Anatomic Limitations

Several factors affect the condition of the edentulous maxilla and may result in a decrease in implant survival or an increase in prosthetic complications. The facial cortical plate of the premaxilla is thin over the roots of the teeth and may be resorbed from periodontal disease or is often fractured during the extraction of these teeth (Figure 25-3). In addition, the facial cortical plate rapidly resorbs during initial bone remodeling, and the anterior ridge loses more than 25% of its width within the first year after tooth loss and 40% to 50% over 1 year, mostly at the expense of the labial plate. As a result, the residual available bone migrates to a more palatal position.^4–7

The patient is more likely to wear and functionally accommodate to a maxillary complete denture compared with its mandibular counterpart. The greater retention, support, and stability compared with the lower denture are well documented. As such, the patient often is able to wear the maxillary removable prosthesis for many years before complications arise. During this time, from a patient’s perspective, the need to replace the maxillary denture is more related to a desire to improve esthetics or for a fixed restoration as the motivating factor. By the time the patient notices problems of stability and retention caused by resorption of the premaxilla, the maxillary bone often has advanced atrophy and may be division C–h or D in volume (Figure 25-4). Therefore, unlike the anterior mandible (in which denture complications occur before advanced bone atrophy), in the completely edentulous maxilla, the anterior bony ridge is often inadequate for ideal endosteal implant insertion even though few denture complications exist. It is the doctor’s responsibility to inform the patient about the continued bone loss in the maxilla before complications arise.

To achieve predictable esthetics for a maxillary anterior or full-arch fixed prosthesis, the hard and soft tissue, volume, and character should be adequate in most aspects. Available bone should be evaluated closely for implant insertion in esthetic regions because of its influence on the soft tissue drape, implant size, implant insertion (angulation and depth), and the final prosthetic result. Bone loss after maxillary anterior tooth loss is rapid and has considerable consequences. Therefore, most multiple maxillary anterior edentulous sites in the esthetic zone require at least some bone and soft tissue augmentation before or during implant insertion and/or at implant uncovery.

As the bone resorbs from division B to C–w in the anterior edentulous mandible, the cross-section of the residual ridge is triangular (with a wide base). As a consequence, an osteoplasty removes the narrower crestal bone and the residual ridge becomes wider, often converted to a division A bone volume. In the maxilla, however, the division B to C–w crest often remains narrow almost to the floor of the nose. An osteoplasty to gain bone width results in a division C–h to D ridge (Figure 25-5). The implant dentist often has difficulty inserting implants in the correct position when augmentation does not restore the region prior to implant insertion (Figure 25-6). Therefore, bone augmentation is more often required to increase bone width in the anterior maxilla compared with the anterior mandible.

In general, the premaxilla requires the most varied surgical approaches to improve success and is the most critical region for esthetics and phonetics. Initially, the anterior maxilla has less bone height than the anterior mandible and may compose only one third of the vertical dimension. Bone grafting is much more predictable for width gains rather than increases in height. Surgical options for division B and C–w bone more often require bone augmentation rather than osteoplasty, as often advocated in the mandibular anterior region. Division B bone grafting may use a synthetic bone component for the graft; division C–w often requires at least some autologous bone as a donor.

In some C–h premaxillae, implants may be inserted for a fixed prosthesis FP-3 or overdenture prosthesis. It should be noted that the opposing landmark is the floor of the nose, and this structure may be modified slightly by nasal elevation of 1 to 2 mm to improve implant support. However, when the premaxilla is less than 7 mm in height and when a D edentulous maxilla is present, this condition requires height augmentation before implant insertion. As a result, the dentist often must resort to the iliac crest or other extraoral donor sites for large volumes of bone. As such, the maxillary completely edentulous patient should understand that the surgical rehabilitation is much more complex and extensive because the volume of bone needed to reconstruct the atrophic maxilla increases. Therefore, notifying patients of their continued maxillary bone loss is even more important than in the anterior mandible rather than waiting until problems with their removable restoration develop.

Biomechanical Limitations

From a biomechanical perspective, the implant-restored anterior maxilla is often the weakest region of the mouth compared with other sections of the mouth. Compromised biomechanical conditions and their consequences include the following (Box 25-1):

As a consequence of these biomechanical factors, not only is implant failure more common, but prosthetic complications are also more often found in maxillary restorations.

The maxillary anterior region with multiple adjacent teeth missing often is restored with an overdenture or a fixed restoration that replaces teeth and the soft tissue drape (FP-3 prosthesis) (Figure 25-12). Whether a denture, an overdenture, or a fixed prosthesis is being fabricated, a full-arch or anterior edentulous maxillary reconstruction begins with the determination of the facial position of the maxillary incisal edge. Its modification at a later step may alter all other determinants of a reconstruction. A baseplate and wax rim (or the patient’s existing denture) may determine the facial support necessary for the labial contour of the maxillary lip. Most often the facial surfaces of the central incisors are 12.5 mm from the most posterior aspect of the incisive papilla.^13,14 The wax rim is initially positioned with this in mind. The farther forward the labial flange and teeth position, the higher the resting position of the lip and the greater the incisal edge exposure. The philtrum of the lip should have a visible depression in the midline under the nose. If the philtrum is too flat, the lip is extended too far, and wax should be removed from the labial aspect of the wax rim.

The position of the maxillary lip also may be determined by the position of the lower lip and chin with the face at the proper vertical dimension. A horizontal line, represented by the Frankfort plane, may be drawn from the highest point of the auditory meatus (top of the tragus) to the lowest point on the margin of the orbit, with the patient’s head in a vertical position. Ideally, a vertical perpendicular line drawn from the Frankfort plane to the lower lip should have the maxillary lip anterior to this landmark 1 to 2 mm and the chin 2 mm posterior to this line¹⁵ (Figure 25-13).

The labial position of the lip in relationship to the premaxillary bone is the primary criterion to determine whether a fixed restoration, a bone graft and fixed restoration, or a maxillary overdenture is indicated. When the labial position of the wax rim is forward of the residual ridge more than 5 mm, a bone graft before implants or a hydroxylapatite graft on the labial plate is required to support the lip for a fixed restoration, or a maxillary overdenture with a labial flange is considered (Figure 25-14).

Key Implant Positions

After the prosthesis type and labial tooth position are determined, the key implant positions are then determined for the maxillary restoration.^16,17 An important parameter in treatment planning is to provide adequate biomechanical position and surface area of support for the load transmitted to the prosthesis. Four guidelines are presented in Chapter 9 for key implant positions in an implant prosthesis. For the edentulous maxilla, these four guidelines may be slightly modified and summarized as no posterior cantilever, no posterior three adjacent pontics, the canine rule, and the first molar sites.

In addition to these four key implant position guidelines, the complete edentulous maxilla should further reduce the increased biomechanical risks by adding a fifth guideline, the five-sided arch¹² (Box 25-2).

Guideline 1: No Posterior Cantilever

The premaxillary teeth may be cantilevered forward from the implants for esthetics and phonetics. Hence, it is more important to place posterior implants connected to anterior implants to increase the anterior-posterior (A-P) distance and counter this affect. There should be little to no posterior cantilever in a complete edentulous maxilla (Figure 25-15).

Guideline 2: No Posterior Three Adjacent Pontics

When the posterior teeth are included in a prosthesis, there should not be three (or more) adjacent pontics.¹¹ Under those conditions, the adjacent abutments must support five or more adjacent teeth, the amount of the force is greater in the posterior regions, and the metal of the restoration flexes 27 times more than a one pontic prosthesis. In addition, the bone density to support the implants is often less in the posterior maxilla, hence the strength of the bone is reduced (Figure 25-16). This further increases the overload risk to the implants.

When all six anterior teeth are missing, at least one implant should usually be positioned between the canine implants in the maxillary arch. However, the “no three pontic” rule may be modified in the front of the mouth because the force is less in the anterior region compared with the posterior region In addition, a lateral incisor is the smallest maxillary tooth, so the length of the span is reduced (compared with the posterior regions).

Guideline 3: The Canine Sites

A fixed prosthesis replacing a canine tooth is at greater risk than almost any other tooth in the mouth. The adjacent maxillary lateral incisor is the weakest anterior tooth, and the first premolar is often the weakest posterior tooth. A traditional prosthodontic axiom indicates that a fixed prosthesis is contraindicated when a canine and two or more adjacent teeth are missing.¹¹ Therefore, if a patient desires a fixed restoration, implants are required whenever the following adjacent teeth are missing: (1) the first premolar, canine, and lateral incisor; (2) the canine, lateral incisor, and central incisor (Figure 25-17); and (3) the canine, first premolar, and second premolar (Figure 25-18).

When any of these three missing teeth combinations are present, a fixed restoration is contraindicated because of the length of the span (three pontics), the amount of force on the abutments (forces greater in the canine region compared with the anterior and the force on the abutments includes the three missing teeth), the direction of the force on the abutments (angled forces to the canine region), the proprioawareness of the canine site, and the need to establish posterior disocclusion of the teeth in excursions (Box 25-3).

Box 25-3

Canine and Two Adjacent Teeth Missing (Contraindicated for Fixed Partial Dentures)

1. Length of span (three teeth)

2. Amount of force (canine and posterior teeth have greater force than anterior teeth)

3. Direction of force (nonaxial load with mandibular teeth in centric and excursions)

4. Proprioawareness of canine is missing

5. Anterior guidance to disclude posterior teeth is necessary in implant restoration.

A tooth-supported prosthesis is less at biomechanical risk than an implant-supported restoration when the canine and two adjacent teeth are missing. Teeth are more mobile than implants; therefore, the stress relief mechanism of the periodontal–ligament complex reduces the flexure, force, and effect of an angled force. Despite this, it is contraindicated for patients with natural teeth to use three pontics in a fixed prosthesis whenever the natural canine and two adjacent teeth are missing. Therefore, under these conditions with implant treatment plans, at least two implants are indicated to support an independent fixed restoration (usually in the terminal positions of the span to eliminate cantilever forces) (Figure 25-19).

Using the missing canine and two adjacent natural teeth guideline, a fixed prosthesis is obviously contraindicated when either (or both) canine(s) and anterior four incisors are missing. For example, when a right canine, right lateral incisor, right central incisor, left central incisor, left lateral incisor, and left canine are missing, the condition is contraindicated for a fixed restoration. Yet in some underengineered treatment plans, implants are placed in each posterior maxillary quadrant, and a fixed restoration with five or six pontics is fabricated to replace the anterior teeth (Figure 25-20). The anterior cantilever from the first premolar sites in this treatment option is more detrimental than a posterior cantilever because of all of the biomechanical issues of the premaxilla. Apparently, the rationale for violating the prosthetic guidelines established in the literature for teeth are the following:

1. To augment a complete premaxilla, autologous bone grafts are often necessary, but synthetic materials can be used to predictably graft in the posterior maxillary sinus. The large volumes of autograft required for the complete premaxilla may require advanced bone graft procedures (which patients do not want, and few doctors are properly trained for) (Figure 25-21).

2. The impression is that implants are more rigid and therefore stronger than natural roots. However, this is false security. The fact that implants are more rigid than teeth makes the three or more adjacent pontics and canine position guidelines more important to follow when the abutments are implants. The rigid abutments magnify the problem of flexibility of the metal and the direction of force applied to the prosthesis. Therefore, the canine is an especially important implant site when the anterior six teeth are missing. When available bone is not present, a bone graft in the canine position is indicated before implant insertion, or an implant in both the lateral incisor and first premolar site is indicated to compensate for the missing canine.

Guideline 4: The First Molar Site

The first molar is an important abutment position in an edentulous maxilla. The bite force in this region increases to 200 lb compared with half this amount in the premolar sites. As a consequence, the first molar natural tooth surface area is more than two times greater than the premolars. In addition, the bone density in the molar region is often poorer than the premolar regions of the jaws. As a result, larger diameter implants or more implants are suggested, not a cantilevered force applied in the molar regions.

An anatomical problem for implant treatment in the posterior maxilla is the rapid expansion of the maxillary sinus after tooth loss. As a result, the edentulous posterior maxilla rarely has enough bone height without sinus grafting. Therefore, a treatment option to cantilever the posterior missing teeth from anterior implants is used to eliminate sinus grafting. Posterior cantilevers from anterior maxillary implants are less predictable than cantilevers from anterior mandibular implants for all the reasons addressed previously in this chapter. In addition, when first molar implants are not present, the A-P distance of the splinted implants is reduced, and the anterior cantilever from the most anterior implants is more of a biomechanical risk (Figure 25-22). Instead of a posterior cantilever, sinus grafting and larger diameter implants (or two smaller diameter implants instead of one larger diameter) are indicated in the first molar region and render improved overall implant success rates and, as important, fewer prosthetic-related complications.

Guideline 5: Five-Sided Arch

The dental arch may be divided into five different components related to their direction of movement (Figure 25-23). The posterior regions (first premolar, second premolar, first and second molars) each move in lateral directions to the midline. The canine positions move in two different oblique directions, and the anterior teeth (laterals and central incisors) move in an A-P direction. When three or more different components of an arch are splinted together, the different force directions are blended together, and the splint has less movement. In addition, when the three or more sections are splinted together, an A-P dimension is present, which also resists lateral forces. The more sections of the arch splinted together, the greater the A-P distance and the more resistant the splint is to any lateral force or cantilever.

In the maxilla, most often at least one implant should be placed in each of the five sections missing teeth and then splinted together when replacing multiple adjacent teeth. This means at least three implants usually are required to replace the anterior six teeth in the premaxilla: one in each canine position and one in any of the four incisor positions^16,17 (Figure 25-24). Previous studies by Bidez and Misch have shown that the force distributed over three abutments results in less localized stress to the crestal bone than two abutments.^17,18 When these three implants are splinted around an arch, connecting at least three segments creates a tripod effect and provides an A-P distance (A-P spread) with mechanical properties superior to a straight line and with greater resistance to lateral forces.

When anterior and posterior teeth are missing, additional posterior implants are usually required (Figure 25-25). Because the premaxillary prosthesis has many biomechanical conditions that increase forces (both in centric and excursions), it may be considered a cantilever from the implant support system. When the implants in all five sections are splinted together, they act as one side of the class I lever with the most anterior implants in the position of the fulcrum, and the incisal edge of the prosthesis as the cantilever length of the lever. The A-P distance of the implants for the anterior cantilever in the premaxillary restoration corresponds to the distance between the center of the most distal implants on each side (in the splint) and the anterior aspect of the most anterior implant. Hence, an implant in the most distal missing tooth position greatly improves the A-P spread and reduces the forces of the premaxilla to the implant system (Figure 25-26).