Towards non-invasive measurements of ligament function: implementation and advancement of ultrasound elastography techniques

Collateral ligament insufficiency due to injury or an imbalance after knee replacement surgery can lead to knee instability or stiffness. The aim of this PhD project was to develop ultrasound techniques that can non-invasively assess the health and function of these tissues by measuring their biomechanical properties. The key to implementing shear wave imaging for measuring material properties was adequate consideration of how shear wave propagation through the collateral ligaments is influenced by the viscous and thin nature of the tissue. Ex vivo tests on tissue-mimicking phantoms and the medial collateral ligaments of porcine joints showed that a Voigt model could effectively capture the viscous effects, while a Lamb wave model accounted for the guided wave phenomenon resulting from the structure’s narrow boundaries. Analysing shear wave propagation with an incorrect model led to material properties that exhibited a dependency on the tissue’s thickness and did not accurately reflect those from mechanical testing. The ultrasound speckle tracking technique, which was used to measure the strain behaviour of the ligaments, also required careful tuning of the post-processing algorithm to account for the complex anatomy and elongation pattern of ligaments. It was found that selection of the algorithmic settings, which conventionally requires a time-consuming trial-and-error approach and validation against ground truth data, could instead be automated by using an optimisation approach that verifies displacement field convergence, then minimises variance between repeat measurements. This optimisation routine was insensitive to anatomical variation and loading conditions, producing tuned algorithms that accurately and reliably measured strains in the medial ligaments of both ex vivo porcine and human knees, and for quasistatic and dynamic loading. Additionally, the non-uniform strain behaviour of in vivo medial ligamentous structures was measured with good reliability, detecting elongation of the anterior bundles and shortening of posterior bundles during knee flexion.