Neurovascular Relationships of S2AI Screw Placement: Anatomic Study

Accepted Manuscript Neurovascular Relationships of the S2AI Screw Placement: An Anatomical Study Amir Abdul-Jabbar, MD, Emre Yilmaz, MD, Joe Iwanaga, DDS PhD, Tamir Tawfik, MD, Thomas M. O’Lynnger, MD MPH, Schildhauer TA, MD, Jens Chapman, MD, Rod J. Oskouian, MD, R Shane Tubbs, PhD PAC PII:

S1878-8750(18)30814-3

DOI:

10.1016/j.wneu.2018.04.095

Reference:

WNEU 7931

To appear in:

World Neurosurgery

Received Date: 16 January 2018 Revised Date:

12 April 2018

Accepted Date: 13 April 2018

Please cite this article as: Abdul-Jabbar A, Yilmaz E, Iwanaga J, Tawfik T, O’Lynnger TM, TA S, Chapman J, Oskouian RJ, Tubbs RS, Neurovascular Relationships of the S2AI Screw Placement: An Anatomical Study, World Neurosurgery (2018), doi: 10.1016/j.wneu.2018.04.095. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Neurovascular Relationships of the S2AI Screw Placement: An Anatomical Study Amir Abdul-Jabbar, MD1; Emre Yilmaz, MD1-3; Joe Iwanaga, DDS PhD2; Tamir Tawfik, MD1; Thomas M O’Lynnger, MD MPH1; Schildhauer TA, MD3, Jens Chapman, MD1; Rod J Oskouian,

Corresponding Author’s Institution:

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MD1,2, R Shane Tubbs, PhD PAC2,4

Swedish Neuroscience Institute, Swedish Medical Center, Seattle, Washington, United States

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Seattle Science Foundation, Seattle, Washington, United States

Department of Trauma Surgery, BG University Hospital Bergmannsheil, Ruhr University

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Bochum, Bürkle-de-la-Camp-Platz 1, 44789, Bochum, Germany 4

Department of Anatomical Sciences, St. George’s University, Grenada

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Disclosure

The authors report no conflict of interest concerning the materials or methods used in this study or

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the findings specified in this paper.

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Corresponding Author:

Emre Yilmaz, MD Swedish Neuroscience Institute Swedish Medical Center 550 17th Avenue Suite 500 James Tower, 5th Floor Seattle, WA 98122, United States phone: (206) 399-1438 fax: (206) 320-5250 e-mail: [email protected]

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Introduction

Spine surgeries with long fusions including pelvic fixation remain challenging and have been associated with poor outcomes, relatively high implant failure rates, and an increased incidence of major complications

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. Complex biomechanical forces and the unique anatomical

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relationships at the lumbo-pelvic junction can present problems when pelvic instrumentation is used . Indications for pelvic fixation include unstable posterior pelvic fractures, lumbo-pelvic fracture

subluxations, tumor resections, non-unions, osteomyelitis with bone loss, pathological fractures, 5-9

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high-grade spondylolisthesis, and spinal deformity surgeries

. Previously described techniques

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for lumbo-pelvic fixation include the Galveston technique developed by Allen and Ferguson and the iliac screw technique used in triangular osteosynthesis described by Schildhauer and Josten 12

.

6,7,10-

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The S2 alar-iliac (S2AI) screw is a modification of the traditional iliac fixation technique using the space between the neuroforamina of S1 and S2 as an insertion point for fixing the sacrum to the ilium. The S2AI screw traverses three cortices giving strong purchase in both the ilium and 13,14

. Also, the increased pullout strength of the S2AI screw can be valuable if bone

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the sacrum

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quality is poor. Connection to the rod construct often requires no side connector and authors have found fewer wound complications due to hardware prominence

15,16

. Furthermore, this technique

causes less tissue morbidity by limiting the size of the incision and reducing paraspinal muscle dissection

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. However, there is a paucity of studies detailing the surrounding anatomy of the

insertion point and relationships to vulnerable neurovascular structures using the S2AI technique. Therefore, the purpose of this cadaveric study was to describe the relevant anatomy including possible vulnerable structures associated with the placement of S2AI screws.

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Materials and Methods

Using fluoroscopy, S2AI screws (Zimmer Biomet, Warsaw, IN, USA) were placed in two fresh, frozen, and thawed predissected adult cadavers (1 male and 1 female) (four sides) through a standard posterior midline exposure. All screws were placed by fellowship-trained spine surgeons

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with the specimens in the prone position. The midline incision was begun at the L4 spinous process and extended distally to beyond the third sacral vertebral body. First, using anteroposterior and pelvic outlet views, the S1 dorsal sacral foramen was identified. The screw insertion point was

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placed 10 mm lateral to a line bisecting the S1 and S2 foramina, adjacent to the sacroiliac joint. Secondly, the S2AI screw entry point was drilled using a high-speed burr (Midas Rex, Medtronic,

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Fort Worth, TX, USA). Using 30°-40° lateral angulation from the midline and 20°-30° of caudal angulation, a pedicle probe was directed toward the anterior inferior iliac spine (AIIS). A mallet was then used to penetrate the sacroiliac joint. The final trajectory was positioned to sit 1-2 cm superior to the greater sciatic notch. The screw length measured between 80 and 90 mm (8-9 mm

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diameter) depending on individual anatomy. A ball tipped probe was then used to ensure there was no breech of the outer cortices of the ilium. Correct screw insertion was verified using the obturator-outlet (30° cephalo-caudal, 30°lateral from midline) and anterior-posterior pelvis

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Results

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radiographic views (Figures 1 and 2).

All screws were placed without breaking through the medial, lateral or caudal cortices (Figure 1, 2, 5A). Removing the bone around the S2AI-screw illustrated the close relationship to the medial (internal) neurovascular structures including the obturator nerve, lumbosacral trunk (ventral rami of L4 and L5), sacral plexus and specifically, S1 ventral ramus, and iliac vein and artery (Figures 3 and 4). By removing the outer cortex of the ilium, the close relationship to the superior gluteal artery, vein and nerve was observed (Figure 3). Additionally, we were able to identify the proximity to the the iliopsoas muscle and the internal iliac vessels (Figure 5B).

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Discussion

Techniques for S2AI screw placement have been described but the insertion point and angle differ from study to study. The previously-described insertion point ranges from the midpoint between the S1/S2 foramina and 2 mm medial to the lateral iliac crest to 1 mm inferior and 1 mm 3,17

. The angulation described ranges from 20° caudally and

30° laterally to 29° caudally and 37° laterally

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lateral to the S1 dorsal sacral foramen

. These ranges illustrate the variable anatomy of

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the sacrum and thus the need for varied trajectories from patient to patient.

Anatomic variations such as sacral dysmorphism are important and should be appreciated 19,20

as they may alter the necessary trajectory for screw placement

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prior to surgery

Furthermore, errors in numeric identification of the vertebrae can cause wrong level surgery

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.

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Thoughtful pre-operative planning and accurate flouroscopic guidance become even more important in these cases. For instance, Matityahu et al. reported a rate of 28.6% of misplaced

anatomy

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sacroiliac screws in patients with a dysmorphic sacrum compared to 11.9% in patients with normal

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Rates of injury of adjacent pelvic structures have been reported in larger studies

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. The

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incidence of neurological injuries in sacroiliac fixation has been noted between 0.5 to 7.7% and is usually associated with screw misplacement, the most common complication in sacroiliac screw fixation. As neurological and vascular injuries can occur in sacroiliac screw fixation the surgeon must be aware of the surrounding nerves and vessles, especially in cases of a fractured sacrum or in patient’s with dysplastic sacral anatomy

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. Misplaced sacroiliac screws have been seen in 3 to 29%

of reported cases and may be caused by suboptimal preoperative planning, poor understanding of the pelvic anatomy, poor imaging quality or lack of surgical inexperience

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. As already

mentioned by Zhu et al. “any excessive deflection of the S2AI screw may puncture the sacral or 4

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iliac cortex, damaging the tissues proximal to the pelvis which may be catastrophic”

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. Shilingford

et al. reported eight (7 lateral, 1 medial) cortical breach using a free-hand technique for S2AI screw placement

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. Likewise, Choi et al. described one cortical violation among 33 S2AI screws

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.

Accordingly, the aim of this study was to illustrate the relationship of neurovascular structures at

lateral relationships were observed: Medial relationships Obturator nerve, Lumbosacral trunk, S1 ventral ramus

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risk when the S2AI technique is used. Based on our cadaveric study, the following medial and

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The obturator nerve arises from the ventral rami of the second to fourth lumbar ventral rami. It descends posterior to the psoas major, emerging from its medial border at the pelvic brim to pass lateral to the common iliac vessels. The incidence of obturator nerve injuries using the S2AI techniqueis unkown. Movement–related pain in the area of the medial thigh and the knee could be

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symptoms of an obturator nerve lesion. The obturator nerve innervates the adductor musculature and if injured, could result in a gait disorder

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. The lumbosacral trunk is the continuation of the

L4 ventral ramus once it joins the L5 ventral ramus. As the lumbosacral trunk, this structure

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descends medial to the obturator nerve and mostly medial to the sacroiliac joint. The lumbosacral

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trunk is the lumbar imput into the lumbosacral plexus and thus, injuries to it can result in dysfunction to branches of this plexus including the sciatic nerve, which would result in significant gait difficulty and potential pain or sensory loss in the L4/L5 dermatomes. Injury to the S1 ventral ramus would affect multiple branches of the sacral plexus including the sciatic and superior and inferior gluteal nerves with significant ambulatory dysfunction and potential pain or sensory loss in the S1 dermatome. Iliopsoas muscle

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The iliopsoas muscle is the most powerful flexor of the hip and runs from the lumbar spine and iliac fossa to the lesser trochanter of the femur. The genitofemoral nerve typically pierces the anterior aspect of the psoas major muscle and is at risk with placement of an S2AI screw

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Injury to the iliopsoas can also indirectly damage any of the branches of the lumbar plexus as they

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traverse the muscle (Figure 5B). Internal iliac vessels

The internal iliac vein ascends posteromedial to the internal iliac artery to join the external

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. Damage to these vessels can have catastrophic results. Elder et al.

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sacroiliac joint (Figure 5B)

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iliac vein, forming the common iliac vein at the pelvic brim, anterior to the lower part of the

reported two “major vessel injuries” in 68 patients treated with S2AI Lateral relationships

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The superior gluteal artery is the largest branch of the internal iliac artery and runs

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posteriorly between the lumbosacral trunk and the S1 ventral ramus. It leaves the pelvis through the greater sciatic foramen superior to the piriformis and then divides into superficial and deep

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branches. The superficial branch enters the deep surface of the gluteus maximus. The deep branch passes between the gluteus medius and lateral surface of the ilium

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(Figure 3).

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Caudal relationships

An excessive caudal insertion of S2AI screws is likely to injure the neurovascular structures deep to the greater sciatic foramen, including the superior and inferior gluteal arteries, veins, and nerve, as well as the sciatic nerve

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.

Conclusions:

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Neurovascular structures including the superior gluteal artery, the sciatic nerve, the obturator nerve, the internal iliac vein and artery and the lumbosacral plexus are potentially at risk when using the S2AI pelvic fixation technique. On the basis of our anatomical study we recommend fluoroscopic guidance to prevent misplacement during the procedure in order to avoid cortical

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breach and resultant injuries to adjacent neurovascular structures.

Acknowledgements

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We wish to thank Zimmer, Inc. for their support and the cadaveric donors used in this study.

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Figure Legends

Figure 1: Obturator outlet view illustrating the S2AI screw. Figure 2: Teardrop view of the S2AI screw. Note the sacroiliac joint (SI joint) and tear drop as seen

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in this view. The SI joint, acetabulum, and iliopectineal lines are outlined. Figure 3: Lateral relationships to sacroiliac screw following removal of outer cortex. The gluteus maximus and medius have been reflexed. Note the piriformis (PF), sacrotuberous ligament (STL), iliac crest (IC), and superior gluteal nerve (SGN) and superior gluteal artery (SGA). The superior

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gluteal nerve is seen exiting the greater sciatic foramen above the piriformis muscle and traveling

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superficial to the gluteus minimus muscle.

Figure 4 A: Illustrating screw localization and trajectory on a cadaveric specimen. Figure 4 B: Removal of the bone around the S2AI screw and visualizing the medial (internal) surface of this bony piece and the nearby major nerves and vascular structures. The ventral rami of

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L4 through S3 are seen as well as the obturator nerve (ON) and common iliac artery (CIA). Figure 5A: Schematic drawing illustrating screw trajectory

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Figure 5B: Schematic drawing illustrating the regional Anatomy

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Neurovascular Relationships of the S2AI Screw Placement: An Anatomical Study

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Pictures

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Figure 2

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cranial

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Figure 3

SGN and SGA PF caudal

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cranial

cranial

sacrum

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L5

L4

S1 S2 ON

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Greater Sciatic Notch caudal

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caudal

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ure 5A

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Highlights • Anatomical studies detailing the surrounding anatomy and relationships to vulnerable

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neurovascular structures using the S2AI technique are lacking • Neurovascular structures including the superior gluteal artery, the sciatic nerve, the obturator nerve, the internal iliac vein and artery and the lumbosacral plexus are potentially at risk • Fluoroscopic guidance (obturator-outlet and teardrop radiographic) is recommended to prevent misplacement during the procedure

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anterior inferior iliac spine common iliac artery iliac crest obturator nerve piriformis S2 alar-iliac superior gluteal artery superior gluteal nerve sacroiliac joint sacrotuberous ligament

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AIIS CIA IC ON PF S2AI SGA SGN SI Joint STL

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Abbreviations

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Edward C. Benzel Editor-in Chief World Neurosurgery

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Dear Dr. Edward C. Benzel,

With regard to our manuscript, “Neurovascular Relationships of the S2AI Screw Placement: An Anatomical Study”, the authors wish to confirm there are no conflicts of interest associated

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with this publication to declare.

Sincerely

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Emre Yilmaz

Any correspondence should be directed to me at the following address:

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Emre Yilmaz, MD Swedish Neuroscience Institute Swedish Medical Center 550 17th Avenue Suite 500 James Tower, 5th Floor Seattle, WA 98122, United States phone: (206) 399-1438 fax: (206) 320-5250 e-mail: [email protected]