Bioabsorbable screw divergence angle, not tunnel preparation method influences soft tissue tendon graft-bone tunnel fixation in healthy bone

Abstract

Bone tunnel fixation of a soft tissue tendon graft is the weak link immediately following ACL reconstruction. This biomechanical study evaluated the influence of extraction drilled or step dilated bone tunnels and bioabsorbable screw divergence on soft tissue tendon graft fixation. From an initial group of 50 available specimens, similar apparent bone mineral density porcine tibiae (1.2 +/- 0.24 g/cm2) were divided into two groups of ten specimens each. Group 1 (extraction drilled) received 9 mm diameter tunnels. Group 2 (step dilated) received 7 mm diameter tunnels that were dilated to 9 mm. Grafts were secured in tunnels using 10 mm diameter, 35 mm long tapered screws. After high resolution CT scanning to evaluate screw divergence, constructs were pretensioned on a servo hydraulic device between 10 and 50 N for 10 cycles, and isometric pretensioned at 50 N for 1 min, prior to 500 sub-maximal loading cycles (50-200 N) and load to failure testing at 20 mm/min. Wilcoxon Signed Rank tests and Mann-Whitney U-tests were used to evaluate group differences. Coefficient of determination values (r2) were calculated to further delineate statistically significant relationships. Tunnel preparation method did not display statistically significant effects on insertion torque, displacement during cyclic testing, relative stiffness during cyclic testing, load at failure, stiffness during load to failure testing or displacement during load to failure testing. Screw divergence < 15 degrees produced lower displacement and greater relative stiffness during cyclic testing and greater load at failure and stiffness during load to failure testing. Screw divergence angle displayed moderate relationships with construct displacement during cyclic testing (r2 = 0.54), stiffness during load to failure testing (r2 = 0.60), and load at failure (r2 = 0.41). Tunnel dilation does not enhance soft tissue tendon graft fixation strength in healthy bone. Bioabsorbable screw divergence of > or = 15 degrees significantly reduces soft tissue tendon graft-bone tunnel fixation.

Intertunnel Relationships in Combined Anterior Cruciate Ligament and Posterolateral Corner Reconstruction

“…If this proves true, then we would be able to avoid tunnel convergence, and the reduced angle could help with the fixation strength as well because it is reported that the fixation strength decreases as the fixation angle increases. 11 This study demonstrated that the safe distances were approximately 30 mm to the ACL tunnel proximally in LCL reconstruction. Within the safety ranges of angles, possible distances that would not violate the far cortex would be longer; however, we believe that it would be safer if we make sure that 30 mm is not exceeded in drilling.…”

In situ comparison of varying composite tibial tunnel interference screws used for ACL soft tissue graft fixation

“…The specially designed clamp enabled the servohydraulic device (MTS 858, Eden Prairie, MN) tensile loading vector to be aligned directly with the tunnel (Fig. 2) [16][17][18]. This direct tensile load on the looped soft tissue tendon graft provided a "worst case" loading scenario.…”

“…Porcine tibiae were however screened for BMD using DXA scanning to better simulate human tibia. Previous soft tissue ACL graft fixation studies have found porcine soft tissue grafts [17,18] and tibiae [16][17][18] to be more representative of young human tissue than cadaveric tissues.…”

Hyperflexion and Femoral Interference Screw Insertion in ACL Reconstruction

“…3,10 However, divergence between the screw and the bone plug in the femoral tunnel can diminish fixation strength. 6,9,13,15,19 In laboratory tests, divergence between the screw and bone plug of 20° has been necessary to diminished fixation strength. 6,9,13,19 For transtibial drilling, multiple strategies have evolved to limit screw-graft divergence, including hyperflexion of the knee during screw insertion.…”

“…6,9,13,15,19 In laboratory tests, divergence between the screw and bone plug of 20° has been necessary to diminished fixation strength. 6,9,13,19 For transtibial drilling, multiple strategies have evolved to limit screw-graft divergence, including hyperflexion of the knee during screw insertion. 2,7,11,16,18,20 There has been less research on limiting divergence with anatomic femoral tunnels.…”