No Cantilevers

The first rule for ideal key implant positions is that no cantilever should be designed in the prosthesis (Figure 8-1). Cantilevers are force magnifiers to the implants, abutment screws, cement or prosthesis screws, and implant-bone interface. Cantilevers on fixed partial dentures supported by teeth have a higher complication rate than prostheses with terminal abutments, including unretained restorations. This is especially noted with parafunction or reduced crown height spaces.⁵ Therefore the ideal key implant positions include the terminal abutment positions when adjacent teeth are missing.

Figure 8-1 Ideal key implant positions include the terminal abutment positions when adjacent teeth are missing. Without a terminal abutment, a cantilever exists on the restoration, which increases complications. This is particularly important when a lateral load or heavy bite force is applied to the cantilever.

The length of the cantilever is directly related to the amount of the additional force placed on the abutments of the prosthesis. Therefore when a 25-lb force is placed along the long axis of an implant, the implant system (i.e., crown, cement, abutment, abutment screw, implant body, implant marginal bone, and implant bone interface) receives a 25-lb load. When a force of the same magnitude (25 lb) is applied on a 10-mm cantilever, the moment force on the abutment is increased to a 250-lb millimeter force. As a result, any part of the implant system is at an increased risk of biomechanical failure (e.g., porcelain fracture, uncemented prosthesis, abutment screw loosening, crestal bone loss, implant failure, implant component or body fracture).

A cantilevered restoration on multiple implants may be compared with a class I lever. The extension of the prosthesis from the last abutment is the effort arm of the lever. The last abutment next to the cantilever acts as a fulcrum when a load is applied to the lever. The distance between the last abutment and the farthest abutment from the end of the cantilever represents the resistance arm and may be called the anteroposterior distance or A-P spread of the implants. The length (usually in millimeters) of the cantilever (effort arm) divided by the resistance arm represents the mechanical advantage. Therefore, when two implants are 10 mm apart with a cantilever or extension of 20 mm, the mechanical advantage is two (20 mm/10 mm). In this example, a 25-lb force on the cantilever results with a 50-lb force on the farthest abutment from the cantilever (25 lb × 2 = 50 lb). The abutment closest to the cantilever (fulcrum) receives a force equal to the sum of the other two forces, or, in this example 75 lb (25 lb + 50 lb) (Figure 8-2). Therefore cantilevers magnify forces to all the abutments supporting the prosthesis.

Figure 8-2 A cantilever on two implants may be considered a Class 1 lever. When the center of each implant is 10 mm apart, with a 20-mm cantilever, a mechanical advantage of 2 is created. Therefore the load on the cantilever will be multiplied by 2 on the far implant, and the implant close to the cantilever receives the total stress of the two loads.

To enforce the rule of no cantilever, the key implant positions when one or two adjacent teeth are missing indicate one implant per tooth. When 3 to 14 adjacent teeth are missing, the key implant positions include the two terminal abutments, one on each end of the prosthesis. A three- to four-unit prosthesis may be fabricated with only these abutments, when nearly all the force factors are low and the bone density is favorable (Figure 8-3). Restorations of 5 to 14 units require additional abutments.

Figure 8-3 A, When three adjacent teeth are missing, the terminal abutments are key implant positions. When all patient force factors are low and bone density is good, these abutments may be adequate to replace the three missing teeth. B, When four adjacent teeth are missing, the terminal abutments are key implant positions. Rarely are these two implants sufficient to replace four posterior teeth.

The ideal treatment plan should eliminate cantilevers. However, in some clinical conditions, a cantilever is the most prudent treatment option. For example, in an edentulous mandible, available bone in the posterior regions may be insufficient for root form implant placement, without advanced procedures (e.g., nerve repositioning, iliac crest bone grafts). An alternative treatment plan may be to cantilever pontics from anterior implants. However, when terminal abutments are not designed in the treatment plan and a cantilever is planned, other force and surface area factors should compensate for the increase in force. When this option is considered, the force factors of parafunction, crown height, masticatory dynamics, implant location, and opposing arch are closely scrutinized. In addition to force modifiers, the A-P distance (A-P spread) of the distal and anterior implants is also a factor. When the implants are in one plane, the cantilever should rarely extend farther than the A-P distance, regardless of how low the patient force factors (see Figure 8-1). When the force factors are unfavorable, the cantilever length should be reduced or eliminated, the implant number increased, the implant size increased, or the implant design surface areas increased. When five or more implants are positioned around an arch, and several different plans exist because of the arch form of the splinted implant, the cantilever may extend as far as 2.5 times the A-P distance when force factors are low and density is favorable (Figure 8-4). However, more than two pontics are not indicated on a posterior cantilever, even under ideal conditions.

Figure 8-4 The anteroposterior distance (A-P spread) of five implants in the mandible is measured from the distal of the last two implants to the mid position of the most anterior implant. Because these splinted implants form an arch, the cantilever may extend up to 2.5 times the A-P distance (when patient force factors are low and bone density is good). Arch shape affects the A-P distance. The ovoid arch form (A) often has an A-P distance of 6 to 8 mm. A square arch form (B) often has an A-P dimension of 2 to 5 mm. A tapered arch form (C) has the greatest A-P distance, larger than 8 mm.

When the cantilever on the prosthesis is represented by only a lateral incisor, the ideal implant position may not include the lateral incisor position when bite forces are low to moderate. The lateral incisor is the smallest tooth in the arch, so the cantilever is limited. The anterior bite forces are lowest in the central and lateral incisor region. The occlusal contact on the pontic may be eliminated and further reduce risk of overload. When a pontic replaces the lateral incisor, the soft tissue drape may be improved compared with an implant. As a result, the ideal implant position based on force and esthetics may be a larger-diameter implant in the canine position (i.e., 5-mm versus 4-mm diameter) and a cantilever pontic to replace the lateral incisor.

The fact that, on occasion, a cantilever may be acceptable when force factors are low and bone density is favorable does not negate the ideal goal that no cantilever should be designed in the prosthesis. Therefore when three or more adjacent teeth are missing, the terminal abutments at each end of the prosthesis are first designed in the treatment plan. When this option is not readily available, additional implants, larger implant sizes, and greater implant surface area design are indicated. In addition, the forces to the cantilevered portion of the prosthesis should be reduced.

No Three Adjacent Pontics

In most prostheses designs, three adjacent pontics are contraindicated on implants, just as they are contraindicated on natural abutments.⁶ The adjacent abutments are subjected to considerable additional force when they must support three missing teeth, especially in the posterior regions of the mouth. In additions, all pontic spans between abutments flex under load. The greater the span between abutments, the greater the flexibility of the metal in the prosthesis. The greater the load, the greater the flexure. This metal flexure places shear and tensile loads on the abutments (see Chapter 25).⁷ The greater the flexure, the greater the risk of porcelain fracture, uncemented prostheses, and abutment screw loosening.

A one-pontic span exhibits little flexure—8 μm or less with precious metal under a 25-lb load.⁸ A two-pontic span flexes eight times more than a one-pontic span, all other variables being equal. A three-pontic span flexes 27 times more than a one-pontic span (Figure 8-5).^7–9 As a result, not only is the magnitude of the force increased to the adjacent abutments when the prosthesis has three pontics (because they are supporting two abutments and three pontics), but the flexure of the metal increases to a point that the incidence of complications make the treatment plan contraindicated, especially when forces are greater (as in the posterior region).

Figure 8-5 Metal flexure is related to the cube of the distance. A, When one pontic is present, the metal flexes (x). B, When the fixed prosthesis has two pontics (2×), the metal flexes 2 × 2 × 2 = 8 times more. C, When three pontics exist (3×), the metal flexes 3 × 3 × 3 = 27 times more than a one-pontic restoration.

The flexure of materials in a long span is more of a problem for implants than natural teeth. Because natural roots have some mobility both apically and laterally, the tooth acts as a stress absorber and the amount of material flexure may be reduced. Because an implant is more rigid than a tooth (and also has a greater modulus of elasticity than a natural tooth), the complications of increased load and material flexure are greater for an implant prosthesis. Therefore because three posterior pontics are contraindicated in a natural tooth-fixed prosthesis, it is even more important not to have three pontics in an implant restoration. Angled forces magnify the amount of the force to the implant system; therefore most maxillary anterior prostheses should also limit the number of pontics in the restoration.

The span of the pontics in the ideal treatment plan should be limited to the size of two premolars, which is 13.5 to 16 mm. When a molar is one of the teeth missing between existing teeth, the missing molar space may be 10- to 14-mm long. Therefore when the span is greater than 14 mm, two pontics should be considered to replace the molar. As a result, when a second premolar and first molar are missing, this span is often treatment-planned to replace three teeth, rather than two. In other words, a missing tooth span is often related to the missing number of roots in the mandible and number of buccal roots in the maxilla. This is especially appropriate for greater patient forces (i.e., moderate to severe parafunction) or softer bone types (i.e., D3 and D4) (Figure 8-6). As a result of these guidelines, an edentulous arch missing 14 natural teeth may have 18 potential implant sites. Rarely are second molars replaced; therefore when 12 natural teeth are considered in an edentulous arch, 14 potential implants sites are present.

Figure 8-6 When adjacent missing posterior teeth include a molar, the missing molars may be regarded as two premolar teeth when the space is 14 mm or more. Therefore when a first molar and second premolar are missing, the treatment plan may consider replacement of three adjacent teeth.

To limit the effect of the complications of three adjacent premolar size pontics, additional key implant positions are indicated in prostheses missing more than four adjacent teeth. To have three potential pontics in a restoration, at least four adjacent posterior teeth or five adjacent anterior and premolar teeth must be missing (when terminal abutments are present). Therefore, when 4 to 14 missing adjacent teeth are to be replaced, key implant positions are located in the terminal abutments and additional pier or intermediary abutments are indicated to limit the pontic spans to 2 premolar-sized pontics or less. Following this rule, a five to seven premolar-sized unit prosthesis has three key abutments (two terminal and one pier) (Figure 8-7). An 8 to 10 premolar-sized unit prosthesis has 4 key implant positions (2 terminal and 2 pier). An 11- to 13-unit prosthesis has 5 key abutments (2 termin/>