Yasushi Nishii

Influence of experimental hemi-occlusion on mandibular morphology and internal structure in growing rabbit

Abstract We induced experimental hemi-occlusion in growing rabbits to investigate the effect of masticatory laterality on the morphology and internal structure of the mandible. Three-dimensional morphometric data from micro-CT were used to compare changes in mandibular morphology and internal structure. Rabbits in the experimental group were placed under general anesthesia and their left maxillary and mandibular molars cut as far as the cervical area using a rabbit molar cutter. Thereafter, the teeth were cut once every 2 weeks. Rabbits were kept until they were 17 weeks old. For morphometric analysis of the mandible, we measured mandibular body length, mandibular length, mandibular ramus height and gonial angle. To analyze cancellous bone structure, we measured total volume, cancellous bone volume, cancellous bone volume density, trabecular thickness, trabecular number, trabecular separation, and trabecular spacing. Mandibular ramus height and gonial angle were significantly greater on the occlusal side in the experimental group than on the non-occlusal side. Cancellous bone volume density, trabecular thickness and trabecular number were significantly greater on the occlusal side in the experimental group than on the non-occlusal side, whereas trabecular separation and trabecular spacing were significantly lower on the occlusal side. The results indicate that differences in mechanical stimuli during the growth phase resulting from masticatory dysfunction or parafunction affect the morphology and internal structure of the mandible.

Stress distribution in the mini-screw and alveolar bone during orthodontic treatment: a finite element study analysis

Objective This study aimed to investigate the reason for high failure rates of mini-screws during orthodontic treatment. We hypothesized that decreasing the length of the mini-screw outside the bone relative to the length inside the bone (outside/inside length ratio) and equalising it to the tooth crown/root ratio would lead to increased stability of the mini-screw against lateral loading when assessed using finite element analysis. Methods We analysed stress distribution of mini-screws in the cortical and trabecular bone and von Mises stress levels when a 2-N force was applied to the head of four mini-screws of 6, 8, 10 and 12 mm in length. The direction of the force was perpendicular to the major axis of the screws. Results Stress levels of screws in cortical bone increased in proportion to the length of mini-screws outside the bone. The length of mini-screws inside the bone did not affect stress levels in the cortical bone. Conclusions The results of this finite element analysis indicate that to stabilize the mini-screw, controlling the screw length outside the bone is more important than controlling the outside/inside length ratio.

Hard palate thickness for temporary anchorage devices placement: Differences in sex and ethnicity

Abstract Purpose The purpose of this study is to determine variations in hard palate thickness among sexes and four ethnic groups to identify optimum area for temporary anchorage devices (TADs) placement. Materials and methods Cone-beam computed tomography (CBCT) volume data on 160 patients were selected. Patients belonged to one of four ethnic groups (Asian, African-American, Caucasian or Hispanic; 20 men and 20 women in each group, 16–45 years old). Hard palate thickness from the incisor fossa to the palatal root of the maxillary first molar was determined on each image and compared between sexes and ethnic groups. Results The data revealed that thickness was greatest at the paramedian area in the incisive canal zone (C-zone) and the median area posterior to the incisive foramen. A significant difference in thickness of palate was confirmed between sexes, and was approximately 0.5 mm and 1 mm greater in men at the median and paramedian, respectively. Among ethnic groups, a trend of decreasing thickness was as follows: Caucasian > Hispanic > African-American > Asian. Conclusion The anterior (incisive canal zone) paramedian and posterior median areas of the hard palate were thicker and suggest that these areas are appropriate for TADs placement. Some differences were also observed local to the paramedian in the C-zone among ethnicities. This indicates that sex and ethnicity should be taken into consideration in selecting sites for placement of TADs.

Crown inclination measured by laser scanner

Abstract We developed the computer measurement procedure to measure crown inclination of three-dimensional (3D) dental images scanned by laser scanner to reduce human error. 20 study models with normal occlusion were scanned by laser scanner. Crown inclinations of each tooth of scanned 3D dental images were measured according to Andrews’ method. Results: The mean value measured by our method and that measured by manual procedure in the same sample did not detect significant differences. In the variances of the measurements, the results of measurement by our method were larger than that by manual procedure for almost all teeth. Compared to other data, the results of our procedure tended to show large variances than that of other report with manual method. On the other hand, compared to non-manual procedure, the data and that of variances tended similar. Our procedure could objectively measure inclinations with less prejudice and or human error of examiner. This procedure would be useful in research and in the clinic.

Dental Implant Treatment with Computer-assisted Surgery for Bilateral Agenesis of Maxillary Lateral Incisors: A Case Report

Here, we report a case of dental implant treatment involving computer-assisted surgery for bilateral agenesis of the maxillary lateral incisors. The patient was a 39-year-old woman with the chief complaint of functional and esthetic disturbance due to maxillary and mandibular malocclusion. The treatment plan comprised non-extraction comprehensive orthodontic treatment and prosthodontic treatment for space due to the absence of bilateral maxillary lateral incisors. A preliminary examination revealed that the mesiodistal spaces left by the absent bilateral maxillary lateral incisors were too narrow for implant placement (right, 5.49 mm; left, 5.51 mm). Additional orthodontic treatment increased these spaces to approximately 6 mm, the minimum required for implant placement if risk of damage to the adjacent teeth due to inaccuracies in directionality of drilling is to be avoided. For dental implant treatment with computer-assisted surgery, preoperative planning/simulation was performed using Simplant® ver.12 software and a toothsupported surgical template fabricated using stereolithography. Two narrow-diameter implants were placed in a two-stage procedure. It was confirmed that there was sufficient distance between the implant fixtures and the roots of the adjacent teeth, together with no exposure of alveolar bone. Following a 4-month non-loading period, second-stage surgery and provisional restoration with a temporary screw-retained implant crown were performed. Cement-retained superstructures made of customized zirconia abutment and a zirconia-bonded ceramic crown were fitted as the final restoration. At 5 years after implant surgery, there were no complications, including inflammation of the peri-implant soft tissue and resorption of peri-implant bone. Computer-assisted implant surgery is useful in avoiding complications in bilateral agenesis of the maxillary lateral incisors when only a narrow mesiodistal space is available for implant placement.

Orthodontic Treatment for Bloch-Sulzberger Syndrome in Patient with Cleft Lip and Palate

We performed orthodontic treatment, fitted prostheses, and provided restorative treatment in a patient with Bloch-Sulzberger syndrome and cleft lip and palate during the early mixed dentition period. We report the case after a subsequent 6-year retention phase including the period of pubertal growth. A girl aged 8 years 4 months visited our hospital with the chief complaint of crowding of the anterior teeth and anterior crossbite. She had bilateral cleft lip, alveolus, and palate; a Class II molar relationship; winging of both the maxillary bilateral central incisors; and spacing in the mandibular anterior teeth arches. Anterior crossbite comprised 0 mm overbite and -1 mm overjet. The crown diameter was at least one standard deviation smaller than normal in both the deciduous and permanent teeth, and the crowns were slightly peg-shaped. Panoramic radiograph confirmed congenital absence of 21 permanent teeth. Cephalometric analysis revealed poor growth of the maxilla, downward growth of the mandible, and lingual inclination of the maxillary central incisors. The diagnosis was skeletal anterior crossbite with cleft lip, alveolus, and palate, accompanied by hypodontia. Orthodontic treatment comprised an edgewise appliance and an expansion arch to improve crowding and anterior crossbite. The appliance was removed 2 years after treatment initiation, followed by crown restorations of the maxillary central incisors and mandibular deciduous anterior teeth. A metal retainer was then fitted to the maxillary dentition. She was subsequently placed in a 6-year retention phase including pubertal growth, during which occlusal stability and esthetics were maintained.

Three-dimensional evaluation of bone void induced in the cortical bone upon insertion of an orthodontic anchor screw

Abstract Purpose The present study evaluated the relationship between the bone void induced in the surrounding cortical bone upon screw insertion, and insertion torque. 3D bone morphometry was analyzed using micro-computed tomography (micro-CT) with different screw diameters and cortical bone thicknesses. Materials and methods Titanium alloy miniscrews with different diameters were used. Specimens were collected from porcine pelvic bones and divided by cortical bone thickness into 48 regions per cm2. Screws were inserted into 32 regions by self-drilling, and maximum insertion torque was recorded. The remaining 16 regions were part of the control group. Specimens were partitioned into regions and micro-CT was used to measure the void volume in bone, including the microdamage surrounding the screw. The proportion of void volume in cortical bone (Vv/Cv) for each region was calculated and compared between the groups. Results Compared with the control group, characteristic microdamage distribution was observed in the group with screws inserted, and a particularly high Vv/Cv was noted on the bone marrow side. In the screw insertion groups, both insertion torque and Vv/Cv were significantly higher in areas with thicker cortical bone. There were no significant differences in the Vv/Cv for different screw diameters, but the influence of the insertion torque was apparent. Conclusion These results suggest that to reduce the bone void to the cortical bone surrounding the screw, it is important to use appropriate insertion torque in accordance with cortical bone thickness.

Crown angulation measured by laser scanner

Abstract The aim of this study was to measure the crown angulation of Japanese subjects with normal occlusion using a laser scanner to minimize human error. Twenty study models with normal occlusion were scanned by laser scanner. Crown angulations of each tooth in scanned 3D dental images were measured according to Andrews’ procedure. In Andrews’ procedure, a plane was established along the mesiodistal contact points of each tooth. The crown angulations of each tooth were measured by projecting the facial axis of the clinical crown (FACC) and a vertical line from the occlusal plane, and the mean and standard deviation were calculated. In maxillary teeth, the canine crown angulation was highest at about 7°, thus indicating a mesial tip, which was seen in all teeth except the second molar. In mandibular teeth, the second molar crown angulation was highest at about 7°, thus indicating a mesial tip, and only the lateral incisor exhibited slight distal tip, while all the remaining teeth exhibited mesial tip. Standard deviations for the maxillary and mandibular teeth ranged from 2.5° to 8°. The standard deviation of the maxillary and mandibular second molars was particularly large.