Acrylic Hybrid Dentures Converted to Monophase Zirconia
New material option provides strength and translucency
Fixed-detachable denture bridges have more than 40 years of documented history.1 This bridge type is also known as a hybrid denture because it is composed of acrylic denture teeth and resin processed to or around a metal screw-retained substructure. This design provides the patient with a restoration that functions as a fixed bridge with the necessary lip support for esthetics.
However, this fixed bridge function allows much higher occlusal forces, resulting in a significant increase in abrasion and breakage of the resin prosthetic teeth.2 Once the acrylic denture teeth have reached a significant level of abrasion, new denture teeth and acrylic must be reprocessed over the existing metal framework. Resin hybrid dentures need to be re-processed with new resin teeth and acrylic approximately every 7 to 9 years.3
This maintenance problem has been solved with monophase zirconia. The term “monophase” is used to signify that there is no layering of ceramics in functional areas. Hence, there is no veneer of porcelain that can fracture away from the zirconia. The newest available zirconia is also exceptionally translucent and, when used with specialized coloring stains, the need for veneering porcelains is eliminated in areas of occlusal load. Twelve millimeters of vertical space is recommended for sufficient strength.
When restoring lighter tooth shades (A to B shades), veneering porcelain is not needed facially. For darker tooth shades, minimal cut-back can be performed and porcelain can be added facially in those nonfunctional areas. Tissue-shaded porcelains also are added gingivally, not affecting functional zones. Functional zones with monophase zirconia are not susceptible to wear, chipping, or breakage.
This article details how to work in conjunction with a zirconia specialty laboratory to convert a traditional or provisional hybrid denture into a monophase zirconia bridge.
Case Example
It is important to note that the restoration being converted must fit passively to the existing implants/abutments, and the tooth arrangement must be verified esthetically and functionally by the restoring dentist and the patient. The existing acrylic hybrid denture (Figure 1) is inserted gingivally into polyvinyl siloxane lab putty, capturing the intaglio/gingival and abutment/implant interfaces.
A wide circumference of the putty is required for stability. While the putty is still moldable, small indentations are made to index later with the covering putty (Figure 2 and Figure 3). Once the top putty portion has set, the mold is separated and the acrylic hybrid denture bridge is removed.
The complete putty mold, along with the articulator mounted with a master model with abutment analogs and bite registration, are sent to the zirconia specialty laboratory. A face-bow may also be included. The zirconia specialty lab will fabricate a resin duplicate of the acrylic hybrid denture.
The resin duplicate is articulated, scanned, and imported into the CAD design program to mill the design for the zirconia Prettau® (Zirkonzahn, www.zirkonzahn.com) bridge. This bridge material has an exceptional bending strength greater than 1,400 Mpa, resulting in a highly fracture- and wear-resistant restoration.
Options for Restoration
The restoring dentist may scan the acrylic hybrid denture on the model and send the open source STL file to the zirconia specialty laboratory. This would negate the need for the putty mold of the acrylic hybrid. The zirconia design is milled in the zirconia block (Figure 4). Space to receive non-engaging titanium interfaces is programmed into the access channels and mating surfaces of the zirconia bridge. California Abutment Luting (CAL) cylinders (Implant Direct, www.implantdirect.com) for the multi-unit abutments are used in this example. The titanium CAL cylinders provide a metal seat for the implant abutment interface and screw seat. The CAL concept also creates a passive fit once they are cemented into the finalized zirconia bridge on the master model or intraorally. Gingival colored porcelain is applied as desired (Figure 5).
Once the glazing is finalized, the metal interfaces of the CAL cylinders are cemented in place with a composite resin cement while attached to the master model in occlusion while on the articulator (Figure 6). Alternately, the CAL cylinders may be cemented into the finished restoration at chairside intraorally if desired. The definitive Monophase Prettau zirconia restoration is then delivered to the restoring dentist (Figure 7).
Summary
Screw-retained monolithic zirconia bridges can now be fabricated from newly available translucent zirconia, providing a superb natural appearance. The definitive restoration is monophase zirconia in the functional load-bearing areas, rendering it resistant to occlusal abrasion, chipping, and breakage.
References
1. Turkyilmaz I, Jones JD. Long term success of 6 implants supporting a mandibular screw-retained fixed dental prosthesis: a clinical report. J Prosthet Dent. 2012;107(5):280-283.
2. Bozini T, Petridis H, Garefis K, Garefis P. A meta–analysis of prosthodontic complication rates of implant-supported fixed dental prostheses in edentulous patients. Int J Oral Maxillofac Implants. 2011;26(2):304-318.
3. Drago C, Gurney L. Maintenance of implant hybrid prostheses: clinical and laboratory procedures. J Prosthodont. 2013;22(1):28-35.
About the Authors
Darwin Bagley, CDT
Senior Director, Technical Services for Attachments International Implant Direct
Calabasas Hills, California
Michael Tischler, DDS
Faculty
UMDNJ, AAID Implant Maxi Course
Camden, New Jersey
Diplomate
International Congress of Oral Implantologists
Wayne, New Jersey
