Primary stability with tibial press-fit fixation of patellar ligament graft: An experimental study in ovine knees.

PURPOSE: To investigate the initial fixation strength and to assess the value of tibial press-fit fixation of the bone-tendon-bone graft in anterior cruciate ligament reconstruction. TYPE OF STUDY: Nonrandomized control trial.
METHODS: For tibial press-fit fixation, the tibial bone block of the bone-tendon-bone graft is countersunk in a bony groove at the distal tunnel outlet and fixed over a bone bridge with 2 No. 6 Ethibond sutures. The bone cylinder harvested from the tibial tunnel using an oscillating hollow saw is plugged into the tibial tunnel parallel to the graft, thus providing for additional anchoring of the graft by tibial press-fit fixation. In a comparative experimental study in 46 ovine knees, this fixation method was assessed for its value in anterior cruciate ligament reconstruction. A tibial bone tunnel was placed in routine manner in each ovine tibia using a target drill unit and an oscillating hollow saw. The complete patellar ligament, proximally attached to the patella and distally to a cylindrical bone block (20 x 8.4 mm), served as graft. Tibial fixation in group A (n = 10) was done using a titanium interference screw (20 x 8 mm), in group B (n = 10) using a titanium staple, in group C (n = 12) using suture fixation over a bone bridge, and in group D (n = 14) using the press-fit fixation described above. In a materials testing machine, all specimens were subjected to continuously increasing load until failure at a velocity of 1 mm/second. Ultimate failure load, stiffness, stress-strain characteristics, and failure mode were evaluated.
RESULTS: Ultimate load to failure was 572 N (range, 473 to 680 N) in group A, corresponding to a fixation stiffness of 17.68 N/mm. For group B, ultimate load to failure was 608.4 N (range, 511 to 727 N) and stiffness of 19.92 N/mm. Bone block dislocation was the failure mode in groups A and B. Group C with exclusive suture fixation showed an ultimate load to failure of 304.5 N (range, 120 to 327 N) and a stiffness of only 6.96 N/mm. The mode of failure was suture cutout caused by the bone block in 9 of the cases and untying of the suture knot in 3 cases. Group D with press-fit fixation showed a significantly higher primary stability of 758 N (range, 513 to 993 N) relative to group C, with a corresponding stiffness of 25.12 N/mm (P <.02). In this group, the mode of failure was ligamentous rupture from the bone block. Regarding mechanical properties, no significant differences were seen between groups A, B, and D.
CONCLUSIONS: Tibial press-fit fixation allows for metal-free fixation with high primary stability. By refilling the bone tunnel, the ligament (with a rather small cross-sectional diameter compared with hamstrings) is safely fixed within the bone tunnel to prevent potential postoperative tunnel enlargement due to movement of the graft within the tunnel. Anchoring the graft at the entrance into the joint, it provides for reduced graft length and adequate elasticity and accomplishes the requirements of fixation at the correct anatomic insertion site.