Os acetabuli is an indirect radiographic sign of femoral retroversion.

BACKGROUND

To report the association between os acetabuli and relevant radiologic measurements in a series of surgically treated hip preservation patients.

METHODS

We retrospectively reviewed 654 hips who underwent preservation surgery between 2012 and 2019, identifying 50 cases with os acetabuli (7.64%). We included 300 hips (277 patients) with radiographs and CT-scans obtained during preoperative assessment, after which the cohort was divided into two groups (with and without os acetabuli). Mean age of the study population was 35 (interquartile range [IQR] 28-42) years old, with 192 (62%) being males. 2 observers measured demographic and radiologic variables. A mixed-effects logistic regression tested the ability of radiologic measurements to predict the presence of os acetabuli.

RESULTS

No between-group differences were found in terms of sex (156/250 vs. 36/50 males,  0.197), alpha angle (64 ± 13° vs. 65 ± 14°,  0.372), Tönnis angle (8 ± 7° vs. 7 ± 7°,  0.152), neck-shaft angle (132 ± 6° vs. 131 ± 5°,  0.199) and CT-acetabular version (16 ± 7° vs. 15 ± 6°,  0.221). Significant differences were found in terms of age (34 ± 9 vs. 39 ± 7 years,  0.002), lateral centre-edge angle (LCEA) (31 ± 9° vs. 34 ± 7°,  0.045), anterior wall index (AWI) (0.45 [IQR 0.37-0.54] vs 0.5 [IQR 0.39-0.6],  0.046), CT-femoral version (17 ± 5° vs. 8 ± 4°,  0.001), cross-over sign (96/250 vs. 28/50,  0.032) and baseline diagnosis (184/250 vs. 44/50 FAIs,  0.043), with the os acetabuli-group being older, with more FAI diagnosis/cross-over sign, and with higher LCEA, higher AWI, and a lower femoral version. After adjusting for confounders, only CT-femoral version (odds ratio 0.32; 95% CI. 0.14-0.73,  0.007) was associated with presence of os acetabuli.

CONCLUSIONS

Presence of acetabular rim fragments was significantly associated with a lower femoral version. Above 20° of femoral version, the likelihood of os acetabuli was almost zero. Treatment of os acetabuli (i.e., fixation vs. removal) should be adjusted for the underlying diagnosis.