Using finely reconstructed helical pelvis CT scans of ninety-three cases and image analysis software, we define the “Safe Zone” for the extra-articular placement of screws during internal fixation of the acetabulum, using a Stoppa approach. Screws should be at most: 11mm from the top of the Sciatic notch, 23mm from the tip of the Ischial Spine, and at most 5mm posterior to the top of the Obturator canal, along the pelvic brim.

The purpose of this study was to identify a “safe zone” in the inner pelvis, to allow extra-articular screw placement using the Stoppa approach.

Acetabulum internal fixation screws can safely (extra-articular position) be placed through the Stoppa approach using three identifiable landmarks.

Surgeons can use these identifiable anatomic landmarks for the safe placement of screws along the inner aspect of the acetabulum.

Study Population: males:females 47%:53%, mean age: 51,3yrs (18–88). Reference measurements (means): Femoral Head (FH): 45,5mm (36–6), Inter-SI joint:177,9mm (102–34). Safe distance to joint: 1) from Sciatic notch: 11mm; 2) from Ischial Spine: 23mm; 3) from Obturator roof: 5mm. The Ischial Spine Distance (ISD) showed clustering (p< 0.05) into two groups according to Femoral Head diameter: FH< 47mm: Safe ISD=23mm ; FH≥47mm: Safe IS=28mm.

Ninety-three Helical Pelvis CT scans with fine reconstruction were done between July 1, 1999-June 30, 2000. Axial images were analyzed using GE Vox Tool® v.3.0.3 image analysis software. The femoral head diameter and the Inter-SI joint distance were used as reference. The distance between three identifiable bony landmarks and the point which would allow the placement of a 4mm screw outside the hip joint were measured. Inter and Intra-observer reliability study showed a difference < 1mm in > 90% of cases.

Surgical approaches which avoid extensive dissection and manipulation of the gluteal musculature are gaining in popularity. The Stoppa is such an approach which gives access to the medial acetabular wall and to the inner pelvis from the SI joint to the symphysis along the pelvic brim. This blind approach does not allow visualisation of the joint and confirmation of screw placement. The present paper offers surgeons these reference points.