Equivalent Clinical Outcomes but Divergent Biological Adaptation After Anterior Cervical Discectomy and Fusion Versus Cervical Disc Arthroplasty: A Longitudinal MRI Cohort Study

Background/Objectives: Anterior cervical discectomy and fusion (ACDF) and cervical disc arthroplasty (CDA) can produce comparable short-term clinical outcomes, but their regional effects on cervical muscle morphology are incompletely characterized. This study compared 12-month clinical outcomes and quantitative magnetic resonance imaging (MRI) changes after single-level C6–7 ACDF and CDA. Methods: This retrospective longitudinal cohort included 100 age-, sex-, and body mass index-matched patients (50 ACDF and 50 CDA). Neck and arm pain were assessed on 0–10 scales, and the Neck Disability Index (NDI) was analyzed as a 0–50-point score. Regional muscle cross-sectional area (CSA) and fatty infiltration (FI) were quantified on standardized preoperative and 12-month axial T2-weighted turbo spin-echo images at C6–7. The primary analysis included four posterior muscles. CSA change was expressed relative to baseline, whereas FI change was expressed in percentage points. Results: Clinical outcomes were statistically indistinguishable at 12 months (neck-pain score, 0.50 vs. 0.36, p = 0.318; NDI, 2.6 vs. 2.2 points, p = 0.479; perfect-outcome rate, 68.0% in both groups). Both groups showed significant regional posterior muscle CSA reduction and FI increase; however, the magnitude was greater after ACDF than after CDA (CSA reduction, −15% to −19% vs. −5% to −9%; absolute FI increase, +5.1 to +8.2 vs. +1.1 to +2.3 percentage points; all between-group q < 0.001). Surgical technique was independently associated with both MRI change metrics. Greater regional CSA reduction was modestly associated with higher concurrent 12-month neck-pain scores (β = −0.26, p = 0.022). This concurrent association does not establish causation or predict outcomes beyond 12 months. Conclusions: ACDF and CDA produced equivalent measured clinical outcomes at 12 months, while CDA showed a more favorable regional posterior muscle MRI profile. Differential device-related artifact was not quantitatively assessed and cannot be completely excluded, although concordant findings across four implant-remote posterior muscles make artifact unlikely to be the sole explanation. The functional and longer-term clinical significance of this imaging difference is not established and requires prospective longitudinal study.