Cyclic mechanical stretching enhances secretion of Interleukin 6 in human tendon fibroblasts

Accelerated rehabilitation after tendon and ligament injuries is widely acc epted to avoid adverse effects of immobilization. However, progressive reha bilitation may also lead to an excessive inflammatory soft tissue response. To investigate the amount of loading necessary to accelerate the healing p rocess without causing damage to the healing tissue, we experimentally stre tched human tendon fibroblasts of healthy tendons 15 and 60 min with 1 Hz a nd an elongation of 5% and measured the secretion of interleukin 6 (IL-6), tumor necrosis factor a (TNF-alpha), transforming growth factor beta1 (TGF- beta (1)), platelet-derived growth factor (PDGF), and fibroblast growth fac tor basic (bFGF). Secretion of IL-6 was significantly induced by 15 min of cyclic biaxial mechanical stretching after 4 and 8 h observation time and b y 60 min stretching and 2 h observation time. The growth factors TGF-beta ( 1), bFGF, and PDGF were secreted by human tendon fibroblasts both in stretc hed cells and controls; however, no increases were related to mechanical st retching. There was no measurable secretion of TNF-alpha in human tendon fi broblasts. These findings suggest that the inflammatory reaction often seen during physiotherapy after tendon and ligament injuries is caused in part by secretion of IL-6 from the stretched human tendon fibroblasts. IL-6 may cause exaggerated proliferation of fibroblasts and synovial cells as seen i n rheumatoid arthritis and arthrofibrosis. However, physiological prolifera tive reactions leading to repair of injured tissue are also possible. IL-6 measured in the synovial fluid may be an important predictor for monitoring and improving therapeutic strategies in terms of tendon/ligament healing.