Biomedical subjects

Joon Hock Yeo

Publications and source records attributed to Joon Hock Yeo.

6 recordsLinked to original sources

PubMed2006

Forces at single point attached commissures (SPAC) in pericardial aortic valve prosthesis.

OBJECTIVE: New pericardial aortic bioprostheses (3F Therapeutics and temporarily stented autologous pericardial valve prosthesis) were developed recently. These valves are designed with commissures connected to the aortic wall at only three single points (single point attached commissures (SPAC)). The aim of this study was to investigate the forces acting on SPAC during varying pressure load. METHODS: Aortic roots with diameters 19, 25, and 29 mm were made using silicone polymer. A bovine pericardial SPAC aortic valve prosthesis was constructed using a 3D-mold and was implanted in the silicone aortic root. The base of the valve was sutured onto the aortic annulus with 4-0 polypropylene running suture and each commissure was sutured to a miniaturized force transducer with only one 3-0 polypropylene U-stitch. Three silicon aortic roots of each size were pressurized up to 200 mmHg and forces on SPAC were measured. RESULTS: All valves remained competent at a pressure of 200 mmHg. Recordings showed a linear correlation between applied pressure and forces measured at SPAC. At a pressure of 80 mmHg (equivalent to diastolic pressure), the forces were 0.44+/-0.22N, 1.15+/-0.18N, and 2.00+/-0.35N in annular diameters 19 mm, 25 mm, and 29 mm, respectively. It was observed, that the main forces were acting along the axial direction and not along the radial direction. CONCLUSIONS: Forces on "single point attached commissures" in pericardial aortic valves were measured. These forces were acting mainly in axial direction and not in radial direction. This knowledge is important for the implantation technique of SPAC pericardial aortic valves.

Animals↗

PubMed2005

Three-dimensional asymmetrical modeling of the mitral valve: a finite element study with dynamic boundaries.

BACKGROUND AND AIM OF THE STUDY: Previous computational studies of the normal mitral valve have been limited because they assumed symmetrical modeling and artificial boundary conditions. The study aim was to model the mitral valve complex asymmetrically with three-dimensional (3-D) dynamic boundaries obtained from in-vivo experimental data. METHODS: Distance tracings between ultrasound crystals placed in the sheep mitral valve were converted into 3-D coordinates to reconstruct an initial asymmetric mitral model and subsequent dynamic boundary conditions. The non-linear, real-time left ventricular and aortic pressure loads were acquired synchronously. A quasi-static solution was applied over one cardiac cycle. RESULTS: The mitral valve leaflet stress was heterogeneous. The trigones experienced highest stresses, while the mid-anterior annulus between trigones experienced low stress. High leaflet stress was observed during peak pressure loading. During isovolumic relaxation, the leaflets were highly stretched between the anterolateral trigone and the posteromedial commissure, resulting in a prominent secondary leaflet stress re-increment. This has not been observed previously, as symmetric models with artificial boundary conditions were studied only in the ejection phase. CONCLUSION: Here, the first asymmetrical mitral valve model synchronized with 3-D dynamic boundaries and non-linear pressure loadings over the whole cardiac cycle based on in vivo experimental data is described. Despite its limitations, this model provides new insights into the distribution of leaflet stress in the mitral valve.

Animals↗

PubMed2004

Flat or curved pericardial aortic valve cusps: a finite element study.

BACKGROUND AND AIM OF THE STUDY: The finite element method (FEM) has frequently been used to investigate the behavior of the aortic valve, but studies on the performance and behavior of free-hand autologous pericardial aortic valves reconstructed using specially designed valve molds have not been performed. The study aim was to demonstrate the effectiveness of a three-dimensional (3-D) cusp of the authors' design (H-Mold) versus a two-dimensional (2-D) (flat) cusp using a FEM to compare stress distribution and leaflet contact properties. METHODS: Solid models of the aortic root and valve cusps were constructed using a computer-aided design package. All models had different free edge lengths and surface areas, but a constant leaflet attachment length corresponding to a 19 mm annulus diameter. A static pressure of 80 mmHg was applied to all models. RESULTS: The maximum von Mises stress value in the H-Mold at the cusp commissure was 34.5% lower than the stress value in the flat leaflet, while the contact area in the H-Mold leaflet was 85.7% greater than that of the flat leaflet. The length of leaflet free edge greatly influenced maximum von Mises stress intensity at the commissures, and the contact area between leaflets was mainly affected by the geometric shape of the leaflet and its surface area. CONCLUSION: 3-D leaflet geometry was found positively to influence leaflet stress distribution and coaptation. This geometry should have a significant impact on the reliability and long-term durability of pericardial aortic valve reconstruction.

Aortic Valve↗

PubMed2003

Comparison of human and porcine aortic valves.

We compared the anatomy of human and porcine aortic valves. Porcine hearts were collected from the abattoir. Human hearts from patients who had died of non-cardiac causes were examined in the mortuary; only undamaged and anatomically normal hearts were used. Silicon casts were prepared by injecting engineering silicon at 80 mm Hg into the aortic arch. Various features of the aortic valve were measured: circumference, length between the commissural end point and central point of coaptation, surface diameter, and surface area. In total, 12 porcine and 12 human aortic valves were studied. The average circumferences of the human and porcine aortic valves were 8.00 +/- 0.2 (SD) cm and 7.90 +/- 1.0 cm, respectively. The central point of coaptation in human valves was skewed toward the left coronary cusp, whereas in porcine valves it was skewed toward the non-coronary cusp. In human aortic valves, the non-coronary cusp had the largest surface diameter and surface area with mean measurements of 3.6 +/- 0.2 cm and 1.230 +/- 0.228 cm(2), respectively; the left coronary cusp was smallest for the same variables with measurements of 3.1 +/- 0.3 cm and 0.898 +/- 0.357 cm(2). In porcine valves, the right coronary cusp had the largest surface diameter and surface area with mean measurements of 3.9 +/- 0.7 cm and 1.716 +/- 0.81 cm(2), respectively; the non-coronary cusp was the smallest for the same variables with measurements of 2.9 +/- 0.5 cm and 1.023 +/- 0.659 cm(2). These differences suggest that when using porcine valves as transplant material (e.g., stentless valves), geometric considerations, such as commissural length, may be important.

Anatomy, Comparative↗

PubMedDate not supplied

Development of squeeze flow in mechanical heart valve: a particle image velocimetry investigation.

Fluid between the reducing flow channel of the valve occluder and the orifice wall tends to be squeezed out of the flow channel, causing a high-speed flow. The squeeze flow is accompanied by a sharp local pressure drop, which may result in potential cavitation phenomenon in a mechanical heart valve (MHV). Limited experimental investigation has been conducted into the flow physics of this squeeze flow phenomenon, which is likely to be the origin of MHV cavitation. We used a pulsatile test loop simulating physiologic flow conditions and an actual-size transparent MHV model for flow visualization. A digital particle image velocimetry (DPIV) system incorporated with a microscope was applied to observe flow within a narrowing channel. A triggering mechanism was designed so that the DPIV system could be timed to capture images when the valve occluder was near its closing position. A series of images within the channel from 1.4 to 0.1 mm were captured. As the gap between the tip of the valve occluder and orifice wall becomes narrower, evidence of high-speed jet flow becomes more apparent. When the flow channel is reduced to around 0.1 mm, flow velocity of up to 2 m/s was noted. A sudden increase in high-speed jet flow causes a corresponding reduction in local pressure, and is a likely source for potential cavitation.

Blood Flow Velocity↗

PubMedDate not supplied

Blood cell counting and classification by nonflowing laser light scattering method.

We present a nonflowing laser light scattering method for automatically counting and classifying blood cells. A linear charge-coupled device (CCD) and a silicon photoelectric cell (which is placed behind a pinhole plate on the CCD) form a double-detector structure: the CCD is used to detect the scattered light intensity distribution of the blood cells and the silicon photoelectric cell to complete the focusing process. An isotropic sphere, with relative refractivity near 1, is used to model the blood cell. Mie theory is used to describe the scattering of white blood cells and platelets, and anomalous diffraction, red blood cells. To obtain the size distribution of blood cells from their scattered light intensity distribution, the nonnegative constraint least-squares (NNLS) method combined with the Powell method and the precision punishment method are used. Both numerical simulation and experimental results are presented. This method can be used not only to measure the mean and the distribution of red blood cell size, but also to divide the white blood cells into three classes: lymphocytes, middle-sized cells, and neutrocytes. The experimental results show a linear relationship between the blood cell (both white and red blood cells) concentration and the scattered light intensity, and therefore, the number of blood cells in a unit volume can be determined from this relationship.

Adult↗