Semi-quantitative assessment of paraspinal muscle fat infiltration in single-level C5-6 cervical spondylosis using gray-level thresholding on conventional MRI: associations with disc degeneration and posture-related segmental angular change.

Background: Cervical spondylosis at C5-6 is common, but conventional imaging may not fully capture paraspinal muscle composition or posture-related segmental angular behavior. This study assessed layer-specific paraspinal muscle fat infiltration (FI) and cross-sectional area (CSA) on conventional MRI and examined their associations with C5-6 disc degeneration and posture-related angular change (PAC). Methods: In this retrospective cross-sectional study, 94 patients with single-level C5-6 cervical spondylosis who underwent cervical MRI and standing lateral radiography between January 2023 and December 2025 were included. At the C5-6 disc level, bilateral splenius capitis (SPL), semispinalis capitis (SCap), and combined multifidus plus semispinalis cervicis (MF + SCer) were manually segmented. FI was calculated as a semi-quantitative area percentage using a fixed 8-bit gray-level threshold (79-255), and CSA was measured on calibrated axial T2-weighted images. PAC was defined as the absolute difference between standing radiographic and supine MRI local C5-6 Cobb angles; patients were classified as low-PAC/relatively stiff (|PAC| ≤ 5°) or high-PAC/relatively mobile (|PAC| > 5°). Results: FI in all three muscle groups increased with higher Pfirrmann grades, whereas CSA did not differ significantly across grades. Pfirrmann grade was independently associated with FI in the SPL, SCap, and MF + SCer after adjustment for age and sex. Compared with the high-PAC group, the low-PAC group showed higher FI in the SPL, SCap, and MF + SCer, and these differences remained significant after adjustment for age, sex, BMI, and Pfirrmann grade. CSA did not differ significantly between PAC phenotypes. Conclusions: In patients with single-level C5-6 cervical spondylosis, paraspinal muscle alteration was characterized primarily by increased FI, whereas CSA reduction was not evident. Compared with CSA, semi-quantitative FI measured on conventional MRI may be more sensitive for detecting observable changes in muscle composition. Higher FI in the superficial and intermediate posterior extensor muscles was associated with a low-PAC/relatively stiff phenotype. Combining PAC with paraspinal muscle FI may provide supplementary imaging information on segmental angular differences between weight-bearing and non-weight-bearing positions; however, the clinical relevance of this combined assessment requires prospective validation using quantitative fat imaging and clinical or functional outcomes.