The purpose of this study is to determine the influence of knee flexion angle for drilling the posterolateral (PL) femoral tunnel during double-bundle anterior cruciate ligament (ACL) reconstruction via the anteromedial (AM) portal on resulting tunnel orientation and length. Methods: In nine fresh cadaveric knees, the ACL was excised and 2.4 mm guide wires were drilled through the PL bundle footprint via an AM portal. We compared knee flexion angles of 90, 110, 130 degrees. AP-, lateral- and tunnel view radiographs were measured to determine tunnel orientation, o’clock position, and direct measurement to determine intra-osseous tunnel length

On AP view, increased flexion resulted in more horizontal tunnels. The angles were 31.9 ± 7.1°, 26.4 ± 8.9° and 23.0 ± 8.1° for 90°, 110° and 130°. The pin orientation was significantly different when comparing 90° and 130°. On lateral view, increased flexion resulted in more horizontal tunnels. The angles were 68.9 ± 19.9°, 50.4 ± 11.6°, 31.3 ± 12.3° for 90°, 110° and 130°. On tunnel view, pin orientation was 22 ± 8.2°, 28.3 ± 6.7° and 35.9 ± 6.2° for 90°, 110° and 130°. Mean o’clock position was 09:00 ± 0:12. Intra-osseous length of the pins did not significantly change with knee flexion. The exit of the pins on the lateral femur with regard to femoral attachment of the LCL was proximal. The distance was 0.1 ± 6.6 mm, 6.4 ± 6.4 mm and 9.2 ± 2.4 mm for 90°, 110° and 130°. This was significant when comparing 90° and 130°. The shortest distance between the exit and the posterior femoral cortex was 4.0 ± 1.8 mm, 9.7 ± 3.5 mm, and 13.2 ± 2.8 mm for 90°, 110° and 130°. All values were significant. Conclusion: At 110°, exit of the PL pin is close to the attachment of the LCL. 90° flexion risks damage to the LCL and posterior cortex blow-out. Thus we recommend drilling the PL tunnel at 130° of knee flexion