Does spinal fusion reduce motion at the operated level and increase stress at adjacent segments?

Spinal fusion is designed to stabilize a painful or unstable motion segment. The evidence strongly supports that it restricts movement at the treated level and moderately supports compensatory mechanical changes at immediately adjacent levels.

Operated-level motion

• Clinical and radiographic studies consistently show markedly reduced flexion–extension motion after fusion. In cervical randomized comparisons, operated-level motion at 1 year was approximately 3.0–4.5° after fusion versus 12.3–13.8° after arthroplasty, with persistence at longer follow-up.
• Lumbar fusion assessment commonly includes <5° angular motion and <3 mm translation, together with bridging bone. Experimental TLIF models reduced flexion–extension from roughly 7.1° to 3.1–3.3°; pedicle-screw supplementation further increased rigidity.
• The degree of residual movement depends on construct design, with bilateral fixation and more extensive instrumentation generally imposing greater restriction.

Adjacent-segment mechanics

• Cadaveric, laboratory, and finite-element studies consistently find that lost index-level motion is partly redistributed to the levels above and below, most clearly during flexion–extension.
• These adjacent levels also experience higher intradiscal pressure, disc/annular stress, and facet-joint loading. Mechanical loading is greater with multilevel fusion than single-level constructs.
• Sagittal alignment matters: finite-element evidence indicates that relatively flat constructs can substantially increase adjacent-disc stress during flexion and increase adjacent-level range of motion.

Clinical meaning

• Increased adjacent motion or stress is a biomechanical finding, not proof of future symptoms. Symptomatic lumbar adjacent-segment disease has been estimated at 5.2%–18.5% in reported series; risk is also influenced by pre-existing degeneration, construct length, alignment, and posterior-element injury.

Bottom line

• Fusion reliably reduces operated-level motion and can increase adjacent-level motion and loading, but these changes do not determine that an individual—including an older adult—will develop symptomatic adjacent-segment disease.