The Influence of Multiplanar Talar Displacement on Articular Ankle Loading: A 3D Finite Element Study

Abstract

Category: Trauma; Ankle Introduction/Purpose: Ankle fractures represent an increasingly common injury world-wide and the decision for operative fixation often hinges on the definition of acceptable stability. Mortise malalignment has generally been considered as the main predictive factor leading to poorer outcomes and the development of post-traumatic osteoarthritis. Following pioneering work by Ramsey & Hamilton, 1-2 mm of lateral talar displacement has commonly been regarded as a cutoff value for surgical decision making. However, isolated lateral talar translation (as studied in previous works) does not fully replicate the multidirectional joint subluxation seen in ankle fractures. Furthermore, previous studies have been limited by low fidelity methodologic techniques. Therefore, the aim of this study was to analyze the influence of multiplanar talar displacement on tibiotalar contact mechanics utilizing finite element analysis (FEA). Methods: 19 patients having undergone advanced computed tomography (CT) scanning of the ankle were included. A female patient (age 18 years, weight 60 kg) without preexisting articular ankle pathology underwent a weightbearing CT-arthrogram of the left ankle. Additionally, 18 ankles derived from the contralateral healthy side in patients who underwent bilateral weightbearing CT scans (WBCT ) for evaluation of a contralateral ankle fracture were included (mean age = 38.70 years, mean weight = 80.00 kg, left/right: 9/9). Segmentation of the WBCT images into 3D models of bone and cartilage was performed semi-automatically, while on the non-arthrography WBCT images, personalized cartilage layers were modeled based on a previously validated methodology. 3D multiplanar talar displacement was simulated to investigate their respective influence on the tibiotalar contact mechanics. Tibial peak contact stress, talar peak contact stress and contact area were extracted from the FEA models for each condition. Results: The talar peak contact stress (in MPa), tibial peak contact stress (in MPa) and contact area (in mm²) for each condition are presented in Table 1. Overall, both the mean and peak contact stress for the talus and tibia incrementally increased when the talus was displaced, for all directions. Correspondingly, the contact area incrementally decreased. Contact stress maps of the talus and tibia were computed for each of the conditions, demonstrating unique patterns of pressure derangement. 1 mm of lateral translation resulted in 11% increase in peak contact pressure and a 12% decrease in contact area. External rotation exhibited the greatest influence on contact stress, reaching a peak talar stress of 510% above baseline after 20 degrees of external talar rotation. Conclusion: In this study, we were able to build on the paradigm of prior uniplanar cadaveric studies and provide a multiplanar mortise malalignment model. The use of advanced computational technologies allowed for more precise quantitative evaluation of articular contact mechanics. With lateral talar translation, our model demonstrated less dramatic increases in tibiotalar contact area compared to previous studies, whereas external rotation had the largest effect regarding pathologic joint loading. Clinically, this study provides a novel understanding of the mechanical sequelae of 3D talar displacement and the importance for adequate mortise alignment.