Formation and development of maxillary first molars with delayed eruption.

Cases of congenitally missing and delayed eruption of the maxillary first molar are rare. However, in recent years, we have experienced cases of suspected delayed eruption of or congenitally missing first molars. The purpose of this study was to analyze the formation of delayed erupted maxillary first molars (M1) (>2 standard deviations), which play important roles in occlusion, and normal eruption of the maxillary first molars (U6). The frequency of M1 among patients born between 1974 and 1994 in one institution with a clear total patient number and personal oral histories was 1.55 % [80 % bilateral eruption in 8 of 806 male patients (0.99 %) and 23 of 1195 female patients (1.92 %)]. To evaluate the formation and eruption of M1 according to Moorrees's tooth formation stages, panoramic X-ray films were obtained every year for 73 patients with M1 from 3 institutions (20 male and 53 female patients, total 131 M1s) without systematic histories or genetic disorders. The development/growth curve of M1 was fitted to both the logistic curve and U6 curve. The M1 development/growth curve was started behind with U6 curve; however, the straight part of the M1 curve exhibited steep inclination compared with the straight part of the U6 curve. The curve of the eruption pathway of M1 also exhibited a sigmoid S shape. These results indicate that the development and migration speed of M1 are faster than that of U6, excluding the delayed start point. These results may help orthodontists in treatment planning for patients with M1.

Repeated bonding of fixed retainer increases the risk of enamel fracture.

The aim of this study was to investigate the influences of repeated bonding, using 2 different orthodontic adhesive systems, on the shear bond strength (SBS) and the enamel surface morphology. Sixty premolars were divided into 2 groups (n = 30), and either Transbond XT (T group) or Fuji Ortho LC (F group) adhesives were used. SBS was measured 24 h after bonding, using a universal testing machine. Then, the enamel surfaces were investigated and the mode of failure was described using adhesive remnant index (ARI) scores. After each debonding, 10 teeth from each group were examined by scanning electron microscopy to determine the penetration of adhesives, the length of resin tags, and the state of the enamel surface. The other teeth were subjected to two more bonding/debonding procedures. In T group, the second debonding sequences had significantly higher bond strengths than the other sequences. The length of resin tags was greatest in the second debonding sequence, although there was no significant difference. In F group, the SBS increased with further rebonding and the failure mode tended towards cohesive failure. In both groups, the ARI scores increased with rebonding. Enamel loss could have occurred with both adhesives, although the surfaces appeared unchanged to the naked eye. From this study, we suggest that enamel damage caused by repeated bonding is of concern. To prevent bond failure, we should pay attention to the adhesion method used for bondable retainers.

Comparison of mesiodistal tooth crown diameters and arch dimensions between modern Mongolians and Japanese.

The purposes of this study were to establish normative data for mesiodistal tooth crown diameters and arch dimensions in Mongolian adults and to compare them with those of Japanese adults. The study materials comprised dental casts of 100 modern Mongolian and 100 Japanese subjects (50 males, 50 females for each) with Angle Class I normal occlusion. The mean ages were 20 years 8 months for the Mongolian subjects and 20 years 0 months for the Japanese subjects. On the dental casts, the mesiodistal tooth crown diameters (excluding wisdom teeth) and dental arch dimensions were measured. The following arch dimensions were measured: inter-canine lingual, inter-premolar lingual, inter-molar lingual, inter-molar central, coronal arch length, basal arch length, and basal arch width. In the Mongolian samples, significant sex differences were noted, and most of the items were significantly larger in males than in females. Significant differences between the Mongolian and Japanese samples were mainly noted in the premolar and molar regions, rather than in the anterior region, and were significantly smaller in the Mongolian samples. In the Mongolian samples, the molar section widths and basal arch width and length were significantly larger in males and females compared with the Japanese samples. These results suggest that the tooth crown size and arch dimensions in the Mongolian samples differed from those in the Japanese samples, and that establishment of the clinical norm for Mongolian adults might be helpful in formulating treatment plans for Mongolian patients, given that these parameters are the basic tools for diagnosis.

Three-dimensional positions of the center of resistance of the maxillary canine distal movement under orthodontic force loading.

Using finite-element analysis, we aimed to determine the center of resistance (CRes) of the maxillary canine for setting orthodontic forces. The inclination of the canine was measured by first loading from the mesial to the distal side of the mesial root surface, then the position and direction of the load that minimized the inclination were investigated. The CRes was defined as the set of midpoints of the minimum distances between two inclination lines. Twenty-one CRes values were calculated from a set of seven lines. These CRes data were then aggregated as a 95% confidence ellipsoid of width 0.170×0.016×0.009 mm with center points 4.269, 0.224, and 4.315 mm in the apical, mesial, and lingual directions from the origin, respectively. Further studies are required to effectively apply the CRes identified in this study to clinical applications.

Influence of third molar space on angulation and dental arch crowding.

The influence of the third molars on mandibular incisor crowding has been extensively studied but remains controversial. The purpose of this study was to ascertain whether, in Mongolian subjects, the lower third molar can affect anterior crowding and/or the inclination of teeth in the lower lateral segments. Panoramic radiographs, 45° oblique cephalograms, and dental casts were taken from Mongolian subjects (age range 18.3-24.1 years, mean 21.0 years) exhibiting impaction of all four third molars and an Angle Class I molar relationship. The Ganss ratio was calculated using panoramic radiographs, whereas the gonial angle and angulation of lower canines, premolars and molars were measured using 45° oblique cephalograms. Little's index of irregularity was calculated using dental casts. Significant relationships between the angulation of the third and second molars and between the first molars and second premolars were found. Conversely, there was no significant correlation between the angulation of third molars, first premolars and canines. The Ganss ratio calculations showed that the lower first and second molars and the second premolars inclined mesially if there was insufficient space for the lower third molars. However, there was no significant correlation between Little's index of irregularity and third molar angulation. Furthermore, although the third molar influences the lateral segments, no obvious relationship between the third molar and anterior crowding was observed. Therefore, the angulation of the third molar appears not to cause anterior crowding.

Three-dimensional location and distribution of the center of resistance in the maxillary first molar applied to occlusal force.

We aimed to investigate the center of resistance (CRes) of the maxillary first molar to set the occlusal force through finite element analysis. The inclination of the molar was measured, with loading to the root on the crown, and the position and direction of the load that minimized the inclination were investigated. The CRes was defined as the set of midpoints of the minimum distances between the two lines. Nine hundred and ninety CRes points were estimated from forty-five lines. The CRes was estimated as the point 1.22 mm (Z), -0.74 mm (X), and 0.23 mm (Y) from the origin in the apical, distal, and buccal side directions, respectively, with an ellipsoid area of 1.578 (Z) mm×0.097 (X) mm×0.100 (Y) mm. Further research is required to make effective use of the CRes identified in this study for clinical applications.

Can hypochlorous acid be a powerful sanitizer to replace alcohol for disinfection ? -Its bactericidal, degradation of the solutions under various storage condition, and steel rust effects.

Coronavirus disease pandemic has led to a shortage of disinfectant alcohol (DA; 76.9-81.4% ethanol solution) in Japan. This study investigated the availability of hypochlorous acid (HA) as an alternative disinfectant to DA in clinical environment. HA is a safe and economical disinfectant with a broad antibacterial spectrum and low risk of hand irritation, which is used worldwide. In this study, HA and sodium hypochlorite solution (SH) were degraded over time and the degradation was accelerated by temperature and lighting. Results of bactericidal effects of HA revealed that the realistic effective concentration should be 200 ppm. Compared with DA, the dental carbon steel burs were more severely rusted by SH and HA. These results indicated that high concentration of HA generated on-site could be an effective and safe disinfectant for use immediately after generation with consideration of object materials in clinical environment, as an alternative to DA.

Semi-powered exoskeleton that regulates the muscular activity of jaw movement for oral functional rehabilitation/training.

The sequential oral functions of mastication and swallowing are well tuned in humans. To prevent oral hypofunction as a risk factor for systemic frailty by oral motor training, semi-powered exoskeleton was developed and evaluated its loading/assist effects by monitoring electromyography signals and saliva secretion in healthy persons. The actuator of the driving unit combined mechanical and powered mechanics and was driven alternatively by an unpowered stainless-steel spring system for jaw-opening training by loading and a powered shape-memory alloy spring system to assist jaw closing. Sequential device movement was controlled by two electronic circuits with two magnetic switching systems to match human jaw movement. This exoskeleton realized a 25% increase of jaw-opening muscular activity for training, 15% saving of jaw-closing muscular activity for assistance, and 15% enhancement of saliva secretion, which could contribute to the prevention of oral frailty by maintenance and strengthening of oral function in the upcoming super-aging society.

Study of delayed development of the upper first molars.

The present study was undertaken to analyze the status of formation of the upper first molar in individuals suspected as having delayed eruption of the upper first molar. The subjects of this study were 51 patients presenting to orthodontic specialists with delayed eruption of the upper first molars. The 95 teeth of these 51 patients were observed. The investigation of the tooth development status included evaluation of the tooth development stage on panoramic radiographs according to the method of Moorrees et al. The upper first molars that showed delayed development were at the stage of initial cleft formation, reflecting a delay by 2.09 years in boys and 2.84 years in girls. In most cases, the delayed eruption was bilateral, and the development stage of the subject teeth did not differ between right and left sides. The plot of the developmental stage of the subject teeth (Y-axis) against the calendar age of the subjects (X-axis) showing delayed upper first molar development was fitted to the logistic curve. The tooth development was at the stage of initial cleft formation was delayed 2.5 years or more, according to these curves.

Estimation of three-dimensional long axes of the maxillary and mandibular first molars with regression analysis.

The purpose of this study was to determine the long axes of molars with multiple roots through ordinary least squares regression (LSR) and to compare them with the axes defined by principal component analysis (PCA). Three-dimensional radiological images of 20 dry skulls were obtained by cone-beam computed tomography (CBCT). Data from maxillary and mandibular first molars were extracted from the CBCT DICOM data with a three-dimensional image visualization system. The obtained data were reconstructed, converted to STL files, and three-dimensional coordinate values were extracted. The long axes were estimated by an algorithm to synchronize the LSR line with the horizontal axis which was translated to the vertical axis. The axes of the molars defined by LSR were compared with the axes of the molars defined by PCA. The coordinate point number of each molar was 5400-5800. The algorithm for determining the tooth axes in this study consisted of four stages containing three steps each. The distance between the two axes calculated by the two methods (LSR and PCA) on the horizontal plane through the origin was less than 10-12 mm and the deviations between them were less than 0.003°. The long axes of the molars estimated by LSR agree almost exactly with the axes estimated by PCA, and the accuracy is sufficient for clinical usage; however, the distance between them would shorten with a more severe convergence condition of the α value at each stage of this LSR system.

Low temperature polytetrafluoroethylene (PTFE) coating improves the appearance of orthodontic wires without changing their mechanical properties.

A strong esthetic demand exists for white-colored rather than metallic-colored dental appliances. Polytetrafluoroethylene (PTFE), which is chemically stable with extremely low friction, is a suitable white-colored coating for dental appliances. In the conventional PTFE coating process, base materials are heated to approximately 400°C, which could change their mechanical properties. Examination of the PTFE-coating of stainless steel, nickel titanium (Ni-Ti), and ß-titanium (Ti-Mo and Ti-Nb) revealed that the conventional 380°C coating, but not the newly developed low temperature coating at 200°C, reduced elasticity and bendability, and changed the crystal structure, especially in Ni-Ti and Ti-Nb wires. PTFE-coating at 200°C resulted in less discoloration, microbial adhesion, and friction against brackets, and higher tolerance of wear than did the conventional 380°C coating. These results suggest that low temperature PTFE coating is an excellent method for improving the metallic appearance of orthodontic wires without changing their mechanical properties.

Influence of shovel-shaped incisors on the dental arch crowding in Mongolian females.

The aim of this study was to investigate the influences of shovel-shaped incisors on the dental arch crowding in the samples of modern young female adult Mongolians (belonging to the Khalkha-Mogol grouping) ranged between 18.5 and 25.0 years, with a mean age of 20.7 years. Materials in this study comprised of 33 dental casts. Plaster replicas of the A.S.U. system plaques were used to facilitate standardization in scoring and classify shoveling regarding both upper central and lateral incisors. Dimensions of the dental arch namely, overjet, overbite, arch depth, arch width and irregularity index were measured. The correlation between the shovel shape of the central incisor and the lateral incisor was highly significant. Significantly relations were found between the grades of shoveling and the mesiodistal diameters of upper first molars and lower incisors. Positive correlation was admitted between the shoveling and upper arch depth anterior, and upper arch depth. It may be considered that the grades of shoveling are indirectly related with upper and/or lower anterior crowding. Although crowding is looks like an expression of disharmony between teeth and alveolar arch, it may be caused by multiple etiologic factors interacted directly and/or indirectly.

Five-axis laser milling system that realizes more accurate zirconia CAD/CAM crowns by direct milling from fully sintered blocks.

Directly milling zirconia computer-aided design (CAD)/computer-aided manufacturing (CAM) crowns from fully sintered zirconia blocks using a five-axis laser milling system, compared with three-axis milling and full sintering by heating milled semi-sintered crowns, was investigated. The mechanical characteristics of zirconia specimens were similar across groups. The order of the marginal gap was three-axis>conventional (lingual thickness of 1.5 mm>0.5 mm)>five-axis group (close to zero). The marginal shape was almost perfectly circular in all groups. The internal corner shape and gap were almost perfect for the five-axis milled crown but not for conventional and three-axis crowns. The roundness of the marginal and internal shapes was almost perfect in the five-axis milling group but not for the three-axis and conventional groups. These small distortions result in large marginal gaps. Results of the present study suggest the superiority of the five-axis milling system in creating a zirconia prosthesis.

Effects of magnetic fields from electric toothbrushes on fluoride- and oral bacteria-induced corrosion of orthodontic metallic wires.

Corrosion of metallic materials in the oral cavity could trigger metal allergy in patients. To clarify the risk elevation of magnetic fields (MFs) exposure on metallic corrosion when combined with fluoride-containing dental care products and indigenous oral bacteria, we investigated electric toothbrush-derived MF-induced corrosion of orthodontic stainless steel (SUS) and nickel titanium (Ni-Ti) wires in the presence of fluoride and oral bacteria, i.e. Streptococcus (S) mutans and S. sanguinis. MFs induced an electric current in the wires under both environments. Oral bacteria corroded SUS wires, and fluoride corroded SUS and Ni-Ti wires as previously reported; however, no additive or synergistic effects of MF exposure on fluoride- and microbiologically-induced metallic corrosion were observed. These results suggest that the MFs from electric toothbrushes do not increase the risk of corrosion of metallic appliances, given that the oral environment of patients is exposed to oral bacteria and fluoride-containing products.

Dental arch widths and mandibular-maxillary base widths in Class III malocclusions from ages 10 to 14.

INTRODUCTION: Evaluations of the dental arch widths and mandibular-maxillary base are needed for a comprehensive dentofacial analysis in subjects with Class III malocclusion. The aim of this study was to analyze the development of the dental arches and the skeletal mandibular-maxillary bases in untreated subjects with Class III malocclusions. METHODS: Two groups of subjects, 1 with Class III malocclusion and the other with Class I malocclusion, were examined. Maxillary skeletal base width, biantegonial widths, and maxillary and mandibular intermolar widths were determined on posteroanterior cephalograms at annual intervals between the ages of 10 and 14 years. RESULTS: Maxillary skeletal base widths and intermolar widths in the Class III subjects were significantly smaller than those in the Class I subjects (P <.05). No statistically significant differences were found among the groups for skeletal mandibular width or intermolar width for the total observation period. The deviations in molar differences increased from ages 10 to 14 in the Class III group. CONCLUSIONS: The main transverse deficiencies in the Class III group were maxillary deficiencies in both skeletal and dental widths. The deviations in molar differences appear to become larger from age 10 to age 14.

Effects of bimaxillary surgery and mandibular setback surgery on pharyngeal airway measurements in patients with Class III skeletal deformities.

INTRODUCTION: The purpose of this study was to compare the short-term and long-term effects of bimaxillary surgery with those of mandibular setback surgery concerning pharyngeal airway measurements at 3 levels: nasopharynx, oropharynx, and hypopharynx. METHODS: The sample included 66 Japanese women in 2 groups who had been diagnosed with Class III skeletal deformities and had undergone surgical-orthodontic treatment. Those in group A (35 patients) underwent bilateral sagittal split ramus osteotomies; those in group B (31 patients) underwent LeFort I procedures with bilateral sagittal split ramus osteotomies. Lateral cephalograms were assessed within 6 months before surgery and at short-term (3-6 months after surgery) and long-term (at least 2 years after surgery) follow-ups. RESULTS: In group A, the pharyngeal airway was constricted significantly at the oropharyngeal and hypopharyngeal levels at both the short-term and the long-term follow-ups. In group B, significant changes were shown at the 3 pharyngeal levels at the short-term follow-up, whereas no significant changes were shown at the long-term follow-up. CONCLUSIONS: These results indicate that, when possible, bimaxillary surgery rather than only mandibular setback surgery is preferable to correct a Class III deformity to prevent narrowing of the pharyngeal airway space, a possible predisposing factor in the development of obstructive sleep apnea.

Dental arch widths and mandibular-maxillary base width in Class III malocclusions with low, average and high MP-SN angles.

OBJECTIVE: To analyze the development of the dental arches and skeletal mandibular-maxillary bases in untreated Class III malocclusions with low averages and high mandibular plane angles in subjects aged 10 to 14. MATERIALS AND METHODS: The records of 50 untreated Japanese girls with Class III malocclusions at age 10 were selected from the files of patients pending orthodontic surgery. The patients included those with low (< or =27 degrees), average (27 degrees through 37 degrees) and high (>37 degrees) mandibular plane angles. The maxillary skeletal base width, biantegonial width, and maxillary and mandibular intermolar width were determined on posteroanterior cephalograms obtained at annual intervals when subjects were between 10 and 14 years of age. The difference between the maxillary and mandibular intermolar width was also calculated and reported. RESULTS: All skeletal and dental transverse widths in the high-angle group were significantly smaller than those in the low-angle group (P < .05) from ages 10 to 14. On the other hand, the maxillary to mandibular molar difference was the same for the three groups (P > .05) at each age. The deviations in molar differences did increase from age 10 to age 14 in all three groups. CONCLUSION: Mandibular plane angles might play a stronger role in the transverse skeletal growth of the maxilla and the mandible than the transverse dental growth of the maxilla and the mandible.

Longitudinal intermaxillary relationships in class III malocclusions with low and high mandibular plane angles.

OBJECTIVE: To analyze the sagittal, vertical, and transverse relationships of the maxilla and mandible in Japanese girls with Class III malocclusions with different inclination of mandibular plane. MATERIALS AND METHODS: This longitudinal study utilized serial posteroanterior and lateral cephalograms of 56 untreated subjects from the age of 8 years until the age of 14 years (low mandibular plane angle group: n = 20; average mandibular plane angle group: n = 15; high mandibular plane angle group: n = 21). Sagittal and vertical growth was analyzed on the basis of lateral cephalograms, and transverse growth was analyzed on the basis of posteroanterior cephalograms. RESULTS: There was no significant difference in intermaxillary sagittal relationships among the three groups from age 8 until 14. On the other hand, there were significant changes in the vertical and transverse intermaxillary relationships during this period. When comparing the three groups at the same age, there were significant differences in vertical and transverse intermaxillary relationships in some ages, whereas no significant difference was found in sagittal relationships in any ages. CONCLUSIONS: The inclination of mandibular plane might play a role in anticipating changes in vertical and transverse intermaxillary relationships from 8 until 14 years of age.

Intraoral electric potential via oral bacterial power generation -A novel mechanism of biofilm formation.

In the early stages of biofilm accumulation, the electric charge of the dental enamel and pellicle surfaces is known to be involved. We therefore investigated the relationship between oral hygiene and intraoral electric potential (IoP) in 45 male participants using a double-blind study. IoP, but not body surface electric potential, was loosely correlated with oral hygiene condition (Oral Hygiene Index; OHI). IoP was also loosely correlated with smartphone use; however, there was no significant correlation between smartphone use and OHI. IoP elevation might be caused by OHI elevation resulting from biofilm formation as an internal factor, with smartphone use as an external factor. This in vitro study revealed the generating capacity of Streptococcus mutans accompanied by biofilm accumulation using a microbial fuel cell. These results suggest that IoP elevation is caused by biofilm accumulation induced by power generation of oral bacteria, resulting in elevation of OHI.

Anchorage effect of osseointegrated vs nonosseointegrated palatal implants.

Palatal implants can be used with a transpalatal arch (TPA) connected with the second premolar to provide anchorage. The purpose of this study was to compare the anchorage effects of an osseointegrated palatal implant (OPI) with a nonosseointegrated palatal implant (NOPI), using finite element analysis. One model, which was composed of two maxillary premolars, periodontal ligament (PDL), alveolar bone, a palatal implant, palatal bone, a bracket, band, and TPA, was created on the basis of the clinical situation. The palatal implant was treated as either NOPI or OPI. The force on the premolars was investigated under three conditions: a distomesial horizontal force, a buccolingual horizontal force, and a vertical intrusive force. The PDL stress was calculated and compared with a model without an implant. The result showed that OPI could reduce PDL stress significantly. (The average stress was reduced by 14.44% for the distomesial horizontal force, 60.28% for the buccolingual horizontal force, and 17.31% for the vertical intrusive force.) The NOPI showed almost the same anchorage effect as OPI. The stress on the NOPI surface was higher than that on the OPI surface, but the stress was not high enough to result in failure of the implant. These results suggested that waiting for osseointegration might be unnecessary for an orthodontic implant.