Effect of orthodontic treatment with preadjusted edgewise appliances on the buccolingual inclination of mandibular canines: a CBCT study.

OBJECTIVES: To assess the effects of treatment with preadjusted edgewise appliances on the buccolingual inclination of mandibular canines and the intercanine distance. SETTING AND SAMPLE POPULATION: The Division of Orthodontics at the University of Minnesota. Thirty patients whose treatment included extraction of mandibular first premolars and 30 patients whose treatment did not include extractions. MATERIAL AND METHODS: The buccolingual inclination of mandibular canines and their linear distance were measured on cone beam computed tomograms before and after treatment in both patient groups. Differences between extraction and non-extraction groups and between pre- and post-treatment measurements were tested for statistical significance, and the correlation between the buccolingual inclination and the intercanine distance was computed. RESULTS: Post-treatment, the buccolingual inclination of mandibular canines was significantly greater in the non-extraction group than in the extraction group. In both groups, the canines became more lingually inclined with treatment (non-extraction group: -2.1°; extraction group: -4.1°). The intercanine distance increased significantly in the extraction group (+1.2 mm) but not in the non-extraction group (-0.5 mm). While there was a significant positive correlation between the buccolingual inclination and the mandibular intercanine distance in both groups before treatment, after treatment this correlation was significant only in the non-extraction group. CONCLUSION: Orthodontic treatment with preadjusted edgewise appliances results in more lingually inclined mandibular canines together with an increased intercanine distance, especially in patients whose treatment involves the extraction of mandibular first premolars.

Changes in rabbit jaw-muscle activity parameters in response to reduced masticatory load.

Mechanical food properties influence the neuromuscular activity of jaw-closing muscles during mastication. It is, however, unknown how the activity profiles of the jaw muscles are influenced by long-term alterations in masticatory load. In order to elucidate the effect of reduced masticatory load on the daily habitual activity profiles of three functionally different jaw muscles, the electromyograms of the masseter, temporalis and digastric muscles were recorded telemetrically in 16 male rabbits between seven and 20 weeks of age. Starting at eight weeks of age the experimental animals were fed significantly softer pellets than the control animals. Daily muscle activity was quantified by the relative duration of muscle use (duty time), burst number and burst length in relation to multiple activity levels. The daily duty time and burst number of the masseter muscle were significantly lower in the experimental group than in the control group at 5% and 10% of the maximum activity during the two weeks following the change in food hardness. By contrast, altered food hardness did not significantly influence the activity characteristics of the temporalis and digastric muscles. The findings suggest that a reduction in masticatory load decreases the neuromuscular activity of the jaw-closing muscles that are primarily responsible for force generation during mastication. This decrease is most pronounced in the weeks immediately following the change in food hardness and is limited to the activity levels that reflect muscle contractions during chewing. These findings support the conclusion that the masticatory system manifests few diet-specific long-term changes in the activity profiles of jaw muscles.

Duty time of rabbit jaw muscles varies with the number of activity bursts.

The relative duration of muscle activity during a specified period (duty time) varies depending on activity level and time of the day. Since both the number and the length of activity bursts contribute to the duty time, it was hypothesized that these variables would show intra-day variations similar to those of the duty time. To test this, we determined duty times, burst numbers, and burst lengths per hour, in relation to multiple activity levels, in a 24-hour period of concurrent radio-telemetric long-term electromyograms of various rabbit jaw muscles. The marked intra-day variation of the burst number resembled that of the duty time in all muscles, and was in contrast to the relatively invariable mean burst length. Furthermore, the duty times were more highly correlated with the number than with the length of bursts at all activity levels. Thus, the variation of the duty time in rabbit jaw muscles is caused mainly by changes in burst numbers.