The effects of prolonged peritendinous administration of PGE₁ to the rat Achilles tendon: a possible animal model of chronic Achilles tendinopathy

Abstract.

We studied the effects of peritendinous Achilles tendon injections of prostaglandin E₁ (PGE₁) on the Achilles tendon of rats. Five groups of Sprague-Dawley rats (n = 24 each) were studied. Groups 1 to 4 received weekly peritendinous injections. In group 1, one side was injected with 800 ng of PGE₁ in 0.5 ml of 0.9% NaCl and the contralateral side was injected with 0.5 ml of 0.9% NaCl. In group 2, one side was injected with 800 ng of PGE₁. In group 3, one side was injected with 0.5 ml of 0.9% NaCl. In group 4, a syringe needle was inserted in the peritenon unilaterally, but no substances were administered. In groups 2, 3, and 4, the contralateral tendon was used as the control. In group 5, treatment was not administered. Eight rats in each group were killed at each time point, after 7, 21, and 35 days of treatment. On day 7, values for average water content and average wet weight of the tendons treated with PGE₁ were significantly higher than those in the control tendons (analysis of variance [ANOVA]; P = 0.02), with a histological picture of acute inflammation. On day 21, approximately half of the PGE₁-treated tendons showed fibrosis of the paratenon, with adhesions and intra-tendinous degeneration, with the other half still showing a picture of acute inflammation. On day 35, all of the PGE₁-treated tendons showed fibrosis of the paratenon, with adhesions and intra-tendinous degeneration. At all time points, there was no evidence of pathology in the tendons that had not received PGE₁. Sham peritendinous injections and injections of normal saline did not produce inflammation in the Achilles tendons. Initially, local administration of PGE₁ produced acute inflammation of the tendon and its surrounding tissues. Prolonged PGE₁ administration produced peri- and intra-tendinous degeneration, providing a cheap, reproducible model of Achilles tendinopathy, which would allow studies of the effects of conservative and surgical management of the condition.