Monolithic Zirconia: Minimizing Adjustments

INTRODUCTION
There has been an ongoing search for a tooth-colored alternative to gold for posterior restorations. Gold has been the standard for many years due to its biocompatibility and excellent physical properties. The advantages of gold are well-documented, but its single greatest flaw is its lack of aesthetic appeal. Contemporary dental patients with high aesthetic demands are resistant to having gold restorations placed.¹ Several alternatives exist, each with its own limitations. Numerous aesthetic all-ceramic systems are available, but most lack sufficient flexural strength to withstand the occlusal demands associated with second molars. Metal-based ceramic restorations require significant reduction occlusally which may compromise resistance/retention form with relatively short clinical crowns.

Currently, there is keen interest in full-contour (monolithic) zirconia restorations.² Many dental laboratories report a significant rise in prescriptions requesting monolithic zirconia restorations.
This article will document a case study of a first premolar restoration in the aesthetic zone.

Background
Zirconia, as a core material, has been well documented for use in clinical dentistry.^3,4 The material has a flexural strength approaching 1,200 MPa and possesses excellent biocompatibility.^5,6 These characteristics make the material viable for full-contour applications.
Studies are presently reaching publication regarding wear characteristics of full-contour zirconia in contact with natural tooth structure. Current conclusions are that the material exhibits wear characteristics consistent with clinically acceptable parameters.^7,8 However, it should be noted that no long-term studies are available at this time. It is the conclusion of existing studies, and the clinical opinion of the authors that restorations requiring adjustments should be reglazed and then polished thoroughly with appropriate rotary instrumentation⁹ (Zirconium Oxide Polishers 94013F.170 [Komet USA]). All studies are based on fully-glazed and polished restorations, which, it must be noted, do not necessarily reflect clinical situations where zirconia has been adjusted post cementation, thus negatively impacting opposing dentition wear. Recent studies recommend thorough polishing of restorations prior to, and after, stain and glazing with the resultant restorations exhibiting wear characteristics similar to enamel to enamel.¹⁰

Typical zirconium oxide-based core materials are starkly white with minimal translucency. This characteristic has been largely controlled by color staining of cores and subsequent application of layering porcelains. This technique is not possible with monolithic zirconia; the final shade of these restorations is typically developed by staining prior to sintering, and with the subsequent application of surface stains and glazing media. Clinical results with this technique have provided varied success, restricting restoration placement to less aesthetically demanding locations.
As with all technology-driven products, these restorations are evolving, with new techniques, materials, and equipment entering the laboratory market.

Wieland Dental has recently released a high translucency zirconia block to address some of the aesthetic shortcomings of this material. The translucency of the parent material is sufficient to enhance the aesthetics of the stained and glazed crown. Wieland Dental has successfully maintained the desirable strength and biocompatibility of the original zirconia product while providing a more aesthetically pleasing final result.

A zirconium oxide restoration is, by its very nature, difficult to adjust and to remove once cemented. It sparks and dulls conventional rotary instruments quickly. It is imperative that clinicians use a light touch and copious water spray to protect the integrity of the zirconia structure and to control heat generation. Heavy-handed adjustment with coarse burs can lead to microfracturing of the monolithic structure, thus potentially affecting the longevity of the restoration.

Adjusting monolithic zirconia is not easy to do by the dental clinician but is something that can be minimized, if not eliminated totally, by understanding why many of our restorations come back “high” from the dental laboratory.

The dental technician is often blamed for an error in technique, yet it is our clinical impression and bite registration technique that is mostly at fault. It has been conclusively shown that there is a 12 times greater accuracy in the maximal intercuspal position when a dual-arch cast is mounted.¹¹ The reason for this increase in accuracy in mounting dual-arch cases, when compared to our traditional method of taking a full-arch impression and an opposing model in alginate, is directly related to the mismatch in accuracy between the material components that are used to fabricate the models that we are trying to articulate.

Vinyl polysiloxane (VPS) is currently the most prevalent material used in North America for taking impressions of crown and bridge preparations, representing about 95% of the impressions sent to the dental laboratory.¹² Many dental practitioners use alginate for generating the opposing model, and this is the major cause of discrepancies in mounting and articulating the models, contributing to the crown and bridge case being too “high.”¹³ The accuracy of models generated from alginate is negatively affected by multiple factors such as: powder-to-liquid ratio, temperature, syneresis, time interval to pour-up, disinfection, humidity, and the powder-to-liquid ratio of the dental stone; but, no matter how accurate the alginate is, the model derived from it will never fit accurately into our current bite registration materials.¹⁴ This is due to the mismatch in the accuracy between the 2 materials, since the finely detailed occlusal anatomy (picked up by the high-flow and stiff VPS bite materials currently used) is not replicated by the alginate; and therefore, the bite registration will not seat accurately on the alginate derived stone model. The only way this combination might work is to remove all the occlusal anatomy from the bite registration, leaving only the cusp tips. Then, the stone model might fit.

A simple solution, for the practitioner who uses full-arch impressions for crown and bridge, is to take an impression of the opposing arch in an alginate substitute (such as COUNTER-FIT [CLINICIAN’S CHOICE]; Algin.X [DENTSPLY Caulk]; Silgimix [Sultan Healthcare]; StatusBlue [DMG America]; Freealgin [Zhermack]; Position Penta Quick [3M ESPE]; AlgiNot SDS [Kerr]; Noginate [Danville Materials]; etc). These are low-cost VPS materials that have all the good qualities of regular VPS impression materials, including excellent occlusal detail that will harmoniously fit with the current generation of VPS bite registration materials. This increased accuracy in mounting the case will drastically reduce the effort and frustration in adjusting occlusion on a restoration that would have otherwise been created by alginate-derived models.¹⁵

For the patient requiring one or 2 crowns, a dual-arch impression using one VPS material eliminates the potential mismatch of the above scenario and, therefore, results in a much better articulation of the models. This impression technique uses less impression material, is easier for the patient and clinician, and is much faster.¹⁶ The dual-arch impression is indicated when the prepared tooth/teeth have a centric stop on either side, the patient demonstrates canine rise, the dentition is intact, the rear bar connector can satisfactorily pass through the retromolar area, and the cuspid can be picked up in the impression.¹⁷ To get consistently accurate results, do not use plastic dual-arch trays; they flex upon impression-taking, they flex repeatedly during the polymerization of the VPS impression material when the patient swallows, and this leads to distortion.¹⁸ Instead, use a metal tray which eliminates flex (such as the QUAD-TRAY XL [CLINICIAN’S CHOICE])¹⁹ with a stiff impression material (such as InFlex [CLINICIAN’S CHOICE] or Aquasil Rigid [DENTSPLY Caulk]) which has a strain in compression of less than 2% as recommended by the ADA.²⁰ These impression materials are specifically designed for the dual-arch technique since the heavy body impression material needs to be stiff to resist flex; that is because these trays typically do not have high sidewalls, and guide the patient into a passive closing. Clenching will move teeth, especially if an occlusal prematurity exists in the teeth adjacent to the preparation, and this can negatively affect your interproximal contacts.

CASE REPORT
Diagnosis and Treatment Planning
A healthy 62-year-old female presented with a chief complaint of an “ugly broken tooth.” Examination revealed a reasonably well-maintained dentition with no missing teeth. The maxillary right posterior quadrant was the focus of intended treatment in accordance with the patient’s wishes. Tooth No. 5 revealed a broken cusp and an old amalgam restoration that was partially missing (Figure 1). Additionally, adjacent teeth required direct composite restorations. The patient admitted a long history of broken restorations, and desired a stronger long-lasting solution.

The treatment plan consisted of direct composite restorations on teeth Nos. 4, 6, and 7, with tooth No. 5 planned for a full-coverage restoration in conjunction with a composite foundation.

Figure 1. Preoperative image.	Figure 2. Temp Tray/Template Impression Material (CLINICIAN’S CHOICE) taken for provisional fabrication.
Figure 3. Initial gross preparation with all old restorative material and caries removed.	Figure 4. Placement of the composite resin foundation.

Clinical Treatment
Initial treatment began with shade selection (shade map, written instructions, and photographs with shade tab included). A preoperative impression (Figure 2) was taken in anticipation of provisional restoration fabrication (Temp Tray/Template Impression Material [CLINICIAN’S CHOICE]). Local anesthetic was administered, followed by gross reduction of tooth No. 5 with a 1,558 cross-cut carbide bur. The preparation goals were to reduce axial surfaces by 1.5 mm and occlusal surfaces by 1.5 mm to 2.0 mm. All previous restorative material and caries were removed (Figure 3). Retentive grooves and potholes were placed to retain the composite foundation. The tooth was acid-etched for 15 seconds (FROST 37% [CLINICIAN’S CHOICE]), followed by a glutaraldehyde-based desensitizer (G5 [CLINICIAN’S CHOICE]). One coat of single bottle adhesive (MPa [CLINICIAN’S CHOICE]) was applied for 10 seconds to the moist prepared dentin, then air-thinned to evaporate the solvent and light-cured for 10 seconds. A dual-cure foundation material was then syringed into the prepared tooth to overfill the preparation (FluoroCore 2+ [DENTSPLY Caulk]) and light-cured; it was then allowed to autocure for an additional 5 minutes.

Next, excess foundation material was removed and final axial contours of preparation were developed (Figure 4). A hemostatic gel (Tissue Goo [CLINICIAN’S CHOICE]) was applied to crestal gingiva prior to intrasulcular margin placement (Figure 5). Initial retraction cord (No. 000 Ultrapak [Ultradent Products]) was placed into the sulcus with hemostatic agent (to act as a lubricant). The combination of hemostatic gel and retraction cord provided an excellent field into which margins could be extended, with final margin placement following the gingival scallop at an intrasulcular depth of 0.5 mm to 1.0 mm. This depth allows concealment of restorative margins while respecting biologic width,^21-23 providing adequate retention/resistance form, and placing all margins on healthy tooth structure. A heavy chamfer margin was prepared using a chamfer profile diamond bur (2008.9 Two Striper [Premier Dental Products]). This bur provides an ideal margin design suited to most all ceramic restorative systems. Occlusal reduction was confirmed at 1.75 mm with the occlusal table mimicking the generalized shape of a premolar.

Hemostatic gel was again applied to peri-marginal gingiva and a No. 1 braided Ultrapack retraction cord was placed into the sulcus.²⁴ This dual-cord technique provides excellent tissue retraction and hemostasis. It is critical that a continuous ring of retraction cord surrounds the prepared margin with no soft-tissue overhang of the cord. The entire preparation was then cleaned with a preimpression cleansing gel (Detail [CLINICIAN’S CHOICE]) that removes all organic residue and residual hemostatic agents which could act to inhibit VPS impression material polymerization.

An inflexible, aluminium-construction, impression tray with thin distal bar (QUAD TRAY XL) was selected. This tray provides excellent lateral support for impression material and will not flex during impression taking. The tray was tried-in to confirm complete seating allowing for maximum intercuspation. A dual-viscosity VPS gun delivery impression system was selected for this procedure (Affinity Light Body RF and Affinity Heavy Body [CLINICIAN’S CHOICE]). The light body material provides appropriate flow characteristics for injecting into the gingival sulcus, while the heavy body material is well suited to the quadrant tray technique. A material with a strain in compression of 1.3% (low flex) can be used effectively with dual-arch impressions to impart more rigidity (InFlex).

The No. 1 cord was removed and the preparation and sulcus dried and inspected. Accessible margins were visually confirmed. Light body impression material was injected into the sulcus using a microtip extension on a gun-delivery mixing tip. The entire preparation was covered with light body material as well as the occlusal table of adjacent teeth. During the injection process, a dental assistant filled both sides of the quadrant impression tray. The tray was placed according to the previously determined path of insertion. The patient was instructed to close into her “normal” bite and not to clench. Complete closure into maximum intercuspation was visually confirmed. Since it is critical to follow the manufacturer’s recommended setting times, a digital timer was started to count down 5 minutes. At the completion of the setting time, the patient was instructed to open while supporting the impression over the quadrant with the prepared tooth. The impression was then removed with a smooth, continuous downward motion with fingers on the side of the impression tray so as not to torque the handle. The impression was inspected for complete capture of the prepared tooth and a continuous 360°-margin (Figure 5). All other teeth in the quadrant were also adequately recorded.

Figure 5. Final vinyl polysiloxane impression (Affinity [CLINICIAN’S CHOICE]).	Figure 6. Provisional material (Temptation NOW [CLINICIAN’S CHOICE]) was injected into the preoperative impression.
Figure 7. Provisional restoration at completion of the preparation appointment.	Figure 8. No bleeding upon removal of the provisional.
Figure 9. Custom chairside staining was done prior to final glaze and polishing.	Figure 10. Facial view of completed restorations.

Figure 11. Occlusal view of completed restorations.

Next, provisional fabrication was initiated. The preoperative impression was modified to allow for missing tooth structure (a small amount of impression material was removed to simulate the missing cusp). Excess impression material was removed from the periphery to facilitate reinsertion of the impression tray. A suitable bis-acryl, gun delivery material (Figure 6) was selected in a shade appropriate to the natural shade (Temptation NOW [CLINICIAN’S CHOICE]). The material was injected into the preoperative impression and seated into the mouth over the prepared tooth. The material was allowed to set for approximately one minute, and then the tray was gently lifted from the teeth. The formed provisional restoration was slightly soft and easily lifted on and off the preparation. The patient was instructed to bite into maximum intercuspation while the material was allowed to set for another minute and a half. Excess provisional material was peeled away and the occlusion was marked with articulating ribbon. The provisional restoration was removed and refined and polished. Marginal adaptation was confirmed by trying back onto the prepared tooth, and final occlusal adjustments were made. Finally, the provisional was polished and coated with a resin glazing material (Glisten [CLINICIAN’S CHOICE]). This step increases wear resistance, decreases the roughness of the bis-acryl to the patient’s tongue, and facilitates easier cleanup of the temporary material. The provisional restoration (Figure 7) was then seated with temporary cement (TempGrip Integrity [DENTSPLY Caulk]) and the patient was dismissed.

A prescription for a full-contour zirconia (Zircast [Drake Precision Dental Laboratory] using the Zenostar system [Wieland Dental + Technik GmbH & Co]) was sent to the dental laboratory with a shade map and clinical photography. The patient returned after 3 weeks for completion of all direct restorative treatment of adjacent teeth and for delivery of the final restoration. The provisional restoration was removed, revealing a noninflamed sulcus (Figure 8). The monolithic zirconia crown was tried-in after a thorough cleaning of the preparation with pumice using a prophylaxis cup. The proximal contacts were ideal and the occlusal contacts required very minor adjustment. After a clinical shade evaluation, surface stains were used to lower the value of the restoration (Figure 9). The restoration was reglazed and thoroughly polished with appropriate rubber wheels (Zirconium Oxide Polishers 94013F.170).

The crown was steam cleaned and thoroughly dried. A universal primer (Monobond Plus [Ivoclar Vivadent]) was applied internally to the restoration to maximize cement-to-restoration bond strength, the tooth was conditioned and primed with a self-etching primer, and the crown was cemented using resin cement (Multilink Automix [Ivoclar Vivadent]). The occlusion was again checked.

Adjacent teeth were restored following placement of the crown. Teeth Nos. 4, 6, and 7 were restored with direct composite (Empress Direct [Ivoclar Vivadent]) restorations.

The combination of a new monolithic zirconia restoration and replacement of faulty adjacent restorations with direct composite resin restorations provided an aesthetically pleasing result for this patient (Figures 10 and 11).

CLOSING COMMENTS
This case demonstrates that meticulous management of soft tissue and tooth structure during preparation creates an excellent field upon which a successful impression can be taken. A well-engineered inflexible quadrant impression tray combined with a stiff VPS impression material assists in providing an extremely accurate impression of both the prepared tooth and the opposing arch. As previously discussed, the accuracy of the bite relationship is without peer. This precise registration results in minimal occlusal adjustments and a much less stressful delivery appointment.F

Acknowledgment
The authors would like to thank the restorative team at Drake Precision Laboratory for their support in making this case a success.

References

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Dr. Holt is retired from active practice. He serves as adjunct faculty at University of North Carolina School of Dentistry. He can be reached at lholt@drakelab.com.

Disclosure: Dr. Holt is director of clinical education and research at Drake Precision Laboratory in Charlotte, NC.

Dr. Boksman was the former director of clinical affairs for Clinical Research Dental and Clinician’s Choice and a former associate professor in operative dentistry at the Schulich School of Medicine and Dentistry. He is now retired from private practice and does part-time freelance consulting and writing. He can be reached at lenboksman@rogers.blackberry.net.

Disclosure: Dr. Boksman, prior to December 31, 2011, was a part-time paid consultant to Clinical Research Dental and Clinician’s Choice with the title of director of clinical affairs. As of January 1, 2012, he is retired but consults for Clinical Research Dental and Clinician’s Choice on a limited basis.