Stress Distributions inside Intervertebral Discs: The Validity of Experimental ‘Stress Profilometry’

Abstract

This paper evaluates a technique for measuring the distribution of compressive stress within cadaveric intervertebral discs. A strain-gauged pressure transducer, side-mounted near the tip of a 1.3 mm diameter needle, was inserted into cubes of disc tissue and into intact discs. Regardless of the position and orientation of the transducer within the tissue or disc, its output was found to be proportional to the compressive force applied to the specimen. The distribution of compressive stress was measured by pulling the instrumented needle through the specimen and the resulting stress profiles were reproducible to within 20 per cent. Profiles obtained at different applied loads showed a similar distribution of stress within the disc, suggesting that the compressive stress at any location and direction increased in proportion to the applied load. Since transducer output was also proportional to applied load, it was reasoned that it must be proportional to compressive stress within the disc. The average vertical compressive stresses acting on various regions within a disc were calculated from the stress profiles and multiplied by the cross-sectional area of each region: the resulting force was then compared with the known applied force in order to assess the calibration coefficient of the transducer. Agreement between the two forces was good, indicating that the calibration coefficient established in a saline bath was applicable to disc tissues also. However, artifactual stress peaks could be generated if the transducer was pulled across a bony asperity. It is concluded that the transducer measures the mean compressive stress acting upon it within disc tissues. Errors associated with the technique are small compared to differences in stress distributions which occur naturally, for example when intervertebral discs are loaded to simulate different postures in a living person.

Dynamic stabilization devices in the treatment of low back pain

“…[24] The nucleus acts almost like a fluid filled bag and distributes load uniformly across the endplate. This has been convincingly demonstrated by McMillan et al [25] and McNally and Adams [26] by stress profilometry in normal cadaver lumbar disc (Figures 1, 2). The nucleus also transmits the load in the lateral direction to the annulus producing tension in the annulus, similar to an inflated car tire.…”

Stress in lumbar intervertebral discs during distraction: a cadaveric study

“…McMillan et al attempted to determine the validity of strain gauge transducer measures in the anulus and found the output of their transducer to be linearly proportional to the vertical force applied to the disc. They reasoned that the output was also proportional to the compressive stress perpendicular to the transducer membrane [21]. Interestingly, they found that the same calibration coefficient was applicable to liquids, nucleus pulposus, and all but the outer 2 to 4 mm of the anulus fibrosus.…”

“…Third, although the output of the transducer we used has been shown to be proportional to the applied compressive stress (perpendicular to the sensing element), it may not provide a highly accurate measure of compressive stress. Nonetheless, it provides a reasonable measure of stress change within specimens [21]. Fourth, we excluded all L5-S1 motion segments from our data.…”

“…Therefore the transducer was oriented to measure both the vertical and horizontal stress values in each specimen. The transducer output from the anulus (oriented to detect vertical or compressive stress) has been shown to be proportional to the compressive stress perpendicular to the transducer sensing surface [21].…”

“…We used the technique of intradiscal stress profilometry to estimate the stress in human cadaver motion segments under 5 conditions [20,21]. We hypothesized that all 3 forms of distraction would significantly reduce nucleus pulposus stress compared to axial loads simulating standing or sitting.…”