Limitations of fusion without pelvic foundation in adult spinal deformity surgery.

INTRODUCTION

Significant spinal corrections require a rigid pelvic foundation (PF) and back muscles related to sagittal alignment. However, criteria for deformity or muscular volume that allow fusion without a PF are lacking. This study aims to assess the feasibility of achieving fusion-ending lumbar spine, focusing on preoperative alignment and back muscle condition, and to analyze how poor alignment progresses over 2 years and the related mechanical failure.

METHODS

Spinal instrumentations involving ≥ 4 levels from thoracolumbar to L5 were categorized from the 2-year sagittal vertical axis (SVA) into Group F (≥ 50 mm) and Group S (< 50 mm). Differences in radiographic measurements (sagittal modifiers, L1/T4 pelvic angles, and others), cross-sectional areas of the multifidus/erector spinae muscles, and mechanical failure rates were evaluated.

RESULTS

The study included 105 patients (average age 70.1 years; 65.7% female), with 60 in Group F and 45 in Group S, averaging 5.8 fused levels. Preoperative sagittal alignment improved postoperatively. Group F had worse postoperative SVA than Group S and showed deterioration over 2 years. Preoperative alignment and multifidus were correlated with 2-year SVA, and multivariate analysis identified significant factors, with cutoff values of 92 mm for SVA and 335 mm² for multifidus. Additionally, Group F showed a higher rate of mechanical failure.

CONCLUSION

References for spinal restoration without PF were evaluated from preoperative alignments and back muscle conditions. Patients who cannot maintain proper alignment for even 2 years might require fusion with a PF to ensure better long-term alignment and prevent mechanical failure.