A positioning method for the transiliac-transsacral screw bone channel based on a manual tracing of a transparent overlay of preoperative computed tomography images in patients with pelvic fracture and a unilateral trans-sacroiliac corridor

Introduction

Pelvic fractures account for 4.2% of all fractures and are considered serious injuries. Unstable posterior pelvic ring injuries, which can be caused by traffic accidents or falling, constitute 40% of pelvic fractures and carry a high risk of mortality (1,2). Unstable posterior pelvic ring injuries include sacral fractures, sacroiliac joint fractures and dislocations, and iliac fractures. Percutaneous sacroiliac screw fixation is considered the gold standard for treating unstable posterior pelvic ring injuries, offering significant advantages including a shorter operation time, reduced soft-tissue injury, and decreased blood loss (3,4).

Traditional methods for transiliac-transsacral screw placement involve ingress-egress view monitoring in the lateral position of pelvis (5). However, this can lead to difficulties in the positioning of the screw channel. The anterior edges of the S1 and S2 vertebrae cannot be clearly displayed in radiographic images due to collinearity and angulation and challenging to accurately assess on radiographic images before and during operation (6). Furthermore, structural variations of the sacrum can range from 30% to 41%. Other factors, including fracture displacement, osteoporosis, C-arm imaging differences, hematoma, soft-tissue swelling, flatulence, and obesity, can impede clear imaging during the procedure (7). These factors may lead to surgical errors and nerve damage, prompting some to advocate for the use of the lateral view of the sacrum as a reference for screw placement (8). Although computer-assisted navigation technology has enhanced screw placement accuracy and safety, the associated complexity of surgical procedures and high costs limit its widespread adoption (9,10). Moreover, despite computer-assisted navigation being the preferred method for implementing percutaneous sacroiliac screw fixation, not all patients with pelvic posterior ring injuries who require surgery have access to computer-assisted navigation.

Preoperative computed tomography (CT) imaging is crucial for assessing posterior pelvic ring injuries and serves as the primary reference for orthopedic procedures, providing detailed anatomical information. Within this context, this study aimed to develop a cost-effective, easy-to-implement, and safe preoperative positioning method for transiliac-transsacral screw placement by combining preoperative CT images with intraoperative radiography, thus providing an alternative to computer-assisted navigation. We present this article in accordance with the PROCESS reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1-2606/rc).

Surgical techniques

Study participants

In this single-center, retrospective study, the clinical records and CT images of a consecutive series of 12 patients diagnosed with unstable posterior pelvic ring injuries were examined. These patients underwent transiliac-transsacral screw fixation at the Guangdong Provincial Second Hospital of Traditional Chinese Medicine between March 2022 and October 2024. All preoperative planning (manual tracing) and surgical procedures were performed by the same senior orthopedic trauma surgeon (with 20 years of experience in pelvic surgery). All procedures in this study were performed in accordance with the Declaration of Helsinki and its subsequent amendments and was approved by the independent Ethics Committee for Clinical Research and Animal Trials of the Guangdong Provincial Second Hospital of Traditional Chinese Medicine (approval No. K202505-001-01). Written informed consent was obtained from the patients for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

The inclusion criteria were as follows: (I) a diagnosis of unstable posterior pelvic ring injury, (II) completion of the transiliac-transsacral procedures, and (III) availability of complete medical records. Meanwhile, the exclusion criteria were (I) abnormal pelvic bone development and (II) incomplete medical records. The selection for transiliac-transsacral fixation was based solely on the fracture pattern and anatomical suitability, with no influence from the surgeon’s preference. The flowchart of patient recruitment is detailed in Figure 1.

Figure 1 Flowchart of patient recruitment.

Patient’s data, including gender, age (years), tile pelvic classification, number of screws used, count of intraoperative radiographs, operative time (minutes), follow-up duration (days), complications, Gertzbein-Robbins scale, and the time to weight-bearing, were collected.

Tile pelvic classification was determined based on Tile’s study (11). The screw position was evaluated according to the Gertzbein-Robbins scale (12). Primary postoperative complications included wound infection, bleeding, delirium, internal fixation loosening, loss of fracture reduction, and neurological symptoms.

Statistical analysis

SPSS 25.0 statistical software (IBM Corp., Armonk, NY, USA) was used for data processing. The image measurement data and continuous variables are expressed as the mean ± standard deviation.

Preoperative design

The CT scans of at the pelvis and sacrum were conducted with the NAEOTOM Alpha system (Siemens Healthineers, Erlangen, Germany). Based on the CT results, it was determined whether a safe corridor for iliosacral screw fixation was present. In patients with significant displacement, this technique was considered feasible only when a unilateral trans-sacroiliac corridor could be clearly identified. Once it was confirmed that the patient was a suitable candidate for this technique, we performed hand tracing on transparent overlays on a computer (Figure 2). The range of bony channels was determined via the following steps.

Figure 2 Procedure of manual tracing performed on a computer.

A 16-row or higher spiral CT axial scan of the pelvis and a calibrated sagittal plane image of the sacrum were acquired. The image axes were defined in the standard sagittal, coronal, and horizontal planes based on three-dimensional (3D) reconstruction results (Figure 3A-3D).

Figure 3 Flowchart of preoperative bone channel positioning. (A) Reconstruction of the pelvic image. (B) Horizontal plane correction. (C) Coronal plane correction. (D) Sagittal plane correction. (E) The outer edges of the L5 vertebra, sacrum, intervertebral disc, and pubic symphysis are delineated on computer. (F) The outer edges of the L5 vertebra, sacrum, intervertebral disc, and pubic symphysis are traced onto transparent paper. (G) The outer edges of the anterior edge of the iliac bone are delineated on a computer. (H) The outer edges of the anterior edge of the iliac bone are delineated on transparent paper. (I) The outer edges of the anterior edge of the sacroiliac joint are delineated on a computer. (J) The outer edges of the anterior edge of the sacroiliac joint are delineated on transparent paper. (K,M) The outer edges of sacral vertebra are delineated on a computer. (L,N) The outer edges of sacral vertebra are delineated on transparent paper. (O) All overlayed lines are pooled to form a projection on the midsagittal plane of the sacrum. The blue blank area is the absolute safe screw channel. (P) Quadrilateral A, B, C, and D represent the S1 vertebral body, and E, F, G, and H represent the S2 vertebral body. The line segments a, b, c, d, e, f, and g represent the distances from points A, C, D, E, F, G, and H to the safe area, respectively.

The sagittal plane image was adjusted to A4 dimensions on the computer. The median sagittal image displayed the pubic symphysis, entire sacrum, and L5 vertebra. The outer edges of the L5 vertebra, sacrum, intervertebral disc, and pubic symphysis were delineated via imaging system tools (Figure 3E). An A4 transparent sheet was placed over the screen, and the boundary lines were traced onto it with a marker (Figure 3F).

• The image was then positioned at the anterior edge of one iliac bone. The anterior edge was delineated and traced onto the same transparent sheet (Figure 3G,3H). All subsequent steps followed the same procedure, and all outlines were superimposed on a single sheet via layer overlay principles.
• Starting from the lateral side of the sacroiliac joint, each CT image layer was reviewed sequentially, laterally to medially. The outlines of the ilium, sacrum, and each sacral disc were delineated and traced onto the transparent sheet. This process continued layer by layer until the median sagittal plane was reached, ensuring all layers were represented (Figure 3I-3N).
• The enclosed central region formed by the superimposed cortical boundary lines on the transparent sheet was considered to represent the common cancellous bone channel across all CT image layers (Figure 3O). This channel is located entirely within the cancellous bone of the sacrum, and this is deal for transiliac-transsacral screw pathway, as the sacral nerve and the sacral canal can be avoided. Prior to surgery, it was essential to measure and evaluate the relationship between the sacral cortical line, intervertebral disc projection, and the safe channel. During the procedure, these markings were used reference points, and the screw was inserted into the safe channel to prevent injury to the sacral nerves, canals, and blood vessels, thereby ensuring surgical safety. A schematic diagram for the anatomical reference points that need to be measured is provided in Figure 3P; in the figure, A, B, C, and D represent the S1 vertebral body; meanwhile, E, F, G, and H represent the S2 vertebral body; the line segments a, b, c, d, e, f, and g represent the distances from points A, C, D, E, F, G, and H to the safe area, respectively.
• Due to the proportional differences between the preoperative map and intraoperative radiographic image, we converted these anatomical references into proportions rather than specific values. The required proportions were a/AC, b/AC, c/BD, d/EG, f/EG, e/FH, and g/FH (Table 1).

Operative procedure

The intraoperative C-arm was measured at the pelvis and sacrum via the Cios Spin (Siemens Healthineers). First, closed reduction was attempted with the skeletal traction and the Starr frame. If this method failed, open reduction was then performed. If necessary, preliminary fixation could be applied to maintain the reduction.

After successful reduction, the patient’s position on the operating table was shifted to the supine position, with the body axis being parallel to the table. To obtain an optimal lateral image with the C-arm, the center was aligned with the target vertebra, the upper and lower end plates were made linear and well-defined, the consistency between the intervertebral disc shape and the CT image was maintained, and the iliac cortical density line (ICD) and ischial tuberosity cutout were positioned to create “a small figure within a big figure” appearance (Figure 4A).

Figure 4 Intraoperative technique. (A) The previously inserted S2 transiliac-transsacral screw and ischial tuberosity cutout depicted as “a small figure within a big figure” image. (B) A custom-designed sliding rail platform to facilitate screw placement. (C) The concentric circle view provided by the hollow cannula indicates the optimal screw entry point and direction (red circle). (D) Confirmation of the position of the guide pin in the outlet view when passing through the sacrum. (E) Confirmation of the position of the guide pin in the inlet view when passing through the sacrum.

The optimal screw placement point was determined via reference to the sacral outline, intervertebral disc projections, and safe channel area on the ideal lateral radiograph. A hollow sleeve was inserted at the optimal needle insertion point and aligned with the projection direction. A custom-designed guided needle fixation system was used to secure the hollow sleeve, and when the hollow sleeve appeared as a concentric circle, this indicated the optimal screw placement direction (Figure 4B,4C). The guided needle was percutaneously inserted, and it was ensured that it was safely positioned throughout the insertion process (Figure 4D,4E). Once the exit and entrance were confirmed, the optimal screw length was determined, and the screw was securely placed into the bone along the guided needle.

Postoperative assessment

A postoperative CT scan of the pelvis was conducted to confirm the preoperative design and evaluate screw placement accuracy. The assessment was based on the Gertzbein-Robbins scale (Figure 5).

Figure 5 Postoperative CT images. (A) Postoperative coronal plane CT image. (B) Postoperative horizontal plane CT image. (C) Postoperative sagittal plane CT image. CT, computed tomography.

The study comprised 12 patients (9 males and 3 females) with a mean age of 46±17 years. Injuries resulted from falls from heights (4), automobile accidents (5), and crush injuries (3). One patient experienced urethral rupture. According to the Tile classification of pelvic ring injuries, there was one case of type B1, five cases of type B2, five cases of type C1, and one case of type C3. Seven patients underwent anterior ring fixation, four received sacral 1 (S1) and sacral 2 (S2) fixation with double-penetrating screws, one received S1 fixation with a single-penetrating screw, and seven received S2 fixation with a single-penetrating screw. Surgical durations ranged from 33 to 99 minutes, with an average of 68±22 minutes. All patients were able to sit up and begin mobilization on the first postoperative day. Except for five patients with multiple traumatic injuries, all patients were allowed partial weight-bearing on the first postoperative day and complete weight-bearing after 2 months. No complications were observed. The Gertzbein-Robbins scale for screw positioning was used to assess the three-dimensional CT reconstruction of the pelvis on the first postoperative day (12). Accordingly, 12 screws were considered grade A and 4 screws grade B (Table 2).

Comments

Previously, posterior pelvic ring injuries—particularly sacral fractures—often went undiagnosed and untreated. These injuries frequently led to neurological symptoms and deficits in the lower extremities, as well as urinary, rectal, and sexual dysfunction. Early attempts at surgical intervention for displaced fractures with neurological involvement were reported by Fardon (13). Subsequently, Roy-Camille et al. (14) pioneered the development of lumbosacral fixation techniques involving plates and screws via open approaches. Denis (15) later classified sacral fractures by direction, location, and level, offering deeper insight into the pathogenesis of associated neurological deficits. In 1989, Matta (16) introduced the technique of open sacroiliac screw placement. Subsequently, Routt et al. (17-21) conducted extensive research on various aspects, such as screw trajectory anatomy, surgical positioning, screw length and number, and reduction techniques. This research gradually developed sacroiliac screw fixation into a standard surgical procedure for treating posterior pelvic ring instability. The advancement of percutaneous sacroiliac screw fixation technology signifies the advent of minimally invasive treatment for posterior pelvic ring injuries.

Screw placement typically involves S1 and S1 screws. The S1 screw was initially employed for posterior pelvic ring injuries. While extensive research has focused on S1 screw placement, dual fixation with S1 and S2 screws has been shown to provide superior stability as compared to single S1 screw fixation (22,23). Furthermore, studies indicate that transiliac-transsacral screws offer enhanced stability and resistance to vertical shear forces than do standard unilateral screws (24,25).

Approximately 76% to 78% of the Chinese population had a safe canal for sacroiliac screw placement at the S1 vertebra, while almost all individuals have a safe canal for such placement at the S2 vertebra (26). There has been increased research interest in accurately identifying safe bone channels and providing operative references during procedures. A common method for placing transiliac-transsacral screws involves utilizing ingress–egress points; however, uncertainties persist regarding the relationships between the anterior edges of the S1 and S2, including alignment, posterior angulation, and anterior angulation (6) Moreover, factors such as anatomical variations, overlapping fractures, and intestinal bloating frequently result in unclear anatomical images during operations (23). Consequently, use of a horizontal “safe channel” in the lateral sacral radiograph has been proposed as the optimal pathway for screw placement. This channel is formed by the anterior edge of the sacral nerve root canal, the anterior edge of the vertebra, and the lower edge of the anterior sacral foramen (27). Ramadanov and Zabler (28) introduced a computationally defined safe zone in the sacral region based on bone density and preliminarily examined the correlation between lateral radiographic views and the safe corridor, thereby establishing a foundation for CT-guided, personalized safe corridor planning according to lateral intraoperative references. Building on this prior work, we propose a simple and feasible technique that visualizes the entire trans-sacroiliac corridor, extending beyond the sacrum, via reference to lateral radiographs.

The critical aspect of percutaneous transiliac-transsacral screw placement is identifying a secure bone channel. We have devised a positioning method for evaluating safe bone channels based on preoperative CT images using a layer overlay principle. Our approach involves manually delineating the outline of all sacral cortical bone on each layer to determine the minimum cortical bone edge. When a blank, enclosed area appears in the hand-drawn sacrum on transparent paper, this signifies an absolutely safe bone channel. This secure channel comprises cancellous bone, positioned away from cortical bone and external neurovascular structures. The inspiration for our design stemmed from the complex, cobweb-like images of the sacrum and sacroiliac joint observed during intraoperative radiography. The channels required for screw insertion are located within the voids of these cobweb-like structures. We applied this design to 12 patients with posterior pelvic ring injuries, documenting and evaluating both intraoperative and postoperative conditions. The results indicated favorable outcomes. It must be acknowledged that computer-assisted navigated screw implantation is a better choice only under certain conditions. However, as mentioned above, it is not available to all patients, and we hope to provide an alternative surgical opportunity for these patients.

It should be noted that transiliac-transsacral screw placement is just one approach that can be used to addressed posterior ring instability. This paper presents a concise and safe method for screw placement, but its application must be guided according to clinical need. A key prerequisite for this technique is achieving adequate reduction and ensuring the presence of a safe bony corridor. Significantly displaced bilateral pelvic fractures, which prevent the formation of a safe corridor, are considered contraindications to this approach. In patients with significant displacement, intraoperative fracture reduction is necessary to restore the safe corridor, and open reduction is performed if needed. For cases with significant displacement or lacking a safe corridor, alternative or supplementary fixation methods should be considered (29). Whenever possible, multiscrew fixation should be applied to enhance stability. Surgery is indicated for patients with minimal displacement if posterior pelvic ring instability is present or if there is persistent pain despite conservative treatment or progressive displacement (e.g., increased pain with posterior pelvic movement) (30). Early stabilization can significantly alleviate pain, promote mobility recovery, reduce early mortality, and improve long-term outcomes (31). Conversely, conservative treatment may be more suitable for patients with impaired limb function or poor general health.

Although our straightforward and efficient design can facilitate valuable preoperative evaluation, certain limitations should be acknowledged. First, this manual drawing design requires proportional image creation without a reliance on computer software or algorithms. Although bone channels can be determined in hand-drawn images, achieving detailed digitalization presents certain challenges. Our approach involves identifying unchanging anatomical landmarks of the pelvis and sacrum in the sagittal plane, which are used as references for intraoperative positioning. Future work in this area will include digitizing CT images with 3D modeling technology. Second, accurate evaluation and implementation of the preoperative design necessitate consistent patient positioning; inconsistencies may compromise accuracy, particularly during intraoperative radiographs. As CT images can be adjusted to a standard horizontal position by radiologist, we posit that the primary challenges in clinical practice lie in positioning patients horizontally and properly aligning the C-arm system. This task requires practice rather than specialized skills, but we believe that the learning curve for this technique is not particularly steep. Finally, our initial experience supports the practicability of this technique, as favorable perioperative outcomes were achieved in a small cohort. However, given the retrospective nature of the study design and the lack of a control group, further prospective comparative and large-scale studies are needed to identify any potential advantages. Additionally, all preoperative planning and surgical procedures in this study were performed by a single senior surgeon, and thus the reproducibility of this technique across different surgeons and institutions remains to be further validated through multioperator studies.

Conclusions

We developed a concise and efficient CT-based method for positioning the transiliac-transsacral screw bone channel in patients with posterior pelvic ring injuries. The method can determine a safe range of bone channels with anatomical landmarks in the sacroiliac region through graphical representation. This approach was successfully applied in a small cohort of patients with posterior pelvic ring injuries, demonstrating convenient implementation and yielding favorable perioperative outcomes. It can be realistically expected that further research will establish this method as a concise and safe alternative to computer-assisted navigation.

Acknowledgments

We would like to thank TopEdit (www.topeditsci.com) for its linguistic assistance during the preparation of this manuscript.

Footnote

Reporting Checklist: The authors have completed the PROCESS reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1-2606/rc

Funding: This work was supported by grants from the Administration of Traditional Chinese Medicine of Guangdong Province, China (No. 20241021) and Guangdong Provincial Second Hospital of Traditional Chinese Medicine Scientific research innovation Foundation (No. SEZYY2023B12).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1-2606/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the independent Ethics Committee for Clinical Research and Animal Trials of the Guangdong Provincial Second Hospital of Traditional Chinese Medicine (No. K202505-001-01). Written informed consent was obtained from the patients for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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