Fracture load of different crown systems on zirconia implant abutments.

OBJECTIVES: The purpose of this study was to evaluate the fracture load of single zirconia abutment restorations using different veneering techniques and materials. MATERIALS AND METHODS: The abutment restorations were divided into 6 groups with 20 samples each: test abutments (control group A), lithium disilicate ceramic crowns bonded on incisor abutments (group B), leucite ceramic crowns bonded on incisor abutments (group C), premolar abutments directly veneered with a fluor apatite ceramic (group D (layered) and group E (pressed)) and premolar abutments bonded with lithium disilicate ceramic crowns (group F). The fracture load of the restorations was evaluated using a universal testing machine. Half of each group was artificially aged (chewing simulation and thermocycling) before evaluating the fracture load with the exception of the test abutments. RESULTS: The fracture load of the test abutments was 705 ± 43N. Incisor abutments bonded with lithium disilicate or leucite ceramic crowns (groups B and C) showed fracture loads of about 580N. Premolar restorations directly veneered with fluor apatite ceramic (groups D and E) showed fracture loads of about 850N. Premolar restorations bonded with lithium disilicate ceramic crowns (group F) showed fracture loads of about 1850N. The artificial ageing showed no significant influence on the strength of the examined restorations. SIGNIFICANCE: All ceramic crowns made of lithium disilicate glass-ceramic, adhesively bonded to premolar abutments showed the highest fracture loads in this study. However, all tested groups can withstand physiological bite forces.

Contribution to the biological assessment of orthodontic acrylic materials. Measurement of their residual monomer output and cytotoxicity.

The acrylic materials used in orthodontics for the fabrication of removable appliances are subjected in the oral cavity to processes of change which influence their physical, mechanical and biological properties. It is therefore essential that every newly developed material must be judged in terms of its clinical value. In the present study, 2 orthodontic cold-cure acrylics, Orthocryl and Forestacryl, and 4 orthodontic photocure acrylics, Triad, Wil-O-Dont, Odontolux and Lux-A-Tech, were investigated and compared with 2 prosthetic acrylic materials: the cold-cure acrylic Palapress and the hot-cure acrylic Paladon. The quantity of residual monomers from methyl methacrylate (MAA) or urethane dimethacrylate (UDMA) eluted from the sample in a given time after the processing was estimated by high pressure liquid chromatography (HPLC). The cytotoxic properties of the materials were examined by Mosmann's proliferation-inhibition test with an established culture of fibroblasts (= MTT test). The hot-cure acrylic Paladon produced by far the smallest amount of eluted residual monomer and the least growth inhibition in the MTT test. The prosthetic cold-cure acrylic Palapress achieved significantly better results than the orthodontic cold-cure materials Orthocryl and Forestacryl. The photocure acrylics released less UDMA than did the cold-cure acrylics MMA. In the cell culture test, all the orthodontic materials examined were assessed as "slightly cytotoxic"; the prosthetic acrylics were graded under ISO-standard 10993-5 as "noncytotoxic". After soaking the plastic material in water for 3 days its cytotoxic properties, as exemplified by the cold-cure acrylic Forestacryl and the photocure acrylic Triad, were reduced, and during the following investigation no more inhibition of growth was observed. It was possible to confirm with the tests used that, for Triad, it is necessary to carefully remove the oxygen-inhibition layer of the photocure acrylic in order to improve the biological properties. The influence of the plastic material on fibroblast cultures was assessed, among other methods, by the quantity of residual monomers liberated. These were significantly reduced after soaking the manufactured substance in water for 3 days. Careful laboratory treatment of the photocure acrylics is necessary in order to improve their biological properties.

Transcutaneous electrical nerve stimulation (TENS): its short-term and long-term effects on the masticatory muscles.

In an electromyographic study on subjects with no functional disturbances of the masticatory muscles, the duration of the post-therapeutic effects of transcutaneous electrical nerve stimulation (= TENS) on the superficial masseter and anterior temporal muscle was analyzed. The myoelectric signals were registered from 20 healthy volunteers in 3 different mandibular positions. The recordings were performed before a 20-minute TENS application with the J-4 Myomonitor and continued with a sequence of follow-up registrations with increasing interval to the initial stimulation. The EMG signals underwent computer-aided analysis and were evaluated by determining the integrated values as a parameter of muscle activity, and after Fourier transformation by 7 describing parameters of the power spectrum (e.g. mean power frequency = MPF). A detailed analysis of variance of all data was used to investigate significant changes of the parameters during the observation period. Muscular response to TENS includes a decrease in muscular activity (= reduction in integrated EMG signals) and a shift in the power spectrum to higher frequencies (increase in MPF). These changes were statistically highly significant for both analyzed muscles and for all different mandibular exercises. As these reactions to TENS are contrary to muscle fatigue, the results can be interpreted as indicating that this type of therapy stimulates a change in the biochemical and physiological muscular conditions, which leads to muscle relaxation. Electromyographically, the post-therapeutic effect lasted for 2 hours in case of normal masticatory muscle activity but for more than 7 hours in case of low muscular loading. The alterations of the integrated EMG values were more persistent than those of the parameters of the power spectrum.

In vitro investigation into the biological assessment of orthodontic wires.

The alloys used in orthodontics are subject in the moist environment of the oral cavity to various corrosion processes. If the products of the corrosion are introduced into a biological system they may cause changes. In the present investigation the corrosion rate of 23 different orthodontic wires (preformed arch wires and straight wires) made from 5 different alloys were examined in a nutrient medium by ICP-AES analysis, and the influence of the corrosion products on the cytotoxicity of a fibroblast culture was investigated using Mosmann's MTT test. The nickel-titanium wires Nitinol, Sentalloy and Original Chinese Wire and the beta-titanium alloy TMA had no effect on the rate of cell proliferation. Nor did stainless steel wires inhibit growth significantly, with the exception of Australian Wire and Wildcat Wire. The manganese-steel alloys Noninium h and Mezanium caused significant reductions in growth rate, which were attributed to the manganese ions released by the corrosion. The most severe growth inhibition was caused by the Co-Cr-Ni alloy Elgiloy, and this reaction is independent of the 4 levels of resilience. The degree of growth inhibition depended upon the concentration of corrosive cobalt and nickel ions in the eluate. In spite of the differences observed, all the orthodontic wires examined are graded under ISO-standard 10993-5 as "non-cytotoxic". The degree of toxicity was found to be determined essentially by the corrosion rate of the alloy and the cytotoxic characteristics of the resulting trace elements.

A study of different calcification areas in newly formed bone 8 weeks after insertion of dental implants in rabbit tibias.

In orofacial implantology there are many types of implants for the different systems. Among these is the implant surface type, e.g., a screw type, cylindrical and laminar. Furthermore, the implants are different in their dimensions, their metal composition, their surface condition, such as smooth, grit or layered surfaces and in their methods of application. Two different self-tapping implants, one smooth and the other grit-blasted, are screwed into the bone, and another one with a plasma of titanium coating, which is also in a screw form but with greater spaces between the screw threads are compared. The greatest amount of bone deposition in the bone/implant interface was encountered in the latter one, the smooth surfaced implant being in second place. All of these systems can alter the implant healing process and to demonstrate this, we injected bone markers in the rabbits over different periods of time so as to observe the different areas of bone deposition in the tibias where the implants had been inserted. The bone tracers used were Alizarin, Calcein and Xylenol-orange. The amount of deposition was calculated by using the method of surface morphometry.

Technical and biophysical aspects in prosthetic occlusal rehabilitation for edentulous teenagers.

The loss of anterior teeth may be transient solved by temporary adhesive bridges (Oslo bridge, Rochette, Ellmann, Neumann, Tschemitscheck, Armatron systems). Provisional crowns and bridges could be used for Brackets fixing. Anterior bridge with subocclusal connector permits transversal expansion. When a successful orthodontic treatment is followed by a prosthetic treatment, it is not important to change the achieved occlusion, by the occlusal framework morphology. The framework design of the metal component can contribute to the improvement of the aesthetic aspect and reduce the cost by using a small quantity of expensive alloy. Cytocompatibility of the used Cr-Co alloy is confirmed by cell culture test.

[Muscle relaxation by transcutaneous electric nerve stimulation (TENS) in bruxism. An electromyographic study]. / Muskelentspannung durch transkutane Elektroneurostimulation (TENS) bei Bruxismus. Eine elektromyographische Studie.

In recent years transcutaneous electrical neuromuscular stimulation (TENS) has become increasingly more common in the treatment of functional diseases of the masticatory muscles and currently the practitioner can choose among a variety of stimulators. In an electromyographic study of 17 adults suffering from nocturnal bruxism and of a control group consisting of 18 adults without any functional disturbances of the masticatory muscles, the effect of this kind of neuromuscular stimulation on the temporal and masseter muscle was examined. The myoelectric signals were registered before and after TENS treatment in 3 different positions of the mandible. Each person was treated 3 times with both the Myo-Monitor (Myo-Tronics, Seattle) and the TNS SM2 MF stimulator (schwa-medico, Giessen). The different effects of continuous low frequency and intermittent high frequency muscular stimulation were studied simultaneously. Muscular activity was determined by computer aided integration of the electromyographically measured raw signals. A fourier analysis of the power spectrum yielded information on the frequency behavior of the studied muscles resulting from the TENS treatment. A statistical analysis of the results led to the following significant conclusions: 1. TENS treatment decreased the values of the registered integrated signals on all test persons, however, the treatment increased the median frequency and the mean power frequency (MPF). Since this effect is contrary to muscle fatigue, these electromyographic results can be interpreted as providing objective proof of a relaxation in the treated muscles. 2. The electromyographic changes after TENS treatment were similar when using either the Myo-Monitor or the TNS SM2 MF stimulator. In addition, the 2 different types of stimulation (high or low frequency) showed the same effects. 3. Compared to the persons in the control group, there was no significant increased muscular activity in patients with nocturnal bruxism. Following TENS treatment both groups showed the same alterations in the electromyograms. The results of the study provide further evidence that TENS treatment is an adequate supportive procedure in the treatment of nocturnal bruxism.

Strength and mode of failure of single implant all-ceramic abutment restorations under static load.

The strength and mode of failure of three different designs of custom-made all-ceramic implant abutments fabricated by milling of In-Ceram sintered ceramic blocks were compared with the conventional CeraOne system under static load. Four test groups were formed with different locations of abutment screws. In three test groups, In-Ceram crowns were fabricated for placement on the all-ceramic abutments, and in one test group, a veneer porcelain was fired directly on the abutment; crowns in the control group were fabricated using the CeraOne system. Ten-mm-long Brånemark implants were placed into a brass block that allowed loading at a 30-degree angle to the long axis. The test group in which the veneer porcelain was fired directly on the all-ceramic abutments was the weakest, and it showed fractures at a mean value of 236 N. The fracture strength of the three other test groups was dependent on the extension of the crown margin relative to the location of the screw head. The test group that had the screw on the top compressing the entire ceramic abutment showed a mean value of 422 N that was similar to the results that were achieved with the CeraOne system (427 N). The weakest link in the all-ceramic single implant restorations was the abutment screw in which the bending began at approximately 190 N.

Cellular and molecular biological events at the implant interface.

Since implants attain and maintain contact with the body environment through their surfaces, the interfacial interaction between implant bed and implant surface could be a determinant of the implant's performance. This study used osteoblasts to test the ability of different ceramic and metallic surfaces to bind fibronectin and to support cell attachment, adhesion, migration and synthesis of extracellular matrix. The results showed active molecular and cellular interaction between the cells and the surfaces. Extracellular matrix synthesis was demonstrated on surfaces that did not subject the cells to toxicity embarrassment. It is concluded that the surfaces of implants should not be designed to be mere bystanders in the interfacial scenario. Instead they should be conceived with a view to enabling them to interact positively with the molecules responsible for would healing, interfacial tissue morphogenesis and repair.

Interfacial reactions of osteoblasts to dental and implant materials.

Scanning electron microscopy was used to study the response of osteoblasts to various surfaces including ceramics and glasses as well as steel and titanium. Hydroxylapatite, tricalcium phosphate, bioglass, steel, and titanium supported cell adhesion. However, the toxic effects of the In-Ceram (Vita, Bad Säckingen, Germany), and feldspar ceramic and glaze were severe enough to cause verrucous necrosis that was identifiable after 2 days of culture. After 10 days in culture only the peripheral portions of these specimens were still occupied by cells; the cells in the central portion of the circular specimens had succumbed to necrosis. It was concluded that scanning electron microscopy is useful in identifying the response of cells to materials. Pathologic changes are not recognizable if they are limited to the internal structure of the cell, but readily discernible when they impinge on the morphologic integrity of the cell surface.

[Integrated and frequency spectra of the electromyograms in patients with temporomandibular symptoms]. / Integral- und Frequenzspektrum der Elektromyogramme bei Patienten mit Kiefergelenksymptomen.

With the aid of bipolar surface electrodes, the electromyographic activity of the anterior temporal and the superficial masseter muscles were recorded bilaterally and evaluated by counting the integrated potentials and mean power frequency using Fourier's analysis. A comparison between 17 patients with myofacial pain dysfunction (average age 21.3 years) and a group of 20 controls (average age 20.5 years) revealed a significantly greater activity in patients with pain syndrome, while no significant inter-group difference in mean power frequency was seen. The differences in the level of activity between the right and left sides in the control group were significantly smaller than in patients with pain dysfunction. An investigation of measures aimed at reducing the right/left asymmetry in a further 14 patients with pain dysfunction revealed the practice of "maximum occlusion with feedback" to be the most effective measure.

[Texture formation of the dental ceramic core material In-Ceram and thermal expansion of its individual components]. / Gefügeausbildung der dentalkeramischen Kernmasse In-Ceram und thermische Dehnung ihrer Einzelkomponenten.

Electron microscopic investigations of the core material In-Ceram show how its texture is formed. While these are processed the Al2O3 crystallites turn idiomorphic. Etching the material with sulphuric acid selectively dissolves the glassy phase and reveals the crystallites. The chemical composition of the two components of the core material is determined by microprobe analysis. While the crystallites are pure Al2O3, the glass consists of La2O3, SiO2, Al2O3 and CaO. Measurements of the thermal expansions of the glass and the Al2O3 frame show the value for the glass to be 5.1% lower than that of the Al2O3 frame. This results in internal pressure and contributes to the high strength of the material.

[Strength of the In-Ceram system under mechanical loads and thermocycling in artificial saliva]. / Festigkeitsverhalten der In-Ceram-Keramik bei mechanischer und thermischer Wechsellast im Kunstspeichel.

The mechanical strength of the new oxide ceramic system In-Ceram introduced by Vita (Bad Säckingen, Germany) was investigated by an in-vitro study for its mechanical strength and suitabilities for use as posterior three-unit bridges. The transverse strength value measured by a three-point bending test (DIN 13,927) is 320 MPa and the maximum vertical load on three-unit bridges until brittle failure occurs is in the range of 1000 to 1350 N. This is up to four times higher than corresponding values of other ceramics, e.g. Dicor (Dentsply) or Optec (Jeneric). The dental bridges were luted to metallic pontics, and a layer of silicon rubber simulating the periodontal membrane, separated the pontic from a resin base in the testing machine. Life value tests were performed before and after 10 days of thermocycling at temperatures alternating between 5 and 55 degrees C in artificial saliva. Although the pontics had a much higher mobility than normal teeth, the fatigue limit was found to be above 600 N which is much higher than the average biting forces acting on the first and second molars.

[Field study on the accuracy of fit and the marginal seal of In-Ceram crowns and bridges]. / Feldstudie über die Passgenauigkeit und das Randschlussverhalten von In-Ceram-Kronen und -Brücken.

It was the objective of this in-vitro field study to evaluate the marginal fit of the In-Ceram system (Vita). All the materials applied in this system were compared with each other under the aspect of their dimensional behavior to determine the influence of this factor on die adaptation. The mean value of the marginal fit of the In-Ceram restorations on the master models was 12 (+/- 10) microns. The PFM bridges showed marginal openings of 9 (+/- 10) microns. The values on the patient models were 38 (+/- 12) microns for In-Ceram, and 44 (+/- 19) microns for PFM. This increase had been anticipated because of the grain size of the Harvard Cement. The dimensional changes of the materials involved were in great harmony. At least one coat of die spacer is necessary to compensate for the contraction of the ceramic and the expansion of the special plaster. Two paints are recommended for crowns and three for bridges. Careful shoulder or chamber preparation is required to create smooth surfaces.

[Qualitative studies on the surface crystallization of dental glass ceramics (Dicor)]. / Qualitative Untersuchungen zur Oberflächen-Kristallisation dentaler Glaskeramik (Dicor).

The crystallization of enstatite at the surface of Dicor glass during 'ceramization' is proved by electron microscopy and x-ray diffraction. Microprobe measurements demonstrate a loss of potassium and an uptake of calcium in the outer region of the glass-ceramic after ceramization due to an ion exchange between glass and ceramization investment. This investment consists of gypsum and leucite. A thermal analysis reveals the decomposition of gypsum in the range of the ceramization temperature. Calcium diffuses into the surface of the glass while simultaneously potassium leaves the glass. The altered composition allows the crystallization of enstatite.