The ideal number of occlusal contacts in different occlusal schemes varies. For example, Peter K. Thomas' occlusal theories suggest that there should be a tripod contact on each occluding cusp (stamp cusp), on each marginal ridge, and in the central fossa with 18 and 15 individual occlusal contacts on a mandibular and maxillary molar, respectively77 (Figure 31-56). Other occlusal contact schemes indicate the number of occlusal contacts for molars may be reduced to five or six contacts, including the dominant cusp (stamp cusp) of the buccal cusps in the mandible and the lingual cusps in the maxilla, the marginal ridges, and the central fossa.
FIGURE 31-56 The ideal number of occlusal contacts varies in the literature. As many as 15 to 18 tripod occlusal contacts have been designed.
Most laboratory technicians wax or bake the occlusal surface and do an occlusal adjustment with the opposing casts until the unrestored natural teeth are in occlusion. No thought is given to how many contacts should be present or where they should occlude. Hence, it is almost impossible for the dentist to control the number and locations of occlusal contacts.
Occlusal contact position determines the direction of force, especially during parafunction. A cantilevered load is a force applied on the mesial or distal from the implant, which acts as a fulcrum. An offset load is a force applied to the buccal or lingual and increases the stress to the implant system. An occlusal contact on a buccal cusp of a mandibular premolar and molar or lingual cusp in the maxilla is usually an offset load when the implant is positioned under the central fossa because the occluding cusp is cantilevered from the implant body (Figure 31-57). In addition, the angle of the cusp tip also introduces an angled load to the implant body.
FIGURE 31-57 An offset load to the implant body increases the stress to the implant system. A buccal cusp occlusal contact in the posterior mandible or lingual cusp contact in the maxilla is an offset load to the implant. B, Buccal; Fn, buccal cusp contact; Fi, central fossa contact; L, lingual.
The most common implant body position for a posterior implant is in the middle of the buccolingual dimension of the bone. The surgeon begins the osteotomy in the middle of the ridge, and the implant diameter maintains 1.5 mm or more of bone on each side. The center of an edentulous ridge more often corresponds to the central fossa of a posterior crown in either arch. On occasion, it may correspond to the natural tooth lingual cusp region but less often is under the buccal cusp in either arch.
The marginal ridge contacts are also a cantilever load on the single-tooth implant crown because the implant is not under the marginal ridge but may be several millimeters away. If the implant body is 5 mm in diameter and replaces a 12-mm molar in the mesiodistal dimension, a marginal ridge contact may create a magnified moment load equal to 3.5 mm times the amount of the force. Hence, a 100-N load will be multiplied to a 350–N-mm force on the marginal ridge (Figure 31-58).
FIGURE 31-58 The ideal occlusal contact on a single-tooth implant crown is directly over the implant. A marginal ridge occlusal contact is an offset load similar to the lingual cusp in the posterior maxilla.
The mesiodistal dimension of the molar crown often exceeds the buccolingual dimension, so the marginal ridge contact may contribute more to the biomechanical risk. In addition, the laboratory often creates an all-porcelain marginal ridge completely unsupported by the metal substructure, which places a shear load on the porcelain. The shear loads further increase the risk of porcelain fracture. The moment forces on marginal ridges also may contribute to forces that increase abutment screw loosening. Therefore, marginal ridge contacts on individual implant crowns or the most mesial or distal splinted crown should be avoided whenever possible.
The marginal ridge occlusal contact is not a cantilevered load when located between two implants splinted to each other. In addition, the metal framework that splints the implants supports the porcelain in the marginal ridge region and minimizes the risks of fracture. The splinted crowns decrease occlusal forces to the crestal bone, reduce abutment screw loosening, decrease the force to the cement interface, increase retention of the crowns, and reduce the force to the bone–implant interface. Hence, adjacent implant crowns should most often be splinted together, and the occlusal contact position may be extended from the most mesial to most distal implants (minus the marginal ridges at each extreme) (Figure 31-59).
FIGURE 31-59 When the implants are splinted together, the occlusal contacts ideally should be in the central fossa, over the implant bodies, and in a zone extending between the implants. The marginal ridges between the implants may also have a secondary occlusal contact.
A posterior screw-retained restoration often requires cantilevered occlusal contacts. The occlusal screw hole rarely is loaded because the obturation material easily wears or fractures. As a result, the occlusal contacts of screw-retained crowns are not often directed over the top of the implant but are offset several millimeters away. This results in a higher moment load to the implant system, yet the occlusal access hole is the best position for the occlusal contact.
The average number of occlusal contacts found on natural posterior teeth of individuals never restored or equilibrated by a dentist and with no occlusal-related pathologic condition has been observed to average only 2.2 contacts (Figure 31-60) with a range of one to three occlusal contacts per tooth.78 If the tooth had an occlusal restoration, the occlusal contact number was reduced to an average of 1.6 occlusal contacts. The number of occlusal contacts on a natural tooth apparently may be reduced to one to three contacting areas without consequence. Therefore, a more simplified occlusal approach than often taught is logical. Hence, if the ideal occlusal contacts per tooth should have minimum offset loads to the implant body, the central fossa is the logical primary occlusal contact position when the implant is positioned in this region.
FIGURE 31-60A to C, An average of 2.2 occlusal contacts (range, 1–3) usually is found on natural posterior teeth.
The central fossa of an implant crown should be 2 to 3 mm wide in posterior teeth and parallel to the occlusal plane. The ideal implant body position for function is most often directly under the central fossa in the mandible and maxilla. The ideal primary occlusal contacts therefore will reside within the diameter of the implant within the central fossa. Secondary occlusal contacts should remain within 1 mm of the periphery of the implant to decrease moment loads. Marginal ridge contacts usually should be avoided unless implants are splinted together. When the implant is positioned closer to a stamp cusp (buccal in the mandible and lingual cusp in maxilla), the cusp angle is flat and the contact is over the implant (Figure 31-61).
FIGURE 31-61A, The occlusal contact position is ideally directly over the implant. When under a cusp tip, the cusp angle is more flat. B, The implant crowns are reduced from the lingual compared with the natural tooth molar crown.
On occasion, when a maxillary posterior tooth is in the esthetic zone, the implant may be 1 to 2 mm to the facial aspect of the midcrest (when bone is abundant) to be closer to the buccal cusp in order to improve the esthetic emergence of maxillary implant crowns. Under these conditions, the central fossa is positioned more facial, the lingual contour of the crown is reduced, and the occlusal contact is over the lingual aspect of the implant body (which is under the central fossa).
Timing of Occlusal Contacts
The most common method a dentist uses to determine the timing of occlusal contacts at the prosthesis delivery is to ask the patient, “How does the bite feel? Is the crown too high?” Jacobs and van Steenberghe evaluated occlusal awareness by the perception of an interference.79,80 When teeth oppose each other, an interference is perceived at approximately 20 microns.24,25 An implant opposing a natural tooth detects an interference at 48 microns; therefore, it is more than twice as poor. An implant crown opposing an implant crown perceives the interference at 64 microns, and when a tooth opposes an implant overdenture, the awareness is 108 microns (five times poorer than teeth opposing each other). Mericske-Stern et al. measured oral tactile sensitivity with steel foils.81 The detection threshold of minimal pressure was significantly higher on implants than on natural teeth (3.2 vs. 2.6 foils). Similar findings also were reported by Hammerle et al. in which the mean threshold value for implants (100.6 g) was 8.75 times higher than that of natural teeth (11.5 g).82 An occlusal adjustment performed by occlusal awareness—“How does the bite feel? Is the implant crown high?”—is a poor indicator for hyper contacts compared with a crown on a natural tooth. As a consequence of decreased quantity and quality of occlusal awareness, a premature occlusal contact may remain on an implant crown after occlusal adjustment.
Controversy has been ongoing regarding whether a rigidly fixated implant may remain successful when splinted to natural teeth.83,84 Because the implant has no periodontal membrane, concerns center around the potential for the “nonmobile” implant to bear the total load of the prosthesis when joined to the “mobile” natural tooth. The mobility of potential natural abutments joined to implants may influence the treatment more than almost any other factor. However, the biomechanical concern for the difference in tooth movement and implant movement should not be restricted to situations in which these entities are directly connected within the same prosthesis. When an implant is placed in a partially edentulous arch, many similar biomechanical elements are present, whether the teeth are splinted to the implant or are independent.
The sudden, initial (primary) tooth movement ranges from 8 to 28 microns in a vertical direction under a 3- to 5-lb load, depending on the size, number, and geometry of the roots and the time elapsed since the last load application.19,20 This tooth movement has been called “primary” tooth movement and is a result of the movement within the periodontal complex. An implant has no initial or primary vertical tooth movement. An implant with a heavy bite force may move apically up to 5 microns. When the initial tooth movement occurs, secondary tooth movement is present during a greater load and reflects the viscoelastic property of the surrounding bone. The vertical secondary tooth movement is minimal and may approach 3 to 5 microns for a natural tooth (Figure 31-62).
FIGURE 31-62 The physiologic vertical movement of a natural tooth is 28 microns with a light force (F). An implant has up to 5 microns of vertical movement but requires a heavy occlusal load.
The secondary tooth movement is similar to the bone–implant movement. In other words, the initial axial movement during a light bite force of an implant has no initial, sudden movement. Contrary to the teeth that move immediately, even with light loads, implants only move under a heavy occlusal load and even under these conditions have almost no mobility. The implant may move up to 5 microns after additional force causes the bone to deform, with little correlation of movement to the implant body length.21 In fact, the mobility of implant “secondary” movement is more related to bone density than any other factor.
When teeth oppose each other, the combined intrusive movements of the contacting elements may be 56 microns (28 + 28 microns). When a tooth opposes an implant, the initial combined intrusive movement is only 28 microns (28 + 0 microns). In other words, when implant prostheses oppose natural teeth, the difference in movement between teeth in the rest of the mouth and the implants causes a condition with greater loads to the implant.
Under a light load, the total combined implant movement when implant crowns oppose each other may remain at 0 microns compared with 56 microns in the rest of the mouth. Therefore, although the occlusal contact design for the natural teeth may be ideal under a light load, premature-like occlusal contacts may exist on the implants, especially with a greater bite force. Because the initial difference in vertical movement of teeth and implants in the same arch may be as much as 28 microns, the initial occlusal contacts should account for this difference, or the implant will sustain greater loads than the adjacent teeth.
The dentist should first evaluate the existing occlusion before implant reconstruction and ideally eliminate occlusal prematurities on teeth before the final evaluation of the occlusion on the implant reconstruction. A decision is then made whether an MI or CO is desired before implant placement.
At the delivery of the implant prostheses, any premature contact on the implant restoration should be eliminated. It is interesting to note that a coating of petroleum jelly on the articulating paper will help release the dye and allow more precise occlusal contact identification on the teeth and implant restoration (Figure 31-63). After this step, the dentist uses thin articulating paper (less than 25 microns thickness) for the initial implant occlusal adjustment in occlusion under a light tapping force (Figure 31-64). The implant prosthesis should barely contact during this light bite force, and the surrounding teeth in the arch should exhibit greater initial occlusal contacts. In other words, only light axial occlusal contacts should be present on the implant crown.
FIGURE 31-63 Petroleum jelly applied to articulating paper helps release the dye and makes the occlusal marks more specific.
FIGURE 31-64 A light occlusal force is applied first to the implant and teeth. The first molar implant crown has less initial contact than the teeth.
After the equilibration with a light bite force is completed, the patient applies a heavier occlusal force and grinds on the articulating paper (Figure 31-65). A plastic articulatory paper is a benefit, so the “paper” will not tear during the heavy bite and grind force on the teeth (e.g., 20 micron, Accufilm; Parkell, Farmingdale, NY). The occlusal contact point on the implant crown should remain axial over the implant body and may be of similar intensity on the implant crown and the adjacent teeth. When greater bite forces are used, all of the occlusal elements react similar under the heavy occlusal load. Hence, to harmonize the occlusal forces between implants and teeth, the dentist evaluates a heavy bite force occlusal adjustment because it depresses the natural teeth, positioning them closer to the less depressed implant position, and therefore permits equal sharing of the occlusal load.33
FIGURE 31-65 The first molar implant crown is evaluated with a heavy bite force during grinding movements (especially in a parafunction patient). The implant crown in this patient needs to be adjusted because the occlusal markings on the lingual cusps and marginal ridge are offset loads.
When all posterior teeth in one quadrant of the same arch are implant supported, the same occlusal timing is suggested. Under a light bite force, the occlusal contacts between the anterior and posterior teeth on the other side are slightly heavier in CO than the implant prosthesis. Under a heavy bite force in occlusion, similar contacts are created around the arch. To evaluate these occlusal contacts, a full-arch articulating paper is required (Figure 31-66).
FIGURE 31-66 To equilibrate the occlusion when multiple implants and natural teeth are in an arch, a full-arch articulating paper is required.
When implant prostheses oppose each other on one side of the mouth, the heavy bite force occlusal adjustment must account for a 56-micron difference in vertical movement between the opposing implant crowns and the rest of the natural teeth. Hence, the light bite force occlusal adjustment should again be performed with a full-arch-size articulating paper, and the implant–implant section should barely contact, but the tooth—tooth anterior and posterior sections have more occlusal contact. Under a heavy bite force in occlusion, similar occlusal contacts are present on both sides of the arch.
It is interesting to note that in a report on porcelain fracture associated with implant crowns, Kinsel and Lin found when the opposing dentition was a denture to an implant prosthesis, no fracture was reported.30 An opposing natural tooth had 3.2% implant crown fracture and a crown on a natural tooth 5.7% fracture, and when an implant crown opposed another implant crown, a 16.2% fracture rate was reported. Hence, the heavy bite force occlusal adjustment becomes more critical when both arches are involved with implant prostheses.
A complete-arch implant-supported prosthesis in one arch opposing complete natural teeth does not require a difference in a light and heavy bite force occlusal evaluation. Likewise, when implants support both maxillary and mandibular prostheses, a light and heavy bite force difference in occlusal timing is not required.
The initial lateral movement of healthy anterior teeth ranges from 68 to 108 microns before secondary tooth movement, or two to four times more movement than their apical movement20 (see Figure 31-4). Horizontal (lateral) implant movements are not immediate and with heavier forces range from 10 to 50 microns21 (Figure 31-67). Therefore, anterior teeth exhibit even greater differences in lateral movements compared with implants than posterior teeth. Hence, one follows a similar equilibration scenario when anterior implants and teeth are not connected and disocclude the posterior dentition during mandibular excursions.
FIGURE 31-67 When a gradually increasing load is applied to a tooth (top left) and an implant (top right), the range of movement is completely different. The tooth (bottom left) moves immediately under very little force (primary tooth movement). As the force intensity is increased gradually, the tooth gradually moves (secondary tooth movement). The primary tooth movement results from the periodontal ligament. The secondary tooth movement results from bone–tooth movement. The implant follows a gradual movement as the force gradually increases. The movement is similar to secondary tooth movement. The occlusal adjustment of implants and teeth in the same arch should compensate for the primary tooth movement, which is sudden and ranges from 56 to 108 microns in a horizontal dimension. The light occlusal contact evaluates the primary tooth movement. The heavy occlusal contact equilibration evaluates the secondary tooth movement and accounts for the slight implant movement.
When anterior teeth disocclude the posterior teeth in excursions, the lateral tooth movement of the posterior teeth (56–73 microns) does not have to be accounted for because no lateral force exists. Because anterior teeth and implants have lateral movement during mandibular excursion that results in greater discrepancies, the occlusal adjustment in this direction is more critical to the implant system. The dentist first uses light force and thin articulating paper to ensure that little to no implant crown contact occurs during the initial occlusal or lateral movement of the teeth. Then the dentist uses a heavier force during CO and excursions to develop similar occlusal contacts on anterior implants and natural teeth (Figure 31-68).
FIGURE 31-68A, The occlusal equilibration of an anterior implant crown is made first with a light occlusal contact in centric occlusion (CO) and during mandibular excursions. B, The anterior implant crown then is equilibrated under a heavy bite force in CO and during mandibular excursions. The difference between primary tooth movement and implant movement is greater in the anterior regions of the mouth.
To compensate for the difference in 100 microns of horizontal movement between maxillary anterior implants and anterior teeth, two modifications are required. The first is to enameloplasty the facial incisal contact of the mandibular incisal edge. The patient is told the height of the tooth is not reduced, only the facial incisal edge. Very often, when maxillary anterior tooth is lost, the opposing mandibular incisor shifts to the facial and makes the implant position and occlusal adjustment more critical. The second modification is often the lingual contour of a maxillary anterior crown is more concave than a natural tooth to accommodate the heavy bite force occlusal adjustment (Figure 31-69).
FIGURE 31-69 The lingual surface of a maxillary anterior implant crown often has a concave appearance to accommodate for the occlusal contact differences compared with the adjacent natural teeth.
The concept of a heavy bite force occlusal adjustment is underestimated by some practitioners. A comparison of the importance of this concept may be made with the restoration of a single posterior natural tooth with a crown. Can a restoring dentist insert a crown from the laboratory without an occlusal adjustment? Despite accurate impressions, bite registrations, face-bows, and full-arch mounted casts, the crown most always requires some occlusal adjustment. Why?
The laboratory cannot equilibrate the occlusion accurately on the working casts. The technician taps two stone casts together to evaluate the occlusal contacts. The stone dies do not move 28 to 108 microns. As a result, the occlusal adjustment in the mouth compensates for the primary and secondary tooth movement. When a heavy bite force occlusal adjustment is not performed at the delivery of an implant crown, the dentist may not be aware that the implant may be overloaded similar to a new crown on a natural tooth that has not been equilibrated in the mouth.
The proposed heavy bite force occlusal adjustment does not encourage tooth migration or changes in tooth position because regular occlusal contacts still occur. The teeth opposing implants are not taken out of occlusion. Brief occlusal contacts on a daily basis maintain the tooth in its original position (similar to the rest of the mouth). In addition, because most teeth in a skeletal class I occlusion occlude with two opposing teeth (with the exception of the mandibular central incisor), the opposing teeth positions are even more likely to remain in the same positions. In other words, the two opposing natural teeth to an implant crown still have occlusal contacts on the adjacent natural teeth to the implant. However, teeth do move over time. Unlike teeth, implants do not extrude, rotate, or migrate under occlusal forces. As such, the restoring dentist may vary the intensity of the force applied to an implant without causing the implant to change its position readily in the bone. On the contrary, natural teeth do exhibit mesial drift, and slight changes in occlusal position do occur over time.
No occlusal scheme will prevent mesial drift and minor tooth movement from occurring. In addition, enamel may wear approximately 30 microns each year. Therefore, an integral part of the IPO philosophy is the regular evaluation and control of occlusal contacts at each regularly scheduled hygiene appointment. This permits the correction of minor variations occurring during long-term function and helps prevent porcelain fracture and other stress-related complications (abutment screw loosening) on the implant prostheses.
Similar biomechanical considerations can be discussed for implants joined to natural teeth and a similar scenario is used for the occlusal equilibration. A light force and thin articulating paper are used, and the implant crown exhibits minimum contact compared with the natural abutment crown in occlusion. A gradient of force is designed on the pontics. A heavy bite force then is used to establish equal occlusal contacts for all the natural teeth and the entire prosthesis, whether implant or natural tooth supported. When possible, lateral forces on the implant abutments are discouraged even in the anterior regions of the mouth.