Kazunori Yamaguchi

A study of force application, amount of retarding force, and bracket width in sliding mechanics

We investigated the relationship of the retraction force to the location of force application, retarding force and bracket width during simulated sliding tooth movement along an arch wire. Point 1 for retraction was located at the center of the bracket, and points 2 and 3 were at 4.0 mm and 6.0 mm from the bracket slot, respectively. Weights of 100 gm, 200 gm, and 400 gm were suspended at 9.0 mm from the bracket slot as the point of simulated center of resistance. Stainless steel standard edgewise wide, medium, and narrow twin brackets were engaged with two elastomeric ligatures on a stainless steel wire (0.016 x 0.016 inch). The bracket was retracted at the rate of 0.1 mm per second for a distance of 2.0 mm. Measurements were repeated six times, and the results were compared with multiple ANOVA tests. For all brackets, with an increase of the retarding weight, the mean retraction force at points 1 and 2 increased but decreased at point 3. The mean retraction force at point 1 for the narrow twin bracket was significantly (p < 0.05) higher than that for the wide twin bracket at all retarding force levels. However, the mean retraction force at points 2 and 3 for the narrow twin bracket was significantly (p < 0.05) lower than for the wide twin bracket at all retarding force levels. These findings indicated that the point of force application, the resistance force of a tooth, and the width of the bracket are crucial in consideration of the tipping moments on the bracket.

Influence of dietary n-3 polyunsaturated fatty acid on experimental tooth movement in rats.

This study was conducted to investigate the influence of dietary n-3 polyunsaturated fatty acid on experimental tooth movement. This acid substantially reduces the production of arachidonic acid. Sixty 4-week-old male Wistar strain rats were divided into experimental and control groups. Animals in the experimental group were fed a purified diet containing 10% refined fish oil (rich in n-3 fatty acid); control animals were fed a diet containing 10% corn oil (rich in n-6 fatty acid). After 6 weeks, the maxillary first molars were moved buccally with an initial force of 20 g for periods of 0, 3, 7, or 14 days. Tooth movement in the experimental group was 80% of that seen in the controls. The number of osteoclasts on the pressure side during tooth movement was nearly 60% of that seen in controls, and the degree of bone resorption was 80%. The data suggest that a diet enriched with fish oil reduces osteoclastic activity and subsequent alveolar bone resorption that is the key to experimental tooth movement.

Influences of vertical occlusal discrepancies on condylar responses and craniofacial growth in growing rats.

The present study was conducted to investigate the influence of vertical occlusal discrepancies on condylar remodeling and the subsequent craniofacial growth in growing rats. Thirty 4-week-old male Wistar strain rats were used. A 1-mm-thick metal plate was bonded onto the occlusal surface of the maxillary molars to increase posterior dentoalveolar height. During the early phase of the experiment, the thickness of the proliferative and maturative/hypertrophic zones in the anterior and superior portions of the condyle was significantly smaller in the experimental group than in the controls. The number of TRAP-positive cells was significantly greater in the experimental group than in the controls. At the end of the experiment, decreased ramus height and a large gonial angle were found in the experimental group. Changes in the intra-articular environment associated with vertical occlusal discrepancies may influence condylar and craniofacial growth in growing individuals, although some adaptive response of the condyle may be induced if growth potential remains.

Reduction of the hypocone of the maxillary first molar and Class III malocclusion.

This study was conducted to examine the incidence of evolutionary changes of the maxillary molars in orthodontic patients and to investigate the association of this event with craniofacial growth. Among 4,892 Japanese patients treated at the orthodontic clinic of Hiroshima University Dental Hospital, 59 patients presented with a defect of the distolingual cusp of the maxillary first molar. They consisted of 52 female patients and seven male patients with a mean age of 14.7 years. Eruption of the maxillary first and second molars was delayed and the third molar was congenitally missing in all of these patients. Thirty-seven patients exhibited Class III malocclusion (Class III evolution group). The Z scores of cephalometric measurements were compared between the Class III evolution group and the Class III control group. The latter group consisted of 37 Class III subjects with no evolutionary dental anomalies and no orthodontic history. The tooth developmental score (TDS) for each age at the initial record was compared with Nollas standard tooth developmental score (STDS) for equivalent ages using paired t-tests. The Z scores for Ptm-A and Ar-A were significantly smaller in the Class III evolutionary group than in the Class III control group (P < .05). The TDS for the maxillary first and second molars were significantly smaller than the STDS for these molars (P < .01). The TDS for the maxillary second molar was significantly larger than the STDS for the maxillary third molar (P < .01). This study revealed that an evolution change of the maxillary molar is more common (P < .05) in female patients than in male patients, is more frequent (P < .05) in subjects with Class III malocclusion, and is related to the anteroposterior undergrowth of the maxilla.