The effect of scanning pathways on trueness and precision in full-arch optical impression.

BACKGROUND: In this study, we investigated the effects of differences in scanning pathways during optical impression on the trueness and precision of full-arch impressions. METHODS: Reference data were obtained using a laboratory scanner. All optical impressions were measured across the dental arch using TRIOS® 3 in four different pathways. The reference and optical impression data were superimposed using the best-fit method. The criteria for superimposition were based on the starting side of the dental arch (partial arch best-fit method, PB) and based on the full arch (full arch best-fit method, FB). The data were compared between the left and right molars (starting and ending sides). The scan deviations for trueness (n = 5) and precision (n = 10) were obtained for each group by calculating the root mean square (RMS) of the deviation at each measurement point. Visual observations using superimposed color map images revealed variations in trueness. RESULTS: There were no significant differences in scanning time or amount of scan data between the four scanning pathways. Trueness did not differ significantly among the four pathways with respect to the starting and ending sides, regardless of the superimposition criteria. Precision with PB was significantly different between scanning pathways A and B, and pathways B and C for the starting sides, and between scanning pathways A and B, and pathways A and D for the ending sides. In contrast, there was no significant difference between the starting and ending side in pathways for FB. Regarding PB, color map images showed a large error range in the direction toward the molar radius for the occlusal surface and cervical regions on the ending sides. CONCLUSION: Differences in the scanning pathways did not affect trueness, regardless of the superimposition criteria. On the other hand, differences in the scanning pathways affected the precision of the starting and ending sides with PB. Scanning pathways B and D were more precise on the starting and ending sides, respectively.

Effects of Cuspal Inclination and Luting Agent on Fracture Load Values in Composite Resin CAD/CAM Crowns.

The purpose of this study was to evaluate whether change in cuspal inclination influences the fracture load values of composite resin computer-aided design/computer-aided manufacturing (CAD/CAM) crowns. Abutment teeth and CAD/CAM crowns were prepared as they would be for treating a mandibular first premolar with two cusps. The CAD/CAM crowns were designed so that 1) the principal stress lines would be radially distributed from the two points of contact with the indenter to the occlusal area of the abutment (Type I), or 2) the principal stress lines would pass outside the occlusal area of the abutment (Types II and III). The CAD/CAM crowns were mounted on the abutments using one of two types of resin or polycarboxylate cement. Fracture load values were measured using a universal tester. The Type I CAD/CAM crowns exhibited the highest mean fracture load value, followed by the Type II crowns and then the Type III crowns, with significant differences seen between all types (p<0.05). The luting agent used (resin or polycarboxylate cement) showed no effect on the fracture load value. With the Type II and Type III crowns, significant differences in the fracture load value were observed between the Super-Bond and Hy-Bond polycarboxylate cement groups (p<0.05).

Patient with Poor Esthetic Anterior Tooth Position Treated with Lithium Disilicate Mesial Cantilever Single-retainer Fixed Partial Denture: A Case Report.

We report a case of fixed prosthetic treatment for poor esthetics due to the position of the maxillary left lateral incisor in a 43-year-old woman. Initial examination revealed no carious lesions, but the tooth axis of the maxillary right canine showed mesial inclination of approximately 15°. Orthodontic treatment was first proposed but was declined by the patient as they did not wish to undergo a prolonged period of therapy. Therefore, recovery by extraction of the maxillary right lateral incisor and prosthetic treatment was proposed as an alternative. The method to be used for application of a 3-unit fixed partial denture and implant treatment was explained to the patient. She refused to give consent to this plan as well, however, due to concerns regarding the need to cut a lot from a non-problematic tooth and the length of time such treatment would require. Therefore, the problem was finally treated by application of a cantilever single-retainer fixed partial denture while giving sufficient consideration to extraction and occlusal contact. Lithium disilicate was used for the material of the prothesis. At 1 year after completion of treatment, no problem was observed with either the prosthetic appliance or the abutment teeth.

A Case of Mandibular Cancer Involving Almost Entire Attached Gingiva.

Here we describe a rare case of mandibular cancer involving almost the entire attached gingiva in a 71-year-old man. First, marginal resection of the entire mandible was performed, followed by one-stage reconstruction comprising application of a split-thickness skin graft onto the wound. This resulted in good alveolar ridge morphology, allowing for a mandibular prosthesis to be installed soon postoperatively. Histopathological analysis revealed a well-differentiated squamous cell carcinoma extending throughout most of the resected attached gingiva, but no malignant features in the stumps. Furthermore, no infiltration into the jawbone was observed, and no vascular or lymphatic invasion or perineural infiltration. At 3 years postoperatively, the patient's clinical course has remained uneventful, with no recurrence or problems arising in the remaining mandible. The patient is also able to eat regularly using the mandibular prosthesis provided.

Influence of Tooth Mold Materials in Composite Resin CAD/CAM Crown Destructive Test.

A range of experimental designs have been used in destructive testing of composite resin CAD/CAM crowns. Various materials have been adopted for the abutment in such tests, including human or bovine dentin, stainless steel, PMMA, and composite resin, the selection of which is made in accordance with study objective or preference of the researcher. The purpose of this study was to determine how the material selected for the abutment material affected fracture load and maximum displacement. Destructive tests were conducted on composite resin crowns of the same design. Three types of material were used for the abutments together with 2 types of adhesive material. Images of each sample were acquired before destruction using a microfocus X-ray CT scanner to confirm the feasibility of a non-destructive test.The load required to fracture the composite CAD/CAM resin crowns depended on the abutment material used, with a decrease being observed in the order of composite resin, stainless steel, and PMMA. Maximum displacement decreased in the order of PMMA, composite resin, and stainless steel. Differences in the material used for setting (adhesive resin or polycarboxylate cement) showed no effect on fracture load. These results indicate that the load required to achieve destruction of resin CAD/CAM crowns varies according to the abutment material used.

Plaque-removing Effects of Interdental Instruments in Molar Region.

Many types of interdental instrument, such as dental floss and interdental brushes, can be purchased easily and sell in large numbers. Many studies have compared the effects of such instruments. Few studies have investigated their relationship with regions of residual plaque, however. The purpose of this study was to compare rates of plaque removal from the mesial and distal surfaces of the bilateral maxillary and mandibular premolars and molars among 3 types of interdental instrument: finger-winding-dental floss, holder-type dental floss, and an interdental brush. Prior to the experiment, the artificial teeth in a jaw model were removed for application of artificial plaque. The teeth were then replaced and the model attached to a phantom. An operator brushed the teeth in a posture close to that adopted in actual tooth brushing with each type of instrument, after which the plaque removal rate was compared among them. The rate of plaque removal using finger-winding floss was the highest, although this type of instrument is less easy to hold than the others. The rate with a handle-equipped interdental instrument showed a decrease in some regions, although it is easy to hold. The results of this study suggest that the type of interdental instrument used should differ depending on the region to be cleaned.

Effect of Differences in Axial Thickness and Type of Cement on Fracture Resistance in Composite Resin CAD/CAM Crowns.

The purpose of this study was to investigate the influence of differences in axial thickness and type of cement on fracture load in CAD/CAM crowns. Assuming the mandibular first premolar to be the abutment tooth, 4 types of crown with different axial thicknesses and radii of curvature were prepared. To unify external design, the morphology of the crown margins was set at 0.15, 0.30, 0.45, or 0.60 mm, thus maintaining uniform axial form of the crowns. The CAD/CAM crowns and abutment teeth were bonded using each of 2 types of resin cement or polycarboxylate cement. The fracture load value was measured using a universal testing machine and the destruction phase observed.No significant difference was observed with change in axial thickness. The fracture load values with each of the 2 types of resin cement used were significantly higher than that with polycarboxylate cement (p<0.01). These results suggest that the fracture load values of CAD/CAM crowns are not influenced by differences in the axial thickness of the crown, and that they are higher when bonding is achieved with resin rather than polycarboxylate cement.

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.

Effect of Connector Design on Fracture Resistance in Zirconia-based Fixed Partial Dentures for Upper Anterior Region.

The purpose of this study was to investigate the influence of the cross-sectional form and area of the connector on fracture resistance in three-unit zirconia fixed partial denture (FPD) frameworks for the upper anterior region. Sixty FPD framework specimens were fabricated using the CAD/CAM system. The cross-sectional form (Type I, II, or III) and area (9.0, 7.0, 5.0, or 3.0 mm(2)) of the connectors differed. The specimens were fixed to a jig capable of applying a load axially to the abutment teeth at an angle of 135 degrees. Each specimen was subjected to fracture load measurements using a universal testing machine and cross-sectional microscopic examination. Fracture load fell significantly with a decrease in cross-sectional area (p <0.01). In terms of cross-sectional form, an isosceles triangle with a gingival base yielded the highest fracture load. These results suggest that the connector of a three-unit zirconia-based FPD framework for the upper anterior region should be triangular, have a gingival base, sufficient height in the loading direction, and a cross-sectional area of >5.0 mm(2).

Basic Finite Element Analysis of Para-periodontal Ligament in All-ceramic Zirconia Fixed Partial Denture.

The purpose of the present study was to investigate the validity of incorporating a para-periodontal ligament in the test mold used in a basic fracture test of a zirconia all-ceramic fixed partial denture (FPD). A simplified three-dimensional finite element analysis model was designed based on the three-unit FPD fracture test. Two types of model, one with and one without a para-periodontal ligament between the abutment and base mold, were fabricated. Microfocus CT of the missing first molar area in a dry human mandible was performed. A three-dimensional model was then fabricated based on the data obtained. A load of 600 N was applied to the center of the pontic and stress distribution observed. The model with the para-periodontal ligament showed stress dispersion to the dental root with rotation of the abutment mold. Stress distribution in the finite element analysis model with a para-periodontal ligament showed greater similarity with that in the mandibular model than with that in the other two models without a para-periodontal ligament.

Effect of connector design on fracture resistance in all-ceramic fixed partial dentures for mandibular incisor region.

Yttrium tetragonal zirconia polycrystal frameworks were prepared for all-ceramic fixed partial dentures (FPDs) for the mandibular incisor region. The effects of the cross-sectional area and morphology of the connector on its strength were evaluated by fracture tests. Nine types of zirconia framework for a 3-unit FPD for a defect of 1 mandibular central incisor were prepared, each differing in cross-sectional area and morphology. Fracture tests were performed by loading until fracture using a universal testing machine. Fracture load was determined and fracture site examined. Significant differences were observed in fracture load according to the morphology and cross-sectional area of the connector (p<0.05, p<0.01, p<0.001). Fracture load differed significantly among all groups according to cross-sectional area, and was also greater when the shape of the connector formed an isosceles triangle widest at the base and the connector had the same height and width. These values still far exceeded 311 N, however, which is the average occlusal force in the incisor region. The results of this study suggest that connector design affects fracture load.

Two cases of prostheses for flap protection after buccal mucosal incision.

Here we describe two patients in whom prostheses were applied for flap protection after buccal mucosal incision. In the first case, the patient was a 65-year-old man with a diagnosis of buccal mucosa squamous cell carcinoma (T2N0M0). Left buccal mucosa squamous cell tumor resection and dermoplasty were performed, followed by alveolar ridge augmentation and buccal mucosal graft in the scar area. The carcinoma recurred, however, and left buccal mucosa carcinoma resection was performed, followed by reconstruction surgery using a free forearm flap. After a 12-week healing period, a molar support was constructed on the unaffected side and a protective prosthesis placed on the affected side. Training in ingestion and swallowing were given postoperatively. The patient in the second case was a 62-year-old woman with a diagnosis of buccal mucosa squamous cell carcinoma (T2N1M0). Right buccal mucosa carcinoma resection and supraomohyoid neck dissection were performed, followed by reconstruction surgery using a free forearm flap. A molar support was constructed on the unaffected side and a protective prosthesis placed on the affected side at 5 months postoperatively. Training was given in ingestion and swallowing postoperatively. The prostheses prevented bite wounds to the flaps in the affected areas due to jaw movement during swallowing or speaking. The postoperative courses were uneventful, and the average masticatory score was 92.5 (85, 100), not affecting daily life. The prostheses were placed after reconstruction surgery using free flaps after buccal mucosa squamous cell carcinoma resection. Dysphasia recovered to the preoperative level by dysphasia and pronunciation training in both cases. The postoperative prognosis was favorable, with the prosthesis preventing damage to the flap.

Effect of framework design on fracture resistance in zirconia 4-unit all-ceramic fixed partial dentures.

The purpose of this study was to conduct static load-bearing tests on 4-unit Y-TZP all-ceramic fixed partial denture (FPD) frameworks with different cross-sectional areas and forms to evaluate the influence of connector design on fracture load. Each of the central, mesial and distal connectors was prepared with one of 2 different cross-sectional areas and one of 3 different forms (one circular and two oval forms) to give a total of 18 designs. Five frameworks were then prepared for each design, making a total of 90. Each framework was cemented to the test model with glass ionomer cement. Fracture load was measured with a universal testing machine at a cross-head speed of 1.0 mm/min. A three-way ANOVA revealed significant differences in fracture load depending on cross-sectional area, central connector cross-sectional form, or mesial/distal connector cross-sectional form (p<0.01). No interaction was observed, however, between any two connector design elements. The results of a Tukey analysis revealed a significant difference between the two connector cross-sectional areas investigated, with an increase in connector cross-sectional area resulting in an increase in fracture load. Fracture load decreased as the height of the mesial or distal connector decreased. Fracture load was significantly higher in frameworks in which the height of the central connector was greater than that of the distal or mesial connector. In conclusion, these results suggest that sufficient height needs to be maintained in the mesial/distal connector to secure a high fracture load in zirconia 4-unit all-ceramic FPDs. Moreover, even when this is not possible, a high fracture load may still be obtained by making the height of the central connector as great as possible. Furthermore, extending the connector cross-sectional area is effective in increasing fracture load.

Occlusal treatment with bite splint improves dyskinesia in Parkinson's disease patient: a case report.

The patient was a 46-year-old woman requesting oral hygiene control. She had been treated for Parkinson's disease at the neurology department of another university hospital for 9 years. When the drugs were effective (drug efficacy), she could get out of bed and change clothes without assistance, albeit slowly. When the drugs were ineffective (no drug efficacy), however, she found it difficult to get out of bed, change clothes, or maintain posture during defecation without assistance. Occlusion was B-1 on the Eichner index, and neither dislocation of the temporomandibular joint nor mandibular tremor was observed. At her first visit, a medical history was taken and periodontal therapy commenced. Informed consent for bite splint therapy was obtained after examination of movement disorder. A bite splint was made for her, and any change in the grade of motor disorder using the bite splint evaluated. In addition, her grip strength was measured when wearing and not wearing the bite splint during periods of drug efficacy and no drug efficacy. The patient could get out of bed, change clothes, and maintain posture during defecation without assistance when wearing the bite splint, even during no drug efficacy. Grip strength in her left hand during drug efficacy and in both hands during no drug efficacy was greater when wearing the bite splint than without the bite splint. When this patient with Parkinson's disease wore a bite splint, her athletic ability and grip strength increased.

Time course analysis of influence of food hardness on head posture and pitching of head during masticatory movement.

The purpose of the present study was to investigate the relationship between mastication and head posture using foods with different degrees of hardness. A total of 12 healthy, dentulous volunteers participated in the study. Each participant was required to chew two types of gummy candy with two levels of hardness while sitting upright. Measurements were conducted using an optoelectric jaw-tracking system with 6 degrees of freedom (Gnatho-Hexagraph II JM-2000®). The horizontal plane perpendicular to the direction of gravitational force served as the reference plane. Analysis of the gradient of the Frankfurt plane (head posture) and pitching of the head during masticatory movement was conducted. The influence of the type of test food on these parameters was evaluated during mastication. During stable mastication, the gradient of the Frankfurt plane was 4.66 degrees on average, close to the horizontal plane. The time course of the Frankfurt plane gradient revealed a tendency toward dorsal flexion during the first to middle phases of mastication, and a tendency toward ventral flexion during the middle to last phases, regardless of the hardness of the test food. The participants were divided into two groups based on change in head posture during chewing. The results showed while there was no change in head posture in the group with marked pitching of the head, head posture did change in the group with little pitching.

Two Cases of Idiopathic Condylar Resorption Due to Temporomandibular Joint (TMJ) Osteoarthritis Remodeled by Different Treatment Modalities.

Condylar resorption is a condition of progressive and significant mandibular head resorption. We treated two patients with condylar resorption caused by temporomandibular joint (TMJ) osteoarthritis. The first patient was a 22-year-old female at the time of the initial examination. She had a history of orthodontic treatment and came to the clinic with a number of symptoms, including difficulty opening the mouth. Idiopathic condylar resorption was diagnosed, an occlusal splint was placed for approximately one year, and the patient was followed up for 12 years. The second patient was a 20-year-old female who had completed non-extraction orthodontic treatment by the age of 17 years. She came to the clinic with esthetic issues, such as an anterior open bite. Implant anchor screws were placed in the alveolar bone of the anterior teeth and intermaxillary fixation was performed. Case 1 had 12 years of follow-up, with CT scan results showing remodeling of the condylar. Case 2 is expected to shorten treatment time. However, the CT scan showed remodeling and improvement in the chief complaint.

Sintering distortion of monolithic zirconia in 4-unit fixed partial denture: Effect of layered structure and vertical milling area.

Layered-type zirconia disks, which offer color gradation from enamel to cervical shade, have been employed in recent years to replicate the shades of natural teeth. The layered structure is effective at replicating colors and has helped popularize monolithic zirconia restorations. However, the sintering shrinkage of zirconia is very large; thus, controlling the sintering distortion is very important. Thus, the objective of this study was to determine the influence exerted by the layered structure of the zirconia disk and the vertical milling area on the sintering distortion. An experimental fixed partial denture (FPD) was designed based on a 4-unit monolithic zirconia FPD. A single-composition (SC)-type disk with no shade and a single-composition-layered (SCL)-type disk with shade gradation were selected for this study. In particular, three milling areas, the top end of the disk (area I), vertical center (area II), and bottom end of the disk (area III), were investigated. Moreover, the sintering distortions generated by the experimental FPDs were measured. Results showed that sintering distortion in 4-unit monolithic zirconia FPDs occurred in all SC and SCL areas. Additionally, the sintering distortions were affected by the layered structure of the zirconia disks, the degree of which depended on the milling area (area I > area II > area III). Thus, when fabricating dental prosthesis using SCL zirconia disks, the milling area must be selected considering both the color adjustment and sintering distortion.

Effect of connector design on fracture resistance of zirconia all-ceramic fixed partial dentures.

The purpose of the present study was to determine the relationship between cross-sectional design and fracture load using a static load bearing test in yttria-stabilized tetragonal zirconia polycrystal ceramic frameworks on a molar fixed partial denture. The test framework was designed as a 3-unit bridge with two abutment teeth at the second premolar and second molar of the mandible. The cross-sectional area of the connector was 9.0, 7.0, or 5.0mm(2). In terms of shape, the cross-section was either circular or oval, with a height/width ratio of 1:1, 3:4, or 2:3. For each of the 9 combinations of cross-sectional area and shape, 5 frameworks were prepared (45 in total). Frameworks were cemented to a metallic test model with adhesive resin cement. After fracture load was measured, the percentage of fracture sites was determined and the fracture surfaces observed. In terms of cross-sectional area, there was a statistically significant difference in fracture load between 9.0, 7.0, and 5.0mm(2). No significant difference in fracture load was observed between any two shapes of connector (p>0.05). The fracture load of all frameworks with a cross-sectional area of 9.0 or 7.0mm(2) was over 880 N, which was recognized as parafunctional occlusal force. Fracture occurred at the distal connector in 82.2% of all frameworks on average. Fracture load decreased as cross-sectional area of the connector became smaller. The cross-sectional shape used in the present study was less influential on fracture load. It appears to be clinical possible to apply a connector with a cross-sectional area of 7.0mm(2). Fracture often occurred at the distal connector between the pontic and the abutment, corresponding to the second molar.

Effect of occlusal groove on abutment, crown thickness, and cement-type on fracture load of monolithic zirconia crowns.

The aim of this study was to investigate the effects of occlusal form of abutment, occlusal thickness of monolithic zirconia crowns (MZC), and cement type on the fracture load of MZC. Abutments were prepared with 2 types of occlusal forms: groove-type and flat-type. These were designed so that thickness at the central fissure region of MZC was 0.3, 0.5, or 0.7 mm. Glass ionomer cement and resin cement were used to lute MZC to their corresponding abutment. Fracture load was determined using a universal testing machine. As a result, groove-type abutment had lower fracture load compared to flat-type abutment; however, the decline in strength was smaller when resin cement was used. Additionally, specimens with larger occlusal thickness had greater fracture load regardless of groove or cement-type. The fracture of MZC occurred on the central fissure region of MZC except for 0.7 mm groove-type MZC luted with resin cement.

Stress distribution in maxillary alveolar ridge according to finite element analysis using micro-CT.

The purpose of the present study was to evaluate stress distribution by finite element analysis in an accurate model simulating trabecular bone using micro-CT. Dentulous and edentulous maxillary jaws of Japanese adult cadavers were used (5 sides each; total, 10 sides). Imaging was performed using a micro-CT, followed by reconstruction with 3-D images. Finite element analysis models were developed using the maxilla with average bone morphometry. A load corresponding to occlusal force was applied in different loading conditions, followed by evaluation of stress distribution. In dentulous maxillas, a load was applied in the dental axis direction to the first molar crown (LD). In edentulous maxillas, a load was applied directly to a circular area 4mm in diameter (LER0) to a cylinder 4mm in diameter and 10mm in height (LER10) corresponding to the first molar area. Stress was concentrated in cortical bone around the first molar, trabecular bone and cortical bone at the maxillary sinus base in LD, cortical bone of the alveolar ridge in LER0, and trabecular bone around the cylinder and cortical bone at the maxillary sinus base in LER10. LER0 showed a stress distribution markedly different from that in LD. Compared with LER0, LER10 showed a stress distribution close to that in LD. A model simulating trabecular bone allows a more accurate evaluation of stress distribution.