A cantilever may be considered a class 1 lever.17 For example, if two implants are 10 mm apart and are splinted with a cantilever of 20 mm, the following mechanics result: the mechanical advantage of the cantilever is 20 mm/10 mm, or 2. Therefore, whatever force is applied to the cantilever, a force twice as great will be applied to the farthest abutment from the cantilever. Whereas the force on the cantilever is a compressive force, the force to the distal abutment is a tensile and shear force. The load on the abutment closest to the cantilever (which acts as a fulcrum) is the sum of the other two components and is a compressive force (Figure 31-36). Hence, in this example, a 100-N force on the cantilever equals a 200-N tensile or shear force on the most distal abutment and a 300-N compressive force on the abutment (the fulcrum) next to the lever.
FIGURE 31-36 A cantilever on two implants may be considered a class 1 lever. When the implants are 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.
Because cement and screws are weaker to tensile loads, the implant abutment farthest from the cantilever often becomes unretained, resulting in the fulcrum abutment's bearing the entire load. Because the implant is more rigid than a tooth, it acts as a fulcrum with higher force transfer. It is a higher risk to cantilever from an implant than a tooth (Figure 31-37). As a consequence, crestal bone loss, fracture, and implant failure are often imminent after the distal abutment becomes no longer connected to the prosthesis. In summary, cantilevers increase the amount of stress to the implant system.
FIGURE 31-37A, A tooth is more mobile than an implant. Therefore, an implant as a fulcrum transmits more tensile and shear load to the distal abutment from the cantilever than a tooth. B, The cantilever to the mesial caused the cement to seal to break in the distal molar. Hence, the fulcrum implant carried all the load, and the implant failed.
The greater the force on the cantilever, the even greater the forces on the implants because the cantilever is a force magnifier. Hence, parafunctional loads are particularly dangerous for biomechanical overload. The greater the length of the cantilever, the greater the mechanical advantage and the greater the loads on the implants. The shorter the distance between the implants, the greater the mechanical advantage and the greater the force on the implant system (Figure 31-38). The cantilever force also varies as a result of implant number.73,74 Cantilevers are known to cause more biomechanical occurrences of implant and prosthesis component failure, particularly failure of prosthesis retaining screws or uncemented prostheses. A clinical report by Lundquist et al. also correlated long cantilevers with increased crestal bone loss around implants.75
FIGURE 31-38A, A panoramic radiograph of a maxillary and mandibular implant fixed prosthesis. The mandibular restoration is cantilevered from implants positioned between the mental foramina. B, A lateral cephalogram demonstrates that the anteroposterior (A-P) distance of the implants is about 6 mm. The prosthesis is cantilevered more than four times the A-P distance. The posterior occlusal load is magnified more than four times to the anterior implants, and the most distal implants receive the total sum of the loads. In addition, the opposing arch is implant supported and with less proprioception and higher bite forces than natural teeth. All of these risk factors make this mandibular restoration less predictable. The cantilever should be reduced, the posterior occlusal contacts should be reduced, an anterior contact occlusal night guard should be worn, and preventive changes of the prosthetic screws should occur every few years.
The goal of IPO relative to cantilevers is to reduce the force on the pontics of the lever region compared with that over and between the implant abutments. To reduce the amount of force that is magnified by the cantilever, the occlusal contact force may be reduced on the cantilevered portion of the prosthesis. A gradient of force type of load that gradually decreases the occlusal contact force along the length of the cantilever is beneficial. In addition, no lateral load is applied to the cantilever portion of the prosthesis (whether it is in the posterior or anterior region). Although the functional forces of occlusion during mastication may not be significantly altered by this technique, parafunctional forces (which are the most damaging) are significantly reduced with a gradient of force occlusal adjustment.
Crown Height and Implant-Protective Occlusion
The implant crown height is often greater than the original natural anatomical crown even in division A bone. If the implant is loaded on the long axis, crown height does not magnify the force (Figure 31-39). However, crown height is a force magnifier (vertical cantilever) when any lateral load, angled force, or cantilever load is applied17 (Figure 31-40). A crown height with any of these conditions acts as a magnifier of stress to most of the implant system (cement or screw retaining the crown, abutment screw, marginal bone, and implant–bone interface). The greater the crown height, the greater the resulting crestal moment with any lateral component of force, including those forces that develop because of an angled load. Angled abutments loaded in the direction of the angled abutment with an increase in crown height are subject to similar greater crestal moment loads because of the lateral load to the implant body and the increased lever effect from the crown height.
FIGURE 31-39 Crown height does not magnify the stress to the implant system when the force is applied in the long axis of the implant body.
FIGURE 31-40 A cantilever load on an implant results in six different moments applied to the implant body. An increase in crown height directly increases two of six of the moment forces.
A 30-degree angled load to an implant body, the implant crown with a 30-degree load, or a 30-degree cusp angle contact results in a similar condition: 50% of the occlusal load is transformed into a horizontal or shear component to the implant system. However, the angled load on an implant crown is at greater risk to the crestal bone than the angled implant body because the crown height acts as a vertical cantilever. Therefore, whatever load is applied to the occlusal table (or cusp angle) is magnified by the crown height. For example, a 12-degree angled load of 100 N on the implant crown results in 21 N of additional load as a lateral force component. However, if the crown is 15 mm high, the final load to the crest of bone and abutment screw is 21 N × 15 mm = 315 N-mm moment force (Figure 31-41). Therefore, the doctor should be aware the noxious effects of a poorly selected cusp angle, or an angled load to the implant crown will be magnified by the crown height measurement.
FIGURE 31-41 The crown height directly increases the effect of an angled force. For example, a 100-N load at a 12-degree angle increases the lateral or shear force component by 21 N. A crown height of 15 mm increases the 21-N force to a 315–N-mm moment force.
If a load perpendicular to the curves of Wilson and Spee is applied to an angled implant body, the increase in load is not magnified by the crown height. The angled implant will increase the force components but will not be magnified by the crown height. Hence, the angle of load to the occlusal surface is more important to control than the angle of the implant body position.
Axial loading of the implant is especially critical when the crown height increases, intensity of force increases, or the duration of force increases (i.e., parafunction). Remember, the crown height is not a force magnifier (lever) when there is no cantilever or lateral load. A long-axis load of 100 N is similar to the implant system, whether the crown height is 10 or 20 mm. Occlusal schemes and crown occlusal anatomy should incorporate axial loads to implant bodies and, when not applicable, should consider mechanisms to decrease the noxious effect of lateral loads. Because horizontal or lateral loads cause an increase in the amount of tension and shear forces to the implant system, these loads should be reduced within the occlusal scheme, especially in mechanical systems that increase the magnitude of the biomechanical load.
Posterior Implant Crown Contour
A buccal or lingual cantilever in the posterior regions is called an offset load, and the same principles of force magnification from class 1 levers apply. In other words, the greater the offset, the greater the load to the implant system. Offset loads may also result from buccal or lingual occlusal contacts and create moment forces, which increase compressive, tensile, and shear forces to the entire implant system (Figure 31-42).
FIGURE 31-42 A cantilever occlusal contact to the facial or lingual is called an offset load. Cantilever or offset loads increase the force by the length of the lever and increase the shear component of the force. A posterior implant most often is placed under the central fossa of the implant crown. A buccal cusp contact is an offset or cantilever load. The ideal occlusal contact is over the implant body. B, Buccal; F, force; L, lingual. (From Misch CE: Contemporary implant dentistry, ed 2, St Louis, 1999, Mosby.)
Wider root form implants can accept a broader range of vertical occlusal contacts while still transmitting lesser forces at the permucosal site under offset loads. Narrower implant bodies are more vulnerable to occlusal table width and offset loads. Therefore, in IPO, the width of the occlusal table is related directly to the width of the implant body.33
The laboratory technician often attempts to fabricate an implant crown with occlusal facial and lingual contours similar to that of natural teeth. When out of the esthetic zone, the posterior implant crown should have a reduced occlusal width compared with a natural tooth. A wide occlusal table favors offset contacts during mastication or parafunction. The narrower occlusal contour of an implant crown also reduces the risk of porcelain fracture. A facial profile similar to a natural tooth on the smaller-diameter implant (e.g., 10-mm tooth versus 4- to 6-mm implant) results in cantilevered restorative materials. This cantilever crown contour is often designed as a ridge lap pontic of a fixed partial denture (Figure 31-43). The facial porcelain most often is not supported by a metal substructure because the gingival region of the crown is also porcelain. As a result, shear forces result on the buccal cusp on the mandibular crown or lingual cusps in the maxillary crown and are more likely to increase the risk of porcelain fracture. This risk is compounded further by the higher impact force developed on implant abutments compared with natural teeth. The extended crown contours not only increase offset loads but also often result in ridge laps or porcelain extension at the facial gingival margin of the implant abutment (Figure 31-44). As a result, home care in the sulcular region of the implant is impaired by the overcontoured crown design. The dental floss or probe may reach under the ridge lap to the free gingival margin, but it cannot enter the gingival sulcus. Hence, daily hygiene is almost impossible to perform. The narrower posterior occlusal table facilitates daily sulcular home care. Thus, a narrow occlusal table combined with a reduced buccal contour (in the posterior mandible) facilitates daily care, improves axial loading, and decreases the risk of porcelain fracture. However, in the esthetic zone, the ridge lap design may be necessary to restore the implant rather than removing it, bone grafting, and replacing the implant.
FIGURE 31-43 The diameter of the implant is smaller than the posterior natural tooth. When the crown contour is similar to a tooth, a facial cantilever is necessary, which often has a ridge lap design (as a pontic in a fixed partial denture).
FIGURE 31-44A, A posterior mandibular implant in the second premolar position. A posterior implant (in the position of the second premolar (in this case) often is inserted under the central fossa position. B, The implant body is narrower than the natural tooth. When the laboratory fabricates an implant crown the same size as the missing tooth, a facial ridge lap crown often results so as to restore the complete tooth contour. The ridge lap crown does not allow sulcular hygiene or facial probing. A ridge lap crown contour was made by the laboratory to restore the full contour of the missing teeth. C, In situ, the crown appears as a crown on a natural tooth, but the cervical aspect is not in the esthetic zone. Hence, offset loads, porcelain fracture, and abutment screw loosening risk are increased. D, The ridge lap was eliminated and the buccal contour reduced. (Note there was no metal work to support the cantilevered porcelain.) E, The modified crown in situ. Daily hygiene is improved and biomechanical risk reduced. The second premolar implant crown restores the function and occlusal aspect of the missing tooth. The esthetic facial cervical region is compromised to improve hygiene and force resistance because this region is not seen during function, speech, or smiling.
Mandibular Posterior Crowns
The posterior mandible resorbs lingually as the bone resorbs from division A to B. As a result, endosteal implants are also more lingual than their natural tooth predecessors. The division C–h and D mandibular ridge shifts to the buccal compared with the maxillary arch. However, endosteal implants typically cannot be inserted because the available bone above the mandibular nerve is inadequate for endosteal implants (Figure 31-45).
FIGURE 31-45 The posterior maxillary and mandibular edentulous arches resorb lingually as bone volumes change from division A to B to B minus width to C minus width. The mandibular posterior arch resorbs facially as the edentulous site becomes C minus height and D bone volume.
The mandibular implant crown should be reduced from the buccal and the maxillary crown reduced from the lingual. Thus, the “stamp cusp” offset load is reduced. The reduced buccal contour in the posterior mandible is of no consequence to cheek biting because the buccal horizontal overjet is maintained (and increased). The lingual contour of the mandibular implant crown is similar to a natural tooth (Figure 31-46). This permits a horizontal overjet to exist and push the tongue out of the way during occlusal contacts (just as natural teeth). As with the natural tooth, the lingual cusp has no occlusal contact.
FIGURE 31-46A, A mandibular implant in the first molar position. B, The first molar crown in situ. The lingual contour is similar to the natural tooth. The buccal contour is reduced in width.
In the posterior mandible, as the implant diameter decreases, the buccal cusp contour is reduced. This decreases the offset length of cantilever load. The lingual contour of the crown remains similar regardless of the diameter of the implant. The lingual contour permits a horizontal overlap with the maxillary lingual cusp, so the tongue is pushed away from the occlusal table during function. The lingual cusp is not occlusal loaded (as with natural teeth) (Figure 31-47).
FIGURE 31-47 The wider the implant body, the wider the occlusal table width of the implant crown. As the mandibular bone width decreases, the implant body may decrease in width. The lingual contour of the implant crown remains similar regardless of the width of implant. The buccal contour is reduced as the implant diameter decreases. A narrow ridge in an esthetic zone may require bone augmentation so that a wider implant may be used to support an implant crown, which appears as a natural tooth. B, Buccal; CF, central fossa; L, lingual.
During mastication, the amount of force used to penetrate the food bolus may be related to occlusal table width. For example, less force is required to cut a piece of meat with a sharp knife (narrow occlusal table) than with a dull knife (wider occlusal table). The greater surface area of a wide occlusal table requires greater force to achieve a similar result. Hence, the wider the occlusal table, the greater the force developed by the biological system to penetrate the bolus of food. However, these functional forces are typically less than 30 psi. The real culprit in biomechanical forces relate to parafunction because forces 10 to 20 times greater may be generated.
Maxillary Posterior Crowns
In the esthetic zone (high lip position during smiling), the buccal contour of the maxillary implant crown is similar to a natural tooth. This improves esthetics and maintains the buccal overjet to prevent cheek biting. But just as with the natural teeth, there is no occlusal contact on the buccal cusp. Ideally, when maxillary posterior implants are in the esthetic zone, they are positioned more facial than the center of the ridge. The lingual contour of a maxillary implant crown should be reduced because it is out of the esthetic zone and is a stamp cusp for occlusion (which is an offset load) (Figure 31-48).
FIGURE 31-48A, A maxillary posterior implant in the esthetic zone is positioned slightly more to the facial position than the central fossa. B, The facial contour of the maxillary first molar implant crown is contoured similar to the adjacent teeth. C, The lingual crown contour of the maxillary first molar implant crown is reduced, compared to the natural tooth.
The ideal functional position for the maxillary posterior implant is under the central fossa when the cervical region is not in the esthetic zone. Hence, the lingual cusp is cantilevered from the implant similar to the buccal cusp of the posterior mandible. Therefore, the reduced lingual contour reduces the offset load to the lingual (Figure 31-49).
FIGURE 31-49 Posterior maxillary implants most often are positioned under the central fossa when the cervical region is not in the esthetic zone. The posterior maxillary lingual contours of implant crowns usually are reduced for improved hygiene and less offset loads to the implants.
The maxillary dentate posterior ridge is positioned slightly more facial than its mandibular counterpart because the teeth have a maxillary overbite. When the maxillary teeth are lost, the edentulous ridge resorbs in a medial direction as it evolves from division A to B, division B to C, and division C to D (see Figure 31-45). As a result, the maxillary permucosal implant site gradually shifts toward the midline as the ridge resorbs. Sinus grafts permit the placement of endosteal implants in the posterior maxilla even in previous division D ridges. However, because of resorption in width, the maxillary posterior implant permucosal site may even be palatal to the opposing natural mandibular tooth.
In the esthetic zone, many of the crown contours are made to resemble the natural tooth as close as possible. However, out of the esthetic zone, in the posterior regions of the mouth, the crown contour should be different than a natural tooth. The implant body buccolingual dimension is smaller than the natural tooth. The center of the implant most often is placed in the center of the edentulous ridge. Because the crest of the ridge shifts lingually with resorption, the implant body is most often not under the opposing cusp tips but rather near the central fossa or even more lingual and in the maxilla may even be under the lingual cusp of the original natural tooth position (Figure 31-50). Most often the laboratory fabricates a posterior implant crown that is similar in size to a natural tooth, with a cantilevered facial contour. In addition, the occlusal contacts are often on the “stamp cusp” of the mandible (buccal cusps). However, these “stamp cusps” are often offset loads (buccal cantilevers) (Figure 31-51).
FIGURE 31-50 The implants in the second premolar and first molar are positioned under the lingual cusps. The laboratory made the buccal crown contour similar to the missing teeth. Hence, a facial offset load is present. The cantilever force should not be compounded by occlusal loads in the central fossa from a mandibular buccal cusp.
FIGURE 31-51 In the maxillary posterior region, the implant may be positioned under the lingual cusp. The laboratory often cantilevers the facial crown profile, to make it appear as a natural tooth.
When the maxillary posterior teeth are out of the esthetic zone, the crown may be designed for a crossbite (Figure 31-52). The lingual overjet prevents tongue biting, the buccal overjet (from the mandibular tooth) prevents cheek biting, the implant is axial loaded by the lingual cusp of the mandible, and hygiene is improved (Figure 31-53).
FIGURE 31-52 When maxillary posterior implants are placed in division B to D bone volumes out of the esthetic zone, the implant crown often is restored in posterior crossbite. The maxillary lingual horizontal overjet prevents tongue biting, the mandibular buccal overjet prevents cheek biting, and the primary occlusal contact is in the central fossa over the implant body. B, Buccal; L, lingual.
FIGURE 31-53A, The maxillary first and second molar implant was placed under the lingual cusp position of the natural tooth. B, The maxillary molar implant crowns are restored in crossbite because they are out of the high smile esthetic zone.
Some authors encourage the placement of implants in the posterior jaws to be staggered to improve biomechanical resistance to loads.76 This concept is most effective when narrower implants are positioned in wider ridges, so the staggered position is increased. However, increasing the diameter of the implants and splinting them together to decrease crestal loads is more efficient than offsetting an implant. Unavoidable, less ideal implant orientations should be accommodated through adjustments in occlusion, increasing implant diameter or number of implants placed to reduce the overall load magnitude applied to any one implant, as well as the resultant interfacial stress and strain profiles.
In summary, restorations mimicking the crown contour and occlusal anatomy of natural teeth often result in offset loads (increased stress and risk of associated complications), complicated home care, and an increased risk of porcelain fracture. As a result, in nonesthetic regions of the mouth, the occlusal table should be reduced in width compared with natural teeth (Figures 31-54 and 31-55).
FIGURE 31-54 A mandibular first molar implant crown. The lingual contour is similar to the adjacent teeth, but the lingual cusp tip is not loaded. The buccal contour is reduced compared with the adjacent teeth.
FIGURE 31-55A, Implants in the esthetic zone (replacing a canine and premolar in this photo) are placed more facial so that the crown emergence may appear natural without using a facial ridge lap crown. B, Implants are used to restore the maxillary canine and first premolars. Natural tooth crowns restore the second premolar and first molar. The canine and first premolar have a reduced lingual contour compared with the crowns on natural teeth.