The influence of mechanical stress on graft healing in a bone tunnel

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      Purpose: To examine the relationship between mechanical stress and tendon-bone healing. Type of Study: Histologic animal study. Methods: Forty-four female Japanese White rabbits underwent transplantation of the extensor digitorum longus tendon into a tibial bone tunnel created perpendicular to the long axis of the bone. After surgery, the animals were returned to their cages and were free to move about without any restriction or immobilization of their extremities. The morphologic differences in tendon-bone junctions in terms of location and time were evaluated at 4, 6, 8, and 12 weeks, and at 6 months. Results: At 4 and 6 weeks, abundant collagen-fiber continuity between the graft and the bone was observed at the upper side of the bone tunnel. These fibers resembled Sharpey fibers. The lower side of the tunnel showed a layer of chondroid cells and newly formed woven bone. At 8 and 12 weeks, the collagen-fiber continuity of the upper side of the tunnel had become attenuated, but it was more organized at the entrance. At the lower side, the woven bone layer was still present at the entrance but had disappeared inside the tunnel. At 6 months, regenerated tendon-bone junction was observed only at the entrance; a direct type of insertion was observed only at the upper side. The interface tissue inside the tunnel had disappeared on both sides. Conclusions: Stress distribution in the bone tunnel was thought to be compressive on the lower side and tensile on the upper side at the entrance, and shear force is dominant inside the tunnel. A comparison of histology and stress distribution suggests that tensile stress enhances the healing process of tendon-bone junctions, compressive stress promotes chondroid formation, and shear load has little or no effect on regeneration of the tendon-bone junction.
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