Sensitivity of an equine distal limb model to perturbations in tendon paths, origins and insertions

As equine musculoskeletal models become common, it is important to determine their sensitivity to the simplifications used. A subject-specific distal forelimb model was created using bones extracted from CT scans to examine movement from in-vivo invasive-marker motion capture. The movements of the sesamoid bones were simulated using the constraints of maintaining an isometric virtual ligament and maintaining contact between the appropriate articular surfaces, creating a variable moment arm for the tendons. The simulation of the proximal sesamoid bones was compared to movement recorded in-vitro. The paths and origins used for the deep digital flexor tendon (DDFT), superficial digital flexor tendon (SDFT) and suspensory ligament (SL) were altered and the effects on their calculated strains during trot stance were examined. The most influential alteration tested was the dorsopalmar changes to the tendon paths at the level of the proximal sesamoid bones, which led to a maximum length reduction of 4 and 2 mm for the SDFT and DDFT, respectively. Alterations to the virtual origins of the SDFT and DDFT were not influential leading to up to a 0.01% effect on strains for a 1 cm dorsopalmar shift. In the SL, the choice of the proximal or distal edge of the proximal attachment site varied the strains calculated by up to 1% (3 mm). These results show that within the anatomically realistic spectrum, changes to tendon paths can have an appreciable effect on calculated strains; however the origin sites chosen are not as influential as changes to paths at the metacarpo-phalangeal joint.