The goal of treating Tile C1.3 pelvic fracture is to restore normal structure of the pelvic ring and rigidly fix it. For pelvic fractures that are difficult to reduce, especially the old fractures, LIF is an important surgical method for the treatment of Tile C1.3 pelvic fractures because of its strong reduction force and solid fixation. In this study, an in vitro biomechanical model of pelvic loading in two-legged stance was developed to factor analyze five LIF techniques, as well as sacral slope. The TOS fixation was shown to enhance the pelvic stability and significantly limited the motion in the coronal plane and cephalad–caudal direction, whereas L4–L5 ULIF did not further improve the immediate stability. BLIF significantly improved the stability in the left–right direction, but its vertical stability was significantly affected by the SS, suggesting that stronger fixation is required at greater SS.

The TOS fixation is a rigid triangular structure combined with the ULIF and the iliosacral screw, which provides multiplanar stability, facilitates early mobilization, reduces mortality, and protects the local neuroanatomical structures from additional injury [25,26,27,28,29]. However, these clinical advantages need to be confirmed by biomechanical studies, especially whether they are sufficiently stable in the basic functional posture of two-legged standing. The present study shows that TOS significantly enhanced the stability in all directions except motion in anterior–posterior direction or flexion–extension, supporting that TOS can provide solid instant stability for Tile C1.3 pelvic fracture.

Our study also shows that ULIF extended to L4 did not significantly improve the immediate stability, i.e., the stability of the L4–L5 ULIF and L4–L5 TOS was similar to that of the L5 ULIF and L5 TOS in the case of different SS. In the view of the principle of treatment of spinal fractures, both the upper and lower two segments of the fractured segment should be fixed to improve the reduction effect on the fracture and to minimize axial rotational motion. However, short fixation can surprisingly provide better stability and clinical efficacy than long fixation according to previous reports [30, 31]. The primary role of ULIF is to resist separation of the fractured posterior pelvic ring, which may explain the lack of stability-gaining effect of fixation extending to L4. In clinical practice, the L5 ULIF or the L5 TOS is enough to achieve strong fixation, which can reduce such complications as surgical invasion and postoperative infection.

This study further demonstrates that in the ULIF, the displacement in the left–right and vertical directions was significantly greater at 40° SS than that at 30° SS. However, the difference between two SS was not significant in either L4–L5 TOS or L5 TOS. We ascribed this phenomenon to the “central fixation” [32] of the iliosacral screw, which may improve the stability of the posterior pelvic ring in patients. L5 TOS is recommended for patients with a large SS to ensure the stability of fixation. In addition, studies have been performed to assess the effect of spinopelvic parameters on lumbar spine loading [33] and found that shear forces were greater at the L5–S1 segment at larger SS. We attribute the weakening of the vertical stability of the posterior pelvic ring with increasing sacral slope to the fact that increased lumbosacral shear requires stronger posterior ring fixation of the pelvic fracture, and additional iliosacral screw fixation can be effective in resisting lumbosacral shear. This study reveals for the first time the biomechanical influence of SS in posterior pelvic ring fixation, suggesting clinical attention to the fact that the vertical stability of posterior pelvic ring fixation is reduced in patients with higher sacral slope, which is also informative for postoperative rehabilitation.

The present study also demonstrated that the BLIF significantly limited left–right displacement when compared with the ULIF. From the perspective of spine biomechanics, bilateral fixation provides a more solid upper-end anchorage for Tile 1.3 pelvic fractures, which should increase the stability of the fracture fixation. However, the BLIF reduced the left–right displacement by only about 0.15 mm, and it is debatable whether it is of clinical significance. In terms of the magnitude of fracture displacement, the effect of SS appeared to be more pronounced, with the results of this study showing that a 10° increase in SS was associated with a 0.37 mm increase in vertical displacement. Both the L4–5 BLIF and the L4–5 TOS had a transversal connector between fracture fragments. The traversal connector of BLIF (the connecting rod) is eccentric fixation with small diameter and doubtful stability, while the one of TOS (the iliosacral screw) is central fixation with larger diameter and stronger stability. The biomechanical analysis also showed that BLIF could only increase the lateral stability compared with ULIF, while TOS could increase the total, vertical, and lateral one. The Tile C1.3 pelvic fracture lacks the total, vertical, and lateral stability, so the BLIF was not an ideal choice for this type of fracture, and we are cautious about it given its surgical costs and risks.

In a study by Roussouly et al. [34], 160 asymptomatic adult volunteers underwent full-length lateral X-rays of the spine in a standard standing position, and the mean sacral slope was 39.9 ± 8.2°. Moreover, in a similar study by Zhou et al. [35], the mean sacral slope was 34.0 ± 7.1°. Therefore, two angles of 30° and 40° were set in the present study to analyze the effect of sacral slope on lumbar–iliac fixation was appropriate, and it also confirmed the effect of pelvic parameters on the lumbar–iliac fixation.

The two-legged standing model used in this study is a validated biomechanical model which was originally developed by Hearn et al. [36] and has been used in many studies to assess the biomechanics of pelvic fixation effectiveness [10,11,12,13]. Specifically, the two-legged standing model is more suitable for biomechanical evaluation of pelvic fracture, as it indicates the physiological loading on the pelvis, where the load from the lumbar spine is transmitted through the sacrofemoral arch to the femur, and the pubic symphysis is distracted through the corresponding para-arch. In the present study, two metal femoral heads supported the bilateral acetabulum without affecting the pelvis motion during loading. The steel cables anchored to the anterior superior iliac spine anterior to the center of the femoral head adjust the SS, simulate the action of the hip flexors [8, 37], and maintain pelvic balance under loading.

In this study, fresh frozen cadaveric spinal–pelvic specimens with preserved major ligamentous provided a better representation of the in vivo condition [38] but with greater individual variability than artificial pelvic models with high homogeneity and better reproducibility [17]. A repeated-measures study design was used in this study, which reduced the effect of individual differences to some extent and still reflected the effects of different factors. Second, changing different internal fixations on the same specimen may affect the strength of the bone–screw interface. In this study, an experienced clinician placed the screws as close as possible to the original screw trajectory and achieved similar screw purchase during instrumentation. In addition, the 500 N axial compressive load was used and should not damage the bone–screw interface based on previous studies [39].

In conclusion, we developed an in vitro Tile C1.3 pelvic fractures model under two-legged standing load and confirmed that TOS enhanced the stability of LIF and significantly reduced the motion of the fracture fragments in the coronal plane and cephalad–caudal direction, whereas L4–L5 ULIF did not further improve the immediate stability. BLIF significantly improved the stability in the right–left direction, but its vertical stability was significantly affected by the SS. This study reveals the biomechanical impact of SS to LIF, suggesting that clinical attention needs to be paid to the vertical stability of posterior ring fixation of pelvic fractures in patients with higher sacral slope.