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Biomedical subjects

D S McNally

Publications and source records attributed to D S McNally.

At least 19 recordsLinked to original sources
PubMed2005

Demonstration of the appearance of the paraspinal musculoligamentous structures of the cervical spine using ultrasound.

The application of ultrasound in the imaging of the neck has primarily focussed on anterior structures (e.g., thyroid gland). Structures located on the posterior aspect of the neck have received little attention. This study illustrates the capability of modern ultrasound equipment in visualising the musculoligamentous structures of the neck, particularly the paraspinal musculature. Ten healthy adult volunteers (6 female; 4 male) underwent ultrasound examination of the cervical spine. A standardised technique for transducer placement was adopted and successive images of the neck of each subject were obtained. Spatial compound (extended field of view) images were obtained in subjects using one of two different ultrasound systems. Images of structures produced by ultrasound were compared to those achieved with magnetic resonance imaging in three subjects. Identification of key landmarks aided orientation and identification of structures. The internal architecture of the musculoligamentous structures of the cervical spine, especially the posterior neck muscles, was demonstrated well using ultrasound. Our study showed that modern ultrasound equipment is capable of producing clear images of the posterior cervical spine musculature and certain bony features.

Adult↗
PubMed2005

A one-dimensional theoretical prediction of the effect of reduced end-plate permeability on the mechanics of the intervertebral disc.

The permeability of the cartilage end-plate (CEP) may play an important role in intervertebral disc (IVD) degeneration by controlling the convective and diffusive transport of metabolites into the nucleus pulposus. A one-dimensional poroelastic model was used to predict the effect of a CEP of lower permeability than the disc tissue on the convective transfer into and out of the IVD. With decreasing CEP permeability, associated with degeneration, the model predicted that the change in disc height with time became more linear; the disc could not rehydrate as quickly; and internal fluid movement was slowed. This study has shown that CEP permeability will only markedly have an effect on fluid movement, and hence convective nutrition, if the permeability of the CEP is reduced to less than that of the disc tissue.

Animals↗
PubMed2005

Rigid-body modelling of shaken baby syndrome.

Recent reassessment of the literature on the shaken baby syndrome (SBS) has revealed a lack of scientific evidence and understanding of all aspects of the syndrome. In particular, studies have been unable to clarify the mechanisms of injury, indicating that impact, rather than shaking alone, is necessary to cause the type of brain damage observed. Rigid-body modelling (RBM) was used to investigate the effect of neck stiffness on head motion and head-torso impacts as a possible mechanism of injury. Realistic shaking data obtained from an anthropometric test dummy (ATD) was used to simulate shaking. In each study injury levels for concussion were exceeded, though impact-type characteristics were required to do so in the neck stiffness study. Levels for the type of injury associated with the syndrome were not exceeded. It is unlikely that further gross biomechanical investigation of the syndrome will be able to significantly contribute to the understanding of SBS. Current injury criteria are based on high-energy, single-impact studies. Since this is not the type of loading in SBS it is suggested that their application here is inappropriate and that future studies should focus on injury mechanisms in low-energy cyclic loading.

Acceleration↗
PubMed2004

Musculoskeletal motion flow fields using hierarchical variable-sized block matching in ultrasonographic video sequences.

We examine tissue deformations using non-invasive dynamic musculoskeletal ultrasonograhy, and quantify its performance on controlled in vitro gold standard (groundtruth) sequences followed by clinical in vivo data. The proposed approach employs a two-dimensional variable-sized block matching algorithm with a hierarchical full search. We extend this process by refining displacements to sub-pixel accuracy. We show by application that this technique yields quantitatively reliable results.

Algorithms↗
PubMed2002

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.

Cadaver↗
PubMed2000

Knoop microhardness anisotropy of the ovine radius.

The Knoop indenter has been used to characterise fully the Knoop microhardness (H(K)) anisotropy of compact bone. 2120 indentations were performed on mature ovine radii and a linear relationship was found between H(K) and the angle between the major diagonal of the indenter and the lamella boundaries (p<<0.001). H(K) increased significantly with ash fraction (p<0.001), but decreased with atmospheric vapour pressure (p<0.05). A significant interaction was found between ash fraction and atmospheric vapour pressure (p<0.01). H(K) significantly varied with indentation position along the diaphysis and around the cortex (both p<<0.001), however radial variation in H(K) was not statistically significant. The variation of ash fraction showed similar trends. These data show that H(K) varies similarly to Vickers microhardness, but in addition, can provide clear information on the anisotropy of Haversian bone without the need for excising many different indentation planes. A large number of indentations are required to obtain low type I and type II errors in the statistical analysis.

Animals↗
PubMed2000

Intervertebral disc structure: observation by a novel use of ultrasound imaging.

The internal structure of intervertebral discs is clinically important in the management of back pain. No current routine imaging modality is able to image disc structure satisfactorily. The aim of this work was to investigate and validate ultrasound imaging so that it might be applied to assessment of structural integrity and degree of degeneration. The optimum imaging technique was determined using a 3.5 MHz probe in one female subject. The applicability of this technique to investigate disc structure in the entire thoracolumbar spine was further investigated in 13 subjects. The optimum disc imaging technique was found to be a posterolateral approach, 1 to 2 cm lateral of the dorsal midline, that revealed structure within the disc not apparent using other approaches. It was demonstrated that posterolateral imaging introduces a smaller reproducibility error in measurements of linear dimensions close to the disc. It is possible to observe internal structure within the disc between T11 and L3 in at least 54% of individuals.

Adolescent↗
PubMed1998

Determination of a standard site for the measurement of bone mineral density of the human calcaneus.

Ultrasound of the calcaneus may be used as a cheap, ionising radiation-free and easy to use indicator of skeletal status, and hence of osteoporotic fracture risk. At present ultrasound is not widely used as it suffers from high precision errors. As ultrasound parameters are determined in part by bone mineral density (BMD), an increase in the accuracy and precision of BMD measurements should reduce the precision error associated with ultrasound measurements. The aim of this study was to define an anatomical site on the calcaneus at which accurate and precise measurements of BMD can be made. Ten dry calcanei and 10 cadaveric feet were scanned using a DXA scanner; 9 anatomically defined regions (1 cm2) were selected in the posterior part of the calcaneus for analysis. The centre of region 1 was positioned halfway along the line joining the anterior border of the calcaneal tubercle and the peak of the posterior superior tubercle, and the remaining 8 regions were placed around this central area. The BMD in these 9 regions was compared with the whole bone BMD and the variability of BMD within each of the 9 regions was measured. The reproducibility of the technique was assessed by taking 10 repeated measurements of 2 bone and 2 cadaveric specimens, each specimen being removed and repositioned between measurements. Region 1 was found to be the most representative of total BMD in cadaveric feet. This region also showed the least variability of BMD and consistently gave the lowest coefficients of variation in the reproducibility study both in the bone and the cadaveric specimens. This region is hence the most suitable site on the calcaneus for measuring absolute values of and changes in BMD. The surface position of region 1 was found to be consistently 5/9 along the line at 45 degrees to the vertical, from the lateral malleolus to the heel. The identification of the surface location of region 1 relative to anatomical landmarks of the foot has enabled the same anatomical site to be measured in all subjects. This allows meaningful intersubject comparisons to be made. Preliminary data suggest that precision errors using ultrasound are also reduced when measurements are taken at this region of the calcaneus. The reduction in the precision error of ultrasound assessment of skeletal status may provide a cheap and safe way to identify individuals at risk from osteoporotic fracture.

Aged↗
PubMed1997

Microhardness anisotropy of lamellar bone.

The Knoop microhardness test has been utilised to observe in-plane microhardness anisotropy of rat tibiae. The elongated rhombohedral geometry of the Knoop indenter enables the Knoop microhardness (HK) to be calculated for a given indenter orientation. Two indenter orientations were used: the major axis of the indenter was aligned along the length of, and across the mid-sagittal section. The statistical analysis demonstrated that the variation in HK was primarily due to the orientation of the Knoop indenter (p < 0.001). HK was consistently greater when the indenter was aligned with the major diagonal radial on the mid-sagittal section.

Animals↗
PubMed1997

The effects of posterior fixation on internal intervertebral disc mechanics.

Posterior fixation of intervertebral discs is used to treat, and occasionally diagnose, discogenic pain since it is thought that it will reduce the internal loading of the discs in vitro. We measured the internal loading of ten intervertebral discs using stress profilometry under simulated physiological loads and then after posterior fixation. Partial discectomies were performed to simulate advanced disc degeneration and the sequence repeated. Posterior fixation had very little effect on the magnitude of the loads acting on the disc and none when disc degeneration was simulated. It did, however, reduce bulging of the anterior annulus under combined bending and compression (p < 0.03). Recent experiments in vivo have shown that discogenic pain is associated with abnormal bulging of the annulus which suggests that the clinical benefit of fixation may be due to this.

Aged↗
PubMed1996

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.

Adult↗
PubMed1996

Effects of hydrostatic pressure on matrix synthesis in different regions of the intervertebral disk.

The intervertebral disk is routinely subjected to compressive loads that alter with posture and muscle activity and can produce pressures > 2 MPa in human lumbar disks in vivo (A. Nachemson and G. Elfstrom. Scand. J. Rehabil. Med. 2, Suppl. 1:1-40, 1979; A. Nachemson and J. M. Morris. J. Bone Jt. Surg. Am. Vol. 46A: 1077-1092, 1964). We measured the effect of load on hydrostatic pressures in bovine caudal disks. With increase in applied load, pressure increased linearly in the nucleus and inner annulus. The resting pressure measured after slaughter (0.19 +/- 0.05 MPa) and the pressure at failure (34 MPa, estimated from the vertebrae/disk segment failure load of 7,430 +/- 590 N) define the limits that can occur in vivo. Because hydrostatic pressure influences matrix synthesis in articular cartilage, we have examined the effects of pressures in the range 1-10 MPa applied for 20 s or 2 h on proteoglycan synthesis in bovine caudal and human lumbar intervertebral disks in vitro. In the nucleus pulposus and inner annulus of bovine disks, application of hydrostatic pressure in the range of 1-7.5 MPa for only 20 s stimulated matrix synthesis over the following 2 h at atmospheric pressure. The maximum stimulation in the bovine disks was seen in the inner annulus after application of 2.5 MPa, where proteoglycan synthesis rates doubled. Exposure to 2.5 MPa also stimulated synthesis in the nucleus pulposus of human disks taken at surgery, whereas 7.5 MPa inhibited synthesis in five out of six specimens. With 2-h continuous exposure to the same levels of pressure, no stimulation was seen in the nucleus of bovine disks, and significant stimulation was only observed at 5.0 MPa in the inner annulus. Exposure to 10 MPa for either 20 s or 2 h inhibited proteoglycan synthesis in these regions of the disks. In contrast, in the outer annulus, where loading does not lead to a rise in hydrostatic pressure in vivo, there was no significant response to hydrostatic pressure over the range of 1-10 MPa in bovine or human disks.

Animals↗
PubMed1996

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.

Adult↗
PubMed1996

'Stress' distributions inside intervertebral discs. The effects of age and degeneration.

We investigated the distribution of compressive 'stress' within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting 'stress profiles' were analysed in order to characterise the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years was examined. Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the 'functional nucleus') by approximately 50%, and the pressure within this region fell by 30%. The width of the functional annulus increased by 80% and the height of compressive 'stress peaks' within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions. We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High 'stress' concentrations within the annulus may cause pain, and lead to further disruption.

Adolescent↗
PubMed1995

An analytical model of intervertebral disc mechanics.

The intervertebral disc is a complex mechanical structure, and it is important to understand the loading of specific structures which might cause damage leading to failure or mechanical impairment. At present it is only possible to model such internal loadings owing to the extreme technical difficulties involved in experimental measurement. The simple analytical model described in this paper makes exact predictions of the loads carried by fibres and also their path within the annulus fibrosus, without pre-defining the fibre configuration. The disc is modelled as an axially symmetric structure comprising a fluid filled centre, retained by a thin, doubly curved, fibre-reinforced membrane under tensile stress. The annulus is taken to consist of two lamellae reinforced by oppositely oriented collagen fibres that are free to follow paths defined by one of two geometrical rules. The predictive power and possible uses of the model are illustrated using boundary conditions experimentally determined from a typical young disc. The model was used to calculate the shape of the membrane surface, fibre path, volume of disc, area of annulus, length of fibre bundle and tension at a point along length of fibre. Equatorial fibre angle could be approximately predicted (to about 5 degrees), since there was only a small range of valid solutions to the model. The predicted surface profiles, fibre loads and angles were found to be in reasonable agreement with published experimental studies. Two examples of how the static model might be used to calculate changes in disc morphology and loading are included to demonstrate how a wide range of experimental data and theoretical behavior might be incorporated. This analytical model is important since it enables exact solutions to be calculated for the forces acting at any point along a fibre, their paths and also the surface geometry, from a small number of physical measurements without the need to estimate the mechanical properties of individual areas of the disc. It facilitates the prediction of the behaviour of the disc under varying load by providing a framework that can be further developed using a wide range and combination of experimental conditions and theoretical relationships.

Adult↗
PubMed1993

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.

Cadaver↗