Pelvic tilt ratio correlates with distinct spinopelvic muscle patterns in adult spinal deformity with sagittal imbalance: a retrospective cohort study.

BACKGROUND

Pelvic tilt ratio (PTr), defined as pelvic tilt divided by pelvic incidence, may provide an individualized measure of pelvic retroversion in adult spinal deformity (ASD). However, the spinopelvic muscle characteristics associated with different degrees of pelvic retroversion remain unclear. This study aimed to characterize radiographic, clinical, and spinopelvic muscle features stratified by PTr and to identify muscle parameters independently associated with pelvic retroversion in ASD patients with sagittal imbalance.

METHODS

Ninety-nine surgically treated ASD patients with preoperative sagittal vertical axis > 50 mm and minimum 2 year follow-up were recruited. Radiographic parameters, patient-reported outcomes, and paraspinal and gluteal muscle parameters were compared across three groups by PTr. LASSO-retained muscle variables, adjusted for age, sex, and body mass index (BMI), were entered into a multivariable linear regression model for PTr.

RESULTS

The Large PTr group showed more severe sagittal malalignment, worse preoperative back pain, and greater postoperative radiographic correction. Higher PTr was associated with smaller cross-sectional areas in all three paraspinal muscles, higher erector spinae fat infiltration (ES FIR), and lower gluteus medius fat infiltration (Gmed FIR). LASSO retained multifidus total cross-sectional area (MF TCSA), ES FIR, and Gmed FIR for the final model. In multivariable linear regression, smaller MF TCSA (B = -3.305, P < 0.001), higher ES FIR (B = 0.458, P = 0.024), and lower Gmed FIR (B = -0.883, P = 0.003) were independently associated with greater PTr.

CONCLUSIONS

In ASD patients with sagittal imbalance, higher PTr was associated with more severe deformity, worse preoperative back pain, and a distinct spinopelvic muscle pattern. Smaller MF TCSA, greater ES FIR, and lower Gmed FIR were independently associated with greater PTr.