Balance or Strength? Reconsidering Muscle Metrics in Sagittal Malalignment in Adult Sagittal Deformity Patients

<b>Background/Objectives</b>: Atrophy of the paraspinal and psoas major muscles is closely linked to sagittal malalignment in adult spinal deformity (ASD). However, most studies overlook the balance between these muscle groups. This study investigates the relationship between trunk muscle balance and sagittal alignment in ASD patients. <b>Methods</b>: A single-institution database was reviewed for patients with sagittal malalignment (PT > 20° and PI-LL > 10°). Standard sagittal parameters were measured based on standing X-rays. The cross-section area (CSA) of trunk posterior muscles (CSA^P: erector spinae and multifidus) and anterior muscles (CSA^A: psoas) at L4 were measured based on a T2-weighted MRI. Patients with prior lateral fusions were excluded. Muscle balance was evaluated by the CSA ratio of trunk posterior to anterior muscles (CSA^P/A). The relationship between sagittal alignment parameters and CSA^P, CSA^A, as well as CSA^P/A were analyzed using linear and quadratic regressions. Akaike information criteria (AIC) compared model fit. Subgroup analyses examined the relationship between sagittal alignment changes and different CSA^P/A levels. <b>Results</b>: A total of 112 patients met inclusion and exclusion criteria. CSA^P correlated linearly with SS (r² = 0.057, <i>p</i> = 0.011), PT (r² = 0.043, <i>p</i> = 0.028), and T4-L1PA mismatch (r² = 0.044, <i>p</i> = 0.027). CSA^A showed no significant linear or quadratic relationships with sagittal spinal alignment parameters. In contrast, CSA^P/A was quadratically associated with LL (r² = 0.056, <i>p</i> = 0.044), SS (r² = 0.134, <i>p</i> < 0.001), PI (r² = 0.096, <i>p</i> = 0.004), L1PA (r² = 0.114, <i>p</i> = 0.001), and T4-L1PA mismatch (r² = 0.094, <i>p</i> = 0.005). Quadratic models of CSA^P/A consistently had higher r² and lower AIC values compared to the linear models of CSA^P for most sagittal alignment parameters, especially in SS, PI, L1PA, and T4-L1PA mismatch (AIC difference ≥4). Higher CSA^P/A is correlated to larger PI (and consequently, larger LL, SS, and L1PA). <b>Conclusions</b>: Trunk posterior-anterior muscle balance (CSA^P/A) demonstrates a stronger relationship with sagittal alignment than individual muscle metrics. Quantitative MRI-based definitions of sarcopenia may need to be adjusted for PI.