Considerations for Full-Arch Implant-Supported Restorations

Fig. 10.1

Emergence profile inadequate. Note ledge as restoration emerges from implant

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Fig. 10.2

Trajectory of implants has been accommodated in fabricating prosthesis. Pre-angled abutments should be selected to correct trajectory of dental implants

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Fig. 10.3

Diagnostic wax tooth try-in with corrected emergence profile

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Fig. 10.4

Provisional restoration fabricated to test aesthetics and phonetics

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Fig. 10.5

Definitive restorations with correct emergence profile

 

    10.2 Restorative Space

    Lack of restorative space is one of the most common occurrences that can compromise a restoration [5]. Inadequate restorative space will result in two scenarios:

    1. (a)

      Restorative complications such as material failure leading to repair or replacement of the veneering materials or complete framework fracture leading to failure of the entire prosthetic restoration

       
    2. (b)

      Changing the treatment plan from one restoration type to another to accommodate the space requirements

       

    Neither of the above scenarios is ideal and can be avoided by good communication between the surgical and restorative team prior to implementation of the therapy. The clinician must evaluate whether the patient exhibits minimal, moderate or advanced resorption to determine the available restorative space and therefore the ideal type of prosthesis to be fabricated.

    General guidelines for space requirements are the following:

    1. (a)

      Monolithic full-contour zirconia-fixed restorations require 10 mm or more of space from the head of the implant to the opposing dentition.

       
    2. (b)

      Porcelain fused to metal/zirconia-fixed restorations requires 12 mm or more of space from the head of the implant to the opposing dentition.

       
    3. (c)

      Acrylic resin bonded to titanium fixed restorations will require 15 mm or more from the head of the implant to the opposing dentition.

       
    4. (d)

      Implant-supported over-dentures will require 16 mm or more of space from the implant to the opposing dentition [6].

       

    10.3 Nature of Opposing Dentition

    The literature is clear that patients with implant-supported restorations have reduced proprioception [7]. This should be kept in mind when treatment planning for opposing arches for fixed restorations. Patients must be aware that the maintenance requirements when there are implants in opposing arches are much higher than when the implants oppose a natural dentition or a denture [8]. The most common complication was fracture and wear of the materials on the occlusal surface (Fig. 10.6).

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    Fig. 10.6

    Milling titanium occlusals can maintain the vertical dimension

    Another study focused on occlusal surface design using noble metals, feldspathic porcelain and acrylic [9]. Type 3 and 4 gold alloy has been favoured as an occlusal material for stability of occlusal contacts and maintenance of the vertical dimension of the restoration. Patients however prefer tooth-coloured materials for the occlusal surface. Historically feldspathic porcelain has been the material of choice but over the last 10 years zirconia has been increasingly used on the occlusal surface. One concern has been the effect of zirconia on the opposing dentition. Laboratory studies have compared zirconia’s wear capacity to that of feldspathic porcelain and it was reported that zirconia was less abrasive to the opposing dentition than feldspathic porcelain [10]. There is a paucity of data on this subject in vivo and further studies are required.

    One clinical situation where the nature of the opposing dentition is of note is when designing fully implant-supported prostheses for the maxillary and mandibular arches together. It has been advocated to restore the maxillary arch with a zirconia-based ceramic prosthesis and an acrylic resin bonded to titanium prosthesis for the mandibular arch [11]. Advantages of using this opposing arch design include the following:

    1. (a)

      Maximum aesthetics with minimal staining over time with use of ceramics in the maxilla

       
    2. (b)

      Absence of reported, “clicking” by patients with opposing ceramic surfaces

       
    3. (c)

      Flexibility and resiliency in the system

       
    4. (d)
      Reduced costs (Fig. 10.7)

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      Fig. 10.7

      Use of zirconia in the maxilla and acrylic resin titanium in the mandible has many advantages

       

    One disadvantage would be increased wear of the mandibular acrylic-based restorations. This can be considered a controlled failure and the patient must be made aware that the acrylic resin teeth will most likely need to be replaced every 5–7 years.

    10.4 Aesthetic Demands

    Similar aesthetics can be achieved when noble alloy and zirconia frameworks are used as a substructure. From an optical perspective both materials block the transmission of light and behave in a similar manner when evaluated aesthetically [12].

    From an aesthetic perspective, ceramic-based restorations with either high noble alloy or zirconia frameworks are less likely to stain over time and result in superior stable long-term aesthetic results compared to acrylic resin bonded to titanium restorations. As mentioned above, ceramics should be considered for maxillary restorations and acrylic resin titanium in the mandible when full-mouth implant rehabilitation is completed.

    10.5 Cantilevers

    10.5.1 Framework Cross-Sectional Area for Cantilevers and Around Screw Channels

    The cantilever section of the prosthesis requires sufficient bulk immediately distal to the terminal implant and around screw access channels (Figs. 10.8 and 10.9). Specifically, the cross-sectional area of the chosen material must be sufficient enough to have strength and rigidity to perform in the intraoral environment. For screw channels, this is typically most important when considering the lingual or palatal cross-sectional area of the material. When focusing on cantilevers, connector size is critical for traditional noble metal alloys, titanium and zirconia frameworks. While there is no specific data on the minimum dimensions required for these frameworks, if space is extremely limited the authors prefer traditional noble metal frameworks. However, for the typical implant-supported restoration, patients who have undergone moderate to advanced resorption will have plenty of room to provide robust connectors with sufficient cross-sectional area. Literature on tooth-supported zirconia-based restorations recommended a minimal connector size of 4 mm × 4 mm and it is the authors’ preference to respect this 16 mm2 area for implant frameworks as well in the area of channels or connectors [13].

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    Fig. 10.8

    Inadequate thickness of zirconia around screw access hole will result in fracture of the zirconia framework

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    Fig. 10.9

    Cantilevers should also be minimized in zirconia as this may result in fracture

    There are few studies to guide clinicians on length of cantilever in the maxilla, especially when utilizing the more contemporary materials such as zirconia. Length of cantilever has a significant effect on failure of these types of restorations and in a meta-analysis it has been shown that the length of cantilever is more significant than the number of implants [14]. Deflection of the cantilever is related to its length; so minute increases in length have a significant impact on the fracture complications of restorations. Lab-based studies on zirconia have shown that

    1. (a)

      The longer the cantilever the lower the load to failure

       
    2. (b)

      The smaller the connector size the less load to failure

       
    3. (c)

      Failure usually occurred in the distal abutment wall [15]

       
    Recommendations for cantilever include the following:

    1. (a)

      Limit distal cantilever.

       
    2. (b)

      Limit buccal cantilever.

       
    3. (c)

      Increase thickness of the framework in the cantilever section distal to the most distal implant.

       
    4. (d)

      Limit occlusion on the cantilever.

       

    10.6 Ease of Fabrication and Passivity

    The gold standard for fabrication of implant-supported restorations has been through traditional waxing and casting and then layering of feldspathic ceramics. The results achieved with these techniques have been predictable and successful [16]. Cost has been one of the major reasons clinicians have moved away from traditional techniques and have started to use CAD/CAM technology in the manufacture of materials such as titanium and zirconia. These techniques provide a much more efficient workflow, and due to the efficiencies gained from these techniques tend to be less costly than conventional fabrication procedures.

    Challenges with screw-retained frameworks have always been achieving passivity. Inaccuracies in fabrication of these frameworks can be traced back to the operator. Often inaccurate impressions and the subsequent technique in fabrication of the master cast have been to blame. Traditional distortions associated with the lost wax process and ceramic firing also contribute to the misfit [17].

    Solutions to obtain passivity involve casting the framework in multiple pieces and soldering. Clinicians have also utilized cement-retained restorations with the lack in passivity being made up by the cement space. The clinician should be cautious in fabricating full-arch cement-retained restorations as this will not allow predictable retrievability [18]. Another approach to achieve a passive screw-retained framework has been the use of the adhesive corrected implant frameworks where individual cylinders were cemented within the framework after it had been cast (KAL technique) [19].

    CAD/CAM technology has eliminated a lot of variables in the fabrication process [20]. An accurate impression, whether it be traditional or optically generated, is critical for an optimal outcome. The laboratory technician’s responsibility is to ensure an accurate pour paying specific attention to correct water powder rations and mixing the die stone with accurate water powder rations for a time specified by the manufacturer. Furthermore, due to the elimination of potential errors the overall workflow has been simplified by utilization of CAD/CAM, which allows frameworks to be produced in fewer clinical steps with less labour in the dental laboratory [21].

    There are several different materials that can be used to fabricate these CAD/CAM frameworks for implant-supported restorations, and these materials include but are not limited to:

    1. 1.

      Acrylic resin bonded or milled to titanium

       
    2. 2.

      High-performance polymers—PEEK

       
    3. 3.

      Milled cobalt chromium

       
    4. 4.
      Zirconia—monolithic:

      1. (a)

        Minimally layered

         
      2. (b)

        Hybrid design with zirconia frameworks and individually cemented crowns (lithium disilicate or zirconia)

         
       

    10.7 Acrylic Resin Bonded or Milled to Titanium

    Framework designs for a full-arch, one-piece, implant-supported acrylic resin and titanium-based restoration have changed significantly since the transition from gold frameworks to titanium [22]. Different manufacturers have different designs available, and despite technological advances frameworks still do not replicate the characteristics familiar to gold frameworks. With titanium frameworks a few key parameters become important:

    1. (a)

      Bulk for strength

       
    2. (b)

      Adequate access for oral hygiene

       
    3. (c)

      Minimal display of metal

       
    4. (d)

      Retention for acrylic

       
    5. (e)

      Adequate space for acrylic resin

       
    6. (f)

      Adequate strength in the cantilever section

       
    7. (g)

      Attention to cross-sectional area

       
    When considering these parameters, there are mainly two types of framework designs to support acrylic resin teeth. One of these designs is the “minimalist” framework where acrylic is wrapped around the bar and encompasses it 360 degrees including the intaglio surface. One advantage is that it can be relined, but little is known about its longevity in terms of biomechanics. Anecdotally, numerous colleagues have experienced fractures of this type of framework. Failures may be due to excessive cantilever or inadequate bar shape and further studies need to explore these issues. The second type of framework may include I- or L-shaped bar designs to maximize rigidity. One advantage of this design is that due to the shape of the titanium framework requiring less bulk of material in any one dimension, adequate space and retention for acrylic resin can be achieved which maximizes thickness in the cantilever area. The evidence base is also lacking with this design. Although these frameworks have served many patients well, particularly in the edentulous mandible, success in the mandible does not automatically translate to success in the maxilla (Figs. 10.10–10.12).

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    Related posts:

    1. and Treatment Planning: A Surgical Perspective
    2. Basis of Immediate Loading and the Biomechanics of Graft-Less Solutions
    3. and Facial Aesthetic Considerations for the Contour of Fixed Prostheses
    4. Integrated Digital Workflow to Guide Surgery and Prosthetics for Full-Arch Rehabilitation: A Narrative Review
    5. Surgical Protocol
    6. Fabrication of Full-Arch Implant-Supported Restorations

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    Feb 19, 2019 | Posted by in Periodontics | Comments Off on Considerations for Full-Arch Implant-Supported Restorations

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