November 9, 2017
by Dr. Natalie Wong
When discussing a full arch fixed implant solution with patients, clinicians spend a considerable amount of time and attention on planning the surgical aspects of the case, the transitional prosthesis and the final full arch restoration. Previous articles by the author have outlined in detail the surgical and prosthetic procedures. The final prosthesis that is selected for a particular patient’s case can impact a patient in terms of cost and convenience. In this article, the author focuses on the types of final fixed prosthetic options that are available to clinicians and the advantages and disadvantages of each.
1. Fixed Complete Denture
A fixed complete denture (also known as a traditional “hybrid” prosthesis) is a screw-retained treatment option that was introduced by Dr. George Zarb and has been used for many years. A metal framework is fabricated, pink acrylic and denture teeth are processed to this framework (Figs. 1-7).
Advantages: This is a popular option due to the low cost associated with fabrication of the prosthesis. Additionally, the cost of repair is minimal as repairs are easily accomplished with repair resin which is added to the defects or areas of concern chairside. Hence, the patient is not impacted without the prosthesis for any length of time as the denture is not sent to the laboratory for repair.
Disadvantages: There are also distinct disadvantages when using this prosthetic option. Acrylic gets its strength from bulk. To optimize the strength of acrylic, a minimum of 12-15 mm of restorative space from the bone to the occlusal plane is required. If there is less than adequate space, the restoration is at risk of fracturing during function. Additionally, acrylic wears at a faster rate than porcelain or natural teeth. As a result, if the prosthesis opposes natural dentition or porcelain restoration, the prosthesis will wear faster leading to flattening of the incisal edges and cusps of teeth. This will greatly reduce chewing efficiency and compromise esthetics. Ultimately, the patient’s vertical dimension of occlusion will decrease if left untreated over a period of time.
Strategies to mitigate some of the issues outlined above include using porcelain denture teeth instead of acrylic denture teeth and/or the incorporation of metal islands and metal occlusal rests in the posterior regions to maintain cusp/fossa relationships. However, this will increase the cost of the final restoration and the use of metal may potentially decrease the esthetic quality of the prosthesis.
2. Metal Framework with Individual Porcelain Crowns
This treatment option utilizes a metal framework as a base, which includes individual crown preparations as part of the framework. Individual crowns are then fabricated and cemented onto the framework by the laboratory. As a result, the final restoration is delivered to the patient as a single unit (Figs. 8-11).
Advantages: The individually cemented crowns give this restoration a more natural look without requiring the implants to be in precise tooth positions. It is important to note at the outset that the use of porcelain has markedly better esthetics (e.g. less staining, better translucency) and better load-bearing characteristics than acrylic to maintain the vertical dimension of occlusion.
Disadvantages: There is an increased risk for fracture with this type of restoration as porcelain can chip, develop craze lines and crack which are then associated with higher costs of repair and complexity.
However, since the crowns are fabricated and cemented individually to the framework the risk and cost of repair is dramatically reduced as the affected crown can be removed and a new crown fabricated very easily using traditional crown and bridge materials and methods. Moreover, if digital CAD-CAM techniques are used to design the initial prosthesis (framework and crowns), the impact to the clinician and patient can be further lessened as the crown can be “ordered” ahead of time using the initial STL design file for the crown and delivered same day for the patient at his/her visit.
3. Zirconia Framework with Porcelain Overlay
In this full arch fixed restorative option, a zirconia framework is made with porcelain laid on top (Figs. 12-19).
Advantages: When compared to metal as a framework, zirconia’s main advantages for use are its biocompatibility, excellent esthetics and strength. The white-coloured base eliminates any grey metal show-through under the porcelain and enhances the natural cosmetic characteristic of porcelain. Additionally, it is exceptionally strong and durable as a base. The use of porcelain over top of this framework provides translucency and esthetics.
Disadvantages: The main disadvantage of this option is related to the properties of porcelain itself. Due to the manner in which porcelain is processed onto the framework as one unit, if there is any chipping or fracturing of the restoration, the entire prosthesis must be removed, sent to the laboratory and the porcelain on the entire restoration must be redone. This is cumbersome for the clinician and can be stressful and expensive for the patient as it will involve temporization and several visits to the dental office.
To minimize this risk, the zirconia framework can also be designed to support the cementation of individual porcelain crowns. The process for repairing a chipped crown is then as outlined in the section above.
4. Full Contour Zirconia (no porcelain)
With this prosthetic option, strength is maximized as the entire fixed restoration is composed of one material, zirconia. A full contour zirconia restoration is milled from one solid block of zirconia. The zirconia is then custom stained to resemble enamel and gingival tissues to optimize esthetics (Figs. 20-25).
Advantages: This material is extremely strong and durable, with minimal wear and the chance of chipping or craze lines is minimal.
Disadvantages: Unfortunately, the decreased translucency of zirconia reduces the esthetics of the prosthesis. Additionally, there are instances where there may be a catastrophic fracture of the framework such that it will break in half. If this were to occur, the entire prosthesis must be removed and remade which, again, is less than ideal for the clinician and patient in terms of both cost and time.
In the event that a catastrophic fracture was to occur, the entire prosthesis must be removed and remade. However, given it is a CAD-CAM restoration, the laboratory can utilize the original STL design file and re-mill the restoration in a timely manner. This minimizes the amount of time that the patient is without his/her prosthesis. Furthermore, to capitalize on the strength of zirconia and optimize esthetics, the framework can be modified and customized. As an example, posterior regions of the full arch framework may be full contour zirconia with the addition of individual porcelain anterior crowns to optimize cosmetics for the patient.
5. Multiple Implant-supported Bridges
Another option that may be considered to restore a full arch with a fixed prosthesis involves the use of traditional PFM bridges with implants as abutments (Figs. 26-28).
Advantages: This is a fairly straight-forward procedure for dentists as it mimics crown and bridge procedures in dentate patients and is thus familiar. With natural breaks in the midline and elsewhere in the arch, the esthetic appears more natural. In addition, in the event of a porcelain fracture, the affected bridge can be replaced fairly easily without affecting the entire arch.
Disadvantages: In this option, a minimum of two implants is required to be placed in very precise positions for every three to four-unit bridge, in contrast to the four implants that can be used for the full arch prostheses fabricated for “All-on-4” cases. As such, the costs will be higher. For example, if 3 bridges are to be used in the mandible (#47-44, #43-33, #33-37), six implants will be required, at a minimum. Implant placement may also require additional bone grafting. Traditional “All-on-4” uses implants in the anterior region to maximize available bone and minimize complications due to potential compromise of vital structures in the posterior regions. The single unit prosthesis is created by using posterior cantilevers bilaterally. The multiple bridge prosthetic option involves the placement of implants in the anterior and posterior regions which requires more skill, precision and sufficient bone volume. Additionally, the delivery of several PFM bridges can prove to be challenging as there are multiple contacts that must be adjusted and idealized which may make it difficult to achieve a passive fit of the prostheses.
All of the above treatment options, with the exception of the traditional crown and bridge abutments, have a distinct advantage as they are constructed as a single unit for strength and stability of the implants as well as the restoration. However, should the framework fracture, it will require removal and potential re-fabrication of the prosthesis which creates significant inconvenience for the patient.
Care should also be taken to consider the patient’s opposing dentition in the selection of the materials for the final restoration. Zirconia may appear to be ideal for strength and minimal wear; however, if it is opposing an acrylic prosthesis, it can lead to increased wear of the acrylic restoration.
Most importantly, during the treatment planning phase, it is necessary to weigh the advantages and disadvantages of each option to optimize the patient’s final outcome. The dental practitioner should consider the indications of each prostheses and discuss them with the patient to ensure clinical longevity and patient satisfaction. OH
Oral Health welcomes this original article.
About the Author
Dr. Natalie Wong graduated from the University of Toronto with her Doctor of Dental Surgery in 1996 and received her Certificate in Prosthodontics from the University of Michigan, Ann Arbor in 2007. Dr. Wong has served as the Co-Director and an Associate-in-Dentistry (clinical instructor) in the Implant Prosthodontic Unit, Graduate Prosthodontic Department, University of Toronto. She has also served as a Clinical Assistant Professor of Oral Implantology, Department of Periodontology, Temple University. She is a Past-President of the ABOI and is currently the Vice President for the AAID, Past-President of the Association of Prosthodontists of Ontario, Founder and Director of the Toronto Implant Institute Inc. which offers one-on-one surgical and prosthetic mentorships.