Basic ScienceAnalysis of the s2 alar-iliac screw as compared with the traditional iliac screw: does it increase stability with sacroiliac fixation of the spine?
Introduction
Solid distal fixation in spinal deformity surgery is necessary to provide the essential support that maintains sagittal and coronal balance. Despite all the advances and developments in spinal instrumentation techniques, arthrodesis of the lumbosacral junction continues to be a challenge in pediatric and adult spinal deformity surgery [1], [2], [3], [4]. The poor bone quality of the sacrum, its complex anatomy, and the tremendous biomechanical forces at the lumbosacral junction contribute to the high rates of instrumentation-related problems [2], [3], [4], [5].
Fixation down to the sacrum requires additional instrumentation to prevent a high failure rate when S1 screws are used alone [2], [5]. Historically, iliac screws have been most commonly used to augment S1 pedicle screws. Several disadvantages have been reported with the use of this technique [1], [5], [6], [7], [8]. Iliac screws often require an offset connector which can create an additional point of failure at the end of the construct. Prominent hardware may also lead to skin necrosis and this makes iliac screw fixation less than ideal [4], [6], [8]. These issues have led to the more recent description of the S2 sacroiliac screw.
The S2 alar-iliac (S2AI) screw technique features inline placement with proximal spinal anchors without the use of offset connectors [1], [2], [9]. The entry point originates distal to the S1 foramen [2], [6], [9]. This reliable and reproducible starting point—along with a trajectory aimed proximal to the sciatic notch—has resulted in accurate S2AI screw placement using the freehand technique [10]. Compared with other methods of lumbosacral fixation, S2AI screws purchase more layers of cortical bone, including the sacroiliac joint, and are buttressed between the inner and outer tables of the ilium, which provide increased stability [10] (Fig. 1). The distal placement of the S2AI screw being positioned anterior to the lumbosacral pivot point can also improve stability [6]. Modern iliac screws and S2AI screws have been associated with relatively low rates of pseudarthrosis [1]. S2AI screws have the clinical advantages of less implant prominence, anchor migration, skin breakdown, and possible decreased infection risk [8].
For comparison purposes, we designed a study to biomechanically examine the strength of two different constructs: one that ends with the S2AI screw and the other that uses the traditional iliac screw. Our methods of assessment included stiffness with flexion, extension, rotation, lateral bending, and axial rotation. We hypothesized that there would not be a statistically significant difference in stiffness measurements between S2AI screw fixation and iliac screw fixation using a 95% confidence interval.
Section snippets
Materials and methods
A total of 10 human cadaveric specimens of the lumbosacral spine and pelvis were acquired (Table 1 [Science Care Inc., Phoenix, AZ, USA]). All soft tissue was removed fromthe specimens whereas all ligamentous structures and joint capsules were left intact. The specimens were separated into two test groups:
- 1.
spines instrumented from L1 to the sacrum with 6.5 mm × 40 mm screws, which included 7.5 mm × 90 mm S2AI screw fixation (n=5), and
- 2.
spines instrumented from L1 to the sacrum with 6.5 mm × 40 mm
Results
Stiffness values were calculated in flexion, extension, lateral bending, and axial rotation movements (Table 2). There was a consistent trend of a reduction in construct stiffness for all iliac screw samples tested as compared with the S2 sacroiliac group. However, none of the groups tested reached statistical significance for a confidence interval of 95%. The power was <0.3 for all comparisons.
Discussion
Biomechanical testing of S2AI screw fixation resulted in comparable stability versus traditional iliac screw configuration. These findings were consistent with flexion, extension, lateral bending, and axial rotation movements. Although these data were not statistically significant (p=.05), our results demonstrate a trend toward greater stiffness in the S2AI screw cohort.
The stability of S2AI screws in the present study is supported by previous research assessing biomechanical and clinical
Conclusions
These data suggest that S2AI screws are just as stable as iliac screws with biomechanical testing in flexion, extension, lateral bending, and axial rotation. Given the similarities of biomechanical testing to human movements, these findings support S2AI screws as a viable option for lumbosacral fixation.
Acknowledgment
The authors thank Stacy T. Cheavens, MS, certified medical illustrator, University of Missouri, for creating the figure illustrations.
References (15)
- et al.
Biomechanical analysis of iliac screw fixation in spinal deformity instrumentation
Clin Biomech (Bristol, Avon)
(2014) - et al.
The mechanics of back-extensor torque production about the lumbar spine
J Biomech
(2003) - et al.
Pelvic fixation in adult and pediatric spine surgery: historical perspective, indications, and techniques
J Bone Joint Surg Am
(2015) Sacropelvic fixation
Spine
(2010)Surgical options for lumbosacral fusion: biomechanical stability, advantage, disadvantage and affecting factors in selecting options
Eur J Orthop Surg Traumatol
(2014)- et al.
Pelvic fixation for adult scoliosis
Eur Spine J
(2013) - et al.
Sacropelvic fixation in adult spinal deformity (ASD); a very high rate of mechanical failure
Eur Spine J
(2015)
Cited by (57)
Non-cannulated S2AI screws have higher rates of hardware failure compared to cannulated S2AI screws: S2AI Screw Failure
2023, Seminars in Spine SurgeryComparative Clinical Efficacy and Safety of Sacral-2-Alar Iliac Screw Versus Iliac Screw in the Lumbosacral Reconstruction of Spondylodiscitis
2023, World NeurosurgeryCitation Excerpt :The IS and S2AI screws are the 2 most commonly adopted lumbopelvic fixation procedures that have been used in recent years. The ability of sacropelvic fixation surgical procedures to significantly enhance construct stability across the lumbosacral junction has received widespread acceptance.21,22 Although IS fixation can satisfy the biomechanical criteria of lumbopelvic fixation, extra incisions and more extensive dissection of soft tissues are necessary to disclose the screw location resulting from the high notch of the screw.
Percutaneous lumbopelvic fixation using modified iliac-screw implantation: A technique description and case report
2022, Asian Journal of Surgery
FDA device/drug status: Approved.
Author disclosures: DGH: Grant: Stryker Spine (E, Paid to the institution), pertaining to submitted manuscript; Royalties: Orthopediatrics (E), outside the submitted work; Stock Ownership: Orthopediatrics (E), outside the submitted work; Consulting: Orthopediatrics, outside the submitted work; Speaking and/or Teaching Arrangements: Biomarin, outside the submitted work; Scientific Advisory Board/Other Office: Medical Advisory Board, Little People of America, outside the submitted work; Research Support (Investigator Salary, Staff/Materials): Biomarin (F, Paid to the institution), Stryker Spine (Paid to the institution), outside the submitted work; Grants: Biomarin (Paid to the institution), Stryker Spine (Paid to the institution), outside the submitted work. TDP: Grant: Stryker Spine (E, Paid to the institution), pertaining to submitted manuscript. FMP: Grant: Stryker Spine (E, Paid to the institution), pertaining to submitted manuscript.
The disclosure key can be found on the Table of Contents and at www.TheSpineJournalOnline.com.
Conflicts of interest and source of funding: The present study received external research support from Stryker Spine (Allendale, NJ).
Author contribution statement: All authors have read and approved the final submitted manuscript. The following is the author contribution: DGH and FMP: substantial contributions to research design, acquisition, and analysis of interpretation of data; DGH, TDP: drafting the paper and revising it critically; DGH, FMP, and TDP: approval of the submitted and final versions.
Previous presentation: The present study was presented as a poster at the 2016 Orthopaedic Research Society's Annual Conference, Orlando, FL; March 5–8, 2016.