Biomechanical analysis of iliosacral and transiliac–transsacral screw combinations for fixation of undisplaced Denis II vertical shear fractures in dysmorphic sacrum

Establishment of intact pelvic model

This study was approved by the ethics committee of Guilin Medical University Affiliated Hospital (Number: 2024QTLL-02) and used pelvic data from a healthy 31-year-old male (173 cm, 70 kg) with normal bone structure and no history of deformity, tumors, or surgery. The volunteer underwent a 64-slice CT scan (0.625 mm thickness). The data were processed in Mimics 21.0 to generate a complete pelvic model including bilateral hip bones and sacrum. Geomagic 17.0 was used for noise reduction, smoothing, and separation of cortical and cancellous bone, with iliac cortical thickness set at 1.5 mm and sacral cortical thickness at 0.45 mm. In Solidworks 2021, the cortical and cancellous bone were assembled, and sacroiliac and pubic symphysis cartilage were created. Ansys 21.0 was then used to assign material properties to bone and cartilage and to model ligaments based on prior studies (Fu et al., 2014). The final pelvic model contains 1,011,005 nodes and 613,587 elements (Fig. 1).

Establishment of fracture model and internal fixation

A pelvic model with unilateral vertical instability injury, including a right-sided transforaminal sacral fracture and fractures of the upper and lower rami of the right pubis, was established through mesh line segmentation. Regardless of the direction of the oblique S1-ISS, the screw trajectory on the pelvic outlet view is from the outer inferior to the inner superior, although the screw trajectory on the inlet view might have an extensive angular range. We divided the probable needle insertion point area of the screw in the inlet view into three equal portions, which we defined as the anterior IS, the middle IS, and the posterior IS in different directions (Fig. 2). The ISS extend up to the midline of the S1 body, while the TTS connect one lateral external iliac cortex to the contralateral external iliac cortex. In seven experimental groups, posterior fixation was performed as follows: Group 1 (G1), S1-ISS; Group 2 (G2), S2-ISS; Group 3 (G3), S1-ISS + S2-ISS; Group 4 (G1), S2-TTS; Group 5 (G5), Anterior S1-ISS + S2-TTS; Group 6 (G6), Middle S1-ISS + S2-TTS; Group 7 (G7), Posterior S1-ISS + S2-TTS (Fig. 3). The ISS and TTS both have a diameter of 6.5 mm.

Loading of forces and setting of contact conditions

A vertical force of 500 N had been given to the upper surface of the sacral as a loading condition that imitated the upper body’s weight, and the bilateral acetabular was fixed in six directions. In addition, a vertical load of 500 N and a torque of 10 N m are applied to imitate the body’s forward flexion, left flexion, right flexion, left rotation, and right rotation. In this finite element model, the contact conditions between cortical bone, cancellous bone, cartilage, and implants were set according to previous studies (Gonçalves et al., 2023).

Assessment by finite element analysis

In this study, a number of parameters will be used to assess the fixation strength for different internal fixation methods. The parameters assessed included the maximum von Mises stress and screw deformation for S1 and S2 segmental screws, vertical displacement of the upper surface of the sacral, sagittal angular displacement of the upper surface of the sacral, and relative displacement of 5 pairs of observation points on the fracture line of the anterior surface of the sacral (Fig. 4).

Intact pelvic finite element model examination

In the process of finite element model validation in this study, we took research concepts from Miller and others (Miller, Schultz & Andersson, 1987; Turbucz et al., 2023; Xu et al., 2020). Specifically, five 294 N translational loads (upward, downward, forward, backward, and lateral) and three 42 N m rotational moments (flexion, extension, and rotation) were applied to the sacral upper surface. Displacements under these loads were measured to assess model reliability. The results closely matched those from previous studies, confirming the model’s validity (Fig. 5).

The maximum von Mises stress and screw deformation for S1 and S2 segment screws

In all motion states, the maximum stress on all internal fixation devices remained below the yield strength of titanium alloy. In the standing position, the maximum von Mises stress in S1 segment screws varied significantly among groups: G1 had the highest stress (104.03 MPa), and G6 had the lowest (39.28 MPa). The stress values were ranked: G1 > G3 > G5 > G7 > G6. For S2 segment screws, the highest stress was in G2 (83.42 MPa) and the lowest in G7 (48.68 MPa), with the ranking: G2 > G4 > G5 > G3 > G6 > G7 (Fig. 6, Table 1).

The screw deformation analysis showed that for the S1 segment screws, G1 had the highest deformation (0.63 mm) and G5 had the lowest (0.30 mm), with the ranking: G1 > G3 > G6 > G7 > G5. For the S2 segment screws, G2 had the highest deformation (0.36 mm) and G5 had the lowest (0.14 mm), with the ranking: G3 > G2 > G6 > G7 > G5. In other motion states, the trends in maximum von Mises stress and deformation for both S1 and S2 segment screws were similar to those in the standing position (Table 1, Supplemental Information 1).

The vertical displacement and sagittal plane angular displacement of the superior surface of the S1

Under the six motion conditions, the trends in vertical displacement and sagittal angular displacement of the sacral upper surface were consistent among groups. G5 had the lowest values (vertical displacement: 0.2949 mm, sagittal angular displacement: 0.4404°), while G2 had the highest (vertical displacement: 0.6194 mm, sagittal angular displacement: 5.1653°). The vertical displacement of the upper surface of the sacral in each group, from highest to lowest, is as follows: G2 > G1 > G3 > G4 > G6 > G7 > G5. The sagittal plane angular displacement, from highest to lowest, is as follows: G2 > G4 > G1 > G3 > G6 > G7 > G5 (Table 2, Supplemental Information 2).

Relative displacement of the observation point of the anterior sacral

In the standing position, the relative displacement of the anterior sacral surface observation points varied significantly among groups. G5 had the smallest displacement (0.0700 mm), while G2 had the largest (0.1425 mm). The ranking from highest to lowest was: G2 > G4 > G1 > G3 > G7 > G6 > G5 (Fig. 7).

Under various motion conditions (flexion, left flexion, right flexion, left rotation), G2 had the highest relative displacement, while G5 had the lowest. During right rotation, G3 had the highest displacement, but G5 still had the lowest (Supplemental Information 3).

Analysis of the biomechanical properties of internal fixation

The study showed that titanium’s yield stress exceeds the maximum stress of screws in all internal fixation techniques, indicating a low risk of fatigue failure (Fu et al., 2014).

Additionally, the maximum von Mises stress in screws used with TSS is significantly lower than that in screws used with ISS alone. This suggests that TSS placement effectively reduces stress concentration and lowers the risk of fatigue failure, consistent with prior findings (Turbucz et al., 2023).

Screw deformation magnitude correlates with its dynamic behavior under cyclic loading. Excessive deformation can induce periodic micro-movement within the bone, leading to screw loosening or thread cutout (Beucler, 2024). In this study, greater screw deformation was observed when Denis II sacral fractures were fixed solely with ISS, potentially increasing the risk of loosening. However, combined ISS and TSS fixation significantly reduced screw deformation, suggesting that TSS can enhance stability, minimize screw oscillation, and mitigate thread cutout.

Sacral stability assessment

One of the main indications employed to evaluate the biomechanical stability of the sacral is the vertical displacement of the upper surface (Fan et al., 2024; Turbucz et al., 2023). Previous research has shown that a single TSS provides superior fixation compared to two ISS, significantly reducing sacral upper surface displacement, even at the S2 level (Bradley et al., 2022; Zhao et al., 2013). In this study, combined ISS and TSS fixation was more effective in reducing vertical displacement than TSS alone. ISS alone struggles to restrict sagittal rotation of the sacral upper surface under stress (Beucler, 2025; Schildhauer et al., 2003). TSS demonstrated superior anti-rotation capabilities compared to ISS alone, likely due to its immobilization of the sacrum and contralateral iliac cortex, providing more stable fixation. Combined ISS and TSS fixation further enhanced rotational stability by offering multiplanar support across two sacral segments.

Stability of the fracture

The relative displacement of the observation point on the anterior sacral surface reflects the impact of internal fixation on fracture stability (Zheng et al., 2021). In this study, the combination of ISS and TSS exhibited the lowest and least deviated relative displacement among all groups. This indicates that ISS and TSS together provide optimal fracture stability.

In this study, the combination of ISS and TSS was found to provide excellent biomechanical and fracture stability in undisplaced sacral dysmorphic fractures. Specifically, the combination of anterior S1-ISS and S2-TSS demonstrated the highest stability. The potential reasons include: (1) Pelvic force conduction originates from the upper sacral endplate, travels through the sacral wing and sacroiliac joint to the arcuate line, and then descends. The anterior S1-ISS aligns more closely with the pelvic physiological structure. (2) The S1-ISS, located anteriorly, is closer to the medial iliac aspect and the anteromedial cortex of the sacral wing. According to Wolff’s law, the elevated trabecular bone density along the screw trajectory results in stronger screw purchase.

Limitation

Despite progress in this study, limitations remain. First, the placement of anterior S1-ISS is technically challenging. The complex sacral anatomy and proximity to major neurovascular structures make nail placement difficult with only X-ray fluoroscopy, often necessitating expensive technologies like computer navigation (Fig. 8). Second, the finite element analysis method used in this study provides theoretical mechanical predictions but lacks verification through cyclic loading tests on cadaver models. This limits the comprehensive evaluation of screw mechanical properties and stability. Future studies should use cadaver experiments to better replicate the actual biomechanical environment. Finally, the study’s findings have not been clinically verified, and long-term radiological and functional assessments have not been conducted on patients. The feasibility, reproducibility, and long-term effects of the surgical construct under daily load must be evaluated in clinical practice. Follow-up research should include multicenter prospective clinical trials to determine the practical utility of the proposed surgical techniques.