OBJECTIVEThis study was about the influence of porcelain thickness on crack at interface.

METHODSThe effect of porcelain thickness on the flaw at the interface between porcelain and metal was studied in three groups with porcelain thickness of 0.5 mm, 1.5 mm and 2.5 mm (metal thickness of 0.5 mm) by means of moire interferometre and interfacial fracture mechanics. The parameter Jc was compared among the three groups and the growing of the flaw was observed.

RESULTSJc and the extreme strength of group with porcelain thickness of 0.5 mm (2.813 N/m and 9.979 N) were lower than those of the groups with porcelain thickness of 1.5 mm and 2.5 mm (5.395 N/m, 19.134 N and 5.429 N/m, 19.256 N). Flaws extend along the interface in the groups with porcelain thickness of 1.5 mm and 0.5 mm.

CONCLUSIONS(1) Fracture resistance of the interface in the groups with porcelain thickness of 1.5 mm and 2.5 mm is similar and it decreases in the group with 0.5 mm thick porcelain. (2) When porcelain is 1.5 mm or 0.5 mm thick, flaws will extend along the interface. When porcelain is 2.5 mm thick, flaws will extend into the porcelain layer.

OBJECTIVETo study the precise stress distribution of the apical foramen area of endodontic endosseous implant, in order to improve the prosthetics of endodontic endosseous implant.

METHODSAfter analysis of the two-dimensional endodontic endosseous implants model with finite element method, left and right areas beside the apical foramen were selected as infinite domains to calculate. D-N interactive method was used to connect the finite and infinite domains.

RESULTSUnder 45 degrees axial right oblique loading, the stress concentration occurred in both infinite domains of the apical foramen. The infinite domain nearing the load side was tension stress concentration, but the other side was compressive stress concentration. Two stress concentration points were just at the central points, which were intersections between implant and dentin. The stress reduced in all directions from these two stress concentration points, but in the ligament, the result was contrary.

CONCLUSIONSThe change of the tooth rotational center is helpful to the tooth stability and carrying capacity after restoration. In the implant area, the diameter of implant at the apical foramen of root shall not be reduced to protect root in clinical work. It is very important to preserve the tissue of periodontal ligament for endodontic endosseous implants.

PURPOSE: This study is aimed to assess changes of stress distribution dependent on different connection lengths and placement of the fixture top relative to the ridge crest. MATERIALS AND METHODS: The internal-conical connection implant which has a hexagonal anti-rotation index was used for FEM analysis on stress distribution in accordance with connection length of fixture-abutment. Different connection lengths of 2.5 mm, 3.5 mm, and 4.5 mm were designed respectively with the top of the fixture flush with residual ridge crest level, or 2 mm above. Therefore, a total of 6 models were made for the FEM analysis. The load was 170 N and 30-degree tilted. RESULTS: In all cases, the maximum von Mises stress was located adjacent to the top portion of the fixture and ridge crest in the bone. The longer the connection length was, the lower the maximum von Mises stress was in the fixture, abutment, screw and bone. The reduction rate of the maximum von Mises stress depending on increased connection length was greater in the case of the fixture top at 2 mm above the ridge crest versus flush with the ridge crest. CONCLUSION: It was found that the longer the connection length, the lower the maximum von Mises stress appears. Furthermore, it will help prevent mechanical or biological complications of implants.
Dental Implant-Abutment Design ; Dental Stress Analysis ; Finite Element Analysis

OBJECTIVETo investigate the effects of different implant thread locations on stress distribution in implant and bone under concentrated load, and discuss the best design of location of implant thread.

METHODSThe stress in implant-bone interface of implants with different thread locations (model A, B, C, D), which were loaded in axial and oblique 450 to axle, were analyzed by Solidworks 2005 plus and Cosmos/works 7.0 software.

RESULTSThe stresses in cervical cortical bone of implant with model C were lower than others. But under oblique load, the stresses of implant and cancellous bone in model C were higher than model A. The stresses distribution of model B was obviously concentrated, and under axial load, all kinds of stresses in this model were higher than others. The stress distribution in model A and D were more even, the stress concentrated more on implant neck, upper part of cortical bone and the lowest thread on implant. The stresses in interface under oblique load were much higher than those under axial load.

CONCLUSIONThe location of thread has effect on stress distribution in implant-bone interface. More attention should be paid to it when we design implants, and we should try to avoid oblique load in implant prosthodontics.

OBJECTIVETo investigate the effect of three types of veneering porcelain on the bending strength of KAVO Y-TZP/porcelain layered structure.

METHODSKAVO zirconia ceramics were used as the substructure. To form Y-TZP/porcelain bilayered structure, a leucite-based veneering porcelain was fired on the zirconia substructures by slip-casting technique with dentin washbake, and two nano-fluorapatite-based veneering porcelains were fired on the zirconia substructures by either slip-casting or pressed-on technique with or without liner coverage. The bending strength was tested according to ISO 6872 standard, and the veneered surfaces of the fracture samples were analyzed by scanning electron microscopy (SEM).

RESULTSFor covering KAVO zirconia core material, the conventional veneering slurry-porcelain combined with liner or wash firing had significant higher bending strength than pressed-on porcelain. SEM showed that the main failure type at the interface was adhesive failure.

CONCLUSIONThin layer sintering using washbake program or liner on KAVO zirconia surface increases the surface wettability, and this procedure may be indispensable when veneering on the surface of dental zirconia.

OBJECTIVEIn terms of the problems of now available dental machinable ceramics, a new type of calcium-mica glass-ceramic, PMC-I ceramic, was developed, and its machinability was compared with that of Vita MKII quantitatively. Moreover, the relationship between the strength and the machinability of PMC-I ceramic was studied.

METHODSSamples of PMC-I ceramic were divided into four groups according to their nucleation procedures. 600-seconds drilling tests were conducted with high-speed steel tools (Phi = 2.3 mm) to measure the drilling depths of Vita MKII ceramic and PMC-I ceramic, while constant drilling speed of 600 rpm and constant axial load of 39.2 N were used. And the 3-point bending strength of the four groups of PMC-I ceramic were recorded.

RESULTSDrilling depth of Vita MKII was 0.71 mm, while the depths of the four groups of PMC-I ceramic were 0.88 mm, 1.40 mm, 0.40 mm and 0.90 mm, respectively. Group B of PMC-I ceramic showed the largest depth of 1.40 mm and was statistically different from other groups and Vita MKII. And the strength of the four groups of PMC-I ceramic were 137.7, 210.2, 118.0 and 106.0 MPa, respectively.

CONCLUSIONThe machinability of the new developed dental machinable ceramic of PMC-I could meet the need of the clinic.

OBJECTIVETo investigate the influences of varying lingual traction forces on the space-closing speed in a typodont model.

METHODSForty-two Angle Class I standard typodont models of bimaxillary teeth protrusion were divided into 7 equal groups. Four regions of the model were paired to groups, and in the odd-numbered models, the top left and bottom left regions served as the experimental group and the top right and bottom right regions as the control group; in the even-numbered models, the regions in the model were grouped oppositely. In the experimental group, the space was closed by niti wire extension spring in the buccal ridge combined with lingual elastic traction of 0, 5, 10, 15, 20, 25 and 30 g. In the control group, the space was closed by exclusive niti wire extension spring in the buccal ridge. The space-closing speed were analyzed in all the groups.

RESULTSThe space-closing speed was significantly lower in the control group than in the experimental groups with lingual traction forces of 5, 10, 15, 20 and 25 g (P<0.05), but a traction force of 30 g resulted in a significantly lower speed than that in the control group (P<0.05). The space closing speed was the greatest in the experimental group with a traction force of 15 g (P<0.05).

CONCLUSIONNiti wire extension spring in the buccal ridge combined with lingual elastic traction results in faster space-closing speed than traditional exclusive niti wire extension spring. The speed is the fastest by applying 15 g lingual traction, which is also associated with the lowest slip resistance.

OBJECTIVETo investigate the effect of different length and diameters on the stability of mini implant and to select optimal length and diameter using continuous variation of parameters.

METHODSTo perform 3-dimensional finite element analysis, finite element models of a maxilla, and mini implants with length of 6-12 mm and diameters of 1.2-2.0 mm were generated. Load of two different forces were applied to the head of mini implant. One type was horizontal force (HF), the other was composite force (CF). The maximum equivalent stress (Max EQV) in maxilla and the maximum displacement (Max DM) of mini implant were evaluated.

RESULTSThe Max EQV in maxilla and Max DM of mini implant decreased as length and diameter increased. When length was more than 9 mm, the evaluation indexes were small and had a less change. Datas indicated that diameter played a more important role in reducing target, and was a more effective parameter in reducing Max EQV when CF was loaded.

CONCLUSIONFrom biomechanical point of view, the choice of the length should not be more than 9 mm. When CF is loaded using the mini implant, diameter exceeding 1.2 mm are optimal design for mini implant.

OBJECTIVETo evaluate the effect of taper and inserting angle on stability of micro-implants in type II bone and to provide theoretical evidence for clinical selection of taper and inserting angle.

METHODSThree-dimensional finite element models of type II bone sections and micro-implants were fabricated. They were designed using different tapers (diameter between 1.1 to 1.6 mm, taper between 0 to 0.0625) and different inserting angles ranging from 30 degrees to 90 degrees (30 degrees, 45 degrees, 60 degrees, 75 degrees, 90 degrees). Effects on the maximum von Mises stresses in mandible and micro-implants, as well as on the maximum displacements in micro-implants, were analyzed under an applied 2 N horizontal force.

RESULTSThe inserting angle had a significant effect on stresses and displacements, with lower distribution of stresses and displacements occurring at 60 degrees. The effect of the taper was also related to the inserting angle. The minimum stresses were 5.013 4 MPa (cortical bone) and 25.1310 MPa (micro-implant) in model D at 60 degrees. The displacements of micro-implants were proportional to the taper.

CONCLUSIONThe inserting angle has a significant effect on stability of micro-implants, and the recommended inserting angle is 60 degrees. The influence of taper on the stability of micro-implants is related to the inserting angle. The micro-implant with 0.0375 taper (maximum diameter is 1.6 mm, minimum diameter is 1.3 mm) is more suitable for type II bones.

OBJECTIVETo analyze the stress distribution of bone around the dental implants with lateral immediate loading of different angles by three dimensional finite element.

METHODSAn adult edentulous mandible was adopted for CT scanning. The CT image was imported to universal surgical integration system to establish a mandible three dimensional mesh model. The real shape of standard thread implant was simulated and the finite element model of mandible with dental implants for immediate loading was established. The models were immediately loaded with 150 N through the angle of 0 degrees, 10 degrees, 20 degrees, 30 degrees. The ANSYS 10.0 was used to analyze the Von Mises stress on the bone around dental implants.

RESULTSThe accurate finite element model of mandible with dental implants for immediate loading was successfully established. The three dimensional finite element analytical results showed: Under axial load, the Von Mises stress of bone contact surface concentrated on the cortical bone of the implant cervix. The strain distribution was even, and centralized at the cortical bone of the implant cervix, cancellous bone of implant bottom and thread contact area. Under different lateral angle load, the Von Mises stress of bone contact surface also concentrated on the cortical bone of the implant cervix, but the maximum value was 4 times of the vertical loading, the strain distributed unevenly, mainly concentrated on the cortical bone of the implant cervix. With the load angle increased, the stress and the strain value also increased.

CONCLUSIONWhen an axial force was immediately loaded, the stress value and the strain concentration value of bone interface around the dental implants are not apparently concentrated, the stress is distributed well. While a lateral force is loaded, the stress values and the strain concentration values of bone interface around the dental implants apparently increase and the strain are distributed unevenly.