To test the hypothesis that stress deprivation induces over-expression of cytokines in the patellar tendon, 40 rats were divided into the following two groups. In the stress-shielded group, we slackened the patellar tendon in the right knee by drawing the patella toward the tibial tubercle with flexible wires. In the control group, we performed a sham operation on the right knee. Animals were killed at 2 or 6 weeks for immunohistological evaluation and biomechanical examination. For IL-1β, TNF-α and TGF-β, the ratio of positively stained specimens to total specimens was significantly higher in the stress-shielded tendons than in the control tendons. The elastic modulus of the stress-shielded tendon was significantly lower than that of the control tendon, while the cross-sectional area of the stress-shielded tendon was significantly greater than that of the control tendon. Therefore, the present study indicated that stress shielding induced the over-expression of IL-1β, TNF-α and TGF-β in patellar tendon fibroblasts with mechanical deterioration of the tendon. Regarding clinical relevance, the present study suggests a possible application of an anti-IL-1β or anti-TNF-α strategy for reducing the mechanical deterioration of tendons and ligaments in response to stress deprivation, although this study did not directly show that over-expression of IL-1β or TNF-α in response to stress deprivation was the causation of mechanical deterioration of tendons.
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