The clinician practicing implant dentistry is confronted with different clinical situations, ranging from a single missing tooth, to an edentulous space with several missing teeth, to complete edentulous ridges. Once a thorough treatment plan is established, different design configurations may be possible for implant-supported prostheses. Each prosthesis design has its advantages and disadvantages. The prosthesis design must be based on the clinical condition to ensure that the appropriate number of implants are placed in the correct prosthetic positions using implants with appropriate dimensions.

After completing this ITI Academy Module, you should be able to: Recognize the design configurations for fixed implant prostheses in single tooth spaces, in short edentulous spaces, in extended edentulous spaces, and in edentulous ridges.

First, we will focus on single tooth spaces. Replacing a single missing tooth with an implant is a treatment option with highly predictable long-term clinical outcomes. According to the clinical situation and location of the implant site, the clinician must choose an implant of the appropriate dimensions and type.

As a basic principle, the mesiodistal space must always be carefully evaluated before placing implants. First, the width of the space must be measured. Then, the minimal distance between tooth and implant must be respected. The minimal distance between an implant and a tooth should be at least 1.5 millimeters at the shoulder of the implant. Taking these considerations into account will ensure an optimal result for the appearance of the interproximal soft tissues.

Next, the appropriate implant diameters are selected according to the measurements of the mesiodistal and bucco-oral dimensions. It is also important to preserve an adequate width of the buccal and oral bone walls. Implants with a reduced diameter have limited indications which must be respected. A reduced diameter may offer the possibility of avoiding bone grafting procedures, provided the ideal prosthodontically determined implant position can still be achieved. Standard or wide-diameter implants may be chosen based on the clinical situation. Two different implant types are shown here. The generic terms for the two implant types shown here are one-piece and two-piece implants. Both types will now be described in more detail.

When inserted into bone, one-piece implants have a transmucosal section which extends into the oral cavity. The transmucosal section is manufactured as part of the implant, and has an built-in fixed prosthodontic platform and an built-in emergence profile. The implant on the right shows a flared emergence profile and a prosthodontic platform built onto the implant, and it is referred to as a soft-tissue level implant.

The two-piece implant is designed to stop at bone level. The emergence profile and prosthetic platform are established by the abutment. This gives more prosthetic flexibility based on the choice of abutment. In sites with relatively small dimensions, this may be an advantage. In the esthetic area, it is also possible to select a tooth-colored abutment for sites with reduced buccal mucosa thickness.

In the anterior region, the fixed-neck design of a one-piece transmucosal implant design may conflict with the available dimensions of the facial aspect. In this situation, the relative flexibility of the prosthetic platform and emergence profile of a two-piece bone-level design may be a better choice. In addition, the metallic-colored transmucosal neck of a one-piece design tissue-level implant may lead to a grayish discoloration of the overlying soft tissues, as can be seen on this clinical image and radiograph of an maxillary left central incisor implant prosthesis. The bone-level design of a two-piece implant may therefore have an esthetic advantage.

Single tooth spaces, Key Learning Points: In single tooth gaps, it is necessary to assess the space in order to define the appropriate implant dimensions. The implant type depends on the clinical situation.

Next we will look at short edentulous spaces. The considerations for implant dimensions and types discussed for single tooth spaces also apply to short edentulous spaces. In addition, the short edentulous space presents the challenge of choosing the most appropriate implant position. When two adjacent teeth are missing there are two different configuration principles for a fixed implant prosthesis, depending on the width of the space: If there is enough mesiodistal space, two implants may be placed. If there is not enough mesiodistal space, one implant is placed in one site and the correct implant position must be carefully defined.

As a basic principle, the mesiodistal space must always be carefully evaluated before placing implants. First of all, the width of the space must be measured. The minimal distance between tooth and implant and between two adjacent implants must be respected. Several studies have indicated that the inter-implant distance between two adjacent implants should be at least 3 millimeters to maintain the bone crest between the implants, and to ensure sufficient space between the prostheses for oral hygiene procedures. The minimal distance between an implant and a tooth should be at least 1.5 millimeters. Taking these considerations into account will ensure an optimal result for the appearance of the interproximal soft tissues and will avoid the risk that the implants are too close together.

The appropriate implant diameters are selected according to these measurements. As stated previously, implants with a reduced diameter have limited indications which must be respected. Standard or wide-diameter implants may be chosen based on the clinical situation. The following slides will describe the configuration principles for the anterior and the posterior region.

When the gap is in the anterior region, two implants with two single crowns is an option that can be considered. However, the mesiodistal space of the edentulous region has to be carefully evaluated. The implant diameter should be selected according to the crown volume. This means that a central incisor usually demands a standard-diameter implant, while a reduced-diameter implant is normally chosen for a lateral incisor. By reducing the implant diameter, the recommended minimal distances between the tooth and the implant or between the adjacent implants can be achieved.

In the clinical situation shown, the mesiodistal space was sufficient to allow the placement of a standard-diameter implant for the central incisor and a reduced-diameter implant for the lateral incisor. Both implants were restored with screw-retained crowns. The advantage of the design principle selected is the segmentation of the restorations.

The prosthodontic design using one implant that is restored with a fixed dental prosthesis - abbreviated FDP - together with a cantilever is a good treatment option in the anterior region if the mesiodistal space is limited. From a prosthodontic point of view, the implant should be placed underneath the crown with the larger volume. The lever arm is thereby reduced and the resulting off-axial forces are lower. In addition, the dimensions of the implant-abutment connection are greater and consequently more appropriate to support a crown with a cantilever. If the bone volume underneath the crown with the larger volume is insufficient, the preference should be to reconstruct the site first and then place the implant. From a surgical point of view, the implant should be placed in the site where the greater bone volume is present to assure primary stability. The location of the implant has to be carefully selected taking all these clinical criteria into consideration.

In this clinical case, the mesiodistal space was not sufficient to allow the placement of two adjacent implants. A standard-diameter implant for the canine was placed to support a crown with a mesial cantilever to restore the lateral incisor.

Two missing adjacent teeth in the anterior region is often one of the most challenging esthetic situations to manage with dental implant therapy. In the study of Tymstra and co-workers, the papilla was always compromised when two adjacent teeth were missing, irrespective of whether two implant crowns or only one implant with a crown and a cantilever were used. This is because of the resorption of the interdental bone peak that results in shrinkage of the papilla.

This case illustrates the esthetic compromise with two missing adjacent teeth. A standard-diameter implant for the central incisor was placed and supported a crown with a distal cantilever to restore the lateral incisor. Pink ceramic has been added to replace the missing soft papilla. The case also demonstrates one major advantage of using one implant to support a cantilever FDP. This configuration allows for compensation of the missing papilla by adding pink ceramic to the interproximal area. In the following section, different clinical situations with short gaps in the posterior region will be discussed using clinical examples.

For a configuration of two implants for two single crowns in a posterior space, the following assessment is made: First the mesiodistal width of the space is measured and rounded down to the nearest millimeter. 6 mm are then deducted from this width to allow 1.5 mm safe distance between the implant and the adjacent teeth and 3 mm between implants. The remaining distance can then be used to identify possible combinations of two implant neck diameters.

Examples of implant-diameter combinations are set out in this table. With a 13-mm gap, deduction of 6 mm for a safe distance to the adjacent teeth and between implants leaves a final distance of 7 mm. This would be enough for two 3.5-mm reduced diameter implants. A 15-mm gap would allow for one 3.5 mm and one 5-mm diameter implant. This combination can be seen in this clinical example where two missing maxillary left premolars have been replaced by two implants with 3.5- and 5-mm diameters. A 16-mm gap would allow two 5-mm diameter implants, while an 18-mm gap, one 5-mm and one 6.5-mm diameter implant. For reduced-diameter implants, refer to the manufacturer’s instructions regarding the need for splinting.

In this clinical situation there is a 14-millimeter gap between a maxillary first molar and a canine. When the minimum distance of 1.5 mm between implants and adjacent teeth and 3 mm between implants have been deducted from the 14 mm, a distance of 8 mm remains for the combined diameters of two implants. One reduced-diameter two-piece implant with a neck diameter of 3.5 and one two-piece implant with a standard diameter of 4.1 mm would fit safely within the available 8 mm. These two implants have been restored with separate single crown prostheses.

The treatment option of one implant with a screw-retained FDP and a cantilever represents a treatment alternative when there is insufficient mesiodistal space to accommodate two implants. In this case, the preference is a two-unit FDP with a mesial cantilever; this could, for example, be a second premolar site cantilevering mesially to replace a first premolar. This configuration principle may also be used when there is insufficient bone volume in a pontic site. A two-unit FDP with a distal cantilever may be used as shown in the clinical example. The distal cantilevers are used to increase the functional unit by one where there is insufficient bone volume in the second tooth site, which is the pontic site. This situation may arise due to the location of the sinus floor or lack of bone width. In a systematic review, Romeo and Storelli reported a higher risk for technical complications when using restorations with a cantilever, which represents a disadvantage.

According to the SAC classification, an edentulous space with three missing teeth is defined as a short edentulous space. On the following slides, the design principles for such gaps in the posterior region will be presented.

When three teeth are missing in the posterior region, a conventional three-unit FDP with pontic on two implants is one design option. In the region of the first molar, a sinus floor elevation may be necessary for implant placement.

In these clinical pictures, the implants are placed in the sites of the first molar and first premolar to support a screw-retained FDP. This configuration of two implants to support a three-unit FPD is considered to be an optimal solution for three missing teeth.

Another treatment alternative in the posterior region is a fixed dental prosthesis with a cantilever. The major indication for this design is to avoid bone augmentation procedures such as horizontal bone augmentation or sinus floor elevation. As a result, morbidity is reduced because less surgery is required for the patient. On the other hand, as already mentioned, a higher risk of technical complications for restorations with cantilevers has been reported, and care should be taken in designing the occlusal scheme.

Another possible solution is the design principle of three implants in the posterior region, which allows for three single crowns or for three splinted crowns to be made. The most common reason for placing three implants is when only short implants can be placed due to limitations in bone height and if bone augmentation is to be avoided because of patient wishes and/or medical issues. A systematic review has shown that short implants with a maximum length of 8 millimeters have the same cumulative survival rate as implants that are longer than 8 millimeters. With this configuration, there needs to be sufficient mesiodistal space to allow three implants to be placed while respecting the minimum distances between implants, and between implants and the adjacent natural teeth. A three-unit FDP with pontic on two short implants is yet another design option. With either option, clinicians are recommended to verify the survival and success outcomes of short implants from the manufacturer selected, especially if implants less than 8 mm in length are being considered.

Short Edentulous Spaces, Key Learning Points: In short edentulous spaces with two missing teeth, the clinical situation must be carefully evaluated to select the appropriate prosthesis design. The mesiodistal space needs to be assessed thoroughly to define the appropriate number of implants and their diameters. The prosthodontic principle of a cantilever has several advantages and disadvantages. In short edentulous spaces with three missing teeth in the posterior region, two implants are generally sufficient to restore three missing teeth. The use of an FDP with a cantilever may avoid the need for bone augmentation. Short implants represent a treatment alternative.

In the next section we focus on extended edentulous spaces, which are defined as spaces with more than three missing teeth. In these clinical situations, the clinician has to select the appropriate number of implants and place those implants according to a predetermined prosthodontic plan.

In the situation where there are four missing teeth in the anterior region, two implants are generally sufficient to replace four teeth with an implant-supported prosthesis. One treatment option is to place the implants adjacent to the neighboring teeth and to restore the missing teeth with an FDP. In this example, the implants have been placed in the sites of the lateral incisors to replace the central and lateral incisor teeth. Another option is to place the implants in the position of the central incisors and to restore the missing teeth with a cantilever fixed dental prosthesis. Yet another option is to place one implant in a lateral incisor site and the other in the contralateral central incisor site. A cantilever pontic is then used to replace the other lateral incisor. Several options are therefore possible with this configuration of four missing teeth and two implants. The decision where to place the implants is based on a careful assessment of the bone volume available and the prosthodontic planning.

The design principle with four implants would allow for four single crowns. This treatment option, however, has major drawbacks. First, when four implants are placed, the minimal distances between teeth and implants as well as in between the implants have to be considered. This results in a complex clinical procedure. And even when taking these considerations into account, the esthetic outcome, for example, the papilla presence between the crowns, will be compromised. Therefore, this prosthetic design cannot normally be recommended.

Configuration of implants in extended edentulous spaces in the posterior region depends on the prosthodontic plan. The first step is to determine the number of teeth to be replaced and whether, in fact, all the missing teeth need to be replaced. Every case needs to be assessed individually, as not all missing teeth need to be replaced. The prosthodontic plan must therefore be developed to suit the clinical situation. The three possibilities are 1. premolar occlusion only as part of a shortened dental arch objective. 2. replacement of the first molar as well as the premolars and 3. replacement of second molar as well as first molar and premolars. Configuration principles for the first two options are the same as for short edentulous posterior spaces, as covered in the previous learning objective. This learning objective will therefore concentrate on configuration principles for replacement of four posterior units.

The number and positions of implants depend on the prosthodontic plan together with local limiting factors, such as nearby anatomical structures or deficiencies in the bone. The prosthodontic plan could involve a one-piece FDP or a segmented alternative with single units or a combination of separate single-unit and multi-unit FDPs. The local factors include anatomical limitations imposed by, for example, the maxillary sinuses, the inferior dental nerve canal and or mental foramen, and available bone volume.

Examples of implant configurations using two implants to support a four-unit FDP can be seen here. In the presence of sufficient bone volume below the maxillary sinus at the implant sites and above the inferior dental nerve canal and mental foramen, implants can be placed in the first premolar and second molar sites as shown. A variation can be seen in this third example where the position of the mental foramen precludes using the first premolar site for implant placement. The alternative could then be to place the anterior implant in the second premolar site and add an anterior cantilever unit as replacement for the first premolar.

Use of three implants to support replacement of four tooth units allows the prostheses to be segmented. Two configuration examples are shown here in which separate single-unit and three-unit FDPs are used. In the first example in the mandible the three-unit FDP implant positions avoid the mental foramen, and the second molar is separate, which can assist in access for oral hygiene. In the second example in the maxilla a separate single unit is replacing the first premolar, whereas the three-unit FDP facilitates splinting of the two implants placed in the grafted sites of the maxillary second premolar and second molar.

Replacement of four missing posterior teeth can in general be achieved with support from two to three implants. Use of four implants adds to the complexity of maintaining recommended distances between the implants and the complexity of the prosthodontic procedures. It also adds to overall cost. The alternatives therefore include options for replacing three units only and providing an opposing occlusal stop via a short cantilever unit.

Extended Edentulous Spaces, Key Learning Points: It is not always necessary to replace every missing tooth in extended posterior edentulous spaces Implant positions are determined by the prosthodontic plan, based on the number of teeth to be replaced together with anatomical limitations and bone volume present For implant configurations in the anterior region with four missing teeth, two implants are in general sufficient For implant configurations in the posterior region with up to four missing teeth, two to three implants are in general sufficient

We will now focus on the edentulous ridge. An edentulous ridge may be restored with a fixed implant-supported prosthesis. Before placing the implants a thorough treatment plan must be established using diagnostic tools. According to the treatment plan, the number of implants need to be defined and assigned to the correct position within the ridge.

One design principle for an edentulous ridge is a full-arch one-piece fixed dental prosthesis. For this design, between four and six implants are necessary. Alternative prosthodontic principles, such as a shortened dental arch and/or the use of cantilever units, may allow the use of a reduced number of implants. The distal implants may be straight or tilted. Implants that are tilted mesially in the bone and distally into the oral cavity may help to avoid a cantilever or to reduce cantilever length and provide additional support to the prosthesis.

The minimum number of implants to support a full-arch bridge is four. If only four implants are used, this will invariably result in a prosthodontic design for a shortened dental arch. The four implants should ideally be spaced evenly apart, most often in the sites of the lateral incisor and the first or second premolar. The distal implants can be straight, or tilted to avoid anatomical structures.

With a design configuration of six implants, the full-arch one-piece FDP can consist of 12 units distributed around the arch. The selection of implant positions must take into consideration the volume of bone available at potential implant sites whilst conforming to the prosthodontic plan. As a general principle, it is desirable to avoid adjacent implants in the esthetic zone. In the example on the left, showing a full-arch one-piece FDP in the maxilla, the anterior implants have been positioned in the lateral incisor sites and the posterior implants occupy the first premolar and first molar positions. In the image on the right, illustrating a full-arch one-piece FDP in the mandible, the implants are located in the canine, first premolar and first molar sites.

In the maxilla, the dimensions of the maxillary sinus often hinder the placement of an implant in the position of the first molar without bone augmentation. Consequently, the placement of the implants in the anterior region often has to be adapted. In this example, the implants are placed in the sites of the central incisors, the canines, and the second premolars. The first molars have therefore been designed as cantilevers.

In some situations, a segmented prosthesis may be indicated. The reasons for this may include patient preference, the need to provide easy removal of short segments for maintenance or repair, or treatment plans which involve the gradual replacement of teeth in segments over a period of time. When a segmented prosthesis is planned in the edentulous ridge, more implants are usually required. It is common for eight to be used in the maxilla and six implants in the mandible. In both jaws the implants are distributed in a way that allows three to four short-span implant-supported FDPs to be designed. In the maxilla, the implants are placed in the sites of the central incisors, the canines, the first premolars and the first molars. In the mandible, the implants are placed in the same location as for a full-arch one-piece fixed dental prosthesis. Variations in the location of the implants and length of individual FDPs are possible, depending on the availability of sufficient bone volume and proximity to anatomical structures.

Edentulous Ridge, Key Learning Points: The prosthodontic principles of a cantilever or a shortened dental arch can reduce the number of implants for a fixed implant prosthesis in the edentulous ridge. Moreover, bone augmentation may be avoided. Tilted implants are a treatment option. A segmented prosthesis design in the edentulous ridge requires a greater number of implants and a strategic distribution of these implants.

Module Implant Configurations for Fixed Dental Prostheses, Summary: In single-tooth spaces, careful assessment of the space is required to define implant dimensions and type. A short edentulous space with two missing teeth may be restored with two single implant crowns or with one implant restored with a cantilever FDP. An edentulous space in the posterior region with three missing teeth should be restored with two implants and an FDP. A cantilever design may be possible. An extended edentulous space in the anterior region with four missing teeth should be restored with two implants and an FDP with a pontic or cantilever design. For an extended edentulous space in the posterior region two to three implants are in general sufficient using one-piece or segmented designs. An edentulous ridge can be restored with a one-piece FDP or three to four segmented FDPs.