Design of all implant prostheses should be based on a prosthodontic plan and should address all specific planning factors identified as relevant. There are a number of aspects that are common to the design of most implant fixed dental prostheses (or FDPs), and it is helpful to arrange these points in a logical design sequence. The first point is selection of the type and configuration of the FDP. The next point is selection of the retention method, followed by consideration of the external aspects and internal structure required to address factors such as esthetics, function, and strength. This module will examine each of these design points in turn. More detailed, separate Academy Learning Modules build on the present Module and address selection of restorative materials for the FDP, selection of abutments, and configuration of the supporting implants.

Once you have completed this module, you should be able to describe the principles for designing FDPs in terms of selection of prosthesis type and configuration of its units, selection of retention method, external aspects, and internal structure.

The first design point to consider is the type of prosthesis required. This will vary according to whether the FDP is intended for an anterior or posterior site. Equally, the design will differ for single units and multiple units. In sites with two or more spaces there are also different options for the configuration, such as cantilevered FDPs or splinted single units. For full arches there is a choice between a one-piece or segmented prosthesis. The configuration of multiple-unit FDPs is usually dictated by the prosthetic plan and specific factors of the clinical case.

As implant therapy has evolved, a definite trend has emerged towards simplifying implant-supported FDPs. This is manifested in the segmentation of long-span prostheses into single units and shorter-span FDPs. This provides for simpler initial construction as well as simpler continuing maintenance. For example, it is less technically demanding to construct passively fitting single units and shorter-span FDPs. Segmentation also allows the clinician to plan ahead for more straightforward management of hardware complications should they occur. Other benefits of segmentation are improved esthetic outcomes and easier access for oral hygiene measures.

The goal of simpler FDPs has also led to a trend towards using two-unit cantilevered prostheses instead of adjacent implants, particularly if the mesiodistal space dimension is reduced. Contrary to data on conventional tooth-supported cantilevered FDPs, the published literature indicates that implant-supported cantilevered FDPs perform well. A systematic review by Romeo and Storelli reported that implant FDPs with cantilever extensions performed as well as those without cantilevers. In addition to avoiding the expense of two implants, two-unit cantilevered prostheses in combination with soft tissue grafting can offer esthetic advantages by overcoming the lack of papilla support between two adjacent implants.

Another way to simplify FDPs is to reduce the overall number of supporting implants. The practice of placing one implant per missing tooth has largely been replaced by either the cantilever modality or by a more traditional multi-unit FDP design. In the anterior example seen here, the use of two implants placed in varying configurations of the central and lateral incisor sites avoids the esthetic challenge of recreating the papilla between adjacent implants. Principles of implant configuration are addressed in detail in the related ITI Academy Learning Module titled 'Implant Configurations for Fixed Dental Prostheses'.

There are a number of exceptions to the principle of FDP segmentation and simplification. Full-arch prostheses supported by less than six implants are difficult to segment and are therefore usually constructed as one piece. Implants for one-piece prostheses should be placed in the interforaminal region to allow for flexure of the mandible. Cantilevers may therefore be needed to replace the posterior teeth. Bone flexure is not a consideration for the maxilla, but limitations on available bone volume in the sinus region can lead to addition of distal cantilevers.

Splinting of prosthetic units should be considered when supported by short implants or implants placed in grafted sites. In these situations, joining the prosthetic units together in order to splint the implants provides mutual support. Splinting of single units is particularly advisable in the presence of confirmed bruxism parafunction or in patients with a history of previous implant failure related to occlusal overload.

Prosthesis Configuration, Key Learning Points: Long-span prostheses can often be segmented into short spans and single units to simplify their construction and maintenance. The benefits of two-unit cantilevered FDPs and other simplified configurations include reduced expense and better soft tissue esthetics. Cantilever designs are often used for full-arch prostheses due to limitations on the positions of supporting implants. Splinting of prosthetic units should be considered for short implants, implants placed in grafted sites, and patients with bruxism.

There is a choice of two different methods of retention for implant-supported FDPs. In common with conventional dental prostheses, the first option is cement retention. The other option is screw retention. Both retention methods have advantages and disadvantages and therefore also their respective indications. Clearer guidelines have evolved regarding the indications for cement versus screw retention, and these are set out in the following slides.

One advantage of cement retention is the simplicity of prosthesis construction, similar to conventional FDPs. There is no screw access hole, and it is therefore easier to control the dimensions of the materials used throughout the FDP. Finally, correct alignment of the implant axis is less critical. However, there is increasing evidence that residue from excess cement is a very real risk, and also evidence that its presence is likely to lead to peri-implant inflammation and associated sequelae such as peri-implantitis. If the cemented FDP must be removed for any reason, this procedure is rarely straightforward.

Based on the list of advantages versus disadvantages, the indications for cement retention include FDPs in which the cement margin is at or above the level of the mucosal margin; situations where the alignment of the prosthesis and implant prevents the use of screw retention; cases in which it is desirable to avoid a screw access hole, for example, to ensure optimal material thickness; or when a simpler prosthodontic process is indicated.

When prescribing cement retention, the following points should be considered. First, a custom abutment may be required to ensure that the cement margin is at or close to the mucosal level to facilitate removal of excess cement. In common with natural tooth preparation, the abutment must have adequate height and design to ensure retention. FDP length is another consideration. Retrieval becomes progressively difficult and unpredictable with an increasing number of linked units.

For screw retention, the outright advantages are retrievability and avoidance of cement. The most significant disadvantage is the need for correct prosthodontic alignment to ensure that the location of the screw hole is in the center of the occlusal surface of posterior prostheses or in the cingulum of anterior prostheses. A visible screw hole can also be an esthetic concern. In addition to the likelihood of screw loosening, there is also some indication that there is a higher incidence of chipping of the veneering ceramic on screw-retained prostheses.

One indication for screw retention is easy retrieval when there is a high risk of complications, such as a patient who exhibits bruxism. Screw retention is also highly indicated when there are multiple linked prosthetic units, such as in long-span and full-arch FDPs . Submucosal margins where cement removal would be difficult also warrant screw retention.

When screw retention is prescribed, the following considerations are relevant. First, prosthodontically determined implant placement is needed to ensure that the implant and screw insertion axes align. Alignment is also important for the esthetic outcome. Unfavorable emergence of screw access hole can result in esthetic compromise, as seen in this example of a screw hole in the buccal cusp tip. Alignment is also needed to ensure adequate material dimensions around the access hole. There is some emerging flexibility on the point of implant alignment because of the availability of specialized drivers and abutments with off-angle access to the screw channel. These components can compensate for poorly aligned implants. Considering the strong advantages of screw retention over cement retention in terms of both retrievability and avoidance of cement-related biological complications, these options are worth exploring when needed. However, the clinician should still aim for optimal implant alignment.

Method of Retention, Key Learning Points: Cement-retained FDPs are indicated when the prosthesis and implant axes do not align or when it is desirable to avoid a screw access hole. Excess cement is associated with peri-implant inflammation; a custom abutment with a supragingival margin may be indicated for cement-retained FDPs. Retrieval of cement-retained FDPs is unpredictable, especially in long-span situations. Screw-retained FDPs avoid the problems associated with cement excess and are retrievable, which is advantageous in high-risk situations such as bruxism. Screw-retained FDPs require that the implant and screw insertion axes are aligned.

There are a number of important design variations to consider for the external aspects of an FDP. External features will vary according to the type and site of the FDP, including anterior versus posterior, single versus multiple units, implant-supported units versus pontics, as well as sites with optimal restorative conditions versus those that present a compromise.

Design considerations include the esthetics of the prosthesis, particularly in situations where it will be visible in the smile or for other reasons of esthetic importance to the patient, as defined in the concept of the esthetic zone by Belser and co-workers. Other considerations are the functional requirements for speech, occlusion, and mastication that the prosthesis must satisfy. The FDP design must also allow access for effective oral hygiene, both for the patient on a daily basis as well as for professional monitoring and peri-implant maintenance.

The esthetic considerations encompass both hard and soft tissue factors. An optimal esthetic outcome should aim to imitate the natural appearance of the missing or adjacent teeth. The parameters for the white esthetic score or WES described by Belser and co-workers in 2009 can be used as descriptors of the desired esthetic design elements. These include the form, outline, volume, and surface texture of an optimal implant prosthesis that blends in with the existing dentition. An important esthetic factor, particularly in anterior sites, is the emergence profile of the FDP. The two clinical images demonstrate how the correct prosthetic contours support the peri-implant tissues and thereby achieve an emergence profile that mimics the contralateral natural tooth. Esthetic factors are covered in more detail in the ITI Academy Learning Module titled 'Esthetic Planning for Implant-Supported Fixed Dental Prostheses'.

For posterior FDPs the same white esthetic score parameters apply, but there may be a greater scope for esthetic compromise when the FDP is not directly in view. Patient acceptance of esthetic compromise may be greater as well. The parameters of the buccal surface in terms of form, outline, and volume are most significant to the esthetic outcome. Similar to anterior sites, the peri-implant tissue support at the point of emergence is important, as seen in this clinical example. In situations of hard and soft tissue defects the FDP may be designed with a longer neck resembling root recession. Alternatively, a pink ceramic component can be added.

The external features of an FDP also contribute to restoring function. It is sensible to imitate the contours and dimensions of the surrounding dentition as much as possible to minimize patient requirements for functional adaptation. This includes surfaces involved in speech and swallowing. These clinical examples demonstrate an incisal edge and palatal surface that closely resemble the contralateral tooth, and thereby are not likely to conflict with speech or occlusion. Similarly, the contours and alignment of these two upper premolar implant crowns should allow for easy functional integration in terms of speech and comfort.

An important functional aspect of prosthetic design is occlusion. Available guidelines recommend axial loading of implants and their prostheses. This is achieved through centered contacts that direct occlusal forces along the implant axis. Lateral loading is avoided by providing for freedom around the centered contacts and by designing flatter cusps than natural teeth. For patients with bruxism or a history of ceramic chipping or fracture, a metal occlusal surface can add strength and reduce the risk of complications. This topic is covered in detail in an ITI Academy Learning Module titled "Occlusion on Fixed Implant Prostheses".

Another functional consideration of the external design is protection of the gingival tissues during mastication of food. The morphology of natural teeth are adapted for this function. The anatomical equator of each tooth, or widest circumference of the crown, serves to deflect the food bolus from direct impact on the gingival tissues during mastication. This morphology should be imitated by implant FDPs, as demonstrated in these two clinical examples. In the anterior example, the implant crown imitates the cervical contour of the contralateral incisor for protection of the immediate peri-implant mucosa. In the posterior example, the lower implant crowns imitate the equator contours of the upper natural teeth.

A third external design consideration is access for oral hygiene. Ready access for effective daily hygiene by the patient as well as for professional monitoring and maintenance is important for the health of the peri-implant tissue and supporting implants. The literature reports that prosthetic contours that prevent access for plaque removal are predictive for peri-implantitis. Designing FDPs with suitable access is therefore a matter of prosthodontic risk management for prevention of peri-implantitis. These diagrams illustrate the contrast between optimal contours, with smooth progressive tapers from mucosal emergence to interproximal contact points, and suboptimal exaggerated contours.

All aspects of an FDP should facilitate access for both patient and professional maintenance. As a general rule, the interproximal surfaces from mucosal emergence to the anatomical equator should be smoothly tapered to allow for biofilm removal and professional probing. Pontics of multi-unit FDPs should be similarly contoured. There should be easy access for insertion of floss and interproximal brushes apical to the connectors. The tissue-fitting surface of pontics should be flat or ovoid so that floss can make contact across the entire surface.

In reality, a smoothly tapered emergence resembles the contours and anatomy of natural teeth. A clinical example is this implant FDP replacement of an upper left first premolar. Note the similarities between the natural tapering of the teeth and the implant FDP. Also note the small interproximal spaces on either side of the FDP, which will assist insertion of floss and brushes. Similar spaces are present in other sites due to gingival remodelling. The same principle has been applied to this single molar unit. The buccal and palatal aspects, as well as the entire crown in general, exhibit a slim tapered emergence.

There are some exceptions to designing contours for hygiene access. Most exceptions are associated with the esthetic requirements of the prosthesis and are more likely to arise in the esthetic zone. Other exceptions relate to the functional requirements discussed earlier, in which FDP contours must facilitate speech, occlusion, and protection from the food bolus during mastication. This image shows two upper lateral incisor FDPs. Their contours support the peri-implant mucosa of the transition zone and are important for the esthetic outcome. This diagram illustrates prosthetic support of the transition zone mucosa. Similarly, this premolar FDP has exaggerated buccal contours to support the peri-implant mucosa at the esthetically determined level.

External Aspects, Key Learning Points: Esthetic considerations for external FDP design include the white esthetic score parameters of form, outline, volume, and texture as well as the emergence profile. The external design should satisfy the functional requirements of speech, swallowing, and mastication while providing protection of the peri-implant mucosa. The occlusal design should promote axial loading. Prosthetic contours that prevent access for plaque removal are a risk factor for peri-implantitis. Prostheses should have a smoothly tapered emergence to facilitate biofilm removal. Tissue surfaces and connectors of multi-unit prostheses should be contoured for effective use of floss and interproximal brushes.

After the prosthesis configuration, method of retention, and external contours have been determined, the internal structure is designed to ensure appropriate and adequate dimensions of the prosthetic materials used. The internal structure is important for strength and durability as well as esthetics of these materials. The final design may need some modification to ensure that these parameters are met. In most cases the internal prosthesis design takes place in the laboratory, but it is important that the clinician is able to assess the internal structure. A clear and specific recommendation from the 5th ITI Consensus Conference states that the prescribing clinician should be involved in the assessment of CAD/CAM designs.

The internal structure can be classified as either combination or monolithic. The most common construction is the combination structure, in which a substructure is covered by a veneering material. The substructure provides strength for the FDP and is most commonly made of metal. The substructure is then veneered with a more esthetic material such as ceramic or resin. Zirconium dioxide ceramic, also known as zirconia, has gained popularity as a substructure material due to its high strength, although the opacity of this material necessitates veneering in esthetic areas with a more translucent ceramic.

Among the monolithic options, all metal constructions may be used for specific indications in which strength is of prime importance. As mentioned previously, a metal occlusal surface can serve as a compromise between the esthetics of the veneered substructure and the strength of an all-metal monolithic FDP. More recently, monolithic structures milled from prefabricated ceramic blocks are gaining use outside the esthetic zone, and surface tinting has improved the esthetics of these FDPs. The ITI Academy Learning Module on 'Dental Materials Selection for FDPs’ discusses these options in more detail.

Multi-unit FDPs will usually have an internal structure similar to single crowns. In esthetically important areas, veneered prostheses are needed to achieve optimal esthetic outcomes. This is the most common construction for multi-unit FDPs. All-ceramic constructions based on zirconia or lithium disilicate glasses are now available for short-span FDPs, although some caution should be exercised in the presence of bruxism or other high-load circumstances. In these situations, metal frameworks may be a better solution.

The main area of difference between single- and multi-unit FDPs is the connection between units. These connectors must be sufficiently strong and of adequate dimension to support the FDP and the veneering material. The minimum dimension of these connectors is related to the strength and stiffness of the material used. The most critical dimension of the connector is generally its occlusogingival thickness, where strength is needed to withstand functional as well as parafunctional loads. Finally, the design of these connectors must take into account the external contours of the FDP, especially in terms of access for oral hygiene.

For constructions with a substructure and veneering material, priority is given to the dimensions of the veneering material to ensure adequate esthetics and strength. Chipping and fracture of veneering ceramics is a documented and frequent complication for implant FDPs. The most predictable way of ensuring adequate dimensions for the veneering material is to first plan the final prostheses to full contour via a real or virtual wax-up, and then use the cut-back technique. In this prosthodontically determined workflow, the initial cut-back for the optimal thickness of the veneering material will ensure that the required support is present in the substructure. As a general rule the veneering material will require a thickness of 1.5 to 2 mm. In areas where the wax-up leaves insufficient dimension for both veneering material and substructure, the veneering may be omitted in favor of substructure only. For example, it may be desirable to have a slimmed-down emergence for optimal oral hygiene access, as seen in this clinical image. It may also be appropriate to omit the veneering from less visible aspects of interproximal connections between prosthetic units. As already discussed, for patients with bruxism it may be advisable to omit veneering from occlusal surfaces.

Despite careful planning, occasionally the implant axis may not be as ideal as intended. In turn this may result in unfavorable access for screw retention. The ideal position is through the cingulum of an anterior FDP or the midpoint of the occlusal surface in a posterior FDP. In some cases cement retention may be chosen to resolve the resulting material and internal structure issues. However, where screw retention is deemed more suitable, specialized drivers and abutments with off-angle screw channel access are increasingly available. These components are designed to overcome alignment problems and are worth investigating.

Internal Structure, Key Learning Points: Single-unit prostheses commonly consist of either a veneered substructure or monolithic ceramic. Multiple unit prostheses commonly consist of a metal substructure and veneering material; all-ceramic prostheses are recommended for short spans only. For high occlusal loads, a veneered substructure with metal occlusal surface is recommended. The connectors in multi-unit prostheses must be of adequate dimensions to withstand occlusal forces. During fabrication, the cut-back technique ensures adequate dimensions for the veneering material. When the implant axis is not ideal, specialized abutments and drivers allow off-angle screw channel access for screw retention.

Design Principles for Fixed Dental Prostheses, Module Summary: Construction and maintenance of FDPs can be simplified through segmenting long spans, reducing the number of implants, and using cantilevers. Cement retention is indicated when the prosthesis and implant axes do not align. Screw retention has significant advantages over cement retention in terms of retrievability and avoidance of cement-related biological complications. The external design of the FDP must satisfy esthetic, functional, and hygienic requirements. The internal design of the FDP must ensure the strength, durability, and esthetics of all materials used.