Achilles tendon loading and deformation during rehabilitation and use of ankle foot orthoses

Achilles tendon rupture is a common injury which most often affects otherwise healthy individuals and which causes long periods of rehabilitation with absence from work and sport activities and frequently sequele with reduced calf muscle performance. The aim of this thesis is to improve understanding of how injury affects the function of the Achilles tendon, and to functionally evaluate different designs of ankle foot orthoses used during rehabilitation after an Achilles tendon rupture. A non-invasive ultrasound based method for estimating tissue motion is also adapted and evaluated for use on the Achilles tendon during motion. In the first study of this thesis an adjustable ankle foot orthosis was investigated regarding its effect on force in the Achilles tendon and muscle activity in the lower leg during walking. Unexpectedly force in the Achilles tendon was found to be higher at certain ankle foot orthoses settings than for barefoot walking. These results raised the question if different designs of ankle foot orthoses affect the Achilles tendon in different ways. To pursue this question, a non-invasive method of studying Achilles tendon biomechanics was required. Advances in tissue displacement and strain estimation in echocardiography lead to the choice of ultrasound based speckle tracking. In the second study of this thesis a commercial ultrasound speckle tracking algorithm was evaluated for estimation of strain in tendon tissue in a series of in-vitro experiments. Results of this study showed high variability in measurement errors which lead to the decision to adapt and use an in-house developed speckle tracking algorithm instead. In the first part of study III, an in-house developed speckle tracking algorithm was adapted and evaluated for displacement estimation in tendons. Results showed a high correlation between estimated tendon displacement and reference values, but a constant underestimation of displacement. For the magnitude and velocity of displacement relevant during range of motion exercise and walking, low coefficients of variation were found. In the second part of study III the in-house developed speckle tracking algorithm was implemented on ultrasound images of uninjured and previously ruptured Achilles tendons during motion. Displacement in superficial and deep tendon layers was estimated and it was found that the non-uniform displacement pattern observed in uninjured tendons was disturbed following injury. Displacement in different parts of the tendons could be distinguished with statistical significance indicating that the speckle tracking algorithm was clinically applicable. In study IV, ultrasound speckle tracking, electromyography of the lower leg muscles and plantar pressure measurement were combined to investigate how the Achilles tendon is affected by the use of three different designs of ankle foot orthoses with varying degrees of dorsiflexion limitation. Results show that the degree of dorsiflexion limitation within an ankle foot orthosis seems to affect tendon displacement patterns and lower leg muscle activity to a greater extent than differences in ankle foot orthosis design. In conclusion, ultrasound based speckle tracking for estimation of tendon motion is feasible and can be used to investigate alterations in Achilles tendon deformation patterns following injury and during use of different designs of ankle foot orthoses.