Abstract: BACKGROUND: Current studies suggest that acetabular coverage may be a risk factor for idiopathic osteonecrosis of the femoral head; however, there is a lack of comparative studies on acetabular coverage among different subtypes of osteonecrosis of the femoral head.
OBJECTIVE: To investigate the association between acetabular coverage and the onset of idiopathic osteonecrosis of the femoral head.  
METHODS: Fifty-four patients (87 hips) with idiopathic osteonecrosis of the femoral head who were admitted to the Wangjing Hospital of the China Academy of Chinese Medical Sciences between January 2018 and June 2024 and a matched group of 162 patients (261 hips) were enrolled, including patients with steroid-induced osteonecrosis of the femoral head, alcohol-induced osteonecrosis of the femoral head, and patients without obvious hip joint disease. Four acetabular morphological parameters were measured on anteroposterior pelvic radiographs using standardized imaging methods: center-edge angle, Sharp angle, acetabular depth ratio, and acetabular head index. 
RESULTS AND CONCLUSION: (1) The central-edge angle in the idiopathic osteonecrosis of the femoral head group was significantly smaller than that in the matched group [(27.4±4.5)° vs. (32.4±5.7)°], and the Sharp angle in the idiopathic osteonecrosis of the femoral head group was significantly larger than that in the matched group [(40.2±3.5)° vs. (38.7±2.9)°]. The acetabular depth ratio (276.4±33.3 vs. 319.3±31.6) and acetabular head index (82.4±5.1 vs. 85.4±4.2) in the idiopathic osteonecrosis of the femoral head group were significantly smaller than those in the matched group (all P < 0.001). (2) Compared with the steroid-induced osteonecrosis of the femoral head subgroup, the idiopathic osteonecrosis of the femoral head group had a smaller central-edge angle [(27.4±4.5)° vs. (33.0±5.2)°] and a larger Sharp angle [(40.2±3.5)° vs. (38.7±3.1)°], lower acetabular depth ratio and acetabular head index (all P < 0.05). (3) Compared with the alcoholic osteonecrosis of the femoral head subgroup, the idiopathic osteonecrosis of the femoral head subgroup also showed a smaller center-edge angle [(27.4±4.5)° vs. (32.7±5.4)°], a larger Sharp angle [(40.2±3.5)° vs. (38.7±3.0)°], and a lower acetabular depth ratio and acetabular head index (all P < 0.05). These results suggest that patients with idiopathic osteonecrosis of the femoral head have characteristic acetabular undercoverage, manifested by a reduced center-edge angle, increased Sharp angle, decreased acetabular depth ratio, and acetabular head index. This anatomical feature may serve as a potential risk factor for idiopathic osteonecrosis of the femoral head and aid in differentiating it from other subtypes of osteonecrosis of the femoral head. It is recommended to focus on acetabular morphology in clinical evaluations to optimize early screening and individualized intervention strategies. 

Key words: osteonecrosis of the femoral head, idiopathic, acetabular coverage, cross-sectional study, risk factor, center-edge angle, sharp angle