Microporous Pure β-Tricalcium Phosphate Implants for Press-Fit Fixation of Anterior Cruciate Ligament Grafts: Strength and Healing in a Sheep Model

Purpose: A sheep study was conducted to test a press-fit technique using microporous pure beta-tricalcium phosphate (beta-TCP) dowels for fixation of the anterior cruciate ligament (ACL) graft. Methods: Microporous (5 mu m) cylindrical plugs of beta-TCP (diameter, 7 mm; length, 25 mm) with interconnecting pores were used. The material featured a novel configuration of structure and surface geometry. Implants were tested by use of press-fit fixation of ACL grafts with and without bone blocks in 42 sheep over a period of 24 weeks. Biomechanical, radiologic, histologic, and immunohistochemical evaluations were performed. Results: In load-to-failure tests at 6, 12, and 24 weeks after surgery, the intra-articular graft always failed, not the fixation. Grafts showed bony fixation in the tunnel at 6 weeks and primary healing at the junction of the tunnel and joint after 24 weeks. Tricalcium phosphate was resorbed and simultaneously replaced by bone. Remodeling was still incomplete at 24 weeks. Conclusions: In the sheep model microporous beta-TCP implants used with press-fit fixation of ACL grafts permit early functional rehabilitation. After 6 weeks, the graft is fixed by woven bone or bony integration. Implanted microporous tricalcium phosphate is resorbed and replaced by bone. Clinical Relevance: In a sheep model we showed that primary healing of ACL grafts with resorption and bony replacement of the fixating implant can be achieved by means of press-fit fixation with pure beta-TCP.