S2-Alar-Iliac & Iliac Screws
- When a spinal fusion is long and ends at the sacrum, the S1 pedicle screws and the LUMBOSACRAL JUNCTION bear enormous cantilever loads, and this is a high-stress zone with a high rate of S1-screw loosening and pseudarthrosis; SACROPELVIC FIXATION extends the construct into the PELVIS to PROTECT the S1 screws, share the load and improve the chance of fusion across the lumbosacral junction.
- The traditional ILIAC SCREW is placed into the ilium (between the inner and outer tables, aiming towards the anterior inferior iliac spine) and gives strong fixation, but it usually requires an OFFSET CONNECTOR to join the laterally placed, off-axis screw to the main rod, and the screw head can be PROMINENT and symptomatic.
- The S2-ALAR-ILIAC (S2AI) screw addresses these drawbacks: its trajectory starts on the S2 ala, crosses the SACROILIAC JOINT and enters the ilium, so it lies more MEDIAL and DEEPER (less prominent) and is roughly IN-LINE with the proximal pedicle screws, which avoids the need for offset connectors; biomechanically it is strong because it crosses the cortical surfaces of the SI joint.
- The S2AI trajectory was introduced specifically to DECREASE STRAIN on the S1 screws and improve fixation across the lumbosacral junction, and it is effective with generally low complication rates - but its path is INTIMATELY ASSOCIATED with major NEURAL and VASCULAR structures and the SI joint, so accurate placement matters; ROBOTIC/navigated guidance achieves the correct trajectory with high accuracy (over 95% reported).
- Recognised COMPLICATIONS of S2AI fixation include SACROILIAC JOINT pain/irritation (because the screw crosses the SI joint - reported in a few percent, often responsive to an SI joint block), SCREW FRACTURE (commonly at the neck) and SET-SCREW DISLODGEMENT, PERISCREW LUCENCY, and the need for revision in a small proportion; distal construct failure can also occur around the lumbopelvic region.
- In summary, sacropelvic fixation is indicated for long deformity constructs reaching the sacrum (and for sacral/pelvic instability or high-grade spondylolisthesis), with the S2AI screw now widely favoured over the traditional iliac screw for its in-line trajectory, lower prominence and avoidance of offset connectors - while being aware of SI-joint symptoms and screw failure as the main trade-offs.
- “Sacropelvic fixation extends long fusions to the PELVIS to PROTECT S1 screws and the lumbosacral junction (high pseudarthrosis zone).
- “Iliac screw = strong but needs OFFSET CONNECTORS and can be prominent; S2AI screw = in-line with pedicle screws, more medial/deeper, crosses the SI joint - generally favoured.
- “S2AI path is near neurovascular structures (robotic/navigation over 95% accuracy); complications = SI joint pain, screw fracture/set-screw dislodgement.
Long fusions to the sacrum overload the S1 screws / lumbosacral junction (high pseudarthrosis). Pelvic fixation protects S1 and shares the load.
S2AI screw (in-line, crosses the SI joint, less prominent, no offset connector) is favoured over the traditional iliac screw. Watch the SI joint and neurovascular structures.
Rationale & the Two Screws
Long fusions ending at the sacrum overload the S1 pedicle screws and the lumbosacral junction - a high-stress zone with a high rate of S1-screw loosening and pseudarthrosis - so sacropelvic fixation extends the construct into the pelvis to protect S1, share the load and improve fusion. The traditional iliac screw gives strong purchase in the ilium but usually needs an offset connector to reach the rod and can be prominent. The S2-alar-iliac (S2AI) screw starts on the S2 ala, crosses the sacroiliac joint and enters the ilium, lying more medial and deeper (less prominent) and roughly in-line with the proximal pedicle screws (avoiding offset connectors), with strong biomechanics from crossing the SI joint cortices. The S2AI path is close to major neural and vascular structures, so accurate placement (often with robotic/ navigated guidance, over 95% accuracy) matters.
Technique Considerations & Complications
- Indications: long deformity constructs reaching the sacrum, and sacral/pelvic instability or high-grade spondylolisthesis - to protect S1 and the lumbosacral junction.
- S2AI vs iliac: S2AI is widely favoured (in-line trajectory, less prominent, no offset connector, crosses the SI joint); the iliac screw remains a strong alternative.
- Accuracy matters: the trajectory lies near neural and vascular structures - robotic/navigated guidance achieves correct placement with high accuracy (over 95% reported).
- Complications to counsel/monitor: SACROILIAC JOINT pain/irritation (the screw crosses the joint; often responds to an SI joint block), SCREW FRACTURE (often at the neck), SET-SCREW DISLODGEMENT, periscrew lucency, and occasional revision; distal construct failure can occur around the lumbopelvic region.
Sacropelvic fixation is what makes a long fusion to the sacrum durable - by protecting the S1 screws and the lumbosacral junction it reduces S1 loosening and pseudarthrosis - but the very feature that makes the S2AI screw strong, crossing the sacroiliac joint, is also the source of its commonest specific complication: SI joint pain or irritation, which is reported in a few percent and often responds to an SI joint block. The trajectory also lies close to major neural and vascular structures, so accurate placement, increasingly with robotic or navigated guidance, is important, and patients should be counselled about SI-joint symptoms, screw fracture and set-screw problems. Choose pelvic fixation deliberately for long sacral constructs and place it precisely.
Evidence & Key Studies
Durability and failure types of S2-alar-iliac screws: 312 consecutive screws
- S2AI screws improve stability across the lumbosacral junction by crossing the cortical surfaces of the sacroiliac joint, and clinical outcomes (back pain, ambulation, ODI) improved significantly.
- SI joint pain occurred in 3.2% (most improved after an SI joint block); periscrew lucency in 2.2%, set-screw dislodgement in 2.2%, and screw fracture in 1.9% (mostly neck fractures).
- Revision for S2AI screw failure was needed in 1.6%; SI-joint irritation occurred with considerable frequency.
Robotic versus freehand S2-alar-iliac fixation: technical considerations
- The S2AI trajectory was introduced to decrease strain on the S1 screws and, by aligning with the proximal pedicle screw instrumentation, avoids the offset connectors needed for traditional iliac screws.
- S2AI screws are effective with low complication rates, but their path is intimately associated with major neural and vascular structures.
- Robotic guidance obtains the correct S2AI trajectory safely and effectively with over 95% accuracy.
According to PubMed, the biomechanical rationale (S2AI screws crossing the SI-joint cortices to improve lumbosacral stability), the improved clinical outcomes and the specific complications (SI joint pain ~3.2%, screw fracture, set-screw dislodgement, periscrew lucency, revision) come from the cited Hyun series; the S2AI trajectory's purpose (decreasing S1 strain), its in-line alignment avoiding offset connectors, its proximity to neurovascular structures and the over-95% accuracy of robotic guidance from the cited Laratta review. The indications for pelvic fixation and the iliac-screw technique are standard, well-established teaching. (See also our Adult Spinal Deformity and High-Grade Spondylolisthesis topics.)
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“Why and when would you extend a long spinal construct to the pelvis, and how?”
“What complications are specific to S2-alar-iliac fixation?”
Mnemonics & Memory Aids
PELVIS
Hook:PELVIS: Protect S1, Extend to pelvis, Lumbopelvic load shared, Vessels/nerves near, Iliac vs S2AI, SI joint crossed (watch pain).
Why & when
- Long fusions to the sacrum overload S1 / the lumbosacral junction (pseudarthrosis zone)
- Pelvic fixation protects S1, shares load, improves fusion
- Also: sacropelvic instability, high-grade spondylolisthesis
Iliac screw
- Into the ilium (between tables, towards AIIS) - strong purchase
- Usually needs an offset connector to reach the rod
- Screw head can be prominent/symptomatic
S2-alar-iliac (S2AI)
- From S2 ala across the SI joint into the ilium
- In-line with pedicle screws (no offset connector), more medial/deeper (less prominent)
- Strong (crosses SI joint cortices); path near neurovascular structures (robotic/navigation over 95%)
Complications
- SI joint pain/irritation (often responds to SI joint block)
- Screw fracture (often at the neck), set-screw dislodgement, periscrew lucency
- Small revision rate; distal lumbopelvic failure