Sacral/Pelvic Fixation

This review article explores the advancements in sacropelvic fixation, comparing traditional and modern techniques, with a focus on iliac and sacral 2 alar-iliac screw fixations. It addresses the biomechanical challenges inherent in securing the lumbosacral junction and discusses the integration of current and future technologies like robotics and augmented reality to improve surgical outcomes. The article underscores the importance of these innovations in enhancing stability and reducing complications in complex spinal surgeries.

Key points

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Indications for sacropelvic fixation include long fusion constructs, high-grade spondylolisthesis, patients at high risk of pseudoarthrosis, revision surgery, sacral trauma, 3 column osteotomies, and tumor resection.
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Iliac screw and second sacral vertebra alar-iliac (S2AI) techniques are the most used pelvic fixation techniques, with current literature supporting the use of both, with a slight favor to S2AI screws, given the reduced postoperative complications.
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Recent advancements in technology, including computed tomography navigation, robotics, and augmented reality, as well as sacroiliac fixation are explored for their potential to enhance surgical precision and outcomes in sacropelvic fixation.

Introduction

The lumbosacral junction’s unique anatomy and mechanical demands present significant challenges for achieving a stable and durable fixation which is necessary for eventual fusion during spinal deformity surgery. The L5-S1 joint is the most mobile segment in the lumbar spine in the sagittal plane with the steepest disc angle and largest amount of shear force across it. Historically, the challenges seen here have led to high rates of nonunion, hardware failure, and subsequent revision surgery. ^, As the understanding of the biomechanical properties of the lumbosacral junction has advanced, so too have the techniques for its stabilization. Current techniques focus on iliac and sacral 2 alar iliac (S2AI) screw techniques. Through examination of recent innovations and clinical outcomes, this article aims to illuminate the progressive strategies that are employed today to navigate the challenges of sacropelvic fixation.

Anatomy and biomechanics

Understanding the anatomy and biomechanics of the lumbosacral region and sacropelvic junction is essential for effective sacropelvic fixation when treating spinal deformity and instability. This area represents the interface where the lower spine meets the pelvis, involving the lumbar spine, sacrum, sacroiliac joints, and the pelvis. Each of these structures plays a role in the overall biomechanics of the spine and influence the techniques used for fixation.

The lumbosacral junction is distinguished by its sagittal mobility. This mobility allows for a considerable range of motion, enabling forward bending and backward extension. However, this region also presents challenges because of the steep angulation of the lumbosacral disc, which is more pronounced than at other spinal levels. A steeper sacral slope can increase the shear forces at the lumbosacral junction, thereby influencing the stability and stress distribution in the lower spine, and the sacroiliac joint connects the sacrum to the ilium providing stability and limited mobility. This is necessary for effective load transfer between the spine and the pelvis and the lower extremities. These joints are supported by strong ligaments but are susceptible to degenerative changes and misalignments that can impact spinal biomechanics.

The biomechanics of the lumbosacral junction, along with the sacrum and pelvis, play a critical role in the structural integrity and functional dynamics of the lower spine. The lumbosacral junction is the site of considerable biomechanical activity because of its transition from the more mobile lumbar spine to the relatively fixed sacral base and is particularly vulnerable to shear stresses due to the slope of the sacrum coupled with the lordotic curvature of the lumbar spine, which intensifies the angulation and force loading at this junction. McCord’s research highlights the importance of stabilization in this region; these findings suggest that constructs extending anterior to the middle osteoligamentous column at L5-S1 are notably more stable. This insight is crucial for surgical planning, emphasizing the need for fixation techniques that traverse or anchor anteriorly to this biomechanical pivot point to enhance stability and reduce the risk of instrumentation failure.

Modern sacropelvic fixation techniques, such as the use of iliac screws and S2AI screws, directly address the biomechanical challenges at the lumbosacral junction by providing anterior stabilization. These screws are designed to anchor into the ilium, bypassing the biomechanical weaknesses of the sacral vertebrae. By anchoring anterior to the pivot point at the lumbosacral junction, these techniques enhance the stability of the fixation. This placement has been shown to increase the overall fusion rates by creating a more stable construct that can withstand the high shear forces at this critical transition zone.

Indications

Sacropelvic fixation in spine surgery is employed to address a variety of spinal conditions and deformities. These indications continue to progress as new technologies and implants become available. Current clinical scenarios that necessitate sacropelvic fixation include long fusion constructs, high-grade spondylolisthesis, and patients at high risk of pseudoarthrosis, revision surgery, sacral trauma, 3-column osteotomies, and tumor resection.

The definition of long fusion constructs is debated but is generally considered to be constructs that extend from the sacrum to at least L2 and cephalad. The long lever arms created by these constructs increase the mechanical load at the lumbosacral junction, thereby amplifying the risk of instrumentation failure. Historically, arthrodesis attempted without regard to the lumbosacral junction resulted in pseudoarthrosis rates above 40%. ^, Pelvic fixation helps distribute these loads more effectively, reducing the risk of pseudarthrosis and improving the overall stability of the construct.

Patients with a high risk of pseudarthrosis, such as those with poor bone quality, osteoporosis, or previous failed spinal surgeries, often benefit from pelvic fixation. ^, The enhanced stability provided by extending the fixation into the pelvis ensures improved biomechanical strength during the formation of an appropriate fusion mass. Additionally, pelvic fixation is frequently indicated in revision spine surgeries where previous interventions have failed. These cases may involve hardware failure, infection, or inadequate fusion, necessitating a more robust and extensive approach to achieve spinal stability. In cases of spinal trauma or tumor resection, structural integrity may be compromised, particularly at the lumbosacral junction, and pelvic fixation can be essential in order to restore stability. It allows for the effective transfer of loads from the upper structure of the spine to the pelvis, mitigating the effects of weakened or removed vertebral or sacral elements.

Historical perspective

Sacropelvic fixation has evolved through years of innovation and refinement. The modern era of spine surgery began with the introduction of the Harrington rod system in the early 1960s. Developed by Paul Harrington, the Harrington rod provided posterior distraction and was effective in correcting thoracic spinal curvatures. However, although it significantly improved spinal alignment, it had limitations, especially in the sacropelvic region where it failed to provide adequate rotational stability or prevent kyphotic collapse over time. In the 1970s, Eduardo Luque advanced spinal fixation techniques by introducing segmental spinal instrumentation. Unlike the Harrington rod, which applied distraction forces, the Luque system used sublaminar wires to anchor each vertebra to a single rod on each side of the spine. The Luque technique was particularly beneficial for patients with neuromuscular scoliosis, offering enhanced segmental fixation, but still resulted in high rates of pseudoarthrosis.

Further advancements were made in the 1980s with the development of the Cotrel-Dubousset instrumentation. This technique introduced pedicle screws, hooks, and multiplanar rods that allowed for correction in all 3 planes of the spine. The Cotrel-Dubousset system provided rotational correction and greater rigidity. Despite its advancements in thoracic and lumbar fixation, the technique still required additional modifications to adequately address sacropelvic instability with hardware failure rates of 44% at the sacral level. The Galveston technique, developed in the 1980s by Allen and Ferguson, marked a significant evolution in sacropelvic fixation. This technique involved the insertion of rods into the pelvis, which were bent to match the contours of the sacrum and pelvis, providing a more robust foundation for the spinal construct. The rods were anchored into the iliac wings, which greatly improved the stability of the lumbosacral junction and were crucial for long spinal fusions extending to the sacrum, especially in cases of adult spinal deformity or high-grade spondylolisthesis. However, implant prominence was an issue because of the starting point of the PSIS as well as implant loosening over time, leading to the described “windshield wiper” effect,. ^,

Modern techniques

Sacral Screws

Multiple options exist for sacral fixation including S1 pedicle screws, S2 pedicle screws, sacral ala, and sacral promontory screws. However, the use of sacral pedicle screws alone as the primary means of fixation in long fusion constructs is not advised because of the high failure rates likely resulting from the large diameter of the sacral pedicle’s soft cancellous bone. ^, This wide pedicle diameter does allow for medial direction of S1 screws, and with the “tricortical” purchase concept described by Lehman and colleagues, the pullout strength of these screws can be increased. These screws are aimed 30 to 40° medially and 15° cephalad to the superior S1 endplate ending with cortical purchase in the sacral promontory. The addition of S2 pedicle screws does not enhance pullout strength given that the construct is posterior to the sacral pivot point. The narrow S2 corridor also makes screw placement here technically more difficult.

Iliac Screws

The iliac screw technique is a common modern technique in sacropelvic fixation ( Fig. 1 ). Over the years, this technique has evolved to include variants such as subcrestal placement and percutaneous placement, each adapting to the unique demands and challenges of spine surgery. Iliac screw fixation demonstrates an advantage over the Galveston rod technique in instrument loosening and pseudoarthrosis rates. ^, This biomechanical advantage is thought to be because of the divergent trajectory to the remainder of the construct. The screws are typically directed toward the anterior inferior iliac spine, which offers strong cortical bone for screw purchase and optimal biomechanical leverage. Another distinct advantage of this technique is the degree of freedom for screw placement and ability to place multiple pelvic screws if desired along the ilium from the iliac tubercle to the posterior superior iliac spine (PSIS). These points of fixation typically lie off the linear axis to the lumbar pedicle screws and are linked to the construct through rod connectors. This placement also increases the ability to create multi-rod constructs which increases biomechanical rigidity. ^,

The weaknesses of the traditional iliac screw technique primarily relate to postoperative discomfort, particularly due to screw prominence. Iliac screw placement is often not in line with lumbar and sacral pedicle screws dictating the need for offset connectors. Incorrect placement in the inferior direction can result in violation of the sciatic notch and potential risk to the sciatic nerve and the superior gluteal artery. ^, Errant placement can also lead to iliac fractures or hardware failure. This technique also requires a large soft tissue dissection leading to increased pain and infection risk. Despite these challenges, with advancements in surgical techniques and imaging technology, the iliac screw method continues to be a reliable choice for achieving durable sacropelvic fixation.

Subcrestal Placement

A notable variant of the iliac screw technique is subcrestal iliac screw placement. This approach involves inserting the screw below the iliac crest, aiming to minimize hardware prominence and reduce soft tissue irritation. ^, An additional benefit is that these screws are often in line with the more superior sacral and lumbar screws, thus obviating the need for offset connectors. Subcrestal placement is particularly advantageous for patients who are thin or have less soft tissue padding over the ilium as it decreases the risk of pain and discomfort post surgery.

Percutaneous Placement

Percutaneous iliac screw placement has gained popularity because of its minimally invasive nature, reducing the surgical impact on the patient. This technique involves inserting the screws through small incisions under fluoroscopic or navigation guidance, which minimizes tissue disruption and can lead to quicker recovery times and reduced infection rates. Rod attachment is then tunneled from the midline incision to the screw.

Sacral 2 Alar-Iliac Screws

The S2AI screw technique represents an advancement developed to address some of the limitations associated with traditional iliac screw placement ( Fig. 2 ). First described in 2007 by Sponseller and Kebaish, this screw placement alleviates the need for extensive rod contouring and offset connectors that were often required with traditional iliac screws. By anchoring the screw at the S2 vertebral level and directing it across the sacroiliac joint into the ilium, the S2AI screw provides a biomechanically sound fixation point that is in line with the spinal rods. One of the most significant advantages of the S2AI technique is the reduced prominence of hardware, which minimizes the risk of pain and soft tissue irritation postoperatively. ^, This aspect is particularly beneficial in patients with lower body mass, where prominent hardware can lead to significant discomfort.

The S2AI technique provides robust biomechanical stability at the lumbosacral junction through quad cortical fixation through the sacroiliac joint. By passing through the SI joint and anchoring into the dense bone of the ilium, it provides resistance to biomechanical stresses such as torsion and shear forces. While this provides a theoretic increase in biomechanical construct strength compared to other types of pelvic fixation, this has not been demonstrated in further biomechanical testing. ^, Despite its benefits, the S2AI technique does come with challenges. Accurate placement of the S2AI screw requires precise surgical technique and a thorough understanding of the complex pelvic and sacral anatomy. There is a risk of breaching the sciatic notch inferiorly and damaging surrounding neurovascular structures.

Iliac vs sacral 2 alar-iliac technique

Multiple recent studies have compared iliac screws and the S2AI screw techniques ( Fig. 3 ). Gao and colleagues performed a systematic review and meta-analysis in 2021 comparing radiologic outcomes and complications of the 2 techniques. They included data from 13 studies involving 722 patients in both the pediatric and adult population. Regarding radiologic outcomes there were no differences between the 2 techniques in postoperative cobb angle, pelvic obliquity, or sagittal vertical axis, except in subgroup analysis of pelvic obliquity in pediatric patients, where S2AI screws were shown to have significant advantage. Hasan and colleagues completed a meta-analysis comparing postoperative complications between the 2 techniques in 2020. Their study included 6 clinical studies directly comparing iliac screws to S2AI screws in 477 patients. Pooled analysis favored S2AI screws with regards to screw prominence, screw loosening, implant breakage, and revision surgery. They reported a trend toward more wound complications with the iliac screw technique; however, this was not statistically significant. Rahmani and colleagues preformed a similar meta-analysis in 2024 comparing iliac screws with S2AI screws in adult spinal deformity. Their review consisted of 15 studies and 1502 patients (889 iliac screws, 613 S2AI screws). They found that the iliac screws had significantly higher odds of revision, symptomatic screw prominence, and wound complications. Additionally, S2AI screws led to a larger postoperative decrease in pain.