Chairside Full-Contour Zirconia
Completing an indirect “single-visit” restoration in less than 90 minutes
Michael Skramstad, DDS
In the past several years, full-contour zirconia (Y-TZP or yttria partially stabilized tetragonal zirconia polycrystalline) has emerged as a popular restorative material for posterior full-coverage indirect restorations. The combination of high flexural strength, excellent fracture toughness, the ability to offer highly conservative preparations, and the capacity to allow conventional cementation has made zirconia a preferred option among dentists. CEREC® (Dentsply Sirona, www.dentsplysirona.com) has provided the capacity to manufacture tooth restorations with various materials for more than 30 years. An increasing number of materials, including polymethyl methacrylates (PMMAs), composites, hybrids, and abutment blocks, all offer completion of the restorative process in a single chairside visit of 90 minutes or less.
While CEREC has also been able to produce zirconia restorations for quite some time, they were traditionally sent to the laboratory as a result of longer sintering times. However, this workflow has changed with Dentsply Sirona’s introduction of milling units with both dry mill and wet mill capabilities, along with CEREC SpeedFire, an all-in-one sintering and glazing furnace that enables the sintering of zirconia in less than 15 minutes.
Reducing Sintering Time
Traditional milling procedures with CEREC are not classified as milling, but grinding. Diamonds are used in a wet environment to shape the restoration by friction—this reflects the grinding process. True milling is a process in which rotary carbide cutters shape the object; this is important when dealing with zirconia because true milling leads to smoother surfaces and margins and less chipping.1,2 Smoother surfaces also allow easier polishing and less wear to opposing enamel.1-4 In the author’s experience, dry milling zirconia with CEREC SpeedFire allows a 50% reduction in sintering times by eliminating the additional drying cycle.
CEREC SpeedFire uses induction technology that allows energy-efficient heating cycles based on ceramic volume and shade. The sintering and glazing furnace has an incredibly small footprint, allowing it to fit conveniently on a countertop. Two key clinical features of this furnace are speed and the integration with CEREC V4.4.2 software; the unit will speed-sinter dry-milled zirconia in roughly 13 minutes and wet mill/grind zirconia (existing milling units) in about 25 minutes. CEREC SpeedFire is also completely software driven when added as a device (Figure 1). The software determines the exact firing time needed for the given restoration and transfers that information wirelessly to the furnace. Additionally, the furnace fires glass-ceramics with no changes to the software integration.
A 48-year-old man presented with a large amalgam restoration and fractured distolingual cusp on tooth No. 19 (Figure 2). Because the patient’s dentition showed signs of both erosion and attrition, the clinician opted to use a higher strength ceramic. Zirconia was selected as the restorative material to enable a minimal occlusal reduction (the depth of the amalgam was roughly only 1.5 mm) and to maximize flexural strength.
Compared to other glass-ceramics, zirconia offers good flexibility on the preparation. The final preparation needed only 1.5 mm of occlusal reduction and approximately a 1-mm supragingival chamfer on the buccal and lingual aspects (Figure 3). Zirconia can tolerate lesser values for both if the clinical need arises. The clear margins, smooth transitions, and minimal preparation, which is less than what is typically required for glass-ceramics, can be seen in the enlarged view of the preparation (Figure 4).
With the release of the CEREC V4.4.2 software, the algorithm for creating proposals was substantially modified with a process called Biojaw. This process calculates the proposal based on the global arch form that is scanned, representing a significant upgrade in the overall workflow with initial proposals that have excellent shape, function, and occlusion. The proposal for this case needed no adjustments to contour, occlusion, or contact (Figure 5). The occlusal anatomy created by Biojaw generated a minimal fissure thickness of 0.8 mm, which would not be adequate for glass-ceramics and would require more occlusal reduction. With zirconia, however, this is well within the acceptable range and allows for less tooth reduction.
The dry milling process began after placing the CEREC zirconia block in the milling chamber. The majority of the bulk reduction was done by the left 25 Shaper carbide bur (Figure 6), while fine-tuning of the anatomy, margins, and intaglio fit were performed by the right Finisher 10 carbide bur (Figure 7). For most molar restorations, the total mill time will be in the range of 10 to 14 minutes. In this case, the mill time was 10.5 minutes.
Another noteworthy feature of the milling process is that the zirconia is milled roughly 22% larger in a non-sintered state. This allows the burs to fit into tighter areas and create more detailed anatomy that results in superb fits and minimal post-cementation adjustments. The sintering process shrinks the crown to the final form based on parameters programmed into the software for that particular zirconia batch.
Once the zirconia crown is milled, the sprue must be carefully removed using fine rubber wheels. The crown is then placed into the CEREC SpeedFire. The software transfers the firing parameters to the oven wirelessly; the dentist needs only to press the “start” button. The sintering process in this case took 13.5 minutes. Once sintering is complete, it is important that the zirconia be polished to a “glass” finish, even if the material will be stained and glazed afterwards. With the zirconia highly polished, the wear on opposing enamel will not differ significantly from polished Empress®2 (Ivoclar Vivadent, www.ivoclarvivadent.com) or gold alloy4 and has demonstrated to be less than feldspathic porcelain.5,6