ObjectiveTo investigate the best knee flexion angle by analyzing the length and orientation of the femoral tunnel through anteromedial portal (AM) at different flexion angles during anterior cruciate ligament (ACL) reconstruction. MethodsTwelve fresh cadaveric knees were selected to locate the center of ACL femoral footprint through AM using the improved hook slot vernier caliper, and to locate the posterior bone cortex using a diameter 3 mm ball at flexion of 90, 100, 110, 120, and 130°. The femoral tunnel length, standard coronal and sagittal plane angles, and the position relation between exit point and the lateral epicondyle were measured; the tunnel orientation on the anteroposterior and lateral X-ray films was also measured. ResultsWith increasing flexion of the knee, the femoral tunnel length showed a first increasing and then stable tendency; significant difference was found between at flexion of 90°and at flexions of 100, 110, 120, and 130°, and between flexions of 100°and 120°(P<0.05). The femoral tunnel showed a trend of decreasing with coronal angle, whereas gradually increasing with sagittal angle. The knee flexion angle had significant difference either among flexions of 90, 110, and 130°or between flexions of 100°and 120°(P<0.05). The exit point of the femoral tunnel located at the lateral epicondyle of the femur proximal to posterior region at flexion of 90°in all knees, and at flexion of 100°in 7 knees, but it located at the lateral epicondyle of the femur proximal to anterior region at flexion of 110, 120, and 130°in all knees. As the knee flexion angle increasing, the angle between femoral tunnel with the tangent of internal-external femoral condyle on anteroposterior X-ray films showed a trend of decreasing gradually, but a trend of increasing gradually on lateral X-ray films. On the anteroposterior X-ray films, significant differences were found in the angle either among flexions of 90, 110, and 130°or between flexions of 100°and 120°(P<0.05). On the lateral X-ray films, there were significant differences in the angle among flexions of 90, 100, 110, 120, and 130°(P<0.05). ConclusionDuring ACL reconstruction by AM, 110°is the best flexion angle, which can get the ideal femoral tunnel.