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Biomedical subjects

H F Chang

Publications and source records attributed to H F Chang.

30 records · Page 2Linked to original sources
PubMed1987

Effects of instantaneous prestretching on force degradation characteristics of orthodontic plastic modules.

The object of this study was to investigate the effect of instantaneous prestretching on the force degradation behavior of three kinds of four-link plastic modules. A simple sliding apparatus was used to prestretch instantaneously the plastic modules to 100, 200 and 300% of their original length. After prestretching, specimens were tested at 20, 25 and 30 mm stretch distances. Force measurements were performed at the following occasions: before prestretching; at the end of prestretching; after 1, 4 and 24 hours; and after 1, 2 and 3 weeks. The following conclusions can be drawn from this investigation: The control specimens which had not been instantaneously prestretched exhibited considerable force loss over the three-week test period. Most of the force loss occurred rapidly during the first few hours, with a much lower subsequent force degradation rate. Instantaneous prestretching to a greater length resulted in a lower force value remaining at the end of prestretching. Instantaneous prestretching of the plastic modules is an efficient method to dissipate the high initial force level which is clinically undesirable. Prestretching of the modules around or slightly longer than the stretch distance could reduce the high initial force level and still maintain the same force level as the control groups. However, if the length of instantaneous prestretching of the four-link plastic module was much longer than the testing stretch distance which corresponds to the average distance from the first molar to the canine in the same quadrant, the prestretching would reduce both the high initial force value and the force value during the three-week test period, as compared with the control group.

Humans↗
PubMed1987

The influence of dental occlusion on electromyographic silent periods.

Electromyographic silent periods (EMG SP) and occlusal contact intensity were studied in 31 young patients (9-18 years of age) having different types of malocclusion. EMG SP was induced from bilateral temporalis and masseter muscles by chin tapping during isometric contraction. The simple silent period (SSP), depressed activity (DP) and double silent period (2SP or 3SP) were classified according to the character of silence. The occlusal contact intensity was obtained by using a polymer wafer for clenching and was analysed with a photocclusion analyser. It was found that the average amplitude of temporalis and masseter muscles during clenching was about 400 microV. The mean SSP was around 34 msec. The total SP was around 40 msec in temporalis and 44 msec in masseter. All of the subjects demonstrated one or more DP in ten taps while 18 of them had 2SP or 3SP. The difference in contact intensity between right and left had a strong correlation with the EMG SSP+DP, SSP+2SP3SP and total SP. Anterior-posterior difference was not as strongly related to SP parameters. Less significant correlation was found between SSP and occlusal intensity. It is concluded that the influence of occlusion on EMG SP is basically related to the uneven occlusal contact between the two sides rather than the total occlusal contact intensity or the difference in intensity anterior-posteriorly.

Adolescent↗
PubMed1986

Acute and chronic effects of haloperidol on plasma and brain homovanillic acid in the rat.

The effects of acute and chronic administration of haloperidol on homovanillic acid (HVA) in plasma and the brain were examined in the rat. Acute haloperidol treatment (1 mg/kg) resulted in highly significant elevations in HVA within 30 min and produced a maximal increase of HVA in 3-6 hr in both plasma and the whole brain. The response of brain HVA to increasing doses (0.05-30 mg/kg) of haloperidol showed an inverted U pattern. Plasma HVA showed a very flat response to lower doses (less than or equal to 5 mg/kg) of haloperidol and a dramatically elevated one to higher doses (greater than or equal to 10 mg/kg). Haloperidol produced a parallel increase in plasma and brain HVA at lower doses (less than or equal to 2 mg/kg) only. After chronic administration of haloperidol for up to 28 days, the response of HVA in plasma correlated mainly with, but tolerated later than, those in the whole brain and the olfactory tubercle.

Animals↗
PubMed1986

The relationship of dental crowding to tooth size, dental arch width, and arch depth.

This study was undertaken to answer the question, "Are there any differences in the mesiodistal tooth diameters, dental arch width and dental arch depth between the crowded and noncrowded dentitions?" The materials submitted for study consisted of the Orthoscan intraoral occlusograms of 89 subjects with well-aligned dentition and those of 74 subjects with gross dental crowding. The tooth size and arch dimensions were measured from these intraoral occlusograms. The data were subjected to statistical treatment for comparison of noncrowded and crowded groups. The findings in this study lead to the following conclusions: The mesiodistal tooth diameters of the crowded group were significantly larger than those of the noncrowded group. The dental arch widths of the crowded group were significantly smaller than those of the noncrowded group. No substantial or significant difference in dental arch depth between the crowded and noncrowded subjects could be noted. From these findings it seems that tooth size and dental arch width are determining factors in the formation of dental crowding. Clinically, these results suggest that in the treatment of dental crowding, extraction of teeth and/or expansion of dental arches should be considered depending on what is the main causative factor.

Adolescent↗
PubMed1985

The effects of lead ion on immune function of rabbit alveolar macrophages: quantitation of immune phagocytosis and rosette formation by 51Cr in vitro.

Experiments by a 51Cr-labeling technique were performed to investigate the effects of lead (Pb2+) on immune phagocytosis and Fc-rosette formation of rabbit pulmonary alveolar macrophages (PAMs). Evidence is presented that Pb2+ at concentrations of 10(-3) and 10(-4)M could inhibit these functions of PAMs. The degree of inhibition corresponded to the concentration of this heavy metal ion in vitro.

Animals↗