P33. Degenerative paraspinal muscles impact thoracic spine compensation in ASD patients

Abstract

BACKGROUND CONTEXT Flattening of the thoracic spine is a common biomechanical compensatory mechanism for adult spinal deformity (ASD). The literature shows that younger ASD patients recruit more thoracic flattening than older patients. It is hypothesized that differences in muscle quality between younger and older ASD patients explains this difference in the recruitment of thoracic compensation. PURPOSE Investigate the relationship between paraspinal muscle degeneration and recruitment of thoracic compensation. STUDY DESIGN/SETTING Retrospective review of a prospective database. PATIENT SAMPLE ASD patients from a prospective database. OUTCOME MEASURES Posterior paraspinal muscle fat infiltration, thoracic kyphosis between T4 and T12 (TK) and pelvic retroversion (PT). METHODS ASD patients with spino-pelvic deformity (defined as SRS-Schwab PI-LL modifier of + or ++) and available pre-op CT imaging were identified from a retrospective database. Patients with previous fusion of the thoracic spine were excluded. Erector spinae and multifidus measurements were manually contoured on CT imagesat three vertebral levels (T2, T10, and L3), at three locations per level: immediately inferior to the superior end plate, in the middle of the vertebral body, and superior to the inferior end plate. Fat infiltration (Fat) within each muscle was defined as the number of voxels with a Hounsfield unit (HU) corresponding to fat (-190 to 0) divided by the total number of voxels. The maximum fat infiltration (MaxFat) was determined by taking the muscle with the highest percentage of Fat. After a Pearson correlation analysis, the cohort was separated into 2 groups: high Fat (HFat) and Low Fat (LFat) based on MaxFat threshold of 35%. Demographics and radiographic parameters including TK and PT were compared between groups. RESULTS Of the 57 patients (mean age 61 yrs, 75% F, BMI 28) that met inclusion, 26% had a fused lumbar spine. Assessed by +/++ SRS-Schwab modifiers, 90% of patients had severe PT malalignment and 75% a severe SVA malalignment. Breakdown of Fat and area by vertebral level was: T2 (35% and 1332mm²), T10 (30% and 1910mm²), L3 (38% and 3915mm²). TK correlated significantly with Fat at all levels from T2 to L3 (r=-0.498, r=-0.519, r=-0.472), FatMax (r=-0.470) and area at T10 (r=-0.341). PT correlated significantly with FatMax (r=0.333) and T10 Fat(r=0.548). After grouping, 31 pts were HFat, and 25 pts were LFat. HFat pts were older (64.2 vs 56.3, p=0.047) and had a greater BMI (30.1 vs 25, p=0.002). The groups did not differ in sex or rates of lumbar fusion. In terms of spinal alignment, HFat pts had a higher T1 pelvic angle (30.7° vs 23.7°), a higher TK (-34.6° vs -22.9°), and a higher PT (30.1° vs 25.2°), without difference in lumbar deformity or other sagittal parameters. CONCLUSIONS Patients with higher fat infiltration in paraspinal muscles show less compensation with the thoracic spine than patients with leaner musculature, despite having similar lumbar deformities. Consequently, patients with degenerative paraspinal musculature have more pelvic retroversion, causing greater gait disruption due to limitation in hip extension. The assessment of fatty infiltration in posterior paraspinal muscles could help identify patient compensatory profile and aid in surgical planning. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.