Cytokine-induced tendinitis: A preliminary study in rabbits

This study was designed to determine the effects of a single injection of a species-specific preparation of cytokines into rabbit patellar tendons and to compare the results with a known model of tendinitis, the collagenase-i njection model. New Zealand White rabbits were divided into two groups and two time periods (4 and 16 weeks) and injected in the midsubstance of the r ight patellar tendon with either cytokines or collagenase under ultrasound guidance to confirm intratendinous needle placement. The left patellar tend on was injected with 0.025 mi of saline solution and served as a control. T he rabbits were returned to cage activity after injection. At death, two ra bbits in each group underwent histological analysis; the remaining eight an imals in each time frame were evaluated biomechanically and then biochemica lly with use of the patella/whole patellar tendon/tibia complex. Histologic results at 4 weeks in the tendons injected with cytokines demonstrated inc reased cellularity, which was resolving by 16 weeks. The matrix appeared un changed. The tendons injected with collagenase demonstrated increased angio genesis of the matrix, hypercellularity, and fibrosis around the tendon at 4 weeks. At 16 weeks, myxoid changes, focal fibrosis, and collagen-bundle d isarray with persistent increase in cellularity were noted. Biomechanically , a significant decrease in ultimate load at 16 weeks was seen in the tendo ns injected with cytokines but no change was seen in cross-sectional area. The tendons injected with collagenase demonstrated a significant increase i n cross-sectional area at 4 and 16 weeks compared with those injected with cytokines. Biochemically, there was no significant difference in collagen c ontent between the two groups at 4 or 16 weeks but the tendons injected wit h collagenase demonstrated a significant increase in crosslinking at 16 wee ks. Our conclusion is that the tendons injected with the cytokine preparati on represent a model of mild, seemingly reversible tendon injury. The cytok ine preparation produces no matrix damage or evidence of collagen degradati on and is species specific.