Spontaneous femoral neck fracture is a rare condition that remains controversial due to limited reported cases. This retrospective study aims to provide further insights into the etiology and characteristics of the disease.We conducted a retrospective review of data from 963 patients with femoral neck fractures. The data encompassed demographic information, medical histories, radiographic records, bone mineral density (BMD) measurements, and pathological examinations. Patients were categorized into two groups: spontaneous femoral neck fracture (SFF) group (30 cases) and control group (933 cases), based on their medical histories. Logistic regression analysis was employed to identify risk factors for SFF. Statistical analysis was performed to compare and elucidate the characteristics of SFF within each group.Logistic regression analysis revealed osteonecrosis of the femoral head, steroid use, and osteoporosis as three significant risk factors for SFF. Furthermore, a higher proportion of Garden type I and II fractures, as well as Pauwels type I fractures, were observed in the SFF group compared to the control group. Within the SFF group, a higher proportion of patients with osteonecrosis exhibited Garden type III and IV fractures compared to those with osteoporosis. Additionally, both magnetic resonance imaging (MRI) and pathological examinations demonstrated that osteonecrosis in the SFF group predominantly occurred at the lateral femoral head-neck junction.Osteonecrosis of the femoral head, particularly involving the lateral head-neck junction, was confirmed as a major risk factor for SFF. Furthermore, SFF exhibits internal heterogeneity based on its different causes.© 2023. The Author(s).