Ultrastructural analyzes on mechanism of effects of stress shielding on the mechanical properties of the in situ frozen patellar tendon.

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1. Effects of in situ freezing on the patellar tendonBiomechanically, freezing reduced the tensile strength of the patellar tendon at more than 3 weeks after treatment. However, reduction of the maximum failurre load was minimum, because freezing increased the cross-sectional area of the patellar tendon. Ultrastructurally, freezing did not change the ultrastructure of the patellar tendon at 3 weeks. At 6 weeks, however, freezing significantly increased the number of thin collagen fibrils having the diameter of less than 90nm and changed the histogram of collagen fibril diameter from bimode into unimode.2. Effects of stress shielding on the patellar tendon frozen in situBiomechanically, stress shielding drastically reduced the tensile strength of the in situ frozen patellar tendon within 1 week. In spite of increase of the cross-sectional area, the maximum failure load significantly decreased. Mechanical response of the patellar tendon to stress shielding occurred independent of living cells. In the patellar tendon frozen in situ, ultrastructurally, stress shielding reduced the total number of collagen fibrils and the ratio of the total area of collagen fibrils to the whole visualized area within 3 weeks, while it increased thick collagen fibrils having the diameter of more than 360 nm.3. Mechanism of the effects of stress shielding on the patellar tendon frozen in situStress shielding affects the matrix of the patellar tendon that include collagen, independent of living cells. Reduction of the total area of collagen fibrils in the cross section, reduction of the frictional force developed between collagen fibrils, and decrease of the tensile strength of collagenfibril itself are considered as causes of the reduction in the tensile strength of the patellar tendon.