• Phillip Pollintine, Manon S L M van Tunen, Jin Luo, Matthew D Brown, Patricia Dolan, and Michael A Adams.
• Department of Mechanical Engineering, University of Bath, Bath, United Kingdom.
• Spine. 2010 Feb 15; 35 (4): 386-94.

Study DesignMechanical testing of cadaveric spines.ObjectiveTo test the hypothesis that, in the ageing spine, vertebrae deform more than discs, and contribute to time-dependent creep.Summary Of Background DataIntervertebral discs and vertebrae deform under load, narrowing the intervertebral foramen and increasing the risk of nerve root entrapment. Little is known about compressive deformations when elderly spines are subjected to sustained physiologic loading.MethodsA total of 117 thoracolumbar motion segments, aged 19 to 96 yrs (mean, 69), were subjected to 1kN compressive loading for 0.5, 1, or 2 hours. Deformations during the first 7 seconds were designated "elastic" and subsequent deformations as "creep". A 3-parameter model was fitted to experimental data in order to characterize their viscous modulus E1, elastic modulus E2 (initial stiffness), and viscosity eta (resistance to fluid flow). Intradiscal pressure (IDP) was measured using a miniature needle-mounted transducer. In 17 specimens loaded for 0.5 hours, an optical MacReflex system measured compressive deformations separately in the disc and each vertebral body.ResultsOn average, the disc contributed 28% of the spine's elastic deformation, 51% of the creep deformation, and 38% of total deformation. Elastic, creep, and total deformations of 84 motion segments in 2-hour tests averaged 0.87, 1.37, and 2.24 mm respectively. Measured deformations were predicted accurately by the model (average r2 = 0.97), but E1, E2, and eta depended on the duration of loading. E1 and eta decreased with advancing age and disc degeneration, in proportion to falling IDP (P < 0.001). Total compressive deformation increased with age, but rarely exceeded 3 mm.ConclusionWhen the ageing spine is compressed, vertebral bodies show greater elastic deformations than intervertebral discs, and creep by a similar amount. Responses to axial compression depend largely on IDP, but deformations appear to be limited by impaction of adjacent neural arches. Total compressive deformations are sufficient to cause foraminal stenosis in some individuals.

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