The hidden asymmetry: facet joint tropism as a clue to spinal malalignment and muscle degeneration in adult spinal deformity.

Abstract

The role of facet joint tropism (FJT) in degenerative spinal disorders such as disc herniation, spondylolisthesis, and lumbar canal stenosis is well-established. However, its association with adult spinal deformity (ASD) remains underexplored. Hence, we aim to study the correlation of FJT with spinopelvic parameters and lumbar paraspinal muscle morphology in ASD patients. We analysed 117 patients with ASD from 2021 to 2024. An absolute value difference (ΔFJA) of more than 10 degrees between the right- and left-facet joint angle (FJA) was defined as FJT. We considered patients with FJT at the apex vertebra as the FJT + group and with ASD but without FJT as the FJT- group. The mean ΔFJAs between the FJT + (n = 45) and FJT- (n = 45) were 17.14 and 5.38, respectively. For Cobb angle (CA) > 40˚ (n = 13), 84.6% (n = 11) belonged to the FJT + group. For CA 10-19˚(n = 28), 78.6% (n = 22) belonged to the FJT- group. Of the radiological parameters, differences in CA (p = 0.012), pelvic incidence (PI) (p = 0.031), grades of vertebral body rotation (VBR) (p = 0.022), facet joint osteoarthritis grades (FJOA) (p = 0.040) and cross-sectional area (CSA) of concave multifidus muscle (MF) (p = 0.010) were statistically significant between both the groups. The CSA of MF was decreased on the concave side (2.45 cm²) compared to the convex side (3.70 cm²) and was negatively correlated with ΔFJA (R² = 0.642, p = 0.020). The ΔFJA had significant positive correlation with CA (R² = 0.550, p = 0.010), PI (R² = 0.624, p = 0.030), grades of VBR (R² = 0.610, p = 0.007), and grades of FJOA (R² = 0.780, p = 0.005). Patients with ASD and FJT exhibited greater Cobb angle, higher PI, higher grades of FJOA and VBR, and lower CSA of concave MF. However, the role of facet joint tropism in adult spinal deformity-whether causal or compensatory-warrants validation through longitudinal, long-term studies. Level III.