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

1.原位冷冻对髌腱的影响从生物力学角度看，冷冻使治疗后3周以上的髌腱的抗拉强度降低。然而，最大失效载荷的减少是最小的，因为冷冻增加了髌腱的横截面积。超微结构观察表明，冷冻3周后，膝盖腱的超微结构无明显变化。冷冻后6周，胶原纤维直径明显增加，直径小于90 nm的细小胶原纤维数量明显增多，胶原纤维直径直方图由双峰变为单峰。应力屏蔽对原位冻结的髌腱的影响从生物力学角度看，应力屏蔽在1周内显著降低了原位冻结的髌腱的抗拉强度。尽管截面面积增大，但最大破坏载荷显著降低。髌腱对应力屏蔽的机械反应独立于活细胞发生。在原位冷冻的髌腱中，超微结构上，应力屏蔽使3周内的胶原纤维总数和胶原纤维总面积与整个可见面积的比值减少，而使直径大于360 nm的粗胶原纤维增多。应力屏蔽对冻结状态下的髌腱的作用机制应力屏蔽影响包括胶原在内的髌腱基质，这种作用不依赖活细胞。横截面上胶原纤维总面积的减少、胶原纤维间摩擦力的减少以及胶原纤维本身抗拉强度的降低被认为是导致髌腱抗拉强度降低的原因。

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.