[Three-dimensional finite element analysis of cement flow in abutment margin-crown platform switching].

OBJECTIVE: To analyze the cement flow in the abutment margin-crown platform switching structure by using the three-dimensional finite element analysis, in order to prove that whether the abutment margin-crown platform switching structure can reduce the inflow depth of cement in the implantation adhesive retention. METHODS: By using ANSYS 19.0 software, two models were created, including the one with regular margin and crown (Model one, the traditional group), and the other one with abutment margin-crown platform switching structure (Model two, the platform switching group). Both abutments of the two models were wrapped by gingiva, and the depth of the abutment margins was 1.5 mm submucosal. Two-way fluid structure coupling calculations were produced in two models by using ANSYS 19.0 software. In the two models, the same amount of cement were put between the inner side of the crowns and the abutments. The process of cementing the crown to the abutment was simulated when the crown was 0.6 mm above the abutment. The crown was falling at a constant speed in the whole process spending 0.1 s. Then we observed the cement flow outside the crowns at the time of 0.025 s, 0.05 s, 0.075 s, 0.1 s, and measured the depth of cement over the margins at the time of 0.1 s. RESULTS: At the time of 0 s, 0.025 s, 0.05 s, the cements in the two models were all above the abutment margins. At the time of 0.075 s, in Model one, the gingiva was squeezed by the cement and became deformed, and then a gap was formed between the gingiva and the abutment into which the cement started to flow. In Model two, because of the narrow neck of the crown, the cement flowed out from the gingival as it was pressed by the upward counterforce from the gingival and the abutment margin. At the time of 0.1 s, in Model one, the cement continued to flow deep inside with the gravity force and pressure, and the depth of the cement over the margin was 1 mm. In Model two, the cement continued to flow out from the gingival at the time of 0.075 s, and the depth of the cement over the margin was 0 mm. CONCLUSION: When the abutment was wrapped by the gingiva, the inflow depth of cement in the implantation adhesive retention can be reduced in the abutment margin-crown platform switching structure.

[A historical review of dental schools founded by dentist Situ Bo in modern time].

About 8 dental schools were founded by Chinese dentists during modern times (1909-1949) in China. Only one still worked after the founding of the People's Republic of China, which was one of the three dental schools founded by dentist Situ Bo. There were no systematic researches on Situ Bo's participation in dentistry education. This paper analyzes the founder's experience in dentistry and in the two schools he founded at the beginning, focusing on the background and process of the schools' construction, management, teaching, teachers and students training, etc. The results show that the establishment of the first two dental schools have laid important foundation for the establishment and development of Shanghai Dental College by the reserve of trained professional teachers and accumulated valuable experience in running schools. The systematic review of this history will help us better understand the establishment and development of Shanghai Dental College and the efforts in dentistry education made by dentists growed up in modern China, as well as the early development process of stomatology.

[Formation of professor Mao Xiejun's thoughts on stomatological education].

Professor Mao Xiejun wrote a report about dental education of China in 1935. From 1948 to 1950, he published three articles containing the educational idea of "developing dentistry into stomatology". When he served as the director of the Faculty of Dentistry of Peking University Medical School in July 1950, he proposed to rename the Faculty of Dentistry into the Faculty of Stomatology,which were approved by the Ministry of Health and the Ministry of Education of the People's Republic of China in one month. The Chinese Medical Association established the Society of Stomatology the next year. Later, dentistry was successively changed into stomatology, and medical content was integrated into dental education, which was of great significance and far-reaching influence. During the developments of the thought of stomatological education in China, Professor Mao Xiejun evidently played a pivotal role. In this paper, the formation process of the thoughts of stomatological education related to professor Mao Xiejun's contribution is elucidated through studying the archives, personal letters, and historical documents, so as to enrich the researches on the history of stomatology in China and to facilitate better understanding and promoting the development of stomatology.

[Prediction, analysis and application of learning curve of tooth preparation for all ceramic crowns of maxillary central incisors].

OBJECTIVE: To predict the learning curve of tooth preparation for all ceramic crowns of maxillary central incisors on phantom head simulators for graduate students participating in standardized dental resident training based on the modified Wright learning curve model, then to analyze and applicate the learning curve. METHODS: Twelve graduate students participating in standardized dental resident training were selected to prepare the resin maxillary central incisors on phantom head simulators for all ceramic crowns 4 times. The results of preparation were evaluated by 3 prosthetic experts with at least 10 years' experience focusing on the reduction, contour, taper, shoulder, finish line, margin placement, adjacent tooth injury, and preparation time for tooth preparation. The learning rate of tooth preparation was calculated by scores of tooth preparation of 4 times. The learning curve of tooth preparation was predicted based on the modified Wright learning curve model. According to the criteria of standardized training skill examinations for dental residents in Beijing, 80 was taken as the qualified standard score. The minimum training times for tooth preparation to satisfy the qualified standard score (80) was calculated, to analyze the characteristics of learning curve and evaluate the effectiveness of tooth preparation. RESULTS: The scores of 4 tooth preparation were 64.03±7.80, 71.40±6.13, 74.33±5.96, and 75.98±4.52, respectively. The learning rate was (106±4)%, which showed the learning curve an upward trend. There were no significant differences between the qualified standard score and the predicted scores of tooth preparation from the 5th preparation to the 13th preparation (P > 0.05). The predicted score of the 14th preparation was higher than the qualified standard score (P < 0.05). CONCLUSION: The trend of the learning curve of tooth preparation for all ceramic crowns of maxillary central incisors on phantom head simulators for graduate students participating in standardized dental resident training is upward, which predicts the minimum training times higher than the qualified standard score is 14 times.

[Effects of pretreatment HIV drug resistance on the virological response of HIV-infected patients after 3-year antiretroviral therapy].

Objective: To investigate the impact of pretreatment drug resistance (PDR) on virological effect among HIV-infected patients having received antiretroviral therapy (ART) after three years. Methods: The baseline survey of PDR among HIV-infected patients was conducted in 2018, with a three-year follow up study. The clinic data and virological laboratory test variables were statistically analyzed. Results: Of the 2 433 participants, 41.6% (1 012/2 433) were aged between 18 and 34, 82.8% (2 015/2 433) were males, 46.9% (1 142/2 433) had education of high school or above, 22.4% (544/2 433) were farmers, 33.8% (823/2 433) were unmarried, 48.1% (1 169/2 433) were infected heterosexually and 41.3% (1 004/2 433) were with CRF07_BC. The prevalence of PDR was 4.5% (109/2 433). The prevalence of virological suppression failure (viral load ≥50 copies/ml) and drug resistance at three years follow up after ART was 8.1%(196/2 433) and 2.5%(60/2 433) respectively. The prevalence of virological suppression failure and drug resistance at three years follow up after ART were 18.3% (20/109) and 7.6% (176/2 324), and 4.6% (5/109) and 2.4% (55/2 324) among participants with PDR and non-PDR, respectively. The results of multivariate logistic regression model showed that illiteracy (aOR=3.26, 95%CI: 1.82-5.86), primary and junior high school education (aOR=1.54, 95%CI: 1.09-2.18), CD4+T lymphocyte count <200/µl (aOR=2.77, 95%CI: 1.75-4.37) and CD4+T lymphocyte count 200-499/µl (aOR=1.55, 95%CI: 1.10-2.18) at a three year follow up visit after ART, missed drugs in the past month (aOR=4.24, 95%CI: 2.92-6.17), and PDR (aOR=2.84, 95%CI: 1.67-4.85) were statistically significant with virological suppression failure on treatment. Conclusions: The prevalence of PDR in China at a low level currently, and the virological suppression failure rate is low after three years of ART. It is necessary to strengthen drug resistance monitoring of HIV-infected patients and pay attention to the influence of PDR on treatment effect.

[Construction and application of virtual patients in prosthodontics].

In recent years, with the rapid development of digital technology, the application of oral virtual patients in prosthodontics, orthodontics, oral and maxillofacial surgery, and other disciplines has gradually increased. Although the focus on oral virtual patients varies in different disciplines, the application of oral virtual patients in assisting the prediction of treatment effects and the formulation of treatment plans will have good prospects. The construction accuracy and presentation effects of oral virtual patients can be influenced by the source of three-dimensional (3D) image, and methods of registration. Based on the studies and clinical experiences of our team, researches of other teams, the source of 3D images, the construction methods, and the clinical applications of virtual patients in prosthodontics will be presented, so as to provide a reference for normalized application and development of oral virtual patients and to offer a future development direction of oral virtual patients.

[Factors associated with death and attrition in HIV-infected children under initial antiretroviral therapy in Guangxi Zhuang Autonomous Region, 2004 - 2019].

Objective: To investigate death and attrition in HIV-infected children under initial antiretroviral therapy (ART) and associated factors in Guangxi Zhuang autonomous region. Methods: This retrospective cohort study was conducted in HIV-infected children under initial ART in Guangxi from 2004 to 2019, data from ART information system of National comprehensive AIDS prevention and treatment information system. Cox proportional hazards models were used to assess factors associated with the death and attrition. Results: In 943 HIV-infected children, the overall mortality and attrition rates were 1.00/100 person-years and 0.77/100 person-years, respectively. The mortality and attrition rates within the first year of ART were 3.90/100 person-years and 1.67/100 person-years, respectively. The cumulative survival rate during the first, second, fifth and tenth year after ART was 96.14%, 95.80%, 93.68% and 91.54%, respectively. Multivariate Cox proportional hazards models results showed that being female (aHR=2.00, 95%CI: 1.17-3.40), CD4+T lymphocytes (CD4) counts before ART <200 cells/µl (aHR=2.79, 95%CI: 1.54-5.06), weight-for-age Z score before ART <-2 (aHR=2.38, 95%CI: 1.32-4.26), hemoglobin before ART <80 g/L (aHR=2.47, 95%CI: 1.24-4.92), initial ART with LPV/r (aHR=5.05, 95%CI: 1.15-22.12) were significantly associated with death; being female (aHR=2.23, 95%CI: 1.22-4.07) and initial ART with LPV/r (aHR=2.02, 95%CI: 1.07-3.79) were significantly associated with attrition. Conclusions: The effect of ART in HIV-infected children in Guangxi was better, but the mortality and attrition rates were high within the first year of treatment. It is necessary to strengthen the training in medical staff and health education in HIV-infected children and their parents in order to improve the treatment effect.

[Clinical outcome of single posterior implant-supported monolithic zirconia crowns fabricated using full digital workflow and conventional workflow: a 3-year follow-up].

Objective: To compare the clinical performance of posterior single implant-supported monolithic zirconia crowns fabricated by full digital workflow and that of those fabricated by conventional workflow. Methods: This is prospective clinical research. Thirty-five patients who participated in a previous study during August 2017 to October 2018 at Department of Prosthodontics, Peking University School and Hospital of Stomatology were included in this 3-year follow-up study. The 35 patients, 17 females and 18 males, aged (49.0±15.4) years (24-86 years old), was allocated into two groups. In the full digital workflow group, intraoral scanning was taken immediately after implant placement, and a full zirconia implant crown was fabricated using model-free computer aided design/computer aided manufacturing (CAD/CAM) procedure (n=14). In the conventional group, a conventional impression was taken 3 months after implant placement and the stone model was produced. A full zirconia implant crown was fabricated using conventional model-based procedure (n=21). Three years following crown delivery, all the prostheses were evaluated in the aspect of color, surface roughness, contour and marginal integrity using modified US Public Health Service criteria (MUSPHS criteria). The soft and hard tissue around implant was evaluated using modified plaque index, probing depth (PD), number of implants with bleeding on probing, marginal bone loss (MBL). The biological and mechanical complication were also recorded. Statistical analysis was performed using independent samples t test, Mann-Whitney U test and Fisher's exact test. Results: The total survival rate of prosthesis and implant was 100% (35/35). No significant difference in MUSPHS criteria ratings on color, surface roughness, contour and marginal integrity of these crowns were found between the full digital workflow group and the conventional group (P>0.05). Sixteen out of the 35 crowns had a contour score of B due to loss of interproximal contact. Ten out of the 35 crowns had the screw hole sealing resin sinking or falling off, four in the full digital workflow group and six in the conventional group. There was no significant difference in the rate of prostheses mechanical complications between the two groups (P=1.000). For all the implants, there was no significant difference in MBL, PD, the modified plaque index, and number of implants with bleeding on probing between the two groups (U=119.50,133.00,142.50, t=-0.53, P>0.05). Conclusions: The clinical performance of implant-supported posterior single monolithic zirconia crowns fabricated by full digital workflow was stable. There was no significant difference in the clinical performance of the single implant crowns between the full digital workflow group and the conventional group.

[Finite element analyses of retention of removable partial denture circumferential clasps manufactured by selective laser melting].

OBJECTIVE: To compare the retentions of different designs of cobalt-chromium (Co-Cr), pure titanium (CP Ti), and titanium alloy (Ti-6Al-4V) removable partial denture (RPD) circumferential clasps manufactured by selective laser melting (SLM) and to analyze the stress distribution of these clasps during the removal from abutment teeth. METHODS: Clasps with clasp arm size A (1.9 mm width/1.1 mm thickness at the body and 0.8-taper) or B (1.2 times A) and 0.25 mm or 0.50 mm undercut engagement were modeled on a prepared first premolar die, named as designs A1, A2, A3, and A4, respectively. The density and elastic modulus of SLM-built Co-Cr, CP Ti, and Ti-6Al-4V were measured and given to different groups of clasps. The density, elastic modulus, and Poisson ' s ratio of enamel were given to the die. The control group was the cast Co-Cr clasp with design A1, to which the density and elastic modulus of cast Co-Cr alloy were given. The Poisson's ratio of all metals was 0.33. The initial 5 N dislodging force was applied, and the maximum displacement of the clasp along the insertion path was computed. The load was reapplied with an increment of 5 N than in the last simulation until the clasp was completely dislodged. The retentive force range of different groups of clasps was obtained. The retentive forces of the SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with equivalent computed retentive force range to the control group were validated through the insertion/removal experiment. The von Mises stress distributions of these three groups of SLM-built clasps under 15 N loads were analyzed. RESULTS: SLM-built Co-Cr, CP Ti, and Ti-6Al-4V clasps with designs B1 or B2, and Co-Cr clasps with design A2 had higher retentive forces than those of the control group. SLM-built CP Ti and Ti-6Al-4V clasps with design A1 had lower retentive forces than those of the control group. SLM-built Co-Cr clasp with design A1 and SLM-built CP Ti and Ti-6Al-4V clasps with design A2 had equivalent retentive forces to those of the control group. The insertion/removal experiment showed that the measured retentive forces of these three groups of SLM-built clasps were (21.57±5.41) N, (19.75±4.47) N, and (19.32±2.04) N, respectively. No statistically significant measured retentive force difference was found among these three groups of SLM-built clasps (P>0.05). The maximum von Mises stress of these three groups of SLM-built clasps exceeded their responding yield strength except for the Ti-6Al-4V one. CONCLUSION: SLM-built Co-Cr circumferential clasps had higher retention than CP Ti and Ti-6Al-4V ones with the same clasp arm size and undercut engagement. The retention of SLM-built circumferential clasps could be adjusted by changing the undercut engagement and clasp arm size. If SLM-built circumferential clasps are used in clinical practice, the Ti-6Al-4V clasp with clasp arm size A and 0.50 mm undercut engagement is recommended considering the long-term use of RPD in the patient's mouth.

[Preliminary clinical application verification of complete digital workflow of design lips symmetry reference plane based on posed smile].

OBJECTIVE: To automatically construct lips symmetry reference plane (SRP) based on posed smile, and to evaluate its advantages over conventional digital aesthetic design. METHODS: Eighteen subjects' three-dimensional facial and dentition data were gathered in this study. The lips SRP of experimental groups were used with the standard weighted Procrustes analysis (WPA) algorithm and iterative closest point (ICP), respectively. A reference plane defined by experts based on regional ICP algorithm, served as the truth plane. The angle error values between the lips SRP of WPA algorithm in the experimental groups and the truth plane were evaluated in this study, and the lips SRP of ICP algorithm of the experimental groups was calculated in the same way. The lips SRP based on posed smile as a reference for aesthetic design and evaluate preliminary clinical application. RESULTS: The average angle error between the lips SRP of WPA algorithm and the truth plane was 1.78°±1.24°, which was smaller than that between the lips SRP of ICP and the truth plane 7.41°±4.31°. There were significant differences in the angle errors among the groups (P < 0.05). In the aesthetic design of anterior teeth, automatically constructing the lips SRP of WPA algorithm based on posed smile and the original symmetry plane by re-ference compared with the prosthetic design, the subjects' scores on the lips SRP of WPA algorithm based on posed smile (8.48±0.57) were higher than those on the original symmetry plane (5.20±1.31). CONCLUSION: Automatically constructing the lips SRP of WPA algorithm based on posed smile was more accurate than ICP algorithm, which was consistent with the truth plane. Moreover, it can provide an important reference for oral aesthetic diagnosis and aesthetic analysis of the restoration effect. In the aesthetic design of anterior teeth, automatically constructing the lips SRP of WPA algorithm based on posed smile can improve the patients' satisfaction in esthetic rehabilitation.

[Application and research progress of digital virtual simulated design in dental esthetic rehabilitation].

In dental esthetic rehabilitation, patients pay great attention to the rehabilitative esthetic effect before teeth preparation, and this is also an important content of doctor-patient communication. Along with the development and combined application of intraoral scan, three-dimensional (3D) face scan, digital design, numerical control machining and 3D printing technology, digital technology is gradually applied to the virtual simulated design before irreversible operation in dental esthetic rehabilitation. Digital technology can be used in dentistry to simulate the esthetic outcome in advance, to assist communication among the dentists, patients and dental technicians, and to realize satisfactory outcome in the final restorations precisely, which, as a result, increases the clinical satisfaction. This review focuses on the application of digital virtual simulated design technology in dental esthetic rehabilitation, analyzes the current research development, deficiency and future prospects, so as to provide guidance for clinical diagnosis and treatment.

[Clinical application of a prefabricated interocclusal recording cap for making interocclusal records of implant-supported fixed prostheses].

Objective: To use a self-developed, prefabricated cap for making interocclusal records in implant-supported fixed prosthetic treatment, and to evaluate its effect of clinical application and accuracy of transferring intra-oral intercuspal position. Methods: Series of prefabricated caps for occlusal recording of implant-supported fixed prostheses were designed based on the healing abutments, and fabricated with three-dimensional (3D) printing. According to the inclusion and exclusion criteria, 12 partially edentulous patients who visited the Department of Prosthodontics, Peking University School and Hospital of Stomatology between November 2020 and September 2021, had lost no fewer than 2 contiguous teeth in distal extension (Kennedy Class Ⅰ or Ⅱ), and had received implant placement 3 months ago were enrolled. Self-control study design was used. Two occlusal records of each included case were obtained by the following 2 methods: for the test group, appropriate prefabricated caps were used; and for the control group, polyvinyl siloxane occlusal record was directly set on the healing abutments. The working casts were mounted on the mechanical articulator using the 2 groups of occlusal records successively. Accuracy of occlusal relationship of the mounted casts was evaluated. Diagnostics test was performed to obtain the sensitivity and positive predictive value, which were determined in photographs by comparing the intra-oral occlusal contact points with those in the mounted casts, point-by-point. Virtual casts were taken by intraoral and extraoral scans in intercuspal position and imported to Geomagic Studio 2014. Then the root mean square values of occlusal clearance space between the upper and lower occlusal surfaces of remaining posterior teeth, and the deviations of the interocclusal position of the occlusal surfaces of the remaining teeth in the mandibular arches when superimposing the maxillary arches of the intraoral and extraoral scans were calculated. As a self-control design, paired t test was used. Results: Twelve participants were enrolled, including 3 men and 9 women aged (52.6±12.1) years, and 36 missing teeth. The prefabricated caps could fit on the healing abutments with good retention and stability. The sensitivity of the test group (0.73±0.14) was significantly better than that of control group (0.63±0.12,P<0.01), with no significant difference in positive predictive value between the 2 groups (P>0.05). The deviations of the interocclusal position of the occlusal surfaces of the remaining teeth were (357.0±140.2) µm for the test group, and (399.4±206.3) µm for the control group, with no significant difference between them (P>0.05). Conclusions: Interocclusal position record based on prefabricated cap in this study for implant-supported fixed prosthetic treatment can improve the consistency between the intra-oral occlusion and the occlusion in dental casts. This technique has good accuracy, clinical convenience and usability.

[Quantitative study of intraoral scanners' accuracy].

Objective: To establish a standard method to evaluate the scanning accuracy of intraoral scanner (IOS) and to investigate six IOS's scanning accuracy and the relationship between different scan span. Methods: Five simplified six abutments full arch model were fabricated by high accuracy (5 µm) milling machine with 7075 aluminum alloy. The machining accuracy, which was verified by a coordinate measuring machine with higher accuracy (0.7 µm), was considered as the reference accuracy. The model with the highest machining accuracy was considered as the test model in IOS's scanning accuracy test, and computer-aided design (CAD) data of the model was used as the reference data. Six IOS scanned the test model 10 times with the same scanning path, obtained 60 test data. CAD data and test data were input into Geomagic Studio 2014. The preparation part above the margin of the abutments of the data was isolated and divided into 4 segments of interest: single crown, three-unit bridge, five-unit bridge, and full arch. The test data were then best-fit aligned to CAD data or each other followed by deviation analysis. Scanning trueness and precision were then calculated. Results: The mid-value of scanning trueness and precision of six IOS in single crown, three-unit bridge, five-unit bridge and full arch were 13.3-29.6 µm and 7.6-20.7 µm, 15.4-30.9 µm and 8.7-26.5 µm, 17.0-66.1 µm and 11.3-44.2 µm, 24.0-107.9 µm and 24.6-150.1 µm respectively. Conclusions: Long-span scanning can affect the accuracy of IOS to a varying extent.

[Evaluation of fracture strength of two kinds of zirconia all-ceramic crowns with different edge compensation angles].

OBJECTIVE: To evaluate the effects of different edge compensation angles on the fracture strength of multilayer zirconia all-ceramic crowns and traditional uniform zirconia all-ceramic crowns. METHODS: The resin tooth preparation specimen of the mandibular first molar with a knife-edge was fabricated. A 3D digital model of the specimen was obtained by scanning it with a 3D dental model scanner. The 3D digital model was imported into computer aided design (CAD) software, and three 3D digital models of the full crown with the same surface shape are designed with the edge compensation angles of 30°, 45° and 60°, respectively. Then, the designed 3D digital model is imported into computer aided manufacturing (CAM) software. Three kinds of multilayer and homogeneous zirconia all-porcelain crowns with different edge compensation angles were fabricated, 10 each for a total of 60. The fracture load of each crown was measured under the electronic universal testing machine. RESULTS: Fracture load of multilayer and uniform zirconia all-ceramic crowns, (4 322.86±610.07) N and (5 914.12±596.80) N in the 30° group, (5 264.82±883.76) N and (5 220.83±563.38) N in the 45° group and (4 900.42±345.41) N and (5 050.22±560.24) N in the 60° group, respectively. The fracture load of multi-layer zirconia all-ceramic crowns in the 30° group was significantly lower than that of homogeneous zirconia all-ceramic crowns(P < 0.05); there was no statistical significance in 45° group and 60° group(P>0.05). In the multi-layer zirconia all-ceramic crowns: the fracture load of the 30° group was significantly lower than that of the 45° group (P < 0.05); there was no significant difference between the 30° group and the 60° group, the 45° group and the 60° group (P>0.05).In uniform zirconia full crown group: the 30° group was higher than the 45° group, the 30° group was higher than the 60° group (P < 0.05), and there was no significant difference between the 45° group and the 60° group (P>0.05). CONCLUSION: The fracture loads of three kinds of uniform and multilayer zirconia all ceramic crowns with different edge compensation angles can meet the clinical requirements. A smaller edge compensation angle is recommended when using traditional zirconia all-ceramic crowns, while 45° is recommended when using multi-layer zirconia all-ceramic crowns.

[Evaluation of methods for fitness of removable partial denture].

OBJECTIVE: To compare the differences and indications of three evaluation methods for fitness evaluation of removable partial denture (RPD). METHODS: A RPD was fabricated and seated on the stone cast of a partially edentulous mandible, and the spaces between RPD and stone cast were recorded with polyvinyl siloxane (PVS) impression material forming PVS replicas. Using cross sectional measurement, the average thicknesses of PVS replicas were measured under stereomicroscope with different numbers of selected measuring points in the denture base, major connector, occlusal rest of the RPD, and the average thicknesses of the PVS replicas measured with different numbers of measuring points were compared using one-way analysis of variance (ANOVA) and independent sample t test. Three kinds of method, including cross sectional measurement, three-dimensional analysis on the stone cast, and three-dimensional analysis on the polyether cast, were applied to measure the average thicknesses of the PVS replicas, and the average thicknesses of the PVS replicas measured by these three evaluation methods were compared with ANOVA. RESULTS: For cross sectional measurement, statistically significant differences were found in the average thicknesses of the PVS replicas in the denture base and the major connector among the different numbers of measuring points (P < 0.05), but no differences were found in the average thicknesses of the PVS replicas in the occlusal rest (P>0.05). There were significant differences among the average thicknesses of the PVS replicas measured by these three evaluation methods in each component of the RPD (P < 0.01). The average thickness measured by three-dimensional analysis on the stone cast and three-dimensional analysis on polyether cast were smaller than that measured by cross sectional measurement (P < 0.05). And there were no differences between the average thicknesses of PVS replicas measured by three-dimensional analysis on stone cast and three-dimensional analysis on polyether cast (P>0.05). CONCLUSION: For cross sectional measurement, the average thickness of the PVS replicas was influenced by the number of measuring points, and the measurement accuracy of cross sectional measurement was not reliable enough. Three-dimensional analysis on stone cast which is suitable for evaluation in vitro and three-dimensional analysis on polyether cast which is suitable for evaluation in vivo can evaluate the fitness of RPD more comprehensively and effectively than that of cross sectional measurement.

[Advances in computer aided design and computer aided manufacturing of removable partial denture].

Computer aided design, numerically controlled milling, additive manufacturing and other digital technologies have been widely used in clinical practice of prosthodontics. The application of these technologies not only changed the design and manufacturing workflow of removable partial denture (RPD), but also improved the production efficiency of dentures, and changed the design and manufacturing concept of dentures. A large number of researches on computer aided design and computer aided manufacturing (CAD/CAM) of RPD have emerged in recent years, including researches on the properties of selective laser melting metal powder and other new dental materials, the innovative design of retainers, connectors, artificial teeth and other denture components, high precision manufacturing of the RPD framework, denture base and artificial teeth, quantitative laboratory evaluations of the accuracy, adaption and mechanical properties of RPD with new structures and made of new materials, clinical evaluations of RPD and others. This paper introduces these recent developments in CAD/CAM of RPD.

[Present status and outlook of prosthodontic treatments based on three-dimensional printing technologies].

Three-dimensional (3D) printing prostheses or prosthodontic devices have been preliminarily used in the clinics of prosthodontics. These prostheses or devices include diagnostic prostheses, crowns and fixed bridges, removable denture frameworks, implant surgical template, and custom-made implants, etc. There are three major types of technologies for 3D printing according to the forming principles, namely, light curing, sintering or melting, and melting-condensation. The materials used in 3D printing prostheses or devices include polymers, metals, and ceramics, etc. The advantages of 3D printing prostheses include material-saving, customized manufacturing for restorations, and batch production. However, the precision and strength of prostheses or devices, long-term clinical effects, and grade of maturity in customized manufacturing need further investigation and improving. This article focused on the present application, and the future development of 3D printing technologies in prosthodontic treatments. It will provide beneficial references for the future studies, development, and clinical application of 3D printing technologies and restorations in prosthodontic fields.

[Application of three-dimensional digital simulated design and implementation in esthetic rehabilitation].

Objective: To explore and evaluate a three-dimensional (3D) digital simulated design and implementation technique in esthetic rehabilitation. Methods: Thirty patients with esthetic deficiency, who came to the Department of Prosthodontics, Peking University School and Hospital of Stomatology from December 2017 to July 2019, were recruited and randomly assigned into 2 groups. Four males and 11 females which were (36.0±10.5) years old in the experimental group, 6 males and 9 females which were (32.0±6.7) years old in the control group, were enrolled. In the experimental group, 3D digital simulated design was used to predict the post-treatment effect; and the final restorations were designed duplicating from the pre-treatment digital design confirmed by the patient and milled. In the control group, the final restorations were manufactured by the dental technician according to the design of two-dimensional (2D) digital smile design. The simulation degree of digital design and the similarity between preoperative design and postoperative rehabilitation were scored by the patients with visual analogue scales (VAS) in both groups; and the satisfaction rate to the restorations was evaluated by the patients. The quality of the restorations was accessed by a prosthodontist who did not know the grouping of patients according to the modified criteria of United States Public Health Service (USPHS). Results: Three-dimensional digital simulated design and implementation technique was successfully established. The VAS score on the simulation degree of digital design in the experimental group (8.5±0.5) was higher than that in the control group (7.2±0.7) (P<0.01); the VAS score on the similarity between preoperative design and postoperative rehabilitation in the experimental group (9.6±0.3) was higher than that in the control group (7.0±0.9) (P<0.01). The satisfaction rate of the patients to the restorations was significantly higher in the experimental group than in the control group (P<0.05). There was no significant difference of the quality of the restorations between the two groups on the anatomic form, the marginal adaption and the surface quality (P>0.05). Conclusions: Three-dimensional digital simulated design and implementation technique can help achieving 3D digital simulated design before treatment and duplication to the final restorations, and can improve the patients' satisfaction in esthetic rehabilitation.

[Investigation of posterior teeth displacement under normal bite force by an intraoral scanner].

Objective: To quantitatively evaluate the change of the long axis angle and the relative displacement of the crown feature points of the posterior teeth under normal bite force utilizing an intraoral scanner, and to provide clinical reference. Methods: From May to December 2019, fifteen graduate volunteers (5 males and 10 females, aged from 22 to 30, with an average age of 25.7 years) from Peking University School and Hospital of Stomatology were recruited to participate in the study. The surface data (U1, L1) of the maxillary and mandibular posterior teeth were scanned by an intraoral scanner i500, and saved as original data. The volunteers were guided to bite in the intercuspal position with normal bite force. The buccal bite data of the posterior teeth were scanned as the basis for registration. The digital casts were imported into Geomagic studio 2013 software and the boundary lines along the gingival margin and mesial and distal contact area of posterior teeth of data (U1, L1) were determined. Long axis of the crown, crown centroid and mesial functional cusp vertex were establishd. The data (U1, L1) were segmented into single tooth. Single tooth was aligned to buccal bite data separately using best-fit alignment command based on the buccal common area of the crown and new casts data (U2, L2) were obtained as the data under bite force. The long axis angle and centroid distance between adjacent teeth (second premolar and first molar, second molar and first molar) were measured and the deviation between data obtained at mouth-open state and that at biting state was calculated. Negative value meant centroid distance became shorter under bite force. The first molar was set as the common area and registrate the U1, L1 to U2, L2. The angle of long axes, and displacement of centroid and of functional cusp vertex between second premolar of two casts were calculated, as well as between second molar of two casts. Wilcoxon signed rank test was used to analyze the differences of teeth displacement between second premolar and second molar, and between maxillary and mandibular jaws with SPSS 26.0 statistical software. Results: The result of second premolar and second molar in the same jaw had no statistical difference (P>0.05). The centroid distance deviation of mandibular second premolar-first molar [-0.022(0.046) mm] was larger than that of maxilla [-0.006 (0.040) mm] (P<0.05). The long axis angle of second premolar itself [0.913°(0.647°)] and centroid distance of second molar itself [0.102 (0.106) mm] on the mandibular jaw were different from that on the maxillary jaw, which were 0.590°(0.550°) and 0.074(0.060) mm respectively (P<0.05). Conclusions: Under bite force, displacement of the second premolar and second molar was present, including the displacement of centroids and deflection of long axes. The mandibular posterior teeth have larger displacement than the maxillary teeth.

[Evaluation of clinical efficacy of a kind of digital complete denture].

OBJECTIVE: To compare the clinical efficacy of a kind of complete dentures fabricated using computer aided design/computer aided manufacturing (CAD/CAM) workflow with that of the complete denture fabricated using conventional workflow. METHODS: Twenty edentulous patients were included in this prospective, single-blind, self-controlled clinical trial. Two pairs of complete dentures were fabricated for each participant: one using the functional suitable denture (FSD) system with CAD/CAM, and the other using conventional fabrication workflow. In the final delivery appointment, the conventional dentures worn by the participant for 3 months at first. Then the participant swapped to wear digital denture for another 3 months. The patients' satisfaction and oral health related quality of life (OHRQoL) were measured using the visual analogue scale (VAS) and oral health impact profile (OHIP-20E) at baseline, 2 weeks, 1 month, 2 months, and 3 months following denture delivery. In addition, the masticatory efficiency of each pair of dentures was measured after three months adaptation period for each type of dentures. RESULTS: The VAS scores of the twenty patients in FSD denture group on general satisfaction, ease of cleaning, ability to speak, esthetics, stability and oral health status on these six domains were higher than that of conventional denture group 3 months after delivery. While the VAS cores in FSD denture group on comfort, ability to chew and ability to chew up were the same as those of conventional denture group. But there was no significant difference (P>0.05). The VAS score on each domain in FSD denture group was higher than that of conventional denture group 3 months after delivery, three domains of which reached statistical significance (P < 0.05), involving functional limitation, psychological discomfort and total. The masticatory performance of the FSD denture (1.20±0.54) was slightly higher than that for the conventional denture (1.16±0.53), but the difference did not reach statistical significance (P=0.691). CONCLUSIONS: The clinical efficacy of the FSD complete denture is comparable to that of the conventional complete denture. As for patient satisfaction and oral health related quality of life, FSD dentures received comparable scores as conventional complete dentures did.