What Is the Best Non-Metallic Material for Maryland Bridges, and How Do You Bond Them?
Jack L. Ferracane, PhD | Steven R. Jefferies, MS, DDS, PhD | Nathaniel C. Lawson, DMD, PhD
Jack L. Ferracane, PhD: Resin-retained bridges are often used for anterior and sometimes, posterior applications to replace a missing tooth when a more definitive solution, such as a dental implant, is not indicated. The benefits of less tooth structure removal, lower cost, and high patient satisfaction have been cited as reasons for their popularity.
A fair amount of literature has been published regarding the clinical success of resin-retained bridges. Overall, studies have shown that single retainer cantilever designs perform better than those with two retainers, with the latter demonstrating a greater incidence of unilateral debonding. The clinical success is consistent in this regard for porcelain-fused-to-metal as well as all-ceramic designs. In general, studies show success rates of 90% and greater, lasting as long as 10 years or more. Study data also indicates that anterior bridges tend to have better outcomes than posterior bridges.
The non-metallic options for Maryland bridges include a variety of different ceramics, most notably those considered to be high strength, such as lithium disilicate or zirconia. Due to its very high strength and toughness, some of the best results have been attributed to zirconia. There have been many studies conducted to examine the clinical outcomes of bridges with cantilever retainer wings made from zirconia, glass-infiltrated alumina, or zirconia ceramics (eg, In-Ceram), but today, the latter materials are used less than zirconia. Although the cumulative literature suggests that high strength dental ceramics generally possess adequate fracture resistance, in contrast, fiber-reinforced dental composites have generally faired poorer as fixed bridges, with reported failures often caused by fracture.
Bonding of all-ceramic retainer bridges is typically achieved with resin cements. To date, the most optimal results seem to have been attained using zirconia ceramics bonded with a phosphate monomer-containing resin cement, such as Panavia, after the surface of the prosthesis has been lightly air abraded with aluminum oxide, typically of rather small particle size (eg, 50 μm).
Steven R. Jefferies, MS, DDS, PhD: The quest for a tooth-colored material with the strength, surface adhesion, and retentive features of electrolytically etched nonprecious alloys is ongoing. Metal frameworks offer excellent strength for connector and pontic stability as well as stable bonding to ceramics, but provide a challenge for anterior esthetics due to show-through of the metal. To overcome this problem, numerous alternative tooth-colored materials have been investigated. To be considered acceptable, any tooth-colored material used for this indication must provide both sufficient mechanical strength and adhesive properties to ensure retention with minimal tooth preparation. With limited mechanical retention, adhesion relies on bond strength to the internal framework surface and the etched enamel surface. As a result, the search for a reliable, non-metallic substitute material for resin-bonded, fixed partial dentures has proceeded through an evolution of tooth-colored materials, including fiber-reinforced composites, slip-cast/glass-infiltrated ceramics, etchable lithium disilicate ceramics, and most recently, monolithic zirconia. Currently, two ceramic materials appear to be preferred for resin-bonded fixed partial dentures in the anterior: lithium disilicate (eg, IPS e.max) and CAD/CAM fabricated, monolithic/yttrium-stabilized zirconia. Whereas zirconia offers a distinct advantage in flexural strength (≈ 1,000 MPa), but cannot be etched and requires alternative surface treatments to achieve surface adhesion, lithium disilicate has a lower flexural strength (≈ 300 MPa to 500 MPa), but has a distinct advantage in framework adhesion and retention due to that fact that its surface can be etched. The internal surface of lithium disilicate frameworks can be etched with hydrofluoric acid, thoroughly rinsed with water, dried carefully with alcohol and oil-free compressed air, treated with a silane coupling agent, and bonded with a conventional or adhesive resin cement. Alternatively, zirconia must be surface cleaned with a special proprietary agent such as Ivoclean (Ivoclar), followed by a two-step, chairside, tribochemical silica-coating/silane coupling agent surface treatment protocol. An adhesive resin luting agent is then used for cementation. Preliminary clinical reports suggest that both materials and cementation methods are viable for selected anterior prostheses.
Nathaniel C. Lawson, DMD, PhD: My mentor John O. Burgess, DDS, MS, and I would recommend a single-wing zirconia Maryland bridge with optional facial veneering porcelain. The advantage of using zirconia for a Maryland bridge is in the incredible strength of the material. The connector of the Maryland bridge is highly susceptible to fracture due to the wedge-shaped geometry created by the gingival embrasure, the thinness of the material where the pontic meets the wing, and the concentration of stress due to the cantilever of the pontic. Because zirconia is the strongest and most durable dental ceramic available, it would be the least likely to fracture under these conditions.
The success of a Maryland bridge also depends on it maintaining its bond to the abutment tooth. The bond between zirconia and resin cement has been shown to very successful in laboratory testing and from anecdotal clinical experience. This bond is dependent on sandblasting the surface of zirconia with alumina and then coating it with an MDP-containing primer.
The design of the preparation and the restoration may also affect the success of the bridge. The abutment tooth may be prepared for either a flat wing, a wing with parallel grooves, or a wing with a circular divot on the cingulum (used for orientation during seating). Because the ceramic substructure is stiffer than a metal substructure, it has no source of stress relief when rigidly bonded to two abutment teeth; therefore, the restoration should be designed with a single wing.
In addition, facial veneering porcelain may be added to increase the translucency of the restoration. However, the lingual aspect of the restoration should not be veneered in this manner because polished zirconia causes approximately 10 times less opposing enamel wear when compared with porcelain. Alternatively, the restoration may be fabricated from a cubic-containing high translucency zirconia, but there will be a sacrifice in the strength of the restoration when using this material.
About the Author
Jack L. Ferracane, PhD, is a professor and chair of the department of restorative dentistry and the division director of biomaterials and biomechanics at Oregon Health & Science University in Portland, Oregon.
Steven R. Jefferies, MS, DDS, PhD, is the associate dean of research and graduate education and a professor in the department of restorative dentistry at the Maurice H. Kornberg School of Dentistry, Temple University, Philadelphia, Pennsylvania.
Nathaniel C. Lawson, DMD, PhD, is an assistant professor and the division director of biomaterials at the University of Alabama at Birmingham, Birmingham, Alabama.
