Zachary Husband OMS III¹; Ethan Kromberg OMS III¹; Aaron Lange OMS III¹; Colton Manfre OMS II¹; David Weinstein MD¹; Shannon M. Constantinides D.N.P.¹; Ian George PhD^2
1OCC: Colorado Center of Orthopaedic Excellence
²Rocky Vista University, College of Osteopathic Medicine (Colorado)

Abstract

Background
L5 nerve radiculopathy is a serious complication associated with iliosacral (IS) screw fixation, a procedure often performed to stabilize pelvic fractures. The proximity of the L5 nerve root to the S1 sacral segment introduces significant risk, particularly when sacral dysmorphism or complex anatomy is involved. Despite advances in surgical techniques, the potential for nerve injury remains a critical concern during IS screw placement.

Purpose/Hypothesis
This study aims to evaluate the risks associated with L5 nerve injury during IS screw fixation and identify factors that increase the likelihood of complications, particularly in cases involving sacral dysmorphism. The hypothesis is that proper preoperative planning, sacral anatomy assessment, and improved surgical techniques can minimize the risk of L5 nerve injury during IS screw fixation.

 Study/Design
This is a systematic review of cohort studies, methodology papers, reviews, and cadaver studies from SCOPUS and PubMed that investigate the relationship between sacral anatomy and L5 nerve root injury during IS screw fixation.

Methods
A comprehensive search was conducted using the keywords “L5 nerve radiculopathy” and “Iliosacral Screw Fixation.” Studies published within the last 15 years were included. Papers involving animal models or published in languages other than English were excluded. A total of 122 studies were identified, of which 15 were selected for detailed review based on their relevance to L5 nerve injury during IS screw fixation. The studies were analyzed for sacral anatomy variations, surgical techniques, and complications.

Results
The studies identified key factors that increase the risk of L5 nerve injury, including sacral dysmorphism, sacral morphology, and improper preoperative planning. Malpositioning of screws in the S1 sacral segment was associated with the highest risk of L5 nerve injury, particularly in cases involving dysmorphic sacra. Cadaver studies revealed that L5 nerve root proximity to the S1 segment significantly limits the margin for error, while imaging studies highlighted the importance of preoperative identification of osseous corridors. Improved techniques, such as the use of guidewires and triangulation methods, were found to enhance the accuracy of IS screw placement.

Conclusion
L5 nerve root injury is a major risk during IS screw fixation, particularly in cases with complex sacral anatomy. Preoperative planning that accounts for sacral morphology, along with the adoption of advanced surgical techniques such as the triangulation method by Zheng and Zao and guidewire use, is essential to minimize the risk of nerve injury. Further research should focus on improving intraoperative navigation and refining surgical techniques to enhance patient outcomes in sacral fixation procedures.

Keywords: Spine, Sacroiliac Fixation, Orthopedics

Background
Due to the unique anatomy of the L5-S1 junction, management of sacral fractures can pose challenges to orthopaedic surgeons. The current standard includes reduction via percutaneous iliosacral screw placement under fluoroscopic guidance.¹ When iliosacral screws are placed, great care must be taken to prevent injury to the L5 nerve root, which is located adjacent to the sacral ala. This issue becomes more complex in the presence of certain osseous morphological features, such as sacral dysmorphia, which is a common anatomic variant in the general population.² Research on this topic has demonstrated that individuals with sacral dysmorphia are at a heightened risk for cortical perforation and neurovascular injury following iliosacral screw placement.² The purpose of this review is to identify factors that may contribute to L5 nerve root injury and subsequent iatrogenic L5 radiculopathy following iliosacral screw placement, as well as specific techniques which may reduce surgical complications. 

Introduction
Iliosacral (IS) fixation is a standard surgical technique performed for the management of traumatic sacral fractures as well as in patients with congenital sacral dysmorphism.¹ IS fixation is also performed in instances of sacral insufficiency/ stress fractures.¹ In order to stabilize the joint, transacral screws or plates can be fixed to the sacrum and ilia.¹ Due to location, any number of surgical complications can arise secondary to hardware instrumentation, including iatrogenic radiculopathy. Radiculopathy, which is generally described as a sharp, burning, or electrical radiating pain caused by compressive forces exerted on the nerve as it transverses and then exits the neural foramina, is a complication which can arise during IS screw placement due to the close proximity of the L5 nerve root to the sacral ala.² Because the L5 nerve root lies adjacent to the sacral ala, L5 radiculopathy can occur as a result of cortical perforation, or cortical breach. ³ There was an increased risk is increased in patients with higher sacral dysmorphism.³ In order to avoid complications related to challenges posed in this regard by local anatomy, the use of various technologies and procedural advancements have become increasingly refined.⁴. In order to establish best practices for placing iliosacral screws during surgery, it is imperative that surgeons understand the anatomy of the L5 nerve root and adjacent structures. Meticulous placement of IS screws has the potential to lessen the risk of nerve injury and radiculopathy as a complication of sacral fracture managemet. The purpose of this paper is to examine current literature surrounding IS screw placement and corresponding L5 nerve injury. Best surgical practices have been outlined in an attempt to highlight the higher-risk aspects of the procedure, as well as methods for IS fixation that have been deemed valid and reliable. 

Methods
SCOPUS and PubMed were reviewed using the keywords: L5 nerve radiculopathy and Iliosacral Screw Fixation. 122 studies were identified that met inclusion criteria based off keywords. Cohort studies, methodology papers, reviews, and cadaver studies were included. Papers including animal models and papers that use a language other than English were excluded. Search criteria was then refined to include only those published within the last 15 years in order to identify current best practices. 15 studies were selected that focused on issues specifically related to L5 nerve root injury. 

Review of Literature
The preoperative planning process is highly dependent on the sacral anatomy present in the region and there have been numerous studies aimed at trying to categorize the various sacral anatomy variations and therefore establishing trends in anatomy that surgeons may utilize while percutaneously placing IS screws. In order to assist with organizing data collected on various types of sacral anatomy, sacral anatomy has been lumped into 3 main categories Ascending, Horizontal, and Descending. Studies that have been performed on the topic have utilized these 3 variations in sacral anatomy in order to make generalizations about the relationship between the sacral anatomy and potential danger to the neurovascular structures.

A 2019 study by Krappinger et al, aimed to highlight the importance of preoperative planning for safe and effective management of sacral fractures. The study included 34 cases in which 59 screws were placed. 2 cases were identified as having been malpositioned, resulting in cortical breach. (n=59 screw attempts) 21 patients with sacral insufficiency fractures and 13 patients with type C pelvic ring injuries underwent fluoroscopically guided SI screw fixation procedures with a specific preoperative planning procedure which utilized a multiplanar reformation tool (MPR) in order to establish “true” inlet and outlet views for percutaneous SI fixation. They determined that defining the ”corridor type” and assessing the “notch” would be useful with regards to the preoperative planning process. Ascending type, Horizontal type, and Descending type were the 3 designations of sacral morphology designations. The “notch” they referred to in the study was an indentation of the anterior cortex of the sacral ala between the SI Joint and sagittal midline. This feature becomes important because it reduces anteroposterior diameter of the osseous corridor and potentially leads to iatrogenic nerve complications. Among the pelvis’ the distribution was ascending type S1 (71%), Descending type (66 and 86% in middle and lower third respectively.) S2 was predominantly Descending type (>95%.)⁴

Navigating the anatomy of patients with sacral dysmorphism becomes more of a challenge in the pre-planning process due to the variations in their anatomy. Many studies have identified this need for more appropriate pre-planning with regards to patients with sacral dysmorphia and have measured trends in osseous corridors in order to determine areas for proper screw placement. 

Sacral dysmorphism bears implications for variations in osseous pathway diameter. In a 2019 study by Yin et al. 100 pelvic computed tomography (CT) scans were completed to evaluate transverse IS screw placement in various locations of the sacrum. Osseous fixation pathway diameter was measured and used as a tool by which to differentiate dysmorphic sacra from normal sacra. Researchers concluded that “the most common cause of sacral dysmorphism was the fusion of the L5 vertebrae to the true S1. This study provided insights into the prevalence of dysmorphic sacra and the various patterns of dysmorphic sacra that exist, however, their claims that drilling into the S1 sacral segment is unsafe remained theoretical based upon the current body of the literature. Drilling into the S1 sacral segment also bears potential implications of cortical breach and contact with L5.⁵

Percutaneous IS screw placement under fluoroscopic guidance has become a mainstay in pelvic surgery. In 2016 Goetzen et al. hypothesized that percutaneous IS screw placement under fluoroscopic guidance provided an effective method for providing posterior pelvic ring stabilization. First, 3 types of osseous corridors (ascending, horizontal, and descending) in 1000 cases were analyzed using plain film radiographs (include true inlet, outlet and lateral views of the sacrum), as well as a 2D fluoroscopic guidance under CT.⁶ Second, the corridor was analyzed for the presence of an anterior indentation of the sacrum between the SI joint and the midsagittal plane with proximity to the nerve root L5. Quantifying percentages of each type of sacrum encountered in the study allowed the facilitators to establish the most frequent sacral morphologies encountered and make predictions about how the L5 nerve root would be effected in each respective procedure the ascending type was the most frequent corridor type in the upper quartile of S1 (71%), relative frequency declining to 5% in the lower quartile. Descending types were found in 15% in the upper quartile of S1 and 86% in the lower quartile. In S2, descending types were found to be the most common.⁶ Ascending corridors have a smaller SI joint diameter than the sacroiliac midsagittal diameter. Descending corridors have a larger SI diameter than midsagittal diameter. 

In order to further discover the potential dangers sacral dysmorphism imposes on the L5 nerve root, Teo et al. performed a review of 36 cases which had received a total of 68 sacroiliac screws as a result of traumatic pelvic fractures. Researchers were interested in the potential for neurovascular injury as a result of surgery and role of sacral dysmorphism in this risk. Research hypothesized that percutaneous placement of IS screws was the most reliable technique utilized by surgeons when operating on the pelvic ring. Results demonstrated that sacral dysmorphism increased the risk of surgery involving the pelvic ring. Cortical breach was present in 3 cases (n=68) 2 of which were documented as having sacral dysmorphism. “The majority of screws placed in the study were percutaneous fixation (86.1%). Intraoperative navigation was used in 47.2% of the patient cohort. 30.6% of the cohort were found to have dysmorphic sacra.³ White cortical breach was present in 3 cases, none appeared to have resulted in injury to the L5 nerve root.

Patient to patient variations in sacral anatomy may bear significance in terms of the potential risks for cortical perforation or nerve injury. In a 2020 study performed by Kurosawa et al. the researchers studied three different types of technical pitfalls encountered during sacaroiliac joint (SIJ) arthrodesis. Although SIJ arthrodesis includes other stages and operative measures than IS screw placement, IS screws placed in cranial segments can perforate the sacrum anteriorly, resulting in nerve root radiculopathy. 3 separate cases with varying sacral anatomy were studied specifically evaluating the lumbosacral transitional vertebrae in each patient and how the variations contributed to surgical pitfalls. In one of the 3 cases (26 year old woman) the most cranial sacral segment was perforated by an SI screw which led to right buttock and lower extremity pain after the procedure. In the case of a 34- year- old man with bilateral SIJ pain, fluoroscopy guided anesthetic injections and manual manipulation of the S1 nerve root before drilling were critical towards avoiding nerve injury.⁷

Another tool being utilized in the realm of pelvic surgery is cement augmentation. A 2020 study by Hartensuer et al. involved 448 patients with 642 screws placed over a 15-year time period. Results of the study indicated that iatrogenic neuropraxia was documented in 2.47% of cases and was associated with IS screw placement (p-value .0001, 642 screw attempts). Researchers commented on the use of cement augmentation as a possible technique associated with a potential decrease in surgical or postoperative complications, Augmentation was mainly performed in fragility fractures and was associated with higher ASA scoring than non- augmented screws (47.3% ASA 3-4 in patients with augmented SI screws vs. 30.8% in patients without augmentation (p<.0391, n=642⁾⁸ 

Knowledge gained by cadaveric study over time has allowed for surgeons to determine proper placement of screws and proper techniques that aid in the avoidance of the L5 nerve root. Establishing safe zones for surgeons to utilize when placing screws is an important part of preoperative planning, and cadaveric studies have allowed for improvements in utilizing these safe zones.]

A 2016 cadaveric study by Zoccali et al aimed to evaluate the importance of the lumbosacral triangle when approaching the management of L5 vertebral body fractures, sacral fractures, sacral and iliac tumors, and extraforaminal decompression of nerve roots. By studying 3 key tissue planes (neurological plexus plane; intermediate plane including the iliosacral tunnel; and the posterior plane including the lumbosacral ligament and posterior muscular plane) it was determined that a lateral approach that corresponded to the medial aspect of the L4 transverse process may decrease the risk of neurovascular damage. In the cadavers studied, one of the commonalities recognized was that L5 significantly medializes as it exits its foramen, approaching the lateral aspect of the L5 body and L5-S1 disk inside the LST. The L4 root may also pass very close to the L5 root in its sagittal trajectory. The underlying anatomy discovered in the cadavers in this region supported the need for optimizing the lateral approach (n=3).⁹

Many of the existing cadaveric studies that have been completed on the topic have been completed in order to determine potential for iatrogenic injury to the L5 nerve root based on it’s close proximity to the S1 sacral segment.When orthopedic surgeons place IS screws in this region the L5 nerve root is within close proximity and so various studies have attempted to uncover potential dangers posed by the close proximity of the structures. 

Proximity of the L5 nerve root to the S1 sacral segment is of utmost importance when considering pre-planning for surgery. A 2022 cadaveric study by Tamagawa et al evaluated complications of S1 pedicle screw misplacement, including L5 nerve root injury. The study included evaluation of 54 L5 nerve roots in 27 cadavers. The ventral rami of the L5 nerve roots were dissected along their courses from the intervertebral foramina to the lesser pelvis. The running angles of the L5 nerve roots from the centerline were measured in the coronal plane. Using a saggital plane, the distance from the sacral ala to the L5 nerve roots was measured. Researchers found that the L5 nerve roots were in close proximity (within 1 mm) in all cases. This not only confirmed the potential risk of contacting the L5 nerve root during surgical operations, however, also indicated that they had poor mobility due to their anatomical location. The results of the study determined that the running angle of L5 changed at the most anterior surface of the ala of the sacrum. The angles of bilateral L5 nerve roots were 13.77 degrees ± 5.01 degrees (5.9 degrees-25.2 degrees), and 14.65 ± 4.71 (4.6-22.2) at intervertebral foramina to their inflection points respectively. Researchers determined that the L5 nerve roots traversed outward when exiting the foramina, indicating a safe zone for S1 pedicle screw placement. They concluded that in order to prevent iatrogenic L5 nerve root injury, surgeons should insert S1 pedicle screws medially with an angle of >0 degrees towards the inside of the S1 anterior foramina and the sacral alar screw laterally with an angle >30 degrees.¹⁰

In a 2021 cadaveric study performed by Ishak et al, a novel method was utilized to measure the movement of the L5 nerve root during flexion and extension of the hip and lower lumbar spine to better understand how the proximity of the L5 nerve root changes throughout patient movement. Researchers hypothesized that even if a misplaced IS screw does not breach cortex and is therefore not a problem at neutral, there may be complications that the patient will experience during movement, such as the nerve root touching the misplaced screw in either flexion or extension. The L5 nerve was fully exposed in 5 cadavers along it’s course from its exiting intervertebral foramen to the sacrum. Psoas major and minor were removed from the cadavers. Movement of L5 was then tested in flexion and extension phases. Flexion was to 100 degrees, Extension 40 degrees. Significant movement or displacement of the L5 nerve root could not be quantified in this study. No mechanical cause for L5 nerve palsy could be identified so the etiology of the condition remains unclear. 

Results of the study indicated that the nerve root did not move significantly during flexion and extension phases, and therefore would appear that the anatomic location of L5 during surgery will most likely remain very similar once the patient is mobile again. (p value was considered <.05 n=5).¹¹ Dynamic motion testing of the L5 nerve root had been an underexplored territory before this cadaver research and the designers of the study were able to assess the confounding variable of movement as well as proximity. 

Due to the importance of L5 location, many studies have been designed to determine the common point of origin of the L5 nerve root. A 2017 study by Pekham et al. evaluated the anatomic location of the L5 nerve root in 108 cases. Using full spine magnetic resonance imaging (MRI), 106 of the 108 cases demonstrated that the L5 nerve root originated from the 24^th (L5) vertebrae. The percentage of perfect agreement with reference standard was 98.1% (95% CI, 93.5%–99.8%.) Studying the anatomic variations between 108 pelvises allowed the researchers to evaluate the most common locations of the L5 nerve root.¹²

With data established for the probable location of the L5 nerve root, researchers can then determine angles of approach. A 2016 cadaveric study by Zoccali et al identified three tissue planes that may assist surgeons to determine proper surgical approach when planning for IS fixation. These planes include the neurological plane, which consists of the L4-L5 nerve root; an intermediate plane consisting of the iliosacral tunnel; and posteriorly, the lumbosacral ligament and myofascial plane. The authors contended that identification of the tissue planes can aid surgeons in planning an approach which will minimize risk to local neurovascular and myofascial anatomy. 

Guidewires are often utilized in order to properly place IS screws. A 2018 cadaveric study by Javidmehr and Golbakhsh evaluated a novel guidewire technique using a lateral sacral X-Ray. Guidewire entry point was determined on the X-Ray. The anterosuperior quadrant of S1 on one hemipelvis, divided into 4 smaller quadrants, whereupon the guidewire was inserte4d into the posteroinferior of the 4 miniature quadrants. In this fashion, the guidewire trajectory resembled a single point in the lateral sacral view, The distance to the sacral canal (important measurement for avoiding neurovascular structures) was determined to be 12.3 ± 4.9 mm, compared to the conventional method 5±2.3 mm (p-value .03)¹³ and provided safer, easier, faster method for the placement of IS screws. (n=91) 

There have been attempts by researchers to determine the ideal location to place sacral screws based on the patient’s external anatomical features. A 2023 study by Zheng and Zhao evaluated 91 cases using a “triangulation method” to identify ideal lateral sacral screw guidewire and IS screw positioning. Creating an area of triangulation encompassing the anterior superior iliac spine (ASIS) and the base of the greater trochanter of the femur, 95% of entry points were within a 12 mm centered radius of the apex of the equilateral triangle. Average distances measured between the needle entry point and average entry point were 0.0566 (0.0512–0.0619)., and the predicted point (regular triangle vertices) was (0.50, 0.87). The predicted point was 0.01 units away from the average point, and the average distance was 1.15 mm. 95% of entry points fell within a circle radius of 12 mm (0.50, 0.87).

As a portion of their statistical analysis, a one-sample t-test was performed on the abscissa and ordinate of the vertex of the regular triangle (0.5, 0.87) and the abscissa and ordinate of the entry point set. In the coordinate system, the average value of the receiving needle entry point’s abscissa and the ordinate was 0.5 (P < 0.001) and 0.87 (P < 0.001)¹⁴

Suggestions for future IS screw procedural success were brought forward in a 2018 paper entitled “Ten Tips to Improve Iliosacral Screw Placement” by Aprato et al. Suggestions made by the researchers mimic many of the common themes brought forward by the other authors, including carefully evaluating the morphology of the sacral ala,flexing the leg to avoid problems with the inlet view, designing vertical and horizontal sacral planes on the skin, carefully choosing the entry point when dealing with crescent fractures, choosing screw placement as orthogonal as possible to the dislocation or fracture, passing midline with screws for sacral fractures, avoiding full length drilling, checking proper screw depth, using a second screw in obese patients, and avoiding sitting after surgery post-op for 40 days.¹⁵

Discussion
Results from this review of literature regarding the risks of L5 radiculopathy during sacral fixation procedures indicate that the largest risk factor for L5 nerve injury is the insertion of screws in the S1 vertebral segment. The anatomy of this segment leaves small margin for error and issues may arise if multiple screws need to be placed in the S1 segment. If the S1 screw placement is absolutely required, the safest measures necessitate that the screw be placed obliquely, and that this only be performed in patients with normal sacral anatomy as dysmorphic sacra were noted in the literature to pose increased risks during surgery. S2 was been identified as the most viable segment for the placement of iliosacral screws and avoiding complications, most notably, L5 nerve injuries along with coinciding radicular symptoms. In order to avoid operating on upper sacral segments, surgeons may want to utilize lower sacral corridors and consider instrumentation of lower segments.

Osseous corridors of the sacrum were determined to be an important factor when deciding where to place iliosacral screws, and understanding the osseous corridors of each individual sacrum will be extremely beneficial in the preoperative planning process. Categorizing various sacra during cadaveric study has allowed researchers to discover prominent trends in the anatomy of the osseous corridors. Trends which become essential when attempting to predict proper trajectories for screw placement in patients. Multiple cadaveric studies have allowed for the discovery of anatomical relationship between the L5 nerve root and the S1 vertebral body, and thus the anatomical proximity of the L5 nerve root as a risk factor during screw placement is becoming more well understood. Studying the movements of the L5 nerve root in flexion and extension also allows for some insight into the amount of movement that the L5 nerve root can undergo postoperatively, when the patient is on the road to recovery from surgery. Any differences in patient anatomy during movement that would differ from the location of these structures during the operation, would become prevalent during this process.

Another concerning aspect of sacral fixation procedures that requires further exploration is how often operating surgeons move astray from the plan created during the pre-planning process. In many of these cases there is evidence resulting in higher chances of cortical breach. This may be due to the complicated anatomy encountered during surgery, which forces the surgeon to move away from the original plan, or it may have to do with differences that are observed when observing the patient open during surgery as opposed to viewing the anatomy on imaging alone. Regardless of the causative factors, this area must be explored more as there are many instances of this where cortical breach is present, and where the L5 nerve root is therefore most susceptible to injury. Researchers can potentially improve the safety of the procedure by further evaluating the rationale behind surgeons moving away from preoperative planning, thereby investigating ways to mitigate this potential area for associated complication.

In order to improve iliosacral fixation procedures, the utilization of techniques such as the “triangulation method” proposed by Zheng and Zhao may be beneficial towards minimizing surgical error. Relying heavily on the lateral view, the transverse plane insertion appears to be the most effective method for placing iliosacral screws. Identifying a single point in the lateral view by placing guidewires on both sides of the pelvis may also be beneficial, as was performed in the 2018 cadaveric study by Javidmehr and Golbakhsh.  Evidence supports that the lateral view is important for screw placement, and so focusing on methods that optimize the lateral view as well as assist with screw insertion in the transverse plane will continue to improve the efficacy of the procedure. These methods will both help avoid nerve root injury as well as maintain distance from the sacral foramina, and prevent cortical complications post operatively. 

Future Research
The focus of future research and the areas which need to be explored are the options for placing instrumentation in the lower sacral segments. There is currently a vast body of literature stating that the S1 segment is unsafe and that S2 is most often the best segment for placement of screws, however, there is a glaring lack of literature focused on placing instrumentation in the lower sacral segments and until surgeons have better options for placing screws other than S1, this will remain an area of question. Until surgeons have viable options for screw placement other than S1, they will continue to utilize the S1 segment.

Further research should include evaluation of the safest methods for iliosacral screw placement. Current standard-of-care techniques include the use of guidewires and fluoroscopic imaging to assist with percutaneously screw placement have been extremely successful thus far. Advancing guidewires under fluoroscopic control, using inlet and outlet views according to planned trajectory, predrilling, using cannulated screws, EMG, NCV, assessing for and utilizing proper tissue planes all contribute to successful preoperative planning as well as surgical operations. Imaging modalities of the future may surpass fluoroscopic guidance procedures. However, these procedures are still considered to be very effective. These methods allow for the best chance to avoid the L5 nerve root during surgery and to place iliosacral screws successfully. Cement augmentation of hardware has also been posed as a viable option for increasing the reliability of the procedure, however, although there is scarce evidence backing this method to date.

Conclusion
Proper placement of SI screws during sacral and pelvic fixation procedures is essential due to the potential risks of iatrogenic radiculopathy. Patients with sacral dysmorphism may require even more effective preoperative planning due to variations present in their sacral anatomy. Intricate presurgical planning and imaging techniques are therefore of utmost importance while evaluating local anatomy as well as developing plans for appropriate surgical management. Methods such as the “Triangulation Method” proposed by Zheng and Zhao allow for appropriate pre surgical planning and avoidance of operational error. Patient positioning, equipment decisions and presurgical planning are all essential components of a successful sacral fixation procedure.

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