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Biomedical subjects

D W Hukins

Publications and source records attributed to D W Hukins.

At least 19 recordsLinked to original sources
PubMed2001

The effect of partial removal of the nucleus pulposus from the intervertebral disc on the response of the human annulus fibrosus to compression.

OBJECTIVE: To determine how partial removal of the nucleus changes the response of the annulus to compression. DESIGN: The deformation of the annulus in the mid-sagittal plane, during compression, was determined from digital video images. BACKGROUND: Several studies have shown that removal of the nucleus changes the external behaviour of the intervertebral disc, but few studies have investigated changes to internal behaviour. METHODS: Six frozen human lumbar discs were bisected in the sagittal plane to produce 12 specimens. The cut surfaces were marked with seven dots of Alcian blue stain. The specimens were sealed, enabling their internal structure to be viewed directly by a digital video recording system, and thawed. The video system recorded the response of each specimen as it was compressed by up to 1.8 mm at a rate of 0.2 mm s(-1). The displacements of the Alcian blue marks were measured using an image analysis program. Magnetic resonance imaging was used to investigate the validity of this technique. RESULTS: Partial removal of the nucleus changed the way that the disc deformed under compression. A highly significant change in direction of movement was seen in the inner posterior region of the annulus. CONCLUSIONS: Partial removal of the nucleus changes the response of the annulus to compression. RELEVANCE: Partial denucleation of the human intervertebral disc is shown to change the direction of bulging of the inner annulus when the disc is compressed. Increases in shear stress, arising from these changes, may lead to further disc degeneration in the form of circumferential tears.

Biomechanical Phenomena↗
PubMed2001

Replacing the nucleus pulposus of the intervertebral disc.

OBJECTIVE: To determine whether replacement of the nucleus with a synthetic material would prevent the effects of nucleus removal. DESIGN: Laboratory experiments on excised tissues and a finite element model. BACKGROUND: Removal of the nucleus from the intervertebral disc causes the inner margins of the annulus to bulge inwards, instead of outwards, during compression. This may cause the annulus to degenerate further. METHODS: Video recordings of sheep discs, sectioned in the sagittal plane, were obtained during compression in a materials testing machine; the cut face of the disc was sealed with a Perspex window. Experiments were repeated with the nucleus removed and then replaced by a synthetic implant. A finite element model of an intact disc was also used to investigate the effect of nucleus replacement. RESULTS: When the nucleus of sectioned discs was replaced with the polymer materials, the inward bulging of the annulus was not observed. The predictions from the finite element model of the intact disc were consistent with this result. CONCLUSIONS: Replacement of the nucleus with a synthetic material can prevent the changes in annulus behaviour that result from removal of the nucleus. RELEVANCE: A suitable implant to replace the nucleus after surgical removal may help prevent inward bulging of the inner layers of the annulus.

Animals↗
PubMed2001

Slippage of a spinous process hook during flexion in a flexible fixation system for the lumbar spine.

This study was undertaken to measure the amount of slippage of a spinous process hook (that forms part of a flexible fixation system) during flexion. Human cadaveric lumbar spines (10) were fitted with the device. A rig was designed to apply flexural displacements to a spine using a materials testing machine. Spherical markers were attached to the spine and hook. As a spine was flexed a digital video camera was used to record the positions of the markers. The movements of the markers were measured using interactive computer software to assess any slippage of the spinous process hook. During flexion the overall mean hook slippage was measured to be 0.10mm (standard deviation 0.04mm). The mean hook slippage, for each of the 10 specimens, was in the range of 0.05-0.14mm. The results imply that slippage of a spinous process hook during flexion is small.

Adult↗
PubMed2001

Viscoelastic properties of the nucleus pulposus of the intervertebral disk in compression.

The viscoelastic properties of the nucleus pulposus were measured in compression for 35 specimens dissected from 9 sheep. Measurements on 19 specimens were made on the day of slaughter; the remaining 16 specimens were stored frozen and thawed before testing. A preload of 0.2 N was applied to each specimen and a cyclic compression of 10 microm applied at eight frequencies in the range 0.1-10 Hz at a temperature of 37 degrees C. Freezing appeared to increase the storage modulus, E ', but not the loss modulus, E ", or tan delta=E "/E '. These parameters, E ', E " and tan delta, had values of 64+/-28 kPa, 24+/-11 and 0.33 kPa+/-0.07, respectively. The value of tan delta passed through a minimum at a loading frequency of 0.9+/-0.2 Hz. The water content of the specimens was 80+/-2%.

Journal Article↗
PubMed2000

Spinous process strength.

STUDY DESIGN: Mechanical testing of cadaveric lumbar spines and dual energy radiograph absorptiometry scanning were performed. OBJECTIVES: To devise a technique to measure the strength of lumbar spinous processes and to determine the bone mineral density of the vertebrae used. SUMMARY OF BACKGROUND DATA: The spinous process has been identified as the weakest part of the anatomy to which a flexible fixation device can be attached. It was unknown if the spinous processes could withstand the forces applied by the device. METHODS: A hook was fitted to the spinous process of 32 lumbar vertebrae. A custom-built rig was designed to secure a vertebra to a materials testing machine. A loop of cord was passed over a bar mounted on the crosshead of the machine and around the two bollards of the hook. As the crosshead was raised, a tension was applied to the cord. Each vertebra was tested to failure. The bone mineral density of each vertebra was then measured using dual energy radiograph absorptiometry. RESULTS: Failure of the specimens occurred by failure of the spinous process, pedicles, or vertebral body. The logarithm (base 10) of the load (N) at which failure occurred was 2.53 +/- 0.3, which corresponded to a mean failure load of 339 N. The bone mineral density of each vertebral body varied between 0.263 and 0.997 g/cm2. A significant linear correlation was found between bone strength and bone mineral density (P < 0.0001). CONCLUSIONS: Specimens with a bone mineral density in the range of 0.263-0.997 g/cm2 failed at a mean load of 339 N when the load was applied through the spinous process hook of a flexible fixation device.

Absorptiometry, Photon↗
PubMed2000

Effect of removing the nucleus pulposus on the deformation of the annulus fibrosus during compression of the intervertebral disc.

Eighteen frozen ovine discs were bisected, in the mid-sagittal plane, to produce 36 specimens. The cut surfaces were marked at the inner and outer annulus boundaries of the annulus fibrosus, both anteriorly and posteriorly, with Alcian blue stain. The sections were sealed by a transparent plate, and thawed. A compression of 1mm at a rate of 0.2mms(-1) was applied. The displacements of the Alcian blue marks were measured from the video images, recorded during the tests, using interactive image analysis software. Before removal of the nucleus, the inner boundaries of the annulus moved outwards during compression (P<0.001, anterior; P=0.01, posterior). However, after removal of the nucleus, both inner boundaries moved inwards (P<0.001, anterior and posterior). The outer boundaries moved outwards both before and after removal of the nucleus (P<0.001). The results showed that total removal of the nucleus changes the response of the annulus to compression.

Animals↗
PubMed2000

Design of an occipito-cervical fixation device.

Metal plates may be used to stabilize the cervical spine. The plates are attached to the posterior of the vertebra by placing screws into the lateral masses. The plating may be extended, in the form of rod or plate, to connect with and support the occiput. Several problems, such as screw loosening and the plate obscuring the surgeon's view as a screw is being inserted, have been identified with present plate systems. This paper describes the initial design for a cervical fixation device to overcome these problems, and the design and development that was undertaken to enable a prototype device to be manufactured.

Bone Plates↗
PubMed2000

Design of spinous process hooks for flexible fixation of the lumbar spine.

A prototype flexible fixation system for the lumbar spine was subjected to tensile testing to failure and cyclic tensile testing in order to determine any regions of weakness. The system consisted of a spinous process hook and two laminar hooks made of stainless steel (316L). Each laminar hook was attached to the spinous process hook by a loop of polyester braid secured by a crimped metal sleeve. In five tensile tests, the system failed by irreversible deformation of the spinous process hook at 2.5 +/- 0.3 kN (mean +/- standard deviation). In three cyclic tests, in which the applied tension varied sinusoidally between 0.04 and 0.4 kN at a frequency of 5 Hz, failure occurred after less than 400,000 loading cycles. This occurred as a result of fatigue crack initiation and propagation in the spinous process hook. A finite element model showed a stress concentration in the region where the crack occurred, which raised the applied stress above the tensile fatigue strength of this stainless steel. The spinous process hook was redesigned for manufacture in a titanium alloy (Ti-6AI-4V ELI) to minimize artefacts in magnetic resonance imaging. Further finite element models showed no unacceptable stress concentrations.

Alloys↗
PubMed2000

Mechanical testing of a flexible fixation device for the lumbar spine.

The aim of this study was to test mechanically a new flexible fixation system for the lumbar spine. This device incorporates loops of polyester braid which are secured by a crimped titanium sleeve. In tensile tests, all loops failed, by slippage through the crimped sleeve, at 434 +/- 25 N (mean +/- standard deviation from five loops) for a single crimp and 415 +/- 15 N (from five loops) for two crimps. The intact system was then tested according to the ASTM standard. In a static test, all five specimens failed by slippage of the braid through the sleeve. Initial slippage occurred between 600 and 700 N, but the mean maximum load sustained was 1090 +/- 140 N. Dynamic tests were performed on ten constructs at a frequency of 5 Hz, under a range of loading conditions. The maximum load applied in any of the tests was 825 N. Two constructs did not complete the required 5 x 10(6) test cycles because of fracture of their spinous process hooks. However, other tests, under the same conditions, showed no signs of failure. Fracture occurred as a result of fretting damage from the recommended stainless steel roll pins.

Biomechanical Phenomena↗
PubMed2000

Development of surgical instruments for implanting a flexible fixation device for the lumbar spine.

A new flexible fixation device for the lumbar spine has been developed. This paper describes the development and evaluation of two surgical instruments required for implanting this device. Prototypes were designed, manufactured and then evaluated for use in surgery. Further evaluation was performed, if necessary, and the design finalized, in accordance with BS EN 12011. This process involved close collaboration between engineers and surgeons.

Equipment Design↗
PubMed1998

Measurement of lumbar spinal flexion-extension kinematics from lateral radiographs: simulation of the effects of out-of-plane movement and errors in reference point placement.

Flexion-extension of the spine is defined in the sagittal plane, which divides the body into right and left halves, and can be measured from lateral radiographs, including those obtained by videofluoroscopy. The effects of axial rotation and lateral bending, which lead to movement out of the sagittal plane, were investigated by generating a three-dimensional computer model of two adjacent vertebrae and projecting it on to the sagittal plane. The projected model was measured as if it were a radiograph, allowing the effects of out-of-plane movement and errors in reference point placement to be calculated. Reference points were defined as the corners of a quadrilateral which fully enclosed the two-dimensional image of each vertebral body, and their projected locations were randomly displaced by 0.5 mm in order to simulate the measurement process. A combination of lateral bending with 1 degree of flexion can lead to a root mean squared (RMS) variation in the calculated position of the instantaneous centre of rotation (ICR) of 292 mm, reducing to 5 mm for 10 degrees of flexion. When the reference point quadrilaterals were divided into two triangles, the RMS variation in the calculated position of the centroid of one of the triangles was only 0.42 mm under the same conditions, remaining constant over the same flexion range. Similar results were obtained when axial rotation was combined with flexion. The calculated centroid provides a robust reference point for kinematic calculations.

Biophysical Phenomena↗
PubMed1998

Hip joint prosthesis design: effect of stem introducers.

The stress levels in the femoral component of a total hip prosthesis (Corin Taper Fit, Corin Medical Ltd, Cirencester, Gloucestershire, UK) were calculated by finite element (FE) analysis. This prosthesis has two holes drilled in the shoulder to engage a stem introducer. There were no unacceptable stress levels around these holes. Instead the maximum stresses were around the periphery of the shaft of the stem, as has been observed for FE analyses of conventional designs. Three prostheses were also subjected to cyclic mechanical testing (peak load 2.3 kN) according to the appropriate British Standard. The holes were examined for cracks, before and after testing, by stereomicroscopy. All three specimens were able to withstand 5 million loading cycles with no evidence of damage. Thus it is possible to design a femoral component with holes in the shoulder, to accommodate a stem introducer, without creating unacceptable stress concentrations.

Computer Simulation↗
PubMed1998

Measurement of lumbar spine flexion-extension using a low-field open-magnet magnetic resonance scanner.

RATIONALE AND OBJECTIVES: The authors investigated the feasibility of using a low-field open-magnet magnetic resonance (MR) scanner to acquire functional flexion-extension images for range of motion (ROM) measurements on the lumbar spine. METHODS: Seventeen healthy subjects with no symptoms of back pain (age range, 22-59 years) were scanned in a low-field open-magnet MR scanner in the flexed, neutral, and extended positions. Each image was downloaded to a computer workstation for subsequent flexion-extension, lordosis, and ROM measurement. RESULTS: Data from two subjects were not analyzed because their images did not show all the lumbar vertebrae. For the remaining 15, there was a large variation in the magnitude of the ROM values (range, 9 degrees-70 degrees; mean 36.4 degrees, SD 16.5 degrees). However, there was a significant correlation between age and ROM (r = -0.63; P < 0.05). CONCLUSIONS: The low-field open-magnet MR scanner provides a method for noninvasive imaging of the lumbar spine, allowing the subject freedom of movement in the horizontal plane. This enables functional flexion-extension images of the lumbar spine to be acquired.

Adult↗
PubMed1998

Collagen orientation and molecular spacing during creep and stress-relaxation in soft connective tissues.

Collagen fibres form cross-helical, cross-ply or quasi-random feltworks in extensible connective tissues; strain-induced reorientation of these networks gives rise to the non-linear mechanical properties of connective tissue at finite strains. Such tissues are also generally viscoelastic (i.e. display time-dependent properties). The hypothesis that time-dependent reorientation of collagen fibres is responsible for the viscoelasticity of such tissues is examined here using time-resolved X-ray diffraction measurements during stress-relaxation and creep transients applied to rat skin and bovine intramuscular connective tissue. Differences in the intensity and angular orientation of the third and fifth orders of the 67 nm meridional D-spacing of collagen molecules were shown before and after the application of loads or displacements. However, no changes in the D-spacing or angular orientation of collagen occurred during the time course of either stress-relaxation or creep in both tissues. This indicates that collagen fibre reorientation is not a primary source of their viscoelastic properties. The non-linear (strain-dependent) nature of the stress-relaxation response in these tissues suggests that relaxation processes within the collagen fibres or at the fibre-matrix interface may be responsible for their viscoelastic nature.

Animals↗
PubMed1998

Comparison of the precision of two vertebral morphometry programs for the lunar EXPERT-XL imaging densitometer.

Comparative precision tests of the vertebral height measurement function of the Aberdeen Vertebral Morphometry System (AVMS) software and Lunar EXPERT-XL software were undertaken using four vertebrae from the same lateral spine dual energy X-ray absorptiometry (DXA) image of the same subject (male, 67 years). Two of the vertebrae were abnormal and two were normal. Three observers inexperienced in morphometry and one experienced observer took part in the study. Repeatability was obtained from 10 sequential measurements of the posterior, middle and anterior heights at the same sitting by the same observer. Intraobserver reproducibility compared the means of one set of measurements for all vertebrae with another taken 1 week later. Interobserver reproducibility compared the means of one set of measurements for all vertebrae from an experienced and an inexperienced observer, and from two inexperienced observers. The AVMS software had significantly higher (p < 0.05) repeatability (mean coefficient of variability, CV = 3.5%) than the Lunar software (mean CV = 5%), significantly higher (p < 0.01) intraobserver reproducibility (mean CV = 4.6%) than the Lunar software (mean CV = 8.5%), and significantly higher (p < 0.05) interobserver reproducibility (mean CV = 4.7%) than the Lunar software (mean CV = 7.5%). In conclusion, the new AVMS method possessed higher precision when measuring both abnormal and normal vertebrae and when used by both experienced and inexperienced observers.

Absorptiometry, Photon↗
PubMed1997

Effect of sodium chloride solutions on mineralized and unmineralized turkey leg tendon.

Previous studies showed that the equatorial diffraction spacing of the collagen molecules in mineralized tissues decreases when the tissue is dried and that the spacing in totally dried tissue is about the same (1.1 nm) whether mineralized or not. Here we report that spacing decreases were observed in both mineralized and unmineralized turkey leg tendon after soaking in various sodium chloride solutions up to 4.0 M concentration. The effect was seen by X-ray diffraction as well as by neutron diffraction. No effect was seen in turkey leg tendon soaked in 3.0 M ethylene glycol solution. The spacing in unmineralized tissue decreased from 1.459 +/- 0.011 nm in 0.15 M saline to 1.403 +/- 0.025 nm in 1.5 M saline, a change of 0.056 +/- 0.03 nm or 3.84%. In mineralized turkey leg tendon the corresponding spacings were 1.387 +/- 0.012 and 1.321 +/- 0.019 nm, a change of 0.046 +/- 0.02 nm or 3.4%. No significant dimensional change was noted in the thickness even though the equatorial diffraction spacing decreased by 3.4%. Electron microscopy showed the collagen fibrils within the mineralized turkey leg tendon to be surrounded by highly mineralized material. Presumably the composition of the extrafibrillar material is different from the intrafibrillar and therefore the extrafibrillar material is a different kind of composite. If it is assumed that the extrafibrillar material does not change dimensions significantly, then the collagen molecules in the fibrils can be mobile within the dimensionally stable cage-like structure.

Animals↗
PubMed1997

An automated method for assessing routine radiographs of patients with total hip replacements.

This paper describes a new, fully automated method of locating objects on radiographs of patients with total joint replacements (TJRs). A statistical computer model, known as an active shape model, was trained to identify the position of the femur, pelvis, stem and cup marker wire on radiographs of patients with Charnley total hip prostheses. Once trained, the model was able to locate these objects through a process of automatic image searching, despite their appearance depending on the orientation and anatomy of the patient. Experiments were carried out to test the accuracy with which the model was able to fit to previously unseen data and with which reference points could be calculated from the model points. The model was able to locate the femur and stem with a mean error of approximately 0.8 mm and a 95 per cent confidence limit of 1.7 mm. Once the model had successfully located these objects, the midpoint of the stem head could be calculated with a mean error of approximately 0.2 mm. Although the model has been trained on Charnley total hip replacements, the method is generic and so can be applied to radiographs of patients with any TJR. This paper shows that computer models can form the basis of a quick, automatic method of taking measurements from standard clinical radiographs.

Computer Simulation↗