A radiological analysis of pelvic fixation trajectories: patient series

Jonathan P Scoville Department of Neurosurgery, Clinical Neurosciences Center, University of Utah, Salt Lake City, Utah

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Evan Joyce Department of Neurosurgery, Clinical Neurosciences Center, University of Utah, Salt Lake City, Utah

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Andrew T Dailey Department of Neurosurgery, Clinical Neurosciences Center, University of Utah, Salt Lake City, Utah

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Marcus D Mazur Department of Neurosurgery, Clinical Neurosciences Center, University of Utah, Salt Lake City, Utah

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BACKGROUND

Three well-defined methods for pelvic fixation are used for biomechanical support in spine fusion constructs: iliac, recessed iliac, and S2-alar-iliac (S2AI) screws. The authors compared the maximum screw sizes that could be placed with these techniques by using image-guidance software and high-resolution computed tomography scans from 20 randomly selected patients. Six trajectories were plotted per side, beginning at recognized starting points (standard or recessed posterior superior iliac spine [PSIS] or S2AI screw) and ending at the anterior inferior iliac spine (AIIS) or supra-acetabular notch (SAN).

OBSERVATIONS

The mean maximum screw length and width ranged from 80.0 ± 32.2 mm to 140.8 ± 22.6 mm and from 8.25 ± 1.2 mm to 13.0 ± 2.7 mm, respectively, depending on the trajectory. Statistically significant differences in length were found between the standard and recessed PSIS trajectories to the AIIS (p < 0.001) and between the standard PSIS-to-AIIS trajectory and the S2AI-to-AIIS (p = 0.007) or S2AI-to-SAN (p < 0.001) trajectories. The most successful trajectory was the PSIS to SAN (95%, 38/40).

LESSONS

The traditional iliac screw trajectory enabled the longest and widest screw trajectories and highest rate of successful screw placement with the fewest theoretical breaches more reliably than recessed and S2AI trajectories. These findings may help surgeons plan for maximum screw purchase for pelvic fixation.

ABBREVIATIONS

AIIS = anterior inferior iliac spine; CT = computed tomography; PSIS = posterior superior iliac spine; S2AI = S2-alar-iliac; SAN = supra-acetabular notch; 3D = three-dimensional

BACKGROUND

Three well-defined methods for pelvic fixation are used for biomechanical support in spine fusion constructs: iliac, recessed iliac, and S2-alar-iliac (S2AI) screws. The authors compared the maximum screw sizes that could be placed with these techniques by using image-guidance software and high-resolution computed tomography scans from 20 randomly selected patients. Six trajectories were plotted per side, beginning at recognized starting points (standard or recessed posterior superior iliac spine [PSIS] or S2AI screw) and ending at the anterior inferior iliac spine (AIIS) or supra-acetabular notch (SAN).

OBSERVATIONS

The mean maximum screw length and width ranged from 80.0 ± 32.2 mm to 140.8 ± 22.6 mm and from 8.25 ± 1.2 mm to 13.0 ± 2.7 mm, respectively, depending on the trajectory. Statistically significant differences in length were found between the standard and recessed PSIS trajectories to the AIIS (p < 0.001) and between the standard PSIS-to-AIIS trajectory and the S2AI-to-AIIS (p = 0.007) or S2AI-to-SAN (p < 0.001) trajectories. The most successful trajectory was the PSIS to SAN (95%, 38/40).

LESSONS

The traditional iliac screw trajectory enabled the longest and widest screw trajectories and highest rate of successful screw placement with the fewest theoretical breaches more reliably than recessed and S2AI trajectories. These findings may help surgeons plan for maximum screw purchase for pelvic fixation.

ABBREVIATIONS

AIIS = anterior inferior iliac spine; CT = computed tomography; PSIS = posterior superior iliac spine; S2AI = S2-alar-iliac; SAN = supra-acetabular notch; 3D = three-dimensional

Pelvic fixation provides a stable base to increase the rate of fusion across the lumbosacral junction after surgery.1–4 It is frequently used for spinal deformity correction that involves long-segment thoracolumbar constructs, pelvic obliquity, high-grade lumbosacral spondylolisthesis, osteotomies in the low lumbar spine, pseudarthrosis at the lumbosacral junction, and other conditions in which increased rigidity is needed at the distal end of a construct.3,5,6 The main advantage of pelvic fixation is provided by stabilization at the lumbosacral pivot point, which has been shown to double the flexion forces necessary to cause failure.7,8 This point is the juncture at which lumbosacral fusion constructs need ventral fixation to endow the entire construct with improved biomechanical stability and promote higher fusion rates.1,3,4,9,10

There are three well-defined methods of extending posterior spinal fusions to the pelvis: the iliac screw, the recessed iliac screw, and the S2-alar-iliac (S2AI) screw (Fig. 1).5,11–13 The iliac screw, the most traditional method, is inserted at the posterior superior iliac spine (PSIS) and traverses the iliac crest toward the anterior inferior iliac spine (AIIS).2,4,11 In contrast, the recessed iliac screw is inserted in a position medial and recessed to the PSIS, but it has the same trajectory as the traditional iliac screw toward the AIIS.5,12,14 The S2AI screw starts at the midpoint between the S1 and S2 sacral foramina, approximately 1 cm lateral to the sacral-iliac joint, traverses the sacrum and sacral iliac joint, and anchors in the ilium.5,15

FIG. 1
FIG. 1

Image-guidance representation of the six trajectories: posterior superior iliac spine (PSIS)-to-anterior inferior iliac spine (AIIS) trajectory (light blue line, A), PSIS-to-supra-acetabular notch (SAN) trajectory (yellow line, B), recessed PSIS-to-AIIS trajectory (gold line, C), recessed PSIS-to-SAN trajectory (pink line, D), S2-alar-iliac (S2AI)-to-AIIS trajectory (green line, E), and S2AI-to-SAN trajectory (orange line, F).

There has been significant debate among spine surgeons as to which of the three methods is best.1,4,13,16–23 The traditional iliac starting point at the PSIS is associated with greater pain from the muscle dissection and prominence of the screw heads, and it frequently requires the use of rod connectors. The recessed iliac screw entry point provides for a lower-profile screw head and eliminates the need for offset connectors, but it is less well studied and some argue it has a poorer biomechanical profile.4,5 The S2-alar entry point allows for a much lower-profile screw head and eliminates the need for offset connectors but requires a more acute angle between the screw head and screw shaft, which may lead to higher rates of failure.4 There have been multiple biomechanical studies demonstrating biomechanical equivalence between S2AI and standard iliac screw fixation as well as case series reporting the advantages of each method over others, especially when comparing traditional iliac bolts with S2-alar screw constructs.1,6,19,21–26 Despite these controversies, it is generally agreed that the more anterior into the iliac crest toward the AIIS that the screw is able to be positioned, the more biomechanical strength is given to the construct, making construct failure less likely.1,5,7 It is also generally agreed that wider screws provide more mechanical advantage than narrower ones, especially when they are able to engage but not break the cortical bone.10

The trajectory through the iliac crest and toward the AIIS is the established trajectory for all three methods of pelvic fixation. Another trajectory aimed at the anterior supra-acetabular notch (SAN) was recently described and compared with the AIIS trajectory in a biomechanical study in which both trajectories were found to have similar biomechanical properties.2–4 The AIIS trajectory has a lower risk for acetabular breach, and placing the screw in the cortical bone can increase fixation. The SAN ending point permits screw placement at the roof of the sciatic notch, which also increased fixation, but there is a greater risk for acetabular breach.3

The purpose of this study was to compare the six possible combinations of entry points and trajectories to determine the technique that is the most reliable for inserting the theoretically longest and widest pelvic fixation screws. We used high-resolution computed tomography (CT) to construct three-dimensional (3D) models through which all six possible combinations of entry points and trajectories could be measured and compared. Our goal was to identify the combination that allows the largest theoretical screw possible and to measure which trajectory allows for the lowest number of violations to the bony anatomy.

Study Description

Model Creation

This radiological study was reviewed and deemed to be exempt by the institutional review board. For the purposes of creating the 3D models, 100 patients who had undergone high-resolution CT of the abdomen or pelvis were retrospectively identified. All the patients had undergone high-resolution CT imaging that included the entire sacrum and ilium. Patients with a history of previous spinal surgery or any disease of the bony spine or pelvis were excluded. CT scans from 20 patients were randomly selected and stratified by sex in a 1:1 ratio from our list using Stata IC vs 15.1 (StataCorp), resulting in CT scans from 10 males and 10 females. Once these patients were identified, the scans were loaded into the Stealth 8 image-guidance system (Medtronic). The software was used to plan screw placement trajectories using the 3D reconstructions and two-dimensional images from the high-resolution CT.

The trajectories were measured beginning at each of the three described starting points with targets at both the AIIS and the SAN. This produced six total screw trajectories: the PSIS to the AIIS, the PSIS to the SAN, the recessed PSIS to the AIIS, the recessed PSIS to the SAN, the S2 to the AIIS, and the S2 to the SAN. Each trajectory was measured on both the right and left sides of the patient, with each counted as an independent measurement, resulting in 40 total samples and 240 total measurements. Both maximum length and maximum width of the trajectory were measured. The total length was measured as a point-to-point measurement from the starting point to the target. As a proxy for the maximum screw width that would be allowed on any trajectory before causing a cortical bone breach, the width was measured from the middle of the trajectory to the cortical bone at the narrowest point on the trajectory and multiplied by 2. If a trajectory reached its target without violating other anatomical structures, it was counted as a successful screw placement and length and width measurements were obtained. However, if a trajectory encroached on other anatomical structures or breached the sacroiliac joint or the pelvic curve, it was considered a failure, and the trajectory was then adjusted to allow for a screw width of at least 8 mm. Once this was done, the length was measured (Fig. 2).

FIG. 2
FIG. 2

Representation of how failures were recorded. If the trajectory breached the cortical bone or would not allow for an 8-mm screw, then it was counted as a failure and the trajectory was changed to allow for at least an 8-mm screw. The length of this new trajectory was then measured. A: In the initially measured trajectory from the recessed PSIS to the AIIS, the trajectory passes through the medial pelvic wall and also breaches the sacroiliac joint; it does not allow for an 8-mm screw. B: The redirected screw trajectory that allows for an 8-mm screw without violating the sacroiliac joint or the medial pelvic wall. Gold lines refer to planned screw trajectory.

Statistical Analysis

Statistical analyses including descriptive statistics and inference were done using Stata IC version 15.1. Using the traditional iliac screw trajectory, PSIS to AIIS, as the gold standard, the Student t test was used to compare the lengths of the other trajectories. Fisher’s exact test was performed to identify statistically significant differences in screw placement successes between the PSIS-to-AIIS trajectory and the other trajectories. An a priori α of 0.05 was chosen to determine significance.

Results

The overall mean age was 54.7 ± 15.6 years, with a range from 23 to 80 years. The most common reason for obtaining a high-resolution CT of the abdomen was cancer staging (n = 14), followed by abdominal pain (n = 10) or distension (n = 4), flank pain (n = 6), trauma workup (n = 4), and jaundice (n = 2).

The mean screw length for the trajectories ranged from 80.0 ± 32.2 mm (recessed PSIS to SAN) to 140.8 ± 22.6 mm (PSIS to AIIS; Table 1). The mean maximum screw width ranged from 8.25 ± 1.2 mm (recessed PSIS to AIIS) to 13.0 ± 2.7 mm (PSIS to SAN). Statistically significant differences in length were found between the PSIS-to-AIIS trajectory and the recessed PSIS-to-AIIS trajectory (p < 0.001), the PSIS-to-AIIS trajectory and the S2AI-to-AIIS trajectory (p = 0.007), and the PSIS-to-AIIS trajectory and the S2AI-to-SAN trajectory (p < 0.001). There was no significant difference in length between the PSIS-to-AIIS trajectory and the PSIS-to-SAN trajectory (p = 0.55). The difference in maximum width was not tested for significance because the widths were adjusted to allow for successful screw placement along a trajectory in which the narrowest part was 8 mm. Nevertheless, the trajectory that allowed for the widest screw was the PSIS-to-SAN trajectory, with the SAN trajectories consistently having wider screw placements than the AIIS trajectories.

Please insert table here

The success rate of screw placements for the trajectories ranged from 15% (6/40) for the recessed PSIS-to-AIIS and recessed PSIS-to-SAN trajectories to 95% (38/40) for the PSIS-to-SAN trajectory (Fig. 3). Using the PSIS-to-AIIS trajectory as the gold standard, we found that there were statistically significant differences in the proportion of successful screw placements with the recessed PSIS-to-AIIS trajectory (p < 0.001), the recessed PSIS-to-SAN trajectory (p < 0.001), the S2AI-to-AIIS trajectory (p < 0.001), and the S2AI-to-SAN trajectory (p < 0.001). The PSIS-to-SAN trajectory was the only trajectory that had a higher success rate numerically than the PSIS-to-AIIS trajectory, but there was no statistically significant difference in the proportion of successful screw placements between the two (p = 0.67).

FIG. 3
FIG. 3

The proportions of successful theoretical screw placements along each trajectory.

Patient Informed Consent

Scans included in the study were likely from trauma patients, who would not have consented; however, we had an institutional review board exemption for the study.

Discussion

Observations

Our study may help guide surgeons when selecting a pelvic fixation approach to maximize implant size. We compared the maximum lengths allowed by all six possible trajectories with a minimal width of 8 mm to identify the most successful trajectory by measuring the amount of cortical bone breaches observed. Overall, the trajectories that began at the traditional iliac screw starting point, the PSIS, had the fewest cortical bone breaches and provided the longest possible screw placement, independent of the target. The SAN trajectories tended to accommodate wider screws than the AIIS trajectories for each of the starting points, although this was not compared statistically. The maximum length of theoretical screw placement varied from trajectory to trajectory, with the shortest length found in the recessed PSIS trajectories, a fact that is supported by the higher failure rates of these trajectories.1,4,14 The recessed PSIS enters the ilium close to the sacroiliac joint and medial pelvic wall, which limits the screw length and width compared with the other trajectories. Although the ideal screw lengths we calculated are longer than those typically used for spinal surgery, our study demonstrates that much longer and wider screws are possible and can be considered for challenging cases.

We observed a difference in the number of breaches among the various trajectories. The PSIS-to-AIIS and PSIS-to-SAN trajectories had significantly fewer failures than all other plotted trajectories, meaning that if a screw were to be placed along trajectories starting at either the recessed PSIS or S2AI, there is a significantly higher likelihood of breaching cortical bone and extending into the adjacent soft tissues. This means that, in practice, the surgeon is more likely to have to deviate from the maximum length trajectory and thereby place a shorter screw. However, if the starting point is the PSIS, then the maximum length trajectory can be used with minimal deviation. Our results are generalizable because we used standard software that surgeons currently use to plan screw trajectories preoperatively—that is, for robotic image guidance or during surgery with real-time image guidance. Although breaches can be controlled intraoperatively by adjusting the starting points or ending points of the screw trajectory using intraoperative image guidance, live fluoroscopy, or freehand methods based on the patient’s individual anatomy, our screw planning technique resembles modern robotic image guidance in which ideal starting points and trajectories are determined from a preoperative CT scan to minimize deviations and maximize precision. The current study did not allow for such course corrections and therefore may explain the higher rate of breaches than described in the literature.27,28 Although we did not fully explore this result, it is interesting to note that there tended to be more failures in women than in men. For example, all the failures in the PSIS-to-AIIS and PSIS-to-SAN trajectories occurred in women. This may be due to anatomical differences in the structure of the pelvis between men and women29 and should be taken into consideration when planning pelvic fixation.

Lessons

Preoperative and intraoperative planning are crucial parts of building a successful construct but are often limited by the types of imaging available. Standard preoperative CT scans of the lumbar spine typically do not include the entire iliac crest out to the AIIS or the SAN, making it difficult to preoperatively identify the best trajectory to guarantee the longest screw placement with the highest likelihood for success. Intraoperative CT scans combined with navigation can greatly improve the chances for successful screw placement. Although, at times, the AIIS or SAN may not be visualized using intraoperative guidance including CT scan or fluoroscopy, the trajectories discussed above may be extrapolated by taking into account certain anatomical landmarks. Additionally, reviewing the evaluation of the entire pelvis with high-resolution CT that includes the entire iliac crest, sacrum, and lower lumbar spine provides the surgeon with the best anatomical information necessary to decide which trajectory is best suited to the patient and may help optimize screw placement.

This study represents an exploratory radiological approach to understanding the maximal possible theoretical screw length and width used to anchor a lumbosacral fusion construct to the pelvis. Most of the limitations are inherent in the study design. This study was not designed to test the actual biomechanical stability of each trajectory when compared with one another but rather to provide a reference for theoretical screw placement in terms of size and safety. The initial cohort of patients was chosen for convenience because of the access to high-resolution CT scans that included the entire iliac crest, but it may not be representative of the population of interest because patients with degenerative spinal disease were generally excluded. The statistical comparisons were performed as exploratory comparisons because no power analysis was done beforehand to identify how many patients would be needed to identify a clinically significant difference. Last, screw size is one of multiple factors that affect biomechanical stability. Bone mineral density, the crossing of cortical surfaces, and anchoring the screw in denser cortical bone located adjacent to the sciatic notch are other factors that should be investigated in future studies comparing pelvic fixation techniques but were outside the scope of our modeling study.

According to this image-guidance study, the trajectory from the traditional iliac screw starting point at the PSIS to the AIIS or the SAN as its target accommodates the longest and widest possible screw without breaching cortical bone. These results may help guide surgeons in selecting an appropriate pelvic fixation method when planning operations.

Acknowledgments

We thank Kristin Kraus, MSc, for editorial assistance.

Author Contributions

Conception and design: Mazur, Joyce, Dailey. Acquisition of data: Joyce. Analysis and interpretation of data: Mazur. Drafting of the article: Mazur, Scoville. Critically revising the article: all authors. Reviewed submitted version of the manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Mazur. Study supervision: Mazur.

Supplemental Information

Previous Presentations

This work was presented at the 2021 International Spinal Deformity Symposium, held December 3–4, 2021, in New York, NY.

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  • Collapse
  • Expand
  • FIG. 1

    Image-guidance representation of the six trajectories: posterior superior iliac spine (PSIS)-to-anterior inferior iliac spine (AIIS) trajectory (light blue line, A), PSIS-to-supra-acetabular notch (SAN) trajectory (yellow line, B), recessed PSIS-to-AIIS trajectory (gold line, C), recessed PSIS-to-SAN trajectory (pink line, D), S2-alar-iliac (S2AI)-to-AIIS trajectory (green line, E), and S2AI-to-SAN trajectory (orange line, F).

  • FIG. 2

    Representation of how failures were recorded. If the trajectory breached the cortical bone or would not allow for an 8-mm screw, then it was counted as a failure and the trajectory was changed to allow for at least an 8-mm screw. The length of this new trajectory was then measured. A: In the initially measured trajectory from the recessed PSIS to the AIIS, the trajectory passes through the medial pelvic wall and also breaches the sacroiliac joint; it does not allow for an 8-mm screw. B: The redirected screw trajectory that allows for an 8-mm screw without violating the sacroiliac joint or the medial pelvic wall. Gold lines refer to planned screw trajectory.

  • FIG. 3

    The proportions of successful theoretical screw placements along each trajectory.

  • 1

    Burns CB, Dua K, Trasolini NA, Komatsu DE, Barsi JM. Biomechanical comparison of spinopelvic fixation constructs: iliac screw versus S2-alar-iliac screw. Spine Deform. 2016;4(1):1015.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 2

    Hsieh PC, Mummaneni PV Introduction to lumbosacral and sacropelvic fixation strategies. Neurosurg Focus. 2016;41(video suppl 1):1.

  • 3

    Santos ER, Sembrano JN, Mueller B, Polly DW. Optimizing iliac screw fixation: a biomechanical study on screw length, trajectory, and diameter. J Neurosurg Spine. 2011;14(2):219225.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4

    Sohn S, Park TH, Chung CK, et al. Biomechanical characterization of three iliac screw fixation techniques: a finite element study. J Clin Neurosci. 2018;52:109114.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 5

    Kebaish KM. Sacropelvic fixation: techniques and complications. Spine (Phila Pa 1976). 2010;35(25):22452251.

  • 6

    Mazur MD, Ravindra VM, Schmidt MH, et al. Unplanned reoperation after lumbopelvic fixation with S-2 alar-iliac screws or iliac bolts. J Neurosurg Spine. 2015;23(1):6776.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7

    McCord DH, Cunningham BW, Shono Y, Myers JJ, McAfee PC. Biomechanical analysis of lumbosacral fixation. Spine (Phila Pa 1976). 1992;17(8)(suppl):S235S243.

  • 8

    Lebwohl NH, Cunningham BW, Dmitriev A, et al. Biomechanical comparison of lumbosacral fixation techniques in a calf spine model. Spine (Phila Pa 1976). 2002;27(21):23122320.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 9

    Hlubek RJ, Godzik J, Newcomb AGUS, et al. Iliac screws may not be necessary in long-segment constructs with L5-S1 anterior lumbar interbody fusion: cadaveric study of stability and instrumentation strain. Spine J. 2019;19(5):942950.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 10

    Karakasli A, Acar N, Uzun B. Straight-forward versus bicortical fixation penetrating endplate in lumbosacral fixation—a biomechanical study. J Korean Neurosurg Soc. 2018;61(2):180185.

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