Chapter 17 Maxillary Arch Implant Considerations: Fixed and Overdenture Prostheses

More than 18 million people, or 10.5% of the adult population of the United States, are completely edentulous. Maxillary dentures usually are tolerated better by patients than their mandibular counterparts. As such, many treatment plans initially concentrate on the problems associated with the mandibular denture (Figure 17-1). However, once the patient enjoys a stable, retentive, and perhaps fixed mandibular prosthesis, often the patient’s attention is brought to the inadequacies of the maxillary prosthesis. In addition to this segment of the population without any teeth, 7% of the adult population wears a maxillary denture opposing some remaining mandibular teeth. This means that 17% of the U.S. population (30 million adults) have no natural maxillary teeth.

Figure 17-1 Implant treatment plans for the completely edentulous patient often address the mandible, as it is the least stable unit. The maxilla is often restored with a traditional denture.

The first chapter of this book addressed the esthetic and psychological consequences of the loss of maxillary teeth. Once patients become aware of the anatomical and esthetic consequences of missing teeth, the desire for implant restorations increases. As a result of patient and doctor education related to the loss of teeth, implant restoration of the edentulous maxilla will become more prevalent.

EDENTULOUS ANTERIOR MAXILLA

In a 20-year review of the literature compiled by Goodacre et al., restorations associated with the edentulous maxilla have the highest early implant failure rate compared with any other situation. In this review, overdentures in the maxillary arch averaged 19% implant failure. Fixed prostheses in the edentulous maxilla had an early implant failure of 10%. In comparison, mandibular overdentures or fixed restorations demonstrated a 3% implant failure rate.¹

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. In addition, the facial cortical plate rapidly resorbs during initial bone remodeling, and the anterior ridge loses 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.²^–⁷

The patient is more likely to wear and functionally accommodate a maxillary complete denture compared with its mandibular counterpart. The greater retention, support, and stability compared with the lower restoration are well documented. As such, the patient often is able to wear the maxillary removable prosthesis for longer periods before complications arise. From a patient’s perspective, the need to replace the denture is more related to a desire 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. Therefore the completely edentulous, anterior bony ridge is often inadequate for ideal endosteal implant insertion.

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. Therefore bone augmentation is more often required in the anterior maxilla compared with the anterior mandible.

It is the doctor’s responsibility to inform the patient about the continued bone loss in the maxilla before complications arise. Bone grafting is much more predictable for width gains rather than increases in height. Division B bone grafting may use a synthetic bone component for the graft; Division C–w often requires autologous bone from the mandible as a donor. However, when an edentulous maxilla requires height augmentation (C–h or D), 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 as 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.

In most patients with available bone, the bone is less dense in the anterior maxilla than in the anterior mandible, where a dense cortical layer surrounds coarse trabeculae of adequate bone strength to provide implant support. In contrast, the maxilla presents thin porous bone on the labial aspect, very thin porous cortical bone on the floor of the nasal and sinus region, and a more dense cortical bone on the palatal aspect.^8,⁹ The trabecular bone is usually fine and is less dense than the anterior region of the mandible. The trabecular bone of D3, often found in the maxilla, is 45% to 65% weaker than the trabecular bone of D2, usually found in the anterior mandible.

To achieve predictable esthetics for a maxillary 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 maxillary anterior edentulous sites require at least some bone and soft tissue augmentation before or during implant insertion and at implant uncovery.

The farther forward the maxillary anterior teeth are positioned from the implants, the greater the moment force leverage on the bone-implant interface, abutment screws, and implants. Yet many dentists attempt to do plastic surgery with plastic, hoping to eliminate vertical lines in the lip by bulking out the labial flange and teeth of an overdenture and positioning the teeth farther forward than the natural tooth position. Patients desiring to eliminate wrinkles in the lips from bone loss should have plastic surgery and bone augmentation, not plastic added to a maxillary prosthesis. This is even more important when the patient desires a fixed prosthesis. Bone and soft tissue augmentation is usually required to restore the natural appearance of the face without the help of a labial denture flange when a fixed restoration is planned. The facial position of the lip relative to esthetics is an important criterion to evaluate at the onset of treatment, before the placement of the implants. This criterion alone may indicate an overdenture rather than a fixed prosthesis.

From a biomechanical perspective, the implant-restored anterior maxilla is often the weakest section compared with other regions of the mouth. Compromised anatomical conditions and their consequences include the following (Box 17-1):

1. Narrow ridges form soon after tooth extractions. Bone augmentation is often necessary and may mandate the need for smaller-diameter implants. Their use results in increased stress concentrations in the implant and contiguous interfacial tissues, particularly at the crestal region.

2. In the premaxilla, esthetics and phonetics dictate that the replacement teeth be placed at or near their original position, often cantilevered from the implants and the residual ridge, which usually is resorbed palatally and superiorly. The use of facial cantilevers results in increased moment loads at the implant crest and often leads to localized crestal remodeling bone loss and soft tissue recession.

3. The arc of closure of the mandible is anterior to the maxillary residual ridge; as a consequence, the moment force is great against the maxillary anterior crowns supported by implants. The force is also directed against the thinner facial bone (Figure 17-2). Oblique centric contacts result in potentially harmful, off-axis load components.

4. All mandibular excursions place lateral forces on the maxillary anterior teeth, with resulting increased stress on the crestal bone of the supporting implants, especially on the labial aspect. These lateral loads in excursion further increases the moment loads applied to the implant (Figure 17-3).

5. A mandibular arch receives a load from the outside of the arch toward the center (on the top left). An arch is constructed for this force direction. A maxillary arch receives a force from within the arch to the outside of the structure (on the right). An arch is not as effective to resist this type of force (Figure 17-4).

6. Reduced trabecular bone density of the maxilla results in compromised bone strength and a weaker implant-bone interface.

7. Absence of thick cortical plate at the crest of the premaxilla results in loss of high-strength implant support and less resistance to angled loads.

8. The accelerated bone volume loss in the incisor region often results in the inability to place central and lateral incisor implants without substantial augmentation procedures.

9. Crown height space (CHS) is often greater than ideal and is a force magnifier to any angled or cantilevered force.

Figure 17-2 The arc of closure is anterior to the maxillary ridge and, as a result, increased forces are applied to the thinner facial cortical plate.

Figure 17-3 Mandibular excursions place lateral forces on maxillary anterior implants, which increase the resultant stresses.

Figure 17-4 During function, the forces are directed against the mandibular arch form, which is designed to reduce stresses (left). In the maxilla, forces are directed outside the arch form (right). An arch is much less effective against outward forces than inward forces.

TREATMENT OPTIONS

The treatment options for the restoration of a complete or partially edentulous patient with all six maxillary anterior teeth missing include a removable partial or complete denture, an implant-supported overdenture, or an implant-supported fixed prosthesis.

An independent, fixed implant-supported restoration has become the treatment of choice for most patients with complete or partial edentulism. A fixed prosthesis presents several advantages over an overdenture for a maxillary edentulous patient. Because many traditional maxillary dentures have adequate retention and stability, function and sore spots are rarely a problem. Therefore little benefit is perceived with an implant overdenture (IOD). The major disadvantage of the maxillary denture is most often the psychological aspect of removable teeth. In contrast, a fixed prosthesis presents significant benefits for the maxillary denture patient. In fact, after 3 years of function, most patients feel the maxillary fixed prosthesis is as good as or better than their natural teeth. On the other hand, an IOD is always considered by the patient as a removable prosthesis. Contraindications to a fixed partial denture include long edentulous spans, poor abutment support, and inadequate edentulous bone for proper prosthetic contour.

The primary reason for a conventional maxillary removable denture is economic reasons or because a patient is unwilling to undergo implant surgery. However, the easiest interim treatment prosthesis for the replacement of several anterior teeth during implant submerged healing is a removable restoration. If bone augmentation is necessary, this prosthesis may need to be used for longer than 1 year before delivery of the final implant restoration.

Sequence of Treatment

Maxillary Labial Lip Position

Whether a denture, an overdenture, or a fixed prosthesis is being fabricated, a full-arch 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 measurements. A baseplate and wax rim may determine 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.^10,¹¹ 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. 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 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 line.

The labial position of the lip in relationship to the premaxilla 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 prior to implants is required for a fixed restoration, or a maxillary overdenture is considered (Figure 17-5). The maxillary anterior region with multiple teeth missing often is restored with an overdenture or FP-3 prosthesis.

Figure 17-5 When the labial position of the teeth needed to support the upper lip is more than 5 mm anterior to the residual ridge, a bone graft or hydroxyapatite graft is required for a fixed prosthesis. Another method to support the maxillary lip is an implant overdenture with a labial flange.

Key Implant Positions

Once the prosthesis type and tooth position are determined, the patient force factors and bone density in the implant sites are evaluated. The key implant positions are then determined for the maxillary restoration.¹²^–¹⁴ 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 were presented in Chapter 8 for key implant positions in an implant prosthesis include:

1. No cantilever.

2. No three adjacent pontics.

3. The canines and first molar implant sites.

4. An arch is a five-sided open pentagon and should have at least one implant in each section of missing teeth.

Of these guidelines, number 3 and number 4 are often violated in the treatment plan of an edentulous maxilla, and therefore deserve further discussion.

Guideline 3: The Canine and First Molar Site

A fixed prosthesis replacing a canine tooth is at greater risk than most any other tooth in the mouth. The maxillary lateral incisor is the weakest anterior tooth, and the first premolar is often the weakest posterior tooth. A traditional prosthodontic axiom indicates a fixed prosthesis is contraindicated when a canine and two or more adjacent teeth are missing.¹⁵ 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 (Figure 17-6); (2) the canine, lateral incisor, and central incisor (Figure 17-7, A); and (3) the canine, first premolar, and second premolar (Figure 17-7, B).

Figure 17-6 When the patient is missing a first premolar, canine, and lateral incisor, two to three implants are indicated.

Figure 17-7 A, A patient missing a canine, lateral, and central incisor requires at least two implants to restore these teeth. B, A patient missing a second premolar, first premolar, and canine requires at least two implants to restore these three missing teeth.

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 (forces greater in the canine region compared with the anterior), and the direction of the force (angled forces to the canine region).

A tooth-supported prosthesis is less at risk than an implant-supported restoration when the canine and two adjacent teeth are missing. Because teeth are more mobile than implants, a stress relief mechanism reduces the flexure, force, and effect of an angled force. Despite this, it is contraindicated to design 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 17-8).

Figure 17-8 A, When all four incisors are missing and the dental arch form is ovoid, three implants are indicated to support the prosthesis. To ensure adequate implant spacing, narrow-diameter implants may be placed in the central and adjacent lateral position. B, When all four incisors are missing and the dental arch is square, two implants in the lateral position may be sufficient to support a final prosthesis. In such cases, the implant diameter should be a standard dimension.

Using the missing canine and two adjacent natural teeth guideline, a fixed prosthesis is also contraindicated when missing a right canine, right lateral incisor, right central incisor, left central incisor, left lateral incisor, and left canine without considerable anterior implant support (Figure 17-9). Yet in some improper treatment plans, implants are placed in each posterior maxillary quadrant and a fixed restoration with six pontics is fabricated to replace the anterior teeth (Figure 17-10). Apparently the rationales for violating the prosthetic guidelines established in the literature for teeth are the following:

1. To augment a premaxilla, autologous bone grafts are usually necessary, whereas synthetic materials can be used to predictably graft in the posterior maxilla. The large volumes of bone required for the premaxilla often requires an iliac crest graft (which patients do not want, and for which few doctors are properly trained) (Figure 17-11).

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 adjacent pontics and canine position guidelines more important to follow when the adjacent 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, an autologous graft in the canine position is indicated before implant insertion, or an implant in both the lateral and first premolar site to compensate for the missing canine may be considered.

Figure 17-9 It is contraindicated to replace three adjacent teeth, which include a canine. This implant prosthesis has at least five pontics that include a canine.

Figure 17-10 A panoramic radiograph of a fixed prosthesis with five adjacent pontics, including a canine. This prosthesis is undersupported, especially as it opposes natural dentition in a younger patient.

Figure 17-11 In the posterior maxillary, a Tatum lateral wall approach is used to graft the floor of the maxillary sinus with synthetic bone materials. For predictable results, the premaxilla and premolar regions most often require autologous bone. When six to eight sites need augmentation, an extraoral bone graft may be indicated.

The first molar is an important abutment position in an edentulous maxilla. The first molar natural tooth surface area is two times greater than the premolars. The bite force in this region increases to 200 lb, compared with half this amount in the premolar sites. 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 are also suggested.

The 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. A trend to cantilever the posterior missing teeth from anterior implants has developed. Posterior cantilevers from anterior maxillary implants are less predictable than cantilevers from anterior mandibular implants for all the reasons addressed in the beginning of this chapter. Instead, sinus grafting and larger-diameter implants (or two implants instead of one) are indicated in the first molar region.

Posterior implants (premolar and molar) without implant support in the premaxilla are sometimes connected with a full-arch bar for a maxillary overdenture. When a bar extends from molar to molar around an arch, the overdenture prosthesis is completely implant supported (RP-4), because it does not move during function or parafunction. As such, the overdenture acts as a fixed restoration (Figure 17-12). The removable implant prosthesis under these conditions should have the same implant support as a complete arch fixed restoration (not less).

Figure 17-12 This overdenture bar extends from second premolar to second premolar. The anterior bar represents a span of eight pontics and is too long for proper prosthetic support. The overdenture, when in place, is rigid over this bar design, and as such, requires as much support as a fixed prosthesis.

Guideline 4: Five-Sided Arch

The maxillary arch may be divided into five segments, similar to an open pentagon (Figure 17-13). The central and lateral incisors represent one segment, each canine a separate segment, and the posterior premolars and molars individual segments. In other words, each segment is essentially a straight line, with little resistance to lateral forces. However, when splinted together, an arch form dynamic becomes evident.

Figure 17-13 The maxillary arch may be treated as an open pentagon, with five straight-line segments. When teeth are missing in multiple segments, at least one implant is required in each section.

At least one implant should be placed in each of the five sections missing teeth and then splinted together when replacing multiple adjacent teeth missing in the maxilla. 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.¹²^–¹⁶ When posterior teeth are also missing, additional posterior implants are required (Figures 17-14 and 17-15).

Figure 17-14 When eight anterior teeth (first premolar to first premolar) are missing, implants should be placed in each segment to provide adequate support.

Figure 17-15 A, The dentate arch form may be different than the residual bone form, as the ridge resorbs apically and away from the original tooth position. In such cases, the prosthesis is designed to restore the proper tooth contour and lip support. B, The fixed FP-3 prosthesis replaces eight adjacent anterior teeth and is supported by six implants. C, Panoramic radiograph of the same patient.

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 Because these three elements are aligned along an arch, connecting at least three segments creates a tripod effect and provides an anteroposterior distance (A-P spread) with mechanical properties superior to a straight line and with greater resistance to lateral forces. The A-P distance for the anterior cantilevers in the premaxilla 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.

A poor treatment option for fully edentulous maxillae is the placement of implants in each posterior quadrant with independent bar segments and an overdenture. This treatment option is prone to failure. The maxillary overdenture rocks back and forth during excursions (if not, the overdenture is really a fixed restoration). The posterior implants are in a straight line and cannot resist the lateral forces. Eventually, almost all the implants on one side are lost. Maxillary complete prostheses and overdentures have a greater incidence of implant failure and complications than mandibular counterparts.^1,^18,¹⁹ These observations further emphasize the need for more implants and fewer pontics in the restoration of a maxilla compared with the mandible.