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Chemistry and Technical Aspects of Monolithic Crowns

2021-11-17 Page view : 51 views

Zirconium is one of the most abundant elements in the earth’s crust. Zirconia is formed by the addition of oxygen to the pure, elemental zirconium metal. The addition of minor components to the zirconia, such as yttrium, can produce a crystal that is both strong and resistant to crack generation because of the unique “transformation toughening” that occurs when zirconia goes from a tetragonal phase to a monolithic phase. It is monolithic phase of zirconia that is so resistant to breakage when used in full-coverage restorations.

Monolithic zirconia restorations begin their journey as chalky white blocks. They are milled to their designed shape and then are soaked in a dyeing liquid to approximate the requested VITA® shade (Vident, www.vident.com). The dyeing liquids have improved over the past 2 years and likely will continue to evolve. The final VITA shades of the MZ crowns now being shipped by laboratories seem to be more consistent than was imaginable only 2 years ago.

Recent changes in coloration protocol for MZ restorations have seen the introduction of a three-zone coloring system. First, the un-sintered restoration is brushed with the desired final color around the cervical zone of the crown. Next, the body of the crown is brushed with the desired body shade. Finally, effect shades are used to characterize the occlusal area of the crown.

After the milled crown has been shade-adjusted in the coloring solution, it is sintered in an oven for 6.5 hours at 1,560ºC. Sintering drives the tetragonal zirconia to its monolithic phase and the gives the milled crown its great resistance to fracture and breakage. During the sintering process, the zirconia shrinks and becomes much more dense. A computer program is used to increase the size of the crown during the milling process to compensate for the shrinkage that occurs during the sintering process.

Resistance to fracture and crack propagation makes full-contour MZ an ideal material to fabricate bridges for posterior teeth. To be sure the bridge will not fracture; each connector must have a calculated number of 27 or higher. The number is determined by measuring the connector area. The height, width, and depth of the connector, when combined, must equal 27 or greater 5. The connector is measured in millimeters and the results are multiplied to obtain the final number.

With adequate connector size, three-, four-, and even six-unit bridges are very predictable using full-contour MZ. Using conventional PFM cementation techniques, these bridges are easily cemented and the clean-up is very simple.

Soft-Tissue Response to MZ

The soft-tissue response to MZ crowns has been very encouraging. They promote a soft-tissue response very similar to porcelain veneers; the tissue stays pink and stippled and healthy. The high biocompatibility of zirconia is evidenced by the success rate when using zirconia medical implants for procedures such as hip replacement. Advantageous clinical properties—such as reduced bacterial adhesion, ease of manipulation, high strength, and polishability, along with superior resistance to fracture—combine to make MZ crowns the ideal choice for posterior full-coverage restorations.

Highly polished ovate pontics are easily cleaned and maintained, and the resistance MZ restorations show to bacterial adhesion encourages a healthy response from surrounding soft tissues. The ease of shaping and polishing MZ restorations provides a nature-mimicking environment that contributes to the health of soft tissues surrounding these crowns and bridges.

Because the MZ crowns are tooth-colored, finish lines may be either supragingival or subgingival. Very often the length of the clinical crown determines the position of the finish line. A short clinical crown may require a subgingival finish line in order to have a significant ferrule for retention of the MZ restoration.