Minimally Invasive Inline Spinopelvic Fixation Without Offset Connectors for Unstable Sacral Fractures: Technical Description and Preliminary Clinical Experience

Discussion

In this study, MIS-SPF using an inline construct without offset connectors was applied for nonosteoporotic unstable sacral fractures caused by high-energy trauma, resulting in favorable clinical and radiographic outcomes. All patients achieved bony union and regained independent ambulation without implant-related complications, supporting the feasibility of this technique in this preliminary case series, with encouraging early clinical and radiographic outcomes.

Spinopelvic dissociation is a severe injury characterized by both vertical and rotational instability. Without appropriate treatment, progressive kyphotic deformity, chronic pain, neurological impairment, and gait dysfunction may occur.7 In particular, severe displacement has been associated with worse neurological and functional outcomes, and surgical stabilization is generally recommended.7 Recently, favorable outcomes have also been reported for less invasive fixation techniques confined to the pelvis, including percutaneous iliosacral screw fixation and transiliac transsacral screw (TITS) fixation.8–11 However, some of these reports include fragility fractures of the pelvis associated with low-energy trauma or osteoporosis, which differ pathophysiologically from true spinopelvic dissociation.12 In contrast, all patients in the present study had high-energy AO type C2/C3 injuries with complete disruption of spinopelvic continuity. In such highly unstable injuries, rigid stabilization using SPF is often required.13,14 Although SPF provides rigid stabilization, traditional open approaches require extensive soft-tissue dissection and have been associated with increased blood loss, wound infection, and soft-tissue complications. Schildhauer et al reported a 16% infection rate following open SPF.10,15 In addition, patients with spinopelvic dissociation frequently sustain multiple associated injuries, including spinal fractures, lower-extremity fractures, pelvic ring injuries, and thoracoabdominal injuries.16 Furthermore, patients with pelvic fractures requiring transcatheter arterial embolization have been reported to be at increased risk of gluteal necrosis and wound complications.10,17,18 Thus, MIS-SPF, which minimizes extensive soft-tissue dissection, may be considered a useful treatment strategy for these severely injured patients.

Recently, an increasing number of studies have reported favorable outcomes for MIS-SPF, including reduced blood loss and lower rates of wound complications compared with conventional open techniques. Koshimune et al reported that the introduction of MIS-SPF reduced operative time from 345 to 208 minutes and blood loss from 520 to 290 mL.19 Williams et al reported favorable perioperative outcomes in MIS-SPF cases, with a mean operative time of 144 minutes and a mean blood loss of 202 mL.20 Liu et al described robotic minimally invasive triangular osteosynthesis and demonstrated reductions in both blood loss (612 mL vs 142 mL) and operative time (185 minutes vs 148 minutes) compared with open procedures.21 Similarly, Pearson et al reported significantly lower blood loss in the MIS group compared with the open group (538 mL vs 171 mL).22 In the present study, the mean blood loss of 154.6 mL and the mean operative time of 149 minutes for the L4–iliac SPF procedures were comparable with values reported in previous MIS-SPF series. These findings suggest that the proposed construct can simplify implant assembly while maintaining perioperative outcomes comparable to existing MIS-SPF techniques.

Maintaining sufficient stability while preserving minimal invasiveness is a crucial challenge in MIS-SPF. Currently reported MIS-SPF construct concepts can largely be divided into 2 approaches. One emphasizes increased stability using crosslinks, offset connectors, and multirod constructs, including techniques such as crab-shaped fixation.19,23 Although these constructs provide strong fixation, the implant configuration becomes more complex. The other prioritizes minimally invasive techniques and procedural simplicity by simplifying the implant construct, such as crosslink-free triangular osteosynthesis or simplified constructs combined with TITS.24–26 However, these simplified constructs may provide insufficient rotational and horizontal stability, which could contribute to fixation failure or nonunion.18,24 In addition, TITS fixation may provide insufficient stability in severely displaced fractures or in cases with disrupted osseous fixation pathways.9 Previous MIS-SPF techniques have primarily focused on reducing surgical invasiveness through percutaneous instrumentation, indirect reduction maneuvers, and more recently through navigation- or robot-assisted techniques.21,25 In contrast, the primary objective of the present technique is not to further reduce surgical invasiveness or improve radiographic outcomes but to simplify construct assembly and surgical workflow. The procedure can be performed through small incisions using conventional fluoroscopic guidance without navigation, robotic assistance, or specialized instrumentation, providing a simple and reproducible surgical strategy.23 Unlike previously reported MIS-SPF techniques that use offset connectors to accommodate the mismatch between the longitudinal rod and iliac screw heads,19,23 direct inline alignment allows continuous rod fixation without additional connectors, thereby simplifying rod insertion and fixation procedures. We propose that sufficient construct stability could be maintained through continuous bilateral longitudinal rod fixation, whereas the transverse rod connecting the caudal iliac screws reinforces rotational and horizontal stability across the posterior pelvic ring. In addition, the overall system can be maintained in a low-profile configuration, which may reduce implant prominence and wound-related complications. A similar low-profile iliac screw concept has been described in the low-profile S1-iliac trajectory and percutaneous modified iliac screw techniques.5,6 In the present study, a low-profile iliac screw trajectory was used to minimize implant prominence. Unlike S2 alar-iliac (S2AI) screws, this trajectory is independent of the sacral osseous corridor and is therefore less likely to be affected by the degree of sacral fracture comminution. However, biomechanical evidence regarding this low-profile iliac screw trajectory remains limited, particularly with respect to pull-out strength and its influence on overall construct mechanics. Furthermore, because implant removal is often required in younger trauma patients, the simplified construct may facilitate secondary implant removal procedures. Unlike conventional crosslinks that connect bilateral rods, the transverse rod directly connects the caudal iliac screws. The use of a crosslink in case 4 represented an early technical modification after sacral decompression rather than the intended construct design, and routine crosslink augmentation is no longer part of our current surgical strategy. However, the present study was not designed to evaluate the mechanical superiority of this construct; further biomechanical validation is required to clarify its stabilizing effect.

This study has some limitations. First, although fracture reduction is an important factor influencing neurologic and radiographic outcomes, quantitative radiographic assessment of reduction quality and postoperative spinopelvic alignment was not systematically performed. In addition, preoperative standing radiographs could not be obtained because of the high-energy nature of these injuries. Consequently, the relationship between reduction quality, spinopelvic alignment, and clinical outcomes could not be evaluated. Second, this was a small retrospective study without a control group; therefore, superiority over conventional SPF techniques cannot be directly demonstrated. In addition, biomechanical evaluation of the present construct was not performed, and the rotational stability provided by the transverse rod configuration requires further investigation in future cadaveric and biomechanical studies. Nevertheless, consistently favorable clinical outcomes and a low complication rate were achieved, suggesting the feasibility and safety of this construct.