Histologic and Biomechanical Analysis of Anterior Cruciate Ligament Graft to Bone Healing in Skeletally Immature Sheep

Purpose

It was our aim to establish an animal model and to investigate the tendon graft–to–bone and physis healing process in skeletally immature sheep after reconstruction of the anterior cruciate ligament (ACL).

Methods

Thirty-two immature sheep aged 4 months underwent a fully transphyseal ACL reconstruction by use of a soft-tissue graft. The animals were subsequently killed after 3, 6, 12, and 24 weeks and analyzed histologically and biomechanically.

Results

There was a transient hypertrophy of the physis tissue at the passing site of the graft. Anchoring Sharpey-like fibers evolved as early as 3 weeks after surgery. A strong expression of collagen III messenger ribonucleic acid within the first 6 weeks preceded this anchoring process. The maximum load to failure of the tendon graft in the reconstructed knees initially decreased to 37.8 ± 17.8 N after 3 weeks and was restored to 522.9 ± 113 N after 24 weeks. Tendon graft stiffness was restored to 86% when compared with the control knees.

Conclusions

The early anchoring by Sharpey fibers was found at 3 weeks with continued maturation to 24 weeks. This development of anchoring fibers corresponded to that of biomechanical strength, starting with 5% of the normal knee at 3 weeks and then 15.2% at 6 weeks, 41.2% at 12 weeks, and 69% at 24 weeks. Tendon graft–to–bone and physis healing in skeletally immature sheep is further characterized by a transient hypertrophy of the physis cartilage. The physis recovers well from the trauma of drilling and placement of a soft-tissue graft. The early development of Sharpey-like fibers results in a solid integration of the graft into bone in a timely manner.

Clinical Relevance

ACL reconstruction in skeletally immature individuals is still controversial. This study describes in detail the histologic and biomechanical stages of tendon graft healing to the bone and physis. These data enrich the existing knowledge of previous studies in adult sheep and may provide a basis for further research in the controversial field of ACL reconstruction during growth.

Key Words: Anterior cruciate ligament reconstruction, Tendon graft, Growth, Skeletally immature, In situ hybridization, Sheep model