Posterior tibial tendon (PTT) dysfunction has commonly been treated with local corticosteroid injections to reduce inflammation. However, a concern with this treatment is potential degeneration and spontaneous rupture of the PTT. This study set out to determine whether mechanical strain may counteract the potentially deleterious effect of corticosteroid treatment on fibroblasts and therefore improve outcomes during recovery from tendinitis. In this study, PTT fibroblasts in vitro were treated with 0 M, 10(-7) M, 10(-6) M, and 10(-5) M triamcinolone acetonide (TA) while incubated under cyclic strains of 0% or 5% for 24 hr. Type I collagen and decorin mRNA expressions were determined by RT-PCR. The results indicated that mechanical strain significantly increased type I collagen and decorin gene expression in the PTT fibroblasts and TA decreased type I collagen and decorin gene expression. Therefore, mechanical strain might be beneficial to PTT after corticosteroid treatment by direct stimulation of fibroblast synthesis.