Abstract
Introduction: ‘Stress profilometry’ involves pulling a pressure transducer through a loaded intervertebral discs in order to characterise the intensity of loading within it. The technique has been used to explore how stress distributions vary with age, spinal level, degeneration, creep loading, and injury. However, can the output of the strain-gauged transducer (which is calibrated in a fluid) really quantify stress perpendicular to its membrane when inserted into the fibrous matrix of degenerated discs?
Methods: Thirteen full-depth cylinders, 7mm in diameter, were cut from inner, middle and outer regions of the anterior and lateral annulus of two human upper-lumbar discs aged 74 and 82 yrs. Specimens were confined within a metal cylinder of internal diameter 7 mm. Two vertical slots on opposite sides of the metal cylinder allowed a pressure transducer, side-mounted near the tip of a 0.9 mm-diameter needle, to be pulled through the annulus sample. Constant compressive loading was applied for 20s to the top of the annulus sample, using a plane-ended 6.9 mm-diameter indenter, while the transducer was pulled through the sample. Transducer output was sampled at 25Hz. ‘Stress profiles’ were repeated with the transducer orientated vertically and horizontally, and with 6-21 values of compressive load, corresponding to stresses up to 3MPa. Average values of measured ‘stress’ were compared to applied stress (compressive force/indenter area).
Results: Measured (average) vertical compressive stress was linearly related to applied stress, with Rsq values averaging 0.97. The gradient of the line averaged 0.98 (range 0.77 – 1.28) indicating that measured stress values approximated to applied stress, and were not merely proportional to it. For horizontal measurements, the Rsq and gradient averaged 0.97 and 0.92 respectively. Abnormal results in 3/13 specimens appeared to be affected by transducer damage and were disregarded.
Conclusion: Stress profilometry can quantify compressive stress within the annulus of degenerated intervertebral discs. This fibrous tissue appears to be sufficiently deformable to allow efficient coupling of stress between the matrix and transducer membrane.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
