A more flattened bone tunnel has a positive effect on tendon–bone healing in the early period after ACL reconstruction

Purpose The purpose of this study was to evaluate whether a flattened bone tunnel has a positive effect on the tendon–bone healing (TBH) process in the early period after anterior cruciate ligament (ACL) reconstruction. Methods Seventy-two New Zealand White rabbits were randomly allocated into two groups, the flattened tunnel (FT) group and the conventional round tunnel (RT) group. We compared the cross-sectional areas and diameters of the bone tunnels between the two groups through computed tomography (CT) scanning. TBH results between the two groups were assessed by histological analysis, micro-CT scanning and biomechanical tests at 4 weeks, 8 weeks and 12 weeks after operation. Results The cross-sectional areas of the bone tunnels between the two groups were almost the same. However, the shape of bone tunnels in the FT group was more flattened. A faster cellular and collagen remoulding process were found in the FT group. Semiquantitative histological analysis of Safranin O staining showed that there was more fibrocartilage formation in the interface region in the FT group ( P