الفهرس يوجد فقط 14 صفحة متاحة للعرض العام
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المستخلص
Casting metals by the lost wax process has been recognized in industry and the arts in many years. No record exists to indicate exactly when and where this type of casting procedure was first developed. All that is known is that somewhere along the line it may have been in ancient Egypt or in ancient China someone conceived of the idea of making a wax replica of an item to be cast, surrounding this replica with an investment material, letting the investment harden, then melting and burning out the wax, thus producing a mold having a highly intricate and accurate cavity. The next steps were melting the metal and pouring it into the cavity.
Commonly used investments consist of a refractory material and a binder. Typically, the refractory material is silica (quartz, cristobalite, or a mixture of the two) and common binders include gypsum or phosphate and metallic oxides. Additionally, silicate-bonded investments are available. The rapid growth in use of metal ceramic and hot pressed ceramic prostheses has resulted in an increased use of phosphate and silicate bonded investments. Although these investments are somewhat more difficult to remove from castings than are gypsum investments, that problem has been reduced recently, and the phosphate bonded investments (PBI) produce satisfactory results. Therefore, PBI materials have become more popular for casting high melting noble metal and base metal alloys in fixed prosthodontics.
Considerations for these investments include: water/powder ratio, concentration of special liquid (silica sol) used, mixing time, spatulation rate, working and setting times, atmosphere during setting, strength and degree of expansion (setting, hygroscopic and thermal) which compensates for the wax and alloy solidification shrinkage. Theoretically, if the shrinkage of the wax and alloy are known, the mold can be expanded an amount equal to such shrinkage, and the problem is solved. Unfortunately, there are variables in the behavior of the materials involved, especially the wax, which cannot be controlled. The overall dimensional accuracy possible with current techniques has not been clearly defined.
There have been numerous reports on attempts to perfect the casting procedure in dentistry by improving materials and techniques. The majority of these efforts deal with the so called “conventional” investing and casting techniques, which usually require at least 1 hour bench set for the investment, followed by a one or two stage wax elimination procedure before casting takes place. The whole process is time consuming and requires approximately 2 to 4 hours for completion. Modern PBI are suited for accelerated bench set and burnout cycles offering many advantages to the patient, dentist, and dental laboratory technician, and have received increased attention as a method of improving productivity. Using this accelerated technique complete crowns can be invested and cast in 30 minutes with comparable results.
However, the issue of marginal fit and accuracy of these crowns remains unclear, provoking the question of clinical acceptability. The objective of the casting process is to provide a metallic duplication of missing tooth structure with as much accuracy as possible. The tolerance limits for the fit and marginal adaptation of a cast restoration are not known. It stands to reason, however, that the more accurate the fit of the casting, the less the likelihood of leakage and secondary caries. Additionally, marginal discrepancies have been shown to adversely affect local periodontal tissue inflammation. There are a number of variables that have proven to directly affect the marginal accuracy of the castings, some of these factors include: sprue design in terms of diameter, length, direction, position and attachment, direct or indirect spruing, ring volume, type of ring and ring liner used, use of wet or dry liner, burnout cycle in terms of time and temperature, technique and relative time of alloy melting, casting technique and the final finishing of the casting. Therefore, the strict adherence to the certified techniques is mandatory for the success of the casting. Yet, further research must be performed to improve and innovate these techniques accompanied by the rise of new materials, such as the accelerated investments, in a continuous quest to perfect the casting procedure.
Thus far, limited studies have shown the effect of different factors on the marginal accuracy of crowns performed with the accelerated (shock heat) casting technique. Therefore, the purpose of this study was to evaluate the effect of some of these factors on the marginal gap distance of base metal crowns performed by the accelerated casting technique, in comparison with the conventional one.