[Show abstract][Hide abstract]ABSTRACT: The purpose was to evaluate the intramuscular reperfusion response characteristics associated with repeated isometric contractions in normal human masseter. Intramuscular blood volume was quantified with a near-infrared spectroscopic device that measured the total haemoglobin (Hb) concentration in the muscle. Electromyographic (EMG) activity from the masseter and total bite forces were also recorded. Sixteen healthy volunteers, eight females and eight males, without masticatory muscle pain participated. They were asked first to clench their teeth for as long as possible at 50% of their maximum voluntary contraction (MVC). This was followed by a 60s rest and then immediately by a standard clenching task (50% MVC for 30s) and a 60s recovery period, immediately after which they were asked to repeat exactly the same procedure, with a final 5 min recovery period after the second 30s contraction. Bite force, EMG and Hb concentration were measured continuously and the duration of the two endurance tasks and the amplitudes of all recorded signals were compared (first trial versus second trial). Specifically, the difference between the lowest Hb (trough) seen during the standardised 30s contractions and the highest (peak) seen just after them was assessed. The trough-to-peak difference in Hb concentration of the second standard contraction task was significantly smaller than that of the first standard task (P<0.05, paired t-test). These data show that with sustained effort the post-contraction vasodilatory reperfusion responses of the human masseter are diminished, suggesting a progressive desensitisation of the vasodilatory system.
No preview · Article · Aug 2002 · Archives of Oral Biology
[Show abstract][Hide abstract]ABSTRACT: Ease of handling and the working time of autopolymerizing resins are directly associated with the setting characteristics during polymerization.
This study assessed the setting characteristics of autopolymerizing resins by measuring viscosity changes and tested the hypotheses that: (1) different products have different viscosity increase patterns during polymerization, and (2) the increased rate of viscosity is not proportional to the length of the setting time.
Viscosity change during polymerization was measured in 5 test materials (Unifast II, ADFA, META FAST, Miky, and Trim) with an oscillating rheometer. The working time was defined as the time required for a 5% reduction in the amplitude of the rheometer trace. The setting time was measured as the time required to reach zero amplitude on the rheometer trace.
The order of working times from longest to shortest was Trim, Unifast II and META FAST, ADFA, Miky (ANOVA, P<.0001). The order of setting times from longest to shortest was Trim, META FAST, Miky, ADFA, Unifast II. Materials that had rapid setting times did not necessarily have short working times. Unifast II showed well-balanced setting properties that satisfied both longer working time and faster setting time.
Autopolymerizing resins have setting characteristics specific to the brand. Knowledge of these characteristics is essential for selection of the appropriate resin product depending on the purpose of clinical usage and the individual dentist's technique.
No preview · Article · Feb 2001 · Journal of Prosthetic Dentistry
[Show abstract][Hide abstract]ABSTRACT: When fabricating a provisional crown with the direct technique, dentists are concerned with margin discrepancies that result from polymerization shrinkage.
This in vitro study examined the effect of water temperature on the fit of provisional crown margins during resin polymerization.
The experiment was designed to simulate a direct technique to fabricate provisional crowns. After mixing autopolymerizing methyl methacrylate resin, the material was placed in a preformed polycarbonate crown. The crown was seated on a prepared premolar-shaped die with a shoulder margin. After 1 minute and 50 seconds, the crown was removed and polymerization was continued under the following conditions: 20 degrees C air, and water at 0 degrees C, 10 degrees C, 20 degrees C, 30 degrees C, 40 degrees C, 60 degrees C, and 80 degrees C. Six minutes after polymerization, the crown was trimmed and reseated on the die. Discrepancy of crown margin was measured with a 3-dimensional digitizer.
Margin discrepancy varied with the condition during resin-polymerization (ANOVA, P <.001). The crowns polymerized in 20 degrees C and 30 degrees C water revealed the best margin fit, showing 3 times more accurate margin fit than those polymerized in 20 degrees C air (Bonferroni/Dunn procedure, P <.01).
Conditions during resin polymerization were significant factors that affected margin fit of provisional crowns using the direct technique. Water temperatures of 20 degrees C and 30 degrees C produced the best fit at the margin of the provisional crown.
No preview · Article · Jan 2000 · Journal of Prosthetic Dentistry