Peri-graft bone mass and connectivity as predictors for the strength of tendon-to-bone attachment after anterior cruciate ligament reconstruction

The present study was designed to compare peri-graft bone mass and microarchitecture with tendon-to-bone (T–B) attachment strength after anterior cruciate ligament (ACL) reconstruction in a rabbit model. Surgical reconstruction using digital extensor tendon in bone tunnel was performed on 58 rabbits. Forty-two of the 58 rabbits were sacrificed at week 0, 2, 6 and 12 after operation respectively. The femur–graft–tibia complexes were harvested for pQCT and micro-CT examination to characterize the spatiotemporal changes of peri-graft bone in T–B healing in conjunction with histological examination. The remaining 16 rabbits were euthanized at week 6 and 12 postoperatively (i.e. 8 rabbits for each time point) for pull-out test after micro-CT examination to investigate the relationship between the T–B attachment strength and peri-graft bone mass/microarchitecture. Peri-graft BMD, BV/TV and connectivity was significantly lower at week 6 than those at time zero although there were no significant changes detected in the first 2 postoperative weeks. In addition, peri-graft bone mass and connectivity was significantly lower on the tibial side than those on the femoral side; and osteoclasts accumulated on the surface of peri-graft bone. Grafted tendon was prone to be pulled out from the tibial tunnel with bone attachment; the weakest point of the complexes shifted from the healing interface at time zero to peri-graft bone at week 6 after operation. With reverse of peri-graft bone at week 12 postoperatively, the weakest point shifted to the intra-osseous tendinous portion. The stiffness of T–B attachment correlated with peri-graft BV/TV (r2 = 0.68, p = 0.001) and connectivity (r2 = 0.47, p = 0.013) at week 6 after operation. T–B healing was a highly dynamic process of emergence and maintenance of peri-graft bone. T–B attachment strength was in relation to peri-graft bone mass and connectivity after ACL reconstruction. The measurement of peri-graft bone should be useful to monitor the quality of T–B healing and guide the postoperative rehabilitation.