Development of a foot and ankle musculoskeletal model: implications for achilles tendinopathy

Abstract

This thesis investigates the mechanics behind Achilles tendinopathies and their respective treatments using a musculoskeletal modelling approach. Specifically, the eccentric heel-drop exercise used to treat Achilles tendinosis and orthotic heel wedges used to treat Achilles tendonitis were investigated, as the mechanics which drive tendon healing are not currently understood, but are believed to work by directly altering the mechanical loading of the Achilles tendon. An inverse dynamics model of the lower limb including the hip, knee ankle and MTP joints was developed to include a musculoskeletal foot and ankle model. An existing muscle geometry dataset was used, but a new algorithm to account for soft tissue and bony constraints at the ankle to ensure physiological musculo-tendon paths around the foot and ankle was developed. Optical motion, forceplate and instrumented pressure insole data was used to derive independent 3D ground reaction vectors necessary for the data inputs for each of the two foot segments modelled. In addition to the moments of the hip and knee, foot and ankle muscle forces and ankle joint reaction forces were also estimated. A cohort of 19 healthy individuals performed the eccentric heel-drop exercise used to treat Achilles tendinosis and walked on a level and up and down an inclined (10°) surface barefoot and in running shoes with and without prefabricated orthotic heel wedges used to treat Achilles tendonitis. Clinical questions regarding changes in lower limb mechanics due to variants of the eccentric heel-drop exercise and orthotic heel wedges were considered as well as model sensitivity to foot models and sources of centre of pressure (CoP) data. CoP data source and number of foot segments modelled did not consistently change the model outputs, with greater or worse similarity between sources depending on the specific phase of stance considered. An example of this are the reduced knee and hip extension moments and increased ankle dorsiflexion moments at heel-strike, but consistent peak ankle joint reaction and Achilles tendon forces due to different CoP inputs. Across all walking conditions, heel wedges were found to have minimal impact on Achilles tendon force, but had a significant impact on knee moments and secondary plantarflexors such as Tibialis Posterior and the toe flexor muscles. The ability of heel wedges to reduce Achilles tendon load during walking was not supported by this thesis. Key observations regarding the eccentric heel-drop exercise were the reductions in peak Achilles tendon force achieved when performing the exercise in running shoes compared to barefoot and with a flexed compared to extended knee. Given the increased difficulty in performing the flexed knee exercise, this questioned the efficacy of the flexed knee version of the task and possible changes to the rehabilitation protocol, incorporating the effect of shoes on peak Achilles tendon force were suggested.