Daniel M. Espino

Investigation of techniques for the measurement of articular cartilage surface roughness.

Articular cartilage is the bearing surface of synovial joints and plays a crucial role in the tribology to enable low friction joint movement. A detailed understanding of the surface roughness of articular cartilage is important to understand how natural joints behave and the parameters required for future joint replacement materials. Bovine articular cartilage on bone samples was prepared and the surface roughness was measured using scanning electron microscopy stereoscopic imaging at magnifications in the range 500× to 2000×. The surface roughness (two-dimensional, R(a), and three-dimensional, S(a)) of each sample was then measured using atomic force microscopy (AFM). For stereoscopic imaging the surface roughness was found to linearly increase with increasing magnification. At a magnification of 500× the mean surface roughness, R(a), was in the range 165.4±5.2 nm to 174±39.3 nm; total surface roughness S(a) was in the range 183-261 nm. The surface roughness measurements made using AFM showed R(a) in the range 82.6±4.6 nm to 114.4±44.9 nm and S(a) in the range 86-136 nm. Values obtained using SEM stereo imaging were always larger than those obtained using AFM. Stereoscopic imaging can be used to investigate the surface roughness of articular cartilage. The variations seen between measurement techniques show that when making comparisons between the surface roughness of articular cartilage it is important that the same technique is used.

MECHANICAL PROPERTIES OF CHORDAE TENDINEAE OF THE MITRAL HEART VALVE: YOUNG'S MODULUS, STRUCTURAL STIFFNESS, AND EFFECTS OF AGING

Youngs modulus and structural stiffness were determined for chordae tendineae of the mitral valve from young (18–26 weeks) and old (over 2 years) porcine hearts. For chordae from the posterior leaflet of the valve, the Youngs modulus values were significantly higher (p < 0.05) for the thinner marginal chordae (59 ± 31 MPa young; 88 ± 21 MPa old) than for the thicker basal chordae (31 ± 4 MPa young; 28 ± 9 MPa old). Marginal chordae (both anterior and posterior) had significantly higher (p < 0.05) value for their Youngs modulus in old (88 ± 21 MPa anterior and posterior) than in young (62 ± 17 MPa anterior, 59 ± 18 MPa posterior) pig hearts. There was no significant difference in structural stiffness between marginal and basal (anterior and posterior leaflets) or between strut chordae (that are associated with anterior the leaflet only) and marginal and basal chordae. However, the value of structural stiffness of chordae was significantly higher (p < 0.05) for old (2.2 ± 0.2 kN/m) than for young (2.0 ± 0.4 kN/m) chordae. These results show that aging affects the properties of chordae and that all chordae need to be included in finite element models of valve function.

Articular cartilage surface rupture during compression: investigating the effects of tissue hydration in relation to matrix health.

This study aimed at investigating articular cartilage rupture by investigating the response of healthy and degenerate cartilage through altering the osmotic swelling environment of surface-intact, cartilage-on-bone specimens. The osmotic environment in healthy and degenerate bovine cartilage was varied by soaking tissues in either distilled water or 1.5 M NaCl saline to render the tissues into a swollen or dehydrated state (respectively). Creep compression was applied using an 8 mm flat-ended polished indenter that contained a central pore of 450 μm diameter, providing a consistent region for rupture to occur across all specimens. In the first set of experiments, surface rupture of healthy and degenerate specimens required similar levels of nominal compressive stress (8 MPa) when dehydrated than when swollen (7 MPa). In the second set of experiments, the time required for surface rupture to occur (for healthy and degenerate specimens) occurred over similar loading times (p>0.05). However, the time required for surface rupture for the swollen specimens occurred over a significantly longer time (approximately one order of magnitude) than that required for the dehydrated specimens (p<0.05). The compressive strains that were measured at rupture in the dehydrated degenerate specimens were significantly lower than those measured in the dehydrated healthy tissues (p<0.05). Rupture in dehydrated degenerate cartilage suggested a weakened articular surface, and it also suggested that dehydrated cartilage may undergo failure due to stress concentrations as it is unable to redistribute stress away from the site of loading.

Development of a transient large strain contact method for biological heart valve simulations

A new 2D method to implement transient contact using Comsol Multiphysics (finite element analysis software that enables multiphysics simulations) is described, which is based on Hertzian contact. This method is compared to the existing (default) contact method that does not enable real transient simulations, but instead performs steady-state solutions where time is a constant. The two types of contact modelling have been applied to simple 2D biological heart valve models, undergoing strains in the region of 10% under 20 kPa pressure (applied over 0.3 s). Both the methods predicted comparable stress patterns, locations of peak stresses, deformations and directions of principal stress. The default contact method predicted slightly greater contact stresses, but spreads over a shorter surface length than the new contact method. The default contact method is useful for contact systems with little transient dependency, due to ease of use. However, where transient conditions are important the new contact method is preferred.

Articular cartilage surface failure: An investigation of the rupture rate and morphology in relation to tissue health and hydration:

This study investigates the rupture rate and morphology of articular cartilage by altering the bathing environments of healthy and degenerate bovine cartilage. Soaking tissues in either distilled water or 1.5 M NaCl saline was performed in order to render the tissues into a swollen or dehydrated state, respectively. Creep compression was applied using an 8 mm flat-ended polished indenter that contained a central pore of 450 µm in diameter, providing a consistent region for rupture to occur across all 105 tested specimens. Rupture rates were determined by varying the nominal compressive stress and the loading time. Similar rupture rates were observed with the swollen healthy and degenerate specimens, loaded with either 6 or 7 MPa of nominal compressive stress over 11 and 13 min. The observed rupture rates for the dehydrated specimens loaded with 7 MPa over 60 and 90 s were 20% versus 40% and 20% versus 60% for healthy and degenerate tissues, respectively. At 8 MPa of nominal compressive stress over 60 and 90 s the observed rupture rates were 20% versus 60% and 40% versus 80% for healthy and degenerate tissues, respectively; with all dehydrated degenerate tissues exhibiting a greater tendency to rupture (Barnard’s exact test, p < 0.05). Rupture morphologies were only different in the swollen degenerate tissues (p < 0.05). The mechanisms by which dehydration and swelling induce initial surface rupture of mildly degenerate articular cartilage differ. Dehydration increases the likelihood that the surface will rupture, however, swelling alters the observed rupture morphology.

Viscoelastic properties of a spinal posterior dynamic stabilisation device.

The purpose of this study was to quantify the frequency dependent viscoelastic properties of two types of spinal posterior dynamic stabilisation devices. In air at 37°C, the viscoelastic properties of six BDyn 1 level, six BDyn 2 level posterior dynamic stabilisation devices (S14 Implants, Pessac, France) and its elastomeric components (polycarbonate urethane and silicone) were measured using Dynamic Mechanical Analysis. The viscoelastic properties were measured over the frequency range 0.01-30Hz. The BDyn devices and its components were viscoelastic throughout the frequency range tested. The mean storage stiffness and mean loss stiffness of the BDyn 1 level device, BDyn 2 level device, silicone component and polycarbonate urethane component all presented a logarithmic relationship with respect to frequency. The storage stiffness of the BDyn 1 level device ranged from 95.56N/mm to 119.29N/mm, while the BDyn 2 level storage stiffness ranged from 39.41N/mm to 42.82N/mm. BDyn 1 level device and BDyn 2 level device loss stiffness ranged from 10.72N/mm to 23.42N/mm and 4.26N/mm to 9.57N/mm, respectively. No resonant frequencies were recorded for the devices or its components. The elastic property of BDyn 1 level device is influenced by the PCU and silicone components, in the physiological frequency range. The viscoelastic properties calculated in this study may be compared to spinal devices and spinal structures.

Effect of the variation of loading frequency on surface failure of bovine articular cartilage.

BACKGROUND Mechanical loading of synovial joints can damage the articular cartilage surface and may lead to osteoarthritis. It is unknown if, independent of load, frequency alone can cause failure in cartilage. This study investigated the variation of articular cartilage surface damage under frequencies associated with normal, above normal and traumatic loading frequencies. METHOD Cartilage on bone, obtained from bovine shoulder joints, was tested. Damage was created on the cartilage surface through an indenter being sinusoidally loaded against it at loading frequencies of 1, 10 and 100 Hz (i.e., relevant to normal, above normal and up to rapid heel-strike rise times, respectively). The frequencies were applied with a maximum load in the range 60-160 N. Surface cracks were marked with India ink, photographed and their length measured using image analysis software. RESULTS Surface damage increased significantly (P < 0.0001) with frequency throughout all load ranges investigated. The dependence of crack length, c, on frequency, f, could be represented by, c=A(log10(f))2+B(log10(f))+Dc=A(log10(f))2+B(log10(f))+D where A = 0.006 ± 0.23, B = 0.62 ± 0.23 and D = 0.38 ± 0.51 mm (mean ± standard deviation). CONCLUSION The increase in crack length with loading frequency indicated that, increased loading frequency can result in cartilage becoming damaged. The results of this study have implications in the early stages of osteoarthritis.

Variation in viscoelastic properties of bovine articular cartilage below, up to and above healthy gait-relevant loading frequencies:

The aim of this study was to determine the variation in viscoelastic properties of femoral head bovine articular cartilage, on-bone, over five orders of magnitude of loading frequency. These frequencies ranged from below, up to and above healthy gait-relevant frequencies, using<1, 1–5 and 10 Hz, respectively. Dynamic mechanical analysis was used to measure storage and loss stiffness. A maximum compressive force of 36 N was applied through a chamfered-end, 5.2-mm-diameter, indenter. This induced a maximum nominal stress of 1.7 MPa. The ratio of storage to loss stiffness increased from near parity (2.5) at low frequencies to 11.4 at 10 Hz. This was the result of a significant logarithmic increase (p < 0.05) in storage stiffness with frequency, from 367 N/mm (0.001 Hz) up to 1460 N/mm (10 Hz). In contrast, the loss stiffness remained approximately constant. In conclusion, viscoelastic properties of articular cartilage measured at frequencies below those of gait activities are poor predictors of its relevant dynamic mechanical behaviour.

Effect of exercise on blood flow through the aortic valve: a combined clinical and numerical study

The aim of this study was to measure the cardiac output and stroke volume for a healthy subject by coupling an echocardiogram Doppler (echo-Doppler) method with a fluid–structure interaction (FSI) simulation at rest and during exercise. Blood flow through aortic valve was measured by Doppler flow echocardiography. Aortic valve geometry was calculated by echocardiographic imaging. An FSI simulation was performed, using an arbitrary Lagrangian–Eulerian mesh. Boundary conditions were defined by pressure loads on ventricular and aortic sides. Pressure loads applied brachial pressures with (stage 1) and without (stage 2) differences between brachial, central and left ventricular pressures. FSI results for cardiac output were 15.4% lower than Doppler results for stage 1 (r = 0.999). This difference increased to 22.3% for stage 2. FSI results for stroke volume were undervalued by 15.3% when compared to Doppler results at stage 1 and 26.2% at stage 2 (r = 0.94). The predicted mean backflow of blood was 4.6%. Our results show that numerical methods can be combined with clinical measurements to provide good estimates of patient-specific cardiac output and stroke volume at different heart rates.

Viscoelastic properties of human bladder tumours

The urinary bladder is an organ which facilitates the storage and release of urine. The bladder can develop tumours and bladder cancer is a common malignancy throughout the world. There is a consensus that there are differences in the mechanical properties of normal and malignant tissues. However, the viscoelastic properties of human bladder tumours at the macro-scale have not been previously studied. This study investigated the viscoelastic properties of ten bladder tumours, which were tested using dynamic mechanical analysis at frequencies up to 30Hz. The storage modulus ranged between 0.052MPa and 0.085MPa while the loss modulus ranged between 0.019MPa and 0.043MPa. Both storage and loss moduli showed frequency dependent behaviour and the storage modulus was higher than the loss modulus for every frequency tested. Viscoelastic properties may be useful for the development of surgical trainers, surgical devices, computational models and diagnostic equipment.