Strains in the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), accessory ligament of the deep digital flexor muscle (inferior check ligament [ICL]) and the interosseus medius muscle (suspensory ligament [SL]) in the right forelimb of 5 ponies were measured using mercury-in-silastic strain gauges a few hours after implantation. Tendon strains were recorded at the walk with normal flat shoes, egg-bar shoes, a 7° increased hoof angle accomplished by application of a heel-wedge and a 7° decreased hoof angle using a toe-wedge, consecutively. Ground reaction forces were recorded with all 4 shoe types preoperatively and with flat shoes post operatively.
The strain patterns of the SDFT, DDFT and SL showed a rapid increase at the beginning of the stance phase, followed by a plateau with a small incline or decline and a rapid decrease at the end of the stance phase. The SDFT had its maximal strain in the first half of the stance phase in all ponies. The DDFT and SL reached their maximal strain in the first half of the stance phase in 2 ponies and in the second half of the stance phase in the other 3 ponies. The ICL was strained maximally in the second half of the stance phase in all ponies.
Averaged over all 5 ponies, the maximal strains in the SDFT, DDFT, ICL and SL with normal flat shoes were 2.4, 1.3, 5.4 and 3.7%, respectively. If an egg-bar was applied the mean peak strain in the DDFT was 0.13% lower and strain in the SL was 0.22% higher. With a heel-wedge, strain decreased in the DDFT and ICL (0.19% and 0.4%, respectively) and increased by 0.24% in the SL. A toe-wedge increased strain in the ICL by 0.8%. All changes mentioned were statistically significant (P<0.1).
The changes in tendon strain as a result of different types of shoeing correlated with changes in calculated torque's of the ground reaction force acting on the coffin joint.