Do Iliac Screws Placed Close to the Sciatic Notch Have Greater Pullout Strength?

Wednesday, February 21, 2024

Presenting Author(s)

Hani Chanbour, MD

Spine fellow
Department of Neurological Surgery, Vanderbilt University Medical Center, Nashville, TN
Nashville, TN, US

Co-Author(s)

AA

Amir Abtahi, MD

orthopedic surgeon
Department of Orthopedic Surgery, Vanderbilt University Medi, United States
MA

Mir Ali, n/a

Medical Student
Hackensack-Meridian School of Medicine, United States
Christopher M. Bonfield, MD

Neurosurgeon
Department of Neurological Surgery, Vanderbilt University Medical Center, Nashville, TN
Nashville, Tennessee, United States
JC

Jeffrey W. Chen, PGY1

resident
Baylor college of medicine, United States
JN

Jeffry Nyman, MD

resident
Vanderbilt University medical center, United States
SR

Steven Roth, MD

Spine fellow
new york presbyterian hospital, United States
BS

Byron Stephens, MD

orthopedic surgeon
Vanderbilt University School of Medicine, United States
Su

Sasidhar uppuganti, MS

researcher
Vanderbilt University medical center, United States
Scott L. Zuckerman, MD, MPH (he/him/his)

Assistant Professor of Neurological Surgery
Vanderbilt University Medical Center
Nashville, Tennessee, United States

Disclosure(s):

Hani Chanbour, MD: No financial relationships to disclose

Introduction: Iliac fixation provides an important anchor in adult spinal deformity surgery constructs, yet optimal screw position in relation to the sciatic notch remains unknown. In twelve cadavers undergoing S2-alar-iliac (S2AI) screw placement, we tested the pullout strength of screws placed in proximity to the sciatic notch (≤5mm) vs. farther away from the sciatic notch (>5mm).

Methods: A biomechanical, cadaver-based study was performed on twelve cadavers undergoing bilateral S2AI screw insertion. The position of the S2AI screw with respect to the sciatic notch was dichotomized as ≤5mm from the sciatic notch on the right side and >5mm on the left side, confirmed with c-arm fluoroscopy. All screws were 100x8.5mm. The primary outcome was the pullout strength of the screw (N). Secondary outcomes were stiffness (N/mm), yield force (N), and work-to-failure (N-mm). The ischial tuberosity was embedded into polymethyl methacrylate and secured ton a custom 3-axis vise grip mounted to a 14.5kN load cell. Pullout testing was performed at 5mm/min. Force and displacement data were collected at 100Hz and evaluated using MATLAB. Mann-Whitney test was performed.

Results: Of 24 S2AI screws, 3 screws could not be tested due to cement-bone interface failure. A positive though non-significant trend of screw pullout strength was found for screws close to the notch compared to farther from the notch (861.8±340.7 vs. 778.7±350.8N, p=0.859). Similarly, screws close to the notch demonstrated a higher trend of stiffness (149.4±145.4 vs. 111.34±128.2N/mm, p=0.320) and force to yield (806.9±352.0 vs. 618.6±342.9N, p=0.455). Conversely, screws farther from the notch had a higher but similarly non-significant area under the force vs. displacement curve (10867.0±9565.0 vs. 14196.6±9578.3N-mm, p=0.455), which might be due to excess sheer/translation force that could not be reliably quantified.

Conclusion : Though placing S2AI screws≤5mm of the sciatic provided stronger fixation in 3 out of 4 biomechanical testing categories, these results were not statistically significant.