Adjacent segment disease after cervical fusion is due to which of the following mechanisms?

• A. Infection at the adjacent level
• B. Degenerative changes at levels adjacent to the fused segment
• C. Spontaneous fusion at adjacent levels
• D. Hardware loosening at the fused segment

Answer: (B)

With cervical fusion, the fused segment no longer moves, so the adjacent levels have to accommodate more motion and bear increased mechanical stress. This shift in biomechanics raises intradiscal pressures and facet joint loading at the levels next to the fusion, accelerating degenerative changes there. Over time this manifests as new or worsened degeneration—disc thinning, facet arthropathy, foraminal stenosis—at the adjacent segments, which is the essence of adjacent segment disease.

Infections would present with systemic or wound-related signs and inflammatory features, which isn’t how adjacent-level degeneration presents. Spontaneous fusion at adjacent levels isn’t a mechanism of ASD, and hardware loosening occurs at the fused segment itself, not the neighboring levels.