Abstract
Introduction: Pathological changes in the elderly spine include intervertebral disc degeneration, apophyseal joint arthritis and osteoporotic fracture of the vertebral body. Such changes are likely to be inter-related through alterations in the sharing of load between the apophyseal joints and the intervertebral disc unit. We describe an accurate, non-destructive method for calculating the load sharing based on measurements of the distribution of stress within the intervertebral disc.
Materials and Methods: Twenty three motion segments, consisting of two vertebrae and the intervening disc and ligaments, were dissected from 17 human lumbar spines. A preliminary “creep” test was used to reduce disc height and water content by an amount equivalent to the diurnal variation seen in vivo. Then, a constant load was applied to each motion segment, using a computer-controlled hydraulic materials testing machine, for a period of 20s while a pressure-transducer, sensitive to spatial variations in compressive stress, was pulled through the disc along its mid-sagittal diameter. Profiles of vertically-acting compressive stress were obtained in each disc positioned in 2° of extension (appropriate for an erect standing posture). The total compressive force acting on the intervertebral disc was calculated by modelling the disc using approximately 20 elliptical rings of known cross-sectional area. The force acting on each ring was given by the product of area and the average compressive stress acting on it, which was obtained from the appropriate region of the stress profile. The total force acting through the disc was obtained by summing up the force contribution from each ring. The force acting on the apophyseal joints was calculated from the difference between applied (known) load and the calculated load acting on the disc. A correction factor was obtained separately for each disc to account for deviations in the cross-section from the elliptical, and variations in the sensitivity of the transducer in disc tissues of different ages. The correction factor was obtained by comparing the applied force with the force calculated from a stress profile measured before creep loading while the disc was in a neutral position, when the load passing through the apophyseal joints is negligible.
Results: The proportion of load passing through the apophyseal joints increased significantly with age (r2=0.48, p< 0.01), from 7% at age 27 yrs to 42% at 82yrs. Similarly, the proportion of load passing through the apophyseal joints increased with degree of disc degeneration (r2=0.5, p< 0.05 Pearson, Chi-square) from 8% in “grade 1” discs to 40% in “grade 4” discs.
Discussion: The compressive load passing through the apophyseal joints is higher than that predicted by previous, inaccurate, methods, or by experiments which failed to reduce the height and water content of the intervertebral disc. Increased load-bearing may be a contributing factor in apophyseal joint degeneration. Also, in lordotic postures, “stress shielding” by the apophyseal joints could contribute to bone loss in the vertebral body, leaving it vulnerable to osteoporotic fracture when the spine is loaded in flexion.
The abstracts were prepared by Dr P Dolan. Correspondence should be addressed to him at the British Orthopaedic Association, Royal College of Surgeons, 35-43 Lincoln’s Inn Fields, London WC2A 3PN.
