November 1, 2004
by Arkady Davidson, DDS, Dan Huber, RDT, DD., Craig Mortimer
Today, more than ever, dentists are being challenged to find alternatives to traditional restorative techniques. Television and the Internet have created a knowledgeable patient who is demanding when dealing with aesthetic concerns. This issue is forcing us to look for new and better ways to deal with what were before straightforward restorative options.
The following case displays a new approach to the challenge of creating a highly aesthetic restoration.
A female patient in her mid-forties presented to the office, complaining that her eight-year old bridge was mobile and caused pain on mastication. The initial examination showed the gingival margins around teeth 2.4 and 2.7 and the soft tissue under the pontics were swollen and inflamed. The patient’s medical history did show she was in good health and had no known allergies or sensitivities to medications.
She had been seeing her dentist regularly for recall examinations and scaling. The clinical examination revealed good oral hygiene. The patient had a four-unit porcelain fused to metal bridge, replacing missing teeth 2.5 and 2.6 with abutments at 2.4 and 2.7. Closer examination revealed that a crack in the pontic area completely separated the bridge into two parts, and the soft tissue over the ridge area and around the abutments was irritated, painful and bled to touch. Radiography showed that tooth 2.4 had been endodontically treated and a post-and-core placed. Both tooth 2.7 and 2.4 showed no pathology and radiographically the periodontal condition was normal. The interocclusal space of the bridge (2.4 – 2.7) was very limited.
It was evident that the broken bridge would have to be replaced immediately. The 3mm vertical was of even greater significance when the patient requested, “No metal should show in any part of the new bridge.” This request presented serious technical problems. Previously, the only restoration that could withstand the biomechanical forces of mastication in this limited vertical space was a porcelain fused-to-metal bridge with metal occlusion. At this point in the treatment plan, I sought the advice of the laboratory as to the availability of metal-free materials suitable for this situation. After lengthy conversations and research, the consensus was that a restoration using milled Zirkon(RT) 1 (Cynovad Cybernetic Innovations for Dentistry, Montreal, Quebec) for the framework would provide both the biomechanical strength and the aesthetics that the patient required. Zirkon(RT) is composed of Yttrium–stabilized HIP Zirconium Dioxide, milled in the cintered state.
Zirconium is a highly stable mineral. It is biocompatible and has exceptionally high resistance to corrosion. Zirconia (zirconium dioxide) has long been used by orthopedic surgeons for complete hip transplants,1 and has recently found its way to dentistry.2
Its superior strength and toughness to stress and biomechanical forces make this material the material of choice when non-metal restorations are required. Zirconium dioxide has a unique property, referred to as the mechanism of transformation toughening (the inhibition of crack propagation). Zirconia (zirconium dioxide)3 has fracture toughness three to six times higher than Zirconium and is tougher than cast iron. The oral milieu represents a type of environment, which makes the extraordinary properties of Zirconium dioxide the material of choice (Table 1).
The bridge was a longer-than-average four-unit bridge and the height of the abutments offered uncertainty that the required six square millimeters of connecting material between segments was available. The abutment tooth 2.7 might need occlusal reduction, and this adjustment would negatively impact on the resistance and retention of the restoration.
These concerns prompted a conference call involving the laboratory and Cynovad Inc. in St-Laurent, Quebec, the manufacturer and milling center for the Zirconium Dioxide. The call did confirm that Zirkon(RT) would withstand the biomechanical forces but crown lengthening on both abutments would be necessary to establish the six square millimeters needed for connecting the copings to the pontics. A reduction of the occlusal height of tooth 2.7 would not be necessary because the Zirkon(RT) coping could be glazed and does not require a layer of porcelain.
Treatment began with the removal of the broken bridge and an evaluation of abutment teeth 24 and 27. The old post-and-core was removed and replaced with a parapost #4 cemented with GC glass ionomer cement and a composite resin-core. To ensure the strength of the framework, a gingivoplasty was completed around the abutment teeth, and along the alveolar ridge in order to create space between the edentulous ridge and the opposing teeth. This required a six-week healing period, during which time a PFM bridge with metal occlusal stops was created to replace the old bridge, and act as a temporary/ permanent restoration. (Fig. 1)
After complete healing of the soft tissue, preparations began for the final working impressions. Retraction cords (Gingibraid nv-0A NTCD, Mississauga, ON, and Ultrapac nv-0 Ultra Dental Product Inc., South Jordan, UT) were placed around the abutments, and the chamfer preparations of the abutments were examined to make sure they conformed to the requirements of a Zirkon(RT) framework. A full-arch impression (Vinyl Polysiloxane–GC Exaflex GC America Inc. Alsip, IL) and CR records (Paltera Resin LS, GC America Inc. Alsip, IL) were taken and sent to the laboratory.
The impressions were poured in the non-light reflecting stone which was necessary for scanning. The dies were cut and scanned, and the design process began. The four-unit bridge was designed with the aid of the CAD software program. The design consisted of a full wax-up and with the help of a virtual articulator (Figs. 2 & 3a); a reduction for the necessary layering of porcelain was requested of the CAD system. The digital image of the adjusted framework (Fig. 3b) was then transmitted to the milling station in Montreal. Four days later, the Zirkon(RT) framework was returned to the laboratory, and a framework try-in was scheduled (Fig. 4).
The try-in (Fig. 5) went well, the frame was positioned, the path of insertion was passive, and no pressure or tension was present. The margins were accurate on the chamfer preparations and fit-check was used to verify the framework’s position. The bite was checked in centric. Protrusive and lateral excursions confirmed no interferences. The bridge was returned to the laboratory for the addition of porcelain. The shade selection was completed after clinical whitening using the one-hour zoom system (Discus Dental Canada) and forty-eight hours later the shade was sent to the laboratory.
The framework was sandblasted and steam cleaned to remove any surface contamination and then opaque was applied to the translucent Zirkon(RT) framework. Opaquing was placed to establish the value and chroma necessary to achieve the desired shade. It is not used to hide the metal substructure normally associated with a regular PFM. The porcelain was built up over the opaque layer, and as with all porcelain procedures, the CTE (Coefficient of Thermal Expansion) was lower than the framework, which has a CTE of 10.6.
The porcelainized Zirkon(RT) framework was returned for cementation (Fig. 6). The aesthetics, shade and occlusal functions were confirmed as well as patient satisfaction. The patient was anaesthetized, teeth 2.4 and 2.7 isolated, and cleaned with alcohol and a fine prophy paste (Denti-Care prophylaxis paste with fluoride – A.R. Medicom Inc., Montreal, Canada). Retraction cords (Gingibraid nr-04, NTCD, Mississauga, ON) were placed around the abutments to prevent cement flowing sub-gingivally and dental floss was placed under the pontics to help clean and remove the cement after the bridge was inserted.
The bridge was cemented with RelyX (3M ESPE Dental Products, London, ON). My choice of this cement was based on clinical studies of RelyX Unicem that show
it to have the highest long-term bond strength values. After stress tests with Zirconium dioxide restorations, even if the abutments are vital, the biocompatibility of Unicem positively influences the pulp cells and the post-operative sensitivity is very low or close to zero. After setting, the excess cement and the retraction cords were removed, and the space between the pontics and the alveolar ridge cleaned with the floss that was in place. (Fig. 7)
Close collaboration between laboratory, dentist and Cynovad created a strong, aesthetic and functional restoration. This CAD-CAM processed restoration was able to overcome significant biomechanical and aesthetic concerns. As the aesthetic revolution continues, there is sure to be a growing demand for metal-free alternatives and the CAD-CAM technology necessary to provide them.
Arkady Davidson, DDS, practices in Thornhill, ON.
Dan Huber, RDT., DD., is President, Lindberg Homburger Modent Dental Studios.
Craig Mortimer is General Manager, Lindberg Homburger Modent Dental Studios.
The authors wish to thank Thomas Gsaenger, Manager of porcelain department and Jim Agoritsas, Manager of fixed framework department, at Lindberg Homburger Modent Dental Studios, for the design and creation of the restoration.
Special thanks to Dr. Bruce Glazer for his editorial assistance.
Oral Health welcomes this original article.
1.Metoxit – High Tech Ceramics History, www.metoxit. com/orthopedicsdental-e
2.Sundar, PHD and Kennedy, PHD: Cercon Zirconia; A Systems Solution for Reliable Metal-free Multiunit Restoration Dentsply Ceramco – Research and Development.
3.Metals and Ceramics Division – Energy* Development Oakridge National Laboratory www.ms.orul/programs/energyeff/cfcc/iof/chap21-2ttz.pdf
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