Level-dependent changes in cervical multifidus composition following anterior cervical discectomy and fusion: bridging quantitative MRI and clinical perspectives

Fatty infiltration (FI) of the cervical paraspinal muscles is associated with greater disease severity and less favorable outcomes in degenerative cervical spine disease. However, longitudinal data on whether muscle composition changes following anterior cervical discectomy and fusion (ACDF) are scarce. This study aimed to evaluate changes in FI and functional cross-sectional area (fCSA) of the cervical multifidus and rotatores after ACDF and to assess whether these changes differ across surgical levels. This single-center retrospective study analyzed patients who underwent ACDF for degenerative cervical stenosis between 2015 and 2018. Patients were included if both preoperative and postoperative cervical spine MRI (obtained 1–5 years after surgery) were available. The multifidus and rotatores colli were segmented bilaterally on T2-weighted axial images. Measurements were categorized by position within the fusion construct as upper (most cranial fused vertebra), middle (intervening vertebrae), and lower (most caudal fused vertebra) levels. FI and fCSA were quantified using Otsu thresholding. P-values were adjusted for multiple testing using the Benjamini-Hochberg procedure. Thirty-five patients met inclusion criteria (median age 58 years, 40% female, 1–4 fused levels). Bilateral FI decreased significantly at the lower level (52.1 ± 15.4% vs. 45.0 ± 14.8%, adjusted p = 0.039), with no significant changes at the upper (adjusted p = 0.858) or middle level (adjusted p = 0.354). Bilateral fCSA decreased significantly at the middle level (165.2 [119.4–196.3] vs. 109.4 [92.0–171.6] mm², adjusted p = 0.018), with no significant changes at the upper (adjusted p = 0.194) or lower level (adjusted p = 0.382) ACDF was associated with level-dependent changes in cervical multifidus composition, including a reduction in FI at the lower surgical level and a loss of fCSA at the middle level. These findings highlight that postoperative muscle remodeling varies by position within the fusion construct, which may have implications for the development of level-specific, targeted postoperative rehabilitation strategies.