Biomedical subjects

D W Hukins

Publications and source records attributed to D W Hukins.

At least 19 recordsLinked to original sources

PubMed1992

A simple model for the function of proteoglycans and collagen in the response to compression of the intervertebral disc.

The nucleus pulposus of the intervertebral disc exerts a pressure which enables it to support axial compression when contained by the annulus fibrosus. The disc was modelled as a thick-walled cylindrical pressure vessel in which the nucleus was contained radially by the annulus. As a result, the stress in the annulus had radial (compressive) as well as tangential (tensile) components. The radial stress at a given point in the annulus was considered to be balanced by the internal pressure which is expected to arise from the attraction of water by proteoglycans. There was a reasonable agreement between the calculated radial stress distribution and published results on the distribution of water within the annulus. As the internal pressure is expected to be isotropic, the annulus was expected to contribute to the axial resistance to compression of the disc; this contribution would be equal, in magnitude, to the radial stress. Predictions of the pressure distribution within the annulus were similar to published experimental measurements made in the radial and axial directions. The tangential stress within the annulus was considered to arise from the restoring stress in its strained collagen fibrils.

Biomechanical Phenomena↗

PubMed1992

Bending and fracture of the femoral component in cemented total hip replacement.

A computer method was used to make 41 measurements on the geometry of insertion of the femoral component in 200 Charnley total hip replacements. Surgery had been performed at least 12 years before, giving results which were classified as: success (90); fracture (56); or loose (54), according to rigid selection criteria. Fracture was associated with heavier patients in which there was poor proximal fixation of the femoral component but adequate distal fixation. Stems with a medial disposition proximally were more common in the fracture group than in the successful or loose groups. Sequential measurements of bending and subsequent fracture were made on the follow-up radiographs of 24 of the 200 cases (6 fracture and 18 successful). These measurements allowed bending to be detected at an earlier stage than by simple inspection of the radiographs.

Aged↗

PubMed1992

Aseptic loosening of the femoral component in cemented total hip replacement.

The purpose of this study was to identify factors which predispose to aseptic loosening of the femoral component in cemented total hip replacement. Its design was based on rigid selection criteria, so that successful and loose replacements which employed the same surgical technique were compared. Measurements of patient anatomy and of the insertion of the femoral component were made, by an accurate computer technique, on initial post-operative radiographs. Loosening was associated with heavier patients with a wider medullary canal which was flared proximally. This difference in anatomy led to differing distributions of cement in the successful and loose replacements. Medial cement-bone demarcation, at the mid-stem level, was also associated with loosening. These findings indicate the importance of optimizing the size of the prosthesis with respect to the femoral morphology.

Bone Cements↗

PubMed1992

Measurement of pH to quantify urease activity.

The rate of change of pH caused by the hydrolysis of urea was measured for urease solutions of 18 different concentrations. Concentrations were converted into an activity, A (measured in IU/cm3), by using a titrimetric method to assay the urease sample. For activities in the range 0.6-38 IU/cm3, A was related to the initial rate of change of pH, (dpH/dt)0, (measured in s-1) by the empirical relationship: A = 549(dpH/dt)0-1423(dpH/dt)2(0). Values of (dpH/dt)0 were sensitive to changes in the urease activity of about 0.6 IU/cm3.

Hydrogen-Ion Concentration↗

PubMed1992

X-ray diffraction by collagen in the fully mineralized cortical bone of cow tibia.

X-ray diffraction patterns were recorded from fully mineralized and demineralized sections of the cortical bone from cow tibia. The results were used to obtain a measure, d, of the mean side-to-side spacing (equatorial diffraction spacing) of the molecules in the type I collagen fibrils of bone. Mineralized bone yielded a d value of 1.23 nm; for the demineralized tissue this value increased to 1.52 nm. Thus it appears that mineralization of bone compacts its collagen molecules within the fibrils.

Animals↗

PubMed1992

Bone composition measured by x-ray scattering.

Ten composite samples consisting of cortical bone and adipose tissue, in known proportions, were made. The intensity of monochromatic x-rays (energy 8 keV) scattered by these samples was determined as a function of the modulus of the scattering vector, K. The ratio of the heights of peaks at K values of around 134 and 22 nm-1 provided a measure of the ratio of adipose tissue to bone mineral in these samples. This method was then used to determine the ratio of adipose tissue to mineral in samples of trabecular bone from 16 vertebral bodies. The results were correlated with measurements of the bone composition determined by ashing (r = 0.66) and histomorphometry (r = 0.66). Furthermore, the ashing and histomorphometry results were correlated with each other (r = 0.68). The feasibility of using higher energy x-rays (35-80 keV) for obtaining the same information from bone within the body is briefly discussed.

Adipose Tissue↗

PubMed1991

Measurement of hip prostheses using image analysis. The maxima hip technique.

A computer-based image analysis system has been developed as a research tool in total hip replacement. The system has been programmed to take multiple measurements from coronal plane radiographs. Poor quality radiographic images can be enhanced and standardised. The measurements which can be obtained include stem subsidence, cup migration, cup wear, and stem loosening. Reproducibility and accuracy were +/- 0.01 mm and +/- 0.5 mm respectively. The present application is in retrospective research, but prospective monitoring of radiographs is planned.

Hip Joint↗

PubMed1990

Mechanism of failure of the ligamentum flavum of the spine during in vitro tensile tests.

Thirty ligamenta flava were dissected from pig lumbar spines with their attachments to the bone of the laminae still intact. These specimens were tested to failure, in tension, at strain rates of 0.03 and 0.2 s-1. Video recordings were made of the specimens during testing. Specimens failed because the ligaments became detached from the bone. The maximum stress that the ligaments were able to transmit was approximately 3 MPa. However, the corresponding strain was higher when testing at the slower strain rate than at the faster rate; these strains were 1.6 +/- 0.5 and 1.0 +/- 0.3, respectively. All specimens were able to transmit stress after the maximum value had been attained because threads of ligament continued to remain attached to bone. Complete failure did not occur until the last of these threads had pulled away. Scanning electron micrographs showed no ligament, other than a few isolated pieces of fibre, adhering to bone after failure had occurred.

Animals↗

PubMed1990

Animal models for osteoarthritis--ensuring experimental validity.

Confounding factors, bias factors and hidden variables affect the design of experiments involving animal models. In a frequently used dog model for osteoarthritis these can arise, for example, because of the influence of age, sex and breed of dog. Controls are required to investigate the progression of osteoarthritis even in experiments forming time series. However, there are no true controls in animal models for joint degeneration. Analysis of data collected from experiments involving animal models depends on the number of factors being varied. The number of animals influences the level of confidence associated with a result. Ignoring statistics can invalidate conclusions drawn from the experiments. Statistical considerations are also important in the presentation of results.

Animals↗

PubMed1990

Comparison of structure, mechanical properties, and functions of lumbar spinal ligaments.

The organization of collagen in the supraspinous, interspinous, and longitudinal ligaments, as well as the ligamenta flava, in lumbar spines from human cadavers has been investigated by polarized light microscopy, scanning electron microscopy, and x-ray diffraction. These experiments were performed on ligaments in situ, with their bony attachments undisturbed, and on excised ligaments at a range of applied strains. Results were related to the composition (investigated by standard histologic techniques) and gross structures (investigated by light microscopy) of the ligaments. More importantly, the results were related to the mechanical properties of the ligaments, which include stiffness, failure conditions, stress relaxation, and hysteresis. Where necessary, results were supplemented by or compared with those obtained from pig ligaments. Mechanical properties were related to postural changes by ligament strains induced in cadaveric specimens, using results from the literature. Thus, ligament structures could be related to their physiologic functions.

Biomechanical Phenomena↗

PubMed1989

Infection of catheterised patients: bacterial colonisation of encrusted Foley catheters shown by scanning electron microscopy.

The surfaces of 32 encrusted urinary catheters were examined by scanning electron microscopy to investigate the association of bacteria with the encrusting deposits. Deposits consisted of struvite crystals surrounded by aggregates of very small crystallites of hydroxyapatite. Underneath these minerals there was a layer of closely packed bacteria. Impressions of bacteria were also observed in hydroxyapatite. Crystals were often engulfed by the bacterial layer, which thus appeared to bind the crystals to each other and to the catheter surface. This thick layer of bacteria associated with crystals may protect both the bacteria from antibiotics and the crystals from acidic bladder washout solutions intended to dissolve them. Furthermore, the existence of this sessile population explains why urease-producing bacteria are not invariably detected in the urine of patients with encrusted catheters. The observation of this bacterial layer (or "biofilm") by scanning electron microscopy provided direct evidence for infection being implicated in catheter encrustation.

Bacterial Adhesion↗

PubMed1989

Resistance of catheters coated with a modified hydrogel to encrustation during an in vitro test.

Mid-shaft specimens were cut from latex catheters coated with a modified hydrogel, latex catheters coated with silicone elastomer, and 100% silicone catheters. These specimens were subjected to controlled in vitro encrustation conditions. During a test period of 11 weeks, there was no significant difference in the quantities of encrusting deposits formed on the three materials.

Catheters, Indwelling↗

PubMed1989

Structure and mechanical properties of the longitudinal ligaments and ligamentum flavum of the spine.

Stress-strain curves were recorded from anterior and posterior longitudinal ligaments and ligamenta flava dissected from pig lumbar spines. Ligaments were examined during extension by light microscopy, to observe crimp structure, and by X-ray diffraction, to determine collagen fibril orientations. Scanning electron microscopy (SEM) was used to examine ligaments fixed at high and low strains. Initial stages of ligament extension involved alignment of collagen fibrils. Collagen fibrils in unstrained ligamentum flavum were much more disoriented than in the longitudinal ligaments. Thus, fibril alignment, and consequent stiffening, occurred at much higher strains than for longitudinal ligaments, allowing ligamentum flavum to exploit the extensibility of its elastin.

Animals↗

PubMed1989

Prosthetic hip failure: retrospective radiograph image analysis of the acetabular cup.

A method has been developed for quantifying movement and wear of the acetabular component (cup) of total hip replacements (THR) from routine postoperative and review radiographs. The method uses both interactive and automatic computer image analysis techniques. Dimensions of the prosthesis are used to scale the measurements and so overcome variation in radiographic alignment. The application of the method is illustrated by retrospective investigations of cup migration and wear using review radiographs taken over a follow-up of at least 12 years.

Acetabulum↗

PubMed1989

Calcium environment in encrusting deposits from urinary catheters investigated by interpretation of EXAFS spectra.

Extended x-ray absorption fine structure (EXAFS) spectra were recorded, above the K absorption edge of Ca, from 10 encrusted urinary (Foley) catheters obtained from 10 different patients. The presence of poorly crystalline apatite was demonstrated in all the deposits, even though this phase could not be detected in the x-ray powder diffraction patterns of most samples because of the obscuring effect of struvite, which was also present. Furthermore, the EXAFS spectra could be interpreted to yield quantitative information on the average short-range structure surrounding the Ca2+ ions in the poorly crystalline apatite.

Calcium↗

PubMed1989

Morphology of mineral deposits on encrusted urinary catheters investigated by scanning electron microscopy.

Struvite and hydroxyapatite were precipitated from artificial urine onto the surfaces of catheter materials by the controlled addition of urease. They were precipitated both together and separately (by omitting components of the artificial urine), and with and without the inclusion of albumin (which was intended to mimic the proteinaceous debris found in infected urine). Precipitates were identified by X-ray powder diffraction and the artificially encrusted surfaces examined by scanning electron microscopy. In the presence of protein, hydroxyapatite was precipitated as a poorly crystalline form which aggregated to form a crust. Struvite crystals could be easily identified under the scanning electron microscope by their relatively large size and characteristic appearance. Fifteen encrusted catheters from patients were also examined by scanning electron microscopy, and a further six using X-ray microanalysis. Their appearance was very similar to that of the materials encrusted in vitro. Encrustation involves the formation of hydroxyapatite and the growth of struvite crystals, intimately associated with bacteria.

Durapatite↗

PubMed1989

Stress in collagen fibrils of articular cartilage calculated from their measured orientations.

Articular cartilage may be considered as a form of pressure vessel in which the internal swelling pressure is balanced by tensile stress in the collagen fibrils. This stress is calculated by analysing the tissue as a series of microscopically small pressure vessels. The previously measured orientations of the collagen fibrils describe the structure necessary for this calculation. The stresses and strains developed in the fibrils are shown to be well within physiological limits.

Animals↗

PubMed1989

Resistance of conformable indwelling urinary catheters to encrustation.

Conformable and conventional indwelling urinary catheters, made of latex coated with Teflon and of the same nominal size, were encrusted in vitro. The extent of encrustation was quantified by dissolving the deposits, determining the concentrations of calcium and magnesium in the resulting solutions (by atomic adsorption spectroscopy), and calculating the surface densities of calcium and magnesium. Surface densities of both elements were significantly less on the conformable than on the conventional catheters. This result can be explained by the pulsed flow of urine through the conformable catheter tending to dislodge deposits adhering to its inner surface. A similar action is expected to occur in vivo.

Calcium↗