Biomedical subjects

S H Teoh

Publications and source records attributed to S H Teoh.

39 records · Page 3Linked to original sources

PubMedDate not supplied

Effects of cyclic temperature changes on hardness of composite restoratives.

The clinical durability of some composite restorative materials may be significantly affected by cyclic temperature changes. This study investigated the effects of cyclic temperature changes on surface hardness of four commercial composite resins (Silux, Z100, Ariston and Surefil). Eighteen specimens of each material were divided into three treatment groups comprising a control and two different thermal cycling regimes. Control specimens were stored in distilled water at 35 degrees C for 178 hours. Thermal cycled specimens were stored in distilled water at 35 degrees C for 173 hours and subjected to five hours (300 cycles) of a thermal cycling regime consisting of the cycle ABAC, where A and B represent the fixed temperatures of 35 degrees C (28 seconds) and 15 degrees C (two seconds) and C, depending on the treatment group, either 45 degrees C or 60 degrees C (two seconds). All specimens were subsequently subjected to hardness testing (KHN) using a digital microhardness tester (load = 500 gf; dwell time = 15 seconds). Results were analyzed using ANOVA/Scheffe's test (p<0.05). The effect of thermal cycling on hardness was material-dependent. While thermal cycling significantly increased the surface hardness of Z100 and Surefil, it significantly decreased the hardness of Ariston. The hardness of Silux was not significantly affected by cyclic temperature changes. For all treatment groups, Z100 was significantly harder than the other composite resins evaluated and Surefil was significantly harder than Silux and Ariston. For both thermal cycled groups, Silux was significantly harder than Ariston.

Analysis of Variance↗

PubMedDate not supplied

Delrin as an occluder material.

Delrin (DR) has been used in biomedical applications for more than 25 years. Because of durability concerns, it was replaced by the expensive Pyrolytic Carbon (PC) in numerous cardiac valves. However, the durability problem could be related to design rather than poor materials selection. Recent reports on brittle fracture of PC, leading to sudden deaths, have prompted a critical comparison between DR and PC in the St. Vincents Mechanical (SVM) heart valves. Three SVM-DR and SVM-PC valves were subjected to accelerated life cycle tests, and examined for wear at 400 million cycles. These results were compared to those of Björk-Shiley Delrin (BS-DR) valves. Wear in BS-DR valves in vivo for more than 17 years were also analyzed and compared. Using a linear (wear depth)-log (cycles) plot, wear rates in mm/log (million cycles) were obtained. The results showed that the wear rates for DR and PC in SVM valves are close. The double reduction in wear rate of the SVM-DR, compared to BS-DR, is probably due to the lower contact stresses of the SVM valves. SVM-DR in vivo should, therefore, have lower wear. The PC discs also showed edge chipping and hairline cracks. The authors conclude that the durability of DR can be improved by design and, since it is more impact resistant than PC, it is a safer, more inexpensive occluder material for cardiac valves.

Biomechanical Phenomena↗

PubMedDate not supplied

In vitro strain measurement of a mechanical heart valve in a pulse simulator.

To ascertain the stress magnitude at the stress concentration areas, in vitro strain measurements on a St. Vincent's mechanical heart valve were carried out in a pulse simulator. Results were combined with a finite element (FEM) stress analysis of the titanium valve housing. The maximum stress at the site of stress concentration of the titanium valve housing was 51 MPa. This is well below the fatigue endurance limit of titanium. The imposed stress on the occluder by the upper strut was less than 2 MPa. This is below the lower stress limit of Delrin and may explain why no fracture of the Delrin disk occluder has been reported. The combined use of microstrain analysis and FEM proved to be essential in the determination of dynamic stresses during the opening and closing of the mechanical heart valve.

Biomechanical Phenomena↗