Mechanical properties of rabbit tendons after repeated anti-inflammatory steroid injections

Abstract

Steroid drug therapy may be a contributing factor to the increased incidence of Achilles tendon rupture among sports participants. To study effects of repeated anti-inflammatory steroid injections, the right front paws of eight male California strain rabbits (4 drug-treated, 4 control) were injected twice per week for 5 weeks with either a suspension of betamethasone sodium phosphate and betamethasone acetate (0.1 mg/kg) or normal saline. Comparisons of selected mechanical properties of extensor digitorum communis tendons excised from right (local effect) and left (systemic effect) paws were made. Tendons from betamethasone-injected paws weighed less than control tendons on an absolute but not on a tendon weight to body weight basis. No significant differences in dynamic stiffness at either 1 or 10% per min strain rates or critical yield loads were found when control vs betamethasone or local vs systemic comparisons were made. Measures of critical yield stress were greater for tendons from betamethasone-injected paws. Although Vogel (10) has reported similar findings for rat skin, this difference in critical yield stress should he viewed with caution due to the limited number of tendons that could be tested without “premature” failure. The small drug dosages caused tendon weight reduction but no changes in mechanical properties indicative of tendons predisposed to subsequent rupture when loaded as described.