The Effect of Graft Pretensioning on Bone Tunnel Diameter and Bone Formation After Anterior Cruciate Ligament Reconstruction in a Rat Model: Evaluation With Micro-Computed Tomography

Abstract

Background: Moderate graft pretensioning in anterior cruciate ligament (ACL) reconstruction is paramount to restore knee stability and normalize knee kinematics. However, little is known about the effect of graft pretensioning on graft-to-bone healing after ACL reconstruction.

Hypothesis: Moderate graft pretensioning will improve bone formation within the bone tunnel after ACL reconstruction, resulting in superior load to failure.

Study design: Controlled laboratory study.

Methods: 67 male Sprague-Dawley rats underwent unilateral ACL reconstruction with a flexor digitorum longus tendon autograft. The graft was subjected to pretensioning forces of 0 N, 5 N, or 10 N. Custom-made external fixators were used for knee immobilization postoperatively. Rats were euthanized for biomechanical load-to-failure testing (n = 45) and micro-computed tomography (μCT) examination (n = 22) at 3 and 6 weeks after surgery. Three regions of each femoral and tibial bone tunnel (aperture, middle, and tunnel exit) were chosen for measurement of tunnel diameter and new bone formation.

Results: Biomechanical tests revealed significantly higher load-to-failure in the 5-N graft pretensioned group compared with the 0- and 10-N groups at 3 weeks (8.58 ± 2.67 N vs 3.96 ± 1.83 N and 4.46 ± 2.62 N, respectively) and 6 weeks (16.56 ± 3.50 N vs 10.82 ± 1.97 N and 7.35 ± 2.85 N, respectively) after surgery ( P < .05). The mean bone tunnel diameters at each of the 3 regions were significantly smaller in the 5-N group, at both the femoral and tibial tunnel sites, than in the 0- and 10-N groups ( P < .05). At 3 and 6 weeks postoperatively, the bone mineral density, bone volume fraction, and connectivity density around the aperture and middle regions of the tibial bone tunnels were all significantly higher in the 5-N group compared with the 0- and 10-N groups ( P < .05). In the aperture and middle regions of the femoral bone tunnels, pretensioning at either 5 or 10 N resulted in increased bone formation compared with the nonpretensioned group at 3 weeks postoperatively. No differences were found in bone formation between any of the 3 femoral tunnel regions at 6 weeks.

Conclusion: Graft pretensioning can stimulate new bone formation and improve tendon-to-bone tunnel healing after ACL reconstruction.

Clinical relevance: Optimal graft pretensioning force in ACL reconstruction can improve bone tunnel healing. Further study is necessary to understand the mechanisms underlying the effect of graft pretensioning on healing at the bone-tunnel interface.

Keywords: ACL reconstruction; bone formation; bone tunnel; graft pretensioning; micro–computed tomography.