Tendon healing in a bone tunnel. Part I: Biomechanical results after biodegradable interference fit fixation in a model of anterior cruciate ligament reconstruction in sheep.

PURPOSE Interference fit fixation of soft-tissue grafts has recently raised strong interest because it allows for anatomic graft fixation that may increase knee stability and graft isometry. Although clinical data show promising results, no data exist on how tendon healing progresses using this fixation. The purpose of the present study was to investigate anterior cruciate ligament (ACL) reconstruction biomechanically using direct tendon-to-bone interference fit fixation with biodegradable interference screws in a sheep model. TYPE OF STUDY Animal study. METHODS Thirty-five mature sheep underwent ACL reconstruction with an autologous Achilles tendon split graft. Grafts were directly fixed with poly-(D,L-lactide) interference screws. Animals were euthanized after 6, 9, 12, 24, and 52 weeks and standard biomechanical evaluations were performed. RESULTS All grafts at time zero failed by pullout from the bone tunnel, whereas grafts at 6 and 9 weeks failed intraligamentously at the screw insertion site. At 24 and 52 weeks, grafts failed by osteocartilaginous avulsion. At 24 weeks, interference screws were macroscopically degraded. At 6 and 9 weeks tensile stress was only 6.8% and 9.6%, respectively, of the graft tissue at time zero. At 52 weeks, tensile stress of the reconstruction equaled 63.8% and 47.3% of the Achilles tendon graft at time zero and the native ACL, respectively. A complete restitution of anterior-posterior drawer displacement was found at 52 weeks compared with the time-zero reconstruction. CONCLUSIONS It was found that over the whole healing period the graft fixation proved not to be the weak link of the reconstruction and that direct interference fit fixation withstands loads without motion restriction in the present animal model. The weak link during the early healing stage was the graft at its tunnel entrance site, leading to a critical decrease in mechanical properties. This finding indicates that interference fit fixation of a soft-tissue graft may additionally alter the mechanical properties of the graft in the early remodeling stage because of a possible tissue compromise at the screw insertion site. Although mechanical properties of the graft tissue had not returned to normal at 1 year compared with those at time zero, knee stability had returned to normal at that time. There was no graft pullout after 24 weeks, indicating that screw degradation does not compromise graft fixation.