Testing of Zirconia FPD Frameworks for Fixed Prostheses: Model Design and Evaluation of Fracture Load

In our previous test model, the abutment teeth and the model base were printed with resin and bonded with a polyether material. Some abutment teeth fractured during the mechanical load test. Therefore, the aim was to develop and to evaluate a new model under mechanical loading until fracture with zirconia FPD frameworks. Up to a fracture load of 1636 N, neither the artificial abutment teeth nor the base model fractured. Furthermore, the artificial abutment teeth did not detach from the base model. Therefore, the model should be suitable for mechanical testing of most ceramic-based framework materials for three-unit FPDs.

Infective endocarditis: prevention strategy and risk factors in an animal model

INTRODUCTION

Infective endocarditis is a serious infection of the endocardium, especially the heart valves, which is associated with a high mortality rate. It generally occurs in patients with altered and abnormal cardiac architecture combined with exposure to bacteria from trauma and other potentially high-risk activities with transient bacteremia.

Comparison of temperature changes in different irrigation methods used in guided piezosurgery: in vitro study

PURPOSE

The study aimed to investigate the cooling performance of three different surgical methods for bone splitting during implant dentistry procedures.

METHODS

An in vitro study was conducted on bovine ribs to test the cooling performance of three osteotomy methods: conventional osteotomy with standard cooling irrigation, template-assisted osteotomy with standard cooling irrigation, and modified template-assisted osteotomy with additional cooling fluid through internal irrigation channels in the surgical guide. A negative control group without cooling was also included. Temperature measurements were taken during the procedures using a contactless infrared thermometer.

RESULTS

The results showed that the conventional osteotomy without cooling resulted in the highest mean of the peak temperatures (114.7 °C ± 45.4), while the modified template-assisted osteotomy with additional cooling achieved the lowest mean of the peak temperatures (29.4 °C ± 7). Statistically significant differences in temperature were observed among the groups (p < 0.001).

CONCLUSION

The study rejected the null hypothesis and demonstrated that the cooling method used during bone splitting procedures significantly affected the temperature development at the surgical site. The modified template-assisted osteotomy with additional cooling provided the best cooling performance. However, caution should be exercised in interpreting the results due to the limitations of the in vitro study. Further research with stronger external validity is needed to confirm these findings and explore their applicability in clinical settings to benefit patients.

Fracture resistance of 3-unit monolithic ZrO2 ceramics FPDs with different preparation designs of the distal abutment - an in-vitro study

INTRODUCTION

Masticatory pressure increases in the distal areas of the dentition. This should be considered when restoring partially edentulous patients with a metal-free fixed partial denture (FPD). An alternative abutment preparation design can be used in order to increase the materials' volume in the most fracture-prone "connector area" of an FPD. The increased size of the connection might positively influence the constructions' mechanical durability, thereby increasing its success and survivability.

AIM

The aim of the present study was to investigate the influence of two preparation designs of the distal abutment on the fracture resistance of three-unit, monolithic, ZrO2 FPDs.

MATERIALS AND METHODS

3D printed replicas of a partially edentulous mandibular segment and a ZrO2, milled in full-contour, three-unit FPDs were used for this investigation. Two experimental groups (n=10 ) were defined based on the preparation design of the distal abutment tooth - classical shoulder preparation 0.8 mm deep, and endocrown preparation with a 2-mm retention cavity. The bridge - mandibular segment replica assembly was done with relyXU200(3M ESPE, USA), light-cured for 10 seconds per side with D-light Duo (GC, Europe). After cementation the test specimens were subjected to loading in a universal testing machine Zwick (Zwick-Roell Group, Germany). Statistical analysis was performed using R and includes descriptive statistics, t-test for quantitative and chi-squared test for qualitative variables.

RESULTS

The results showed no difference between the two studied groups in the maximum force required to fracture the test specimens [t=-1.8088 (17.39), p-value=0.087; P>0.05]. 95% of the fracture lines were located in the distal connector.

CONCLUSIONS

Within the limitations of this study, it can be concluded that both tested preparation designs show similar results in terms of the load required to fracture the test specimens. Furthermore, it is confirmed that the distal connector is the weakest area of an all-ceramic 3-unit FPD in the posterior area.

A new technology for the removal of corundum residues on dental implants

Surface modification is an important measure to improve dental implants. Corundum residues, which are part of current dental implant blasting, on Straumann dental implants, were found to have disappeared in recent publications. We further evaluated this new cleaning technology by evaluating the surface of four different Straumann implants using scanning electron microscopy (SEM) and energy-dispersive radiographic spectroscopy (EDX). The involved technology fits to a Straumann patent involving a dextran coating allowing easy corundum particle removal by aqueous solution.

Implant Placement in the Esthetic Zone: More Efficiency in Guided Surgery

The aim of this case report is to describe the combination of a surgical guide with a temporary restoration to streamline the implant process. A 54-year-old male patient presenting with partial edentulism underwent computer-aided template-guided implant placement for the replacement of the missing upper second right incisor. The presented technique was used during the surgical procedures; it introduced the integration of a surgical guide into the temporary fixed partial denture. Using computer-aided design, computer-aided manufacturing technology and virtual implant planning, a temporary-implantation fixed partial denture was constructed, and a guidance sleeve was implemented into it. The implant bed preparation was then performed using the bridge as a surgical guide. After osteotomy, the guidance sleeve within the bridge was sealed, and the bridge was temporarily incorporated for submerged healing of the implant. The use of a temporary restoration as a surgical guide seems to make the digital workflow of guided implant placement more efficient by achieving a representative clinical outcome.

Pull-off resistance of a screwless implant-abutment connection and surface evaluation after cyclic loading

PURPOSE

The aim of this study was to investigate to what extent cyclic load affects the screwless implant-abutment connection for Morse taper dental implants.

MATERIALS AND METHODS

16 implants (SICvantage max) and 16 abutments (Swiss Cross) were used. The screwless implant-abutment connection was subjected to 10,000 cycles of axial loading with a maximum force of 120 N. For the pull-off testing, before and after the same cyclic loading, the required force for disconnecting the remaining 6 implant-abutment connections was measured. The surface of 10 abutments was examined using a scanning electron microscope 120× before and after loading.

RESULTS

The pull-off test showed a significant decrease in the vertical force required to pull the abutment from the implant with mean 229.39 N ± 18.23 before loading, and 204.30 N ± 13.51 after loading (P<.01). Apart from the appearance of polished surface areas and slight signs of wear, no visible damages were found on the abutments.

CONCLUSION

The deformation on the polished abutment surface might represent the result of micro movements within the implant-abutment connection during loading. Although there was a decrease of the pull-off force values after cyclic loading, this might not have a notable effect on the clinical performance.

The Influence of Implant Site Preparation and Sterilization on the Performance and Wear of Implant Drills

PURPOSE

The aim of this study was to investigate if repeated use of pilot and depth drills for implant site preparation in combination with resterilization leads to any significant changes in cutting edges or in preparation times.

MATERIALS AND METHODS

With drills of different lengths and widths (pilot, parallel, and tapered), osteotomies were performed under constant conditions in pig jaws. After each osteotomy, the drills were processed and sterilized. The required time for finishing the osteotomy was noted and analyzed. At the end of each osteotomy cycle, consisting of 10 osteotomies and 10 sterilizations, the cutting edge wear of the drill was analyzed using scanning electron microscopy. Unused but sterilized drills served as a control group.

RESULTS

There was no statistically significant difference in the evaluated wear between pilot drills and depth drills as well as between the test and the control group. After 60 osteotomies, implant drills showed considerable signs of wear at the major cutting edge (P > .05). Osteotomy led to a significant increase in the drilling time. While pilot drills showed an increase of drilling time of the factor 33.3, and the tapered drills of the factor 5.37, the parallel drills remained constant in their performance.

CONCLUSION

Within the limitations of this study, it can be concluded that the process of sterilization as well as the number of performed osteotomies play a major role in the wear of the cutting edge and the performance of implant drills.

Ridge augmentation-The new field of computerized guided surgery: A technical note for minimal-invasive bone splitting

Different instrumentation procedures of the alveolar ridge expansion technique (ARST) with or without Guided Bone Regeneration have proven to be effective for successful implant placement in cases of alveolar bone width between 3mm and 6mm. Conventional bone splitting techniques require flap arising. This technical note demonstrates a method for flapless guided bone splitting. For this purpose, a newly developed surgical guide with internal irrigation channels was used. Using CAD-CAM additive technology, a narrow slot along the field of interest and a pin of a cooling pipe was designed and implemented in a surgical guide template. The bone split was performed flapless through the surgical guide while the cooling pipe was connected to it. During surgery, the piezo-driven instrument was moved within that slot, and the irrigation solution was directly rinsing it at point of entry through the irrigation channel. This procedure was performed on a 3.3 mm wide alveolar ridge achieving over 3 mm of bone gain. The described method combines several positive aspects. The micro-invasive flapless surgical procedure might improve postoperative healing. Additionally, sufficient cooling of the bone might lead to less thermal affection of bone cells and less resorption of the cortical bone. However, systematic studies are needed to confirm the observations of the presented case report.

Use of interactive instructional tools in virtual 3D planning

OBJECTIVES

The aim of this study is to analyze the effectiveness of screencast training videos for users from different professional backgrounds on the basis of the quality and quantity of virtual 3D implant planning as evaluated using assessment software.

METHODS

Participants in the study were dental students (n = 21), dental technicians (n = 16), or practicing dentists (n = 16) with no knowledge of the implant planning software CoDiagnostiX or any experience in virtual implant planning. They were randomly divided into 2 groups: The participants of the control group were presented training videos on CBCT (cone beam computed tomography) and implant planning, additional to the traditional software manual. The participants of the experimental group received, additionally, a training video on the planning software, produced in the form of a screencast. Then 3 implant planning procedures were performed by all participants. The quality of implant planning was assessed using the Treatment Evaluation Tool software, and time for planning was measured.

RESULTS

Paired sample tests showed significantly shorter planning times for the experimental groups consisting of dentists and students (P < 0.001). A comparison of the results of the individual groups reveals that students and dental technicians who had not received prior screencast training were significantly less precise in their planning (P < 0.001).

CONCLUSION

This study suggests a promising outcome for the future use of screencasts in the acquisition of software skills for dental software. There is a need for tools for objective self-assessment of effectiveness in software training.

Guided Implant Placement Using an Internally Cooling Surgical Template: A Technical Note

Cooling irrigation during implant bed preparation is mandatory to avoid overheating. Due to the surgical guide design, external cooling systems do not reach the point of entry of the implant burr. Here, a new technique for irrigation during guided implant surgery for direct rinse of the burr is described. Using computer-aided design/computer-aided manufacturing additive technology, a pin of a cooling pipe was designed and implemented in a surgical guide template. The implant bed preparation was performed while the cooling pipe was connected to the surgical guide. During surgery, the irrigation solution was directly rinsing the burr at the point of entry through the irrigation channel. The use of a cooling surgical guide seems to improve the cooling of the bone during implant bed preparation. This might lead to less thermal effect of bone cells. However, systematic studies are needed to confirm the observations of the presented case report.

The color stability and wear resistance of provisional implant restorations: A prospective clinical study

Objectives

The aim of the present clinical study was to determine the stability of color and resistance against abrasion and attrition of provisional single implant restorations.

Material and Methods

A group of 16 patients were treated with provisional crowns made of Telio CAD. Shortly before the insertion the crowns were photographed and scanned using a 3D‐laser scanner. After 8 weeks of clinical usage, the crowns were photographed and scanned again. The vertical occlusal wear and color changes between the restorations were measured.

Results

The occlusal plane of the original crown showed a statistically significant reduction of 0.052 mm ± 0.037 mm 8 weeks after placement (p < .05). For the stability of color, a change in red, green and blue was described. All three scopes (red, green and blue) showed a statistically significant reduction (p < .05).

Conclusions

This prospective clinical study showed that Telio CAD experienced a significant occlusal reduction and color change after an intraoral placement of 8 weeks.

A retrospective study of the accuracy of template-guided versus freehand implant placement: A nonradiologic method

OBJECTIVE

The aim of this retrospective case study was to evaluate implant placement accuracy in inexperienced clinicians with use of a surgical guide template vs the freehand method for preoperatively planned implant position.

STUDY DESIGN

Oral implant therapy was performed by inexperienced dental practitioners (<20 implants placed) on eligible patients after cone beam computer tomography (CBCT)-based software planning. Two patients' groups were formed according to the surgical technique: guided and freehand implant placement. The casts used for the fabrication of reconstructions were scanned by using a 3-dimensional (3D) laser scanner, the implant positions were superimposed with preoperative planning data, and the 3D deviations were calculated.

RESULTS

Twenty adult patients were included in this retrospective study (20 guided and 21 freehand implant placements). Mean calculated deviations with use of a surgical guide template were significantly lower compared with those of the freehand method with regard to angulation (P = .002), apical position (P = .002), and basal position (P = .012). No significant differences in implant placement accuracy were detected between the groups according to the implant position (premolars/molars; upper jaw/lower jaw) (P > .05).

CONCLUSIONS

Computer-aided planning and the use of surgical guides in accordance with CBCT images may help inexperienced clinicians to place implants with high accuracy.

Biomaterial shell bending with 3D-printed templates in vertical and alveolar ridge augmentation: a technical note

OBJECTIVES

Alveolar ridge and vertical augmentations are challenging procedures in dental implantology. Even material blocks with an interconnecting porous system are never completely resorbed. Shell techniques combined with autologous bone chips are therefore the gold standard. Using biopolymers for these techniques is well documented. We applied three-dimensional (3-D) techniques to create an individualized bending model for the adjustment of a plane biopolymer membrane made of polylactide.

STUDY DESIGN

Two cases with a vertical alveolar ridge defect in the maxilla were chosen. The cone beam computed tomography data were processed with a 3-D slicer and the Autodesk Meshmixer to generate data about the desired augmentation result. STL data were used to print a bending model. A 0.2-mm poly-D, L-lactic acid membrane (KLS Matin Inc., Tuttlingen, Germany) was bended accordingly and placed into the defect via a tunnel approach in both cases. A mesh graft of autologous bone chips and hydroxylapatite material was augmented beneath the shell, which was fixed with osteosynthesis screws.

RESULTS

The operative procedure was fast and without peri- or postoperative complications or complaints. The panoramic x-ray showed correct fitting of the material in the location. Bone quality at the time of implant placement was type II, resulting in good primary stability.

CONCLUSIONS

A custom-made 3-D model for bending confectioned biomaterial pieces is an appropriate method for individualized adjustment in shell techniques. The advantages over direct printing of the biomaterial shell and products on the market, such as the Xyoss shell (Reoss Inc., Germany), include cost-efficiency and avoidance of regulatory issues.

The influence of pressure changes on the retentive force and coronal microleakage of different types of posts in endodontically treated teeth during simulated dives

OBJECTIVES

We assessed the influence of a simulated diving environment on the interfacial microleakage and retentive forces of different post types in root-canal-filled teeth.

MATERIALS AND METHODS

One-hundred-and-twenty extracted, single-rooted teeth were endodontically treated and were randomly divided into three groups according to the post and cement used: ER Post/Harvard cement (Titanium), CeraPost/DentinBuild Evo (Zirconia), DT Light Post/Calibra (FRC). Each group was randomly divided into two equal subgroups, a control group, and an experimental group, subjected to simulated dives to 456 kPa in a diving chamber. For 10 specimens of each subgroup the pull-out strength and the coronal microleakage were measured.

RESULTS

Significant differences in the linear coronal penetration were observed between the Titanium and FRC groups (experimental group P ≤ 0.001; control group P = 0.02). Diving simulation had no significant impact on the microleakage for the three post types. The FRC groups showed significantly higher retentive strength values compared to the Titanium and Zirconia groups before and after simulated diving. The pull-out strength of the titanium experimental group was significantly less than the control group (P = 0.008).

CONCLUSIONS

Following root canal treatment the combination of fibre-reinforced posts and resin cement should be preferred for patients requiring retention for tooth restorations using posts that are likely to be exposed to hyperbaric conditions.

Influence of the preparation design and artificial aging on the fracture resistance of monolithic zirconia crowns

PURPOSE

The aim of this study was to evaluate the fracture resistance and fracture behavior of monolithic zirconia crowns in accordance with the preparation design and aging simulation method.

MATERIALS AND METHODS

An upper first molar was prepared sequentially with three different preparation designs: shoulderless preparation, 0.4 mm chamfer and 0.8 mm chamfer preparation. For each preparation design, 30 monolithic zirconia crowns were fabricated. After cementation on Cr-Co alloy dies, the following artificial aging procedures were performed: (1) thermal cycling and mechanical loading (TCML): 5000 cycles of thermal cycling 5℃-55℃ and chewing simulation (1,200,000 cycles, 50 N); (2) Low Temperature Degradation simulation (LTD): autoclave treatment at 137℃, 2 bar for 3 hours and chewing simulation; and (3) no pre-treatment (control group). After artificial aging, the crowns were loaded until fracture.

RESULTS

The mean values of fracture resistance varied between 3414 N (LTD; 0.8 mm chamfer preparation) and 5712 N (control group; shoulderless preparation). Two-way ANOVA analysis showed a significantly higher fracture loads for the shoulderless preparation, whereas no difference was found between the chamfer preparations. In contrast to TCML, after LTD simulation the fracture strength of monolithic zirconia crowns decreased significantly.

CONCLUSION

The monolithic crowns tested in this study showed generally high fracture load values. Preparation design and LTD simulation had a significant influence on the fracture strength of monolithic zirconia crowns.

Bioactive and biodegradable silica biomaterial for bone regeneration

Biosilica, a biocompatible, natural inorganic polymer that is formed by an enzymatic, silicatein-mediated reaction in siliceous sponges to build up their inorganic skeleton, has been shown to be morphogenetically active and to induce mineralization of human osteoblast-like cells (SaOS-2) in vitro. In the present study, we prepared beads (microspheres) by encapsulation of β-tricalcium phosphate [β-TCP], either alone (control) or supplemented with silica or silicatein, into the biodegradable copolymer poly(d,l-lactide-co-glycolide) [PLGA]. Under the conditions used, ≈5% β-TCP, ≈9% silica, and 0.32μg/mg of silicatein were entrapped into the PLGA microspheres (diameter≈800μm). Determination of the biocompatibility of the β-TCP microspheres, supplemented with silica or silicatein, revealed no toxicity in the MTT based cell viability assay using SaOS-2 cells. The adherence of SaOS-2 cells to the surface of silica-containing microspheres was higher than for microspheres, containing only β-TCP. In addition, the silica-containing β-TCP microspheres and even more pronounced, a 1:1 mixture of microspheres containing β-TCP and silica, and β-TCP and silicatein, were found to strongly enhance the mineral deposition by SaOS-2 cells. Using these microspheres, first animal experiments with silica/biosilica were performed in female, adult New Zealand White rabbits to study the effect of the inorganic polymer on bone regeneration in vivo. The microspheres were implanted into 5mm thick holes, drilled into the femur of the animals, applying a bilateral comparison study design (3 test groups with 4-8 animals each). The control implant on one of the two hind legs contained microspheres with only β-TCP, while the test implant on the corresponding leg consisted either of microspheres containing β-TCP and silica, or a 1:1 mixture of microspheres, supplemented with β-TCP and silica, and β-TCP and silicatein. The results revealed that tissue/bone sections of silica containing implants and implants, composed of a 1:1 mixture of silica-containing microspheres and silicatein-containing microspheres, show an enhanced regeneration of bone tissue around the microspheres, compared to the control implants containing only β-TCP. The formation of new bone induced by the microspheres is also evident from measurements of the stiffness/reduced Young's modulus of the regenerated bone tissue. The reduced Young's modulus of the regenerating bone tissue around the implants was markedly higher for the silica-containing microspheres (1.1MPa), and even more for the 1:1 mixture of the silica- and silicatein-containing microspheres (1.4MPa), compared to the β-TCP microsphere controls (0.4MPa). We propose that based on their morphogenetic activity on bone-forming cells in vitro and the results of the animal experiments presented here, silica/biosilica-based scaffolds are promising materials for bone repair/regeneration.

The Randomized Shortened Dental Arch Study

The scientific evidence concerning prosthodontic care for the shortened dental arch (SDA) is sparse. This randomized multicenter study aimed to compare two common treatment options: removable partial dental prostheses (RPDPs) for molar replacement vs. no replacement (SDA). One of the hypotheses was that the follow-up treatment differs between patients with RPDPs and patients with SDAs during the 5-year follow-up period.

Two hundred and fifteen patients with complete molar loss in one jaw were included in the study. Molars were either replaced by RPDPs or not replaced according to the SDA concept.

A mean number of 4.2 (RPDP) and 2.8 (SDA) treatments for biological or technical reasons occurred during the 5-year observation time per patient. Concerning the biological aspect, no significant differences between the groups could be shown, whereas treatment arising from technical reasons was significantly more frequent for the RPDP group. When the severity of treatment was analyzed, a change over time was evident. When, at baseline, only follow-up treatment with minimal effort is required, over time there is a continuous increase to moderate and extensive effort observed for both groups ( Controlled-trials.com number ISRCTN97265367).

Wear behavior of dental Y-TZP ceramic against natural enamel after different finishing procedures

OBJECTIVE

The aim of this in vitro study was to evaluate the influence of different finishing procedures on the wear behavior of zirconia against natural enamel.

METHODS

64 quadratic specimens (10 mm × 10 mm × 2 mm) were cut from a commercial hipped dental Y-TZP ceramic. Four different groups with 16 specimens each were formed according to the following finishing procedures: PZ (polished), RR (fine-grit diamond), GR (coarse-grit diamond), GZ (glazed). Polished specimens of a leucite-reinforced glass ceramic (Empress CAD) were used as a control (GC). The materials were subjected to the Ivoclar wear method (Willytec chewing simulator, 120,000 cycles, 5kg weight) with 80 natural caries-free cusps of first upper molars as antagonists. Wear was analyzed for both the enamel cusps and test specimens by measurement of the vertical substance loss with a laser scanner. Surface roughness was measured by means of a white-light interferometer.

RESULTS

The surface roughness was significantly different among the polished, diamond-finished, and glazed ceramic specimens (ANOVA, post hoc Bonferroni p<0.05). The results of the one-way ANOVA indicated that the finishing technique significantly affected enamel wear (p<0.05). The post hoc test indicated that the specimens finished with the coarse diamond caused significantly higher antagonist wear than the polished ones. Polished zirconia showed the lowest wear of the antagonist enamel, with a mean value of 171.74 (SD = 121.68), and resulted in enamel wear that was not significantly different from that of the glass ceramic control group. No significant linear correlation could be found between pre-testing surface roughness and abrasive wear.

SIGNIFICANCE

If zirconia is used without veneering material for crowns and fixed dental prostheses (FDPs), the surface must be well-polished if occlusal adjustments with coarse diamonds are performed. The polishing step reduces the wear of the opposing enamel.

In vitro investigation of the performance of different restorative materials under cast circumferential clasps for removable dental prostheses

OBJECTIVES

The objective of this in vitro study was to investigate the behavior of different composite restorative materials under the load of cast circumferential clasps for removable dental prostheses (RDPs).

METHODS

In 60 human molars, standardized mesial-occlusal-distal cavities were prepared. The cavities were restored with the following materials: Definite, Tetric Ceram, SureFil, Heliomolar RO, Ariston pHc, and Oralloy, and provided with a rest seat. The rest seats were subjected to 5,000 cycles of thermal cycling and 1,200,000 masticatory cycles in a mastication simulator via cobalt-chromium circumferential clasps cast to standardized frameworks in a laboratory model designed to simulate the biomechanics of a free-end denture base. Fracture analysis of the restorations was performed by light microscopy. Before and after loading, material wear was measured with a 3D-laser scanner, and an analysis of the marginal quality was performed in an SEM at ×200 applying the replica technique.

RESULTS

No significant differences in the fracture behavior among the composite materials were found; the amalgam control group showed a significantly higher fracture resistance. Regarding the wear of the materials, the composites Definite and SureFil exhibited a behavior similar to that of amalgam. The other composites demonstrated higher wear rates. The initial marginal quality was significantly worse for Ariston pHc. The marginal adaptation decreased significantly after thermal and mechanical loading for Definite and Ariston pHc.

CONCLUSIONS

In terms of the investigated aspects of mechanical performance, the tested composites seemed to be inferior to amalgam. Further clinical studies are needed to evaluate the ability of composite restorations to provide support for RDP clasps.

CLINICAL RELEVANCE

The use of composites as direct restoration materials should be avoided in teeth, which serve as abutments for clasp-retained RDPs.

Introducing and evaluating MorphoDent, a Web-based learning program in dental morphology

MorphoDent, a computer-assisted learning program designed to teach the anatomy of the adult dentition, was introduced into the curriculum of dentistry at the University of Saarland in Homburg, Germany. Thirty-six second-year students of the School of Dentistry were introduced to morphoDent alongside the traditional lectures of dental anatomy. Questionnaires that evaluated the students' perceptions of virtual learning as well as aspects of the functionality of the program educational objectives were developed and distributed to the students. Online tests were performed and compared with a traditional examination. All questionnaires were returned. A wide variety of aspects dealing the pedagogic implications of e-learning were evaluated by the students, with the overall result being that students are aware of the needed hardware and technical skills and expect the quality of their studies to improve by implementation of e-learning. All sophomores except one felt that morphoDent had helped them in learning dental morphology and reported enjoying the virtual anatomical examination. One-third reported technical difficulties in operating the program. The average exam score was 2.53 (SD: 0.66) for the traditional and 2.23 (SD: 0.59) for the online dental anatomy test. Students expressed their positive attitude towards e-learning in general and indicated that the use of photorealistic 3D models of human teeth supported the process of individual learning of dental anatomy. The online test could be rated as a good option for students' self-performance rating.

Investigations of subcritical crack propagation of the Empress 2 all-ceramic system

INTRODUCTION

The mechanical properties and slow crack propapagation of the all-porcelain system Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein) with its framework compound Empress 2 and the veneering compounds "Empress 2 and Eris were examined.

METHODS

For all materials, the fracture strength, Weibull parameter and elastic moduli were experimentally determined in a four-point-bending test. For the components of the Empress 2 system, the fracture toughness K(IC) was determined, and the crack propagation parameters n and A were determined in a dynamic fatigue method. Using these data, life data analysis was performed and lifetime diagrams were produced. The development of strength under static fatigue conditions was calculated for a period of 5 years.

RESULTS

The newly developed veneering ceramic Eris showed a higher fracture strength (sigma(0)=66.1 MPa) at a failure probability of P(F)=63.2%, and crack growth parameters (n=12.9) compared to the veneering ceramic Empress 2 (sigma(0)=60.3 MPa). For Empress 2 veneer the crack propagation parameter n could only be estimated (n=9.5). This is reflected in the prognosis of long-term resistance presented in the SPT diagrams. For all materials investigated, the Weibull parameter m values (Empress 2 framework m=4.6; Empress 2 veneer m=7.9; Eris m=6.9) were much lower than the minimum demanded by the literature (m=15).

SIGNIFICANCE

The initial fracture strength value alone is not sufficient to characterize the mechanical resistance of ceramic materials, since their stressability is time-dependent. Knowledge about the crack propagation parameters n and A are of great importance when preclinically predicting the clinical suitability of dental ceramic materials. The use of SPT diagrams for lifetime calculation of ceramic materials is a valuable method for comparing different ceramics.

Enterovirus 71 isolated from cases of epidemic poliomyelitis-like disease in Bulgaria

Virological and serological studies of an epidemic disease in Bulgaria, 1975, were carried out. Epidemiologically, clinically and pathomorphologically, the disease simulated almost all known forms of poliomyelitis, acute stem encephalitis, encephalomyocarditis and aseptic meningitis. The studies completely rules out the participation of polioviruses and provided comprehensive evidence for the etiological role of a peculiar enterovirus subsequently identified as enterovirus (EV) type 71 known in the literature since 1974. Altogether, in 1975 and 1976 from 65 cases of poliomyelitis-like disease (PLD) 92 strains of EV71 were isolated, including 37 strains from the brain and medulla, 1 from the cerebrospinal fluid, 10 from mesenterial lymph nodes and tonsils and 44 from faeces. In addition, in 282 convalescent cases of the disease, diagnostic seroconversion or high titers of antibody to this virus were demonstrated. The most successful virus isolation was achieved by inoculation of green monkey kidney cell cultures and newborn white mice. Bulgarian strains of enterovirus 71 regularly caused paralysis in monkeys and morphological poliomyelitis-like lesions in their CNS, and paralysis and myositis with Zenker necrosis in newborn white mice, cotton rats, Syrian hamsters, and 3-week-old cotton rats. The diseased rodents had much more virus in their mucles than in brains.