Article: PDF OnlyBiomechanical Definitions of Spinal InstabilityPOPE, MALCOLM H. PhD*; PANJABI, MANOHAR PhD†Author Information From *the University of Vermont and †Yale University. Spine: April 1985 - Volume 10 - Issue 3 - p 255-256 Buy Abstract Instability is a mechanical entity, and an unstable structure is one that is not in a optimal state of equilibrium. In the spine, stability is affected by restraining structures that, if damaged or lax, will lend to altered equilibrium and thus instability. Instability is defined as loss of stiffness. © Lippincott-Raven Publishers.