Abstract
Introduction: There are extensive differences in structure and composition between cervical and thoracolumbar discs, yet practically nothing is known about the time-dependent “creep” behaviour of cervical discs.
Methods: 41 cadaveric cervical motion segments aged 48–89 yrs were subjected to a static compressive load of 150N for 2 hrs. Specimen height was recorded by the displacement of the actuator of the testing machine. Digitized radiographs were analysed to obtain dimensions of the vertebrae and discs. A three-parameter solid viscoelastic model was fitted to experimental data using nonlinear regression. Model parameters represent compressive stiffness of the wet tissue (E2) and of the drained solid matrix (E1), and tissue viscosity (η1).
Results:Model and experimental data were in good agreement (r2> 0.98) and the average absolute error was always < 2%. E1 was 11% and 39% lower than published values for thoracic and lumbar discs, respectively, whereas E2 was 43% and 53% higher. The ratio E2/E1 for cervical discs (1.63) was greater than for thoracic (1.01) and lumbar (0.66) discs. η1 for cervical discs was 108% and 21% higher than in thoracic and lumbar discs, resulting in a creep rate (E1/η1) which was lower by 51% and 43% respectively. Comparisons between younger (mean age 58 yrs) and older (79 yrs) cervical discs showed that in the latter, η1 was reduced by 32% (p=0.01), E2 reduced by 18% (p=0.06), whereas E1/η1 was increased by 47% (p=0.02).
Discussion: Cervical discs appear to resist water loss more than thoracolumbar discs, but this resistance falls in old age.
Correspondence should be addressed to Ms Alison McGregor, c/o BOA, SBPR at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
