Mount Sinai study highlights sex-based pelvic differences’ effect on spinal screw, rod placement during surgical procedures

Mount Sinai Study Highlights Sex-Based Pelvic Differences’ Effect on Spinal Screw, Rod Placement During Surgical Procedures
Findings aimed at tailoring treatment to patients’ specific anatomy

Journal: Spine Deformity

Title: Impact of sex on S2-alar-iliac pelvic screw position and lumbosacral rod alignment in adult spine deformity

Senior Author: James D. Lin, MD, MS, Assistant Professor of Orthopedics (Spine Surgery), Associate Director of the Mount Sinai Spine Surgery Fellowship for the Mount Sinai Health System

Bottom line: This study examined how sex-based pelvic anatomical differences affect S2 alar-iliac (S2AI) screw placement and rod alignment in the coronal plane during spine surgery. It is commonly known that males have a narrower pelvis, which results in a more medial S2AI screw starting point and greater rod convergence compared to females.

Surgeons often encounter challenges in aligning rods across the lumbosacral junction, especially in male patients. These observations suggested that pelvic morphology might play a role in screw positioning and alignment. Researchers aimed to objectively quantify these differences to inform preoperative planning and improve surgical efficiency.

Why this study is unique: To the team’s knowledge, this is the first study to describe sex-based differences in S2AI screw placement and their impact on lumbosacral rod alignment during surgical procedures.

Why the study is important: Men’s and women’s pelvises are shaped differently. Because of this, screws placed in the pelvis during spine surgery often sit in different positions depending on sex. In men, this can make it harder to connect the screws to the rods in a straight line, leading to extra rod bending or adjustments during surgery.

Understanding these differences is important because S2AI screws are a cornerstone of modern spinal deformity surgery. Misalignment at the lumbosacral junction can complicate construct assembly, particularly with pre-contoured rods, and may influence biomechanical stability and long-term outcomes. In the age of personalized medicine, we are increasingly able to treat each individual patient with a custom plan based on their individual sex, anatomy, and goals.

How the research was conducted: This study was a retrospective cohort analysis evaluating the impact of sex on pelvic screw placement in male and female adult deformity patients. Data was collected from a single institution and included cases performed by three surgeons experienced in both freehand and navigated S2AI screw placement. Preoperative and postoperative X-rays and computed tomography (CT) scans were analyzed to assess screw placement and anatomical differences.

185 complete cases were included in the analysis.

Results: The findings revealed that males had significantly smaller distances between the posterior superior iliac spine (PSIS) compared to females (7.1 cm vs 8.08 cm), aligning with conventional knowledge about sex differences in pelvic anatomy.

Additionally, researchers observed that the alignment of the rod was significantly more convergent distally in males versus females (−12.3 degrees vs + 7.6 degrees), indicating poor alignment of the S2AI screw with the rest of the construct proximally. The narrower male pelvis results in a more medial starting point for S2AI screws, causing rods to angle medially rather than achieving the desired parallel alignment, reflecting a novel sex-based difference in S2AI screw placement.

What this study means for patients: For patients, this means surgeons can better tailor procedures to their anatomy, potentially leading to shorter surgeries, fewer hardware complications, and more reliable outcomes.

What this study means for doctors: Clinicians performing multilevel spinal fusions should anticipate sex-based differences in screw alignment. Adjustments—such as modified pedicle screw trajectories, alternate pelvic fixation methods, or the use of intraoperative navigation/robotics—may be necessary to reduce rod contouring.

What the next steps are for this work: Future studies should investigate whether these anatomic differences translate into differences in clinical outcomes such as implant failure, pseudarthrosis, revision rates, or patient-reported outcomes. Biomechanical modeling and prospective trials could further validate strategies to optimize lumbosacral alignment based on sex-specific anatomy.

Quotes: “In today’s era of personalized medicine, understanding the differences in anatomy between individuals is key to delivering the best possible surgical care. Our study is the first to show that men and women may benefit from different approaches when placing pelvic screws during spine surgery, due to differences in pelvic shape,” Dr. Lin said. “These findings underscore the importance of patient-specific planning, which—when combined with advanced navigation technologies and custom implants—can significantly improve alignment and outcomes in spinal deformity surgery. We’re especially grateful to our dedicated medical students and trainees at Mount Sinai, whose hard work and commitment made this research possible.”

About the Mount Sinai Health System

Mount Sinai Health System is one of the largest academic medical systems in the New York metro area, with 48,000 employees working across seven hospitals, more than 400 outpatient practices, more than 600 research and clinical labs, a school of nursing, and a leading school of medicine and graduate education. Mount Sinai advances health for all people, everywhere, by taking on the most complex health care challenges of our time—discovering and applying new scientific learning and knowledge; developing safer, more effective treatments; educating the next generation of medical leaders and innovators; and supporting local communities by delivering high-quality care to all who need it.

Through the integration of its hospitals, labs, and schools, Mount Sinai offers comprehensive health care solutions from birth through geriatrics, leveraging innovative approaches such as artificial intelligence and informatics while keeping patients’ medical and emotional needs at the center of all treatment. The Health System includes approximately 9,000 primary and specialty care physicians and 10 free-standing joint-venture centers throughout the five boroughs of New York City, Westchester, Long Island, and Florida. Hospitals within the System are consistently ranked by Newsweek’s® “The World’s Best Smart Hospitals, Best in State Hospitals, World Best Hospitals and Best Specialty Hospitals” and by U.S. News & World Report's® “Best Hospitals” and “Best Children’s Hospitals.” The Mount Sinai Hospital is on the U.S. News & World Report® “Best Hospitals” Honor Roll for 2025-2026. 

For more information, visit https://www.mountsinai.org or find Mount Sinai on Facebook, Instagram, LinkedIn, X, and YouTube.

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