Evaluation of Mechanical Stress Distribution Across the Calcaneus following Calcaneal Autograft: A Finite Element Analysis

Abstract

Category: Other Introduction/Purpose: The purpose of this study was to utilize finite element analysis to evaluate progressive bone removal from the calcaneus at various angles and its effect on the mechanical stress distribution at the calcaneus, and potential increase in fracture risk. Methods: A calcaneus CT scan was imported into Autodesk Fusion360 (Autodesk, v.2.0.14569). After rendering the initial mesh body using tetrahedral elements, a copy of the calcaneus was made. A simulated force equal to the average weight of person (876N) was applied to the middle and posterior talus facets while the calcaneus was oriented in the heel strike position during gait, with constraints applied around the heel. The initial harvest site was modelled as a 6mm diameter 30mm deep cylinder. From this center point, the diameter of the harvest site was increased to 8, and 10mm, with the depth being adjusted such that there was no risk of breaching the cortex on the opposite side. For each iteration, the safety factor, stress (MPa), and strain were recorded. This was repeated at an angle of 90°, 45° and a combination of 45° and 90°. Results: The 90° model demonstrated that as the diameter of the harvest site increased, there was a reciprocal increase in stress across the calcaneus. The number of cycles to failure (fracture) for all diameters was 10^6-10^7. The 45° model demonstrated that as the diameter of the harvest site increased, there was a reciprocal increase in stress applied across the calcaneus with a corresponding decrease in stress at the harvest site. The number of cycles to failure increased from 10^3 at 6mm to 10^4-10^5 at 10mm. The combination of the 45°-90° model demonstrated that as the diameter of the harvest site increased, there was a reciprocal increase in stress applied across the calcaneus. The cycles to failures decreased as the diameter of the harvest site increased. Conclusion: This finite element analysis demonstrated that an increase in the diameter of the harvest site led to an increase in stress along the calcaneus. There was variation in the correlation between size of the diameter and the number of cycles to failure depending on the angle of the harvest. The greatest amount of stress applied at the calcaneus was with the 45°-90° model. It was determined that harvesting at an angle of 45° at 6mm produced the lowest number of cycles to failure.