Histological and biomechanical studies of inter-strand healing in four-strand autograft anterior cruciate ligament reconstruction in a rabbit model.

In order to observe the remodeling process of the inter-strand of the four-strand graft used for anterior cruciate ligament (ACL) reconstruction, and to find the histological and biomechanical influences on this process by braiding the four-strand graft, we conducted this study in a rabbit model. The rabbit's ACLs were reconstructed with bilateral four-strand semitendinous tendons which are in parallel or braided shape. The specimens of the two groups were collected at 3, 6, 12, 26 and 52 weeks after the operation. Gross and histological observations were done and the biomechanical properties of the specimens of the 26 and 52 weeks were compared. The result showed that in regular group, at 3 weeks, the strands which were in necrosis status were still separated with each other. At 6 weeks, the grafts were enveloped with hyperplastic synovium, some outer part of the gaps between strands were filled with synovium and fused together, while the inner parts were still separated. At 12 weeks, the graft was still in remodeling progress, but the remodeling degrees of each strand were different. Some strands had fused but some were still separated with "big" interval. At 26 and 52 weeks, some inter-strand gaps were hard to discriminate, some strands were connected with connective tissue and some separated ones still showed up. The braided ones manifested a similar but more synchronized remodeling process and showed a higher inter-strands fusing rate. The biomechanical test showed that the strength of the reconstructed ACL of the regular unbraided tendons was 36% of the contralateral normal ACL, while the braided ones reached 67%. The graft strengths of the specimen of the 26 and 52 weeks in the braided graft group were significantly higher than that of the unbraided group, while the stiffness of the 52-week specimen of the braided group was significantly higher than that of the unbraided group also. From this study, we concluded that as a graft for reconstruction of the ACL, the four-strand hamstring tendon needs to pass through the necrosis, revascularization, and ligamentation progress, but different strands were not in a synchronous process. The inter-strand gap may be completely fused, partial fused, fused but connected with connective tissue or still separated. By braiding the strands, the fusing percentage of the graft could be elevated and the biomechanical properties could be improved.