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Biomedical subjects

P L Loh

Publications and source records attributed to P L Loh.

6 recordsLinked to original sources
PubMed2004

Influence of dietary simulating solvents on the hardness of provisional restorative materials.

OBJECTIVE: To evaluate the influence of dietary simulating solvents on surface hardness of chemical, light and dual-cure provisional restorative materials. METHODS: The provisional materials evaluated were Temporary Bridge Resin (TBR) (Dentsply), Protemp Garant (PG) (ESPE-3M), Luxatemp Automix (LA) (DMG), Unifast LC (UF) (GC), Luxatemp Solar Plus (LS) (DMG) and Provipoint DC (PP) (Vivadent). Materials were manipulated according to manufacturers' instructions and placed in customized molds. Immediately after setting/light-polymerization, the materials were stored in the following dietary simulating solvents at 37 degrees C for 1 week: (a) heptane, (b) 100% ethanol solution (ES), (c) 75% ES, (d) 50% ES, (e) 25% ES, and (f) distilled water. Specimens stored in air were used as control. After the one-week conditioning period, hardness testing (KHN; n = 6) was carried out with a digital microhardness tester (load=100 gf; dwell time=15 s). Hardness ratios were plotted against the solubility parameter of the solvents and the Environmental Index was subsequently computed. Data were subjected to ANOVA/Scheffe's test at significance level 0.05. RESULTS: For all materials, conditioning in the various ES resulted in significantly lower KHN values compared to conditioning in air and water. TBR, UF and PP were also significantly softened by conditioning in heptane. Ranking of Environmental Index was as follows: LS>LA>PG>TBR>UF>PP. SIGNIFICANCE: All provisional materials were significantly softened by aqueous ethanol solutions. Materials based on bis-acryl resins were generally more resistant to damage from dietary solvents.

Composite Resins↗
PubMed2003

Fabrication of a new composite orthodontic archwire and validation by a bridging micromechanics model.

A new technique based on tube shrinkage is proposed for the fabrication of composite archwires. Compared with a traditional pultrusion method, this new technique can avoid any fiber damage during the fabrication and can provide the archwire with a required curvature in its final clinical usage. The present paper focuses on the technique development and mechanical design and validation in terms of constituent materials by using a micromechanics bridging model. Prototype archwire has been fabricated using fiberglass and an epoxy matrix, with a wire diameter of 0.5mm and a 45% fiber volume fraction. Tensile and three-point bending tests have shown that the mechanical performance of the prototype composite archwire is comparable to that of a clinical Ni-Ti archwire. Another purpose of the present paper is to provide an efficient procedure for a critical design of composite archwires. For this to be possible, the ultimate load especially flexural load carrying ability of the composite archwire must be assessed from the knowledge of its constituent properties. However, difficulty exists in doing this, which comes from the fact that the failure of the utmost filament of the composite archwire subjected to initially the maximum bending stress does not imply its ultimate failure. Additional higher loads can still be applied and a progressive failure process is generated. In this paper, the circular archwire was discretized into a number of parallel laminae along its axis direction, and the bridging micromechanics model combined with the classical lamination theory has been applied to understand the progressive failure process with reasonable accuracy. Only the constituent fiber and matrix properties are required for this understanding. Nevertheless, the ultimate bending strength cannot be obtained only based on a stress failure criterion. This is because neither the first-ply nor the last-ply failure corresponds to the ultimate failure. An additional critical deflection (curvature) condition must be employed also. By using both the stress failure and the critical deflection conditions, the predicted load-deflection up to the ultimate failure agrees well with the measured data. Thereafter, different mechanical performances of composite archwires can be tailored before fabrication by choosing suitable constituent materials, their contents, and the archwire diameters. Several design examples have been shown in the paper.

Computer Simulation↗
PubMed1996

An evaluation of microwave-polymerized resin bases for removable partial dentures.

PURPOSE: The hardness, porosity, and adaptation of removable partial dentures fabricated with one heat-polymerized denture base resin and two resins designed for microwave polymerization were evaluated. MATERIALS AND METHODS: Five prostheses were evaluated for each resin. Adaptation of the denture bases to the master cast was evaluated by spatial orientation and mean weight of residual impression material. The prostheses were than embedded in epoxy resin and sectioned for evaluation of resin hardness (Knoop hardness) and microporosity. RESULTS: There were no significant differences in the adaptation of the acrylic resin bases for Acron MC and Ch Lucitone. There was no significant difference in the mean Knoop hardness values for any of the resin bases near and away from the metal. None of the denture bases showed porosity greater than 100 microns. CONCLUSIONS: Both resin bases formulated for microwave polymerization were effectively polymerized around metal frameworks without adverse effects on resin hardness or porosity. Justi Denture Base material had poorer base adaptation than the other two resins.

Acrylic Resins↗
PubMed1995

Interocclusal distance in patients with different skeletal patterns.

A pilot study was conducted to measure the interocclusal distance in patients with class I, II and III skeletal patterns. Ten patients were randomly selected for each skeletal pattern group. The measurements of interocclusal distance were obtained from lateral cephalometric radiographs of each patient. The results indicated that there was a significant difference (p < 0.05) between the interocclusal space of skeletal class II and class III patients.

Adolescent↗
PubMedDate not supplied

Viability of magnetic denture retainers: a 3-year case report.

The purpose of this article is to report on the use of a neodymium-iron-boron magnet enclosed in a laser-welded stainless steel casing as a viable method of retention for removable partial dentures. This mode of retention dispenses with unsightly clasps, at the same time providing direct tooth support via the overdenture abutment. Magnetic attachments are simpler to incorporate into the prosthesis than mechanical attachments, which require springs, coplanar surfaces, or screws. Overdentures of this nature have the added advantage of being more stable and retentive than conventional partial dentures, and they are easily removed and seated without the patient having to grapple with clasps and complex paths of insertion. This particular magnetic attachment exhibited no deterioration after 3 years in function, and the results so far have been promising.

Aged↗