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Although low back pain (LBP) is the most commonrheumatologic symptom that leads to consultation with ageneral practitioner, the identification of a specific symp-tomatic cause remains elusive in the majority of patientswith mechanical LBP. Degenerative lumbar intervertebralinstability as a cause of chronic LBP is an important factorin determining indications for spinal surgery.The concept of lumbar instability remains a subject ofconsiderable debate. In spite of many efforts, a widelyaccepted definition of the condition has not been clearlyestablished. The best diagnostic methods and the mostefficacious treatment approaches are controversial, and therelationship between radiologic evidence of instability andsymptom production is not always clear cut.This perspective discusses the role of imaging as apotential unifier of varied clinical labels and therapeuticoptions.PathomechanismsA workable definition of instability is based on a loss ofmotion segment stiffness, such that force application to thatsegment produces greater displacement than would be seenin a normal state [1, 2]. In a biomechanical sense, stiffnessis defined as the ratio of the load applied to a structure andthe resulting motion. Stability to the lumbar spine isprovided by the restraining structures working in conjunc-tion (i.e., disks, joints, ligaments, and muscles) [3].Degenerative processes, leading to laxity of restrainingstructures, result in altered equilibrium and instability [2–6]. The degenerative processes of the lumbar spinegenerally initiate from the intervertebral disk with progres-sive biochemical and structural changes leading to amodification in its physical properties of elasticity andmechanical resistance and eventual collapse of the inter-vertebral disk.Three clinically relevant consequences of acquiredcollapse of the intervertebral disk are: (a) pathologicchanges in the vertebral bodies, with osteophyte develop-ment; (b) anterior bulging of the flaval ligaments andposterior bulging of the posterior longitudinal ligament,with consequential narrowing of the central spinal canal;and (c) posterior bulging of redundant posterior disksurface, with narrowing of the central spinal canal and ofthe inferior recesses of the neural foramina. Moreover,weakening of the disk structure permits the adjacentvertebrae to slide back and forth over each other, resultingin laxity of the ligamentous network binding the vertebrae

Degenerative lumbar intervertebral instability: what is it and how does imaging contribute?