Relationship between occlusal curvatures and bite force in humans

PurposeIt is clinically believed that humans with a broad and rounded dental arch accompanied by a shallow curve of Spee have good masticatory function. However, the relationship between them has not been elucidated fully. The purpose of this study was to quantitatively examine the relationship between occlusal curvatures and bite force.Materials and methodsBite force was measured and analyzed in 20 young adults with healthy permanent dentition and normal stomatognathic function using a bite force recording system. Study models of the mandibular dental arch were scanned with CCD cameras, and X, Y and Z coordinates of the cusp tips of the molars were recorded. Their occlusal curvatures were analyzed using a three-dimensional measurement system.ResultsBite force showed significant positive correlations (0.52 < R < 0.57) with the radius of the anteroposterior occlusal arc of the molar region and the radii of the translateral occlusal arcs in the second premolar, first molar and second molar regions, and showed significant positive correlation (R = 0.71) with the radius of the occlusal sphere of the molar region.ConclusionsIt was suggested that occlusal curvatures is related with the magnitude of bite force, and that the three dimensional spherical analysis of occlusal curvatures is useful predictor of the masticatory function.     

Mandibular dental changes following serial and late extraction of mandibular second premolars

Objectives:To determine changes in occlusal curves and dental tipping occurring from mandibular second premolar serial extraction, early extraction of deciduous mandibular second molars with missing second premolars, and late second premolar extraction compared with untreated controls.Materials and Methods:Information was collected from 85 subjects at three time points: T0, prior to serial extraction; T1, after serial extraction and drift prior to orthodontic treatment, and pretreatment for the late premolar extraction patients; and T2, posttreatment. Untreated age- and gender-matched controls were used for comparison. Three occlusal curves were measured on digitized mandibular casts, and dental tipping was assessed using lateral cephalograms.Results:At T0, there were no significant differences among groups. At T1, there was significant steepening of Monson''s sphere and the curve of Wilson between early and late extraction and control groups. At T2, the differences in Monson''s sphere and the curve of Wilson were fully corrected. At T1, there were significant differences in the tipping of mandibular 6''s, 4''s, and 3''s between the early extraction groups compared with the late extraction and control groups. At T2, these differences in tipping were fully corrected. There were no differences in mandibular incisor tipping between groups at T1 or T2.Conclusions:Serial extraction produced steeper occlusal curves and significant tipping of mandibular first molars, first premolars, and canines after extraction and physiologic drift (T1). Accentuated occlusal curves and tooth tipping were fully corrected following orthodontic treatment (T2). Mandibular incisor position was unchanged by serial or late second premolar extraction.     

Monolithic crowns fracture analysis: The effect of material properties,cusp angle and crown thickness

ObjectivesThis study aimed to investigate the collective influence of material properties and design parameters on the fracture behavior of monolithic dental crowns.MethodsThree-dimensional (3D) models (N = 90) with different combinations of design parameters (thickness, cusp angle and occlusal notch geometry) and material type (lithium disilicate, feldspar ceramic, zirconia, hybrid resin ceramic and hybrid polymer-infiltrated ceramic) were developed for the failure analysis using extended finite element method (XFEM) to identify the stress distribution, crack initiation load, fracture surface area and fracture pattern. Analytical formulation, in vitro fracture tests and fractographic analysis of dedicated models were also performed to validate the findings of the XFEM simulation.ResultsFor all material types considered, crowns with a sharp occlusal notch design had a significantly lower fracture resistance against occlusal loading. In most of the models, greater crown thickness and cusp angle resulted in a higher crack initiation load. However, the effect of cusp angle was dominant when the angle was in the low range of 50° for which increasing thickness did not enhance the crack initiation load.SignificanceComparing the critical load of crack initiation for different models with the maximum biting force revealed that for the studied monolithic materials excluding zirconia, a design with a rounded occlusal notch, 70° cusp angle and medium thickness (1.5 mm occlusal) is an optimum combination of design parameters in terms of tooth conservation and fracture resistance. Zirconia crowns exhibited sufficient strength for a more conservative design with less thickness (1.05 mm occlusal) and sharper cusp angle (60°).     

Effect of honey in preventing gingivitis and dental caries in patients undergoing orthodontic treatment

ObjectivesThis study was conducted to investigate the following: (1) the effects of chewing honey on plaque formation in orthodontic patients, (2) the effect of chewing honey on dental plaque bacterial counts, (3) determine if honey possesses antibacterial effects on bacteria recovered from plaques.MethodsFemale orthodontic patients (n = 20, 12–18 years of age) participated in this randomized controlled study. The effects of honey were compared to treatment with either 10% sucrose or 10% sorbitol that served as positive and negative controls, respectively. The pH of plaque was measured using a digital pH meter prior to baseline and at 2, 5, 10, 20, and 30 min after chewing honey or rinsing with control solutions and the numbers of Streptococcus mutans, Lactobacilli, and Prophymonas gingivalis in respective plaques were determined. The antibacterial activity of honey was tested against commonly used antibiotics using the disk diffusion method.ResultsSignificant differences in pH were observed in the honey and sucrose groups compared to the pH observed in the sorbitol group (p ? 0.001). The maximum pH drop occurred at 5 min in both the honey and sucrose groups; however the pH in the honey group rapidly recovered 10–20 min after exposure and did not drop below the critical decalcification pH of 5.5. On the other hand, the pH following sucrose exposure fell <5.5 and was associated with a 30 min recovery time. The pH observed for the sorbitol group did not change over time. Bacterial counts were significantly reduced in the honey group compared to the other treatment groups (p ? 0.001) and honey significantly inhibited the growth of all studied strains compared to inhibition observed with antibiotics (p ? 0.001).ConclusionsHoney can be used as an alternative to traditional remedies for the prevention of dental caries and gingivitis following orthodontic treatment.     

Two-year follow-up of changes in bite force and occlusal contact area after intraoral vertical ramus osteotomy with and without Le Fort I osteotomy

This study was performed to examine the longitudinal changes in bite force and occlusal contact area after mandibular setback surgery via intraoral vertical ramus osteotomy (IVRO). Patients with mandibular prognathism who underwent IVRO (surgical group: 39 men and 39 women) were compared with subjects with class I skeletal and dental relationships (control group; 32 men and 35 women). The surgical group was divided into two subgroups: 1-jaw surgery (n = 30) and 2-jaw surgery (n = 48). Bite force and contact area were measured in maximum intercuspation with the Dental Prescale System before treatment, within 1 month before surgery, and at 1, 3, 6, 9, 12, and 24 months postsurgery. A linear mixed model was used to investigate the time-dependent changes and associated factors. Bite force and contact area decreased during presurgical orthodontic treatment, were minimal at 1 month postsurgery, and increased gradually thereafter. The 1-jaw and 2-jaw subgroups showed no significant differences in bite force. The time-dependent changes in bite force were significantly different according to the contact area (P < 0.05). The results of this study suggest that bite force and occlusal contact area gradually increase throughout the postsurgical evaluation period. Increasing the occlusal contact area may be essential for improving bite force after surgery.     

Fatigue resistance of ultrathin CAD/CAM ceramic and nanoceramic composite occlusal veneers

ObjectiveThe fracture resistance of different ultrathin occlusal computer-aided design/computer-aided manufacturing (CAD/CAM) veneers was investigated under cyclic mechanical loading to restore combined enamel-dentin defects.MethodsEighty-four molars were reduced occlusally until extensive dentin exposure occurred with a remaining enamel ring. Twenty-four molars were ground flat for examination of highly standardized specimens, of which 8 were treated with uniformly flat 0.3 mm IPS Empress CAD and 0.3 and 0.5 mm IPS e.max CAD restorations. Sixty-four molars were anatomically prepared until dentin exposure and were restored using occlusal veneers with fissure/cusp thicknesses of 0.3/0.5 mm from 3 different dental CAD/CAM materials: IPS Empress CAD, IPS e.max CAD and Lava Ultimate CAD/CAM. Teeth were etched with 37% phosphoric acid, and occlusal veneers were bonded using an adhesive luting system (Syntac Primer, Adhesive, Heliobond and Variolink II). Specimens were placed under cyclic mechanical loading in a chewing simulator (1 million cycles at 50 N) and were examined for cracks after each cyclic loading sequence. The anatomical 0.3/0.5 mm IPS e.max CAD specimens experienced an additional 1 million cycles at 100 N. Kaplan–Meier survival curves and log-rank tests were used for data analysis.ResultsAll highly standardized and 0.3/0.5 mm IPS e.max CAD specimens tolerated cyclic loading. One anatomical Lava Ultimate CAD/CAM and 10 IPS Empress CAD specimens showed cracks.SignificanceUltrathin occlusal veneers of lithium disilicate ceramic and nanoceramic composite showed remarkably high fracture strength under cyclic mechanical loading. These veneers might be a tooth substance preserving option for restoring combined dentin–enamel defects.     

Long-term clinical evaluation of direct resin composite restorations in vital vs. endodontically treated posterior teeth — Retrospective study up to 13 years

ObjectivesThis retrospective study evaluated and compared the survival rate of Class II posterior direct resin based composite (RBC) restorations made in vital teeth (VT) and endodontically treated teeth (ETT). The influence of risk factors on the long-term performance of restorations was also investigated.MethodsPatients (n = 245) receiving RBC posterior restorations between 2004 and 2012 were selected. A total of 597 restorations (485 in VT, 112 in ETT) with minimum 2.5–3 mm remaining cusp thickness, made with the same brand of RBC and adhesive, were evaluated using the USPHS criteria. Data were analyzed with Mann–Whitney, Chi-square and Fisher’s Exact Test, Extended Cox-regression and Kaplan–Meier analysis (p < 0.05). Relative risk ratio was estimated for each evaluated parameter.ResultsThe mean observation period was 8.6 ± 2.3 years. An annual failure rate in VT and ETT of 0.08% and 1.78%, respectively, was detected. The reasons of failures included restoration fracture, secondary caries in VT; vertical root fracture, cusp fracture, restoration fracture, secondary caries and loss of adhesion in ETT. Significantly better performance was observed in RBCs of VT for each evaluated parameter. Among the evaluated risk factors only occlusal stress affected negatively the survival of RBC in ETT (Hazard Ratio 37.1; CI_95% 8.4–163.7).SignificanceAlthough, there is significant difference in the success rate of RBCs in VT (98.97%) and ETT (76.8%), the long-term (6–13 years) durability of Class II RBCs with 2.5–3 mm cusp thickness in ETT is also clinically acceptable. The presence of occlusal stress decreases the survival of RBCs in ETT.     

“A comparative evaluation between single noncompression titanium miniplate and three dimensional titanium miniplate in treatment of mandibular angle fracture” – A randomized prospective study

PurposeThe aim of this study was to compare efficacy between the single non-compression titanium miniplate and single three-dimensional titanium miniplate in mandibular angle fracture treatment.Method and materialsA prospective study of 20 patients with mandibular angle fractures. Patients were randomly categorized into two groups with 10 patients in each group. Group-I patients were treated with single 2.0 mm conventional titanium miniplate, Group-II patients were treated with single 2.0 mm three-dimensional titanium miniplate according to Champy's principles. Parameters such as stability of fracture fragments, occlusion, mouth opening, additional fixation required and complications were evaluated at different time intervals.ResultsIn Group-I, about 40% (n = 4) of patients showed unstable fracture fragments on immediate postoperative day whereas in Group-II only 10% (n = 1) of patients with fracture instability. In Group-I, 30% (n = 3) had mild occlusal derangement and 20% (n = 2) had deranged occlusion on immediate postoperative day, where as in Group-II only 20% (n = 2) of patients had mild occlusal derangement. Additional fixation required in 30% (n = 3) of patients in group-I, and 10% (n = 1) in Group-II. 20% (n = 2) of patients in Group-I developed infection. All patients in both the groups had inadequate mouth opening on immediate post operative day, later resumed normal mouth opening. 10% (n = 1) in group-I and 20% (n = 2) of patients in group-II presented with postinjury/preoperative inferior alveolar nerve sensory disturbance with no incidence of postoperative sensory disturbance. None of the patients in both the groups had malunion, nonunion, plate fracture, and loosening of plates and screws.Conclusion3-D titanium miniplates showed more favorable results compared to single conventional titanium miniplate with respect to initial interfragmentary stability and complications.     

Digitization of simulated clinical dental impressions: Virtual three-dimensional analysis of exactness

ObjectivesTo compare the exactness of simulated clinical impressions and stone replicas of crown preparations, using digitization and virtual three-dimensional analysis.MethodsThree master dies (mandibular incisor, canine and molar) were prepared for full crowns, mounted in full dental arches in a plane line articulator. Eight impressions were taken using an experimental monophase vinyl polysiloxane-based material. Stone replicas were poured in type IV stone (Vel-Mix Stone; Kerr). The master dies and the stone replicas were digitized in a touch-probe scanner (Procera^® Forte; Nobel Biocare AB) and the impressions in a laser scanner (D250, 3Shape A/S), to create virtual models. The resulting point-clouds from the digitization of the master dies were used as CAD-Reference-Models (CRM). Discrepancies between the points in the pointclouds and the corresponding CRM were measured by a matching-software (CopyCAD 6.504 SP2; Delcam Plc). The distribution of the discrepancies was analyzed and depicted on color-difference maps.ResultsThe discrepancies of the digitized impressions and the stone replicas compared to the CRM were of similar size with a mean ± SD within 40 μm, with the exception of two of the digitized molar impressions. The precision of the digitized impressions and stone replicas did not differ significantly (F = 4.2; p = 0.053). However, the shape affected the digitization (F = 5.4; p = 0.013) and the interaction effect of shape and digitization source (impression or stone replica) was pronounced (F = 28; p < 0.0001). The reliability was high for both digitization methods, evaluated by repeated digitizations.SignificanceThe exactness of the digitized impressions varied with shape. Both impressions and stone replicas can be digitized repeatedly with a high reliability.     

Evaluation of factors affecting the continuing use and patient satisfaction with removable partial dentures over 5 years

PurposeThe purpose of this retrospective study was to investigate factors that affected the continuing use of RPDs and the patients’ satisfaction 5 years after delivery.MethodsSixty-seven patients treated with 90 RPDs delivered at the Tohoku University Hospital (Sendai, Japan) between 1996 and 2001 participated in this study. The patients were re-examined 5 years after delivery. Data were collected from clinical records and a questionnaire was used to evaluate the patients’ delivery use of the RPDs. The RPD treatment was divided into three categories labeled as ‘successful’, ‘remake’, and ‘failure’. Twelve factors that might affect the continuing use and 15 factors regarding satisfaction were evaluated. Stepwise logistic regression analysis was used to assess statistical significance.ResultsFifty-five RPDs were regarded as successful, 21 were remake, and 14 failure. Statistically significant associations were found between the continuing use and the patient's age (P = 0.002), location of the edentulous area (P = 0.047), number of occluding pairs of teeth (P = 0.038), number of occlusal rests (P = 0.038), pain while using RPDs (P = 0.006), color of the artificial teeth (P = 0.021), and tooth shape and set-up (P = 0.022).ConclusionsThese findings suggest that the continuing use of RPDs is related to factors such as the patient's age, location of edentulous area, number of occluding pairs of teeth, and number of occlusal rests, satisfaction including pain while using RPDs, color of the artificial teeth, and tooth shape and set-up.     

Surface roughness of impression materials and dental stones scanned by non-contacting laser profilometry

ObjectiveTo analyze differences in the way dental materials digitize on a non-contacting laser profilometer (NCLP).MethodsThree Type IV dental stones and 15 impression materials were mixed according to the manufacturer's instructions and expressed against a glass block to record its surface characteristics. From each material an area of 6 × 40 mm was scanned on the NCLP and the Ra, Rq and Rt roughness values measured from 20 randomly selected transverse profiles. The surface of the impression materials was subsequently poured in Moonstone? (Bracon Ltd., Etchingham, England) dental stone and the same roughness values obtained from these casts. Differences in roughness values from the dental materials were compared using ANOVA and differences in roughness between impression materials and the Moonstone? casts compared using paired t-tests.ResultsThere were significant differences in roughness values between individual materials within each type (impression material or dental stone) (p < 0.05). The roughness of the dental stones varied between Ra = 0.87 and 0.99 μm, Rq = 1.09 and 1.23 μm, and Rt = 5.70 and 6.51 μm. The roughness values of the impression materials varied between Ra = 0.75 and 4.56 μm; Rq = 0.95 and 6.27 μm and Rt = 4.70 and 39.31 μm. Darker materials showed higher roughness values compared to lighter materials (p < 0.05). The roughness of the Moonstone? casts varied between Ra = 0.80 and 0.98 μm; Rq = 1.01 and 1.22 μm, and Rt = 5.04 and 6.38 μm. Roughness values of some impression materials were statistically significantly lower when the surface was reproduced in Moonstone? (p < 0.01).SignificanceDigitization of dental materials on optical profilometers was affected by color and transparency.     

Three-dimensional analysis of dentolabial relationships: effect of age and sex in healthy dentition

Morphological changes in three-dimensional (3D) dentolabial relationships during ageing were assessed in healthy individuals. 38 subjects with healthy dentition were analysed. They were divided into a youthful group (21–34 years, mean 26 years, SD 4) and an older group (45–65 years, mean 53 years, SD 5). Stone labial and dental models were made, digitized and 3D virtual reproductions of dentolabial morphology were obtained. From the digital reconstructions, the relative positions of the labial commissure and of the maxillary dental clinical crowns in the vertical direction were obtained. Sex and age effects were compared using two-way analysis of variance. Lip position relative to the teeth was significantly different in youthful and older subjects (P < 0.01). No statistically significant effects of sex in dentolabial relationship were demonstrated, but a sex × age effect was found in the anterior labial segments (P < 0.05). The perioral soft tissues drop down in older subjects and the soft tissue descends on the entire labial arch. These differences may help the clinician when estimating, planning and evaluating surgical, orthodontic and prosthetic treatments.     

Digital replacement of the distorted dentition acquired by cone beam computed tomography (CBCT): a pilot study

During cone beam computed tomography (CBCT) scanning, intra-oral metallic objects may produce streak artefacts, which impair the occlusal surface of the teeth. This study aimed to determine the accuracy of replacement of the CBCT dentition with a more accurate dentition and to determine the clinical feasibility of the method. Impressions of the teeth of six cadaveric skulls with unrestored dentitions were taken and acrylic base plates constructed incorporating radiopaque registration markers. Each appliance was fitted to the skull and a CBCT performed. Impressions were taken of the dentition with the devices in situ and dental models were produced. These were CBCT-scanned and the images of the skulls and models imported into computer-aided design/computer-aided manufacturing (CAD/CAM) software and aligned on the registration markers. The occlusal surfaces of each dentition were then replaced with the occlusal image of the corresponding model. The absolute mean distance between the registration markers in the skulls and the dental models was 0.09 ± 0.02 mm, and for the dentition was 0.24 ± 0.09 mm. When the method was applied to patients, the distance between markers was 0.12 ± 0.04 mm for the maxilla and 0.16 ± 0.02 mm for the mandible. It is possible to replace the inaccurate dentition on a CBCT scan using this method and to create a composite skull which is clinically acceptable.     

Fracture strength of CAD/CAM composite and composite-ceramic occlusal veneers

PurposeTo determine the effect of material type and restoration thickness on the fracture strength of posterior occlusal veneers made from computer-milled composite (Paradigm MZ100) and composite-ceramic (Lava Ultimate) materials.Methods60 maxillary molars were prepared and restored with CAD/CAM occlusal veneer restorations fabricated from either Paradigm MZ100 or Lava Ultimate blocks at minimal occlusal thicknesses of 0.3, 0.6, and 1.0 mm. Restorations were adhesively bonded and subjected to vertical compressive loading. The maximum force at fracture and mode of failure were recorded. 2-Way ANOVA was used to identify any statistically significant relationships between fracture strength and material type or thickness. Spearman's rank correlation coefficient was used to analyze mode of failure with regard to fracture strength.ResultsThe average maximum loads (N) at fracture for the Paradigm MZ100 groups were 1620 ± 433, 1830 ± 501, and 2027 ± 704 for the material thicknesses of 0.3, 0.6, and 1.0 mm, respectively. The Lava Ultimate groups fractured at slightly higher loads (N) of 2078 ± 605, 2141 ± 473, and 2115 ± 462 at the respective 0.3, 0.6, and 1.0 mm thickness.Statistical analyses revealed that, while no significant difference existed among the various restoration thicknesses in terms of fracture strength (P > 0.05), the material type was found to be influential (P = 0.04). The maximum load at fracture (N) for Lava Ultimate averaged over all thicknesses (2111 ± 500) was significantly higher than that of the Paradigm MZ100 (1826 ± 564). No correlation between mode of failure and fracture strength was found.ConclusionsUnder the conditions of this study, the maximal loads at fracture for these “non-ceramic” occlusal veneer restorations were found to be higher than human masticatory forces. Occlusal veneers made from the two materials tested are likely to survive occlusal forces regardless of restoration thickness, with those fabricated from the composite-ceramic hybrid material being more likely to survive heavier loads.     

The viscoelastic behavior of dental adhesives: A nanoindentation study

ObjectivesIn order to predict the long-term performance of dental adhesives, it is necessary to understand their mechanical properties. The objective of this study was to use a new nanoindentation technique to characterize the in-plane linear viscoelastic properties of dental adhesive layers.MethodsThe dental adhesives used were Clearfil SE Bond (CSE) and Clearfil Tri-S Bond (CTS) by Kuraray Medical and Single Bond (SIB) and Single Bond Plus (SBP) by 3M ESPE. A thin film of each adhesive was made on a micro-glass slide, and was then tested on a nanoindenter system (ENT 1100, Elionix) with a Berkovich indenter at a constant loading rate of 0.1 mN/s up to a maximum load of 1.8 mN. The load–displacement data of the loading segment were fitted to a curve to find best fit parameters for a generalized Kelvin viscoelastic model, from which creep compliance and Young's modulus were calculated. The modulus results were compared to the values calculated by the nanoindentation device.ResultsThe experimental data fitted well to the viscoelastic model for all materials (R > 0.9999). SIB and CTS showed higher creep compliance compared to SBP and CSE. The modulus values obtained using the model were 4.0, 2.6, 2.4 and 4.2 GPa for CSE, CTS, SIB and SBP, respectively. The nanoindentation default software designed for time-independent materials significantly overestimated the modulus values up to 2.5 times.ConclusionAs generally expected for polymer materials, the adhesives tested showed time-dependent viscoelastic behavior. The mechanical evaluation techniques developed for time-independent materials ignore this behavior and may not be appropriate for dental adhesives.     

Effects of layering techniques on the micro-tensile bond strength to dentin in resin composite restorations

ObjectivesThe purpose of this work was to investigate the effects of layering techniques in resin composite restorations on the micro-tensile bond strength to the dentin of the occlusal cavity.MethodsHuman premolars were extracted and randomly divided into four groups. The occlusal enamel was then removed to expose a flat superficial dentin surface. Cavities 3.5 mm long and 3.5 mm wide were prepared to a depth 3 mm below the dentin surface. The adhesive Single Bond was applied according to the manufacturer's instructions. The teeth were then restored with Z100 resin composite as follows: Group 1 was restored in horizontal increments (three layers). Groups 2 and 3 were restored in different oblique increments (three layers). Group 4 was restored in oblique increments (four layers). After 24 h storage at 37 °C in water, all the teeth were sectioned to obtain bar-shaped specimens with a bonded surface area of approximately 0.9 mm × 0.9 mm. Dentin micro-tensile bond strength was measured at a crosshead speed of 0.5 mm/min. The results obtained were statistically analyzed using one-way ANOVA and SNK test at a significance level of P = 0.05. All fractures were then observed under a scanning electron microscope (SEM).ResultsThe results showed that there is a significant difference between the strength of the micro-tensile bonds to the dentin of occlusal cavities depending on which of the four layering restorative techniques was used (P < 0.01). SEM observation showed that failure patterns were most evidently interfacial cohesive failure.SignificanceLayering techniques in resin composite restorations affected the micro-tensile bond strength between the resin composite and the dentin. But the outcomes related to only Single Bond, as the same using other adhesives might have different outcomes.     

Indentation size effect of Y-TZP dental ceramics

ObjectiveThe purpose of this study was to investigate and analyze the indentation size effect (ISE) in Vickers hardness of monolithic yttria partially stabilized zirconia (Y-TZP) dental ceramics without and with the addition of dental dye A3. The ISE is analyzed using the Mayer law, a proportional specimen resistance (PSR) model and a modified proportional specimen resistance (MPSR) model.MethodsTwo samples of Y-TZP dental ceramics, trade names BruxZir (provided by Glidewell Laboratories, CA, USA), were investigated. The first sample was polished Y-TZP and the second sample was polished Y-TZP with the addition of dental dye A3, by VITA Classical Shade Guide. The Vickers hardness was measured under the following loads: 0.49 N, 0.98 N, 1.96 N, 4.90, 9.81 N and 29.42 N. Thirty indentations were made on each sample, under each load. Relationships between the applied load, F, and the resulting indentation size, d, have been analyzed by the Mayer law, the PSR model and the MPSR model.ResultsThe Meyer index (n) for both Y-TZP dental ceramics is less than 2, which indicates that hardness is dependent on test loads. The PSR model and the MPSR model were used to calculate “true” Vickers hardness or load-independent hardness.SignificanceAll applied mathematical models are suitable for the data analysis, which is confirmed with high correlation coefficients, but the best correlation between measured values and mathematical models was achieved with the MPSR model with a correlation coefficient of 0.9999.     

Mathematical model for assessing true irradiance received by luting materials while curing through modern CAD/CAM resin composites

Statement of problemMeasurement of irradiance passing through a dental restoration for properly curing a dual- or light-polymerized luting composite is imprecise due to surface reflection.ObjectiveTo provide a mathematical correction of measured transmitted irradiance for predicting true transmitted light intensity through CAD/CAM restorations.MethodsA total of 432 specimens were fabricated. Seven modern CAD/CAM resin-based composites (RBCs) and one CAD/CAM glass-ceramic (control group) were sectioned and polished into specimens of 0.5–5 mm thickness (in 0.5 mm steps, n = 6). Irradiance of a violet-blue LED light curing unit (LCU) (power modes: Standard, High and Plasma) was measured after passing through each specimen with a spectrometer. Data was compared based on 95% confidence intervals and using univariate ANOVA followed by Tukey HSD (α = 0.05).ResultsThe measured transmitted irradiance passing through the specimens decreased exponentially. Significantly highest values of transmitted irradiance were measured for 0.5 mm thick specimens for all materials (p < 0.05). The decadic absorption coefficient for CAD/CAM-RBCs ranged from 0.292 mm⁻¹ to 0.387 mm⁻¹ while the control group (glass-ceramic) reached a significantly lower value of 0.283 mm⁻¹. The reflection ratio for all materials ranged from 12.6% to 18.5%.SignificanceA correction can be implemented to predict the true transmitted irradiance after passing through a dental restoration as function of initial irradiance, specimen thickness and material specific parameters. For a practitioner, this model may be applied depending on the specific treatment conditions, the individual LCU's radiant emittance and restoration thickness for the tested materials.     

The influence of vertical dimension of occlusion changes on the electroencephalograms of complete denture wearers

PurposeThe present study was conducted to identify how changes in the vertical dimension of occlusion (VDO) affect the sensory perception and activity of the brain in complete denture wearers using an electroencephalogram (EEG).MethodsSubjects were 21 individuals wearing complete dentures who regularly visited the Division of Prosthodontics at Tsurumi University Dental Hospital for checkups (12 males and 9 females, average age: 76.6). Based on their original dentures, two duplicate dentures with different VDO (−3 mm and +5 mm) were fabricated. EEG activity and occlusal force were measured before and after gum chewing with each denture in all subjects. Negative indicator scores for psychological conditions and stable neuronal activity (Dα) were calculated using EEG data. Statistical analysis was performed using the Wilcoxon test to compare changes in the sensory perception, activity of the brain, and occlusal force (α = 0.05).ResultsAfter gum chewing with the +5-mm denture, a significant increase was observed in the negative indicator score (p < 0.05). No significant difference was found in the Dα values before and after gum chewing with any of the dentures (p > 0.05). A significant decrease was observed in the occlusal force between the original denture and the −3-mm denture (p < 0.05).ConclusionPsychological condition and occlusal force were influenced by immediate changes in the VDO of the complete denture.     

A cone-beam CT based technique to augment the 3D virtual skull model with a detailed dental surface

Cone-beam computed tomography (CBCT) is used for maxillofacial imaging. 3D virtual planning of orthognathic and facial orthomorphic surgery requires detailed visualisation of the interocclusal relationship. This study aimed to introduce and evaluate the use of a double CBCT scan procedure with a modified wax bite wafer to augment the 3D virtual skull model with a detailed dental surface. The impressions of the dental arches and the wax bite wafer were scanned for ten patient separately using a high resolution standardized CBCT scanning protocol. Surface-based rigid registration using ICP (iterative closest points) was used to fit the virtual models on the wax bite wafer. Automatic rigid point-based registration of the wax bite wafer on the patient scan was performed to implement the digital virtual dental arches into the patient's skull model. Probability error histograms showed errors of ≤0.22 mm (25% percentile), ≤0.44 mm (50% percentile) and ≤1.09 mm (90% percentile) for ICP surface matching. The mean registration error for automatic point-based rigid registration was 0.18 ± 0.10 mm (range 0.13–0.26 mm). The results show the potential for a double CBCT scan procedure with a modified wax bite wafer to set-up a 3D virtual augmented model of the skull with detailed dental surface.