Biomechanics of the Achilles tendon and midportion Achilles tendinopathy

Abstract

The Achilles tendon is the largest tendon in the human body and correspondingly it transmits large forces during everyday activities. Such frequent loading makes it prone to injury. Achilles tendinopathy is the most common injury of the Achilles tendon with two-thirds of cases being midportion Achilles tendinopathy. Midportion Achilles tendinopathy is a debilitating condition for both athletes and people with a sedentary lifestyle. People affected with midportion Achilles tendinopathy often significantly reduce their activity level, in some cases even quitting physical exercise completely. This has a negative effect on their overall health. The exact mechanism of midportion Achilles tendinopathy is not clear. Two mechanical hypotheses have been proposed: differential loading within the Achilles tendon and interaction of the plantaris tendon. The first objective of this thesis was to clarify the role of these two hypotheses. The potential for differential loading within the Achilles tendon was analysed using musculoskeletal modelling, while the interaction of the plantaris tendon was investigated by categorizing the plantaris tendon geometry using MRI and ultrasound. The differential loading within the Achilles tendon was modelled by including the Achilles tendon fibre bundle rotation in the musculoskeletal modelling. It was observed that the medial gastrocnemius force increased as the degree of rotation of the fibre bundle increased. As the medial gastrocnemius fibre accounts for the lateral aspect of the Achilles tendon, such increased load could also increase the risk of midportion Achilles tendinopathy. The process of predicting the muscle forces, using constrained optimization, was further analysed and a novel presentation of ‘Lagrange Vector’ was proposed to facilitate the understanding of the model prediction process. The plantaris tendon geometry was also investigated by analysing MRI scans in a case-control study. The plantaris tendon geometry was associated with plantaris-involved midportion Achilles tendinopathy. The odds ratio of the plantaris directly inserting into the Achilles tendon was significantly higher in the symptomatic group. Rehabilitation is one of the first lines of treatment for midportion Achilles tendinopathy. Such rehabilitation induces tendon healing by loading the tendon in a controlled manner. However, the relationship between external loading and Achilles tendon force during different rehabilitation activities is not clear. Thus, the second objective was to analyse the Achilles tendon force during rehabilitation. It was observed that the force exerted on the Achilles tendon was not associated with the direction (eccentric or concentric) but strongly correlated with the maximum dorsiflexion angle reached during the exercises. As the ankle dorsiflexed, the Achilles tendon force increased. These findings could be used to estimate the tendon loading during rehabilitation and modify the current rehabilitation to improve clinical results. This thesis investigates the Achilles tendon and midportion Achilles tendinopathy. For the two mechanical hypotheses, the role of plantaris tendon in midportion Achilles tendinopathy was further clarified, but the intra-tendon loading simulated cannot be validated as the tendon force measurement in vivo was infeasible. For the rehabilitation, the role of dorsiflexion angle during the rehabilitation was identified and a regression equation that clarifies the role of external loading and dorsiflexion angle was proposed. Future work including a prospective study comparing the plantaris tendon geometry for patients diagnosed with midportion Achilles tendinopathy and a prospective study comparing the effectiveness of the modified Achilles tendon rehabilitation regimen is now required.