BackgroundThe “In–Out–In” (IOI) posterosuperior screw was common in screw fixations of femoral neck fractures. The impacts of the IOI screw on the blood supply of the femoral head have not yet been clarified. The nutrient foramen was damaged when the screw was present in their corresponding cortex surface. This study aimed to evaluate the damage degrees of the nutrient foramina in the femoral neck as the IOI posterosuperior screw was placed in different posterosuperior locations.MethodsOne hundred and eight unpaired dry human cadaveric proximal femurs were scanned by a three-dimensional scanner. Digital data obtained from the proximal femur surface were employed for subsequent analysis. All nutrient foramina in the femoral neck were identified and marked in each subject. A simulation of the anteroposterior, lateral, and axial views was then performed, and regions of interest (ROIs) for IOI posterosuperior screws, with 6.5 mm diameter, were determined in the posterosuperior femoral neck on the axial graphs. Nutrient foramina were counted and analyzed in ROIs and femoral neck, and its damage from the IOI posterosuperior screw was also calculated in different conditions of screw placement. Pairedt-tests were used for comparative analyses before and after damage.ResultsMost nutrient foramina were located in the subcapital region and the least in the basicervical region in the femoral neck, while the most were located in the transcervical and the least in the subcapital in the ROIs. In addition, most nutrient foramina in ROIs were located in the superior–posterior area of the femoral neck. There were four main locations of IOI posterosuperior screws where the decrease in the nutrient foramina was statistically significant (P< 0.01). The risk zone determined by these locations was located in a posterosuperior square of ROIs with an edge length of 9.75 mm.ConclusionTo minimize iatrogenic damage to the blood supply of the femoral head, screw positions could be assessed in anteroposterior and lateral radiographs using a risk zone. The IOI posterosuperior screw in ROIs can be applied to fix femoral neck fractures when feasible in clinical practice. This study could provide surgeons with more alternatives for screw placement in the posterosuperior femoral neck.