The influence of a local injection of cortisol on the mechanical properties of tendons and ligaments and the indirect effect on skin.

The effects of local cortisol treatment on the mechanical properties of muscle tendon and ligament in rats were studied. In addition to the local effects, possible systemic effects as reflected in the properties of the skin, remote from the site of the injection, were investigated. Two groups of rats were given cortisol injections, 20 mg/kg, every third day for 24 days. In one of the groups the injections were given bilaterally around the peroneal tendons and in the other they were given into both knee joint cavities. Specimens from the peroneal tendons, the posterior cruciate ligament with bone attachments, and strips of skin from the dorsum, were tested in a materials testing machine. The local treatment resulted in increased "tensile strength" of the peroneus brevis tendon and increased maximum load and stiffness of the peroneus longus tendon with no change in the collagen content in any of the samples. The posterior cruciate ligament always broke at one of the two sites of attachment to the bone and the maximum load value at these points was decreased after local cortisol treatment. The systemic effects of this local cortisol treatment on the skin were decreased thickness and fat content, increased collagen concentration and higher "tensile strength" and failure energy. From this study it is concluded that local cortisol injections for 24 days increase the strength and stiffness of muscle tendons, but decrease the strength of the bone attachments of ligaments. This treatment also results in a systemic effect which shows itself as increased strength of the skin remote from the site of the injections.