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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
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↗
PubMed1996

Migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine.

STUDY DESIGN: Magnetic resonance images were obtained of the lumbar spines of three volunteers in neutral, flexed, and extended postures. OBJECTIVES: To measure migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine in living people. SUMMARY OF BACKGROUND DATA: Results of experiments on bisected cadaveric spines have indicated that the nucleus migrates posteriorly during flexion and anteriorly during extension in nondegenerate discs. Degenerate discs may have faults or fissures that result in abnormal motion of the nucleus. METHODS: Proton density weighted, sagittal, magnetic resonance images were obtained from the lumbar spines of three volunteers. Measurements of the positions of the anterior and posterior margins of the nucleus and of flexion and extension angles were made on tracings of the images corresponding to neutral, flexed, and extended postures. RESULTS: The observed frequency (22 of 24 measurements) at which the margins of the nucleus migrated in the directions predicted by results of cadaveric studies was significantly greater than the frequency that would be expected by chance (P < 0.001). The two exceptions may be a result of disc degeneration. There was a significant (P < 0.05) linear correlation between the migration of the anterior margin and the flexion-extension angle and a highly significant (P < 0.001) correlation for the posterior margin and the flexion-extension angle. CONCLUSIONS: Flexion of an intervertebral disc in a living person tends to be accompanied by posteriorly directed migration of the nucleus pulposus within the disc. Extension tends to be accompanied by an anteriorly directed migration.

Adolescent↗
PubMed1996

Euler buckling as a model for the curvature and flexion of the human lumbar spine.

The upright thoraco-lumbar spine resembles an Euler column buckled in the second mode (n = 2) when viewed in the sagittal plane. An advantage of n = 2 buckling is that further load can be carried without adopting a stooped posture. Flexion of the spine is considered as the first quarter cycle of an Euler pendulum. This is possible if the antagonistic muscles which control movement increase the bending stiffness, EI, to a value of about 15 N m2. If the muscles are incapable of increasing EI sufficiently to support the weight of the body, or any excess load, the spine will be dynamically unstable. This conclusion is consistent with a static model which considers spinal instability as 'loss of stiffness' and a dynamic model which suggests that it arises from ineffective adaptive control. The flexed spine resembles an n = 1 buckled column.

Humans↗
PubMed1996

Analysis of load-relaxation in compressed segments of lumbar spine.

Load-relaxation was measured in 12 segments of human cadaveric lumbar spine. Each segment consisted of an intact intervertebral disc attached to half of its adjacent vertebrae with the posterior elements removed. Six specimens were each compressed at six different strains (corresponding to initial loads of 0.5-2.5 kN) and, for each strain, the load-relaxation was measured for a period of 20 min at room temperature. These load-relaxation curves were used to plot three isochrones for each specimen. All isochrones were linear (r values in the range 0.95-0.99). This result indicated that a linear model could be used to represent load-relaxation. Four specimens were tested at a single strain (corresponding to an initial load of about 2 kN) at 37 degrees C for a period of 4-6 h. Load was plotted against the logarithm of time. The resulting plots did not show any peaks, indicating that relaxation effects did not predominate at any particular times during load-relaxation. However, it was possible to model the load-relaxation as a simple linear system which can be represented as two Maxwell elements in parallel. These elements were characterized by relaxation times of 16 +/- 8 min and 4.6 +/- 0.8 h. Fourier transformation of the load-relaxation curves showed a gradual increase in the storage modulus and a gradual decrease in the loss modulus for frequencies of about 1 Hz and above. At these frequencies, the spine cannot function as a shock-absorber in pure compression.

Aged↗
PubMed1996

Quantifying the kinematics of natural and prosthetic knee flexion.

A new method has been developed for quantifying knee kinematics during flexion. This method was used to measure knee kinematics from lateral radiographs taken at different angles of flexion with the two femoral condyles superimposed in each image, thus standardizing the plane of flexion-extension. When applied to the radiogaphs of five healthy male volunteers (age range 21-26 years), it showed that flexion was accompanied by translation between the articular surfaces. Knee kinematics were also measured in five patients after total knee replacement (TKR) surgery with a Kinemax Modular Total Knee prosthesis (Howmedica, Warsaw, Indiana). In the TKR patients, a translation was detected in three out of the five patients. This indicates that the prosthesis is capable of restoring normal kinematics, but does not always do so.

Adult↗
PubMed1995

Measurement of the time course of bending of the back in the sagittal plane.

A simple, inexpensive method has been developed for measuring the position of the back, projected onto the sagittal plane, during flexion. Light-emitting diodes (LEDs) were attached to the skin overlying the T6, L1, L3 and L5 vertebrae. They started to flash, at a controlled frequency, at the beginning of movement from an upright to a flexed position. The positions of the markers together with the initial (upright) and final (flexed) positions of the back were recorded on a single photograph. A plumb line provided a vertical reference. Times were determined from the flashing frequency. Vertical and horizontal components of the position were measured from the photograph; their resultant represented total displacement at a given instant in time. The results were scaled so that the displacements were expressed as a fraction of their maximum value, and time was expressed as a fraction of the total movement time. The reproducibility of the technique was tested by making repeated measurements from a single healthy volunteer. At any instant, the horizontal, vertical and resultant displacements of the T6 marker could be determined to within 7% of their maximum values; the resultant displacement of the L5 marker could be determined to within 10%. The results from the other markers were less reliable.

Adult↗
PubMed1994

Seeded growth of hydroxyapatite in the presence of dissolved albumin.

Hydroxyapatite (HAP) crystals were grown from a supersaturated solution by the addition of a suspension of seed crystals at a controlled pH value of 7.4 and a temperature of 37 degrees C. The degree of supersaturation was comparable to that in biological fluids and was such that all HAP precipitated would be expected to deposit on the seeds. Albumin was added to some of the solutions to give a concentration in the range 75-250 micrograms cm-3. Samples of solution were removed at known times after the addition of seed crystals and their calcium ion concentrations were determined by atomic absorption spectroscopy. The decrease in the dissolved calcium concentration was taken to be a measure of crystal growth. In the absence of seeds, no decrease in calcium concentration occurred. The initial rate of HAP growth decreased linearly with albumin concentration, i.e., albumin was found to inhibit crystal growth. Inhibition kinetics were consistent with a Langmuir model in which a single albumin molecule was capable of binding to more than one growth site on the crystal surface. Comparison with published results indicated that albumin was a less potent inhibitor of HAP growth than phosphoproteins but was a more potent inhibitor than magnesium or citrate ions.

Body Fluids↗
PubMed1994

Seeded growth of hydroxyapatite in the presence of dissolved albumin at constant composition.

Hydroxyapatite (HAP) crystals were grown from a supersaturated solution by the addition of seed crystals at a temperature of 37 degrees C. Throughout the experiment, the pH and calcium ion concentration were monitored by electrodes in the solution, and maintained at constant values by the addition of ammonia solution (maintaining a pH value of 7.4 during HAP precipitation) and calcium nitrate solution. Triammonium orthophosphate solution was added at the same time as calcium nitrate, so that the ratio of calcium to phosphate ions in the solution was appropriate to the precipitation of HAP. The volume of calcium nitrate solution which needed to be added was used as a measure of HAP crystal growth. Albumin was added to some of the supersaturated solutions to give a concentration in the range 50-250 micrograms cm-3. HAP crystals grown at different albumin concentrations were examined by scanning electron microscopy. After an initial period of 30 min, the growth of HAP crystals increased linearly with time. This behavior was attributed to initial growth healing such dislocations as steps and kinks; growth at screw dislocations was considered to occur throughout the period of the experiment and to give rise to the linear growth phase. The linear growth rate decreased linearly with the albumin concentration. HAP crystals grown in the absence of albumin formed spherical aggregates; the size of these aggregates decreased with the albumin concentration.

Calcium↗
PubMed1993

End-plate displacement during compression of lumbar vertebra-disc-vertebra segments and the mechanism of failure.

Seventeen specimens of lumbar discs, attached to the caudal and cranial halves of the adjacent vertebral bodies, were subjected to a maximum compressive load of 5.5 kN in six stages. The time between each stage was about 15 seconds. At each stage of compression, a radiograph of the specimen was recorded, and the bulging of the end-plate into the caudal vertebra was measured using a displacement transducer. After compression, the ash content of a bone sample and the water content of a sample of the nucleus of the disc were measured for each specimen. Sections through the specimens were examined by light microscopic study. Eight specimens did not fail, although end-plate displacement occurred during compression. The remaining nine specimens experienced fracture or permanent deformation of the end-plate. Specimens that failed had significantly lower rigidity of the end-plate and underlying trabecular bone; this rigidity was correlated with bone ash content.

Adult↗
PubMed1993

A method for the kinematic evaluation of the knee following anterior cruciate ligament injury and reconstruction.

A quantitative method for assessing the kinematics of the knee in the sagittal plane has been developed in order to evaluate the role of the anterior cruciate ligament following injury and reconstruction. Measurements were made on a series of lateral radiographs obtained at different angles of flexion with the limb weight-bearing and the foot and ankle rotated so that the condyles of the femur overlapped. The kinematics of the joint were then defined by recording the path of the tip of the medial tibial spine as flexion proceeded, using a coordinate system based on the femur. This method overcomes the problems inherent in quantifying knee kinematics by using the pathway of the centre of rotation. In an amputated knee, tibial positions could be specified to within approximately 1.2 mm. There were no significant differences between results obtained at the beginning and end of a six month period for the normal knees of two patients; the standard deviation of the measured tibial positions was approximately 1.6 mm.

Anterior Cruciate Ligament↗
PubMed1993

Acidification of urine is not a feasible method for preventing encrustation of indwelling urinary catheters.

Encrusting deposits form on urinary catheters as a result of infection by bacteria which produce urease. Urease catalyses the hydrolysis of urea to produce ammonia, with the result that the pH of the urine increases and the deposits are precipitated. It has been suggested that acidification of urine may prevent this rise in pH and so prevent encrustation. Experiments were performed in vitro to determine whether acidification could be used to prevent the rise in pH when urease is present in urine. It was found that, in the presence of urease, the initial decrease in pH resulting from acidification was countered by further urea being converted into ammonia. Thus, although acidic washout solutions may dissolve encrusting deposits, acidification of urine is not a feasible method for preventing catheter encrustation.

Acid-Base Equilibrium↗
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↗