Dental CAD/CAM Materials PDF

Dental CAD/CAM materials Materials available for the fabrication of fixed restorations can be categorized in several different ways: by strength, composition, purpose, and fabrication method. There are basically three applications for CAD/CAM materials: A) die materials, B) pat...

Dental CAD/CAM materials Materials available for the fabrication of fixed restorations can be categorized in several different ways: by strength, composition, purpose, and fabrication method. There are basically three applications for CAD/CAM materials: A) die materials, B) pattern materials, and c) restoration materials. Die materials Dies for the fabrication of fixed restorations must fulfill the following requirements: They must be an exact reproduction of the prepared tooth. They must be free from voids and distortions. They must capture the entire tooth preparation, as well as the area 0.5– 1.0mm apical to the finish line. They must allow adequate access and visualization of the margin. Definitive dies are conventionally fabricated from gypsum. They have the advantages of being inexpensive, quick and easy to produce, and dimensionally accurate. On the other hand, gypsum materials disadvantages are poor abrasion resistance and air bubbles incorporation of during mixing leading to voids in the final die. Dies can also be fabricated from a digital impression which have many advantages as it can be: 1. Digitally manipulated, stored, shared. 2. Converted into an analog die, either by computer-aided machining or printing. 1  Pattern materials Patterns for cast alloy and pressed all ceramic fixed restorations are usually formed free hand from wax. Wax is a weak material that is prone to distortion due to handling and temperature variations. Patterns can be fabricated from digital data as well, designed on a computer and then either milled from wax or resin blanks or printed in resin. The advantage of resin CAD/CAM patterns: 1. Strong. 2. Resistant to temperature variations. 3. They can be fabricated of large or complex frameworks. Restorative materials Materials for fabrication of fixed restorations by the CAD/CAM process are produced by milling blocks. Compared to restorations fabricated by hand in a dental laboratory, the blanks are produced under controlled conditions to increase the material homogeneity and reduce voids and inclusions, which weaken the material. The types of materials for CAD/CAM production of final restorations include: Composite Composite/ceramic hybrids Ceramics Glass-ceramics Alloys 2  Materials for CAD/CAM fixed restorations can be produced by Hard milling Soft milling Hard milling Hard milling is described as the milling of a restoration from a blank of material that is already in its final structural form the polished or glazed prior to cementation. Materials for hard milling are those materials that are milled in their final state, no further heat treatments to transform the material into its final hardness and strength. They are generally milled at full contour, and the only treatments necessary after milling are polishing and/or staining and glazing. The types of hard milled materials consist of composites, composite/ceramic hybrids, and ceramics. These materials are most often employed for the fabrication of single unit restorations using a dental milling machine. Composites Composites are resins reinforced with particles of inorganic filler. They are soft and easy to mill. However, their disadvantages include low wear resistance, tendency for marginal microleakage, and questionable long-term color stability compared to ceramics. Newer composites for CAD/CAM applications are being produced with higher filler content, which improves mechanical properties and reduces the disadvantages listed earlier. These materials represent a low-cost alternative for limited use as a definitive single tooth, full-coverage restoration or as a durable provisional restoration. 3  Composite/ceramic hybrids Resin-Matrix Ceramics The resin-matrix ceramics are materials with an organic matrix that is highly filled with ceramic particles (>50 wt%). The rationale for developing resin-matrix ceramic materials was to: 1. Obtain a material with modulus of elasticity compared to dentin. 2. Develop a material that is easier to mill, crystallize and adjust. 3. Facilitate repair or modification with composite resin. Ceramics Ceramics produced for in office hard milling applications are generally low to medium in strength. They contain high glassy matrix content, making them translucent and easy to mill. The high amount of glass renders them brittle and susceptible to fracture at relatively low loads and also makes them prone to chipping in thin areas (margins) during milling, requiring careful examination for flaws prior to cementation. Once the milling is complete, the final restoration should be polished or submitted to a glazing heat treatment prior to cementation. 4  Existing materials for in office hard milling can be classified into three groups by composition of their crystalline phase: Feldspar containing ceramics Leucite containing ceramics Zirconia-reinforced lithium silicate Feldspar and leucite containing ceramics: are the weakest materials (∼140 MPa) for full coverage restorations and should be limited to restoration of anterior teeth. Their high glassy content imparts a translucency similar to tooth structure, and therefore, they are also candidates for ceramic veneers. These materials can be etched and silanated prior to luting with a resin cement to provide a strong bond to tooth structure (enamel). Zirconia-reinforced lithium silicate: has been recently introduced which contains fine-grained zirconia dispersed within a lithium silicate glass-ceramic. The unique property of one manufacturer’s version of this material is that it can either be luted in the as-milled state or after an additional strengthening heat treatment. The material in the as- milled state has reportedly high translucency and low strength (∼200 MPa), whereas after a 30-minute heat treatment, the strength increases to around 370MPa. 5  Zirconium oxide (zirconia) ceramics: can be fabricated by milling a block of densely sintered material, but are most commonly milled in the soft, partially sintered “green” stage. Hard milling of fully sintered zirconia ceramics was attempted early in their introduction to dentistry. The hard-milled zirconia appeared to have more accurate margins, as the sintering shrinkage had already taken place. However, hard milling could induce flaws and cracks into the material, leading to failure. Tool wear was also an issue when milling such a hard substance. So, hard milling of dental zirconia is only mentioned here in a historical context. Alloys Hard-milled alloys are generally produced by laboratories, utilizing large blanks of alloy to mill out crowns or frameworks. Titanium and titanium alloy blanks are routinely used for milling out implant abutments and superstructures. Milling of titanium and chromium cobalt overcomes the main issue with casting of base metal alloys; poor castability and marginal accuracy. Advantages of base metal alloys are that they are inexpensive relative to gold alloys, have high stiffness and can be conventionally veneered with ceramic. 6  Soft milling Soft milling refers to milling of a material in a “green” form that must undergo additional laboratory heat treatment in order to produce its final mechanical properties. This usually requires staining or layering with additional ceramics. Materials for soft milling are generally those that cannot be cut easily with diamond burs in their final state. The soft, partially sintered “green” or partially crystallized blank is then fully sintered or crystallized by a heat treatment after milling. The sintering of partially sintered green pieces is accompanied by significant (∼20%) shrinkage of the material, whereas crystallization of partially crystallized pieces does not. Due to their opaque nature, the application of veneering ceramic to produce acceptable esthetics. These materials can be classified into four groups by composition of their crystalline phase: Lithium disilicate glass-ceramics Magnesium aluminum oxide-containing ceramics Aluminum oxide (alumina)-containing ceramics Zirconium oxide (zirconia)-ceramics Lithium disilicate glass-ceramics Lithium disilicate glass-ceramics are milled in an intermediate “blue” (metasilicate) state then submitted to a tempering heat treatment that completes the (disilicate) crystallization process and almost triples the strength (∼360 MPa). Lithium disilicate glasses are produced in high and low translucencies which make them suitable for full coverage crowns in the anterior regions. However, lithium disilicate can be used in posterior regions limited 3-unit FDP applications and the most distal abutment not extended beyond 2nd premolar. 7  Magnesium aluminum oxide ceramics Magnesium aluminum oxide ceramics (Inceram Spinell) are fabricated by milling a framework in the partially sintered state, sintering at high temperature, infiltrating the porous framework with a low-fusing glass, then veneering with conventional ceramics. The fact that it is the weakest of the soft-machined materials (∼280 MPa) is only indicated for production of copings for full coverage restorations limited to restoration of anterior teeth. The glass infiltration step content imparts a translucency similar to lithium disilicate ceramics with the advantage of extensive intrinsic coloration can be achieved when the veneering porcelains are applied. Aluminum oxide (alumina) ceramics Aluminum oxide (alumina) ceramics can be fabricated by several methods, by milling: A framework in the partially sintered state, sintering at high temperature, infiltrating the porous framework with a low-fusing glass, then veneering with conventional ceramics (Inceram Alumina). A digitally enlarged refractory die, isostatically pressing a coping of alumina powder onto the die, sintering, then veneering with conventional ceramics (Procera Alumina), or a digitally enlarged restoration or framework from a partially sintered blank, then veneering with conventional ceramics. Alumina-based ceramics are not used without adding an esthetic veneer of porcelain, as they are rather opaque (see Figure 5.16-right). These materials have high strength (∼600 MPa) and the advantage of employing extensive intrinsic coloration. 8  Soft-milled zirconium oxide (zirconia) ceramics Soft-milled zirconium oxide (zirconia) ceramics requires either the dies or the restoration itself be milled at an increased size to compensate sintering shrinkage. Zirconia is a rather opaque material, usually supplied to dental laboratories as chalky white blanks. Once they are milled to their designed shape, the restoration or framework is either soaked in or painted with in a dyeing liquid to approximate the shade. After the milled crown has been shaded with the coloring solution, it is sintered in a high-temperature furnace for several hours. During the sintering process, the zirconia shrinks and becomes much denser. Full contour restorations (monolithic) are now possible without application of an esthetic ceramic veneer. Monolithic restorations have eliminated the incidence of veneering porcelain chipping and saved a fabrication step, as no veneering porcelain needs to be applied and contoured. 9