Background: In the field of anterior cruciate ligament (ACL) reconstruction, there is a renewed
emphasis on placing the ACL graft in an anatomic based position. Post-operative evaluation of
ACL reconstructive surgery may include graft location and graft orientation relative to the native
ACL. The orientation of the native ACL in terms of its angle of inclination has not been clearly
established. The purpose of this study was to evaluate the angle of inclination of the native ACL
in both the sagittal and coronal planes, and to evaluate these findings based on sex, BMI, and
skeletal maturity. A secondary purpose was to evaluate the native ACL inclination angle of
patients with a history of ACL rupture.
Material and Methods: Inclusion criteria for the first phase of the study included patients
undergoing routine magnetic resonance imaging (MRI) of the knee at a single outpatient
orthopedic center that had an intact ACL on MRI. Patients were excluded if they had significant
cartilage, meniscal or ligamentous damage. Using established methods, measurements of the
angle of inclination were made on MRIs in both the sagittal and coronal planes. Patients were
then compared based on sex, BMI, and skeletal maturity. In the second phase, the inclination
angle of intact, native ACLs in a subset of patients who had a subsequent history of ACL rupture
were compared to normative data.
Results: In the first phase, 188 patients were included (36 skeletally immature/152 skeletally
mature; 97 male/90 female). The overall angle of inclination was 74.28º±4.75º in the coronal
plane and 46.88º±4.89º in the sagittal plane. With regard to sex, there was no difference in the
angle of inclination in either the coronal (M: 74.15º±4.84º; F: 75.21º±5.46º; (P=0.16)) or sagittal
plane (M: 46.02º±4.88º; F: 46.77º±4.99º; (P=0.83)). Skeletally immature patients (coronal:
71.82º±6.06º; sagittal: 44.67º±5.49º) were significantly different in both coronal and sagittal
planes (P=0.044 and 0.009, respectively) from skeletally mature patients (coronal: 75.32º±4.71º;
sagittal: 47.4º±4.65º). In the second phase, no difference was found between patients with
subsequent ACL rupture (n=30; coronal: 70.44º±4.87º, sagittal: 47.53º±5.16º) and those without
a history of ACL rupture (P=0.59 and P=0.74, respectively).
Conclusion: This study adds to the current body of literature on the native ACL angle of
inclination and suggests three key findings. First, while the previously suggested inclination
angle of the native ACL in the coronal plane is consistent with our findings, our data suggests
that the angle in the sagittal plane may be less vertical. Secondly, when using the inclination
angle for post-operative evaluation, variation based on skeletal maturity should be taken into
account. Thirdly, the lack of a difference between patients with and without a history of ACL
rupture suggests the absence of a predisposition for such injuries with regard to the angle of the
ligament.