Over the past two decades, the incidence of fragility and insufficiency fractures of the pelvis (FFP) and sacrum (SIF), has increased due to increased life expectancy [[1], [2], [3]]. These fractures commonly occur in osteoporotic patients following low-energy trauma, such as a fall from standing height, or even in the absence of trauma. To enhance existing FFP classification and treatment selection, Rommens and Hofmann developed a comprehensive system for FFP [4], expanding upon the Association of Osteosynthesis (AO) classifications [5]. The severity of FFP is determined by fracture localization and the degree of instability it creates. According to the Rommens and Hofmann classification, FFP Type I fractures are the least severe and may be managed conservatively, while Type IV fractures are the most unstable and typically require surgical intervention [4,5].

The gold-standard surgical treatment for FFP Type IV fractures—which correspond to AO Type C fractures—is triangular osteosynthesis (TOS) [[6], [7], [8], [9]]. Traditional TOS combines two key approaches: (1) lumbopelvic fixation to address vertical instability by securing L4/L5 to an iliac screw, and (2) transverse fixation to address rotational instability using an iliosacral screw. Biomechanical studies have shown the TOS construct provides sufficient stability to enable early weight-bearing [[10], [11], [12]]. However, TOS implantation can be technically demanding and relatively invasive, with infection rates and wound healing complications reported in up to 26 % of cases [13]. Aiming to address this problem, modifications of the traditional TOS construct have been developed to reduce post-operative pain [14,15] or allow for a minimally invasive approach [13]. However, these solutions still require meticulous surgical techniques and fixation stability can be compromised by poor sacral bone quality, requiring cement augmentation [3,[16], [17], [18]].

To add to the modifications of the traditional TOS construct, Riesner et al. has introduced an emerging triangular minimally invasive spinopelvic stabilization (TMSS) technique [19]. While the TMSS technique utilizes the same lumbopelvic fixation as the TOS technique, it differs by incorporating a second, inferior iliac screw to enhance rotational stability and compression across the fracture. Unlike TOS, where rotational stability is largely dependent on the bone quality of the sacrum, TMSS relies on iliac screw fixation. Clinically, this technique has demonstrated adequate stabilization of FFP Type IV fractures in osteoporotic bone [19], however, its biomechanical properties have not yet been explored.

This study biomechanically evaluates the stability of Rommens & Hofmann Type IV U-shape sacral fractures treated with the traditional TOS technique and the emerging TMSS technique in a cadaveric model. Fracture displacement was analyzed under a loading scenario simulating post-operative mobilization at 3–4 weeks. To the authors knowledge, no prior biomechanical studies have directly compared the stability of these two constructs in a bilateral FFP cadaveric model. The authors hypothesized that the TMSS construct would demonstrate comparable or reduced fracture displacement compared to traditional TOS under cyclic loading conditions.