A proper occlusal scheme is a primary requisite for long-term implant prosthetic survival, especially when parafunction or a marginal foundation is present. A poor occlusal scheme increases the magnitude of loads and intensifies mechanical stresses (and strain) to the implant system. These factors increase the frequency of complications of the prosthesis and bone support. Crestal bone loss may lead to anaerobic sulcus depths and periimplant disease states. These conditions may also cause tissue shrinkage and loss of interdental papillae and poor esthetic conditions. All of these complications may be caused by biomechanical stress as a result of occlusal loads (functional or parafunctional).
The implant-protective occlusion (IPO) concept refers to an occlusal plan specifically designed for the restoration of endosteal implants, providing an environment for reduced biomechanical complications and improved clinical longevity of both the implant and prosthesis.32,33 The biomechanical rationale for this concept was published by the author after long-term clinical evaluation and biomechanical studies (and was originally called medial-positioned, lingualized occlusion).34 This concept was specifically designed for fixed prostheses in either partial or complete edentulous patients. Clinical considerations for this concept are drawn from basic prosthetic concepts, bone biomechanical principles, and finite element analyses to reduce noxious occlusal loads and establish a consistent occlusal philosophy.
A primary goal of an occlusal scheme is to maintain the occlusal load that has been transferred to the implant system within the physiologic and biomechanical limits of each patient. These limits are not identical for all patients or restorations. The forces generated by a patient are influenced by ranges of parafunction, masticatory dynamics, implant arch position and location, arch form, and crown height. The treatment plan philosophy for dental implants varies greatly and depends on these several parameters. The implant dentist can address these force factors best by selecting the proper position, number and implant size, increasing bone density when necessary by progressive bone loading, and selecting the appropriate occlusal scheme using stress-relieving design elements.
Implant and natural tooth position, number, size, and prosthesis design produce a myriad of possible combinations. However, consistent occlusal patterns may be established. The following guidelines are used to restore fixed implant–supported prostheses. A slightly different occlusal concept of the author is presented for complete dentures or removable prosthesis type 5 (RP-5) implant overdentures (implant and soft tissue support) and is called medial-positioned, lingualized occlusion (see Chapter 33).
The IPO principles for fixed prostheses address several conditions to decrease stress to the implant system, including existing occlusion, implant body angle to occlusal load, cusp angle of implant crowns, mutually protected articulation, cantilever or offset loads, crown height, crown contour, occlusal contact position, timing of occlusal contacts, and protection of the weakest component (Box 31-3).
• No premature occlusal contacts or interferences
• Mutually protected articulation
• Implant body angle to occlusal load
• Cusp angle of crowns (cuspal inclination)
• Cantilever or offset loads
Occlusal Considerations for Fixed Implant Prostheses
Maximal intercuspation (MI) is defined as the complete intercuspation of the opposing teeth independent of condylar position, sometimes described as the best fit of teeth regardless of the condylar position.24Centric occlusion (CO) is defined as the occlusion of opposing teeth when the mandible is in centric relation (CR).25 This may or may not coincide with the tooth position of MI. Its relationship to CR (a neuromuscular position independent of tooth contact with the condyles in an anterior, superior position) is noteworthy to the restoring dentist. The potential need for occlusal adjustments to eliminate deflective tooth contacts as the mandible closes in CR and the evaluation of their potential noxious effects on the existing dentition and the planned restoration is important to evaluate.
Correction of the deflective contacts before treatment presents many advantages and may follow a variety of approaches depending on the severity of the incorrect tooth position: selective odontoplasty (a subtractive technique), restoration with a crown (with or without endodontic therapy), or extraction of the offending tooth. The existing occlusion is best evaluated with facebow-mounted diagnostic casts on an articulator mounted with an open-mouth bite registration in CR. (This process was addressed in Chapter 16.)
Controversy exists as to the necessity to have MI harmonious with CO (CR occlusion). A vast majority of patients around the world do not have such a relationship, yet they do not exhibit clinical pathology or accelerated tooth loss. Therefore, it is difficult to state that these two positions must be similar. However, it is important to evaluate the existing occlusion and the mandibular excursions to consciously decide whether the existing situation should be modified or be maintained. In other words, dentists should determine whether they are going to ignore or control the occlusion of the patient (Figure 31-8).
FIGURE 31-8 The patient has mild bruxism (minimum tooth wear). The engram position of teeth wear indicates in the left excursion the mandibular first premolar occludes with the maxillary cuspid. The implant crowns for the posterior missing teeth should not have lateral forces. The cervical abfraction of the premolar and slight increase in mobility indicates that there is an increased biomechanical risk.
Many dentists begin to evaluate the occlusion of the patient when the final implant prosthesis is delivered to the patient. However, this time frame is often too late to properly restore the patient (Figure 31-9). The underlying question that helps determine the need for occlusal correction before restoration of the implant patient is the observation of negative symptoms related to the existing condition. This may include temporomandibular joint (TMJ) conditions, tooth sensitivity, mobility, wear, tooth fractures, cervical abfraction, or porcelain fracture.24,25 The fewer and less significant the findings, the less likely an overall occlusal modification is required before restoration of the patient. However, to properly assess these conditions, the dentist must not ignore them before treatment.
FIGURE 31-9A, A maxillary first molar is replaced with an implant. The lateral forces should be applied to the anterior teeth during mandibular excursions. B, The implant crown is seated, and the right mandibular excursion is evaluated. The premolars exhibit a slight working interference on the buccal cusps. The maxillary premolars have slight gingival recession and initial cervical abfraction regions below the cementoenamel junction. The excursive force is reduced when the posterior teeth do not interfere during excursions. Therefore, the buccal cusp inclines of the premolars should be reduced. If the incisal edge of the canine continues to wear in the future, the occlusal contacts will need to be modified further when the posterior teeth contact during excursions.
As a general rule, the more teeth replaced or restored, the more likely the patient is restored to CO. For example, if a completely edentulous mandible is to be restored with an implant-supported fixed prosthesis, CO provides consistency and reproducibility between the articulator and the intraoral condition. The slight changes in occlusal vertical dimension (OVD) and its relationship to the position of anterior implant abutments to the direction of force may be studied and implemented on the articulator without the need to record a new occlusal vertical position on the patient. On the other hand, when one anterior tooth is being replaced, the existing MI position is often satisfactory to restore the patient even though a posterior interference and anterior slide into full interdigitation may be present (with little clinical variance from the ideal conditions). However, in a partially edentulous patient, the existing occlusion should be evaluated to determine if noxious conditions are present.
Premature Occlusal Contacts
A fundamental biomechanical formula is stress equals force divided by the area over which the force is applied (S = F/A).17 Therefore, during either maximum intercuspation or CO, no occlusal contacts should be premature, especially on an implant-supported crown. Premature occlusal contacts often result in localized lateral loading of the opposing contacting crowns.35 Because the surface area of a premature contact is small, the magnitude of stress in the bone increases proportionately (i.e., S = F/A). All of the occlusal force is applied to one region rather than being shared by several abutments and teeth. In addition because the premature contact is most often on an inclined plane, the horizontal component of the load increases the shear crestal stresses and the overall amount of stress to the entire implant system. The occlusal porcelain, the abutment screw, and the cement retaining the crown are all at increased risk because shear loads render more complications.
This is a general criterion for natural teeth, but the concept is much more important on implant prostheses with their higher impact force and less occlusal awareness for the several reasons previously addressed. Myata et al. evaluated premature contacts on implant crowns in monkeys (Macaca fascicularis).11 The crestal bone was histologically evaluated on implant crowns with 100 microns, 180 microns, and 250 microns of premature contacts for 4 weeks.12 The crowns with 100-micron premature contacts had little bone changes. The 180-micron group demonstrated a V-shaped pattern of bone loss for several millimeters. The 250-micron implant crowns for 4 weeks had a large V-shaped defect around the implants that extended for more than two thirds of the implant body (Figure 31-10). The implant is rigid, and the premature implant load cannot be released by increased mobility or occlusal material wear as with a natural tooth.
FIGURE 31-10A, An implant crown with a 100-micron premature contact for 4 weeks displayed little crestal bone change. B, An implant crown with a 180-micron premature contact for 4 weeks had 2 to 3 mm of crestal bone loss. C, An implant crown with 250-micron premature contact for 4 weeks had marginal bone loss of more than two thirds of the implant length.
Isidor et al. evaluated excessive premature contacts on implants in monkeys over a 20-month period on eight integrated implants.13 Implant failure occurred in six of eight implants between 2 and 14 months. The implants that did not fail had greater bone density and crestal bone loss with osteoclastic activity within the threads of the implants (Figure 31-11). The premature contact on an implant system contributes to a higher risk of early abutment screw loosening, porcelain fracture, early loading failure, and crestal bone loss.
FIGURE 31-11A, Three fourths of integrated implants with premature occlusal contacts failed from 2 to 14 months after loading. B, The implants that did not fail had crestal bone loss. C, The implants that did not fail had osteoclastic activity within the threads. (From Isidor R: Histological evaluation of peri-implant bone at implants subjected to occlusal overload or plaque accumulation, Clin Oral Implants Res 8:1–9, 1997.)
The elimination of premature occlusal contacts is especially important when habitual parafunction is present because the duration and magnitude of occlusal forces are increased. The elimination of premature contacts is more critical than in natural teeth because of the lack of proprioception and the implant inability to move and dissipate the forces. Because of increased proprioception, an initial premature occlusal contact on a tooth often affects the closure of the mandible to result in an MI position different from CO. A premature contact on an implant crown does not benefit from such protective features; as a result, the implant system is at increased risk. Therefore, occlusal evaluation in CO and MI and adjustment as necessary in partially edentulous implant patients are more critical than in natural dentition because the premature contacts can result in more damaging consequences on implants compared with teeth.36
Implant Body Orientation
Forces acting on teeth and dental implants are referred to as vectors (defined in magnitude and direction).17 Occlusal forces are typically three dimensional, with components directed along one or more of the clinical coordinate axes. The primary forces of occlusion can be resolved into a combination of components in any given plane. The same magnitude of force can have dramatically different effects on the implant system, solely because of the direction of the applied load. This is especially noted on implant support systems because they are more rigid.
The teeth are designed primarily for long-axis loads. The natural tooth roots in the majority of the mouth are perpendicular to the curves of Wilson and Spee. Although chewing is in an elliptical “tear drop” pattern, when the teeth finally contact, the forces are in the long axis of the roots, especially during power biting (Figure 31-12). The apical movement of teeth is minimal compared with their lateral movement. The maxillary anterior teeth receive a lateral load. The consequences of a lateral force to a tooth are reduced because of the increased tooth mobility, which decreases the effects of the lateral force component of a load.
FIGURE 31-12 The natural teeth occlude perpendicular to the curves of Wilson and Spee.
Implants are also designed for long-axis loads. Two-dimensional finite element analysis by Binderman in 1970 evaluated 50 endosteal implant designs and found that all designs sustained stress contours concentrated primarily at the transosteal (crestal) region.37 In addition, less stress was observed under a long-axis load compared with angled loads. Since then, two- and three-dimensional finite element analyses by several authors have yielded similar results38–51 (Figure 31-13).
FIGURE 31-13 A three-dimensional finite element analysis of an implant with a long-axis load. The stresses are mainly at the crestal region and primarily have compressive forces.
An axial load over the long axis of an implant body generates less overall stress and a greater proportion of compressive stress compared with an angled force to the implant body. When an implant body is loaded along its long axis, a 100-N force results with an axial force component of 100 N, and no lateral force component is observed. Therefore, the implant body should be positioned perpendicular to the curves of Wilson and Spee, just as with natural teeth.
Most anatomical variations of the bone (e.g., bony concavities) are located on the facial aspect and influence implant body inclination. An implant body may be positioned with a 15-degree angle to avoid the facial concavity and therefore is positioned at 15 degrees to the occlusal load. This angled implant may be restored during prosthetic reconstruction with a 15-degree angle abutment. From the level of the crest of the ridge to the occlusal plane, the implant abutment looks similar to one in an axial implant body. Hence, the laboratory technician and restoring dentist often treat the angled implant and axial implant in similar fashion. However, in the 15-degree angled implant body, the load to the facial bone increases by 25.9% compared with an axial load32 (Figure 31-14). If the implant surgeon places the implant body with a 30-degree angulation, the buccal force component of any occlusal load will result in a 50% increase of the load applied to the facial bone.32
FIGURE 31-14 An implant loaded in the long axis does not increase the buccal force component of the load (far left). A 15-degree angle increases the buccal force component by 25.9% (middle). A 30-degree angle load increases the force by 50%. When the forces are applied along the long axis of an implant body, stresses are concentrated on the crestal region (far left). The intensity of the stress is not increased as a result of the position of the implant. The implant body in the center is 15 degrees off the long axis. With an angled abutment of 15 degrees, the implant restoration is similar to the previous situation. However, now 25.9% greater stress is on the crestal bone; all other factors are similar. The implant body on the far right is 30 degrees off the long-axis load. With a 30-degree angled abutment, the crown may appear similar. However, the abutment screw, abutment–implant connection, and implant–bone interface are subject to a 50% increase in stress on the facial aspect of the system. (From Misch CE: Contemporary implant dentistry, ed 2, St Louis, 1999, Mosby.)
Hence, the risk of crestal bone loss is increased with an angled implant.52,53 In addition, the greater force is applied to most of the implant system. The occlusal porcelain may be loaded in the long axis with the angled abutment, but the abutment screw loosening and implant component fracture risks increase in direct comparison to the load applied to the bone. Therefore, although the restoring dentist may place a 30-degree angled abutment and restore the case similar to the axial implant, the conditions and risks of early loading failure, crestal bone loss, and loose abutment screws are dramatically different (Figure 31-15).
FIGURE 31-15 As the angle of the implant body load direction increases, the stresses to the entire crown implant–bone system increase. B, Buccal; L, lingual. (From Misch CE: Contemporary implant dentistry, ed 2, St Louis, 1999, Mosby.)
Force Direction and Bone Mechanics
The noxious effect of offset or angled loads to bone is exacerbated further because of the anisotropy of bone. Anisotropy refers to the character of bone whereby its mechanical properties, including ultimate strength, depend on the direction in which the bone is loaded and the type of force applied. For example, cortical bone of human long bones has been reported as strongest in compression, 30% weaker in tension, and 65% weaker in shear54 (Figure 31-16). Porcelain, titanium components, and cements are also weakest to shear components of a load. Therefore, IPO attempts to eliminate or reduce all shear loads to the implant system because the bone, porcelain, titanium components, and cement are weakest to shear loads.
FIGURE 31-16 The strength of the bone before fracture depends on the type of force applied to the bone. Bone is strongest to compression forces, 30% weaker to tensile forces, and is only 35% as strong to shear loads. Therefore, whenever possible, bone should be loaded with compressive loads. (From Misch CE: Contemporary implant dentistry, ed 2, St Louis, 1999, Mosby.)
Any occlusal load applied at an angle to the implant body may be separated into normal (compressive and tensile) and shear forces. As the angle of load to an implant body increases, the amount of compressive and tensile forces is modified by the cosine of the angle. Hence, the force is slightly reduced. However, the angled component of force is a shear force, and the shear force is the amount of force times the sign of the load, which considerably increases the load. The force the bone observes is the sum of the compressive, tensile, and shear forces. For example, a 100-N force applied at 12 degrees off-axis will increase the total force to the bone by 100 N × cosine 12 degrees = 97.81 N + 100 N × Sine 12 degrees = 20.79 N. The total force is 97.81 N + 20.79 N = 118.60 N (or almost a 20% increase in total force).The greater the angle of load to the implant long axis, the greater the compressive, tensile, and shear stresses (Figure 31-17).
FIGURE 31-17 A 12-degree angled force increases the force to the implant system by 18.6%.
In finite element analysis, when the direction of the force changes to a more angled or horizontal load, the magnitude of the stress is increased by three times or more.51,52 In addition, rather than a primarily compressive type of force, tensile and shear components are increased more than 10-fold compared with the axial force. In a photoelastic block with implants inserted, the strain contours in the bone may be observed (Figure 31-18). The axial-loaded implants have less strain in the system (left side and lower right of figure). The angled implant has more strain lines indicating greater loads (right upper implant).
FIGURE 31-18 A photoelastic study of opposing implants in a maxilla and mandible. One of the maxillary implant bodies is angled in relation to the direction of load. The number of stress contour lines in the material is similar for the three implants with a long-axis load. The stress contour lines are increased for the angled implant body.
An angled load to the implant long axis increases the compressive forces at the crest of the ridge on the opposite side of the implant, increasing the tension component of force along the same side as the load. The greater the angle of force to the long axis of the implant body, the greater the potentially damaging load at the crest of the bone. For example, three-dimensional finite element analysis demonstrates that a vertical load on an implant with 100% bone contact may have compressive stress of 4000 psi (27.6 MPa) and almost no tensile stress at the bone-to-implant crest interface.51 With a load at a 45-degree angle on the same implant design, the compressive stress may increase to 14,000 psi (96.6 MPa), and on the opposite side, tensile stress may increase to 4000 psi (27.6 MPa). Hence, the compressive stresses are tripled, and the tensile stress increases 1000-fold with a load from a 45-degree angle.
The stress contours in the bone simulant of the three-dimensional studies resemble the clinical pattern of early crestal bone loss on implants. Therefore, not only does the magnitude of stress increase under angled loads, but it also evolves into a more noxious shear component, which is more conducive to bone loss and screw loosening.39 The greater the angle of the force, the greater the shear component. Bone is 65% weaker to shear load. Hence, the amount of the force increases, and the strength of the bone decreases. It has been reported that angled occlusal forces decrease the ability of successful bone repair on natural teeth. It may also impair successful bone remodeling around an implant.55
Not only is the bone weakest to shear loads, but forces applied at an angle to the bone also further affect the physiologic limit of compressive and tensile strengths of bone.54,56 A force applied at a 30-degree angle may decrease the bone strength limits by 10% under compression and 25% with tension (Table 31-2). A 60-degree force reduces the strength 30% under compression and 55% under tension. Therefore, not only does the crestal bone load increase around the implant with angled forces, but also the amount of stress the bone may withstand (i.e., the ultimate strength) decreases in shear, tension, and compression. The greater the angle of load, the lower the ultimate strength of bone. Therefore, IPO attempts to eliminate lateral or angled loads to an implant-supported prosthesis because the magnitude of the force increases and the strength of the bone decreases.