How subject-specific biomechanics influences tendon strains in Achilles tendinopathy patients: A finite element study

Abstract

The treatment of Achilles tendinopathy is challenging, as 40% of patients do not respond to the existing rehabilitation protocols. These rehabilitation protocols do not consider the individual differences in the Achilles tendon (AT) characteristics, which are crucial in creating the optimal strain environment that promotes healing. While previous research suggests an optimal strain for AT regeneration (6% tendon strains), it is still unclear if the current rehabilitation protocols meet this condition. Consequently, this study aimed to investigate the influence of a selection of rehabilitation exercises on strains in patients with Achilles tendinopathy using subject-specific finite element (FE) models of the free AT. Secondly, the study aimed to explain the influence of muscle forces and material properties on the AT strains. The 21 FE models of the AT included the following subject-specific features: geometry estimated from 3D freehand ultrasound images, Elastic modulus estimated from the experimental stress-strain curve, and muscle forces estimated using a combination of 3D motion capture and musculoskeletal modelling. These models were used to determine tendon strain magnitudes and distribution patterns in the mid-portion of the AT. The generalized ranking suggested a progression of exercises to gradually increase the strains in the mid-portion of the AT, starting from the concentric and eccentric exercises and going to more functional exercises, which impose a higher load on the AT: bilateral heel rise (0.031 ± 0.010), bilateral heel drop (0.034 ± 0.009), unilateral heel drop (0.066 ± 0.023), walking (0.069 ± 0.020), unilateral heel drop with flexed knee (0.078 ± 0.023), and bilateral hopping (0.115 ± 0.033). Unilateral heel drop and walking exercises were not significantly different and they both fell within the optimal strain range. However, when examining individual strains, it became evident that there was diversity in exercise rankings among participants, as well as exercises falling within the optimal strain range. Furthermore, the strains were influenced more by the subject-specific muscle forces compared to the material properties. Our study demonstrated the importance of tailored rehabilitation protocols that consider not only individual subject-specific morphological and material characteristics but especially subject-specific muscle forces. These findings make a significant contribution to shape future rehabilitation protocols with a foundation in biomechanics.