Summary

In vivo analysis of graft bending angle (GBA) during dynamic weight bearing activities after anatomic ACL reconstruction showed substantially steep GBA at footstrike, especially with flexible drills (mean ± SD: 80.5 ± 9.0º during running), and higher GBA was significantly associated with more anterior location of proximal femoral exits and correlated to bone tunnel widening 6 months after surgery.

Abstract

Introduction

Acute graft bending at the femoral tunnel aperture may increase stresses between the bone and the reconstructed graft. We aimed to determine in vivo dynamic graft bending angle (GBA) in ACL reconstructed knees, correlate the angle to tunnel positions and tunnel widening (determined from CT scans), and evaluate effects of two femoral drilling techniques on GBA. We hypothesized that the steep graft bending could occur during dynamic activities, especially while running, and flexible drills would increase bending angles, leading to greater bone tunnel enlargement.

Methods

Anatomic ACL reconstruction was performed on 54 patients (Avg. age 21) using the trans-portal technique. 37 were male and 17 were female. Rigid (n=20) or flexible (n=34) drills were used to create the single femoral tunnels. The three-dimensional (3D) locations of the femoral and tibial tunnel aperture centers and the proximal femoral tunnel exit were identified from a high-resolution CT scan 6 months postoperatively. Tibio-femoral kinematics were determined using dynamic stereo x-ray analysis shortly before footstrike through midstance. GBA was calculated from the 3D angle between the graft and femoral tunnel vectors, determined at each motion frame from the dynamic positions of the CT-identified landmarks. Cross-sectional area of the femoral tunnel was measured using 3D imaging software, and compared to the initial area from the dilators used at the surgery to assess tunnel widening (TW). The mean GBA, tunnel locations in 3 directions and bone tunnel widening were compared between the two drill groups using Student's t-test. The relationships between GBA and tunnel locations, or TW were analyzed using Pearson’s correlations.

Results

Use of flexible drills resulted in significantly higher GBA during walking (80.6º ± 7.8º; p<0.0001) and running (80.5º ± 9.0º; p=0.0249) than rigid drills (walking: 67.5º ± 9.3º, running: 74.1º ± 9.6º). It led to greater tunnel widening (p=0.0033) than rigid drills.
Femoral and tibial apertures were located in anatomic position in both groups, but femoral exits were located more anteriorly (p=0.0005) in flexible drill group. More anterior location of femoral exits were highly correlated with higher GBA during walking (R=0.63, p<0.0001) and running (R=0.45, p<0.005), and TW was moderately correlated with GBA in both walking (R=0.48, p=0.0004) and running (R=0.37, p=0.008). GBA was significantly higher during running than walking (mean difference from footstrike through mid-stance: 3.1º; p<0.001).

Discussion

High graft bending angles were identified during both walking and running after anatomic ACL reconstruction, especially with flexible drills. Moderate correlations with early (6 month) tunnel enlargement suggest that motion of the knee and graft, combined with a steep GBA which is formed by anatomic tunnel positions and anterior exit positions, may contribute to tunnel enlargement. This was the first study to analyze in vivo GBA dynamically. As for limitation, this is a secondary analysis of data collected for a prospective trial and the use of drill type was not randomized, but consecutive.

Significance

The relationships between high GBA and increased bone tunnel widening may suggest high stress between the graft and femur at footstrike during dynamic motion after anatomic ACL reconstruction.