Implant dentistry has shifted to prosthetically guided implant planning and placement. This shift has influenced the range of available dental materials to restore single crowns and partially and fully edentulous jaws with dental implants. This article presents an overview of the available options to restore dental implants. The qualities that define an ideal restoring material are discussed along with the most commonly used materials with their advantages and limitations. Because clinicians face different clinical scenarios in their practice, the discussion sums up the best techniques and materials that are useful in addressing these nonideal situations.
Key points
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With the multiple choices to restore dental implants in single, partial edentulous, and fully edentulous cases, the clinician should be aware of all necessary specifications in the restoring material to achieve an ideal outcome.
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The common materials used in dental implant prostheses are commercially pure titanium, titanium alloys, zirconia, cobalt-chromium alloys, and multiple resin-based immerging materials. The advantages and limitations of these materials are discussed.
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The clinician faces different clinical situations at the dental practice, some of which are complicated and require different techniques or materials to achieve an acceptable outcome. These scenarios are presented with suggested treating protocols.
Introduction
Over the past 50 years, implant dentistry has evolved to provide a long-term successful and predictable treatment with many biologic and mechanical advantages over conventional fixed and removable treatments. It also has shifted from the surgical placement of implants according to the availability of bone to prosthetically guided implant planning and placement. This shift has influenced the range of available dental materials to restore single crowns and partially and fully edentulous jaws with dental implants.
This article explores the main specifications required for a dental implant prosthesis. It also addresses the commonly used prosthetic materials and different scenarios that may present in a dental practice and the material of choice for each scenario.
What is considered an ideal material?
Abutments and superstructures for dental implants should ideally possess the following fundamental characteristics:
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The material should not produce harmful toxicologic or allergic effects to the patient or the operator.
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The physical and mechanical properties of the material should withstand functional load and the challenging oral environment (eg, fracture resistance, modulus of elasticity, solubility, thermal conduction).
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The mode of fabrication should be inexpensive and feasible for the dentist and technician.
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It should enhance the esthetic outcome of color and contour.
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The connection should fit passively in order not to cause wear at the prosthesis-implant interface.
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The mode of insertion and removal should be convenient for the dentist to provide maintenance services.
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The material should promote proper oral hygiene measures and prohibit or eliminate oral plaque accumulation.
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The material should be reparable when an adverse reaction occurs.
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Reliable manufacturers should be used to ensure availability of spare parts (abutments, screws, plastic retentive components for bars and single attachments).
What are the commonly used materials?
Commercially Pure Titanium, Grade 4 and Titanium Alloy (Ti-6Al-4V)
Titanium and titanium alloys have been used in orthopedic reconstructions for a long time and they have been the materials of choice to fabricate dental implants, abutments, and prosthesis. The durability, corrosion resistance, and biocompatibility are the attractive characteristics that give titanium its importance. Nevertheless, difficulties in porcelain application and casting has motivated the search for other fabrication methods, such as machining, or using monolithic materials, such as ceramics and polymers.
Grade 4 commercially pure titanium and titanium alloys (specifically Ti-6Al-4V) are used to fabricate stock and customized abutments/frameworks to retain or support dental prosthesis. These objects are used during the interim and definitive phases for fixed and removable prosthesis.
Fixed
Implant-supported single crowns or fixed dental prostheses are cement- or screw-retained. Implant prostheses were historically fabricated as screw-retained prostheis. Cement-retained prosthesis has gained popularity because of its cost effectiveness and similarity to tooth-borne restorations. In addition, it has comparable survival rate to screw-retained prostheses. However, there is a shift back to screw retention because more studies showed higher biologic complications associated with cement-retained prosthesis, and because of the retrievability advantage of screw-retained prosthesis.
Abutments
Prefabricated (stock) abutments have an emergence profile that was developed to resemble the cross-section of teeth in different oral locations. However, stock abutments often fail to match the patient’s anatomy. They are cost-effective and are easily adjusted by laboratory technicians or clinicians. Straight or angled stock abutments (different angulations) with different transgingival heights are commercially available ( Fig. 1 ).
Hybrid abutments (titanium inserts) ( Fig. 2 ) were developed by bonding an individualized monolithic ceramic crown (zirconia or lithium disilicate) to the abutment to protect the implant interface from wear by all-zirconia abutments, which manifest clinically as a titanium tattoo. Titanium inserts have an increased fracture strength of ceramic abutments and crowns and reduced wear of the implant connections.
Customized implant abutments began with the introduction of the universal clearance limited abutment to bypass the transmucosal cylinders of the Brånemark implant system. This helped in developing a customized emergence profile and allowed restoration of angled implants in the case of limited interocclusal space. However, the abutment did not limit the accumulative error associated with casting. The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) abutments ( Fig. 3 ) helped in overcoming the problems associated with the universal clearance limited abutments, improving the gingival health of the restored implants, and reducing the cost. CAD/CAM abutments show good survival and success rates, provide better soft tissue reaction (less recession), and have less incidence of screw loosening than conventional stock abutments. Custom abutments show comparable, if not better, clinical outcomes when compared with conventional abutments.
Framework
Casting titanium frameworks have been popular because of their biocompatibility and corrosion resistance, which is derived from the thin passivating oxide layer. These advantageous properties are attained by undergoing a sensitive casting and finishing processes because titanium has a high melting temperature (1668°C) and high rate of oxidation greater than 900°C. Therefore, it requires special investments and a casting machine with arc-melting capability and cooling cycles. Titanium crystalline structure changes shape at 883°C, when the alpha-hexagonal phase becomes a beta-cubic phase. This conversion affects the bond between titanium and dental ceramic; therefore, dental ceramics staked on titanium are fired at temperatures less than 800°C.
The distortion and porosity induced by the classic lost-wax casting technique are eliminated by using CAD-CAM milling ( Fig. 4 ). It is logical that larger frameworks with a greater number of implants benefit from the advantages of CAD/CAM technology. The accuracy of CAD/CAM ensures passive fit of frameworks/abutments to limit movement and bacterial leakage. Abduo found that CAD/CAM implant frameworks fabricated with titanium and zirconia have a high level of accuracy with a passive fit that surpasses the one-piece casting or laser-welded frameworks. A systematic review by Kapos and Evans showed the use of CAD/CAM frameworks for implant-supported restorations provides comparable prosthesis survival rate and technical and biologic complications to that of conventional techniques.
Removable
An implant-retained overdenture (IOD) with two nonsplinted implants is the treatment of choice for edentulous mandible, according to McGill consensus. However, splinted anchorage (ie, bar) provided a higher implant survival rate compared with nonsplinted anchorage (ball/locator attachments or telescopic crowns) in implant-supported maxillary overdenture.
Single attachment
Single attachments are implant screw-retained abutments that snap into a corresponding secondary housing within the intaglio surface of the IOD. These attachments are divided into resilient and rigid.
The resilient attachments (Ball, locator, and magnets) are usually prefabricated. Locators, which are widely used, offer up to 40° of compensation for nonparallel implants. Prosthesis retained with resilient attachments allows some movement during mastication and requires mucosal support.
Rigid attachments are custom-made abutments that are cast or milled (Atlantis Conus concept, Dentsply Sirona, York, PA). Retention of rigid attachment comes from the metal friction fit between the 5° and 6° tapered abutment and the corresponding metal cap within the prosthesis. Rigid attachments provide support to the IOD preventing any vertical movement during mastication. They also correct implant misalignment.
Locator attachments provide a better access for oral hygiene compared with bars and telescopic crowns on implants. They showed minimal scores of peri-implant health indicators (plaque, gingival, bleeding, and calculus indices) and fewer maintenance appointments over 3 years. Well-designed studies are warranted to explore the advantages of using rigid attachments over resilient ones.
Bar
Bar attachments ( Fig. 5 ) provide rigid retention and they compensate for implant misalignment. They can provide distal extensions to support an IOD in a class II jaw relation when implants are placed in the anterior maxilla region. However, bars require more interocclusal space (15 mm) compared with locator (8–10 mm) and they are less hygienic. The lack of proper oral hygiene and the negative pressure that forms under a bar-supported IOD predispose to the formation of hyperplastic gingivae, which may require surgical removal.
Zirconia/Zirconium-Alumina
Zirconia is a polycrystalline ceramic and it has mechanical properties advantage over other dental ceramics. These physical properties depend largely on the preparation techniques and the design of the final prosthesis. Color and decreased bacterial plaque adhesion have given zirconia a wide range of applications in dentistry.
Abutment
The gingival architecture around the implant has a major role in esthetic outcome. In addition, color is one of the key esthetic parameters. The undesirable shine-through effect of the underlying metal abutment in thin soft tissue phenotype compromises the peri-implant mucosa shade. Zirconia abutments demonstrate less effect on optical outcomes of peri-implant mucosal tissue when compared with conventional titanium abutments. However, it causes a higher wear rate on the implant connection compared with the titanium abutment. It also has a higher rate of the zirconia abutment fracture. In multiple unit restorations, a higher chance of prosthesis misfit is observed with one-piece zirconia frameworks.
To eliminate the previously mentioned problems with a one-piece zirconia abutment and superstructure, two-piece zirconia abutment ( Fig. 6 ) was developed, allowing a customized zirconia abutment to be cemented on top of a prefabricated titanium base. Because of the metal-to-metal contact between the implant internal connection and the titanium base, two-piece zirconia abutments can achieve higher strength than one-piece abatements. A prefabricated titanium abutment provides a passive fit to the internal connection of the implant and minor misfit from the sintering process of zirconia is corrected with resin cement.
