Three-Dimensional Computed Tomography-Based Mapping of the Intraosseous Safe Zone for Acetabular Cup Screw Fixation: A Retrospective, Single-Center Cohort Study.

Background: Conventional safe zones are defined by the proximity of acetabular cup screws to intrapelvic vessels; however, this relationship varies among individuals. This study aimed to define the intraosseous safe zone in which the screws remain entirely within bone without penetrating the medial wall. Methods: Three-dimensional computed tomography data for 30 hips of female patients with normal acetabular morphology were analyzed. A hemispherical cup was positioned virtually, and screw trajectories were evaluated in 24 directions using a clock-face coordinate system referenced to the ipsilateral anterior superior iliac spine. Bone thickness was measured in each direction, and intraosseous safe zones were defined for screw lengths of 15, 20, 25, 30, 40, and 50 mm. Heatmaps were generated to visualize the distribution of safe regions, and the angular range in which screws remained intraosseous in all cases was determined. Results: The median patient age was 50.1 years. The intraosseous safe zone was consistently located in the posterosuperior region for all screw lengths. The extent of the safe zone significantly decreased as the screw length increased (p < 0.01), whereas shorter screws expanded the safe zone toward the posteroinferior region. The angular range of the intraosseous safe zone was consistently the greatest at the 11 and 11:30 o'clock positions for 15, 20, and 25 mm screws. Conclusions: The findings provide a useful framework for preoperative planning and conceptualizing safe screw trajectories in total hip arthroplasty. Clinical Relevance: Directing screws toward the posterosuperior acetabulum, particularly around the 11 o'clock region, may enhance safety by maintaining intraosseous fixation and avoiding intrapelvic neurovascular injury. Shorter screws may expand safe fixation options toward the posteroinferior region.