Biomechanical analysis of stress around the tilted implants with different cantilever lengths in all-on-4 concept.

BACKGROUND: Many clinical studies have reported the high success rate of the All-on-4 concept. In the present study, we aimed to compare the stress distribution with different tilted distal implants and cantilever lengths in an All-on-4 system using the two-dimensional photoelastic method and to establish the All-on-4 implant photoelastic model by computer-aided design (CAD) and rapid prototyping (RP).  METHODS: The data of the human edentulous mandible were acquired by computed tomography (CT). Three human edentulous mandible All-on-4 implant models with different distally inclined implant holes were fabricated using Mimic, Geomagic Studio software, and a light solidifying fast shaping machine. Then the final photoelastic models were established through the traditional method. Each of the three models had four NobelSpeedy Replace implants between the interforaminal regions. The two posterior implants were placed 0, 15, and 45 degrees distally before the mental foramen. The four implants were splinted by wrought cobalt-chromium alloy frameworks. Each of the three photoelastic models was submitted to a 150 N vertical load at five points on the framework: the central fossa of the mandibular first molar, and 0 mm, 5 mm, 10 mm, and 15 mm of the cantilever length. The stress produced in the models was photographed with a digital camera, and the highest value of the stressed fringe pattern was recorded. RESULTS: The All-on-4 implant photoelastic model established by CAD and RP was highly controllable and easy to modify. The position and inclination of implants were accurate, and the frameworks could be passively emplaced. The stress values were higher around a single tilted implant compared with the distal implant in All-on-4 with the same inclination. The 0-degree distal implant and 45-degree distal implant demonstrated the highest and lowest stress when loading at the central fossa of the mandibular first molar, respectively. With the same inclination of distal implant, the peri-implant bone stress increased as the length of cantilever increased. CONCLUSION: The method of establishing the All-on-4 implant photoelastic model by CAD and RP was highly controllable, convenient, fast, and accurate. The tilted implants splinted in the fully fixed prosthesis with reduced cantilever lengths did not increase the stress level compared with the vertical distal implants.And this illustrated that the influence of cantilever on stress distribution was greater than the influence of implant inlination.

[Kinetic spectrophotometric study on hemolytic process of polysorbate 80].

To establish kinetic assay method for the analysis of hemolysis and to investigate dynamic hemolytic process of polysorbate 80. The UV-VIS spectrum of heme changes when hemoglobin is released continuously during the hemolytic process. Therefore, dynamic hemolytic curve was determined as a new way to characterize the kinetic process of interaction between polysorbate 80 and red blood cells. The effect of polysorbate 80 on blood cells could be perfectly investigated by the hemolytic dynamics. Dynamic hemolytic parameters of polysorbate 80 were calculated according to the hemolytic curves. The constants of hemolytic rate and maximum hemolytic rate of polysorbate 80 had fine linear relationships at the range of 1-20 mg x mL(-1) and 5-20 mg x mL(-1), respectively. In comparison with the present official method such as macroscopic observation and static spectrophotometric methods, kinetic spectrophotometry has the advantages of real time, on-line determination, sensitive, objective, good reproducibility and 2-dimensional information acquired. Therefore, as a biological technique, kinetic spectrophotometry could be applied to evaluate the quality of polysorbate 80 and to screen other solubilizing excipients.

[Study on selecting the suitable solubilizing excipients for Shengmai injection].

OBJECTIVE: To select the suitable solubilizing excipients for Shengmai injection by the mean of hemolysis. METHODS: The hemolysis ratio of Shengmai injection, solubilizing excipients and different proportion of them were determined by absorption spectrometry. RESULTS: In the experimental concentration, Shengmai injection itself had no hemolytic effect. Polysorbate 80, Polyethylene glycol 400, Polyethylene glycol 600, Poloxamer 188 solution had no hemolytic effect to appear, but all the experimental concentrations of Polysorbate 20 had hemolytic effect. When combining with different proportion of Polysorbate 20, Polyethylene glycol 400, Polyethylene glycol 600, Poloxamer 188 and Polysorbate 80 of high concentration (0.75% - 1%), Shengmai injection had hemolytic effect. CONCLUSIONS: The absorption spectrometry is more reliable for the judgement of hemolysis than macroscopic observation, and it is helpful to improve the accuracy to select the solubilizing excipients for the TCM injection. Polysorbate 80 can be used as solubilizing excipient of Shengmai injection, which is safe in the concentration range of 0.1% to 0.5%.

[Effects of dental cements on the stress distribution in amalgam-restored teeth].

OBJECTIVE: To evaluate the effects of 5 kinds of base materials on the stress distribution and deflection in amalgam- restored teeth. METHODS: A computerized two-dimensional finite-element of the tooth model first molar was established. Various dental cements with a thickness of 0.5 mm and amalgam restorations with thickness of 2 mm in the prepared cavity were simulated and subjected to microhardness test (load=45 kg). The stresses and deflections along the cavity wall interface were analyzed by finite-element method. RESULTS: High stress and deflection occurred mainly at the line angle of the cavity, this could be magnified by using cements with lower elasticity modulus. The maximum compressive stress at the line angle induced by common ZOE or Ca(HO)(2) was increased 3.3 times or 2 times and deflections increased by 40% and 25% respectively that by Zinc phosphate and glass ionomer. CONCLUSION: Modulus elasticity of cement plays an important role in affecting the fracture strength of the amalgam restored tooth. The lower the elasticity modulus, the higher the stresses and deflections induced in the dentine. From the point of mechanical property, zinc phosphate is one of the best dental cements.

[An experimental study of the wear behavior of dental feldspathic glass-ceramic and lithium disilicate glass-ceramic].

OBJECTIVE: To investigate the tribology characteristics of two ceramic materials in vitro:feldspathic glass-ceramic (veneer porcelain) and lithium disilicate glass-ceramic (heat-pressed ceramic), and to evaluate the wear resistance of different ceramic materials from the dynamic chewing perspective. METHODS: Wear tests were performed in simulated oral environment with stainless steel ball antagonists (r = 3 mm), veneer porcelain (CERAMCO 3) and heat-pressed ceramic (IPS e.max Press HT type) in the chewing simulator. The tribological tests were carried out under artificial saliva lubrication condition in room temperature with a vertical load of 10 N for 1.2×10(6) cycles (f = 1.5 Hz, uniform circular motion, revolving speed = 90 r/min, radius = 0.5 mm). The wear volumes were measured using three-dimensional profiling, and surface microscopic morphology were observed using scanning electron microscopy at time point of 200 000, 400 000, 600 000, 800 000, 1 000 000, and 1 200 000 cycles. RESULTS: In a simulated oral environment, the wear rates of veneer porcelain were (0.001 20 ± 0.00 018) , (0.000 10 ± 0.000 03) , (0.000 50 ± 0.000 05), (0.000 10 ± 0.000 02) , (0.004 10 ± 0.000 38) , and (0.019 00 ± 0.003 53) (×10(-4) mm(3)/cycles) at 200 000, 400 000, 600 000, 800 000, 1 000 000, 1 200 000 cycles. The wear rates of heat-pressed ceramic were (0.139 50 ± 0.030 94), (0.124 40 ± 0.031 20), (0.054 80 ± 0.005 38), (0.038 80 ± 0.006 10), (0.011 10 ± 0.003 75), (0.198 90 ± 0.045 80) (×10(-4) mm(3)/cycles) at 200 000, 400 000, 600 000, 800 000, 1 000 000, 1 200 000 cycles. Three stages were observed in the wear loss process of the two materials: running-in stage, steady wear stage and severe wear stage. In running-in and steady wear stage, the shallow wear tracks of veneer porcelain were produced by the fatigue effect.While in severe wear stage, the wear tracks turned into ploughing. In running-in stage, the surface of heat-pressed ceramic was characterized by dense and shallow ploughing. In steady wear stage, the wear tracks turned into flake peeling produced by fatigue effect. At last, the whole layer was worn off by the effects of ploughing. CONCLUSIONS: In a simulated oral environment, the wear rate and wear loss of heat-pressed ceramic are greater than that of veneer porcelain.

[Selection of optimal occlusal thickness and shoulder thickness in mandibular first molar all-ceramic crown: a three-dimensional finite element analysis].

OBJECTIVE: To investigate the effect of different occlusal thickness and shoulder finish line depth on stress distribution of all-ceramic crowns and to select optimal occlusal thickness and shoulder finish line depth using continuous variation of parameters. METHODS: This analysis was performed using mandibular first molar finite element model. The range of occlusal thickness was set from 1 mm to 3 mm, and that of shoulder finish line depth was from 0.4 mm to 1.2 mm. Load of 225 N was applied perpendicularly to the occlusal surface of the tooth at all buccal cusps to simulate functional occlusal force. The maximum equivalent stresses in crown, cement layer, abutment, periodontal ligament, alveolar bone were calculated, and the sensitivities of stresses to the variables were also evaluated. RESULTS: The maximum equivalent stresses in crown, cement layer and abutment decreased as occlusal thickness and shoulder finish line depth were increased, while no obvious change were found in maximum equivalent stresses in periodontal ligament and alveolar bone. When occlusal thickness exceeded 1.87 mm and shoulder finish line depth exceeded 0.66 mm, the tangent slope rate of the maximum equivalent stress response curves ranged from -1 to 1. Data indicated that occlusal thickness played a more important role in reducing maximum equivalent stress in cement layer than finish line depth did, and shoulder finish line depth was a more effective parameter in reducing maximum equivalent stress in crown and abutment than occlusal thickness was. CONCLUSIONS: Occlusal thickness exceeding 1.87 mm and shoulder finish line depth exceeding 0.66 mm are optimal design for ceramic crown on mandibular first molar from biomechanical point of view.

[Analysis of reliability of the strength of all-ceramic crown on mandibular first molar].

OBJECTIVE: To analyze the reliability of all-ceramic crown on mandibular first molar under randomized biting force, and to explore the failure possibility of all-ceramic crown. METHODS: Finite element (FE) models of all-ceramic crown on mandibular first molar were established using the Algor FE software. All-ceramic crown was loaded at different occlusal areas and positions with random variables of biting force, and the reliability of all-ceramic crown was analyzed. RESULTS: The occlusal contact positions were located on the fossa, inclined surface and dental cusp. When the occlusal contact area was 0.4 mm(2), The reliability of all-ceramic crown on mandibular first molar were 100% (fossa), 99% (inclined surface), and 97% (cusp) respectively. When the occlusal contact area was 1.5 mm(2), the reliabilities of all-ceramic crown on mandibular first molar were 100% (fossa), 100% (inclined surface), and 99% (cusp) respectively. When the occlusal contact area was increased to 12.6 mm(2), the reliability of all-ceramic crown was 100%. CONCLUSIONS: Premature occlusal contact and steep cusp inclination on all-ceramic crown on mandibular first molar will result in stress concentration and the crown will exhibit high failure probabilities. Chewing hard food is another contributing factor to the failure of all-ceramic crowns.

[An 8-year follow-up study of Cerec2 computer aided design and computer aided manufacture of all-ceramic crowns].

OBJECTIVE: To evaluate the clinical performance of the crowns made by Cerec2 computer aided design and computer aided manufacture (CAD/CAM) system. METHODS: Twenty four teeth were restored with Cerec2 CAD/CAM crowns. Eight years after delivery of the prostheses, the all-ceramic crowns were evaluated using modified standard of California Dental Association quality evaluation system. Patients filled out a satisfaction questionnaire at the same time. And Kaplan-Meier analysis was carried out. RESULTS: Three out of 24 restorations were rated as failure. Kaplan-Meier analysis showed that the success rate of Cerec2 crowns eight year following delivery was 88%. CONCLUSIONS: Crowns made by Cerec2 CAD/CAM system can survive in oral condition and function well for 8 years.

[Stress analysis of different fixed prosthesis designs following molar hemisection therapy].

OBJECTIVE: To evaluate mechanical behavior of different fixed prosthesis following molar hemisection therapy for clinical reference. METHODS: Finite element method and photoelastic method were employed to investigate the stress distribution around the reserved distal root of the first mandibular molar with various fixed prosthesis designs. The restorations of single crown of the first mandibular molar, combined crowns of the first and second mandibular molars, combined crowns of the second mandibular premolar and first mandibular molar, and combined crowns of the second mandibular premolar, the first and second mandibular molars were designed. The same load was simulated on the occlusal surfaces of the molars. RESULTS: The results of two studies showed that the stress around the reserved root in the single crown model was over 3 times as much as that of the normal model, the first mandibular molar with normal roots and a crown. For the combined crowns of the first and second mandibular molars, the stress was almost twice of the normal stress. While in the models of the combined crowns of the second mandibular premolar and first mandibular molar, as well as the combined crowns of the second mandibular premolar, the first and second mandibular molars, the stress distributions were similar to that in normal conditions. CONCLUSIONS: Both the results indicated that the restoration of two-unit dual-ended combined crowns has favorable stress distribution and is applicable for the reserved root following the hemisection therapy.