Posterior Approach to Sacroiliac Joint

What it exposes. The posterior approach gives direct access to the outer table of the ilium, the posterior SI joint capsule and the posterior sacrum, with the PSIS as the constant landmark. It is the workhorse exposure for SI joint disruption, sacral fractures with posterior-ring instability, and crescent fractures of the ilium. Why posterior (and not anterior). The anterior approach to the SI joint is now rarely used because of its high L5 nerve-root injury rate (reported around 5 to 15 percent). The posterior route keeps the L5 root safe because the nerve lies anterior to the sacral ala, roughly 1-2cm anterior to the SI joint — dissection that stays posterior to the joint never encounters it. The posterior approach and percutaneous screws achieve equivalent or better stability with far lower neurological morbidity. Primary indications.

• Acute SI joint disruption — vertical shear pelvic injuries (Young-Burgess APC-III, LC-III, VS patterns) and combined-mechanism injuries with posterior instability.
• Denis Zone I and II sacral fractures with SI joint involvement.
• Failed closed reduction requiring direct visualisation.
• Complex posterior pelvic ring injuries — crescent fractures of the ilium with SI joint extension, comminuted sacral fractures needing buttress plating, dysmorphic sacral anatomy precluding percutaneous fixation, and transiliac-transsacral injuries. Relative indications.
• Percutaneous screw failures — malpositioned screws requiring revision, loss of fixation in osteoporotic bone, inadequate reduction after a percutaneous attempt.
• Chronic SI joint instability — symptomatic malunion after nonoperative management, progressive deformity with neurologic symptoms, and failed conservative management beyond six months. Contraindications.
• Absolute — active infection overlying the proposed incision; severe soft tissue injury (for example a Morel-Lavallée lesion) precluding safe closure; medical comorbidities prohibiting surgery.
• Relative — obesity (BMI greater than 40, consider staged fixation); prior posterior pelvic surgery with scarring; neurogenic bowel requiring prone-positioning caution; osteoporosis requiring augmentation planning. Open vs percutaneous — the key decision. Examiners test understanding of when each technique applies: dysmorphic anatomy, the need for direct reduction, the quality of closed reduction, comminution, and surgeon experience all factor in. Percutaneous iliosacral screws suit simple disruptions with good anatomy and an adequate reduction; open posterior plating is reserved for dysmorphic sacra, comminution, reduction needs, osteoporotic bone, and revision cases.

The exposure is built around prone positioning on a radiolucent table with the PSIS as the landmark, verified fluoroscopy, and then a stepwise subperiosteal dissection to the posterior SI joint.

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Positioning. General anaesthesia with endotracheal intubation and a Foley catheter. The patient is placed prone on a radiolucent Jackson table or chest rolls, arms on arm boards at less than 90 degrees abduction. Hips are extended to relax the gluteal muscles, the pelvis is tilted to correct any rotational deformity, and leg length is equalised with a bump. Pad every pressure point — gel pads under the chest (avoid brachial plexus compression), forehead padding (avoid facial nerve compression), knee padding (avoid peroneal nerve compression), and feet off the end of the table (avoid compartment syndrome). Landmarks. Palpate the posterior superior iliac spine (PSIS) — the primary landmark — the iliac crest extending laterally from it, the sacral spinous processes as the midline reference, and the ischial tuberosity as the caudal landmark (rarely palpable prone). The SI joint lies 2-3cm medial to the PSIS and runs obliquely, about 25 degrees from the sagittal plane. Fluoroscopy — verify before incision. A large C-arm with pelvic capability must be able to deliver an AP pelvis, an inlet view (40 degrees caudal tilt), an outlet view (40 degrees cephalad tilt), and a lateral sacral view (difficult prone). Confirm all views are obtainable before draping; the inlet and outlet views are the safety net for every screw.

Fixation technique selection. The choice between percutaneous screws and open posterior plating drives morbidity, time, and biomechanics.

Structures at risk, by layer

Extensile options. Extend the incision inferiorly across the iliac crest for transiliac plating in bilateral instability, and escalate to lumbopelvic (triangular) fixation for spinopelvic dissociation and vertically unstable transforaminal sacral fractures. Closure is as described above — interrupted mattress skin sutures over the high-tension crest, meticulous fascial closure, and a drain for extensive dissection. Complications and their prevention.

• Intraoperative — neurovascular injury (less than 2 percent): L5 root injury is rare because the root is anterior; sacral nerve injury comes from screw malposition into the foramina (prevent with fluoroscopy and anatomic knowledge, recognise with neuromonitoring); superior gluteal vessel injury follows excessive inferior dissection (limit to the superior border of the notch). Loss of reduction and hardware malposition (screw into canal, foramina, or anterior to sacrum) are prevented by maintaining reduction forceps and using multiple fluoroscopic views.
• Early (within 1 month) — wound complications in 2-5 percent (superficial dehiscence is most common; risk factors obesity, smoking, diabetes, tension — prevent with meticulous tension-free closure; deep infection less than 1 percent needs irrigation and debridement; seroma/haematoma benefits from a drain). Neurologic: expected cluneal numbness, transient sciatic symptoms from positioning, permanent deficit rare (less than 1 percent). Thromboembolic events are elevated in pelvic trauma — chemical and mechanical prophylaxis are mandatory.
• Late (beyond 1 month) — hardware loosening or breakage, prominent hardware over the PSIS (may need removal), SI joint arthritis from residual instability or malunion, and chronic pain syndromes. Malunion occurs in 5-10 percent from inadequate reduction; nonunion in less than 5 percent with adequate fixation.

• Percutaneous iliosacral screw fixation (S1 and S2 corridors) — the workhorse for reducible disruptions with favourable sacral anatomy, performed through a stab lateral to the main wound.
• Open posterior SI plating and transiliac tension-band plating — for dysmorphic anatomy, comminution, osteoporosis, and revision cases.
• Triangular (lumbopelvic) osteosynthesis — for vertically unstable transforaminal sacral fractures and spinopelvic dissociation.
• Open reduction of sacral fractures (Denis Zones I-III) needing buttress plating, and crescent fractures of the ilium with SI joint extension.
• Drainage or debridement of posterior pelvic infections and haematomas, and removal of prominent iliac-crest hardware.

Epidemiology. Unstable posterior pelvic ring injuries arise almost exclusively from high-energy mechanisms — road traffic collisions, falls from height, and crush or industrial injuries — and concentrate in young men and, increasingly, in elderly patients with low-energy fragility fractures of the sacrum. Associated injuries are present in roughly 80 percent of operative cases; haemorrhage from the posterior pelvic venous plexus and superior gluteal vessels is a leading cause of early mortality, so haemodynamic control precedes definitive fixation.

Registry and outcome signals. Across series, image-guided percutaneous iliosacral screw fixation is first-line for reducible disruptions with favourable osseous corridors; open posterior plating or triangular/lumbopelvic constructs are reserved for dysmorphic anatomy, comminution, or fixation that cannot be reliably achieved percutaneously. The anterior approach to the SI joint is now rarely used because of its high L5 nerve-root injury rate (reported around 5 to 15 percent); the posterior approach and percutaneous screws achieve equivalent or better stability with lower neurological morbidity. Global practice variation. In high-resource settings, CT-based planning, intraoperative 3D imaging or navigation, and trans-sacral screws are increasingly standard. In resource-limited settings without reliable fluoroscopy or navigation, open posterior reduction and plating remains a safe default because it permits fixation under direct vision without dependence on advanced imaging. Antibiotic prophylaxis follows local antimicrobial-stewardship guidance (typically a first-generation cephalosporin such as cefazolin at induction, re-dosed for long procedures); thromboprophylaxis is mandatory given the high venous thromboembolism risk of pelvic trauma.