Overview
This technique was described by Routt et al., who also reported its outcomes and complications and studied the anatomic and radiographic variations of the upper sacrum that affect screw placement. Their work is essential reading before performing it.
Relevant Anatomy: The Safe Zone
The sacral alar slope is the inclined anterosuperior surface of the sacral ala, running from proximal-posterior to distal-anterior. The L5 nerve root and the iliac vessels lie directly anterior to the ala in this region.
Boundaries of the safe zone into the S1 body:
Superior: the cortex of the alar slope
Anterior: the anterior surface of the vertebral body
Posterior and inferior: the S1 nerve root foramen
Iliac cortical density (ICD): The anterior cortical thickening of the iliac side of the SI joint. On an accurate lateral sacral view, it is used to estimate the alar slope.
In 94% of nondysplastic upper sacral segments, the ICD coincides with the alar slope on CT, so it is a useful marker of the anterior border of the safe zone.
Sacral dysplasia (dysmorphism): The alar slope may be steeper, narrowing the safe zone. Routt et al. found dysplasia in 28 of 80 patients with pelvic fractures. A recessed sacral ala relative to the dense iliac bone is best shown on CT.
Pitfall: Screws can look intraosseous on inlet and outlet views and still be extraosseous on CT, with L5 root injury. The lateral sacral view and the ICD must be used.
Sacroiliac vs Transsacral–Transiliac Screws
Sacroiliac screw | Transsacral–transiliac screw | |
|---|---|---|
Trajectory | Usually perpendicular to the SI joint | Parallel to the foramina and to the anterior vertebral cortex |
Corridor | Ipsilateral S1 body | Must cross the whole sacrum without neural compromise |
Imaging Requirements
The surgeon must know the variations of upper sacral anatomy, and imaging, including the lateral sacral view, must be excellent.
Average C-arm tilt from vertical (Graves and Routt): inlet view 25°, outlet view 42°.
Routt et al.: The posterior pelvis must be accurately reduced so that the greater sciatic notches and both ICDs superimpose on the lateral image. With this as a requirement, the ICD as the anterior marker, and awareness of anterior sacral recession, there were no screw placement errors in 51 consecutive patients.
The Starr–Reinert reduction frame allows directed forces to be applied, and may allow substantial percutaneous reduction with the patient supine.
Surgical Technique (Technique 61.10)
Planning and Positioning
Preoperative CT review is essential: Measure the inlet and outlet angles and study the sacral anatomy with the patient supine.
Position the patient supine on a radiolucent table, with a soft support under the lumbosacral spine to lift the patient off the table.
Place the C-arm on the side opposite the injured hemipelvis.
Obtain AP, inlet, outlet and lateral sacral views to confirm adequate visualization. Mark the inlet and outlet C-arm positions so you can switch between them quickly.
Reduction
Reduce the posterior pelvis first. Useful aids:
Traction
Schanz screws in the iliac wings
An anterior external fixator
Prior anterior internal fixation
Starting Point and Trajectory
On the lateral sacral view, identify the anterior and posterior borders of the S1 vertebral body. The exact starting point depends on the number of screws planned, the injury type, and the patient's anatomy:
Sacral fracture, or isolated anterior widening of the SI joint: A transverse screw is best.
Combined anterior and posterior SI joint injury: A screw directed posterior-to-anterior and inferior-to-superior, perpendicular to the joint, is best.
Mark the entry on the skin and make a 1 cm stab incision oriented along the sacral long axis.
If a cannulated guide is used, advance it onto the lateral ilium.
On the lateral view, place the tip of the guide at the ideal starting point and tap it in with a mallet so it does not slip.
Adjust the trajectory on inlet and outlet views so that the wire will enter the S1 segment safely.
Guidewire and Screw
Advance the guidewire, confirming a safe path on both inlet and outlet views.
On the lateral sacral view, the wire should lie inside the S1 body, below and behind the ICD.
In a severely dysmorphic pelvis, the wire tip may lie just superior to the ICD on this view. Checking the lateral sacral view as the wire crosses the ala helps confirm safe passage in these patients.
Measure the screw length.
Insert the screw over the wire, checking its position on inlet and outlet views.
Confirm the final screw position on AP, inlet and outlet views.
Postoperative Care
When comfort allows, start walking with crutches or a walker, with touch-down weight bearing on the injured side.
Highly unstable bilateral injuries may need bed-to-chair transfers and a wheelchair.
Weight bearing usually begins 6–12 weeks after injury, depending on fracture pattern and bone quality.
Start VTE chemoprophylaxis immediately after injury unless contraindicated, and resume it immediately after fixation. Aspirin 81 mg twice daily was equivalent to LMWH in the PREVENT CLOT trial.
Reference
Beebe MJ. Fractures of the acetabulum and pelvis. In: Azar FM, Sawyer JR, Throckmorton TW, eds. Campbell's Operative Orthopaedics. 15th ed. Philadelphia: Elsevier; 2026. Chapter 61, Technique 61.10.