Posterior Pelvic Ring ORIF (Sacral and Crescent Fractures)

Surgical Indications

Absolute Indications for Open Posterior ORIF

• Irreducible posterior pelvic ring displacement greater than 1 cm after attempted closed reduction
• Comminuted sacral fractures with sacral dysmorphism that precludes safe percutaneous iliosacral screw placement
• Crescent fractures with intra-articular SI joint extension or significant displacement
• Spinopelvic dissociation (U-type, H-type, T-type sacral fractures) with or without neurologic deficit
• Open pelvic fractures with posterior ring involvement requiring debridement and stabilisation
• Associated lumbosacral plexus injury requiring direct exploration and decompression

Relative Indications

• Failed percutaneous iliosacral screw fixation with loss of reduction
• Delayed presentation with malreduction that cannot be corrected percutaneously
• Poor bone quality requiring augmented fixation (osteoporosis, metabolic bone disease)
• Concomitant acetabular fracture requiring extensile exposure

Contraindications

Absolute:

• Haemodynamic instability precluding prolonged surgery (damage-control orthopaedics first)
• Active infection at the surgical site
• Severe soft-tissue degloving (Morel-Lavallée) not yet debrided and stable

Relative:

• Patient factors precluding prone positioning (severe pulmonary injury, spinal instability)
• Anticipated poor wound healing (steroids, smoking, diabetes) — consider staged or percutaneous alternatives

Evidence for Operative Treatment

Timing and Damage-Control Principles

Early stabilisation within 72 hours reduces mortality and pulmonary complications in polytrauma patients. However, posterior pelvic approaches in the presence of degloving injuries carry high wound complication rates. Many centres now advocate a staged approach: anterior external fixation or percutaneous anterior ring fixation for resuscitation, serial debridement of Morel-Lavallée lesions, and definitive posterior ORIF once soft tissues permit (typically 5-14 days).

Fixation Constructs — Evidence Summary

Key Evidence

Posterior Pelvic Ring Anatomy Relevant to ORIF

Osseous Anatomy

The posterior pelvic ring comprises the sacrum (five fused vertebrae) and the posterior ilium. The SI joint is a true synovial joint anteriorly with a fibrous posterior component. The sacral promontory and ala form the anterior boundary; the posterior sacral surface is subcutaneous with thin overlying soft tissue.

Key bony landmarks:

• Posterior superior iliac spine (PSIS) — palpable and the starting point for many posterior approaches
• Posterior inferior iliac spine (PIIS) — inferior to PSIS, marks the superior border of the greater sciatic notch
• Sacral foramina (four pairs) — S1 and S2 most relevant; lie 2-3 cm medial to the SI joint
• Sacral ala — the lateral mass of the sacrum; the L5 nerve root crosses its anterior surface

Ligamentous Restraints

The posterior SI ligaments (interosseous, short posterior, long posterior) are the strongest stabilizers of the SI joint. In crescent fractures the posterior fragment remains attached to the sacrum via these ligaments, allowing indirect reduction through manipulation of the ilium.

Neurovascular Structures at Risk

L5 nerve root Exits the L5-S1 foramen and courses laterally across the sacral ala approximately 2 cm medial to the SI joint before entering the greater sciatic notch. It is vulnerable during anterior retraction of the iliacus or during screw placement across the sacral ala.

Sacral nerve roots (S1-S4) Exit through anterior and posterior sacral foramina. The S1 root supplies the majority of the sciatic nerve contribution; injury produces foot plantarflexion weakness and perineal sensory loss.

Superior Gluteal Neurovascular Bundle Exits the greater sciatic notch superior to the piriformis muscle, 2-3 cm inferior to the PSIS. It supplies the gluteus medius and minimus; laceration causes abductor weakness and can produce a gluteal compartment syndrome.

Sciatic nerve Lies anterior to the piriformis in the greater sciatic notch; excessive medial retraction or posterior displacement of fracture fragments can stretch or compress the nerve.

Soft-Tissue Envelope

The posterior sacral skin is thin (less than 1 cm in many patients) and lies directly on bone. The gluteus maximus originates from the posterior ilium and sacrum and must be elevated as a flap during the posterior approach. Morel-Lavallée lesions (closed degloving) are common in high-energy pelvic trauma and create large dead spaces prone to infection.

Positioning and Preparation

Patient position: Prone on a radiolucent table with chest rolls and padded prominences. The arms are abducted less than 90 degrees. A urinary catheter is placed. Bilateral hip and knee flexion (slight) relaxes the sciatic nerve. The entire posterior pelvis from the lumbar region to the gluteal crease is prepped and draped; both iliac crests must be accessible.

Anaesthesia: General endotracheal anaesthesia with muscle relaxation for reduction. Consider neuromonitoring (somatosensory and motor evoked potentials) for spinopelvic dissociation cases with neurologic deficit. Arterial line and large-bore venous access are routine.

Imaging setup: High-quality fluoroscopy with C-arm. Obtain anteroposterior, inlet, outlet, and lateral sacral views before incision. Obturator-oblique and iliac-oblique views are added for iliac screw placement. Consider intraoperative CT or navigation when available.

Consent discussion: Neurologic injury (L5 or sacral root, 3-8 percent), wound breakdown and infection (10-20 percent), hardware prominence requiring later removal (15-25 percent), nonunion or malunion (5-10 percent), and the need for staged procedures if soft tissues are compromised.

Posterior Approach to the Sacrum and SI Joint — Step-by-Step

Step 1: Incision and Soft-Tissue Handling

A longitudinal midline incision is made 2 cm lateral to the PSIS extending from the L4 spinous process to the level of the PIIS (approximately 12-15 cm). For bilateral exposure a single midline or two separate paramedian incisions may be used. The skin is incised sharply; subcutaneous fat is minimal. Identify and preserve any perforating vessels.

Safety note: In the presence of a Morel-Lavallée lesion the incision must be planned through viable skin; previous degloving planes are avoided.

Step 2: Exposure of the Posterior Ilium and Sacrum

The gluteus maximus is elevated from the iliac crest and sacrum in a lateral-to-medial direction. The muscle is reflected laterally to expose the posterior ilium and the SI joint. The erector spinae muscles are elevated from the sacral spinous processes and laminae. The posterior SI ligaments are identified but not divided unless necessary for reduction.

The greater sciatic notch is palpated; a malleable retractor is placed to protect the superior gluteal bundle and sciatic nerve. The L5 nerve root is identified on the sacral ala if anterior exposure is needed.

Step 3: Reduction of the Posterior Ring

For crescent fractures the posterior iliac fragment is controlled with a 5 mm Schanz pin placed in the PSIS as a joystick. Pointed reduction clamps are applied across the SI joint or fracture line. For sacral fractures, Schanz pins in the iliac wing and sacrum allow derotation and compression. Intraoperative fluoroscopy (inlet, outlet, lateral) confirms reduction before fixation.

For spinopelvic dissociation the lumbar spine is realigned with the pelvis using Schanz pins in L5 or the iliac wings; distraction or compression is applied under fluoroscopic guidance.

Step 4: Fixation Constructs

Tension-band plating A 3.5 mm or 4.5 mm reconstruction plate is contoured from the posterior ilium across the sacrum to the contralateral ilium. At least three screws on each side are required. The plate lies posterior to the SI joint and resists rotational forces.

Transiliac internal fixator (TIFI) Two iliac screws (7.0-8.0 mm diameter, 60-80 mm length) are placed in each ilium, connected by a transverse rod. The rod lies subcutaneously or subfascially. This construct is useful when soft-tissue cover is poor.

Lumbopelvic fixation Pedicle screws are placed in L5 (and L4 if additional points are needed) under fluoroscopic or navigated guidance. Iliac screws (7.5-8.5 mm, 70-90 mm) or S2-alar-iliac (S2AI) screws are placed in the ilium. The rods connect the lumbar screws to the iliac screws, restoring continuity between the spine and pelvis. Cross-connectors add rotational stability.

Supplementary iliosacral screws When a safe corridor exists on preoperative CT, 7.3 mm cannulated screws can be placed percutaneously or open across the SI joint into the S1 body. They are used to augment plate or lumbopelvic constructs.

Step 5: Wound Closure and Soft-Tissue Management

Meticulous haemostasis is obtained. Two deep drains are placed (one along each SI joint). The gluteus maximus is reattached to the iliac crest and sacral fascia with heavy absorbable sutures. Subcutaneous tissue is closed in layers; skin is closed with staples or nylon. A sterile negative-pressure dressing may be applied over the wound if soft-tissue swelling is significant.

Patients are log-rolled only for the first 48-72 hours; no direct pressure is placed on the posterior wound.

Post-operative Protocol

Immediate Post-operative (Day 0-7)

• ** mobilisation**: Log-roll only for first 48-72 hours; no direct pressure on posterior wound. Begin bed-to-chair transfers with assistance once drains removed.
• DVT prophylaxis: Low-molecular-weight heparin from 12 hours post-operatively; continue for 4-6 weeks or until fully mobile.
• Drains: Remove when output less than 30 mL per 24 hours (typically day 2-4).
• Wound care: Daily inspection; negative-pressure dressing if used remains for 5-7 days.
• Neurologic monitoring: Serial examination of L5 and S1 root function; any deterioration prompts urgent CT.

Weight-Bearing and Rehabilitation

• Non-weight-bearing or toe-touch weight-bearing on the affected side for 6-12 weeks depending on fracture pattern and construct rigidity.
• Lumbopelvic constructs allow earlier mobilisation (within 1-2 weeks) because of superior stability.
• Physical therapy: Begin gentle hip and knee range-of-motion exercises once wound stable. Progressive strengthening from 6 weeks. Core stabilisation and gait training from 8-12 weeks.
• Return to work: Sedentary work at 3-4 months; manual labour at 6-9 months.

Follow-up Schedule

• 2 weeks: wound check, radiographs (AP, inlet, outlet)
• 6 weeks: radiographs, assess for loss of reduction
• 3 months: CT to confirm union if radiographs equivocal
• 6-9 months: consider hardware removal if symptomatic prominence

Special Case: Spinopelvic Dissociation

Recognition and Classification

U-type, H-type and T-type sacral fractures disconnect the lumbar spine from the pelvis. Sagittal CT reformats show L5-S1 kyphosis and anterior sacral displacement. Neurologic deficit (bowel, bladder, sexual dysfunction, lower-limb weakness) is present in 60-80 percent of cases.

Fixation Principles

Lumbopelvic fixation with pedicle screws in L5 (and L4 if needed) connected to iliac or S2AI screws is the standard. At least four points of fixation (two lumbar, two iliac) are required. Supplementary iliosacral screws are added when safe corridors exist. Sacral decompression (laminectomy) is performed if neurologic deficit is present and imaging shows root compression.

Outcomes

Union rates exceed 90 percent with modern constructs. Neurologic recovery occurs in approximately 60 percent of patients with preoperative deficits. Hardware prominence requiring removal occurs in 15-25 percent. Patients should be counselled about the high likelihood of later hardware removal.