Donal McNally

Internal intervertebral disc mechanics as revealed by stress profilometry

A technique was developed for measuring the distribution of stress within loaded cadaveric intervertebral discs. A strain-gauged membrane mounted on the side of a 1.3-mm diameter needle was pulled through the disc at constant speed. The orientation of the membrane was changed by rotating the needle, so that profiles of vertical and horizontal components of compressive stress could be obtained. The measurements were reproducible and did not perturb the tissue to any significant extent. Stress profiles varied considerably between discs and were highly dependent on the severity of degenerative changes. They also showed that the mechanical behavior of individual disc tissues was dependent not only on their location, but also on the loading and loading history of the disc. The new insight into internal disc mechanics revealed by stress profilometry may lead to a greater understanding of the mechanisms of disc function and failure.

In vivo stress measurement can predict pain on discography

Study Design An in vivo experimental investigation of internal disc mechanics and discogenic pain. Objectives To test the hypotheses: 1) The pattern of internal loading of intervertebral discs in vivo is similar to that measured previously in vitro; 2) stress concentrations also are found in clinically degenerate discs in vivo; and stress concentrations are associated with discogenic pain. Summary of Background Data Stress concentrations corresponding to potentially painful loading patterns of the intervertebral disc and endplate have been observed in vitro. Methods The distribution of stress within the lumbar intervertebral discs of patients with chronic discogenic pain was measured using stress profilometry. The severity of their pain was assessed using provocative discography. Results Discogenic pain was found to be associated with anomalous loading of the posterolateral anulus (P < 0.001) and nucleus (P < 0.01). Painful discs were found to have a 38% wider posterolateral anulus (P < 0.023) than painless discs and to have a 63% lower mean nuclear stress (P < 0.017). Conclusions Stress profilometry is an effective investigation of the mechanics of intervertebral discs in vivo. Discogenic pain is caused by changes in the pattern of loading of the posterolateral anulus or nucleus pulposus.

Posture and the compressive strength of the lumbar spine

Summary The effect of posture on spinal compressive strength was examined in a series of three experiments on cadaveric material. Lumbar ‘motion segments’, consisting of two vertebrae and the intervening disc and ligaments, were compressed while positioned in various angles of flexion and extension. In the first experiment load sharing between the disc, the apophyseal joint surfaces, and the intervertebral ligaments was inferred from measurements of intradiscal pressure (IDPI. Results showed that extension caused the apophyseal joints to become load-bearing, and damage could occur at compressive loads as low as 500 N. Flexion angles greater than about 75% of the full range of flexion (as defined by the posterior ligaments) generated high tensile forces in these ligaments, and caused substantial increases in IDP. The optimum range for resisting compression therefore appeared to be O-75% flexion. The second experiment compared the distribution of compressive stress within the disc at the endpoints of this range, and showed that at 0% flexion high stress concentrations occur in the posterior annulus of many discs, whereas an even distribution of stress was usually found at 75% flexion. However, the third experiment showed that there was no significant difference in the compressive strength of motion segments positioned in 0% and 75% flexion. A comparison of the range of flexion/ extension movements in vivo and in vitro led us to conclude that in life a position of moderate flexion is to be preferred when the lumbar spine is subjected to high compressive forces. Relevance The experiment suggests that the normal lumbar lordosis should be flattened during manual handling to avoid injury to the osteoligamentous lumbar spine.

The internal mechanics of the intervertebral disc under cyclic loading

The mechanics of the intervertebral disc (IVD) under cyclic loading are investigated via a one-dimensional poroelastic model and experiment. The poroelastic model, based on that of Biot (J. Appl. Phys. 12 (1941) 155; J. Appl. Mech. 23 (1956) 91), includes a power-law relation between porosity and permeability, and a linear relation between the osmotic potential and solidity. The model was fitted to experimental data of the unconfined IVD undergoing 5 cyclic loads of 20 min compression by an applied stress of 1MPa, followed by 40 min expansion. To obtain a good agreement between experiment and theory, the initial elastic deformation of the IVD, possibly associated with the bulging of the IVD into the vertebral bodies or laterally, was removed from the experimental data. Many combinations of the permeability-porosity relationship with the initial osmotic potential (pi(i)) were investigated, and the best-fit parameters for the aggregate modulus (H(A)) and initial permeability (k(i)) were determined. The values of H(A) and k(i) were compared to literature values, and agreed well especially in the context of the adopted high-stress testing regime, and the strain related permeability in the model.

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

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.

Development and validation of a new transducer for intradiscal pressure measurement

Potentially damaging tensile stresses in the annulus fibrosus are directly related to the hydrostatic pressure in the centre of an intervertebral disc: the design and development of a miniature strain gauge pressure transducer is described for measuring such pressures. Static calibration tests in bulk liquid demonstrated that measurements made with the transducer were of sufficient accuracy and stability for in vitro and in vivo investigations of spinal mechanics, and a study of the dynamic behaviour of the transducer demonstrated that it had a frequency response suitable for in vitro and in vivo investigations. Tests within loaded cadaveric discs showed that the transducer could be used to make repeatable measurements which were free from significant artefacts, when the disc was subjected to forces of up to 4000 N and when deformed in full flexion/extension.

Can intervertebral disc prolapse be predicted by disc mechanics

The hypothesis was tested that stress concentrations in the posterior anulus of an intervertebral disc predispose it to prolapse under high compressive loads and anterolateral bending. The distribution of compressive stress inside the intervertebral discs of 22 cadaveric lumbar motion segments was measured with the specimens loaded in pure compression and in compression combined with anterolateral bending. Each motion segment was then loaded to failure in combined compression and anterolateral bending. Failure occurred in the vertebral body (n = 12) or posterolateral anulus (n = 10); the latter group showed a significantly greater incidence of stress concentrations (P < 0.001) in the posterior anulus, when loaded in compression and bending. It was concluded that some discs are predisposed to prolapse because of damaging, localized concentrations of stress in the posterior anulus in combined anterolateral bending and compression.

2009 ISSLS Prize Winner: What influence does sustained mechanical load have on diffusion in the human intervertebral disc?: an in vivo study using serial postcontrast magnetic resonance imaging.

Study Design. An in vivo study of the effects of mechanical loading on transport of small solutes into normal human lumbar intervertebral discs (IVD) using serial postcontrast magnetic resonance imaging (MRI). Objective. To investigate the influence of a sustained mechanical load on diffusion of small solutes in and out of the normal IVD. Summary of Background Data. Diffusion is an important source of disc nutrition and the in vivo effects of load on diffusion in human IVD remains unknown. Methods. Forty normal lumbar discs (on MRI) in 8 healthy volunteers were subjected to serial post contrast (Gadoteridol) 3 Tesla MRI in 2 phases. In phase 1 (control), volunteers were scanned at different time points – precontrast and 1.5, 3, 4.5, 6, and 7.5 hours postcontrast injection. In phase 2, 1 month later, the same volunteers were subjected to sustained supine loading for 4.5 hours. MRI scans were performed precontrast (preload) and postcontrast (postloading) at 1.5, 3, and 4.5 hours. Their spines were then unloaded and recovery scans performed at 6 and 7.5 hours postcontrast. In house software was used to analyze images. Results. Repeated-measures ANOVA and pairwise comparisons at different time points in the central region of the loaded disc (LD) compared to the unloaded discs (UD) revealed significantly lower signal intensity ratios (P1.5h:P3h:P4.5h<0.001:<0.001:<0.002) indicating reduction in transport rates for the LDs. Signal intensity ratios continued to rise in LD for 3 hours into recovery phase,whereas UD at the same time point showed a decrease (mean ± SD = 0.08 ± 0.08 vs. −0.21 ± 0.03). Conclusion. Sustained supine creep loading (50% body weight) for 4.5 hours retards transport of small solutes into the center of human IVD and it required 3 hours of accelerated diffusion in recovery state for LD to catch-up with diffusion in UD. The study supports the theory that sustained mechanical loading impairs diffusion of nutrients entering the disc and quite possibly accelerates disc degeneration.

High pressures and asymmetrical stresses in the scoliotic disc in the absence of muscle loading.

BackgroundLoads acting on scoliotic spines are thought to be asymmetric and involved in progression of the scoliotic deformity; abnormal loading patterns lead to changes in bone and disc cell activity and hence to vertebral body and disc wedging. At present however there are no direct measurements of intradiscal stresses or pressures in scoliotic spines. The aim of this study was to obtain quantitative measurements of the intradiscal stress environment in scoliotic intervertebral discs and to determine if loads acting across the scoliotic spine are asymmetric. We performed in vivo measurements of stresses across the intervertebral disc in patients with scoliosis, both parallel (termed horizontal) and perpendicular (termed vertical) to the end plate, using a side mounted pressure transducer (stress profilometry)MethodsStress profilometry was used to measure horizontal and vertical stresses at 5 mm intervals across 25 intervertebral discs of 7 scoliotic patients during anterior reconstructive surgery. A state of hydrostatic pressure was defined by identical horizontal and vertical stresses for at least two consecutive readings. Results were compared with similar stress profiles measured during surgery across 10 discs of 4 spines with no lateral curvature and with data from the literature.ResultsProfiles across scoliotic discs were very different from those of normal, young, healthy discs of equivalent age previously presented in the literature. Hydrostatic pressure regions were only seen in 14/25 discs, extended only over a short distance. Non-scoliotic discs of equivalent age would be expected to show large centrally placed hydrostatic nuclear regions in all discs. Mean pressures were significantly greater (0.25 MPa) than those measured in other anaesthetised patients (<0.07 MPa). A stress peak was seen in the concave annulus in 13/25 discs. Stresses in the concave annulus were greater than in the convex annulus indicating asymmetric loading in these anaesthetised, recumbent patients.ConclusionIntradiscal pressures and stresses in scoliotic discs are abnormal, asymmetrical and high in magnitude even in the absence of significant applied muscle loading. The origin of these abnormal stresses is unclear.

Computer vision elastography: speckle adaptive motion estimation for elastography using ultrasound sequences

We present the development and validation of an image based speckle tracking methodology, for determining temporal two-dimensional (2-D) axial and lateral displacement and strain fields from ultrasound video streams. We refine a multiple scale region matching approach incorporating novel solutions to known speckle tracking problems. Key contributions include automatic similarity measure selection to adapt to varying speckle density, quantifying trajectory fields, and spatiotemporal elastograms. Results are validated using tissue mimicking phantoms and in vitro data, before applying them to in vivo musculoskeletal ultrasound sequences. The method presented has the potential to improve clinical knowledge of tendon pathology from carpel tunnel syndrome, inflammation from implants, sport injuries, and many others.