Sacropelvic Fixation (S2-Alar-Iliac & Iliac Screws)

Why the pelvis. When a long fusion ends at the sacrum, the S1 pedicle screws and the lumbosacral junction bear enormous cantilever loads, and this high-stress zone has 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 junction. It is indicated for long deformity constructs reaching the sacrum, for sacral or pelvic instability and for high-grade spondylolisthesis.

The iliac screw. The traditional anchor is placed into the ilium between the inner and outer tables, aiming towards the anterior inferior iliac spine, and gives strong fixation. Because the screw sits laterally and off-axis to the main rod, it usually needs an offset connector to join the construct, and its head can be prominent and symptomatic.

The S2-alar-iliac screw. The S2AI screw starts on the S2 ala, crosses the sacroiliac joint and enters the ilium. It was introduced specifically to decrease strain on the S1 screws and improve fixation across the lumbosacral junction, and crossing the cortical surfaces of the SI joint makes it biomechanically strong.

Its trajectory lies roughly in line with the proximal pedicle screws, which avoids offset connectors, and it sits more medial and deeper, so it is less prominent. It is effective with generally low complication rates and is now widely favoured, although the iliac screw remains a strong alternative.

Prominence, measured. A meta-analysis of 18 studies and 1,462 patients turned "S2AI is less prominent" from an assertion into a measured difference. Symptomatic screw prominence occurred in 1.9% (12 of 648) with S2AI against 6.3% (51 of 814) with iliac screws. None of the 12 symptomatic S2AI screws was removed, against 14 of the 51 iliac screws (27.5%), so iliac prominence is both commoner and more often needs a second operation.

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The pivot point. McCord and colleagues defined a lumbosacral pivot point around the middle column at the L5-S1 disc. The bending moment a distal anchor must resist depends on how far anterior to this point it reaches.

Why S1 screws loosen. S1 screws end at or behind the pivot point, so they see a huge cantilever load and loosen. Iliac and S2AI screws extend the construct anterior to the pivot point, giving a long lever arm and far greater resistance to flexion, which is the biomechanical reason pelvic fixation protects S1.

Anterior column support. An interbody cage or ALIF at L5-S1 also improves load-sharing: it offloads the posterior screws, reduces S1 strain and lowers lumbosacral pseudarthrosis. Pelvic fixation and anterior column support are complementary solutions to the same high-stress junction.

S2AI entry and trajectory. The entry is just inferior and lateral to the S1 dorsal foramen, on the S2 ala. Aim caudally, laterally and anteriorly to cross the SI joint into the ilium between the inner and outer tables, directed above the sciatic notch towards the AIIS.

Iliac screw entry and trajectory. The entry is at or just below the PSIS, aiming towards the AIIS between the tables. The entry is often recessed, or an offset connector used.

Confirming the corridor. On an obturator-outlet fluoroscopic view the bony corridor is seen as the "teardrop", which confirms a safe path between the tables. Confirm containment with intra-operative imaging or navigation; detailed robotic and navigation technique is covered in Navigation & Robotics in Spine Surgery.

Accuracy. The S2AI path is intimately associated with major neural and vascular structures and the SI joint, so accurate placement matters. Robotic guidance obtains the correct trajectory with over 95% accuracy, a figure from a technical review rather than a comparative trial.

What to avoid. Accurate placement depends on knowing exactly what a malpositioned screw endangers:

• The sciatic notch, with the sciatic nerve and superior gluteal vessels - stay above it
• The hip joint and acetabulum, if the screw runs too caudal or anterior
• The anterior iliac cortex, near the iliac vessels

The joint it crosses. The 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. In a consecutive series of 312 S2AI screws the recognised complications were:

• SI joint pain 3.2%, most improving after an SI joint block
• Periscrew lucency 2.2%
• Set-screw dislodgement 2.2%
• Screw fracture 1.9%, mostly at the neck
• Revision for screw failure 1.6%

Distal construct failure can also occur around the lumbopelvic region. Patients should be counselled about SI-joint symptoms, screw fracture and set-screw problems.

Failure in proportion. Pelvic fixation failure is usually discussed as "screw problems", but in practice the construct more often fails around the screws than at them. In 253 adult-deformity patients instrumented over at least six levels and followed for two years, pelvic fixation failed in 11 (4.3%), by these mechanisms:

The junction becomes the weak point. Seven of the eleven failures were rod fracture or pseudarthrosis at the lumbosacral junction, not a failure of the pelvic anchor. Adding a pelvic screw protects S1, but the junction it protects then becomes the highest-strain point in the construct, and it is the rod crossing that junction that breaks.

What protects it. In the same cohort two construct choices were protective and one alignment variable was a risk factor:

• More rods crossing the lumbopelvic junction. Non-failures had a mean of 3.8 rods crossing it against 2.9 in failures (p = 0.009), and on multivariate analysis the number crossing was protective, OR 0.15 (p = 0.002)
• An accessory rod taken down to S2 or the ilium rather than stopping at S1 (OR 0.2, p = 0.004), present in 54.2% of the non-failures against 18.2% of the failures. Stopping the accessory rod at S1 leaves the very junction that fails without any buttress
• Residual coronal malalignment, measured as a worse post-operative C7 coronal vertical axis, was an independent risk factor (OR 1.5, p = 0.028)

The modifiable variable. Coronal malalignment is the one genuinely modifiable intra-operative variable in that list. Correcting coronal balance protects the lumbopelvic construct, while leaving the patient coronally off-centre loads it eccentrically until something breaks, so correct the coronal plane, not only the sagittal.

The cohort. Mean pelvic screw diameter and length were 8.6 mm and 87 mm, an L5-S1 interbody was used in 74.7%, all rods crossing the lumbosacral junction were cobalt-chrome, and all pelvic screws were closed-headed tulips.

The multirod vocabulary. "Add another rod" is imprecise, and the supplementary rods have distinct definitions worth using correctly. The accessory rod is the one relevant at the lumbosacral junction.