In Vivo Analysis of Dynamic Graft Bending Angle in Anterior Cruciate Ligament–Reconstructed Knees During Downward Running and Level Walking: Comparison of Flexible and Rigid Drills for Transportal Technique

Purpose To determine the in vivo dynamic graft bending angle (GBA) in anterior cruciate ligament (ACL)–reconstructed knees, correlate the angle to tunnel positions and tunnel widening, and evaluate the effects of 2 femoral tunnel drilling techniques on GBA. Methods Patients with an isolated ACL injury undergoing reconstruction from 2011 to 2012 were included. Transportal techniques were used to create femoral tunnels. Tunnel locations were determined by 3-dimensional computed tomography. Tibiofemoral kinematics during treadmill walking and running were assessed by dynamic stereo x-ray analysis 6 months and 2 years postoperatively. The GBA was calculated from the 3-dimensional angle between the graft and femoral tunnel vectors on each motion frame. The cross-sectional areas of femoral tunnels were measured at 6 months and compared with the initial size to assess tunnel widening. Results A total of 54 patients were included. Use of flexible drills resulted in significantly higher GBAs during walking (80.6° ± 7.8°, P < .001) and running (80.5° ± 9.0°, P = .025) than rigid drills (walking, 67.5° ± 9.3°; running, 74.1° ± 9.6°). Their use led to greater tunnel widening of 113.9% ± 17.6%, as compared with 97.7% ± 17.5% for rigid drills ( P = .003). The femoral and tibial apertures were located in similar anatomic positions in both groups, but the femoral tunnel exits were located more anteriorly ( P < .001) in the flexible drill group. A higher GBA was highly correlated with anterior location of femoral exits ( r = 0.63, P < .001) and moderately correlated with greater tunnel widening ( r = 0.48, P < .001). Conclusions High GBAs were identified during dynamic activities after anatomic ACL reconstruction with a transportal femoral tunnel drilling technique. The GBA was greater when flexible drills were used. The high bending angle resulted from the more anterior location of the femoral tunnel exits, and it correlated with early bone tunnel widening at 6 months. These results suggest that a high GBA may increase stress at the bone-graft interface and contribute to greater tunnel widening after anatomic ACL reconstruction, although the clinical impact should be further investigated. Level of Evidence Level III, retrospective comparative study.