Background context:

The concept of sagittal spinal malalignment is well established in spinal surgery. However, the effect of musculature on its development has not been fully considered and the position of the pelvis is mostly seen as compensatory and not necessarily a possible cause of sagittal imbalance.

Purpose:

This study aimed to investigate the influence of the posterior paraspinal muscles (PPM, erector spinae & multifidus) and the psoas muscle on spinopelvic and spinal alignment.

Study design/setting:

Retrospective cross-sectional study.

Patient sample:

Patients undergoing posterior lumbar fusion between 2014 and 2021 for degenerative conditions at a tertiary care center, with preoperative lumbar magnetic resonance imaging (MRI) within 12 months prior the surgery and a preoperative whole spine radiograph were included.

Outcome measures:

PPM and psoas muscle measurements including the cross-sectional area (CSA), the functional cross-sectional area (fCSA), the amount of intramuscular fat (FAT), and the percentage of fat infiltration (FI). Spinopelvic measurements including lumbar lordosis (LL), pelvic tilt (PT), sacral slope (SS), pelvic incidence (PI), and sagittal vertical axis (SVA).

Methods:

A T2-weighted MRI-based quantitative assessment of the CSA, the fCSA and the amount FAT was conducted, and FI was further calculated. The regions of interest included the psoas muscle and the PPM on both sides at the L4 level that were summarized and normalized by the patient’s height (cm²/m²). LL, PT, SS, PI and SVA were determined on standing lateral radiographs. Spearman correlation was used to calculate the crude relationship between spinopelvic and muscle parameters. Multiple linear regression models with age, sex, LL, PT, SS, and SVA set as independent variables were established to determine the association with spinal muscle outcome measures.

Results:

A total of 150 patients (53.3% female) were included in the final analysis with a median age of 65.6 years and a median BMI of 28.2 kg/m². Significant positive correlations were observed between PT (ρ=0.327), SVA (ρ=0.256) and PI (ρ=0.202) and the FI_PPM. Significant negative correlations were detected for the PT and the fCSA_PPM (ρ=-0.202) and PT and the fCSA_Psoas (ρ=-0.191). Furthermore, a negative correlation was seen for PI and SVA and FI_Psoas. PT (β=0.187; p=0.006), SVA (β=0.155; p=0.035), age (β=0.468; p<0.001) and sex (β=0.235; p<0.001) significantly predict FI_PPM (corrected R²=0.393) as independent variables.

Conclusion:

This study demonstrated the potential role of posterior paraspinal muscles and psoas muscle on pelvic retroversion and elucidated the relation to sagittal spinal malalignment. Although we cannot establish causality, we propose that increasing FI_PPM, representing loss of muscular strength, may lead to increased pelvic retroversion and thus might be the initiating point for the development of the sagittal imbalance. These findings might challenge the well-known theory of increased pelvic retroversion being a compensatory mechanism for sagittal spinal balance. Thus, muscular weakness might be a factor involved in the development of sagittal spinal malalignment.