Background High volume image guided injections (HVIGI) have recently been shown to significantly reduce pain and improve function in patients with resistant Achilles tendinopathy. However, the mechanism by which HVIGI treatment works has yet to be identified.

Methods Subjects will be recruited from the London Independent Hospital for either open surgery for Achilles tendinopathy or for intervention by HVIGI. Tissue from the Kager's fad pad will be removed during open surgery for Achilles tendinopathy and paratendinopathy. Mechanical loading of the Kager's sample will be performed using an MTS Biomex 100 at varying applied loads in order to plot the Young's Modulus. Concurrent to this, patients undergoing HVIGI treatment will be recruited to undergo imaging by MRI scan during high volume injection to measure fluid distribution during and immediately after injection. Geometries of the tendon and Kager's fat pad will be reconstructed from images using Mimics software, which constitutes the computational domain of numerical modelling. Poroelastic theory will be adopted to model both Achilles tendon and Kager's fat pad with their distinctive parameters. Mechanical responses of the whole system will be compared before and after the injection. The fluid pressurisation and resultant stress/strain distribution are crucial to understanding the working mechanisms for HVIGI.

Results Parametric studies of the injection treatment will be conducted numerically by adjusting different loading speeds and loading time intervals between injections and volumes. This is for the first time Kager's fat tissue has been mechanically characterised. In combination with poroelastic modeling, we will be able to obtain the detailed strain and stress information and injected fluid movement, which are important to understand the mechanism underlying the high volume image guided injection.

Discussion It is expected that results from this study will elucidate the unknown mechanism by which HVIGI works, particularly the effect on neovasculature associated with Achilles tendinopathy