Long term local cortisol treatment of tendons and the indirect effect on skin. An experimental study in rats.

The mechanical properties of muscle tendon after long term local cortisol treatment were studied as well as those of skin with regard to possible systemic effects. Rats received cortisol injections 10 mg/kg into each hind limb, around the peroneal tendons every third day for 55 days. A control group was injected in the same way, but with saline. The local cortisol treatment did not alter the mechanical properties of the peroneous longus tendon, even though its dry weight and hydroxyproline content was reduced. A systemic effect on skin from the dorsum was observed. The thickness and hydroxyproline content were reduced, and in spite of that, the strength of the skin specimens increased. The hydroxyproline/nitrogen ratio in purified, insoluble skin collagen was not changed after the cortisol treatment. Two different effects of corticosteroids on collagenous tissues are suggested to act here: (1) within the first one to two weeks corticosteroids induce a relatively fast increase in the stability of the collagenous tissue, (2) followed by a progressive thinning and reduction in collagen of the tissue, caused mainly by an inhibited collagen synthesis. Thus the strength of the muscle tendon is not reduced, even though its collagen content is reduced after local cortisol treatment for 55 days.