The effects of intratendinous and retrocalcaneal intrabursal injections of corticosteroid on the biomechanical properties of rabbit Achilles tendons.

BACKGROUND: The use of corticosteroid injections in the treatment of retrocalcaneal bursitis is controversial. We assessed the effects of corticosteroid injections, both within the tendon substance and into the retrocalcaneal bursa, on the biomechanical properties of rabbit Achilles tendons. The systemic effects of bilateral corticosteroid injections were also studied.

METHODS: The rabbits were divided into three treatment groups. The rabbits in Group I received injections of corticosteroid into the Achilles tendon on the left side and injections of normal saline solution into the Achilles tendon on the right, those in Group II received injections of corticosteroid into the retrocalcaneal bursa on the left side and injections of saline solution into the Achilles tendon on the right, and those in Group III received injections of corticosteroid into the Achilles tendon on the left side and injections of corticosteroid into the retrocalcaneal bursa on the right. These injections were given weekly for three weeks. At four weeks after the final injection, the tendons were harvested and were tested biomechanically to determine failure load, midsubstance strain and total strain, modulus of elasticity, failure stress, and total energy absorbed. The site of failure was also documented. The groups were compared according to the location of the injections, the type of injection (steroid or saline solution), and the total systemic load of steroid.

RESULTS: Specimens from limbs that had received intratendinous injections of corticosteroid showed significantly decreased failure stress compared with those from limbs that had received intratendinous injections of saline solution (p = 0.008). Specimens from limbs that had received intrabursal injections of corticosteroid demonstrated significantly decreased failure stress (p = 0.05), significantly decreased total energy absorbed (p = 0.017), and significantly increased total strain (p = 0.049) compared with specimens from limbs that had received intratendinous injections of saline solution. Specimens from limbs that had received intratendinous injections of corticosteroid were biomechanically equivalent to specimens from limbs that had received intrabursal injections of corticosteroid. Specimens from rabbits that had received bilateral injections of corticosteroid demonstrated significantly decreased failure load (p = 0.011), modulus of elasticity (p = 0.015), failure stress (p = 0.03), and total energy absorbed (p = 0.015) compared with those from rabbits that had received unilateral injections of steroid.

CONCLUSIONS: Local injections of corticosteroid, both within the tendon substance and into the retrocalcaneal bursa, adversely affected the biomechanical properties of rabbit Achilles tendons. Additionally, tendons from rabbits that had received bilateral injections of corticosteroid demonstrated an additive adverse effect, with significantly worse biomechanical properties compared with tendons from rabbits that had received unilateral injections of corticosteroid.