Primary anterior cruciate ligament tears are associated with a shift from varus-dominant to neutral and valgus coronal alignment phenotypes.

PURPOSE: The role of coronal alignment in primary anterior cruciate ligament (ACL) tears remains unclear. Coronal alignment phenotypes in ACL-injured patients and healthy controls were compared using Hirschmann's functional classification and the coronal plane alignment of the knee (CPAK) system.

METHODS

A total of 192 patients with unilateral ACL tears and 193 age- and sex-matched healthy controls were compared. Full-length standing radiographs were analysed using mediCAD software to measure hip-knee-ankle (HKA) angle, femoral mechanical angle (FMA), tibial mechanical angle (TMA) and joint line convergence angle (JLCA). Knees were classified according to Hirschmann's functional phenotypes and CPAK system. Group comparisons used chi-square and t-tests; paired t-tests compared ACL-injured and contralateral intact knees (p ≤ 0.05).

RESULTS

Overall, 384 knees from ACL-injured patients and 386 knees from controls were compared with no demographic differences. Paired analyses revealed significant differences in HKA (ACL-intact minus ACL-injured mean difference, 0.45°; 95% CI, 0.03-0.87°; p = 0.034) and JLCA (ACL-injured minus ACL-intact mean difference, 0.46°; 95% confidence interval [CI], 0.28-0.64°; p < 0.001) between injured and intact knees. Healthy knees were predominantly varus (65.0%), while ACL-injured knees shifted toward neutral (43.8%) and valgus (21.9%), with varus decreasing to 34.4% (p < 0.001; Cramér's v = 0.33). ACL-intact knees demonstrated an intermediate distribution (35.9% varus, 50.6% neutral, 13.5% valgus). CPAK type II predominated in patients, whereas type I was most common in controls (p < 0.001). Valgus-associated CPAK phenotypes (types III and VI) were more frequent in ACL-injured knees than in ACL-intact and healthy knees.

CONCLUSION

Primary ACL tear was associated with a shift from varus-dominant alignment toward more neutral and valgus phenotypes. The intermediate phenotype distribution observed in contralateral ACL-intact knees is consistent with a possible bilateral constitutional predisposition, whereas the greater shift in ACL-injured knees suggests an additional injury-related or post-injury component. These findings should be interpreted as associations rather than evidence of pre-injury causal mechanisms.

LEVEL OF EVIDENCE: Level III, cross-sectional comparative study.