Influence of femoral tunnel exit on the 3D graft bending angle in anterior cruciate ligament reconstruction

Abstract

Purpose: To quantify the influence of the femoral tunnel exit (FTE) on the graft bending angle (GBA) and GBA-excursion throughout a full range of motion (ROM) in single-bundle anterior cruciate ligament (ACL) reconstruction.

Methods: Three-dimensional (3D) surface models of five healthy knees were generated from a weight-bearing CT obtained throughout a full ROM (0, 30, 60, 90, 120°) and femoral and tibial ACL insertions were computed. The FTE was simulated for 16 predefined positions, referenced to the Blumensaat's line, for each patient throughout a full ROM (0, 30, 60, 90, 120°) resulting in a total of 400 simulations. 3D GBA was calculated between the 3D directional vector of the ACL and the femoral tunnel, while the intra-articular ACL insertions remained unchanged. For each simulation the 3D GBA, GBA-excursion, tunnel length and posterior tunnel blow-out were analysed.

Results: Overall, mean GBA decreased with increasing knee flexion for each FTE (p < 0.001). A more distal location of the FTE along the Blumensaat's line resulted in an increase of GBA and GBA-excursion of 8.5 ± 0.6° and 17.6 ± 1.1° /cm respectively (p < 0.001), while a more anterior location resulted in a change of GBA and GBA-excursion of -2.3 ± 0.6° /cm (+ 0.6 ± 0.4°/ cm from 0-60° flexion) and 9.8 ± 1.1 /cm respectively (p < 0.001). Mean tunnel length was 38.5 ± 5.2 mm (range 29.6-50.5). Posterior tunnel blow-out did not occur for any FTE.

Conclusion: Aiming for a more proximal and posterior FTE, with respect to Blumensaat's line, reliably reduces GBA and GBA-excursion, while preserving adequate tunnel length. This might aid to reduce excessive graft stress at the femoral tunnel aperture, decrease femoral tunnel widening and promote graft-healing.

Level of evidence: IV.

Keywords: ACL; Anterior cruciate ligament; Femoral tunnel; GBA; Graft bending angle.

Fig. 1

Tibial ACL insertion. Definition of 3D tibial ACL insertion based on Stäubli et al. [29] and described in the text

Fig. 2

Femoral ACL insertion. Definition of 3D femoral ACL insertion based on Bernard et al. [2] and described in the text

Fig. 3

Femoral tunnel exit (FTE). Definition of femoral tunnel exit and orientation of rows (proximal to distal) and lines (anterior to posterior)

Fig. 4

Schematic illustration of different flexion grades and measurement of graft bending angle (GBA). GBA: Graft bending angle. The tibia moved around the femur for 0, 30, 60, 90, 120° of flexion (A). 3D GBA between ACL (pink) and femoral tunnel (violet) illustrated from an anterior and lateral view (B)

Fig. 5

Mean graft bending angle (GBA) for all subjects and FTE among flexion grades. GBA: Graft bending angle. SD: Standard deviation. Boxplots depicts mean (line), 1st and 3rd quartile (box), minimum and maximum (whisker). Significant differences between groups after Bonferroni correction marked with * (90° vs. 120° p = 0.002), remaining: p < 0.001; ANOVA). Red dotted line: FTE with the greatest mean GBA-excursion; ΔGBA = 102.1 ± 6.3° (most distal-anterior FTE). Green dotted line: FTE with the smallest mean GBA-excursion; ΔGBA = 21.7 ± 6.9° (most proximal-posterior FTE)

Fig. 6

Mean GBA among flexion grades and femoral exit points. GBA: graft bending angle. 3D bar chart depicts GBA (y-axis) for each flexion° and for each FTE. All bar charts are oriented as in the model on the top left, listing the FTE with respect to the Blumensaat’s line (x-axis (rows; proximal to distal) and z-axis (lines; posterior to anterior)