Attachment Systems for Retention of Full-Arch Fixed Implant Restorations

Written by: Drs. Joseph Massad and Swati Ahuja

INTRODUCTION

Fixed, full-arch, screw-retained implant prostheses (Figure 1) supported on titanium endosseous implants were first described in clinical studies conducted by a research group led by Per-Ingvar Brånemark.^1,2 These early protocols involved placement of 4 to 6 endosseous implants in the anterior region of the mandible to support a fixed prosthesis.^1,2 Long-term follow-up of this treatment concept, as reported by Brånemark and co-workers, demonstrated 10-year success rates ranging from 78.3% to 80.3% in the maxilla and 88.4% to 93.2% in the mandible.³ Currently, full-arch, fixed, implant-supported prostheses are a commonly prescribed treatment modality for the edentulous patient.^4-6 

Factors Affecting the Long-Term Success of Fixed Implant Restorations

The long-term success of fixed, full-arch restorations depends on several factors, including the availability of adequate restorative space, the prosthetic materials selected, and the mode of retention of the prosthesis.^6-8

Restorative Space.Restorative space is the 3D oral space available to receive the proposed prosthodontic restoration (Figure 2).⁹ In edentulous patients, the available restorative space is bounded by the occlusal plane, supporting tissues of the edentulous jaw, facial tissues (cheeks and lips), and the tongue.⁹ Factors such as interocclusal rest space, phonetics, and aesthetics must also be considered when defining available restorative space.⁹ The minimum vertical restorative space required for fabricating a full-arch, fixed, screw-retained zirconia and a hybrid (metal-acrylic) restoration is 12 mm and 15 mm, respectively.^8-10 Designing prostheses without considering the restorative space can result in restoration with inadequate contours, increased fracture susceptibility, and poor aesthetics.⁹ Insufficient restorative space may lead to the removal of existing non-restorable implants.¹¹

Prosthetic Materials for Full-Arch Restorations.The material selected for fabricating an implant-supported restoration influences its success rate.¹² Traditionally, metal-acrylic and metal-ceramic restorations were used, however, today, newer materials such as monolithic zirconia, ceramic-veneered zirconia, ceramic-veneered titanium, lithium disilicate, hybrid ceramics, milled PMMA (polymethyl methacrylate), PEEK (polyether ether ketone), and 3D-printed resins are now considered viable options for the fabrication of full-arch implant prostheses.¹² 

Zirconia. Zirconia has become a preferred material for the fabrication of full-arch, implant-supported prostheses due to its superior aesthetics compared to metal-ceramic restorations, as well as its excellent mechanical properties and biocompatibility.¹² Zirconia restorations have highly polished surfaces that promote favorable tissue contact and help maintain the health of the peri-implant tissues without causing adverse systemic reactions.^12,13 

Strengthening Zirconia With a Metal Framework.Zirconia is a ceramic, and like all ceramics, it is stronger in compression than in tension.¹⁴ Hence, it may be susceptible to fracture when used to support a long cantilever or when the forces are of greater magnitude and/or the restorative space is less than ideal. To enhance the durability and performance of zirconia restorations, incorporating a metal framework into the prosthesis design has proven to be an effective strategy (Figures 3a to 3d).^15-18 However, when the joint between zirconia and metal is flat, the likelihood of fracture increases, particularly in high-stress areas. To address this issue, the metal structure may be designed to partly overlap the zirconia (Figure 3e), thereby reducing the risk of fracture and enhancing the restoration’s overall structural integrity.

Adhesive Cementation of Zirconia to Metal Framework.Achieving a reliable bond between zirconia and metal or titanium remains a significant challenge in prosthodontics. This protocol offers a step-by-step guide to help overcome these difficulties and ensure a stronger, more durable bond:

• Fit-check the anodized titanium framework and zirconia structure to eliminate overextensions and/or interferences. Next, finish, polish, clean, and air-dry the zirconia structure and the anodized titanium bar.
• Air-abrade the external surface of the anodized titanium framework, the internal surface of the zirconia structure, and the portion of the polished zirconia surface that will be overlapped by the metal using 50-µm aluminum oxide.¹⁹
• Apply a primer (Visalys Restorative Primer [Visalys]) to the air-abraded metal and zirconia surfaces, and air-dry after 60 seconds.^19,20
• Use a dual-cure composite with Active Connect Technology (ACT) (Visalys CemCore [Visalys]) to coat the primed surfaces. Ensure bubble-free coverage.
• Carefully seat the metal framework onto the zirconia structure, and light-cure the entire assembly with a handheld unit till the elastic state is achieved. Eliminate the excess cement and then place the prosthesis in a light-curing unit for 5 minutes. Remove the prosthesis from the curing unit and thoroughly examine it for proper bonding (Figure 3f). 

PMMA. Conventionally processed PMMA materials have several disadvantages, including increased polymerization shrinkage, susceptibility to microbial adhesion, radiolucency (lack of radio-opacity), potential for allergic reactions, and degradation of the mechanical properties of the material over time.²¹ The use of milled PMMA circumvents most of these disadvantages. CAD/CAM frameworks are more accurate compared to the casted frameworks developed by the lost wax technique.²² They have a passive fit, thereby decreasing the prosthesis movement and bacterial leakage.²² 

CAD/CAM frameworks may be milled or printed. Milled CAD/CAM frameworks are more accurate but significantly more expensive compared to printed CAD/CAM frameworks and conventional cast frameworks.²³ 

Printed titanium frameworks (Figure 4a) are cost-effective and perform well clinically.²³ Recently, full-arch implant restorations have been made using milled PMMA resin (Figure 4b) combined with a printed titanium frame (AvaMax hybrid prosthesis [AvaDent]) (Figure 4c).²⁴ These prostheses are stronger because the PMMA resin is injected under high pressure (about 180 bar) as it bonds with the titanium frame.²⁴ This method produces a more durable and affordable restoration compared with traditional metal-acrylic or zirconia options.²⁴

The design features a digitally engineered titanium core shaped like a space truss, integrated into a dense acrylic base (Figures 4d and 4e).²⁴ During injection, the PMMA flows through the titanium lattice, preventing separation between acrylic and metal, improving fracture resistance, and reducing flexibility compared with PMMA alone.²⁴ A second design, known as the Montreal-style, uses the same injection process with a titanium truss framework that covers the entire tissue-side (intaglio) surface and slightly overlaps the outer PMMA on the buccal and lingual sides (Figures 4f to 4h).²⁴ Both styles significantly improve strength and hardness compared with older designs.²⁴

Retention Systems for Full-Arch Fixed Implant Restorations

The retention of fixed, full-arch, implant-supported restorations may involve screws, cement, abutments, or a combination of these systems.^7,25,26 Choosing the appropriate retention system is challenging and affects the long-term success and survival of the restorations.^7,21,22 Retrievability is an important factor to consider when choosing the type of retention.⁷ 

The full-arch, screw-retained hybrid prosthesis is retrievable, thereby permitting easier management of mechanical as well as biological complications after the placement of the restoration and during the maintenance phase.^7,21,22 Screw-retained restorations require less vertical restorative space compared to cement-retained restorations.⁸ Since there is no cement used, there is no fear of leaving cement material, which may affect the health of the peri-implant tissues.^7,25,26 

However, there are a few disadvantages associated with screw-retained restorations. The fabrication of screw-retained prostheses is time-consuming as it requires several complex clinical and laboratory steps.^7,25,26 Screw retention from the bone-level platform may make it difficult to fabricate a passive restoration, thereby necessitating the use of multi-unit abutments which are quite expensive.²⁶ Thus, the overall cost of rendering this treatment is high. At the time of placement/removal of these restorations, small screws must be placed/removed from the mouth and if they accidentally fall into the oral cavity, there may be a possibility of aspiration. There may be accessibility issues, especially in the posterior quadrants of the mouth in patients with a limited oral opening.²⁵ Screw-retained restorations are susceptible to screw loosening, screw fracture, fracture of prosthetic material around the screw access holes (as the screw channel compromises the stiffness of the material), and loss of covering material in the screw access holes. Achieving a truly passive fit in screw-retained restorations remains a challenge.^7,25,27

An Abutment-Based Retentive System for Fixed Implant-Supported Prostheses

The LOCATOR FIXED Attachment System (Zest Dental Solutions) (Figure 5a) permits the fabrication of a retrievable fixed restoration that can be easily removed by the dental practitioner for maintenance of hygiene or performance of repairs.²⁸ 

The same legacy LOCATOR abutments that are used for retaining removable implant prostheses are utilized to support and retain fixed implant prostheses by simply switching the conventional LOCATOR housings and inserts with LOCATOR FIXED housings (that provide superior and rigid engagement) and PEEK inserts (Figure 5b). It is a simple yet rigid abutment-based retentive system; it eliminates the need for cement, screw-access channels, retention screws, and composite filling materials.²⁸ The LOCATOR FIXED attachment system now includes angled abutments. This innovation eliminates the need for the multi-unit abutments (MUAs), which simplifies the clinical procedure while reducing both cost and complexity. 

The procedural steps used to fabricate a fixed, full-arch prosthesis with the LOCATOR FIXED Attachment System are similar to those used to fabricate an implant overdenture supported by the conventional LOCATOR Attachment System.²⁸ The retentive housings can be picked up in the prosthesis through a chairside procedure (similar to the LOCATOR Attachment System), thereby permitting the fabrication of a passive restoration (Figures 6 to 8).²⁸ Because both the clinical steps and laboratory procedures are greatly simplified with this fixed attachment system, prostheses supported by the system can be fabricated and placed quickly and cost-effectively.²⁸ Compared with screw-retained restorations, the placement and the removal of the LOCATOR FIXED restoration is less complicated and less time-consuming. As the restoration has no coping screws and no access channels to fill the problems such as screw loosening, screw fracture, material fracture adjacent to the screw axis holes, and loss of the screw access filling material are eliminated.²⁸ This attachment system requires less vertical restorative space compared to other abutment-based retentive systems. Also, it provides a pleasing aesthetic result owing to the elimination of the screw access holes.²⁸