom-written code. Results: Amongst tibial component designs inserted without sidewalls and with line-to-line fit, relatively large micromotions were observed. Micromotion differences at heel strike between Peg Designs 1 and 2 may be attributable to sidewall height, while Peg Design 3 was less susceptible to the proximal forces and dorsiflexion moments dominant in early stance (Fig. 1d), potentially because of its angled anterior pegs. Micromotions in Peg Design 3 were largest near toe off, where the plantarflexion moment and decreased proximal force may have permitted motion along the pegs. Differences in micromotion between the different implant designs over the stance phase of gait were greatly reduced when interference press-fit was modeled (Fig. 1e). Conclusion: This study provides new insight on the relative initial stability afforded by different TAR fixation features across designs. In the absence of interference press-fit, all designs showed micromotions large enough to be concerning. However, when interference press-fit was modeled, all designs performed similarly in terms of initial stability during simulated gait. While a better understanding of the relationships between TAR implant features, patient bone density profiles, and initial stability is still needed, our findings suggest that implant interference press-fit plays an important and dominant role in device performance.