OBJECTIVES: Through the investigation of the microhardness and microstructure of permanent tooth enamel at various eruption stages during childhood, this research offers references for the early prevention of childhood dental caries. METHODS: Forty-five premolars extracted due to orthodontic reasons were collected and screened. These premolars were divided into three experimental groups according to the time since eruption: Group A (erupted for 0-1 year), Group B (erupted for 1-3 years), and Group C (erupted for 3-5 years). Additionally, the third molars that were extracted due to impaction and had not erupted were selected as the control group, with 15 teeth in each group. Samples were prepared, and the surface microhardness, microstructure, and elemental composition of the enamel were measured using Vickers microhardness tester, scanning electron microscope, and electron probe, respectively. RESULTS: Compared with that in the control group, the microhardness of enamel in groups A, B, and C increased with prolonged eruption time, the surface porosity structure decreased considerably, the contents of Na and Mg on the surface decreased, and that of F increased (P<0.05). CONCLUSIONS The microhardness and microstructure of enamel in permanent teeth at different stages vary. Permanent teeth are at a substantially higher risk of caries within one year after eruption, and early prevention should be emphasized.
Dental Enamel/ultrastructure* ; Humans ; Hardness ; Dental Caries/prevention & control* ; Microscopy, Electron, Scanning ; Tooth Eruption ; Bicuspid/chemistry* ; Dentition, Permanent ; Child ; Child, Preschool

OBJECTIVES: This study aimed to remove occlusal veneers of varied thicknesses and compositions by Er:Yag laser in vitro and analyze the interfacial microstructure between veneers and tooth that irradiated by laser, by which experimental evidence could be provided to support the non-invasive removal of occlusal veneerby laser. METHODS: Fresh mandibular premolars extracted for orthodontic requirements were collected for tooth preparation. Three kinds of ceramic materials (Vita Suprinity, Vita Mark Ⅱ, and Upcera Hyramic) were selected to fabricate occlusal veneer with different thicknesses (1.0, 1.5, and 2.0 mm). One week later, Er:Yag laser (2.5 W and 3.5 W) was used to irradiate and remove the occlusal veneer and recorded the timespan. After the removal operation, the micro-morphologies of samples were examined by scanning electron microscope. RESULTS: Upcera Hyramic veneer failed to be removed (>20 min); the operation span at 2.5 W, Vita Suprinity (96.0 s±16.0 s) was longer than Vita MarkⅡ(84.5 s±19.5 s) in the 1.0 mm group (P<0.05), and Vita Suprinity (246.5 s±13.5 s) was longer than Vita MarkⅡ(170.0 s±14.0 s) in the 1.5 mm group (P<0.05). At 3.5 W, Vita Suprinity (381.0 s±24.0 s) was longer than Vita MarkⅡ(341.5 s±26.5 s) in the 2.0 mm group. CONCLUSIONS Increasing laser power could shorten the operation span and facilitate the removal of occlusal veneers with the same thickness and composition. The occlusal veneer was sustained when insufficient laser power was applied. With the same laser power and ceramic thickness, laser penetration could interfere with the integral of the ceramic structure when the laser interacted with the bonding layer. With the same ceramic composition and laser power, the operation span and laser power increased with the thickness of the occlusal veneer. However, the laser was incapable of removing occlusal resin veneer directly.
Lasers, Solid-State ; Materials Testing ; Dental Porcelain/chemistry* ; Ceramics/chemistry* ; Bicuspid ; Dental Veneers

OBJECTIVEThis study aims to evaluate the influence of different bone types on the stress distribution in tooth implant-supported fixed partial prostheses by using finite element (FE) analysis.

METHODSFour FE models of mandibular arch containing one implant splinted to the mandibular second premolar were built according to bone types I, II, III, and IV. Dynamic loads of 250 N were applied to the buccal and lingual cusps of the prostheses in different directions to simulate the masticatory cycle. The maximum Von Mises stresses were calculated using the FE analysis software.

RESULTSThe maximum Von Mises stresses of the cortical bones were 89.229, 91.860, 125.840, and 158.420 MPa, increasing from type I to type IV, respectively. The maximum Von Mises stresses of the trabecular bone were 58.584, 43.645, 21.688, and 18.249 MPa, decreasing from type I to type IV, respectively. During the process of dynamic loading, the maximum Von Mises stresses of the cortical and trabecular bones followed the order buccal to tongue loading>tongue to buccal loading>vertical loading.

CONCLUSIONThe results showed that bone type significantly influenced the stress distribution in bones, and that for tooth implant-supported fixed partial prostheses, bone types I and II were a better choice than bone types III and IV. More caution should be exercised when restoring missing teeth using tooth implant-supported fixed partial prostheses in softer bone regions.

Bicuspid ; Bone and Bones ; chemistry ; Dental Implants ; Dental Prosthesis, Implant-Supported ; Dental Stress Analysis ; Denture, Partial, Fixed ; Finite Element Analysis ; Humans ; Stress, Mechanical

OBJEVTIVETo evaluate the effect of different restoration methods on fracture resistance of endodontically treated teeth.

METHODSFifty intact extracted maxillary first premolars were randomly divided into 5 groups. Medial-occlusal cavity models were established in all the test groups (B-E) according to the same standard, followed by treatments with defect exposure only, defect filling with light cured composite resin, indirect resin inlays, or light cured composite resin combined with Biosplint fiber. Each specimen was tested using a universal test machine at 1.00 mm/min until fracture and the fracture load was recorded. The load angle was 45 degree to the long axis of the teeth, and the load was pointed to the middle of the lingual surface on the buccal cusp. The fracture resistance was analyzed statistically.

RESULTSThe mean load to cause fracture of the samples in each group group A to E was 1.27∓0.41, 0.23∓0.17, 0.55∓0.31, 0.89∓0.40, and 0.98∓0.34 kN, respectively, showing significant differences between the groups.

CONCLUSIONThe fracture resistance of the teeth is reduced after endodontic therapy, but can be increased significantly by restoration with composite resin inlay or light cured composite resin combined with Biosplint fiber.

Bicuspid ; physiopathology ; Composite Resins ; chemistry ; Dental Materials ; chemistry ; Dental Restoration, Permanent ; methods ; Dental Stress Analysis ; Humans ; Inlays ; Maxilla ; Tooth Fractures ; physiopathology ; Tooth, Nonvital ; physiopathology ; therapy

To investigate whether masticatory fatigue affects the fracture resistance and pattern of lower premolars restored with quartz-fiber post-core and full crown, 44 single rooted lower premolars recently extracted from orthodontic patients were divided into two groups of 22 each. The crowns of all teeth were removed and endodontically treated and then restored with quartz-fiber post-core and full crown. Twenty-two teeth in one group were selected randomly and circularly loaded at 45° to the long axis of the teeth of 127.4 N at a 6 Hz frequency, and the other group was not delivered to cyclic loading and considered as control. Subsequently, all teeth in two groups were continually loaded to fail at 45° to the long axis of the teeth at a crosshead speed of 1 mm⋅min(-1). The mean destructive force values were (733.88±254.99) and (869.14±280.26) N for the experimental and the control group, respectively, and no statistically significant differences were found between two groups (P>0.05). Bevel fracture and horizontal fracture in the neck of root were the major fracture mode of the specimens. Under the circumstances of this study, it seems that cyclic loading does not affect the fracture strength and pattern of the quartz-fiber post-core-crown complex.
Acid Etching, Dental ; methods ; Adult ; Bicuspid ; Bite Force ; Chromium Alloys ; chemistry ; Crowns ; Dental Prosthesis Design ; Dental Restoration Failure ; Dental Stress Analysis ; instrumentation ; Humans ; Materials Testing ; Methacrylates ; chemistry ; Phosphoric Acids ; chemistry ; Post and Core Technique ; instrumentation ; Quartz ; chemistry ; Resin Cements ; chemistry ; Stress, Mechanical ; Tooth Fractures ; physiopathology ; Tooth Root ; injuries ; Tooth, Nonvital ; rehabilitation

OBJECTIVETo evaluate the effect of carbamide peroxide (CP) bleaching agents at different concentrations and with different carriers on the micro-leakage of composite resin interface.

METHODSClass V cavity (2 mm in diameter and 2 mm in depth) preparations were made at the enamelo-cemental junction on the buccal and lingual surfaces of 35 extracted human premolars. The cavities were filled with hybrid composite resin. The teeth were stored for 24 h in distilled water at 37 degrees celsius; before thermocyling for 500 times between 5 and 55 degrees celsius;. The teeth were then randomly assigned into 7 groups, and in groups 1-6, the bleaching gels containing 10% or 20% of CP were applied on the buccal and lingual surface of the teeth for two weeks (6-8 h/day, 37 degrees celsius;, 100% relative humidity) using Carbopol, PVP or Poloxamer as the thickening carriers, respectively. The seventh group served as the control without bleaching treatment. Nail polish was applied to the surface of the tooth, and all the teeth were immersed in ammoniacal silver nitrate solution followed by developing solution. The teeth were finally sectioned through the midline of the restoration and observed under stereomicroscope. SEM micrographs were also made to observe the interface.

RESULTSWith the same bleaching agent, the micro-leakage in the gingival wall was slightly greater than in the occlusive wall, but the difference was not significant. Only 20% CP with Poloxamer as the thickening agent significantly increased the leakage of dentine-resin composite interface, and 10% and 20% CP with Carbopol or PVP as the thickening agents and 10% CP with Poloxamer produced minimal effects on filling the micro-leakage.

CONCLUSIONThickening carriers and the concentration of CP (20% or below) have no significant effect on micro-leakage of composite resin.

Bicuspid ; Bleaching Agents ; chemistry ; Composite Resins ; chemistry ; Dental Caries ; therapy ; Dental Leakage ; chemically induced ; Dental Restoration, Permanent ; methods ; Humans ; Peroxides ; chemistry ; Tooth Bleaching ; adverse effects ; Urea ; analogs & derivatives ; chemistry