Dynamic shear properties of the temporomandibular joint disc

Shear stress might be an important factor associated with fatigue failure and damage of the temporomandibular joint disc. Little information, however, is available on the dynamic behavior of the disc in shear. Since the disc is an anisotropic and viscoelastic structure, in the present study the dependency of the dynamic shear behavior on the direction and frequency of loading was examined. Ten porcine discs were used for dynamic shear tests. Shear stress was applied in both anteroposterior (A-P test) and mediolateral (M-L test) directions. The dynamic moduli increased as the loading frequency increased. The dynamic elasticity was significantly larger in the A-P test than in the M-L test, although the dynamic viscosity was similar in both tests. The present results suggest that non-linearities, compression/shear coupling, and intrinsic viscoelasticity affect the shear material behavior of the disc, which might have important implications for the transmission of load in the temporomandibular joint.     

Functional magnetic resonance imaging of human jaw movements

This study used functional magnetic resonance images (fMRI) to examine brain activity during clenching, gum chewing, and tapping tasks. It has been considered difficult to obtain sufficient fMRI data during jaw movement because the head motion associated with the jaw movements creates artifacts on the images. To avoid these artifacts, larger pixels were used, thus allowing some head motion of the subjects, and data from subjects where the heads were evaluated to have moved more than 0.5 mm were discarded. Further, all pixels obtained by fMRI were evaluated and pixels positively synchronized with the task, which were considered to show brain activity, were selected. Sufficient fMRI data was obtained from 30 experiments, 10 sets for each task. During the clenching and tapping tasks, the activated pixels were in the sensory, motor and pre-motor cortexes, and in the sensory and motor cortexes but not in the pre-motor cortex during the gum chewing task. There appears to be no significant differences between right- and left-hemispheres. It is conceivable that there are differences between voluntary jaw movements (clenching and tapping tasks) and mastication (gum chewing task) concerning the control of jaw movements.     

Inhibitory effect of synthetic histatin 5 on leukotoxin from Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a gram-negative bacterium strongly implicated in the pathogenesis of juvenile periodontitis. This periodontal pathogen synthesizes a leukotoxin that destroys human polymorphonuclear leukocytes (PMNs), and this toxin is thought to be responsible for the virulence of A. actinomycetemcomitans. It was therefore of interest to assess whether major virulence factors of periodontal pathogens were neutralized by salivary components. This study focuses on the effect of histatins, components of the nonimmune oral defense system, on leukotoxin activity. Leukotoxin was extracted with polymyxin B from freshly grown anaerobic cultures of A. actinomycetemcomitans strain Y4. PMNs isolated from blood of healthy human volunteers were incubated in a cytotoxicity assay containing PMNs (10(7) cells/ml) and leukotoxin preparation (0-500 microg/ml) in Hanks' balanced salt solution at 37 degrees C for 0-120 min with or without synthetic histatin 5 (0-500 microM). Cytotoxicity was measured by release of lactate dehydrogenase (LDH) at different time intervals. Histatin 5 neutralized the toxic effect of the leukotoxin preparation in a concentration-dependent manner, with an IC(50) value of 150 microM. When PMNs were preincubated with histatin 5 (300 microM), washed and subsequently exposed to leukotoxin, no protective effect was observed. This observation suggests a mechanism of inhibition whereby histatin 5 either directly neutralizes the leukotoxin or interferes with the leukotoxin-PMN interaction. The inhibitory effect of histatin 5 on leukotoxic activity may suggest a new biological function of histatins in the oral cavity as a naturally occurring secondary antibiotic.     

Differential scanning calorimetric studies of nickel titanium rotary endodontic instruments

Differential scanning calorimetric (DSC) analyses were performed between -130 degrees and 100 degrees C on specimens prepared from nickel-titanium (NiTi) rotary endodontic instruments: ProFile (n = 5), Lightspeed (n = 4), and Quantec (n = 3). The ProFile and Lightspeed instruments were in the as-received condition, whereas the Quantec instruments were randomly selected from a dental clinic and had unknown history. The DSC plots showed that the ProFile and Lightspeed instruments analyzed had the superelastic NiTi property, with an austenite-finish (Af) temperature of approximately 25 degrees C. Differences in DSC plots for the ProFile instruments and the starting wire blanks (n = 2) were attributed to the manufacturing process. The phase transformation behavior when the specimens were heated and cooled between -130 degrees and 100 degrees C, the temperature ranges for the phase transformations, and the resulting enthalpy changes were similar to those previously reported for nickel-titanium orthodontic wires having superelastic characteristics or shape memory behavior in the oral environment. The experiments demonstrated that DSC is a powerful tool for materials characterization of these rotary instruments, providing direct information not readily available from other analytical techniques about the NiTi phases present, which are fundamentally responsible for their clinical behavior.     

Dental unit waterlines: review and product evaluation

Dental practitioners must be knowledgeable regarding microbial contamination and biofilm formation in dental unit waterlines. Education should stress the need for improvement in the quality of water delivered to patients during treatment. Manufacturers must also play an important role by providing training and education regarding the proper use and maintenance of their systems. Dental facilities, both public and private, need reliable methods to prevent the development of biofilms within DUWs. These methods must be economical and require minimal effort to use on the part of the dental staff. In order for the system to work efficiently, the effluent water that is produced must be compatible with dental materials and be potentially free from toxic or carcinogenic materials. There are numerous models of water filtration units and chemical flushes available to the dental practitioner. However, the Food and Drug Administration have not approved all products currently on the market. Our evaluation of Zerosil, a new waterline-cleaning product, indicates that it is very easy to use and is extremely effective in killing the commonly found microorganisms in dental unit waterlines, as well as eliminating existing biofilms. It is also economical and requires minimum staff time to keep the waterlines clean. Following the initial treatments during week one, the water emanating from the DUWs was free from any viable microorganisms. This effect was present the entire three weeks in which the waterlines were treated. The elimination of viable microorganisms continued into a fourth study week, even though no further treatment of the DUWs was performed. Although the manufacturer recommends weekly treatment of DUWs following the initial treatment regimen, this result indicates that the product has a longer lasting effect than previously thought. Finally, the product can be delivered through any of the commercially available reservoir/bottle water delivery systems. From our study, Zerosil appears to meet the demanding requirements of keeping dental unit waterlines clean. Based on the research that has been done thus far, no universal treatment protocol can be recommended. A combination of approaches may offer the best available assurance of high-quality dental treatment water. Independent water reservoir systems, when used with a periodic chemical treatment protocol, have demonstrated safety and efficacy. Until we reach a point when a recommendation based on thorough evaluations can be made, dental offices should follow current ADA, OSAP, and CDC guidelines: flush waterlines for two to three minutes at the beginning of each day and for 20 to 30 seconds between each patient, and anti-retraction valves should be installed to prevent oral fluids from being drawn into dental waterlines. It is expected that in the near future, the dental practitioner will have a choice of proven systems and products to deal with this issue. Until that time, one should carefully evaluate any product or system being considered to prevent the formation of biofilms in DUWs.     

Breadth of the mandibular condyle affected by disturbances of the sex hormones in ovariectomized and orchiectomized mice

It is well known that the size of the condyle is different between males and females in humans. It has also been documented that ovariectomy (OVX) and orchiectomy (ORX) influence the remodeling of the condylar head in experimental animals. However, influences of sex hormones on the condyle in terms of its breadth have not been examined. With these considerations, this study was designed to examine the breadth of the mandibular condyle in OVX and ORX mice by histomorphometry 8 weeks after surgery. In the OVX mice, the condylar breadth was significantly larger than in the controls, while no significant differences in the breadth were found between the ORX mice and the controls. The thickness of the condylar cartilage layers exhibited similar findings to the breadth of the condyle. On the contrary, the trabecular bone volume was significantly smaller in the OVX and ORX mice than in the corresponding controls, and the difference was more substantial in female mice than in the males. These findings emphasize that a reduction of sex hormone levels, induced by OVX and ORX, significantly influences condylar morphogenesis.     

Wear of nickel-titanium endodontic instruments evaluated by scanning electron microscopy: effect of ion implantation

The present work analyzes possible increases in resistance to wear of nickel-titanium endodontic instruments that have undergone a process of ionic implantation. ProFile .04 taper #25 instruments were subjected to ionic implantation with bands of nitrogen ions of 250 KeV, currents in the order of 10 microA/cm2, and doses of 2 x 10(17) ions/ cm2. The instruments were used to make preparations in acrylic endodontic training blocks. Scanning electron microscopic investigations showed that after 60 s of work inside the endodontic training blocks nonionic implanted control instruments showed small modifications in their blades, and their tips showed the first signs of wear. After 240 s the control instruments showed consistent signs of wear and frequent changes to their surfaces. After 240 s of use the ionic-implanted instruments did not present any significant changes in the micromorphology of their surfaces. The implanted instruments did not manifest the typical signs of wear and did not show the surface changes that quickly affect the working life of untreated endodontic instruments manufactured from nickel-titanium.     

Treatment timing for rapid maxillary expansion

The aim of this study was to evaluate the short-term and long-term treatment effects of rapid maxillary expansion in 2 groups of subjects treated with the Haas appliance. Treatment outcomes were evaluated before and after the peak in skeletal maturation, as assessed by the cervical vertebral maturation (CVM) method, in a sample of 42 patients compared to a control sample of 20 subjects. Posteroanterior cephalograms were analyzed for the treated subjects at T1 (pretreatment), T2 (immediate post-expansion) and T3 (long-term observation), and were available at T1 and at T3 for the controls. The mean age (years: months) at T1 was 11:10 for both the treated and the control groups. The mean ages at T3 also were comparable (20:6 for the treated group and 17:8 for the controls). Following expansion and retention (2 months on average), fixed standard edgewise appliances were placed. The study included transverse measurements on dentoalveolar structures, maxillary and mandibular bases and other craniofacial regions (nasal, zygomatic, orbital, and cranial). Treated and control samples were divided into 2 groups according to individual skeletal maturation. The early-treated and early-control groups had not reached the pubertal peak in skeletal growth velocity at T1 (CVM 1 to 3), whereas the late-treated and late-control groups were during or slightly after the peak at T1 (CVM 4 to 6). The group treated before the pubertal peak showed significantly greater short-term increases in the width of the nasal cavities. In the long-term, maxillary skeletal width, maxillary intermolar width, lateronasal width, and lateroorbitale width were significantly greater in the early-treated group. The late-treated group exhibited significant increases in lateronasal width and in maxillary and mandibular intermolar widths. Rapid Maxillary Expansion treatment before the peak in skeletal growth velocity is able to induce more pronounced transverse craniofacial changes at the skeletal level.     

Nickel and chromium levels in the saliva and serum of patients with fixed orthodontic appliances

The aim of this study was to evaluate the concentrations of nickel and chromium ions in salivary and serum samples from patients treated with fixed orthodontic appliances. A second aim of this study was to determine any significant changes in these concentrations during any period of the treatment time. Saliva and blood samples were collected from 100 patients ranging in age from 12 to 33 years. Twenty samples from each group were obtained. The groups were as follows: In the first group, saliva and blood samples were collected before insertion of the fixed appliances. In the second, third, fourth, and fifth groups, samples were collected at 1 week, 1 month, 1 year, and 2 years after appliance insertion. The serum was prepared by centrifuging the blood samples at 3000 rpm for 10 minutes. The fixed appliances consisted of an average of 4 bands and 20 bonded brackets. No palatal or lingual appliances welded to bands or extraoral auxiliary appliances were used. The spectrophotometric determinations were carried out using electrothermal atomic absorption spectrophotometry. The results indicated certain differences in the amounts of nickel and chromium released from fixed orthodontic appliances during different periods of treatment. The Mann-Whitney U-test from the SPSS statistics program was used to analyze the significance of the differences between no-appliance samples and those obtained with the appliances present. In the serum, there were statistically significant increases in ion concentration in the second-year groups. In saliva samples, nickel and chromium reached their highest levels in the first month and decreased to their initial level in the rest of the groups. It can be concluded that fixed orthodontic appliances release measurable amount of nickel and chromium when placed in the mouth, but this increase doesn't reach toxic levels for nickel and chromium in the saliva and serum.     

Changes in mechanical properties of poly-l-lactic acid mini-plate under functional load simulating sagittal splitting ramus osteotomy

The purpose of this study was to investigate how the characteristics of a poly-l-lactic acid mini-plate changed with dynamic loading in an environment with hydrolytic degradation. A mandible osteosynthesis model was prepared with specimen poly-l-lactic acid mini-plates to simulate sagittal splitting ramus osteotomy. The model was then subjected to dynamic loading, and changes in specimen shape and surface quality were observed. Specimen bending strength was then measured, and degree of hydrolytic degradation estimated. Neither dynamic loading nor degree of load clearly affected degree of hydrolytic degradation. The specimens maintained their original shape and bending strength for up to 4 weeks with dynamic loading of 40 N or less in an environment with hydrolytic degradation. At 8 weeks, under the same conditions, the specimens showed cracks or fractures, or both, together with a clear decrease in bending strength. The results suggest that dynamic loading causes cracking in a poly-l-lactic acid mini-plate, and that growth of these cracks decreases bending strength over time, leading to fatigue fracture.