Internet Edition

Issue No. 4, October 1998


Effects of Carbon Dioxide laser and organic matrix on enamel demineralization.
by C.S. Hsu* and J. Wefel**
(* Department of Preventive Dentistry, National University of Singapore
** Dows Institute for Dental Research, College of Dentistry, University of Iowa, Iowa City, Iowa, USA)

This research won the 1998 IADR/Colgate Research Award in prevention and Dr Hsu was awarded the 1998 Staff Achievement Award from NUS

Picture features Dr. Hsu operating the polarized light microscope, a powerful tool for qualitative assessment of treatment effects on enamel structures. Using microradiographs, the mineral loss can be quantified. The graphs illustrate mineral changes in four enamel sections of one of the tooth samples, which recieved four different treatments.

In the past few decades, accumulated evidence has clearly demonstrated inhibitory effects of laser irradiation on enamel demineralization. However, the exact mechanisms of this inhibitory effect of lasers on enamel remain unclear. The purpose of this study was to explore the potential roles of carbon dioxide laser, and organic matrix (OM) on demineralization of human enamel. Twenty fourhuman molars were collected, cleaned, and cut into two halves. One (NOM) half of each tooth was randomly selected and the lipid and protien content of it were extracted. The other (OM) half of each tooth was used as the matched control. Each tooth half was painted with acid resistant varnish except for two window areas. All the left windows were assigned to recieve a laser treatmentand the right windows served as the matched controls of the left windows. The carbon dioxide laser irradiation had an energy density of 0.334 J/cm2, a repetition rate of 20 Hz, a pulse width of 5 msec, and a duration of one second each time for four times. Following all the treatments, each tooth half was submitted to a pH cycling environment for a 4-day period to create a caries-like lesion. Micrographs of tooth sections were taken for quantification of mineral content in enamel. The mean mineral loss (with standard deviation) of the control, the lased, the 'NOM control', and the 'lased NOM' subgroups are 3955 (1911), 52 (49), 4565 (1311), and 1191 (940), respectively. The ANOVA analysis showed significant effects of laser irradiation (p=0.0001), organic matrix (p=0.0125), and the laser-OM interaction (p=0.0377). The laser irradiation caused more than 98% reduction in mineral loss, and this percent reduction dropped to about 70% when organic matrix in enamel was removed. In the polarized light microscopy examination, the birefringence change of enamel was very little after the laser irradiation. Based on the results of previous studies (Sato, 1983, Caries Res; Palamera et. al., 1987 JDR), it is suggested that the laser-induced temperature rise in enamel was likely to be less than 4000 degrees celsius. In the SEM examination, there is very little or no enamel melting that can be visualized after laser irradiation. it is thus indicated that,in this study, the enamel melting is not a major mechanism involved in the laser-induced reduction in enamel demineralization. The results in this study also suggested that a clinically applicable carbon dioxide laser irradiation can induce an almost complete inhibition of enamel demineralization. and these laser effects were likely due to both the purification of enamel hydroxyapatite and the organic plugging of the diffusion pathways in enamel.

Above shows the carbonated organic matrix, "cooked" by laser, partially blocking the periprismatic space, which constitutes the highway system and controls the ion diffusion and caries formation in enamel.

There are two points in this study worthy of mention. Firstly, it showed that melting of enamel is not a main mechanism involved in the caries-protective effect of laser. The enamel melting has been one of the most frequently cited mechanisms in the past few decades and believed by many at the present time. Instead, we proposed a new theory, the "organic plugging" theory. Secondly, this study obtained a 98% caries protective effect, greater than 85%, the current world record set by Featherstone's team in the University of San Francisco.

The functionally graded post project
Prof Chew Chong Lin, Dr Loh Poey Ling, Dr Ganesh VK, Dr Ramakrishna S, A/Prof Teoh SH

One of the most exciting breakthroughs in the history of research for the Faculty is the development of a patent on the project entitled "Fibre-Reinforced Composite Product with graded stiffness", which is jointly carried out with the Institute of Materials Research and Engineering (IMRE). The research project resulted in the development of a fibre reinforced composite material for dental applications. The patent was filed in June 1998 for the technology which allows a variation in the braid angle of the fibre reinforcement in the composite material. Potential applications would include restoring non-vital teeth which require a flexible dental post and in orthodontics.

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