Chapter 34 Premaxilla Surgery: Implant Insertion, Bone Spreading, Nasal Floor Elevation, and Incisive Foramen Implants

INTRODUCTION

Seventeen percent of the U.S. population (30 million adults) has no maxillary natural teeth. A large segment of the population is also missing multiple maxillary anterior teeth, often from trauma or failed fixed partial dentures (FPDs). Therefore the premaxilla is a region often in need of implant abutments for either fixed or overdenture prostheses. In a 20-year review of the literature by Goodacre et al., restorations associated with the edentulous maxillae have the highest early implant failure rate (19% for overdentures and 10% for fixed prostheses). In comparison, mandibular overdentures or fixed restorations have some of the highest implant survival rates (3% 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 surgical complications. After tooth loss, the facial cortical plate rapidly resorbs during initial bone remodeling, and the anterior ridge loses 25% of its width within the first year, as well as 40% to 50% within the next 3 to 5 years, mostly at the expense of the labial contour. As a result, the residual available bone migrates to a more palatal position.²^–⁷ The farther forward the maxillary anterior teeth are positioned from the more palatal bone position implants, the greater the moment force leverage on the bone-implant interface, abutment screws, and implants. Coupled with an angled force in both centric and excursions, more stress is transmitted to premaxillary implants than those in anterior mandibles. This often mandates more implants and larger-diameter implants with bone augmentation by bone-spreading or bone graft procedures before or in conjunction with implant placement.

In most patients with available bone, the bone is less dense in the anterior maxilla than in the anterior mandible. 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 in the premaxilla is usually fine and less dense than the anterior region of the mandible. This usually results in more overload implant failures or crestal bone loss. However, this type of bone lends itself to bone spreading or a compaction technique to insert implants.

In the anterior maxilla, the moderately resorbed bone categories of Division B to C–w often remain narrow almost to the floor of the nose; therefore osteoplasty to gain width is inappropriate. Once the tissues are reflected, the unusual anatomy may confuse the surgeon, and the resultant angulation of implant insertion complicates the restoration of the prosthesis (Figure 34-1). On occasion, osteoplasty to gain bone width results in a Division C–h or D bone volume. The incisive foramen may be used as an implant site in a C–w or C–h bone volume. Nasal elevation and implant insertion may also modify the opposing landmark in C–h bone volume. Therefore unique force factors and implant insertion procedures are more often observed in the anterior maxilla compared with the anterior mandible.

Figure 34-1 The maxillary bone often remains narrow to the floor of the nose. As a consequence, osteoplasty to gain width is often inappropriate. The surgical placement of implants in bone of minimal width is more challenging and may result in poor angulation and difficulty in restoring the implants.

PREMAXILLA EDENTULOUS TREATMENT PLANS

Key Implant Positions

If a patient desires a fixed restoration, implants are required whenever any of the following three combinations of adjacent teeth are missing: (1) the first premolar, canine, and lateral incisor; (2) the canine, lateral incisor, and central incisor; and (3) the canine, first premolar, and second premolar. When any of these conditions exist, 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 region), and the direction of the force (angled forces to the canine region).¹⁰

Under the condition of a missing canine and two adjacent teeth, at least two implants are indicated to support an independent fixed restoration (usually in the terminal positions of the span to eliminate cantilever forces). Therefore the canine implant position is one of the more important key positions in an edentulous premaxilla.

When one canine region cannot be used to place an implant in the edentulous premaxilla, at least one implant on each side of the missing canine is required to compensate for this vital position (a first premolar and lateral incisor implant). An incisor and canine position implant in the contralateral section can be splinted to these other two implants to act as abutments for the fixed or overdenture restoration in the edentulous premaxilla.

PREMAXILLA ARCH FORM AND IMPLANT NUMBER

The distance between two horizontal lines, drawn from the two canine tips and the facial of the central incisors, determines the dental arch form in the anterior maxilla. (Figure 34-2). A distance less than 8 mm corresponds to a square dental arch form. Lateral and central incisors are not cantilevered very much facially from the canine position, and therefore mandibular excursions and occlusal forces exert less stress on the canine implants. Implants in the canine positions to replace the six anterior teeth may suffice when the force factors are low and if they are splinted to additional posterior implants (Figure 34-3).

Figure 34-2 Two parallel lines perpendicular to the midline are drawn in the premaxilla to determine the dentate arch form. The anterior line is through the facial aspect of the central incisors. The posterior line is drawn through the tips of the canine cusps. A distance between these two lines of less than 8 mm is a square dentate arch form. An ovoid dentate arch form is 8 to 12 mm. A tapered dentate arch is more than 12 mm.

Figure 34-3 In a square arch form, implants in the canine region splinted to posterior implants may be sufficient with low patient force factors because the anterior cantilever is less than 8 mm.

An ovoid dentate arch form corresponds to a distance from the canine to central incisor horizontal lines between 8 and 12 mm. At least three implants should be inserted into the premaxilla, one in each canine and preferably one in a central incisor position (Figure 34-4). The central incisor position increases the anteroposterior (A-P) distance from the canine to central and provides improved biomechanical support to the prosthesis. In long-term edentulous maxillae, this most likely will require bone augmentation before implant insertion (Figure 34-5). When patient force factors are low to moderate, the anterior implant may be positioned in a lateral incisor site.

Figure 34-4 An ovoid dentate arch form has implants in the canine and an additional implant in the premaxilla. When posterior teeth are missing, additional posterior implants splinted to the anterior component are necessary.

Figure 34-5 A, An FP-3 prosthesis for an ovoid arch form. B, An intraoral view of maxillary and mandibular FP-3 prostheses. C, A panoramic radiograph of the FP-3 prostheses. The maxilla was restored with an iliac crest graft and bilateral sinus grafts. Three implants are found in the premaxilla (canines and central incisor) splinted to posterior implants. D, The high smile line lip dynamics of the maxillary FP-3 prosthesis.

When the distance between the canine and central incisor horizontal lines is greater than 12 mm, the dentate arch form is tapering and the restoration of a tapered dental arch form places the greatest forces on anterior implants, especially during mandibular excursions (Figure 34-6). The anterior teeth create a significant facial cantilever from the canine position. As such, four implants should be considered to replace the six anterior teeth. When more than six anterior teeth are missing, additional posterior implants should also be splinted to the anterior segment (Figure 34-7).

Figure 34-6 A tapered dentate arch form is subject to greater anterior moment forces, because the teeth are cantilevered farthest from the canines. As a result, two canine and two additional implants are usually indicated in the premaxilla.

Figure 34-7 A, An intraoral view of the maxillary FP-3 prosthesis. The edentulous maxilla with an ovoid to tapered dentate arch form should have three to four implants in the premaxilla and additional posterior implants to support a full-arch prosthesis. B, A panoramic radiograph illustrating a cantilevered restoration in the mandible and a fixed maxillary restoration with four anterior implants and four posterior implants splinted together without a cantilever.

As a consequence of bone resorption, the residual ridge form becomes more squared. The edentulous ridge arch form may be different from the dentate arch form (which is determined by the final teeth position in the premaxilla and not the arch shape of the residual ridge). The number and position of implants are related to the arch form of the final dentition (restoration), not the existing edentulous arch form. The worst-case scenario is a patient requiring restoration of a dental tapered arch form with a square residual ridge form. When force factors are greater than usual in any arch form, four implants in the premaxilla should be splinted together and share any lateral forces during excursions. In this patient condition, four implants in the premaxilla are required to compensate for the cantilevered anterior tooth position. These implants should be connected to additional posterior implants, preferably to include implants as far distal as the second molar sites, to increase the A-P position of the implants (Figure 34-8).

Figure 34-8 A, A square residual ridge arch may need to be restored with a tapered dentate arch form. As a result, the prosthesis is cantilevered from the more palatally positioned implants. B, An intraoral view of a tapered dentate arch form and a square residual ridge arch form. The four premaxillary implants should be splinted to posterior implants to compensate for the facial cantilever. C, Intraoral view of the prosthesis. A greater number of implants is indicated to compensate for the biomechanical force factors.

The edentulous square-to-ovoid residual bone arch form dimension often does not accommodate interimplant spacing for more than four anterior implants. The largest interimplant distance is typically found in a tapered arch form. As the radius of the circle becomes smaller (from labial resorption, arch shape, and/or patient size), the premaxilla interimplant distance is reduced. As a result, no more than four implants are typically used to replace the anterior six teeth, regardless of the force factors.

Restorations with posterior pontics that are cantilevered should not be placed on maxillary anterior implants. The forces are two to five times greater in the posterior regions compared with the anterior region of the mouth. A cantilever magnifies these higher bite forces. As a result, the anterior implant abutments with a cantilever may receive a tenfold greater load compared with implants without a cantilever. If posterior teeth also are being replaced in the prosthesis, then additional implants are required. Seven to 10 implants often are inserted to restore a completely edentulous maxilla with a fixed prosthesis, especially when opposing natural dentition or a fixed restoration.

It should be noted that most full-arch maxillary prostheses are FP-3 fixed restorations or RP-4 overdentures. In either scenario, mesiodistal implant position does not have to strictly correlate with tooth position. The faciopalatal position may also be more flexible, because the emergence profile of the prosthesis is usually outside of the esthetic zone and because the gingival aspect of the restoration separates the clinical crown from the implant site. As such, the implant sites are determined more for biomechanics, interimplant spacing, or available bone, rather than a strict tooth position (as in an FP-1 prosthesis) (Figures 34-9 and 34-10).

Figure 34-9 When the bone loss occurs in the premaxilla, the smaller radius of the arch mandates only four implants to be positioned from canine to canine.

Figure 34-10 A FP-3 prosthesis does not require an exact tooth position for the implants, because the gingival porcelain separates the implants from the teeth. The implants in the canine positions are slightly more distal than desirable, and the patient’s left lateral incisor implant is too mesial. Although the implant positions are not ideal in this intraoral view of the previous FP-3 prosthesis, no esthetic compromise exists.

When the natural canines are present and the missing teeth are the four maxillary incisors, the implant number is not as dependent on the dental arch form. Therefore the most common scenario is to place three Division B root forms (3.5 mm wide) in the lateral incisors and one central incisor region. When four anterior teeth are missing in a square dentate arch form, it may be easier to develop acceptable esthetics around central incisor pontics, compared with two implants in the central incisor regions. Therefore when biomechanics are favorable, the treatment plan may include only two implants in the lateral incisor sites. This two-implant treatment option is often designed for a square dental arch form in an older female, with little to no parafunction to fabricate an FP-1 prosthesis and a high lip line, when the natural canines are present.

MULTIPLE ANTERIOR IMPLANT DIAMETER

Several conditions should be considered for the proper implant diameter: tooth size, distance from an adjacent tooth, interimplant distance, facial bone dimension, and loading forces. A primary factor for the implant size when multiple anterior teeth are missing is the distance between the adjacent teeth or implants. The horizontal dimension of a wedge-shaped bone defect around an implant at the crest of the ridge from the biological width, implant design, or occlusal overload ranges from 0.5 to 1.4 mm.^11,¹² Therefore when implants are placed adjacent to each other, a minimum interimplant distance of 3 mm is suggested to accommodate for eventual crestal bone loss and maintain interseptal bone levels.¹¹ This is of importance for an FP-1 restoration because the interseptal bone height in part determines the incidence of presence or absence of a lack of interdental papillae between the teeth or implants, with the most ideal results having a distance of 5 mm or less from the interproximal bone to the interproximal crown contact.^13,¹⁴

Implants with 3 mm or greater interimplant space permit the development of papilla between the implants. When two adjacent teeth are missing in the premaxilla, the natural teeth adjacent to the edentulous span provide the interdental papilla support for proper esthetics, and an FP-1 restoration is often fabricated (Figure 34-11). However, the interdental papilla between the two implants is more difficult to restore.

Figure 34-11 A, The two implants are farther than 3 mm apart. The interimplant papilla is easier to develop with the additional space. B, The two adjacent implant crowns have distal papilla developed from the adjacent natural teeth. The interimplant papilla is slightly reduced in height; however the crown shape lowers the interproximal contact, and the interproximal region is acceptable.

The usual range of interimplant papillary height is 2 to 4 mm, even when the implants are 3 mm or more apart.¹⁵ Therefore when an FP-1 restoration is desired, the prosthesis design (square-tooth forms) and implant positions may need to be altered accordingly to improve the esthetic result (Figure 34-12). Implants less than 3 mm apart often have a depressed interimplant papilla. As a result the crown contour is modified to a square form to lower the interproximal contact to the soft tissue. Although this technique reduces the black triangular space over the papilla, it is not an ideal result (Figure 34-13).

Figure 34-12 When two implants are adjacent to each other and an FP-1 prosthesis is desired, the implants should be 3 mm or farther apart to help develop the interproximal soft tissue papilla regions. The implants are too loose; therefore the papilla is often reduced in height between the implants, so the crowns should be more square in shape to eliminate the space above the lack of soft tissue height.

Figure 34-13 A, When four adjacent incisors are missing, the soft tissue drape is often inadequate in the interimplant region. B, When four anterior teeth are missing, the papillae are often depressed, and square-crown forms reduce the height requirement of the interimplant papillae.

When four anterior adjacent teeth are missing, the soft tissue drape is usually inadequate to restore the height of the interimplant papillae; therefore the anatomy rarely permits an FP-1 prosthesis design. FP-2 or FP-3 prostheses are most always fabricated, dependent on the high lip dynamics of smiling within the esthetic zones (Figure 34-14).

Figure 34-14 FP-3 prostheses are often the more adequate prosthetic option to restore the interproximal papilla.

The difference in the emergence profile from an anterior crown between a 4-mm-diameter implant and a 5-mm-diameter implant is negligible and is often not clinically relevant. However, it is more difficult to control the creation of an interdental papilla for FP-1 prosthesis on larger-implant diameters. Therefore when in doubt, a smaller-size diameter implant should be selected in the esthetic zone; thus a 3.5- to 4-mm-diameter implant often is used in the central implant position for an FP-1 prosthesis. Likewise, a 3.0- to 3.5-mm-diameter implant often is used for a lateral incisor FP-1 restoration.

When multiple adjacent teeth are missing in the premaxilla, rarely is an FP-1 prosthesis and ideal soft tissue contour a realistic goal. Instead, most often FP-2 restorations are fabricated for low smile line patients and FP-3 restorations for ideal to high smile line patients. The exact mesiodistal tooth position is not required for an FP-2 or FP-3 restoration or a removable overdenture. It is more important to have adequate width of bone on all the sides of the implants. Therefore the interproximal spaces of bone may be used without complication in either the anterior or posterior regions of the mouth. As a result, the mesiodistal position of the implants is often less specific in the edentulous premaxilla.

MAXILLARY OVERDENTURE TREATMENT PLANS

The edentulous maxilla has the lowest implant survival for removable implant restorations.^1,^16,¹⁷ All reports concur that maxillary bone tends to be of poorer quality and volume and presents several biomechanical disadvantages. To compensate for the poor local conditions, a greater number of implants should be treatment planned, along with a greater A-P distance. With these concerns in mind, the minimum implant number for a completely edentulous maxillary overdenture prosthesis is four implants splinted in the premaxilla. The most usual sites are one central (or lateral) incisor position, bilateral canine positions, and at least one in the contralateral central or premolar site. Additional implants are often indicated when force factors are greater or for a RP-4 prosthesis overdenture.

PREMAXILLA SURGERY

Maxillary Lip Support

The maxillary lip support above a fixed prosthesis should be evaluated before surgery. The maxillary lip should be more facial than the mandibular lip at repose, yet a depressed philtrum should be maintained, especially in a younger patient. As a general rule, when a vertical line is drawn along the patient profile, the maxillary lip should be 1 to 2 mm in front of the lower lip, and the chin should be 2 mm behind the lower lip position when the patient has the jaw in a vertical rest position. If the patient wears a labial flange on the removable prosthesis that provides acceptable maxillary lip support, or one exists at the prosthetic try-in, and a fixed prosthesis is desired, then a soft tissue template may be fabricated from the removable prosthetic try-in to indicate the soft tissue support required (Figures 34-15 and 34-16).

Figure 34-15 The flange on a removable prosthesis may be removed to evaluated the volume of graft required for lip support for a fixed prosthesis.

Figure 34-16 A, A soft tissue template evaluates the amount of dense hydroxyapatite (HA) required on the facial bone above the implants to support the maxillary lip similar to the removable restoration. B, The soft tissue is approximated over the HA graft. C and D, An FP-3 prosthesis supports the lip below the facial soft tissue from the HA graft and the prosthesis.

When the maxillary lip requires greater horizontal support, yet adequate bone is present to place implants, an acellular tissue graft (AlloDerm) and/or or dense hydroxyapatite (HA) graft may be used on the facial aspect of the bone before closure of the soft tissue over the implant (Figure 34-17). A bone graft may be used to support the maxillary lip above the prosthesis. This is often indicated when insufficient bone is present for the proper implant size, position, and/or number (Figure 34-18). The added labial tissue thickness will support the maxillary lip above the cervical regions of the teeth, increase the crestal tissue thickness, and improve ridge contour. In addition, the soft tissue augmentation to support the maxillary lip may also create sufficient volume of tissue to allow the sculpture of an interdental papilla around the implant crowns at stage II uncovery.