Morphometric Evaluation of Anterior Cruciate Ligament Orientation and Tibial Footprint Location Using Magnetic...

Background/Objectives: The anterior cruciate ligament (ACL) plays a key role in knee stability, biomechanics, and proprioception, and is one of the most frequently injured and reconstructed ligaments in both athletes and the general population. The anatomical placement of femoral and tibial tunnels close to the native ACL insertion sites is critical for long-term clinical outcomes and graft survival. This study aimed to define sagittal and coronal ACL alignment and tibial footprint morphology on magnetic resonance imaging (MRI) in healthy knees, to explore sex- and side-related differences, and to provide population-specific reference values. Methods: In this retrospective cross-sectional study, knee MRIs acquired between 2018 and 2021 were screened, and knees with an intact ACL and without deformity or joint pathology that could alter alignment were included. After applying inclusion and exclusion criteria, 636 knees (320 right, 316 left) from 545 individuals (338 women, 298 men; 15-80 years, mean age 34.87 ± 11.65 years) were analyzed. On sagittal images, the sagittal ACL angle (S-ANGLE) was measured on the slice where the ligament appeared maximally visualized. The midpoints of the ACL were identified on two adjacent sagittal slices, and a line drawn through these midpoints was used to represent the central axis of the ligament; the angle between this line and the tibial plateau was recorded as the S-ANGLE. For anteroposterior localization of the tibial footprint, an anteroposterior reference distance (S-long) was defined as the length measured parallel to the tibial plateau, extending from the midpoint of the tibial tuberosity (corresponding to the insertion site of the patellar ligament and used as a topographic anterior landmark) toward the posterior aspect of the proximal tibia. A perpendicular line was drawn from the anterior end of S-long to establish the anterior reference boundary. The distance from this anterior reference line to the midpoint of the ACL tibial footprint along the same anteroposterior axis was defined as S-short. The sagittal footprint percentage (S-PERCENTAGE) was calculated as (S-short/S-long) × 100, representing the size-normalized sagittal anteroposterior position of the ACL tibial footprint midpoint. On coronal images, the ACL-tibial plateau angle (C-ANGLE), mediolateral tibial length (C-LONG), and distance from the medial edge to the ACL insertion (C-short) were obtained; C-PERCENTAGE was calculated analogously. Medial mechanical proximal tibial angle (mMPTA) was used to confirm physiological coronal alignment. Non-parametric tests were applied, with p < 0.05 considered statistically significant. Results: Women had significantly greater sagittal ACL angles than men, whereas anteroposterior distances measured from the midpoint of the tibial tuberosity (used as an anterior topographic landmark) and oriented parallel to the tibial plateau (S-LONG) and mediolateral tibial lengths (C-LONG) and absolute distances to the ACL tibial footprint were larger in men. In contrast, normalized sagittal and coronal footprint percentages (S-PERCENTAGE, C-PERCENTAGE) did not differ meaningfully between sexes, indicating the preservation of the relative ACL tibial insertion site despite size differences. Small but statistically significant side-to-side differences were observed in some coronal parameters; however, absolute differences were small and did not substantially modify the overall alignment pattern. Conclusions: This study provides large-sample, population-specific reference values for ACL orientation and tibial footprint location in both sagittal and coronal planes in healthy knees. The combination of higher sagittal ACL angles and shorter anteroposterior distances reference measured from the midpoint of the tibial tuberosity and oriented parallel to the tibial plateau (S-LONG) in women may represent a structural substrate contributing to the higher ACL injury rates reported in females. The morphometric data presented here may assist in individualized ACL reconstruction planning, MRI-based assessment of tibial tunnel position, and the design of knee-related biomedical implants and devices.