Dental Materials: Properties And Manipulation, 9e (Dental Materials: Properties Amp; Manipulation (
Wan Cabiness
The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.
A wide range of polymers are commonly used for various applications in prosthodontics. Polymethyl methacrylate (PMMA) is commonly used for prosthetic dental applications, including the fabrication of artificial teeth, denture bases, dentures, obturators, orthodontic retainers, temporary or provisional crowns, and for the repair of dental prostheses. Additional dental applications of PMMA include occlusal splints, printed or milled casts, dies for treatment planning, and the embedding of tooth specimens for research purposes. The unique properties of PMMA, such as its low density, aesthetics, cost-effectiveness, ease of manipulation, and tailorable physical and mechanical properties, make it a suitable and popular biomaterial for these dental applications. To further improve the properties (thermal properties, water sorption, solubility, impact strength, flexural strength) of PMMA, several chemical modifications and mechanical reinforcement techniques using various types of fibers, nanoparticles, and nanotubes have been reported recently. The present article comprehensively reviews various aspects and properties of PMMA biomaterials, mainly for prosthodontic applications. In addition, recent updates and modifications to enhance the physical and mechanical properties of PMMA are also discussed.
Dental Materials: Properties And Manipulation, 9e (Dental Materials: Properties amp; Manipulation (
Download Zip https://geags.com/2yLV14
Impression materials are used to copy the teeth and surrounding oral structures by creating a dental impression poured with dental plaster to fabricate a dental cast. This procedure provides a tridimensional and accurate mouth replica, allowing dental work even in the absence of the patient.
Dental models enable dentists to perform a better diagnosis and treatment planning since the teeth can be meticulously visualized and studied from angles that are difficult to see in the patient's mouth. Particular treatment, such as removable and fixed prostheses, can be executed thanks to dental casts. The final restoration or prosthesis fit depends on how accurately the impression material has recorded the tissue details.
Wax was the only impression material used in dentistry until the mid-19th century when gutta-percha first appeared. Then in 1857, Charles Stent created a thermoplastic modeling compound similar to today's impression compound. Still, the problem with this material was that it was rigid and could not reproduce undercut areas. All the impression materials used until that date became rigid after setting and could not copy the oral tissues accurately. Thus, there was always a need for an impression material that could remain elastic even after setting. That is when agar, a reversible hydrocolloid manufactured from algae, was introduced in dentistry. Although this jelly-like material was elastic, it required a complicated procedure to be used as an impression material.
When the algae used to manufacture agar was unavailable during the second world war, Americans used local algae to manufacture another elastic impression material known as alginate, which has gained popularity since then.
Alginate and agar have disadvantages, like dimensional instability and low tear strength, which led to the manufacture of elastomeric (also known as rubber-based) impression materials. First came polysulfide, then condensation silicone followed by polyether, and then addition silicones.[1][2]
With the advancement in technology, digital dentistry is also making its way into the field.[3] However, there is always a scope for new impression materials, as no impression material is 100% accurate until the date.
Dental impression materials are used to copy oral tissues, create a dental impression, and then pour gypsum into them to produce a dental cast. A dental cast is used to analyze and diagnose a clinical case, manufacture removable and fixed prostheses, and build occlusal splints and mouthguards. Beyond the teeth, impression materials are utilized to fabricate maxillofacial prostheses such as obturators (maxillectomy and cleft lip and palate), ear prostheses, and prostheses of the eyes.
The wettability of the impression material determines its ability to record fine details, which depends on its viscosity. The more the impression material wets the tissues, the more it will come in close contact with them and capture fine details. The more viscous materials will have limited flow and very few wetting features.[4] In fixed dental prostheses, an impression is expected to reproduce 20 to 70 microns and 100 to 150 microns in removable prosthodontics.
Type 1 sheet impression compound is softened in a water bath with warm water and is placed in a stock tray to copy edentulous ridges. The water bath temperature is kept at around 55-60 degrees Celsius, above the impression compound's fusion temperature.[6] The impression compound is a mucocompressive material, meaning that it impresses the mucosa when subjected to compression. These impressions must be corrected with a second impression made in a custom impression tray fabricated on the primary cast of the patient.
It sets in two steps: first, by the hydrolysis of ZnO to form zinc hydroxide (water is required to initiate the reaction). The second stage is a typical acid-base reaction in which zinc hydroxide reacts with eugenolic acid to form zinc eugenolate, forming a chelate and water as a byproduct. The final set structure consists of ZnO particles embedded in the matrix of zinc eugenolate.
The impression paste's setting time can be altered in clinical set up according to need, either decreased by adding a drop of water during mixing or increased by adding vaseline during mixing. The material has a good flow, which records fine details.
Alginate (Fig3): Alginate is a hydrocolloid widely used as an impression material in dentistry. It is derived from alginic acid, present in brown algae. It is an irreversible and elastic impression material.[13]
The water is added first to the rubber bowl and then the powder; the manufacturer gives the proportions. The mixture is mixed vigorously against the walls of the bowl in a figure of eight motion. The mixing time is 45-60 seconds according to the type of alginate. Then the mixture is loaded into the selected impression tray (tray border should be a maximum of 3 mm short of vestibular depth) and placed into the patient's dental arch. After the alginate attains sufficient elasticity, the impression is removed. There should be a minimum of 3 mm thick material between the tray and tissues not to get torn. The impression should be removed swiftly, simultaneously breaking the seal by inserting fingers in the buccal vestibular region. The impression must be cast within half an hour as hydrocolloids tend to deform because of imbibition and syneresis.[14] The impression could be kept in 100% humidity and cast within 24 hours instead. But for optimum dimensional stability, an alginate impression should be poured immediately.[15]
First, calcium sulfate reacts with trisodium phosphate to form calcium sulfate and sodium sulfate; the second stage starts after all the trisodium phosphate gets consumed, which provides optimum working time in the manipulation of alginate. In the gelation reaction, potassium alginate reacts with calcium sulfate to form calcium alginate gel.
First, the material is boiled in the first compartment and changed to sol from gel state. In the second compartment, the material is stored at 65 degrees Celsius and used when needed. The agar is brought to 45 degrees Celsius in the tempering section to impress oral tissues comfortably. The water circulates through the tubing and converts the sol into a gel state.
Elastomers can present in different consistencies, including putty, heavy body, medium body, and light body. The putty type is available in two jars containing base and catalyst; the medium body is dispensed in two collapsible tubes as base and catalyst; and the light body in syringes.
The base paste containing hydrosilane-terminated molecules reacts with an accelerator paste containing siloxane oligomers with vinyl end groups and a platinum catalyst. Although no by-product is formed, a secondary reaction leads to hydrogen gas production. That is why it is recommended to wait at least 30 minutes rather than have a pitted cast.[19]
Polysulfide: these are the first elastomers to be introduced. Although messy to manipulate, it is useful when a long working time is needed. Polysulfides are not recommended except for complete dentures.
The polysulfide prepolymer reacts with lead oxide, forming polysulfide rubber and water. The reaction byproduct significantly affects the dimensional stability of the impression; therefore, the impression should be cast immediately. Polysulfide is the most flexible of elastomeric materials, and it can be easily removed from undercut areas.[24]
Elastomers are typically constituted by a base and a catalyst. The light body comes in a syringe, the regular body is available in collapsible tubes, and the putty is dispensed in two jars. The mixing can be done manually or by automix systems, e.g., disposable intraoral syringes, automatic mixing machines, and dispensers with attached cartridges.
The advantages of intraoral scanning are too many: the material and armamentarium used in analog impressions are avoided (e.g., impression trays, impression material, gypsum); the communication between the clinician and laboratory technician is improved since the image can be modified, recaptured, a soft copy can be stored; and the cross infections can be minimized due to absence of physically stored casts.
However, digital impressions require an expensive setup, images of completely edentulous arches are less accurate, the presence of blood and saliva obscures subgingival finish lines, and they do not record complete occlusal information for comprehensive prosthodontic treatments.[26][27] But, for single units and segmental dentistry, the intraoral scanners are highly accurate.[28]
7fc3f7cf58