Iliosacral (Sacroiliac) Screw Fixation

Surgical Indications

Absolute Indications

• Vertically unstable posterior pelvic ring injury (Tile C, Young-Burgess APC III or LC III) with greater than 1 cm displacement of the hemipelvis
• Sacroiliac joint dislocation with greater than 1 cm displacement after closed reduction
• Unstable sacral fracture (Denis zone I or II) with greater than 1 cm displacement or comminution that precludes non-operative management
• Bilateral posterior-ring instability or sacral insufficiency fracture with spinopelvic dissociation

Relative Indications

• Rotationally unstable posterior-ring injury (Tile B, APC II, LC II) in a polytraumatised patient when early mobilisation is required
• Sacral fracture or SI joint injury with associated neurological deficit that may benefit from decompression and stabilisation
• Elderly patient with osteoporotic sacral insufficiency fracture causing intractable pain and inability to mobilise

Contraindications

Absolute:

• Active infection at the planned screw entry site or within the pelvis
• Inadequate fluoroscopic visualisation (obesity, bowel gas, spinal hardware) without access to navigation or CT guidance
• Patient unable to tolerate prone or lateral positioning due to associated injuries

Relative:

• Dysmorphic sacrum with inadequate S1 or S2 corridor (less than 10 mm width) — consider trans-sacral or alternative fixation
• Severe comminution of the sacral body that precludes reliable screw purchase
• Open pelvic fracture with contaminated wound at the planned percutaneous entry site

Evidence for Percutaneous Iliosacral Fixation

Biomechanical and Clinical Outcomes

• Percutaneous iliosacral screw fixation provides immediate biomechanical stability equivalent to open posterior approaches when the screw is placed accurately within the osseous corridor
• Closed reduction and percutaneous fixation reduces surgical time, blood loss and wound complications compared with open posterior approaches in the acute trauma setting
• Navigation-assisted or 3D-fluoroscopy-guided placement improves screw accuracy and reduces radiation exposure to the surgeon compared with conventional 2D fluoroscopy alone

Trans-sacral versus Unilateral Iliosacral Screws

• Trans-sacral screws crossing the midline into the contralateral ilium provide superior torsional and axial stability for bilateral posterior-ring injuries and sacral insufficiency fractures
• A single trans-sacral S1 screw combined with a unilateral S2 screw offers equivalent stability to two unilateral iliosacral screws in biomechanical models
• Clinical series report lower rates of screw loosening and loss of reduction when trans-sacral constructs are used in osteoporotic or bilateral injuries

Supplementary Anterior Fixation

• Rotationally unstable injuries (APC II-III) require anterior-ring stabilisation in addition to posterior screws; isolated posterior fixation allows late rotational displacement
• Symphysis pubis plating or external fixation combined with accurate iliosacral screws restores pelvic-ring stability and permits earlier mobilisation than posterior screws alone

Osseous Corridors of the Sacrum

S1 Osseous Corridor

• The S1 corridor is the primary and largest safe passage for iliosacral screws
• It runs from the outer ilium through the sacroiliac joint, across the sacral ala and into the S1 vertebral body
• On the true lateral view the corridor is bounded anteriorly by the anterior cortex of the sacral ala and posteriorly by the sacral canal
• The corridor is widest at the level of the superior S1 endplate and narrows inferiorly toward the S1 foramen

S2 Osseous Corridor

• The S2 corridor is smaller and more inferior than S1
• It is used when the S1 corridor is inadequate (dysmorphic sacrum) or when additional fixation is required
• On the outlet view the screw must remain superior to the S2 foramen and inferior to the S1 endplate
• S2 screws are particularly useful in bilateral constructs or when combined with a trans-sacral S1 screw

Dysmorphic Upper Sacrum

• Present in up to 30 percent of the population
• Key radiographic features on the true lateral view:
  • The anterior cortex of the sacral ala lies posterior to the sacral promontory
  • The superior S1 endplate is steeply inclined (greater than 30 degrees)
  • The S1 foramen appears anterior to the promontory on the inlet view
• In dysmorphic sacra the safe S1 corridor may be less than 10 mm wide and is often unsafe for a standard iliosacral screw
• Recognition on the true lateral view before guide-wire insertion is mandatory

Neurovascular Structures at Risk

L5 Nerve Root

• Exits the L5-S1 foramen and courses anterior to the sacral ala immediately in front of the sacroiliac joint
• Most vulnerable structure during anterior cortical breach of an S1 screw
• Injury produces ipsilateral foot-drop, weakness of ankle dorsiflexion and numbness in the L5 dermatome

S1 Nerve Root

• Travels within the S1 foramen and exits anteriorly
• At risk from medial or inferior screw breach into the S1 foramen
• Injury produces ipsilateral plantar-flexion weakness, loss of ankle jerk and sensory loss on the sole of the foot

Cauda Equina

• Lies within the sacral canal medial to the S1 and S2 bodies
• At risk from screws that cross the midline or breach the sacral canal
• Bilateral injury produces saddle anaesthesia, bowel and bladder dysfunction and bilateral leg weakness

Iliac Vessels

• Common iliac vein and artery lie immediately anterior to the sacroiliac joint
• Anterior cortical penetration of the ilium or sacrum can cause life-threatening retroperitoneal haemorrhage
• The superior gluteal artery exits the greater sciatic notch and is at risk from an inferior trajectory

Greater Sciatic Notch Contents

• Sciatic nerve, superior and inferior gluteal neurovascular bundles
• Inferior screw trajectory risks direct injury to these structures

Positioning and Preparation

Patient position: Prone on a radiolucent table with the pelvis centred over the table break. The ipsilateral hip is slightly flexed to relax the iliopsoas and improve access to the posterior ilium. Arms are abducted 90 degrees. A radiolucent headrest and chest rolls are used. The lower limbs are draped free to allow closed reduction manoeuvres.

Anaesthesia: General endotracheal anaesthesia with muscle relaxation to facilitate closed reduction. Arterial line and large-bore intravenous access are recommended in polytrauma patients. Cell-saver should be available.

Fluoroscopic setup: A large C-arm is positioned on the contralateral side of the table. The surgeon must be able to obtain true lateral, inlet and outlet views without moving the patient. The C-arm is draped sterilely. Navigation or 3D fluoroscopy is used when available, particularly in dysmorphic or obese patients.

Reduction: Closed reduction is performed before skin preparation. Traction through the ipsilateral lower limb, manual compression of the iliac crest and percutaneous Schanz-pin manipulation of the hemipelvis are used to restore length, rotation and translation. An external fixator or temporary skeletal traction is applied to maintain reduction during screw insertion.

Consent: Counsel regarding risk of nerve-root injury (L5 or S1, 1-5 percent), vascular injury (less than 1 percent), screw malposition requiring revision (3-8 percent), loss of reduction, infection, wound complications and the need for supplementary anterior fixation.

Percutaneous Iliosacral Screw Technique — Step-by-Step

Step 1: Obtain the Three Critical Fluoroscopic Views

True lateral view:

• The C-arm is rotated until the two greater sciatic notches overlap perfectly and the anterior cortex of the S1 ala lies posterior to the sacral promontory.
• This view defines the anterior and posterior limits of the S1 corridor and identifies dysmorphic anatomy.
• If the anterior ala lies anterior to the promontory the sacrum is dysmorphic and the S1 corridor is unsafe.

Inlet view:

• The C-arm is tilted cephalad approximately 30-40 degrees until the S1 endplate appears as a single line and the pubic symphysis is superimposed on the S2 body.
• This view confirms that the screw remains posterior to the anterior sacral cortex and lateral to the midline sacral canal.

Outlet view:

• The C-arm is tilted caudad approximately 30-40 degrees until the S1 and S2 foramina appear as clear circles and the superior S1 endplate is seen in profile.
• This view confirms that the screw remains superior to the S1 foramen and inferior to the superior endplate.

Step 2: Mark the Entry Point and Incision

On the true lateral view the ideal entry point on the outer ilium lies at the intersection of a line drawn from the anterior cortex of the S1 body to the posterior ilium and a line from the superior S1 endplate to the inferior aspect of the greater sciatic notch.

A 2-3 cm longitudinal incision is made over the posterior ilium at this point. The gluteal fascia is incised and a blunt trocar is passed through the gluteus maximus to the outer cortex of the ilium.

Step 3: Insert the Guide-Wire

A 3.2 mm guide-wire is advanced through the trocar under fluoroscopic control on all three views.

True lateral: The wire must remain within the corridor bounded by the anterior cortex of the ala and the sacral canal. The tip stops at or just short of the anterior cortex of the S1 body.

Inlet: The wire must remain posterior to the anterior sacral cortex and lateral to the midline.

Outlet: The wire must remain superior to the S1 foramen and inferior to the superior endplate.

The wire is advanced until it reaches the contralateral sacral body or ilium (for trans-sacral screws). Position is confirmed on all three views before proceeding.

Step 4: Measure, Drill and Insert the Screw

Screw length is measured with a depth gauge. A washer (minimum 10-12 mm diameter) is mandatory on every screw.

A 6.5 mm or 7.3 mm cannulated partially-threaded screw is inserted over the guide-wire. The screw is advanced until the washer compresses against the outer ilium and the threads engage the contralateral sacral body or ilium.

Final position is confirmed on all three views. The screw tip must lie within the contralateral sacral body or ilium without breaching the anterior cortex.

For bilateral injuries or insufficiency fractures a second screw (S2 or trans-sacral) is placed using the same technique.

Step 5: Supplementary Anterior Fixation

If the injury is rotationally unstable (APC II-III or LC III) an anterior-ring procedure (symphysis pubis plating or external fixator) is performed after the posterior screws are in place.

Step 6: Wound Closure and Dressing

The small percutaneous incisions are closed with absorbable sutures. A sterile dressing is applied. No drain is required.

Post-operative Protocol

Early Phase (Day 0-14)

• Mobilisation: Touchdown weight-bearing (10-15 kg) on the affected side from day 1 if reduction is anatomic and fixation is stable
• DVT prophylaxis: Low-molecular-weight heparin or oral anticoagulant for 6 weeks
• Analgesia: Multimodal regimen including paracetamol, NSAIDs (if renal function permits) and short course of opioids
• Wound care: Daily inspection of percutaneous incisions; remove dressings at 48 hours
• Imaging: Post-operative radiographs (AP, inlet, outlet) and CT scan to confirm screw position and reduction quality

Intermediate Phase (2-8 weeks)

• Weight-bearing: Continue touchdown weight-bearing until 6-8 weeks
• Physiotherapy: Gentle range-of-motion exercises for hips and spine; no active hip abduction against resistance
• Follow-up: Clinical review at 2 and 6 weeks with radiographs

Late Phase (8-12 weeks)

• Weight-bearing progression: Advance to partial weight-bearing at 8 weeks if bridging callus is visible; full weight-bearing by 12 weeks
• Return to work: Light duties at 3 months; heavy manual work or sport at 6 months
• Long-term surveillance: Annual radiographs for 2 years to monitor for late displacement or screw failure

Special Case: Trans-sacral Screw Fixation

Indications

• Bilateral posterior-ring injuries
• Sacral insufficiency fractures with spinopelvic dissociation
• Osteoporotic patients with poor bone quality
• Revision cases with failed unilateral fixation

Technique Modifications

• The guide-wire must cross the midline and engage the contralateral ilium
• Inlet and outlet views must be perfectly symmetric to ensure the wire does not breach the sacral canal
• A single trans-sacral S1 screw combined with a unilateral S2 screw provides excellent stability
• Washers are used on both ends of the trans-sacral screw

Outcomes

• Higher biomechanical stability than unilateral constructs
• Lower rate of screw loosening in osteoporotic bone
• Slightly higher technical demand and radiation exposure during insertion

Special Case: Dysmorphic Sacrum

Recognition

• True lateral view shows anterior cortex of ala posterior to sacral promontory
• Steeply inclined S1 endplate (greater than 30 degrees)
• S1 foramen appears anterior to promontory on inlet view

Management

• Abandon S1 corridor
• Use S2 corridor or trans-sacral construct
• Consider navigation or intra-operative CT to confirm safe trajectory
• May require open posterior approach with direct visualisation if percutaneous corridors are inadequate

Special Case: Sacral Insufficiency Fractures

Patient Population

• Elderly, osteoporotic patients with low-energy falls or atraumatic pain
• Often bilateral or H-shaped (spinopelvic dissociation)

Fixation Strategy

• Trans-sacral S1 screw (or two trans-sacral screws) provides immediate stability
• Percutaneous technique minimises surgical morbidity in frail patients
• Early mobilisation with protected weight-bearing reduces complications of prolonged recumbency