Mechanical properties of collagen fascicles from in situ frozen and stress-shielded rabbit patellar tendons

Objective. To know the effects of stress shielding on the biomechanical pro perties of collagen fascicles obtained from in situ frozen patellar tendons (an autograft model). Design. Collagen fascicles of approximately 300 mu m in diameter tt ere obt ained from in situ frozen rabbit patellar tendons and also from in situ fro zen and stress-shielded ones, and their mechanical properties and fibroblas t density were determined. Background Stress shielding changes the mechanical properties of in situ fr ozen patellar tendons in which there exist no fibroblasts, The mechanisms o f this phenomenon have not been studied well. Method. Patellar tendon of both in situ frozen group and in situ frozen and stress-shielded group were frozen in situ by liquid nitrogen to kill fibro blasts. Then, in the in situ frozen and stress-shielded group, no tension w as applied to the tendons for 2, 3, and 6 weeks, while normal tension was a pplied to the tendons of the in situ frozen group. Tensile properties of th e collagen fascicles obtained from these tendons were determined using a mi crotensile tester, and were compared to the collagen fascicles from nonfroz en, stress-shielded patellar tendons. Results. Tangent modulus and tensile strength of collagen fascicles from th e in situ frozen and stress-shielded group progressively decreased with the time of stress shielding; however, these decreases were much smaller than those of the fascicles obtained from non-frozen, stress-shielded tendons. A lthough there were few fibroblasts in the patellar tendon of the in situ fr ozen and stress-shielded group at 2 weeks, the modulus and strength of the fascicles from the posterior portion were significantly lower than those in the in situ frozen group. In addition, the reduction of strength caused by stress shielding was much smaller in collagen fascicles than in bulk patel lar tendons. Conclusion. The mechanical properties of collagen fascicles in in situ froz en tendons (an autograft model) are affected by stress shielding even under acellular condition.