The 'safe zone' for extra-articular screw placement during intra-pelvic acetabular surgery

Intra-articular screw placement during acetabular surgery must be avoided. To date, no anatomic guidelines exist to prevent intra-articular screw placement in the treatment of complex acetabular fractures by the ilioinguinal or Stoppa approach.

Ninety-three pelvic computed tomography scans were analyzed and the femoral head diameter (FHD) and the intersacroiliac joint distance (ISIJD) were referenced as anthropometric parameters. A safe zone for screw placement avoiding the acetabulum for specific anatomic landmarks such as the ischial spine, sciatic notch, and obturator canal was defined and correlated to the FHD and ISIJD.

The mean FHD was 45.5 mm (standard deviation, 3.96). The mean ISIJD was 117.9 mm (standard deviation, 6.32). The correlation for the distance of the ischial spine in relation to the femoral head diameter was 0.59 (P = 0.0101) and for the ISIJD 0.274 (P = 0.0501). A safe zone of 28 mm from the ischial spine aiming laterally for a FHD greater than 47 mm and 23 mm for a FHD less than 47 mm could be identified. The safe zone for potential screw placement near the sciatic notch was 11 mm (correlation FHD 0.234, P = 0.0501; ISIJD 0.412, P = 0.0101). A safe zone of 5 mm in relative distance of the obturator canal could be determined for screws on the pubic ramus.

This study identifies a safe zone for screw placement in the treatment of acetabular fractures when operated by an anterior approach. These findings have the potential to prevent further injury to the acetabular and femoral cartilage as a result of the surgical trauma and minimize intraoperative irradiation time.