Longitudinal and Transverse Instability of Ion Acoustic Waves.

Ion acoustic waves are found to be susceptible to at least two distinct decay processes. Which process dominates depends on the parameters. In the cases examined, the decay channel where daughter modes propagate parallel to the mother mode is found to dominate at larger amplitudes, while the decay channel where the daughter modes propagate at angles to the mother mode dominates at smaller amplitudes. Both decay processes may occur simultaneously and with onset thresholds below those suggested by fluid theory, resulting in the eventual multidimensional collapse of the mother mode to a turbulent state.

Numerical investigation of transients in the SSPX Spheromak.

Nonlinear plasma simulations of the Sustained Spheromak Physics Experiment demonstrate the role of transient effects in establishing a toroidal magnetic structure that confines internal energy. Magnetohydrodynamics modeling with temperature-dependent transport coefficients compares well with experimental measurements and shows that the second current pulse improves confinement by keeping the q profile from falling below the value of 1/2, suppressing resonant m = 1, n = 2 fluctuations.

Increasing the magnetic helicity content of a plasma by pulsing a magnetized source.

By operating a magnetized coaxial gun in a pulsed mode it is possible to produce large voltage pulses of duration approximately 500 mus while reaching a few kV, giving a discrete input of helicity into a spheromak. In the sustained spheromak physics experiment (SSPX), it is observed that pulsing serves to nearly double the stored magnetic energy and double the temperature. We discuss these results by comparison with 3D MHD simulations of the same phenomenon.

Observation of the parametric two-ion decay instability with thomson scattering.

We present the first direct experimental observation of the parametric two-ion decay instability of ion-acoustic waves driven by a high intensity (5 x 10(15) W cm(-2)) laser beam in a laser produced high-Z plasma. Using two separate Thomson scattering diagnostics simultaneously, we directly measure the scattering from thermal ion-acoustic fluctuations, the primary ion waves that are driven to large amplitudes by the high intensity beam, and the two-ion decay products. The decay products are shown to be present only where the interaction takes place and their k spectrum is broad.

Observation of saturation of energy transfer between copropagating beams in a flowing plasma.

Experiments demonstrate energy and power transfer between copropagating, same frequency, beams crossing at a small angle in a plasma with a Mach 1 flow. The process is interpreted as amplification of the low intensity probe beam by the stimulated scatter of the high intensity pump beam. The observed probe amplification increases slowly with pump intensity and decreases with probe intensity, indicative of saturation limiting the energy and power transfer due to ion-wave nonlinearities and localized pump depletion. The results are consistent with numerical modeling including ion-wave nonlinearities.

Use of inductively coupled plasma-emission spectroscopy and mercury vapor analyses to evaluate elemental release from a high-copper dental amalgam: a pilot study.

STATEMENT OF PROBLEM: The use of dental amalgam as a direct restorative material has been a subject of controversy for many years. The potential safety of amalgam has been questioned because of leakage of elements such as mercury, copper, tin, and silver. PURPOSE: This study evaluated the elemental leaching from Tytin dental amalgam placed in deionized water for 2 months. Both mercury vapor and elemental (silver, copper, tin, and mercury) analyses were performed. MATERIAL AND METHODS: Two capsules of Tytin amalgam were triturated (one for the precipitate and liquid analysis, and the other for the mercury vapor analysis) and stored in a polypropylene tube with 10 mL deionized water for 60 days at room temperature. The amalgam pellet then was removed and rinsed with deionized water. The resulting liquid was separated from a precipitate, and 2 separate analyses were run: one on the liquid without any precipitate and another on the precipitate. Elemental analyses for copper (Cu), tin (Sn), mercury (Hg), and silver (Ag) were determined by inductively coupled plasma-emission spectroscopy with a Perkin-Elmer P2000 spectrometer. Mercury vapor analyses were performed daily for 60 days with a Jerome 431-X vapor analyzer. RESULTS: The maximum amount of copper (80 microg), silver (2.6 microg), mercury (15 microg), and tin (550 microg) was found in the precipitate. The maximum amount of mercury vapor released was 67 microg/m(3)/d. CONCLUSION: Under the conditions of this in vitro study, there was a significant amount of elemental leaching and mercury vapor release from the Tytin amalgam over a 60-day period.

GABA-transaminase, the liver and infantile autism.

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter of the mammalian brain and the enzyme responsible for catabolism (breakdown in the liver during regulation) is GABA-Transaminase (GABA-T). Recently, Cohen has shown that extremely high GABA levels in the urine and blood were observed for an autistic child. The finding that elevated levels of GABA in the urine and blood are present for an autistic child could explain why autistic features (such as self-stimulatory behavior and language delays, etc.) are found. Increasing the GABA-T enzyme activity for this autistic patient could result in less plasma GABA (after liver regulation) entering into the bloodsteam and brain and it is postulated that this could result in a reduction of the autistic features (such as self-stimulatory behavior and language delays, etc.) due to abnormal development of the axon(s) in the corpus callosum.

Comparison of the flexural strength of six reinforced restorative materials.

This study calculated the flexural strength for six reinforced restorative materials and demonstrated that flexural strength values can be determined simply by using physical parameters (diametral tensile strength and Young's modulus values) that are easily determined experimentally. A one-way ANOVA analysis demonstrated a statistically significant difference between the two reinforced glass ionomers and the four composite resin materials, with the composite resin being stronger than the glass ionomers.

The relationship of post design to the long-term success of endodontically restored teeth.

Artificial cores replace missing tooth structure and contribute to the retention and stability of the subsequent restoration. The post is the anchoring mechanism for the core, which supports the final restoration. Stability of the post and core is the most important factor concerning the long-term success of the post-and-core restoration. The amount of insertional and functional stresses produced and distributed by a post are also associated with retention. An optimal way of maximizing retention, minimizing insertional stresses, and distributing functional stresses may be through the use of a multi-tiered parallel threaded split-shanked post.

Torsional resistance of crowns cemented to composite cores involving three stainless steel endodontic post designs.

STATEMENT OF PROBLEM: There are no studies analyzing the effects of a crown in relationship to torsional resistance for a titanium-reinforced core material supported by a post. PURPOSE: This study investigated the effects of post design on the torsional resistance of a crown supported by a titanium-reinforced composite core material (Ti-Core) and 3 endodontic posts (AccessPost, Flexi-Post, and ParaPost). MATERIAL AND METHOD: Three groups of 10 specimens per group were studied. A total of 30 recently extracted human single-rooted (incisors) teeth with their crowns removed were used. Post holes preparations were created according to manufacturer's instructions. All posts were cemented using Flexi-Flow with titanium cement. Scotchbond Multi-Purpose bonding system was used as the bonding system for all groups. All cores were fabricated with Ti-Core with titanium core material with hard copper bands as the matrix. The Ti-Core core material was allowed to set for 1 hour. Metal crowns were fabricated and cemented with zinc phosphate cement. Specimens were placed in a special jig and a clockwise torsional force was applied. Torsional force was measured on a Lebow 1102-200 torque transducer and recorded on a Hewlett Packard 7015B X-Y recorder in inch x ounce. A 1-way analysis of variance (ANOVA) was used to test for statistical significance (P < .001). RESULTS: The torsional values (inch x ounce) were AccessPost 77.2 +/- 30.3, Flexi-Post 162.2 +/- 51.2, and ParaPost 60.9 +/- 28.4. SNK and Scheffé comparison tests revealed that the Flexi-Post group had significantly greater resistance to torque loading than the AccessPost and ParaPost groups, which were statistically similar to each other. CONCLUSION: Post design has an effect on torsional resistance of a crown supported by titanium-reinforced core build-up and post. The threaded split-shank design of the Flexi-Post dowel offers significantly greater resistance to torsional loading than the passive posts studied.

Retention of a core material supported by three post head designs.

STATEMENT OF PROBLEM: There are few studies on the retention of core build-up materials to prefabricated post head designs, especially for a new ceramic post design. PURPOSE: This in vitro study compared the retention of 2 types of core build-up materials (Ti-Core titanium-reinforced composite and a GC Miracle Mix silver-reinforced glass ionomer) supported by 3 post head designs (Flexi-Post, AccessPost, and Cerapost dowel). For all test groups, heads of the posts were placed into the core material at a 90-degree angle to the surface. Test specimens (unloaded) were then placed into a special jig and retention test was performed using an 810 MTS testing machine. MATERIAL AND METHODS: This study consisted of 6 groups with 10 specimens per group. A 3 x 2 factorial design was used to test for statistical significance and results were considered significant when P <.05. RESULTS: Results of the retention test were as follows: group 1, Flexi-Post/Ti-Core 220.0 lb (982.1 N), group 2, AccessPost/Ti-Core 212.7 lb (949.6 N), group 3, Cerapost/Ti-Core 41. 8 lb (186.6 N), group 4, Flexi-Post/Miracle Mix 43.4 lb (193.8 N), group 5, AccessPost/Miracle Mix 61.6 lb (275 N), and group 6, Cerapost/Miracle Mix 7.5 lb (33.5 N). Results of the ANOVA revealed a highly significant difference between posts and cores (P <.0001). CONCLUSION: Post head designs of the stainless steel AccessPost and Flexi-Post dowels offers greater retention then the smooth ceramic head design of the Cerapost dowel. In addition, the composite core material (Ti-Core) offers greater retentive strength values than the glass ionomer material (Miracle Mix).

Pilot study of the cyclic fatigue characteristics of five endodontic posts with four core materials.

This pilot study concerned the cyclic fatigue of five different endodontic post systems (AccessPost(R), Flexi-Flange(R), Flexi-Post(R), ParaPost(R), and Vlock(R)) with four different core materials (Tytin(R) Silver amalgam, Ti-Core(R), Ketac-Silver(R) and G-C Miracle Mix(R)). In vitro cyclic fatigue was performed using a fatigue machine that simulates masticatory fatigue forces. An instantaneous force of 5 lb was applied to each post and core combination for a test configuration of 4 000 000 repetitions or until failure occurred. The type of failure and number of repetitions at failure was recorded for each sample tested. Two-way ANOVA was used to compare groups. All posts/core samples with Ti-Core and Tytin Silver amalgam completed the test configuration of 4 000 000 cycles without failures. All posts/core samples with Ketac-Silver failed before the 4 000 000 test cycle configuration, all as a result of core failures. All posts/core samples with G-C Miracle Mix failed in a similar way. The Newman-Keuls multiple comparison test illustrated that Ti-Core and Tytin Silver amalgam were similar to each other and were superior to both G-C Miracle Mix and Ketac-Silver. Similar fatigue test results are found in the literature.

Retention of four endodontic posts cemented with composite resin.

Retention was compared of an active post system (Flexi-Flange), a metal passive prefabicated post system (ParaPost), a passive prefabricated burnout post system (ExactaCast) with and without grooved dentin walls, and a zirconium oxide ceramic post design (Cerapost). The Flexi-Flange with Flexi-Flow Natural cement obtained the highest retentive value of 270.4 lb. The Cerapost cemented with Universal Cement had the lowest retention value of 23.4 lb. The ExactaCast with grooved dentin walls was significantly stronger than the ExactaCast without grooving, the ParaPost, and the Cerapost. The two ExactaCast groups and the ParaPost group had higher retention than the Cerapost group.

An in vitro study of the cytotoxicity of two root canal sealers.

The purpose of this in vitro study was to determine the cytotoxicity of two root canal sealing materials (AH26 and AH-Plus). This cytotoxicity test (agar diffusion test) was conducted based on the procedures described in the International Organization for Standardization. The biological reactivity of a mammalian monolayer, L929 mouse fibroblast cells, in response to the tested agents was determined. After the 48-h observation period, the cell cultures exposed to the test articles discs for AH26 and AH-Plus exhibited severe reactivity (grade 4). The positive control article exhibited moderate reactivity (grade 3). No signs of reactivity (grade 0) were noted for the negative control article or the negative control discs. The tested samples of AH26 and AH-Plus are considered cytotoxic and do not meet the requirement of the agar diffusion test. Similar cytotoxicity results have been found in the literature for AH26 and other root canal sealing cements.

The evolution of instrumentation and obturation leading to a simplified approach.

The evolution of endodontic treatments has included the introduction of rotary crown-down nickel-titanium instrumentation and the use of thermoplasticized gutta percha to obturate canals. Although these techniques have provided dentists with solutions to problems encountered with earlier generation endodontic procedures, they have also presented deficiencies that are now receiving some critical review. As a result, a systematic technique has been developed that provides a simplified approach for root canal instrumentation and predictable obturation. This article reviews the aspects of earlier endodontic techniques and provides the step-by-step clinical protocol involved with simplified endodontic techniques.

Comparison of the retentive properties of six prefabricated post overdenture attachment systems.

STATEMENT OF PROBLEM: There are few research studies concerning prefabricated posts overdenture attachment systems using multiple pull cycles. PURPOSE: This study tested 6 prefabricated post overdenture attachment systems (AccessPost Overdenture, ERA white and ERA grey, Flexi-Overdenture, O-SO and ZAAG) measured to simulate wear over a 2000 pull cycle and compared the in vitro retention, measured (pounds). MATERIAL AND METHODS: Records of the pounds of force necessary to separate each system were made at the beginning of testing, at pull 10 and then at every 50th pull until the 2000th pull was attained. All pulls were performed manually with a Shimpo force gauge. Absolute force (AF) and relative force (RF) were compared, AF being the force required at any pull to separate the overdenture device, whereas RF is the force required at the first pull. RESULTS: There was considerable difference of the force required at pull 1 for the ERA white, O-SO and ZAAG anchor systems, then the AccessPost overdenture, and finally the Flexi-Overdenture and ERA grey attachment systems. CONCLUSION: At pull 2000, there was no statistical difference between any of the attachments studied. The amount of force necessary to offer functional retention to a specific patient wearing an overdenture may vary considerably, depending on the many factors in fabricating the overdenture, and the age, strength, musculature, and the expectations of the patient.

Pilot study of the effects of three bonding systems on the torsional resistance of a titanium-reinforced composite core.

STATEMENT OF PROBLEM: There are no studies analyzing the effects of a bonding agent on torsional resistance for a titanium-reinforced core material bonded with a multistep bonding system. PURPOSE: This pilot study investigated the effects of multistep bonding agents on torsional resistance of a titanium-reinforced composite core material (Ti-Core core material). MATERIAL AND METHODS: A total of 40 recently extracted human molars with their crowns removed were placed into acrylic resin blocks and divided into 4 groups of 10 specimens. Only multistep bonding systems were used (AllBond-2, ScotchBond Multi- PURPOSE: Tenure A&B, and the control without bonding agent). Specimens were prepared according to manufacturer's instructions. The control group was etched with a 37.5% phosphate acid. All cores were fabricated with Ti-Core titanium core material with No. 1 hard copper bands as the matrix. Ti-Core core material was applied and allowed to cure for 1 hour, then the cores were placed in 100% humidity (water) for 1 week. Test specimens were placed in a special jig and a clockwise torsional force applied. Torsional force was measured on a Lebow 1102-200 torque transducer and recorded on a Hewlett Packard 7015B X-Y recorder in inch x ounce then converted into Newton-meter. One-way analysis of variance (ANOVA) was used to test for statistical significance (P <.05). RESULTS: Torsional values (Newton-meter) were AllBond-2, 0.1980, ScotchBond Multi- PURPOSE: 0.1890, Tenure A&B, 0.2142; the control group recorded a value of zero. Statistical analysis revealed that bonding systems did not differ from one another. CONCLUSION: Use of a bonding agent increased the core's resistance to torsional forces. Lack of a bonding agent dramatically reduced the resistance to torque.

Comparison of the retentive and photoelastic properties of two prefabricated endodontic post systems.

This in vitro study compared the retention and photoelastic stress patterns from two loading conditions, vertical (133.2 N, 30 pounds) and oblique at a 26 degrees angle (133.2 N, 30 pounds) for two prefabricated post systems. The post studied were: (1) Flexi-Post (a split-shank threaded post) and (2) C-Post (a carbon fibre bound in an epoxy resin, passive double taper tier post). Two groups with 10 specimens per group were subjected to retentive forces with a universal testing machine (MTS 810 Material Testing Machine). In addition, two photoelastic test blocks were prepared with simulated root canals for each post studied. After cementation the photoelastic blocks were photographed before vertical and oblique loading and after loading. One-way analysis of variance (ANOVA) for retention data revealed a highly significant difference (P<0.0001) between groups. The Flexi-Post had a statistically higher mean retention force of 1180.6 N (265.9 pounds), while the C-Post had a mean of 171.8 N (38.7 pounds). Photoelastic analysis indicated minimal stresses for both the Flexi-Post and C-Post in the unloaded state. The C-Post showed asymmetrical apically stress patterns, while loaded in both states. The multi-tiered system of the Flexi-Post clearly distributes stress symmetrically, while the C-Post distributes stresses asymmetrically. The symmetric, even stresses and statistically higher retentive strength for Flexi-Post are more favourable than the asymmetric, uneven stresses and relatively low retentive strength for the C-Post.

Effects of three bonding systems on the torsional resistance of titanium-reinforced composite cores supported by two post designs.

STATEMENT OF PROBLEM: There are no studies that have investigated the effects of bonding agents on the resistance to torque of a composite core supported by a prefabricated post. PURPOSE: This study investigated the ability of bonding agents to resist rational forces applied to titanium-reinforced composite cores supported by 2 post designs. MATERIAL AND METHODS: Two post designs (AccessPost and ParaPost dowels) with titanium-reinforced core material and 4 bonding conditions (AllBond-2, ScotchBond Multi-Purpose, Tenure A&B, and without a bonding agent) were studied. A total of 80 specimens in 8 experimental groups were prepared and subjected to clockwise torsional forces until failure. RESULTS: Torsional values ranged from 53.31 in. x oz (ParaPost/ScotchBond Multi-Purpose) to 72.31 in. x oz (AccessPost/ScotchBond Multi-Purpose). Two-way of analysis of variance failed to demonstrate a significant difference among posts or type of bonding system. CONCLUSION: The use of a multistep bonding agent had no significant effect on the torsional resistance of a core supported by a passive endodontic post.

Rethinking endodontics: attaining total obturation of the root canal system with a simplified system.

A sequence of steps constituting an innovative way of cleansing, shaping and obturating a root canal system is described. The advantages of this sequence over other instrumentation and obturation techniques are discussed.