Bone Height

The available bone should be closely evaluated because it will greatly influence the soft tissue drape, implant size, implant position (angulation and depth), and ultimately the final esthetic outcome. Hard tissue topography is a prerequisite to an optimal, esthetic implant restoration. Available bone volume is necessary, and the position of the osseous crest is specific. The model midcrestal position of the edentulous site should be 2 mm below the facial CEJ of the adjacent teeth. On occasion, the bone crest may be above this position when the interproximal bone height is higher and/or a bone graft was performed at extraction. The interproximal bone should be scalloped 3 mm more incisal than the midcrestal position.

The position of the interproximal crest of bone is an important anatomical consideration, especially for the development of the interproximal soft tissue height.^16,17 Becker et al.¹⁸ classified the range of interproximal bone height above the midfacial scallop from less than 2.1 mm (flat) to scalloped (2.8 mm) to pronounced scalloped (<4.1 mm). The flat anatomy should correspond to a square-shaped tooth, the scalloped to an ovoid-shaped tooth, and the pronounced scalloped to a triangular-shaped tooth (Figure 33-3). However, these relationships do not always exist. When a flat interdental-to-crest dimension is found on triangular teeth, the interproximal space will usually not be filled with soft tissue, because the dimension of the interproximal contact to the bone will be greater than 5 mm (Figure 33-4).²¹

Figure 33-3 A triangular tooth form corresponds to the greatest width and height of interdental papillae and the most incisal interproximal contact position on the crown.

Figure 33-4 When natural teeth have an interproximal crown contact to interseptal bone distance of 5 mm or less, the interdental papilla almost always fills the interproximal space. When the distance is 6 mm, the interproximal space is not filled with soft tissue almost 40% of the time; at 7 mm the interproximal space is filled with an interproximal papilla 25% of the time.

Often the osseous crest may be more apical than ideal, in both the implant site and the adjacent tooth roots. Under these conditions, ideal crown contour, soft tissue emergence, and interproximal tissue conditions are less likely (Figures 33-5 and 33-6). Bone and soft tissue changes after maxillary anterior tooth loss are rather rapid and of considerable consequence. As a result, many maxillary anterior edentulous sites require at least some bone and/or soft tissue modification before, in conjunction with, and/or at implant uncovery. Under perfect conditions, the implant body should not be inserted until the bone and soft tissue are within normal limits.

Figure 33-5 A periapical radiograph of an implant and crown placed in the maxillary right lateral incisor region. The interproximal bone adjacent to the canine is ideal; the interproximal bone on the central incisor is 4 mm below the cement-enamel junction (CEJ).

Figure 33-6 The lateral incisor implant crown from Figure 33-5. The interproximal distal papilla is acceptable as a result of the canine crestal bone level. The mesial papilla is depressed, and a “black hole” is evident. This is related to the crestal bone level on the central incisor. In addition, the triangular tooth form translates into a more incisal interproximal crown contact position, which further enhances the triangular defect. The distal bone loss on the central also increases the risk of root exposure (as shown).

Mesiodistal Space

An adequate mesiodistal space is necessary for an esthetic outcome of an implant restoration and the interproximal soft tissue of the adjacent teeth. A traditional two-piece implant should be at least 1.5 mm from an adjacent tooth. When the implant is closer than this to an adjacent tooth, any bone loss related to the microgap, the biological width, and/or stress might cause the implant and adjacent tooth to lose bone. This may compromise interproximal esthetics and/or sulcular health of the implant and natural tooth^19–21 (Figure 33-7).

Figure 33-7 If bone loss occurs on an implant placed closer than 1.5 mm to a tooth (on the distal), then bone will also be lost on the tooth. As a result, the distance from the interproximal crown contact to the interproximal bone increases, and the risk of soft tissue shrinkage and loss of interdental papilla increases. When the implant is greater than 1.5 mm from the tooth (on the mesial); bone loss on the implants does not cause bone loss on the tooth root. The interproximal crown contact to interproximal bone relationship remains ideal, and the interdental papilla is maintained.

The smallest-diameter implant body offered by most commercial companies is 3.2 mm. However, the crest module of these two-piece implants is usually 3.5 mm or more. Therefore the mesiodistal edentulous space for a two-piece implant should be 6.5 mm or greater.

One-piece dental implants may be fabricated in 2.5-to 3.0-mm^22–24 diameters to accommodate a reduced mesiodistal dimension criterion. These implant designs do not have a microgap, and the vertical defect is narrower than most two-piece implant systems. As such, they may be placed as close as 1 mm from an adjacent tooth, and therefore can accommodate a 5-mm mesiodistal missing tooth space. However, because these implants have the abutment post attached to the implant body, they must be immediately restored the day of surgery. These implants are addressed in Chapter 35.

Faciopalatal Width

Most of the conditions that lead to single-tooth loss result in the loss of some or all of the facial bone in the region of the missing tooth. In addition, a 25% decrease in faciopalatal width occurs within the first year of tooth loss and rapidly evolves into a 30% to 40% decrease within 3 years. As a result, even an intact alveolus 6 to 8 mm wide is often inadequate in width after 1 year for a Division A root form implant in a central incisor position, and after 3 years it almost never presents adequate available bone for the proper-size implant. The bone width loss is primarily from the facial region, because the labial plate is very thin compared with the palatal plate, and facial undercuts are often found over the roots of the teeth (Figure 33-8). A bone graft is often necessary to restore the proper anatomy of the ridge and to avoid a compromised implant position more palatal and apical.

Figure 33-8 Before the loss of a maxillary anterior tooth, the bone around the roots most often is present (A). The cause of extraction often causes a loss of the thin labial plate of bone over the root (B). After extraction, the residual ridge most often is decreased in width (Division B). After 6 months to 1 year, the residual ridge continues to resorb and becomes Division B–w (C). Eventually, the residual ridge forms a C–w bone volume that is slightly deficient in height and less than 2.5 mm in width (D). This bone volume often extends almost to the floor of the nose (E).

The amount of available bone width (faciopalatal) should be at least 2.0 mm greater than the implant diameter at implant insertion. Therefore a 3.5-mm implant requires at least 5.5 mm of bone width. Bone augmentation in width is very predictable. In many instances it is performed before implant placement; however, it may be performed at the time of implant insertion, especially when no dehiscence of the implant is visible. It should be emphasized the implant diameter measurement is at the crest module of the implant. Most 3.75-mm diameter implant bodies are 4.1 mm at the crest module. In these situations, the mesiodistal limitation is 7.1 mm and the faciolingual width limitation is 6.1 mm.

Implant Size

The first factor that influences the size of an implant is the mesiodistal dimension of the missing tooth. The average mesiodistal dimension of a central incisor is 8.6 mm for a man and 8.1 mm for a woman, a lateral incisor is 6.6 mm for a man and 6.1 mm for a woman, and a canine is 7.6 mm for a man and 7.2 mm for a woman. Obviously, the implant body should not be as wide as the natural tooth or clinical crown. Otherwise, the emergence contour and interdental papillae region cannot be properly established.

The mesiodistal dimensions of the maxillary central incisor at the cervix (preferably 1 mm below the free gingival margin) averages 6.4 mm, the lateral incisor dimension is 4.7 mm, and canine natural teeth at the cervix are 5.6 mm (Table 33-1).^25,26 However, these dimensions are also too large for an implant.

Table 33-1 Maxillary Teeth Dimensions

The bone level on natural teeth is 2 mm below the CEJ; the natural tooth dimensions at this bone level are reduced to 5.5 mm for central incisors, 4.3 mm for lateral incisors, and 4.6 mm for canines. Therefore in theory, the latter dimensions most closely resemble the consummate implant diameter to mimic the emergence profile of a natural tooth. However, this dimension is usually too large to adequately restore the soft tissue drape of the missing anterior tooth.

The second factor that determines the mesiodistal implant diameter is the necessary distance from an adjacent tooth root.²⁰ Initial vertical bone loss around an implant during the first year of loading is variable and ranges from 0.5 to more than 3.0 mm.

The height of the interseptal (interimplant) bone in part determines the incidence of presence or absence of the interdental papillae between the teeth.²¹ When the distance from the interseptal bone to interproximal contact is 5 mm or less, the papilla fills the space. When the distance is 6 mm, a partial absence of papilla is seen 45% of the time, and at 7 mm the risk of a compromise in the interproximal space is 75%. Therefore the intraseptal bone height is relative to the maintenance of the interdental papilla and should be preserved. As a consequence, the implant should be at least 1.5 mm from the adjacent teeth whenever possible, and the interseptal bone on the adjacent teeth should be within 5 mm of the desired interproximal crown contact position.

In summary, two mesiodistal parameters determine the preferable implant size. The suggested width of the single-tooth implant should correspond to the width of the missing natural tooth, 2 mm below the CEJ. The distance between the roots of the adjacent teeth should also be measured. The implant diameter + 3 mm (1.5 mm on each side) should be equal to or less than the distance between the adjacent roots, at the crest of the ridge (which is 2 mm below the interproximal CEJ).

The next dimension that determines the width of an anterior implant is the faciopalatal dimension of bone. The width of bone should allow at least 1.5 mm on the facial aspect of the implant so that if a vertical defect forms around the crest module, then that defect would not become horizontal and change the cervical contour of the facial gingival. Because of its initial reduced volume, facial bone tends to be labile, and its resorption is responsible for most of the compromised long-term esthetic results in the anterior maxilla.^27–29 The faciopalatal width dimension is not as critical on the palatal aspect of the implant, because it is dense cortical bone, more resistant to bone loss, and not within the esthetic zone. Facial bone grafting at the time of implant insertion is frequently needed, because the bone volume in width is often compromised (Figures 33-9 to 33-11).

Figure 33-9 An implant placed in the central incisor position, with less than 1.5 mm of bone on the facial.

Figure 33-10 A layer of freeze-dried bone (MinerOss) is placed over the autograft on the facial bone over the implant.

Figure 33-11 A layer of autograft, collected from the drills during osteotomy preparation, is placed on the facial bone over the implant.

The width of the implant should not only mimic the emergence of a natural tooth, but also help to preserve the bone and health of the adjacent teeth. The natural intraroot distance of the two central incisors distance is approximately 2 mm. However, the natural roots of the central to lateral and lateral to canine are usually less than 1.5 mm apart and often only 0.5 mm of space exists between them. As a consequence, the archetypal size of the single-tooth implant is usually smaller in diameter than the natural tooth root.

The typical diameters of the implant used to replace the average-size tooth often results in a 4.0- to 5.2-mm implant for a central incisor, a 3.0- to 3.5-mm implant for a lateral incisor, and a 3.7- to 4.2-mm implant for a canine. The difference in the emergence profile of a 4-mm-diameter implant and a 5-mm-diameter implant is negligible and often not clinically relevant for an anterior tooth, because 0.5-mm difference occurs on each side of the implant. Therefore when in doubt, the surgeon should use a smaller-diameter implant. As such, a 4-mm-diameter implant may often be used in the central-implant position for a single-tooth replacement. Likewise, a 3.0- to 3.5-mm implant is often used for a lateral incisor single-tooth restoration.

Mesiodistal Position

The maxillary anterior single-tooth implant should be located precisely in three planes. From a mesiodistal aspect, the implant most often is placed in the middle of the space, with equal amount of interproximal bone toward each adjacent tooth. On occasion, the central incisor implant is positioned slightly to the distal of the intratooth space (Figure 33-12). The incisive foramen is variable in size and position. When a central incisor implant is planned and the foramen between the existing central incisor root and implant site is larger than usual, the remaining bone may be inadequate. The foramen may also expand off to one side of the midline within the bony canal. When the central incisor implant is placed, the implant may encroach on the canal and result in a probing pocket depth of 10 mm or greater on the mesiopalatal surface of the implant. As a precaution, the surgeon should reflect the palatal tissue when placing a maxillary central incisor implant, probe the foramen, and if necessary, place the implant in a more distal position. This usually requires a smaller-diameter implant than usual to remain 1.5 mm or more from the lateral incisor. On occasion, the contents of the foramen must be removed and a bone graft inserted, to decrease the size of the incisive canal (Figure 33-13).

Figure 33-12 The best implant position is under the incisal edge of the final crown, or slightly more palatal (A). The ideal mesiodistal implant position for a central incisor is 0.5 to 1.0 mm more distal than the midtooth position. This decreases the risk of encroachment on the incisive canal (B). The best mesiodistal position for a cuspid is centered in the cuspid position (C).

Figure 33-13 On occasion, the incisive canal extends into the site of the missing central incisor. As a result, the foramen should be enucleated and filled with a bone graft to restore the site and allow implant placement.

Faciopalatal Position

The midfaciopalatal position of the implant is in the middle to slightly palatal 0.5 mm of the edentulous ridge of adequate contour. This approach permits the use of the greatest-diameter implant. The crestal bone should be at least 1.5 mm wider on the facial aspect of the implant and 0.5 mm on the palatal aspect. Therefore for a 4-mm-diameter implant, a minimum 6-mm faciopalatal width of bone is required for the central or canine position, and 5.5 mm of bone width is required for a lateral incisor with a 3.5-mm implant. Bone spreading in conjunction with implant placement or bone grafting on the facial aspect of the edentulous site may be used when the ridge is less wide than is desirable. The thickness of bone on the facial aspect of a natural root is usually 0.5 mm thick. As a result, the implant is 1 mm or more palatal than the facial emergence of the adjacent crowns at the free gingival margin.

Implant Angulation

The implant center is positioned in the faciopalatal center of the edentulous ridge and the midmesiodistal position. The implant body angulation from this point is considered next. In the literature, three faciopalatal angulations of the implant body are suggested: (1) a facial angulation so that emergence of the final crown will be similar to adjacent teeth, (2) under the incisal edge of the final restoration, and (3) within the cingulum position of the implant crown (Figure 33-14).

Figure 33-14 Three implant positions are found in the literature related to the final crown position. A, A position below the incisal edge is best used for a cemented crown in the esthetic zone. B, An implant is in the position of the natural root of the tooth. Although this makes sense, it places the implant too facial, and an angled abutment is usually necessary. C, An implant in the cingulum position that is used when a screw-retained crown is the treatment of choice. This position requires a facial ridge lap of porcelain when used for FP-1 prostheses in the esthetic zone.

Facial Implant Body Angulation

Researchers often theorize that a maxillary anterior implant body angulation be positioned at the facial emergence of the final crown. The facial implant position is predicated on the concept that the facial emergence of the implant crown at the cervical should be in the same position as a natural tooth. At first, this makes some sense. However, the crown of a natural tooth has two planes, and its incisal edge is palatal to the facial emergence of the natural tooth by 12 to 15 degrees (Figure 33-15). This is why anterior crown preparations are in two or three planes. The implant body is more palatal than a natural root, so 1.5 mm of bone exists facially. In addition, because the implant is narrower in diameter than the faciopalatal root dimension, when the implant body is oriented as a natural tooth and has a facial emergence, a straight abutment is not wide enough to permit the two or three plane reduction to bring the incisal edge of the preparation more palatal. As a result, the incisal edge of the preparation remains too facial. Therefore when the implant is angled to the facial emergence of a tooth, an angled abutment of 15 degrees must be used to bring the incisal edge more palatal.

Figure 33-15 A natural tooth has very thin facial cortical bone over the root, and the incisal edge of the crown is 12 to 15 degrees palatal to the facial emergence profile. This is not an ideal position for an implant. The bone to the palatal region is better suited for an implant and allows the implant to be positioned under the incisal edge.

Most two-piece angled abutments have a design flaw that compromises facial cervical esthetics (Figure 33-16). The metal flange facial to the abutment screw is thinner than a straight abutment and may result in fracture (especially because angled loads are placed on the facial-positioned implant). The manufacturers thicken the profile of the abutment on the facial aspect to reduce the risk of fracture. However, this design flaw brings the cervical facial margin more facial and wider than the implant body, which is already as facial as the adjacent tooth. As a result, the implant crown margin is facially overcontoured. The restoring doctor then has to prepare the facial aspect of the abutment metal flange for esthetics, which weakens it and makes it prone to fracture. Because the doctor modified the abutment, the implant company is held harmless but, nonetheless, the patient and doctor are at risk.

Figure 33-16 Most angled implant abutments, regardless of manufacturer, have an extra metal flange, opposite to the angled post. This increases the thickness of metal so that the component will not fracture. However, the extra metal flange is more facial than the implant body and results in increased risk of tissue shrinkage and/or metal seen through the thin facial soft tissue.

When the surgeon attempts to align the implant body with the facial aspect of adjacent teeth, the implant may inadvertently be inserted too facial. No single method exists to restore proper esthetics when the implant abutment is located above the free gingival margin of the adjacent teeth. At best, the final crown appears too long and too facial. Soft tissue grafts and/or bone augmentation do not improve the condition once the implant is already incorrectly inserted (Figure 33-17).

Figure 33-17 When implants are placed too facial, no prosthetic method is available to improve the poor esthetic condition.

The natural maxillary anterior teeth are loaded at a 12- to 15-degree angle, because of their natural angulation in comparison with the mandibular anterior teeth. This is one reason the maxillary anterior teeth are wider in diameter than mandibular anterior teeth (which are loaded in their long axis). The facial angulation of the implant body often corresponds to an implant body angulation, which leads to 15 degrees off axial loads and increases the force to the abutment screw-implant-bone complex by 25.9%, compared with a long axis load. These offset loads increase the risks of abutment screw loosening, crestal bone loss, and cervical soft tissue marginal shrinkage. As a result, implants angled too facially compromise the esthetics and increase the risk of complications (Figure 33-18).

Figure 33-18 An implant that is too close to a tooth and/or too facial increases the risk of tissue shrinkage and poor cervical esthetics. The central implant crown and the natural lateral incisor have tissue recession as a result of the bone loss and improper implant position. (too facial and too close to the tooth)

Cingulum Implant Body Angulation

A second angulation suggested in the literature is more palatal, with an emergence under the cingulum of the crown. This may also be the result of an implant insertion in a width-deficient ridge (Division B), because the bone is lost primarily on the facial (see Figure 33-8, C). This position is often the goal when a screw-retained crown is used in restoration. The prosthesis fixation screw (to retain a maxillary anterior crown) cannot be located in the incisal or facial region of the crown for obvious reasons.

The cingulum implant position may cause a considerable health compromise. The implant body is round and usually 4.0 to 5.5 mm in diameter. The labial cervical contour of the implant crown must be similar to the adjacent teeth for the ultimate esthetic effect. Because the long axis of the implant for a screw-retained crown must emerge in the cingulum position, this most often requires a facial projection of the crown or “buccal correction” facing away from the implant body. The facial ridge lap must extend 2 to 4 mm and is often similar in contour to the modified ridge lap pontic of three-unit fixed prosthesis (Figure 33-19).

Figure 33-19 An implant placed in the cingulum position requires a ridge lap on the implant crown to restore the facial contour of the tooth.

The modified ridge lap crown has become a common solution to correct the esthetics of the restoration when the implant is placed in narrow bone or follows a palatal angulation position.^30,31 However, plaque control on the facial of the implant is almost impossible. Even if the toothbrush could reach the gingival sulcus, no hygiene device could be manipulated to a right angle to proceed into the facial gingival sulcus. As a result, although an acceptable esthetic restoration may be developed, especially with the additional cervical porcelain, the hygiene requirements and present implant dentistry standards render this approach unacceptable (Figure 33-20).

Figure 33-20 A, The maxillary left central incisor (right side) with a ridge lap crown presents acceptable esthetics. B, However, probing on the facial aspect measures the distance to the implant but cannot evaluate facial bone loss, because it cannot be directed apically to the pocket depth. C, Periodically, the left central incisor implant becomes inflamed, and the bleeding index increases as a result of the inability to clean the implant sulcus on the facial.

Some authors argue that an improved contour may be developed subgingivally with a palatal implant position. To create this contour, the implant body must be positioned more apical than desired. This position may prevent food from accumulating on the cervical “table” of the crown. However, the subgingival ridge lap does not permit access to the facial sulcus of the implant body for the elimination of plaque, as well as to evaluate the bleeding index or facial bone loss. Therefore the maintenance requirements for the implant facial sulcular region do not permit the surgeon to consider this modality as a valid primary option.

Greater interarch clearance is often needed with an implant palatal position, because the permucosal post exits the tissue in a more palatal position. Inadequate interarch space may especially hinder the restoration of Angle’s Class II, division 2 patients, with the implant in this position. The bony ridge should be augmented if too narrow for the model implant diameter and position, or an alternate treatment option should be selected. The anterior single-tooth implant should use a cement-retained crown, so the cingulum screw position is not necessary.