Date Published: February 3, 2019
Author(s): Vahhab Zarei, Sharon C. Yson, Joan E. Bechtold, Jonathan N. Sembrano.
Study Design. Mathematical Model. Objectives. To investigate the relationship between pelvic osteotomy opening angle (OA) and its effect on spinopelvic sagittal parameters as well as the resting length of surrounding muscles. Methods. Predictive equations correlating OA with spinopelvic parameters were derived using geometric relationships. A geometric model calculated spinopelvic parameters (SVA, pelvic incidence [PI], PT, and T1 pelvic angle [TPA]) produced by progressively increasing the OA. These values were compared to optimal balance criteria in the literature. Four muscles crossing the osteotomy site were evaluated: Gluteus Medius (GMED), Gluteus Maximus (GMAX), Piriformis (P), and Tensor Fascia Lata (TFL). Insertion points were obtained from an OpenSim software model. GMAX and GMED were subdivided into 3 (anterior, middle, and posterior). Results. OA correlated negatively with PI, TPA, and SVA and positively with PT. From baseline SVA of 22 cm, OA 21° reduced SVA to 5cm. OA 23° reduced TPA to 14°. OA 30° increased PT to 20°. OA 26° decreased PI-LL to 10°. OA range of 26°-30° resulted in optimal sagittal deformity correction. OA correlated with SR positively for TFL and anterior GMED and negatively for the rest of muscles. For this OA, the SR approximately decreased 6%, 5%, 6%, 8%, and 5% for posterior GMED, anterior GMAX, middle GMAX, posterior GMAX, and P, respectively. It increased 8% and 4% for anterior GMED and TFL, respectively. Conclusion. Predictive relationships between osteotomy OA and spinopelvic parameters were shown, providing proof of concept that sagittal balance may be achieved via pelvic osteotomy.
Surgeons have known for many years that a flattened lumbar lordosis (flatback syndrome) has adverse effects on surgical outcome. Lazennec et al. (2000) recognized that fusion should not only be the main goal of lumbar spine surgery; they have shown that failure to restore normal sagittal alignment correlated with postfusion pain . However, it was not until the mid-2000s when Glassman et al. demonstrated clear statistical correlation between sagittal alignment and outcomes as measured by validated tools that primacy was placed on restoring and/or preserving sagittal alignment when performing spinal fusion surgery . Even in patients who present with coronal deformity (i.e., scoliosis), sagittal alignment was still shown to be significantly more predictive of clinical outcome compared to coronal deformity correction .
The linear relationships between osteotomy opening angle (OA) and selected sagittal spinopelvic radiographic parameters are shown on Figure 6. OA correlated negatively with PI, TPA, and SVA and positively with PT. From baseline SVA of 22 cm, a 21° OA reduced SVA to 5 cm. A 23° OA reduced TPA to 14°. A 30° OA increased PT to 20°. A 26° OA decreased PI-LL to 10°. Thus, an OA range of 26° to 30° resulted in optimal sagittal deformity correction. Within this range, PI decreased by 19°-21° from baseline.
In this study we primarily sought to examine the feasibility of a proposed pelvic osteotomy procedure in producing effective spinal sagittal alignment change via geometric mathematical modeling. Specifically, we looked at the relationships between pelvic osteotomy opening angle and commonly used spinopelvic radiographic parameters and calculated the amount of opening angle necessary to bring these parameters within the range of what is considered acceptable in the literature. Secondarily, the relationships between osteotomy opening angle and resting length of muscles spanning the osteotomy site, as characterized by the muscle-stretch ratio, was examined.