TECHNICAL NOTEJ Neurosurg Spine 28:543–547, 2018

Spinopelvic fixation is essential for reinforcing long fu-sions to the sacrum in spinal deformity surgery. This instrumentation technique has also been applied in

spinal trauma, tumor, and degenerative conditions.4,9,13 Although several methods for pelvic fixation have been described, iliac screw and S-2 alar/iliac (S2AI) screw fixa-tion are currently the most commonly used methods.5,6,8

Traditional iliac screw fixation improves stability and fusion rate.10,14 However, the screw head can be prominent and the technique has been associated with symptomatic hardware and wound dehiscense.4,7 Compared with S2AI screws, iliac screws have been associated with higher rates of reoperation, surgical site infection, and symptomatic screw prominence.3 In a series of 67 patients, Tsuchiya et al. found that 34.3% required screw removal for symp-tomatic prominence, and they reported 7 cases of screw breakage.14

The S2AI screw trajectory was developed so that the screw head would sit below the posterior superior iliac spine (PSIS); recent literature has shown a decreased complication rate compared with iliac screws.3,6 However, S2AI screws do not always provide sufficient pelvic fixa-tion. In a series of 20 patients, Guler et al. reported a 35% implant failure rate due to screw pullout and/or head-shaft disengagement.5

To avoid the complications associated with iliac screws and to supplement pelvic fixation, we describe an S-1 alar/iliac (S1AI) screw trajectory that may be used in addition to S2AI screws or independently if the patient’s anatomy

limits the ability to place S2AI screws. We also present 2 clinical case examples demonstrating its implementation.

Surgical Technique: S1AI TrajectoryPreoperatively, the patient’s sacropelvic anatomy should

be thoroughly reviewed on plain radiographs and/or CT. The availability of a wide range of screw diameter and length options, as well as a full complement of side-to-side connectors, should be confirmed.

The starting point for the S1AI screws is just lateral to the junction of the S-1 superior articular facet and the posterior sacral ala, which is in proximity to the entry point for S-1 pedicle screws. It is 3–5 mm above the su-perolateral corner of the S-1 foramen (Fig. 1). This entry site allows the S1AI screw heads to remain collinear with lumbar pedicle screws and S2AI screws, limiting the need for offset or side-to-side connectors. Figure 1 provides an image of a cadaveric specimen with the entry site marked with a pedicle probe; the white star in the figure indicates the S-1 foramen.

The trajectory of the screw path runs in a caudal and lateral direction through the ala and into the ilium. The surgeon should direct the S1AI screw 40°–50° laterally in the transverse plane, similar to S2AI screws.2,11 Fluo-roscopic imaging (including anteroposterior and oblique pelvic radiographs) is used to identify the teardrop (PSIS–anterior inferior iliac spine [AIIS] corridor) and to confirm the screw is within the plane of the posterior ilium (Fig. 2).

ABBREVIATIONS AIIS = anterior inferior iliac spine; PSIS = posterior superior iliac spine; S1AI = S-1 alar/iliac; S2AI = S-2 alar/iliac.SUBMITTED July 30, 2016. ACCEPTED August 17, 2017.INCLUDE WHEN CITING Published online February 2, 2018; DOI: 10.3171/2017.8.SPINE16904.

S-1 alar/iliac screw technique for spinopelvic fixationJ. Mason DePasse, MD,1 Mauricio Valdes, MD,2 Mark A. Palumbo, MD,3 Alan H. Daniels, MD,3 and Craig P. Eberson, MD4

Divisions of 3Spine Surgery and 4Pediatric Orthopaedic Surgery, 1Department of Orthopaedics, Alpert Medical School of Brown University, Providence, Rhode Island; and 2Hedley Orthopaedic Institute, Phoenix, Arizona

Spinopelvic fixation provides an important anchor for long fusions in spinal deformity surgery, and it is also used in the treatment of other spine pathologies. Iliac screws are known to sometimes require reoperation due to pain resulting from hardware prominence and skin injury. S-2 alar/iliac (S2AI) screws do not often require removal, but they may provide inadequate fixation in select cases. In this paper the authors describe a technique for S-1 alar/iliac screws that may be used independently or as a supplement to S2AI screws. A preliminary biomechanical analysis and 2 clinical case ex-amples are also provided.https://thejns.org/doi/abs/10.3171/2017.8.SPINE16904KEY WORDS spinopelvic fixation; spine deformity; technical note; surgical technique; sacral

J Neurosurg Spine Volume 28 • May 2018 543©AANS 2018, except where prohibited by US copyright law

J. M. DePasse et al.

J Neurosurg Spine Volume 28 • May 2018544

Figure 3 upper provides comparison trajectories for tradi-tional iliac screws, S1AI screws, and S2AI screws, while Fig. 3 lower compares the trajectories of the S1AI screw to the S-1 pedicle screw.

Depending on the patient’s sacral anatomy, screws up to 10.5 mm in diameter and 100 mm in length may be placed. This determination can be made intraoperatively, or in select cases may be made on preoperative CT through assessment of alar thickness as well as the PSIS-AIIS corridor if cut angle reformatting is available. Polyaxial screws (with or without a favored angle head) are recom-mended for the S1AI and S2AI screws to minimize rod bending and contouring and to simplify seating of the rod.

Biomechanical AnalysisBiomechanical analyses comparing S2AI screws to

traditional iliac screws have been performed,1,12 and have shown that there is no significant difference in stability and stiffness between constructs. To validate the feasibil-ity of spinopelvic fixation utilizing S1AI screws only, a preliminary biomechanical analysis with a single cadav-eric specimen was performed using methodology similar to that used by Burns et al.1

An unembalmed human lumbopelvic specimen was stripped of all muscle tissue, though care was taken to preserve ligamentous tissue and intervertebral discs. Mul-tiaxial pedicle screw instrumentation (Stryker) was placed from L-4 to L-5 bilaterally, then 8.5 × 80 mm screws were

placed along the described S1AI trajectory bilaterally. All screw positions were confirmed with both direct ball-tipped probe palpation and visualization, and fluoroscop-ic imaging. The constructs were completed with titanium rods and set screws.

The cephalad end of the spine segment and the pelvis were then rigidly embedded utilizing a urethane potting compound (Smooth-On Inc.), and a custom apparatus ap-plied pure moments about 3 principal anatomical axes with a biaxial servohydraulic load frame (Instron Corp.). As in the Burns et al. study,1 the torsional stiffness of the specimen was tested in flexion, extension, right lateral bending, and left lateral bending.

Measured torsional stiffness values (in Newton-me-

FIG. 1. Gross image of a cadaveric specimen with the S1AI entry site marked with a pedicle probe and the S-1 foramen marked with a white star. Figure is available in color online only.

FIG. 2. Fluoroscopic anteroposterior (upper) and oblique “teardrop” (lower) trajectory of the S1AI screw marked with a pedicle probe on a cadaveric specimen.

J Neurosurg Spine Volume 28 • May 2018 545

J. M. DePasse et al.

ters per degree [N-m/°]) of the L4-pelvis construct with S1AI screws were: 8.84 N-m/° in flexion, 7.69 N-m/° in extension, 42.57 N-m/° in right lateral bending, and 29.56 N-m/° in left lateral bending. These values are similar to or greater than those measured by Burns et al. for their L-5 pelvis construct with L5–S1 pedicle screws and S2AI screws. The authors reported mean torsional stiffness of 9.38 ± 2.11 N-m/° in flexion, 8.39 ± 1.66 N-m/° in ex-tension, 13.97 ± 3.03 N-m/° in right lateral bending, and 13.34 ± 1.88 N-m/° in left lateral bending.1

Although this analysis utilizes only a single specimen, the torsional stiffness of the lumbopelvic construct with S1AI screws was similar in flexion-extension and greater in lateral bending as compared with S2AI constructs. Fur-ther biomechanical analysis is warranted; however, these preliminary data suggest that the biomechanical proper-ties of spinopelvic fixation constructs with S1AI screws are similar to constructs with S-1 pedicle screws and S2AI screws.

Clinical CasesCase 1

A 64-year-old woman presented with lower-extremi-ty weakness and pain due to critical spinal stenosis and progressive sagittal and coronal deformity. Her plain ra-diographs on presentation are shown in Fig. 4. The pa-tient was treated with lumbar decompression and fusion, with pedicle screw instrumentation from T-9 to L-5, S1AI screws, and a 4-rod construct (Fig. 5). S2AI screws were not used due to a large psoriatic plaque overlying the sa-

FIG. 3. Upper: Anteroposterior view of the pelvis demonstrating the S1AI screw (blue), S2AI screw (red), and traditional iliac screw (black). Lower: Inlet view of the pelvis demonstrating the trajectory of an S1AI screw (blue) and an S-1 pedicle screw (black). Images adapted from: Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918. The original figure is in the public domain. Figure is avail-able in color online only.

FIG. 4. Case 1. Preoperative anteroposterior (left) and lateral (right) radiographs demonstrating coronal and sagittal imbalance.

FIG. 5. Case 1. Left: Anteroposterior radiograph demonstrating S1AI screws and a 4-rod construct. Right: Lateral radiograph demonstrating T9–L5 pedicle screws and S1AI screws.

J. M. DePasse et al.

J Neurosurg Spine Volume 28 • May 2018546

crum, limiting the caudal extent of the skin incision. At the 1-year follow-up, there was no evidence of hardware failure or screw loosening on plain radiographs (Fig. 6).

Case 2A 58-year-old woman presented with severe back pain

and progressive lumbar scoliosis with coronal imbalance resulting from a congenital L-5 butterfly vertebra (Fig. 7). The patient was treated with pedicle screw instrumenta-

tion from T-10 to L-4, a single screw into the right L-5 butterfly vertebra, bilateral S1AI and S2AI screws, and a 4-rod construct (Fig. 8). Advanced imaging postopera-tively demonstrated the screw trajectories in 3 dimensions (Fig. 9). At the 1-year follow-up, the patient reported dra-matic improvement in back pain with no evidence of hard-ware failure or loosening (Fig. 10).

DiscussionSpinopelvic fixation is a technique used for a wide

range of spinal column pathology, most commonly in an-

FIG. 6. Case 1. One-year follow-up anteroposterior (left) and lateral (right) radiographs demonstrating no loosening or hardware complica-tions.

FIG. 7. Case 2. Preoperative coronal CT image demonstrating coronal deformity associated with an L-5 butterfly vertebra.

FIG. 8. Case 2. Left: Anteroposterior radiograph demonstrating S1AI and S2AI screws and a 4-rod construct. Right: Lateral radiograph dem-onstrating T10–L5 pedicle screws, and S1AI and S2AI screws.

FIG. 9. Case 2. Left: Three-dimensional CT reconstructed image of the construct, demonstrating S1AI and S2AI trajectories. Right: Coronal CT image showing pedicle screw fixation into vertebrae and the ilium. Figure is available in color online only.

J Neurosurg Spine Volume 28 • May 2018 547

J. M. DePasse et al.

choring long constructs to the pelvis in adult deformity.8,13 Traditional iliac screws can result in skin complications that may require revision.3 In certain situations, S2AI screws may not provide sufficient fixation strength to pre-vent screw pullout, breakage, or rod-screw interface fail-ure.5 We present a technique for S1AI screws that may be used independently or in combination with S2AI screws for additional fixation strength.

In general, S1AI screws cannot be used with standard S-1 pedicle screws. The S1AI screw uses a similar entry point and achieves fixation in the sacral ala and ilium in-stead of the pedicle and vertebral body, which achieves ad-ditional sacropelvic purchase. It is possible that the use of S1AI screws may increase stability for longer constructs and reduce the rate of nonunion, which would be especial-ly important for patients with lumbopelvic dissociation or large deformity correction. Further biomechanical testing will be required for verification. S1AI screws may also function as an excellent salvage alternative for loss of S-1 pedicle screw fixation or in cases of broken S-1 screws, which limit replacement of screws into the S-1 pedicle.

As noted in the technique description, it is important to use intraoperative fluoroscopy for placement of S1AI screws due to the risk of penetrating the inner table and entering the pelvis. The S1AI screw, similar to the S2AI screw, must be angled toward the teardrop of the ilium or apex of the acetabulum, which requires a significant de-gree of caudal angulation. Placement that is too horizontal is most likely to result in violation of the anterior cortex of the ilium and penetration into the iliacus muscle.

Although future biomechanical and clinical study will be required, S1AI screws provide a viable alternative to S-1 pedicle screws and strong supplemental fixation to S2AI screws.

References 1. Burns CB, Dua K, Trasolini NA, Komatsu DE, Barsi JM:

Biomechanical comparison of spinopelvic fixation con-structs: iliac screw versus S2-alar-iliac screw. Spine Deform 4:10–15, 2016

2. Chang TL, Sponseller PD, Kebaish KM, Fishman EK: Low profile pelvic fixation: anatomic parameters for sacral alar-iliac fixation versus traditional iliac fixation. Spine (Phila Pa 1976) 34:436–440, 2009

3. Elder BD, Ishida W, Lo SL, Holmes C, Goodwin CR, Kosz-towski TA, et al: Use of S2-alar-iliac screws associated with less complications than iliac screws in adult lumbosacropel-vic fixation. Spine (Phila Pa 1976) 42:E142–E149, 2017

4. Emami A, Deviren V, Berven S, Smith JA, Hu SS, Bradford DS: Outcome and complications of long fusions to the sa-crum in adult spine deformity: Luque-Galveston, combined iliac and sacral screws, and sacral fixation. Spine (Phila Pa 1976) 27:776–786, 2002

5. Guler UO, Cetin E, Yaman O, Pellise F, Casademut AV, Sa-bat MD, et al: Sacropelvic fixation in adult spinal deformity (ASD); a very high rate of mechanical failure. Eur Spine J 24:1085–1091, 2015

6. Ilyas H, Place H, Puryear A: A comparison of early clinical and radiographic complications of iliac screw fixation versus S2 alar iliac (S2AI) fixation in the adult and pediatric popu-lations. J Spinal Disord Tech 28:E199–E205, 2015

7. Ishida W, Elder BD, Holmes C, Goodwin CR, Lo SFL, Kosztowski TA, et al: S2-alar-iliac screws are associated with lower rate of symptomatic screw prominence than iliac screws: radiographic analysis of minimal distance from screw head to skin. World Neurosurg 93:253–260, 2016

8. Kebaish KM: Sacropelvic fixation: techniques and complica-tions. Spine (Phila Pa 1976) 35:2245–2251, 2010

9. Kuklo TR: Principles for selecting fusion levels in adult spinal deformity with particular attention to lumbar curves and double major curves. Spine (Phila Pa 1976) 31 (19 Suppl):S132–S138, 2006

10. Kuklo TR, Bridwell KH, Lewis SJ, Baldus C, Blanke K, Iffrig TM, et al: Minimum 2-year analysis of sacropelvic fixation and L5–S1 fusion using S1 and iliac screws. Spine (Phila Pa 1976) 26:1976–1983, 2001

11. O’Brien JR, Yu WD, Bhatnagar R, Sponseller P, Kebaish KM: An anatomic study of the S2 iliac technique for lum-bopelvic screw placement. Spine (Phila Pa 1976) 34:E439–E442, 2009

12. OʼBrien JR, Yu W, Kaufman BE, Bucklen B, Salloum K, Khalil S, et al: Biomechanical evaluation of S2 alar-iliac screws: effect of length and quad-cortical purchase as com-pared with iliac fixation. Spine (Phila Pa 1976) 38:E1250–E1255, 2013

13. Shen FH, Mason JR, Shimer AL, Arlet VM: Pelvic fixation for adult scoliosis. Eur Spine J 22 (Suppl 2):S265–S275, 2013

14. Tsuchiya K, Bridwell KH, Kuklo TR, Lenke LG, Baldus C: Minimum 5-year analysis of L5–S1 fusion using sacropelvic fixation (bilateral S1 and iliac screws) for spinal deformity. Spine (Phila Pa 1976) 31:303–308, 2006

DisclosuresDr. Palumbo reports being a consultant to Stryker. Dr. Daniels reports being a consultant to Stryker, Globus, and Orthofix.

Author ContributionsConception and design: Valdes, Palumbo, Daniels, Eberson. Acquisition of data: DePasse, Valdes, Daniels. Analysis and inter-pretation of data: DePasse, Valdes. Drafting the article: DePasse. Critically revising the article: DePasse, Palumbo, Daniels, Eber-son. Reviewed submitted version of manuscript: Palumbo, Dan-iels, Eberson. Study supervision: Eberson.

CorrespondenceJ. Mason DePasse: Warren Alpert Medical School of Brown Uni-versity, Providence, RI. jmdepasse@gmail.com.

FIG. 10. Case 2. One-year follow-up anteroposterior (left) and lateral (right) radiographs demonstrating no loosening or hardware complica-tions.