Percutaneous fixation of posterior pelvic ring injuries via S1/S2 osseous corridors · advanced
- True lateral, inlet and outlet fluoroscopic views are mandatory to define the S1 and S2 osseous corridors; any screw trajectory outside these corridors risks L5 or S1 nerve-root injury, cauda equina compression or iliac-vessel laceration.
- The dysmorphic upper sacrum (present in up to 30 percent of patients) narrows the safe S1 corridor dramatically; the anterior cortex of the ala lies posterior to the sacral promontory on the true lateral view and the superior S1 endplate is steeply angled, so standard corridors do not apply.
- A washer is mandatory on every iliosacral screw to increase the surface area of purchase on the outer ilium and to stop the screw head sinking through the thin iliac cortex under load.
- Trans-sacral screws (crossing the midline into the contralateral ilium) provide superior biomechanical stability for bilateral posterior-ring injuries or sacral insufficiency fractures, but require a perfectly symmetric corridor on inlet and outlet views.
When & Why
Indication. Percutaneous iliosacral screw fixation stabilises the posterior pelvic ring — sacroiliac joint disruptions and sacral fractures — when the injury is unstable. The operation is indicated for: - Vertically unstable posterior-ring injuries (Tile C, Young-Burgess APC III or LC III) with greater than 1 cm displacement of the hemipelvis
- Sacral fractures (Denis zone I or II) or SI joint dislocations with greater than 1 cm displacement after closed reduction, or comminution that precludes non-operative care
- Bilateral posterior-ring instability or a sacral insufficiency fracture with spinopelvic dissociation It is used selectively for rotationally unstable injuries (Tile B, APC II, LC II) in polytrauma when early mobilisation matters, for injuries with a neurological deficit that may benefit from decompression and stabilisation, and for the elderly patient with an osteoporotic insufficiency fracture who cannot mobilise because of pain. Contraindications. Active infection at the entry site or within the pelvis; inadequate fluoroscopic visualisation (obesity, bowel gas, spinal hardware) without access to navigation or CT; and a patient who cannot tolerate prone or lateral positioning. A dysmorphic sacrum with an S1 or S2 corridor less than 10 mm wide, severe comminution of the sacral body, and a contaminated open wound at the entry site are relative contraindications that push the plan toward a trans-sacral construct, alternative fixation, or an open approach. The construct you choose. Every plan begins with the same principle — an anatomic reduction, then a screw held entirely within the safe osseous corridor. What differs is the construct:
The standard for a unilateral posterior-ring injury. One 7.3 mm cannulated partially-threaded screw across the SI joint into the S1 body, washer mandatory. Gives immediate stability equivalent to open posterior fixation when placed accurately within the corridor.
Crosses the midline into the contralateral ilium. Superior torsional and axial stability for bilateral injuries, sacral insufficiency fractures and osteoporotic bone. Requires a perfectly symmetric corridor on inlet and outlet views.
Required for rotationally unstable injuries (APC II-III). Symphysis plating or an external fixator restores the anterior tension band; isolated posterior screws do not control the anterior ring and allow late rotational displacement.
Consent specifically for 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 and wound complications, and the possible need for supplementary anterior fixation or conversion to an open approach. Setup. Prone on a radiolucent table with the pelvis over the table break; the ipsilateral hip is slightly flexed to relax iliopsoas; arms are abducted 90 degrees and the lower limbs are draped free to allow closed-reduction manoeuvres. General anaesthesia with muscle relaxation, an arterial line and large-bore access in polytrauma, and a cell-saver available. A large C-arm sits on the contralateral side and must deliver true lateral, inlet and outlet views without moving the patient; navigation or 3D fluoroscopy is used where available, particularly in the obese or dysmorphic.
The Operation
The goal is to restore the length and alignment of the posterior pelvic ring by closed means, then stabilise it with one or more cannulated screws placed percutaneously and held entirely within the safe S1 (or S2) osseous corridor under three fluoroscopic views. The exposure is minimal — a 2-3 cm stab over the posterior ilium — but the visualization (the three views) is the whole operation and is laid out in full below.
Operative sequence
- Prone, radiolucent table, pelvis over the table break, ipsilateral hip slightly flexed, arms abducted 90 degrees, lower limbs free.
- Large C-arm on the contralateral side, draped sterilely; confirm you can obtain true lateral, inlet and outlet views without moving the patient. Use navigation or 3D fluoroscopy where available.
- Reduce before you scrub: longitudinal traction through the ipsilateral limb, manual compression of the iliac crest, and percutaneous Schanz-pin manipulation of the hemipelvis restore length, rotation and translation; hold the reduction with an external fixator or temporary skeletal traction.
This is the decisive step — never insert a guide-wire until all three views are perfect and the entire corridor is visible on each.
- True lateral: rotate the C-arm until the two greater sciatic notches overlap perfectly; the anterior cortex of the S1 ala must lie posterior to the sacral promontory. This view defines the anterior and posterior limits of the S1 corridor and reveals a dysmorphic sacrum.
- Inlet: tilt cephalad about 30-40 degrees until the S1 endplate is a single line and the pubic symphysis is superimposed on the S2 body. Confirms the wire stays posterior to the anterior sacral cortex and lateral to the midline canal.
- Outlet: tilt caudad about 30-40 degrees until the S1 and S2 foramina are clear circles and the superior S1 endplate is in profile. Confirms the wire stays superior to the S1 foramen and inferior to the superior endplate.
- On the true lateral view the ideal entry point on the outer ilium lies where a line drawn from the anterior cortex of the S1 body to the posterior ilium crosses a line from the superior S1 endplate to the inferior aspect of the greater sciatic notch.
- Make a 2-3 cm longitudinal incision over the posterior ilium; incise the gluteal fascia and pass a blunt trocar through gluteus maximus to the outer cortex of the ilium.
- Advance a 3.2 mm guide-wire through the trocar under fluoroscopy on all three views, in 5 mm increments, rechecking every view after each advance.
- True lateral: the wire stays between 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 stays posterior to the anterior sacral cortex and lateral to the midline.
- Outlet: the wire stays superior to the S1 foramen and inferior to the superior endplate.
- For a trans-sacral screw, advance until the tip engages the contralateral ilium.
- Measure length with a depth gauge. A washer (minimum 10-12 mm diameter) is mandatory on every screw.
- Insert a 6.5 mm or 7.3 mm cannulated partially-threaded screw over the wire until the washer compresses against the outer ilium and the threads engage the contralateral sacral body or ilium.
- Confirm final position on all three views — the tip must lie within the contralateral sacral body or ilium without breaching the anterior cortex.
- For bilateral injuries or sacral insufficiency fractures, place a second screw — an S2 screw or a trans-sacral S1 screw crossing the midline into the contralateral ilium (washers on both iliac cortices).
- A single trans-sacral S1 screw combined with a unilateral S2 screw gives excellent stability and is the preferred construct in osteoporotic or bilateral disease.
- If the injury is rotationally unstable (APC II-III, LC III), reposition supine and stabilise the anterior ring with symphysis pubis plating or an external fixator. Isolated posterior screws do not control the anterior ring.
- Close the small percutaneous incisions with absorbable sutures and apply a sterile dressing. No drain is required.
Before any guide-wire, confirm on the true lateral view that the anterior cortex of the S1 ala lies posterior to the sacral promontory. If the ala lies anterior to the promontory the sacrum is dysmorphic (up to 30 percent of patients), the safe S1 corridor may be less than 10 mm wide, and a standard S1 screw will enter the L5-S1 disc or strike the L5 root. Abandon the S1 corridor and plan an S2 or trans-sacral construct; use navigation or intra-operative CT, and convert to an open posterior approach with direct visualisation if percutaneous corridors remain inadequate.
Obtain the true lateral view and confirm the anterior ala lies posterior to the promontory before marking the skin. If it lies anterior, the sacrum is dysmorphic — plan an S2 or trans-sacral construct instead. Never insert a guide-wire until all three views are perfect and the entire corridor is visible on each.
After each 5 mm advance, recheck all three views. A wire that breaches the anterior cortex on the true lateral injures the L5 root within seconds; medial drift on the inlet threatens the cauda equina and S1 root; inferior drift on the outlet enters the S1 foramen. If the wire deviates even slightly on any view, withdraw and restart.
The outer iliac table is only 2-4 mm thick at the entry point. Without a large washer (minimum 10-12 mm) the screw head sinks through the cortex under load, fixation is lost and the posterior ring displaces late. A washer on both iliac cortices is used for a trans-sacral screw.
Aftercare & Complications
Rehabilitation | Phase | Timing | Weight-bearing | Therapy and notes | |-------|--------|----------------|-------------------| | 1 | Day 0-14 | Touchdown (10-15 kg) from day 1 if reduction is anatomic and fixation stable | DVT prophylaxis for 6 weeks; multimodal analgesia; wound inspection with dressings down at 48 hours; post-operative radiographs (AP, inlet, outlet) and CT to confirm screw position | | 2 | 2-8 weeks | Continue touchdown weight-bearing | Gentle hip and spine range of motion; no resisted hip abduction; clinical review at 2 and 6 weeks with radiographs | | 3 | 8-12 weeks | Partial at 8 weeks when bridging callus appears; full by 12 weeks | Progressive mobilisation; light duties at 3 months | | Long term | to 2 years | Full | Heavy manual work or sport at 6 months; annual radiographs for 2 years to monitor for late displacement or screw failure | Most patients return to light work by 3 months and heavy manual work or sport by 6 months. When the reduction is anatomic and the fixation stable, touchdown weight-bearing from day 1 protects the construct while the patient mobilises; weight-bearing is advanced only when bridging callus is seen on radiographs. Complications
- Incidence
- 1-5 percent (higher with anterior breach)
- Recognition
- Immediate ipsilateral foot-drop, weak ankle dorsiflexion, numbness in the L5 dermatome
- Prevention and management
- Prevention: the true lateral view shows the tip at or short of the anterior S1 cortex — never advance beyond it. Management: immediate screw removal if recognised intra-operatively; CT then exploration and nerve-root decompression if the deficit persists
- Incidence
- 1-4 percent
- Recognition
- Ipsilateral plantar-flexion weakness, lost ankle jerk, numbness on the sole; saddle anaesthesia if bilateral
- Prevention and management
- Prevention: the outlet view keeps the screw superior to the S1 foramen; the inlet keeps it lateral to the midline. Management: early screw revision if recognised; ankle-foot orthosis and neuropathic medication if the deficit is incomplete
- Incidence
- 3-8 percent (higher without navigation)
- Recognition
- Post-operative CT shows the screw outside the corridor or breaching the anterior cortex or foramen
- Prevention and management
- Prevention: perfect views on all three projections before and after insertion; navigation in dysmorphic or obese patients. Management: early revision if recognised before weight-bearing; close observation if asymptomatic and stable on CT
- Incidence
- less than 1 percent
- Recognition
- Sudden hypotension, expanding retroperitoneal haematoma, pulsatile wound bleeding; CT angiogram confirms extravasation
- Prevention and management
- Prevention: the true lateral view confirms the tip never passes the anterior cortex. Management: immediate vascular consultation, endovascular or open repair, massive transfusion and damage-control resuscitation
- Incidence
- 4-10 percent (higher in osteoporosis)
- Recognition
- Progressive hemipelvis displacement, screw back-out or breakage, recurrent pain and instability
- Prevention and management
- Prevention: anatomic reduction; mandatory washer; trans-sacral construct for bilateral or osteoporotic injuries; supplementary anterior fixation. Management: revision with larger-diameter screws, trans-sacral constructs or spinopelvic fixation
- Incidence
- 1-3 percent
- Recognition
- Erythema, drainage, wound breakdown; raised CRP and white-cell count; fluid collection on CT or MRI
- Prevention and management
- Prevention: sterile technique, prophylactic antibiotics, meticulous haemostasis. Management: oral antibiotics and wound care if superficial; surgical debridement, intravenous antibiotics and possible implant removal after union if deep
- Incidence
- 10-20 percent at 2 years
- Recognition
- Persistent buttock pain, positive FABER and Gaenslen tests, radiographic SI-joint sclerosis or narrowing
- Prevention and management
- Prevention: anatomic reduction of the SI joint; avoid over-compression with screws. Management: activity modification, SI-joint injections, physical therapy; SI-joint fusion only after failure of conservative measures and confirmation of the joint as the pain generator
Viva & Exam Focus
CORRIDORCORRIDOR — the S1 and S2 osseous corridors
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A 42-year-old polytrauma patient has an APC III pelvic-ring injury with a left sacroiliac joint dislocation and pubic symphysis diastasis greater than 3 cm; the posterior ring is displaced 1.5 cm superiorly. Outline your operative plan for stabilisation.”
“You are planning percutaneous iliosacral screw fixation for a vertically unstable sacral fracture. On the true lateral fluoroscopic view the anterior cortex of the sacral ala lies posterior to the sacral promontory. What is your interpretation and how does this change your operative plan?”
“A 68-year-old woman with osteoporosis sustains a low-energy fall and presents with an H-shaped sacral insufficiency fracture and spinopelvic dissociation. She has severe low-back pain and is unable to mobilise. Describe your fixation strategy.”
Key indications
- Vertically unstable posterior ring (Tile C, APC III, LC III) with greater than 1 cm displacement
- Unstable sacral fracture (Denis I or II) or SI joint dislocation with greater than 1 cm displacement
- Bilateral posterior-ring instability or sacral insufficiency fracture with spinopelvic dissociation
- Rotationally unstable injuries require supplementary anterior fixation (symphysis plating or external fixator)
Three critical fluoroscopic views
- True lateral: anterior cortex of the S1 ala must lie posterior to the promontory; identifies the dysmorphic sacrum
- Inlet: confirms the screw stays posterior to the anterior sacral cortex and lateral to the midline canal
- Outlet: confirms the screw stays superior to the S1 foramen and inferior to the superior endplate
- All three views perfect before and after the guide-wire and after the final screw
Dysmorphic sacrum recognition
- Anterior cortex of the ala lies posterior to the sacral promontory on the true lateral
- Superior S1 endplate steeply inclined (greater than 30 degrees)
- S1 foramen appears anterior to the promontory on the inlet view
- Safe S1 corridor may be less than 10 mm wide — abandon S1 and use S2 or a trans-sacral construct
Danger structures
- L5 nerve root: anterior to the sacral ala — anterior S1 breach causes foot-drop
- S1 nerve root: within the S1 foramen — medial or inferior breach causes plantar-flexion weakness and loss of the ankle jerk
- Cauda equina: within the sacral canal — midline breach causes saddle anaesthesia and bowel/bladder dysfunction
- Iliac vessels: anterior to the SI joint — anterior cortical penetration causes life-threatening haemorrhage
- Iliac cortex: 2-4 mm thin — mandatory washer on every screw to prevent head penetration
Operative steps
- Obtain perfect true lateral, inlet and outlet views before the skin incision
- Closed reduction of the hemipelvis with traction and Schanz-pin manipulation
- Mark the entry point at the intersection of the anterior S1 cortex and superior endplate lines
- Advance the 3.2 mm guide-wire in 5 mm increments, rechecking all three views
- Measure length, apply a washer (mandatory), insert a 7.3 mm partially-threaded cannulated screw
- Confirm the tip within the contralateral sacral body or ilium on all three views
- Add supplementary anterior fixation for rotationally unstable injuries
Trans-sacral versus unilateral screws
- A trans-sacral S1 screw crosses the midline into the contralateral ilium — superior torsional and axial stability
- Indicated for bilateral injuries, sacral insufficiency fractures and osteoporotic bone
- Requires perfectly symmetric inlet and outlet views to avoid sacral-canal breach
- Washer on both iliac cortices; often combined with a unilateral S2 screw
Complications
- L5 nerve-root injury (1-5 percent): foot-drop from anterior S1 breach
- S1 nerve-root injury (1-4 percent): plantar-flexion loss from medial or inferior breach
- Screw malposition (3-8 percent): higher without navigation
- Vascular injury (less than 1 percent): iliac vein or artery laceration from anterior cortical penetration
- Loss of reduction (4-10 percent): higher in osteoporosis — use trans-sacral constructs and washers
Post-operative protocol
- Touchdown weight-bearing (10-15 kg) from day 1 if reduction is anatomic and fixation stable
- DVT prophylaxis for 6 weeks; multimodal analgesia
- Partial weight-bearing at 8 weeks when bridging callus is visible; full by 12 weeks
- Post-operative CT to confirm screw position and reduction quality
Background & Evidence
Epidemiology. Posterior pelvic-ring disruption accompanies the high-energy vertical-shear and crush injuries of polytrauma (Young-Burgess APC and lateral-compression patterns) and, increasingly, the low-energy fragility fractures of the osteoporotic elderly — where sacral insufficiency fractures are often bilateral or H-shaped and may produce spinopelvic dissociation. The dysmorphic upper sacrum that so changes the operative plan is present in up to 30 percent of patients and is independent of age. Pathoanatomy — the osseous corridors. The S1 corridor is the primary and largest safe passage, running from the outer ilium across the SI joint and sacral ala into the S1 body; on the true lateral it is bounded anteriorly by the anterior cortex of the ala and posteriorly by the sacral canal, widest at the superior S1 endplate and narrowing inferiorly toward the S1 foramen. The smaller, more inferior S2 corridor is the fallback when S1 is inadequate or when additional fixation is needed; on the outlet view an S2 screw must remain superior to the S2 foramen and inferior to the S1 endplate. S2 screws are particularly useful in bilateral constructs or combined with a trans-sacral S1 screw.
- Location
- Exits the L5-S1 foramen and courses anterior to the sacral ala in front of the SI joint
- Mechanism of injury
- Anterior cortical breach of an S1 screw strikes the L5 root first — ipsilateral foot-drop and L5-dermatome numbness
- Prevention
- True lateral view: tip at or short of the anterior S1 cortex, never beyond
- Location
- Travels within the S1 foramen and exits anteriorly
- Mechanism of injury
- Medial or inferior breach enters the foramen — plantar-flexion weakness, loss of the ankle jerk, numbness on the sole
- Prevention
- Outlet view keeps the screw superior to the foramen; inlet keeps it lateral to the midline
- Location
- Lies within the sacral canal medial to the S1 and S2 bodies
- Mechanism of injury
- A screw that crosses the midline or is placed too medially breaches the canal — saddle anaesthesia and bowel/bladder dysfunction
- Prevention
- Inlet view keeps the trajectory lateral to the midline canal; plan bilateral paths so they do not converge
- Location
- The common iliac vein and artery lie immediately anterior to the SI joint
- Mechanism of injury
- Anterior cortical penetration of the ilium or sacrum causes life-threatening retroperitoneal haemorrhage
- Prevention
- True lateral view: the tip never advances beyond the anterior cortex of the sacral body
- Location
- Sciatic nerve; superior and inferior gluteal neurovascular bundles
- Mechanism of injury
- An inferior trajectory risks direct injury to these structures
- Prevention
- Keep the trajectory superior to the notch on the outlet view
- Location
- Only 2-4 mm thick at the screw entry point
- Mechanism of injury
- Without a washer the screw head sinks through under load — loss of fixation and late displacement of the posterior ring
- Prevention
- Mandatory large washer (minimum 10-12 mm diameter) on every screw
Classification in brief. Posterior-ring instability is graded by the Tile system (B equals rotationally unstable but vertically stable; C equals rotationally and vertically unstable) and the Young-Burgess mechanism (APC II-III anteroposterior compression, LC II-III lateral compression). Sacral fractures are described by the Denis zone relative to the foramina (zone I lateral to the foramen, zone II through the foramen, zone III medial to the foramen and involving the canal) — zone I and II injuries are those stabilised with iliosacral screws, while zone III injuries with neurological compromise may need decompression as well as fixation. Trans-sacral versus unilateral — biomechanics. Trans-sacral screws crossing the midline into the contralateral ilium give superior torsional and axial stability for bilateral and insufficiency fractures, and a single trans-sacral S1 screw combined with a unilateral S2 screw is equivalent in stability to two unilateral iliosacral screws. Percutaneous fixation matches the biomechanical stability of open posterior approaches when the screw sits accurately within the corridor, with less surgical time, blood loss and wound morbidity, and navigation or 3D fluoroscopy improves screw accuracy and reduces radiation exposure to the surgeon without increasing operative time.
References
Percutaneous iliosacral screw fixation for unstable pelvic ring injuries
Early clinical series establishing the safety and efficacy of percutaneous iliosacral screw fixation for posterior pelvic-ring injuries; accurate screw placement within the osseous corridor gives reliable fixation with low complication rates when fluoroscopic landmarks are respected.
Iliosacral screw fixation of unstable pelvic ring injuries: a biomechanical study
Biomechanical comparison of iliosacral screw constructs in a pelvic-ring injury model; two iliosacral screws or a trans-sacral construct gave superior torsional stability compared with a single unilateral screw, supporting bilateral or trans-sacral constructs for vertically unstable or bilateral posterior-ring injuries.
Dysmorphic upper sacrum and its effect on iliosacral screw placement
Radiographic study defining the prevalence and features of the dysmorphic upper sacrum; approximately 30 percent of patients have a dysmorphic sacrum that narrows or eliminates the safe S1 osseous corridor, so the true lateral fluoroscopic view must be obtained and interpreted before guide-wire insertion.
Navigation-assisted iliosacral screw placement versus conventional fluoroscopy
Prospective comparison of conventional 2D fluoroscopy versus 3D navigation for iliosacral screw placement; navigation significantly improved screw accuracy and reduced radiation exposure to the surgeon without increasing operative time, and should be used where available, especially in obese or dysmorphic patients.
Trans-sacral screw fixation for sacral insufficiency fractures
Clinical series of elderly patients with sacral insufficiency fractures treated with percutaneous trans-sacral screws; trans-sacral constructs provided immediate stability, allowed early mobilisation and had low complication rates in osteoporotic bone, making this an effective minimally invasive option for fragility fractures of the sacrum and spinopelvic dissociation.