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Chapter Summary
• Context: Spinal arthrodesis alters spine biomechanics by eliminating motion across treated segments and redistributing mechanical loads onto adjacent mobile levels. Over time, degenerative changes can emerge at these neighboring junctions. The chapter clearly distinguishes two concepts that must not be conflated: radiographic adjacent segment degeneration (ASDeg) and symptomatic adjacent segment disease (ASDis), where structural degeneration correlates with new clinical symptoms. This distinction is vital because radiographic abnormalities occur far more frequently than the need for reoperation. Risk factors include patient genetics, bone quality, fusion length, facet joint violation, lumbosacral alignment, and sagittal imbalance.
• Chapter Objective: To explain the pathophysiology, biomechanics, and risk factors of adjacent segment degeneration vs. disease, detail clinical and radiographic diagnostic workup, and present evidence-based prevention and treatment strategies, ranging from conservative care and selective decompression to fusion extension, motion preservation, and minimally invasive techniques.
• Degeneration vs. DiseaseAdjacent segment degeneration (ASDeg) refers to asymptomatic radiographic changes (disc space narrowing, osteophytes, facet arthrosis, listhesis). Adjacent segment disease (ASDis) requires structural changes to produce clinical symptoms: radiculopathy, claudication, myelopathy, or mechanical pain. Treating asymptomatic radiographic degeneration with surgery is an error.
• Biomechanics and Risk FactorsFusion increases intradiscal pressure, segmental motion, and facet joint loading at adjacent mobile levels. In the cervical spine, subaxial alignment and T1 slope mismatch drive adjacent loading; in the lumbar spine, loss of physiological lordosis (PI-LL mismatch) and pelvic tilt increase mechanical strain. Surgical factors include iatrogenic superior facet joint violation by pedicle screws, excessive fusion length, and rigid construct termination.
• Diagnostic ApproachDynamic standing radiographs evaluate segmental instability, listhesis, and global sagittal balance. Contrast-enhanced MRI assesses disc herniation, canal stenosis, and neural compression. CT evaluates fusion status, facet violation, and screw positioning, ruling out pseudarthrosis and hardware loosening.
• Prevention and TreatmentPrevention begins during primary surgery: preserving adjacent supra/interspinous ligaments and capsule, avoiding facet impingement, restoring individualized sagittal lordosis, and optimizing bone metabolism. Mild symptomatic cases are managed conservatively (rehabilitation, core strengthening, epidural injections). When surgery is needed, options include standalone decompression for stable stenosis, extension of fusion with alignment restoration for unstable segments, or motion preservation (cervical arthroplasty) in selected patients.
• Clinical Application: When an arthrodesis patient presents with new pain, the key clinical question is: "Do these radiographic adjacent findings explain the current symptoms?". Complete biomechanical evaluation must assess global alignment, fusion solidity, and neural compression. Isolated stenosis without instability allows targeted minimally invasive decompression, whereas gross instability, severe listhesis, or sagittal malalignment requires fusion extension with deformity correction.
