Loading protocol

Tendon specimens were subject to a cyclical loading programme, using a hydraulic biomechanical testing machine (ESH, Brierley Hill, UK) and the output sampled using an analogue to digital converter connected to a laboratory computer (Fig. 2). This programme has been chosen as it has been used in testing Achilles tendon repair models to represent the early stages of rehabilitation (Akizuki et al., 2001; Demetracopoulos et al., 2014; Lee et al., 2009): passive ankle range of motion (100 N) and walking in a controlled ankle motion walking boot with a 2.5 cm heel lift (190 N). The present study included 4 phases of cyclical loading:

“Pre-conditioning” with 10 cycles at 100 N at frequency 1 Hz.

An additional sequence of 90 cycles at 100 N.

A further 100 cycles at 190 N.

Finally the tendon was loaded until complete failure.

The biomechanical testing jig and experimental set up

After each phase of loading the separation of the tendon ends was measured using the vernier caliper (Ortiz et al., 2012). Clinical failure was considered to be an end-to-end separation of greater or equal to 20 mm, comparable to elongation noted in non-operative management (Heikkinen et al., 2017). If a phase of loading could not be completed the number of cycles that were completed before failure was noted.

Additionally the mode of failure of each repair model was noted. Cyclical loading of the tendon was digitally recorded using a camera to determine the specific characteristics and pattern of the loading and failure of the tendon repair model.