Cast Gold and Porcelain fused-to-metal crowns have a proven track record and are predictable.1,2 They are forgiving materials that can be used in less-than-ideal conditions such as uneven tooth reduction, sharp line angles and varying finish lines. Masking the underlying metal can result in high value opaque crowns. Gum recession may show an unaesthetic crown-tooth junction displaying different colours and opacities. The underlying root structure often refracts the dark colour of the metal.3 Attempts to hide margins around recessed areas are generally not successful. (Fig. 1)
All-ceramic crowns ameliorate these potential problems. They have better translucency and vitality. Supra-gingival margins are often possible and invisible if the colour is not being significantly changed. If sub-gingival margins are eventually exposed due to gingival recession, the esthetic result is not as detrimental as with porcelain fused to metal crowns.
Preparations for all-ceramic crowns require more time and precision. Even and adequate reduction is important to ensure maximum strength.4 Shoulders preparations with rounded line angles and edges are a requirement to allow thickness of the material at the margins and to decrease tensions in the ceramics.5 The intaglio surface cannot be adjusted since it creates internal stresses that increases the likelihood of fracture.6 Cementation protocols are specific for each material and must be closely followed. Attention to detail and a cooperative patient are required for long-term success. Dentists and lab technicians must understand the different all-ceramic materials available and their characteristics to select the optimal choice.
Lithium disilicate is a leucite reinforced glass-ceramic material with a long track record of predictable service.7 It has a flexural strength near 400 MPa and is available in pressable and millable forms. It can be used as a monolithic material and glazed to maintain the best strength or layered with porcelain for the best appearance. In general, monolithic lithium disilicate is preferred when used in molars for high esthetic demanding patients and in bicuspids. Lithium disilicate layered with ceramic is general used in bicuspids, incisors and cuspids where the best long-lasting natural depth of colour and translucency is required. Lithium disilicate is recommended for inlays, onlays and crowns but not for bridges. (Fig. 2)
The material has good translucency and the colour of the cement can affect the final colour of the restoration. Teeth with dark cores, severe discolourations or cast metal posts must be blocked out with opaquers so that they do not show through the lithium disilicate. Adequate tooth preparation is vital to the lifetime of the restoration. At least 1.1mm of tooth preparation is preferred.8 Rounded shoulder finish line margins with rounded internal line angles reduce destructive forces within the restoration which can lead to premature fractures, the leading cause of failure.9
Restorations ideally should be bonded to provide the optimal strength and resistance to fracture.10 Crowns with a minimal thickness of 1.5mm can be conventionally cemented with resin-modified glass ionomer. Lithium disilicate may be your material of choice if the best esthetic outcome is desired, occlusion is stable and non-destructive, the colour of the underlying tooth structure is not severely discoloured, and the patient is cooperative.
DIFFERENT FORMS OF ZIRCONIA
Zirconia exhibits different crystal structures at varying temperatures. (Fig. 3) At room temperature it is in the softer monoclinic form. Upon heating to 1100oC the crystalline structure undergoes a phase transformation to a tetragonal orientation. Further heating to 2370oC results in a cubic orientation. Once allowed to cool, zirconia returns to its weaker monoclinic form.
Zirconia can maintain a multiphase form containing cubic, tetragonal and monoclinic forms at room temperature through the incorporation of stabilizers. Yttrium oxide has been successfully used imparting significantly improved physical properties. The resulting material is known as Ytterium Tetragonal Zirconia Polycrystals or Y-TZP.11 The amount of yttrium oxide added, affects the physical and optical qualities of the zirconia. 3 mole % yttrium oxide (3Y-TZP) results in the strongest form but is opaque. Higher concentration 5 mole % yttrium oxide (5Y-TZP) makes the material more translucent but is not as strong.
HIGH STRENGTH ZIRCONIA
High strength zirconia (3Y-TZP) is very strong with flexural strengths of 1100-1200 MPa. It is opaque and was initially used to replace metal as a substructure and layered with porcelain. Later it was used in the monolithic mode to reduce the occurrence of chipping between the zirconia and layering porcelain. Initial concern was that zirconia would wear opposing tooth structure but that has been shown to not be a problem if the occlusal surface is properly polished, not left glazed.12 The material has a low fracture rate.13 Initially there were a significant number of crowns which came loose. Zirconia crowns become contaminated by salivary phosphates when tried in the mouth blocking effective bonding between the cement and the crown. This can be remedied by treating the intaglio surface with high pH cleansers immediately prior to cementation.14 (Fig. 4)
Monolithic high strength zirconia crowns can be used in all areas including molars and in posterior bridges. They are indicated when strength is the primary concern and are a good value. Layered high strength zirconia works well with badly stained teeth, when conventional cementation is preferred and on anterior teeth where occlusion is very tight. (Fig. 5) In close situations, the palatal surfaces are zirconia only and the facial is layered. (Fig. 6) There has been a higher incidence of fracture between zirconia and the overlaying porcelain which is highly dependent upon the materials used and the skill and experience of the laboratory technician.15-16 Porcelain fused to zirconia crowns have higher residual stresses than porcelain fused to lithium disilicate crowns and are more affected by the ratio between the thickness of the core and the porcelain.17
HIGH TRANSLUCENT ZIRCONIA
High translucent zirconia (5Y-TZP) is a more esthetic zirconia material with flexural strengths of 550-750 MPa.Recommended uses include first molars of high-esthetic demanding patients, premolars and in budget anterior crowns. They can be seated with all types of
cements according to the esthetic requirements, the strength needed and the retention of the tooth preparation. Self-adhesive resin cements are ideal to use in many instances.
Newly developed liquid ceramics are an effective way to enhance the natural appearance of high translucent zirconia.18 They are thicker than traditional glazes and add a 100–200-micron thick layer to the outer surfaces. This brushed on layer gives a more 3-dimensional effect to the facial surfaces and increases the durability of the colour enhancements at a lower cost than traditional layered restorations. (Fig. 7)
Zirconia has been developed that contains several different crystal lattice forms. High strength (3Y-TZP) is present in the cervical portion of the crown giving the best strength and high translucent (5Y-TZP) is in the occlusal/incisal areas giving the best appearance. This combination can be used in crowns and in bridges giving the best features of different materials in one restoration.
FACTORS DETERMINING OPTIMAL CROWN MATERIAL SELECTION
Optimal crown material selection must consider many factors. Strength is obviously a primary consideration. The esthetic expectations of the patient are a major contributing factor. Occlusal habits and occlusion must be thoroughly evaluated to insure longevity and stability. The specific teeth to be restored plays a role. The colour of the underlying tooth structure must be studied. The amount of tooth preparation necessary and occlusal clearance and should be calculated. The periodontal biotype should be evaluated.19 The size and type of proposed and existing core buildups or post and cores must be determined. The appearance of the surrounding teeth must be considered when doing single crowns.20 The preferred type of cement to be used for seating should be deliberated. Finally, the cooperation and tolerance of the patient along with cost is of upmost importance. All these factors should be evaluated in consultation with specialists, the dental laboratory and the patient. It is important to realize that there is not one best material for any single factor, but an optimal material for all the factors considered together. (Table 1)
A 30-year-old female was referred to my office who had 22 porcelain fused to metal crowns placed when she was 18 to improve her appearance. (Fig. 8) Her chief complaints were the black triangles between her teeth, the black lines at the crown margins, the dark roots visible where the tissue had receded, and the colour of the crowns. (Fig. 9) Full mouth radiographs, photographs and study models were taken. Decay and significant discolourations were noted around many of the crown margins. Several teeth had undergone endodontic root canal therapy. The vertical dimension of occlusion was satisfactory.
The photographs were studied, and a RED Proportion template was superimposed over her smile.21 (Fig. 10) A 66% RED Proportion was selected. A composite mockup of the anterior 6 teeth was made, a vacuum form splint fabricated, and the 6 crowns were removed to see the condition of the underlying tooth structure. Severe stains were prevalent with multiple colours of varying intensities. (Fig. 11) Decay and old restorations were removed, and composite buildups placed. The provisional restorations were customized with composite to mimic the desired size and shape of the final restorations. (Fig. 12) The patient was asked to evaluate the esthetics and ease of speaking for several weeks.
After seeing the condition of the underlying tooth structure, large core buildups, and dark varied underlying tooth structure it was decided to use high strength zirconia overlaid with porcelain. This would give maximum core strength, best optical blocking of discolourations, better control of achieving uniform colour of all restorations. Conventional cementation with resin-modified glass ionomer cements would provide fluoride release and easier removal of sub-gingival excess cement.
The patient was pleased with the provisionals and the remaining maxillary teeth from first molar to first molar were prepared. (Fig. 13) The second molars were not shaped to maintain a vertical stop to keep the existing vertical dimension of occlusion. Photos of the prepared teeth with stump shade guide, seated provisionals, desired shade tab, and impressions of the provisionals were sent to the lab. (Figs. 14-15) This provides them with a physical and visual blueprint of the desired length, position, and shape of the final restorations.
The crowns were fabricated by the laboratory. The patient returned and the crowns were tried in with adjustments made on the opposing arch to maintain ideal anatomy of the new crowns. Immediately prior to cementation the intaglio surface of each crown was coated with a high pH zirconia cleaner for 30 seconds and thoroughly washed and dried to remove the phosphate contamination from saliva in the mouth. Resin-modified glass ionomer was used to cement the crowns. All excursive movements were then thoroughly adjusted. (Fig. 16)
The patient returned and the lower bicuspids and first molars were prepared. She insisted that the lower anterior teeth would be treated conservatively only with tooth whitening and composite restorations. The lower crowns were tried in and seated. The patient returned in 2 weeks to refine all aspects of the occlusion and to take photographs. (Fig. 17) The patient felt she looked much younger and was pleased with the results.
A 67-year-old male had talked about a cosmetic improvement for years. (Figs 18,19) Finally he said he would like to proceed with his long-discussed esthetic treatment as soon as possible. Master Diagnostic ModelsTM had been fabricated 10 years earlier and the patient said that was the way he wanted his teeth to look. (Fig. 20) Even though there had been significant wear it was determined that the vertical dimension of occlusion did not need to be opened and there was adequate room to achieve the desired effect.
The patient was most interested in having whiter teeth. Strength was important but we had adequate space to remove many of the destructive occlusal elements though the re-shaping of the new crowns. The newer high-translucent zirconia crowns overlaid with liquid ceramics offer good strength, nice esthetics and easier cementation. His caries rate was low so self-adhesive resins would give adequate bonding and greater ease of use.
The maxillary teeth were prepared (Fig. 21) and provisionals placed. (Fig. 22) Since there was no change in the vertical dimension of occlusion, preparations extended from first molar to first molar. Photos of the desired shade tab, preparation shade and preparations were taken. The patient returned to the office 2 days later and was asked about the appearance. He was pleased so photos and impressions of the provisionals were taken and sent to the lab.
The crowns were returned from the lab. (Fig. 23) They were tried in individually and then together. Different try-in pastes were used and the opaque white cement selected. A cementation order was determined. With multiple crowns and slightly different paths of insertion, a sequence should be rehearsed that allows unrestricted complete seating. Seating multiple teeth at the same time allows the dentist to keep the maximum amount of cervical interproximal ceramic material avoiding food traps and speeds up the cementation procedure. The intaglio surface of each crown was decontaminated with the zirconia cleaner immediately prior to cementation. If a resin cement contains MDP, no further treatment is needed to the crown, otherwise an MDP primer is recommended for the intaglio surface of the crown. The tooth was treated with chlorhexidine for 20-30 seconds and excess moisture removed. This step disinfects the surface and slows down the degradation of the dentin bond from matrix metallo-proteases (MMP) which can attack the collagen fibers in the hybrid layer.22 The crowns were seated using self-adhesive resin cement. (Fig. 24) Occlusal adjustments were made on the opposing arch which was to be prepared next.
The mandibular arch was prepared in a similar fashion. (Fig. 25) The crowns were tried in and seated. The patient returned 2 weeks later for final occlusal adjustments and photographs were taken. (Fig. 26) The patient could not stop smiling and was very pleased.
A 32-year-old female had difficulty chewing her food and rarely smiled. Complete photographs and study models were taken. (Figs. 27-28) The patient was acutely biting into her incisive papilla which was red and swollen. The maxillary incisal edges flared outward and completely overlapped the mandibular incisors when the teeth were together. A full mouth rehabilitation was considered but several teeth may have required endodontic root canal therapy and the up righted plane of the restored occlusal surfaces would not have favorably coincided with the long angle of the roots creating stresses on the bone and periodontium. We referred her to the orthodontist to evaluate placing the teeth in more favorable positions.
The orthodontist discussed orthognathic surgery which the patient declined. He said he could open up the vertical dimension, expand the arch, and re-align and retrude the maxillary incisors. Brackets were placed (Fig. 29) and the orthodontics proceeded for 18 months. (Fig. 30) Following removal of the appliances, retainers were worn for 9 months. (Fig. 31) The smile was designed using the RED Proportion template and a Master Diagnostic ModelTM was created. (Figs. 32-33) Since the patient had a desire for optimal esthetics, exhibited low wear, showed very little staining and was very cooperative, layered lithium disilicate crowns were selected. The maxillary teeth were prepared and photographs taken of the desired shade and of the stump shade. (Figs 34-36) Provisionals were fabricated as well as bite records taken with an increased vertical dimension of 1mm.
The maxillary crowns were fabricated to match the tooth positions of the MDM and the provisionals. (Fig. 37) The crowns were tried in individually and together. Try-in pastes were used to determine the ideal shade. The intaglio surface was treated with phosphoric acid to clean and acidify the surface. Two component non-hydrolyzed silane was placed on the inside surface and allowed to sit for several minutes. Mixing the silane immediately prior to placement insures the freshest and most effective material. The crowns were bonded in 2 at a time starting with the central incisors using adhesive resin cement. Any adjustments to the occlusion were made to the opposing arch. Though the bite had been opened the remaining increase in dimension would be obtained with the lower crowns. This would give the patient time to get used to her opening in phases. (Fig. 38)
The posterior mandibular teeth were prepared. (Fig. 39) The patient decided to just tooth whiten the relative unrestored mandibular anterior teeth. The provisionals were constructed at the final vertical dimension of occlusion using the template made of the ideal lower teeth made by the laboratory. (Fig. 40) A rigid bite material was used to record the desired final vertical dimension to ensure the best accuracy.
The crowns were returned from the laboratory and checked. (Fig. 41) They were tried in, seated and adjusted. The patient returned a day later and then 2 weeks later for final adjustments and photographs. (Figs. 42-43) She said that the bite felt comfortable and was ecstatic with her new smile.
Three cases have been shown which help to demonstrate the treatment planning decisions when selecting the optimal all-ceramic materials to use. The primary factors of strength, occlusion, esthetics, periodontal biotype, cost, ease of use and patient cooperation are instrumental to making the best choice. Dentists in consultation with their dental laboratory should use this process to plan the best choices when using the latest all-ceramic crowns.
Oral Health welcomes this original article.
Acknowledgement: The author would like to thank Dr. Amin Mason for the excellent orthodontic treatment and coordination, Jenny Wohlberg CDT of Valley Dental Arts for her fabrication of the beautiful crowns and moral support, Dr. Elliot Mechanic for demonstrating and reinforcing the concept of orthodontic treatment prior to more conservative treatment and Dr. Irwin Smigel for educating me on the role of the teeth on the face and ultimately on the lives of our patients.
Master Diagnostic ModelTM is a trademark of Valley Dental Arts. Stillwater, MN.
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About the Author
Daniel Ward is in private practice in Columbus, Ohio. He is a Diplomat of the American Board of Aesthetic Dentistry, a Fellow of the American Society for Dental Aesthetics, Fellow in the Academy of General Dentistry and Fellow in the American and International College of Dentists. He has lectured internationally and authored numerous articles in the field of proportional smile design. Email: firstname.lastname@example.org
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