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Biomedical subjects

David W L Hukins

Publications and source records attributed to David W L Hukins.

8 recordsLinked to original sources
PubMed2006

Cluster analysis as a method for determining size ranges for spinal implants: disc lumbar replacement prosthesis dimensions from magnetic resonance images.

STUDY DESIGN: Statistical analysis of clinical radiologic data. OBJECTIVE: To develop an objective method for finding the number of sizes for a lumbar disc replacement. SUMMARY OF BACKGROUND DATA: Cluster analysis is a well-established technique for sorting observations into clusters so that the "similarity level" is maximal if they belong to the same cluster and minimal otherwise. METHODS: Magnetic resonance scans from 69 patients, with no abnormal discs, yielded 206 sagittal and transverse images of 206 discs (levels L3-L4-L5-S1). Anteroposterior and lateral dimensions were measured from vertebral margins on transverse images; disc heights were measured from sagittal images. Hierarchical cluster analysis was performed to determine the number of clusters followed by nonhierarchical (K-means) cluster analysis. Discriminant analysis was used to determine how well the clusters could be used to classify an observation. RESULTS: The most successful method of clustering the data involved the following parameters: anteroposterior dimension; lateral dimension (both were the mean of results from the superior and inferior margins of a vertebral body, measured on transverse images); and maximum disc height (from a midsagittal image). These were grouped into 7 clusters so that a discriminant analysis was capable of correctly classifying 97.1% of the observations. The mean and standard deviations for the parameter values in each cluster were determined. CONCLUSIONS: Cluster analysis has been successfully used to find the dimensions of the minimum number of prosthesis sizes required to replace L3-L4 to L5-S1 discs; the range of sizes would enable them to be used at higher lumbar levels in some patients.

Adult↗
PubMed2006

Stress transfer in collagen fibrils reinforcing connective tissues: effects of collagen fibril slenderness and relative stiffness.

Unlike engineering fibre composite materials which comprise of fibres that are uniform cylindrical in shape, collagen fibrils reinforcing the proteoglycan-rich (PG) gel in the extra-cellular matrices (ECMs) of connective tissues are taper-ended (paraboloidal in shape). In an earlier paper we have discussed how taper of a fibril leads to an axial stress up-take which differs from that of a uniform cylindrical fibre and implications for fibril fracture. The present paper focuses on the influence of fibre aspect ratio, q (slenderness), and Young's modulus (stiffness), relative to that of the gel phase, E(R), on the magnitude of the axial tensile stresses generated within a fibril and wider implications on failure at tissue level. Fibre composite models were evaluated using finite element (FE) and mathematical analyses. When the applied force is low, there is elastic stress transfer between the PG gel and a fibril. FE modelling shows that the stress in a fibril increases with E(R) and q. At higher applied forces, there is plastic stress transfer. Mathematical modelling predicts that the stress in a fibril increases linearly with q. For small q values, fibrils may be regarded as fillers with little ability to provide tensile reinforcement. Large q values lead to high stress in a fibril. Such high stresses are beneficial provided they do not exceed the fracture stress of collagen. Modulus difference regulates the strain energy release density, u, for interfacial rupture; large E(R) not only leads to high stress in a fibril but also insures against interfacial rupture by raising the value of u.

Animals↗
PubMed2006

Mitral valve repair: an in-vitro comparison of the effect of surgical repair on the pressure required to cause mitral valve regurgitation.

BACKGROUND AND AIM OF THE STUDY: The study aim was to compare mitral valve repair techniques in vitro. Rupture or elongation of the mitral valve chordae tendineae is a known cause of mitral regurgitation, and can be corrected by edge-to-edge repair, chordal replacement, or chordal transposition. METHODS: A test apparatus was used to apply pressure to porcine mitral valves. Mitral valve specimens were tested intact (n = 50), after they had been experimentally damaged, and after repair. Each test was repeated ten times. Experimental damage consisted of severing either the anterior leaflet strut, and attached marginal chordae (n = 30) or posterior leaflet chordae (n = 20). Valves with damaged anterior leaflets were repaired by either: (i) edge-to-edge repair; (ii) chordal replacement; or (iii) chordal transposition. Valves with damaged posterior leaflets were repaired by the first two techniques. Each repair method was repeated on ten specimens. RESULTS: Mitral valves repaired using the edge-to-edge repair (p = 0.002) and chordal replacement (p = 0.038), after rupture to anterior leaflet chordae, recovered significantly better than specimens repaired by chordal transposition. There was no statistical difference in recovery between edge-to-edge repair and chordal replacement (p > 0.05). There was no statistical difference (p > 0.05) in the recovery of the pressure withstood by valves repaired by edge-to-edge repair and chordal replacement, after rupture of posterior leaflet chordae. CONCLUSION: These results showed that edge-to-edge repair and chordal replacement are well suited for the repair of both the anterior and posterior leaflets.

Animals↗
PubMed2005

The role of Chordae tendineae in mitral valve competence.

BACKGROUND AND AIM OF THE STUDY: The study aim was to understand the role of different mitral valve chordae tendineae, and how damage to them affects valve competence. METHODS: A test apparatus was used to apply pressure to porcine mitral heart valves that were intact and have had selected chords severed. Anterior leaflet strut and marginal chords were selectively severed, as were posterior leaflet basal and marginal chords. Commissural chords were also severed. RESULTS: Severing anterior leaflet marginal chords (p = 0.018) and commissural chords (p = 0.018) significantly reduced mitral valve competence. Severing posterior leaflet marginal and basal chords, and anterior leaflet strut chords, had no significant effect in reducing the pressures that the valves could withstand. Severing a mixture of posterior leaflet basal and marginal chords significantly reduced the pressure withstood by the valves (p = 0.004). CONCLUSION: The study results confirmed that anterior leaflet marginal chords, but not strut chords, are vital for valve competence. Commissural chords were also shown to be vital for mitral valve competence. Several posterior leaflet chords had to be severed to affect mitral valve competence.

Animals↗
PubMed2003

Preliminary evaluation of the Sternum Screw: a novel method for improved sternal closure to prevent dehiscence.

Sternal dehiscence is a relatively rare but serious complication of sternal closure with an unacceptable mortality and morbidity rate. The 6 mm cannulated Sternum Screws are a novel approach that aims to prevent dehiscence. The screws are placed on either side of the sternotomy and wire threaded through the cannula and closed customarily. The Sternum Screws make the bone non-penetrable at the same time retaining the familiarity of conventional stainless steel wire. This novel technique was tested in sheep sterna to compare its efficacy to conventional wire closure. Using tensile testing in a testing machine, randomised controlled closures of the Sternum Screws and No. 5 stainless steel wires were evaluated until system failure. Seventeen matched pairs were tested. The Sternum Screw closure was on average 36% stronger, 284+/-43 N (mean+/-SD) compared to conventional wire closure alone, 215+/-38 N (mean+/-SD) [p<0.0001 by t-test]. System failure in 82% of Sternum Screw closures, however, was due to wire breakage or untwisting rather than the screw itself. In all these cases the screw remained intact in bone. 71% of conventional wire closures failed by dehiscing through the sterna. The mean forces required for wire dehiscence and wire failure in the Sternum Screws may be achieved in vivo during large coughs. The study shows there is merit in further evaluating the approach as a method of preventing dehiscence. It also highlights the use of alternative wiring techniques increase the tensile strength of the closure yet retain the familiarity and versatility of conventional wire.

Animals↗
PubMed2003

Stochastic finite element analysis of biological systems: comparison of a simple intervertebral disc model with experimental results.

Statistical methods allow the effects of uncertainty to be incorporated into finite element models. This has potential benefits for the analysis of biological systems where natural variability can give rise to substantial uncertainty in both material and geometrical properties. In this study, a simple model of the intervertebral disc under compression was created and analysed as both a deterministic and a stochastic system. Factorial analysis was used to determine the important parameters to be included in the stochastic analysis. The predictions from the model were compared to experimental results from 21 sheep discs. The size and shape of the distribution of the axial deformations predicted by the model was consistent with the experimental results given that the number of model solutions far exceeded the number of experimental results. Stochastic models could be valuable in determining the range and most likely value of stress in a tissue or implant.

Animals↗
PubMed2002

Sternum screw: analysis of a novel approach to the closure of the chest after surgery.

BACKGROUND: To show the benefits of using a novel approach to closure of the median sternotomy through a mechanical model and mechanical testing. Simple cannulated screws are placed on either side of the sternotomy. Conventional stainless steel wire is passed through the cannula of each screw and the sternotomy is closed in the usual manner. METHODS: Hertzian contact analysis was used to estimate the stress between the wire and the sternum. Mechanical testing was used to compare using wire on its own with a sternum screw plus wire. Ten samples of balsa wood (sternum substitute) had wire placed through a hole in them, while a further ten samples were fitted with a cannulated screw and had wire passed through the screw cannula. The wire was connected to a materials testing machine, which applied tension to the wire until the wire or screw cut through the wood. RESULTS: The analysis showed that the mean stress between the wire and the sternum decreases with increasing wire diameter. At low diameters of wire the stress in the sternum can be comparable to the failure stress of bone. Using a cannulated screw reduces the stresses in the sternum. The mechanical testing showed that the wire cut through the wood at a mean load of 104 N, whereas the sternum screw cut through the wood at a mean load of 209 N (p = 0.007, Mann-Whitney Test). CONCLUSIONS: Closing a median sternotomy with cannulated screws plus wire should reduce the occurrence of sternal dehiscence.

Biomechanical Phenomena↗