Quantitative ultrasound for the evaluation of intratendinous deformation in the pre-insertional Achilles tendon

Background

Achilles tendinopathy remains one of the most prevalent overuse injuries in elite athletes as well as recreational runners. The Achilles tendon is the thickest tendon of the human body, consisting of three main elements: the cells, the extracellular matrix and collagen. Despite this well-designed architecture, the tendon is highly susceptible to overuse. The typical histological finding of “tendinosis”, a non-inflammatory, degenerative histological aspect of chronic tendinopathy, has been well documented. Research regarding the pathophysiology and risk factors continues, with hypotheses of compression and changes in vascularization being investigated. Nonetheless, a mechanical aetiology appears to be more widely accepted. Although aetiology of tendinopathy has not been fully understood, therapeutic loading programs have emerged as being the treatment of choice for many clinicians, despite limitations in the success rates. Eccentric loading regimes have long time been considered the golden standard but also other programs have emerged, leading to a heterogeneous approach and outcome. There is a need for optimization of existing therapeutic programs.

As mentioned before, the Achilles tendon has a unique structure-function relationship with an important role in creating a biomechanical advantage during contraction and also providing energy storage whilst walking and running. It is, however, likely that these mechanical features make the Achilles tendon vulnerable to overuse. Strain, a description of mechanical deformation, referring to the relative differences in particle displacement within a material, is an often-used parameter to describe the elastic properties of tendon material. As opposed to Young's modulus, strain can be measured non-invasively and is thus the preferred means to characterize the biomechanical properties of tendon tissue. In vitro research has provided us with numerous data and well-known stress-strain curves. In- vivo research has also investigated strain in the Achilles tendon and its response to loading or unloading or its profile in the presence of tendinopathy. This was valuable in giving us insight in the global strain values to be expected in tendons and its mechanical behaviour.

Some studies have shown evidence for a non-homogeneous strain distribution in the Achilles tendon. In vitro and laboratory based studies have supported these findings. As mechanotherapy appears to work at the cellular level in a beneficial manner, knowledge on local mechanical deformation is crucial for optimizing therapy. Currently, only indirect approaches to provide qualitative information on mechanical properties of tendon exist, with elastography seeming most promising.

Mapping of the intratendinous local strain distribution has been done in the past using invasive techniques. Fortunately, advancements in the field of real-time ultrasound scanning have provided the possibility to perform in vivo, non-invasive measurements by manual tracking of a reference point within the field of view. This technique was further automated using speckle-tracking algorithms. At KU Leuven, prof. D’hooghe and his team have further investigated this technique in cardiology and optimized the registration process. Preliminary research in the field of musculoskeletal medicine on tendons has shown some promising results, although important limitations still remain.

General hypothesis and aim

The overall aim of this research proposal is to deliver an ultrasound-based tool to provide the clinician with knowledge on local intratendinous strain distribution in the Achilles tendon. In doing so, individuals at risk for the development of pathology could be identified and existing therapeutic loading programs further optimized. To reach this goal an existing ultrasound based speckle-tracking technique from the field of cardiology will be adapted to the musculoskeletal field. After determining the reliability of this technique, the validity will be defined through comparison with clinical data and current available indirect techniques. The final part will consist of clinically oriented research in prevention and treatment.