Can compressive stress be measured experimentally within the annulus fibrosus of degenerated intervertebral discs?

The aims were to assess the ability of a pressure transducer to measure compressive stress within the annulus fibrosus of degenerated intervertebral discs. Measurements could help to explain the mechanisms of disc failure and low back pain. The methods used were as follows. Thirteen full-depth cores of annulus, 7 mm in diameter, were removed from the middle and outer annuli of two severely degenerated human discs and constrained within a metal cylinder. Then static compressive forces were applied by a plane-ended metal indenter of diameter 6.8 mm, while a strain-gauged pressure transducer, side mounted in a needle of diameter 0.9 mm and calibrated in saline, was pulled through the tissue. The transducer output was converted into stress, and the average measured stress was compared with the nominal applied stress. Measurements were repeated at up to 21 load levels, with the transducer oriented vertically and horizontally. The results showed that the measured and applied stress were linearly related (average r2 = 0.98) with a mean gradient (calibration factor) of 0.98 (vertical stress) and 0.92 (horizontal stress). Gradients ranged between 1.28 and 0.73. Damaged transducers grossly under-recorded 'stress' even though their output remained proportional to applied load. It was concluded that pressure transducers can measure compressive stress inside a degenerated human annulus. The tissue is sufficiently deformable to allow efficient coupling of stress between the matrix and transducer membrane. Damage to the transducer can give misleading results.