The clinician practicing implant dentistry is confronted with different clinical situations in the edentulous posterior maxilla. Once a thorough treatment plan is established, different design configurations may be possible for implant-supported prostheses. The prosthesis design has to be based on the clinical condition to ensure that the appropriate number of implants is placed in the correct prosthetic positions using implants with appropriate dimensions. However, each prosthesis design has its advantages and disadvantages.

The treatment plan for the edentulous posterior maxilla determines the number of units that will be replaced by the implant-supported prosthesis. At least two units must be replaced in the edentulous posterior jaw to allow for two chewing units. It has been shown that the prosthetic concept of a shortened dental arch allows for sufficient oral function (Käyser 1982). In addition, replacing missing teeth with dental implants was shown to improve the subjective oral health of adults up to the premolar areas, but not necessarily in molar areas (Ponsi et al. 2011). The advantages of the concept of a shortened dental arch are numerous. In the maxilla, a sinus lift procedure may be avoided, which may positively influence the patient-reported outcome measures by reducing treatment costs (Esposito et al. 2014). However, the replacement of a molar in the maxilla may be indicated for esthetic reasons, even though an antagonist is missing in the mandible.

Ideally, the fixation mode of the planned implant-supported prosthesis in the posterior jaw is defined prior to implant placement. Implant reconstructions can either be screw-retained or cemented on standardized or customized abutments. A systematic review has shown that no individual fixation method has an advantage over any other (Sailer et al. 2012). Cemented reconstructions exhibit more biological complications: Bone loss exceeding 2 mm and soft tissue recession tend to occur more frequently at cement-retained reconstructions (Sailer et al. 2012). Soft tissue complications due to excess cement are reported in several clinical studies (Bornstein et al. 2003; Abboud et al. 2005). Screw-retained reconstructions exhibit more technical problems, such as loosening or fracture of the abutment screw and chipping of the veneering ceramic (Weber &Sukotjo 2007; Torrado et al. 2004). Regarding implant-prosthesis design, the angulation and horizontal position of implants is more critical in screw-retained implant prosthesis. The position of the screw access hole has to be in the center of the prosthetic unit to allow enough space for the surrounding restorative materials and reduce the risk of technical complications (Torrado et al. 2004). Independently of the retention mode, the implant prosthesis design must allow for appropriate oral hygiene. It was shown that 48% of the implants presenting peri-implantitis were those with no accessibility for proper oral hygiene with respect to 4% of the implants with accessibility (Serino & Strom 2009).

Replacing missing teeth in the edentulous posterior jaw presents the challenge of choosing the most appropriate implant position. As a basic principle, the mesiodistal space must always be carefully evaluated before placing implants. First of all, the width of the gap needs to be measured. The minimum distance between tooth and implant and between two adjacent implants needs to be respected. Several studies have indicated that the inter-implant distance between two adjacent implants should be at least 3 millimeters (Tarnow et al. 2000; Gastaldo et al. 2004; Kourkouta et al. 2009). The distance between an implant and a tooth should be at least 1.5 millimeters (Esposito et al. 1993). Taking these considerations into account will ensure an optimal result for the appearance of the interproximal soft tissues.

In general, the implant diameter should be selected according to the crown volume. This means that a premolar may require a reduced-diameter or standard-diameter implant, whereas a standard- or wide-diameter implant may be chosen for a molar site. The recommended minimum distances between the tooth and the implant or between adjacent implants can be achieved by reducing the implant diameter. The manufacturer's instructions may only recommend the use of their reduced-diameter implants for a limited number of indications. A reduced-diameter implant may also help preserve an adequate width of the buccal and oral bone walls and therefore avoid horizontal bone augmentation procedures.

The selection of the implant type according to the prosthetic position has to be considered too. The one-piece implant has a transmucosal design with a fixed neck and therefore an inbuilt fixed prosthetic platform and emergence profile. Therefore, this implant type offers an appropriate design for use in the posterior region. In contrast, the two-piece implant is designed to stop at bone level. The emergence profile and prosthetic platform are established by the abutment. The two-piece implants allow more prosthetic flexibility determined by the choice of abutment. In sites with relatively small dimensions and in esthetic sites, this may be an advantage. The smaller dimensions of the prosthetic components may also be an advantage of the one-piece implant as it allows the size of the screw access hole to be reduced. Therefore, two different implant types may support a multi-unit reconstruction, such as a one-piece implant in the first premolar site and a two-piece implant in the first molar site.

In a short edentulous space with two missing teeth in the posterior maxilla, two implants may be placed. The minimum distance between the adjacent tooth and implant of 1.5 mm and between the two implants of 3 mm need to be respected. In contrast, if there is not enough mesiodistal space for two implants, one implant is placed in one site and the correct implant position must be carefully defined. The prosthetic design of a two-unit FDP with a cantilever supported by one implant represents a valuable treatment option. The preferred retention mode for a cantilever FDP may be screw-retained as a higher rate of loss of retention has been observed with cemented cantilever restorations (Wittneben 2013). This configuration may also be used when there is insufficient bone width in one of the sites of the two prosthetic units (Figs 1 - 5).

A two-unit FDP with a distal cantilever may be used to avoid a vertical bone augmentation procedure due to the sinus floor location (Figs 6 - 13). Systematic reviews reported high survival rates after 5 years of 98.7% for implants supporting cantilever FDPs and 94.3% for cantilever FDPs (Aglietta et al. 2009). The survival rate of FDPs and the risk of technical complications for fixed implant restorations with (Romeo & Storelli 2012) and without a cantilever (Pjetursson et al. 2012) was the same. However, the length of the cantilever unit should be limited to a premolar (Kim et al. 2014). In addition, when using cantilevers appropriate implant dimensions need to be selected. Finally, the occlusal contact points of cantilever FDP should be carefully designed to avoid any interference with the cantilever unit in laterotrusion and protrusion.

One possible prosthetic design in which three missing teeth are replaced in the posterior maxilla is a conventional three-unit bridge supported by two implants. One implant is placed in the prosthetic position of the mesial unit, while the second implant is located in the site of the distal unit. In the region of the first or second molar, a sinus floor elevation may be necessary before or simultaneously with implant placement (Figs 14 - 19). However, these techniques are associated with increased postoperative morbidity, higher treatment costs, and a higher risk of complications during patient rehabilitation (Esposito et al. 2009; Esposito et al. 2014).

Short implants can make vertical bone augmentation unnecessary. 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 (Atieh et al. 2012). However, early implant loss was significantly associated with short implants. 73% of the failed short implants were lost before loading, indicating that biomechanical factors have an inferior impact.

In terms of the crown-to-implant ratio (C:I ratio), short implants have higher C:I ratios as compared to longer implants. The C:I ratio is the relationship between the length of the implant-supported prosthesis and the length of the implant. It was assumed that the higher forces resulting from a longer lever arm might cause resorption of the marginal bone around short implants (Rangert et al. 1997). Clinical studies demonstrated that a higher C:I had no influence on the marginal bone level and the implant survival rates (Blanes et al. 2007; Schneider et al. 2012; Tawil et al. 2006).

There are relatively few studies focusing only on implants shorter than 6 mm. Consequently there are limited data on implants within this category. In a prospective multicenter study, 28 patients with a severely resorbed posterior mandible received a fixed partial denture supported by 4-mm-long implants (Slotte et al. 2015). One short implant was placed for each tooth unit of the multi-unit fixed dental prosthesis and all short implants were splinted. The implant survival rate was 92.2% after 5 years. Compared to recent systematic reviews, the survival rate is only slightly lower.

In a clinical situation with more than three missing teeth in the posterior maxilla, the clinician has to select the appropriate number of implants and place those implants according to the prosthetic planning. The same design principles apply as discussed for the short edentulous space with two and three missing teeth. Prosthetic options, such as a shortened dental arch and a cantilever, allow for the use of a reduced number of implants.

Independently of the implant prosthesis design, it has to be ensured that patients who are rehabilitated with dental implants are provided with proper oral hygiene instructions. Following the prosthetic design principles for implant prosthesis in the posterior maxilla will result in a proper prosthetic design allowing access for oral hygiene around the implants and therefore help towards long-term success.