Effects of recombinant equine growth hormone on in vitro biomechanical properties of the superficial digital flexor tendon of Standardbred yearlings in training

Objective-To determine whether recombinant equine growth hormone (rEGH) would alter the in vitro biomechanical properties of the forelimb superficial digital flexor tendon (SDFT) in exercising young Standardbred horses. Study Design-Randomized complete block design. Animals-Twelve Standardbred yearlings. Methods-Horses were trained for 12 weeks on a high-speed treadmill (10% positive incline). rEGH was administered intramuscularly (IM) daily (1 mu g/kg during week 4; 20 mu g/kg for weeks 5-9) to 6 horses (treated group), whereas 6 horses (control group) were administered an equivalent daily volume of sterile water IM. At 12 weeks, horses were euthanatized and left forelimb SDFTs were collected and stored (-70 degrees C). A section from the mid-region of the SDFT was held in cryoclamps with a 4cm interspace distance and distracted at 10 mm/s until failure. The variables evaluated were maximal load at yield and failure, ultimate and yield tensile stress and strain, tendon stiffness, and mode of failure. Data were analyzed using unpaired, two-tailed, Student's t-test. Statistical significance was set at P <.05. Results-Yield and ultimate tensile stress were significantly lower in the rEGH-treated horses compared with controls. There was a trend toward increased maximal displacement, increased ultimate tensile strain, and decreased tendon stiffness in rEGH-treated horses compared with controls. Tensile stress and cross-sectional area, and tensile stress and stiffness were significantly correlated at yield and failure points. Conclusions-rEGH, administered at the manufacturer's recommended dose rates to maturing Standardbred horses in training, does not significantly augment the in vitro biomechanical properties of the forelimb SDFT. Clinical Relevance-Administration of rEGH to young horses in training is unlikely to enhance the physiologic adaptation of the SDFT to exercise stress. (c) Copyright 2005 by The American College of Veterinary Surgeons.