Can lumbar fusion increase stress and motion at adjacent spinal segments?

Lumbar fusion predictably redistributes load and motion to untreated levels, making the proposed pathway credible; however, degeneration and pain after surgery remain multifactorial rather than inevitable consequences of the procedure.

Biomechanical evidence

• Human cadaveric L2–S1 testing found that L4–5 fusion significantly increased intradiscal pressure and facet forces at both adjacent levels (L3–4 and L5–S1), directly demonstrating increased adjacent-segment loading.
• Finite-element studies consistently predict greater adjacent-disc stress after fusion. Stress rises with longer fusion constructs, and reduced lordosis at the fused level increases both adjacent-level intradiscal pressure and motion.
• Motion changes are less uniform clinically. In dynamic biplane radiography, all 7 patients maintained or increased upper-adjacent-level flexion motion at 6 months. Yet, across a review of 39 in-vivo kinematic studies, excessive upper-adjacent motion was reported in only about 20–30% of patients.

Degeneration and symptoms

• Increased loading and/or motion provide biologically coherent mechanisms through which fusion may contribute to adjacent-segment degeneration, but available clinical evidence does not isolate either biomechanical change as an independent cause.
• Aging, pre-existing disc or facet disease, obesity, fusion length, and sagittal alignment also influence adjacent-level degeneration. Longer constructs and malalignment are particularly relevant modifiers.
• Degenerative imaging findings alone do not establish a pain generator. Adjacent-segment disease is clinically meaningful when new or worsening adjacent-level pathology is concordant with back pain, radiculopathy, or claudication; many radiographic changes are incidental.

Clinical implications

• New pain after fusion warrants assessment for adjacent pathology but also nonunion, hardware-related problems, infection, alignment/instability, and non-spinal causes. MRI is useful for radicular symptoms or claudication; radiographs and CT assess alignment, fusion integrity, and hardware.

Bottom line

• Fusion can increase adjacent-level mechanical stress and, in some patients, motion. These changes plausibly contribute to adjacent degeneration, which can cause pain when clinically concordant, but they do not by themselves prove that postoperative pain or degeneration was caused by fusion.