Background: Alpha angle and femoral intracondylar notch type are anatomical markers linked to ACL injury, but region-specific data combining both measures, with stratified analysis by age, sex, and body mass index (BMI), remain scarce.

Methods: This case-control study enrolled 110 patients with confirmed ACL injury and 110 controls with intact ACL, frequency matched by age and sex, at three hospitals in Babol, Iran (2023). Assessors measuring alpha angle and notch type (A, U, or W) were not blinded to case/control status. Notch-type distribution was compared using an omnibus chi-square test; alpha angle, being non-normally distributed, was compared using the Mann-Whitney U test and summarized as median [interquartile range, IQR], with the Hodges-Lehmann shift as the principal effect estimate. Multivariable logistic regression adjusted for age, sex, and BMI.

Results: Age, sex, and BMI distributions were similar between groups. Notch-type distribution differed significantly (type A: 88.2% vs. 12.7%; type U: 10.9% vs. 80.9%; type W: 0.9% vs. 6.4%; omnibus χ²=125.3, P<0.001). Alpha angle was higher in the ACL group (median [IQR] 45.0° [42.4-46.0°] vs. 42.6° [41.0-45.0°]; Hodges-Lehmann shift 1.0°, 95% CI 1.0-2.0°, P<0.001). Type A notch (adjusted OR 70.4, 95% CI 26.3-188.0) and alpha angle (adjusted OR 1.53/degree, 95% CI 1.21-1.92) were independently associated with ACL injury after adjustment for age, sex, and BMI.

Conclusion: Type A notch and increased alpha angle are independently associated with ACL injury after adjustment for age, sex, and BMI. The alpha-angle difference (~1°) was small and should be interpreted cautiously pending formal measurement-reliability testing.