Reliability of 3D quantitative displacement of femoral neck fracture site measured via CT and its correlation with femoral head avascular necrosis: a retrospective cohort study

Background: Conventional two-dimensional (2D) imaging measurements of femoral neck fractures (FNFs) merely reflect angular displacement. We developed a novel measurement method for measuring the three-dimensional (3D) quantitative displacements of FNF site via CT. This study aimed to introduce this technique, assess its reliability, determine the correlation of 3D displacement with femoral head avascular necrosis (AVN), and develop a prediction model. Methods: A retrospective cohort study was conducted on 200 FNF patients who underwent closed reduction and internal fixation from January 2018 to January 2022 and were followed for at least 3 years. The vertical, horizontal, and separation displacements of the fracture site were measured using computed tomography (CT) 3D reconstruction technique. One attending orthopedic surgeon, one orthopedic resident, and one radiology resident independently performed measurements using both 2D and 3D imaging techniques on all 200 patients; the radiology resident repeated the measurements after a 3-month interval. Inter- and intra-observer reliability were assessed using intraclass correlation coefficients (ICCs). Bivariate correlation identified variables associated with AVN, followed by binary logistic regression to evaluate the relationship between displacement parameters and AVN. Patients were divided into training and validation sets (7:3 ratio) for development and validation of a prediction model. Results: The 3D measurement method showed higher ICCs [inter-observer: 0.963, 95% confidence interval (CI): 0.949-0.974; intra-observer: 0.962, 95% CI: 0.945-0.975] than the 2D measurement method [inter-observer: 0.723 (95% CI: 0.639-0.794); intra-observer: 0.776 (95% CI: 0.685-0.844)]. Bivariate correlation showed that both 2D and 3D displacements significantly correlated with AVN [all P