It has been demonstrated that shorter femoral tunnel lengths are created with the knee in lesser degrees of flexion (19). This may explain why the results of our study showed no difference in tunnel length between flexible reamers with the knee flexed 90 degrees and rigid reamers with the knee flexed 115 degrees. Tunnel length is an important determinant of graft fixation potential. It is recommended to have a minimum femoral tunnel length of 25 mm when using an interference screw and 35 mm when using suspensory-type fixation (19). Although there were 4 tunnels in the flexible group and 3 tunnels in the rigid group that were less than 35 mm, no tunnel was less than 30 mm in this study.
Distance of the femoral tunnel from the posterior cortex has also been shown to decrease in lower angles of knee flexion. Basdekis et al. reported that rigid guide pins may routinely result in violation of the posterior femoral cortex if placed in 90 degrees of knee flexion (19). Similarly, Steiner et al. reported 3 of 6 (50%) of rigid guide pins in a position that would result in violation of the posterior cortex of the femur compared to none in the flexible group (16). In this study, there were no cases of posterior wall “blow-out”; however, tunnels drilled with rigid reamers were significantly closer to the posterior cortex of the femur.
Obliquely oriented femoral tunnels provide more rotational stability than vertical tunnels (1-5,13,19). Scopp et al. demonstrated that oblique tunnels oriented 60 degrees to vertical in the axial plane restored tibial internal rotation in 30 degrees of knee flexion to the same value as for knees with intact ACLs, whereas less oblique tunnels (30 degrees to vertical) had significantly more tibial internal rotation (5). In this study, flexible reamers were superior to rigid reamers in producing obliquely oriented tunnels. Similar to our study, Larson et al. found that the use of flexible reamers through the AM portal produced tunnels of greater obliquity than rigid reamers. The method of measuring this angle was slightly different, but the flexible reamers produced tunnels that were approximately 10 degrees more oblique (18). The difference in our study was approximately 17 degrees.
Limitations of this study include the technique of tunnel placement, which was not identical to that performed in the operating room. With soft tissues dissected removed, visualization and guide placement is much simpler. We assessed only tunnel position and orientation and did not perform actual reconstructions with subsequent biomechanical analysis that may demonstrate clinically relevant differences for ACL reconstructions performed with flexible reamers.
Femoral ACL tunnel placement using flexible reamers produced tunnels with greater obliquity. The tunnels were farther from the posterior cortex of the femur and there was no difference in length of tunnels compared to those created with rigid reamers. The use of flexible reamers for femoral ACL tunnels produced acceptable tunnels without requiring excessive knee flexion.
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