Minimizing post-processing of zirconia hybrid restorations
By Alexander Wünsche, CDT
After many years spent developing a protocol for fabricating successful zirconia hybrid restorations, it is time for a revolution. Amann Girrbach launched its new software module, Ceramill M-Gin, to offer a less complex workflow. The company also developed a new cutback tooth library and a new milling strategy to fabricate complex cases with a minimum of post-processing. This article will present a step-by-step workflow for creating the final zirconia restoration: the REliable All-on-X (REAX) Bridge.
After the laboratory received the final abutment- or implant-level impression and all other records, a master cast was poured. A verification jig was made to verify the poured master cast. Non-engaging titanium temporary abutments were used and connected with clear acrylic pins and GC Pattern Resin (GC America Inc., gcamerica.com).
After try-in of the verification jig, a bite registration was taken. A screw-retained bite block was created, using non-engaging temporary abutments to retain the bite block on the implants; two implants should be used for this.
The master cast and opposing were articulated using the bite registration. After case analysis, which can be handled the same as for full dentures, a wax setup was created for an esthetic and phonetic try-in. An analysis with the patient confirmed VDO, tooth shape, and shade.
Once the wax setup was sent back to the laboratory, the digital process could begin.
All records were scanned with a Ceramill Map400 scanner (Amann Girrbach, amanngirrbach.com), which uses white light, a technology the author has found to be extremely accurate in comparison with other scanning technology. Both models were scanned in full articulation with an articulation transfer. This was helpful in comparing a mush bite scan of the opposing.
The implant location scan bodies were used for digitization. They functioned as the “digital impression coping,” providing the exact position and angulation of the implant. All scans were merged together and finalized.
With the construction software launched, the virtual articulation could be processed. To make sure all functions of the articulation are working throughout the design process, the technician should not close the virtual articulator window, so occlusion can be controlled at all times.
The implant positions and platforms were confirmed and aligned with an easy one-point alignment workflow and finished with a best-fit matching step, followed by marking of the emergence profile and creating the bottom of the emergence profile.
To control the tooth setup, the library teeth can be moved and shaped easily on the arch. The new update includes a convenient chain mode, so teeth can be moved together in different styles, horizontally or vertically, organically so contacts and relationships to neighboring teeth are not lost (Figure 1).
Next, the gingiva was applied to the restoration. The gingiva extension to the ridge was added with marking points, and they were connected. It is critical to avoid any ridge overlap, so a proper hygienic design can be accomplished (Figure 2). For an improved hygienic underneath surface, cleaning channels can be added around the implants to aid the patient in flossing.
The gingiva was shaped in the papilla and tooth margin area to the esthetic and functional design (Figure 3 and Figure 4). It is important to set the transition between the pink and the white in the correct spot; photos of the patient with the immediate temporary in place and without can be helpful. If a cutback for esthetic porcelain layering in the anterior facial area is needed, the new module has the Knut Miller cutback library* built in, with multiple different cutback styles (Figure 5). (*This upgrade module is available for purchase).
The facial shape of papillae and interproximal of the teeth were designed with a combination of esthetic and cleansability considerations.
After the design was finished with the free form tools in the Ceramill Mind software (Amann Girrbach), it was merged and saved for milling. The designed restorations were milled first in Ceramill Temp (Amann Girrbach) as a prototype to be tried in and adjusted as necessary to correct and improve the final Ceramill Zolid (Amann Girrbach) restoration.
The design was nested with the Ceramill Match 2 software (Amann Girrbach) for milling in the Ceramill Motion 2 milling machine (Amann Girrbach). For this type of case, the placement of a supporting bar lingually or palatally is important to avoid any distortion or breakage during sintering. The correct placement of the connectors to this supporting bar is critical, because curved areas must be connected and secured for sintering (Figure 6). The supporting bar must not reach or extend the volume of the actual restoration, as this could have a distortion effect as well. The Ceramill Match 2 software ensures that the supporting bar is milled to the correct size and thickness.
The Ceramill Motion 2 can mill zirconia full-arch cases in high definition, with a 5-axis outside milling strategy specially developed for undercut areas, which allows for premium esthetics after milling. Once the milling process was finished, the restoration was taken out of the machine and evaluated (Figure 7 and Figure 8).
Next, the technician fine contoured the interproximal and occlusal anatomy with fine diamonds or carbides and fine diamond discs. Texturing or major shaping was no longer necessary, as the restoration was milled in high definition. Gloves should be worn throughout this process, as any oil from the skin can affect the saturation of the pre-sintering colors.
The pre-sintering coloring was the final step before the sintering process. Ceramill Liquids New Formula (Amann Girrbach) were used. Available in all 16 VITA shades and multiple effect colors, the liquids make it possible to create the perfect base color for a beautiful finish (Figure 9). For better differentiating, the marking colors Ceramill Liquid Eye (Amann Girrbach) are useful.
The technician started with dentin over the cervical with a three-time stroke, followed by a two-time stroke and one-time stroke over the body and incisal area. The incisal was colored carefully with violet and grey, and orange was applied occlusally. The coloring process was finished by applying gingiva or rose color to the gingiva area. For a natural effect, the dentin color was extended over the tissue at every tooth to help support a root look underneath the tissue.
When all color liquids were applied and sufficiently dried, the restoration was stood upright in a sintering tray on beads and put into the Ceramill Therm 2 sintering furnace (Amann Girrbach).
A 12-hour sintering program was used before the sintered restoration was removed and cooled to 200°C. The supporting bar then was removed carefully with diamond burs or discs by water cooling.
No fitting was necessary, as the restoration fit perfectly onto the abutments. It is important to clean the surface by steaming, not sand blasting, before the porcelain layering process starts.
Multiple techniques can be used to layer the porcelain (Figure 10). The author chose to “pre-glaze” the framework with GC Lustre Paste (GC America Inc.), developing first characterization and foundation before layering with GC ZrFs .
For all firing processes, the heating and cooling strategy is critical. A 450°C starting temperature, 25°C heating per minute, and 25°C cooling per minute down to 450°C are ideal. Each firing is relatively long, but it is safer because zirconia retains heat well; rushing the cooling down of the material creates stress and the zirconia can break or crack. Micro cracks can cause the finished restoration to break even months after the case is seated in the patient’s mouth.
When the porcelain process was finished and the restoration was stained and glazed, it was cemented onto the abutments. Multilink Hybrid cement (Ivoclar Vivadent, ivoclarvivadent.com) is intended for cementing zirconia restorations onto abutments benchside. The abutment and restoration were air abraded on the cement surface, and Monobond (Ivoclar Vivadent) was applied on both surfaces and air dried for 60 seconds. Multilink was applied to the abutments, and the restoration was seated onto the model with constant pressure for 7 minutes’ curing time, with no light curing necessary. The restoration then was unscrewed, excess cement was removed, and the zirconia hybrid restoration was ready to be seated (Figure 11 and Figure 12).
Alexander Wünsche, CDT, is the President of Zahntechnique Dental Laboratory in Miami, Florida