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J Am Dent Assoc, Vol 138, No 3, 321-329.
© 2007 American Dental Association
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CLINICAL PRACTICE

A review of the functional and esthetic requirements for dental implants

Binnaz Leblebicioglu, DDS, PhD, Swati Rawal, BDS, MS and Angelo Mariotti, DDS, PhD


   ABSTRACT
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 ABSTRACT
THE ANATOMICAL BASIS FOR...
 ANATOMY AND MORPHOLOGY OF...
 CONCLUSION
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Background. The esthetic replacement of teeth has become an important standard for implant dentistry. While defining this goal has not been difficult, the ability to restore implants esthetically has been fraught with obstacles and sometimes has not been attainable. The purpose of this review is to summarize essential anatomical and surgical considerations for cosmetic implant dentistry.

Methods. This article provides a summary of the predominant findings from clinical studies and case reports that help develop implant surgical guidelines for better esthetic outcomes.

Results. Soft- and hard-tissue requirements for placing an implant in an ideal position are defined. The authors discuss the best treatment approaches as well as the limitations associated with esthetic implant placement. They evaluate the available data specifically for the maxillary anterior sextant, since this anatomical region has higher esthetic demands.

Conclusions. Several parameters and various surgical techniques have been developed to manipulate soft- and hard-tissue contours and to control the esthetic outcome for implant-supported restorations.

Clinical Implications. It is essential for practitioners to understand the anatomical basis for and limitations of implant dentistry in the esthetic zone.

Key Words: Alveolar bone; gingiva; dental implant; regeneration

Abbreviations: GBR: Guided bone regeneration

Esthetic demands posed by dental implant-supported restorations are increasing in the maxillary anterior region. In this sextant, the natural dentition is surrounded with a scalloped gingival margin and a pyramid-shaped interdental papilla. Gingival architecture is determined mainly by the anatomy of the teeth and the position and the size of the contact surfaces,1,2 and all of these features are framed by the lip line.3

Seibert4 defined the limitations of soft-tissue grafting for a better esthetic appearance around anterior fixed partial dentures by creating a classification for alveolar ridge defects. This was modified by Palacci and Ericson5 to classify the amount of vertical and horizontal loss of soft tissue, hard tissue or both in the maxillary anterior region (Figure 1Go). Therefore, it is important that the clinician take into account soft-tissue considerations—including the height of the papilla and the presence of attached gingiva—to fulfill the patient’s esthetic needs.6–9


Figure 1
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  		Figure 1. Palacci’s classification for vertical and horizontal loss of soft tissue, hard tissue or both in the maxillary anterior region. On the basis of vertical loss, Class I is an intact or slightly reduced papilla (A). Class II has limited loss of papillae. Class III has severe loss of papillae. Class IV represents absence of the papillae (B). On the basis of horizontal loss, Class A shows intact or slightly reduced buccal tissue (A). Class B has limited loss of buccal tissue. Class C has severe loss of buccal tissue (B). Class D has extreme loss of buccal tissue, often in combination with a limited amount of attached mucosa.

 
Consequently, a preoperative treatment plan for implants in the anterior maxilla requires an evaluation of the hard and soft tissue to determine the type and size of fixture needed. The dentist should visualize the expected restorative outcome by means of a temporary restoration with the desired emergence profile. Both the restorative dentist and dental implant surgeon should use this template and work out all the steps involved in reaching expected treatment outcomes. In this article, we characterize soft- and hard-tissue prerequisites for placing an implant in an ideal position in the maxilla so that it can be restored with esthetically acceptable soft-tissue contours.


   THE ANATOMICAL BASIS FOR IMPLANT SELECTION
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 THE ANATOMICAL BASIS FOR...
ANATOMY AND MORPHOLOGY OF...
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Osseous structure and architecture. Alveolar process of the maxilla. The clinician should examine the alveolar process of the patient’s maxilla in relation to the nasal cavity, the floor of the maxillary sinus and the incisive canal.10 In general, the canine occupies a neutral position between the two cavities, while incisors are below the floor of the nasal cavity and the premolars and molars are below the maxillary sinus.10 The relationships of the apexes of teeth to the nasal floor depend on two factors: the height of the face and length of the roots. In people with a relatively short alveolar process and long roots, the central incisor actually may reach the thin compact bony plate that forms the floor of the nasal cavity.10 Thus, the dentist should examine the height of the alveolar process and the length of the roots before deciding on the appropriate implant length.

Alveolar sockets are placed eccentrically. The axis of the root and the socket are more nearly vertical than is the axis of the alveolar process as a whole.10 Therefore, the alveolar bone proper on the labial surface of the roots fuses with the external plate of the alveolar bone. In addition, a wedge-shaped area of spongy bone generally is found between the alveolar bone proper and the palatine plate of the alveolar process.10 As a result, the buccal plate often is fractured and collapses during tooth extraction and may need to be regenerated for a better implant diameter selection and correct localization.

Owing to the proximity of the incisive canal and its contents to the maxillary central incisors, positioning an implant in this area requires careful consideration.11 Terminal branches of nasopalatine nerves and the greater palatine artery, together with the nasopalatine artery, pass through the incisive canal. Investigators12 have reported difficulties and anatomical limitations regarding the location of the incisive canal in relation to maxillary implants replacing central incisors.

Quality of available bone. The maxilla has thin porous bone on the labial aspect, very thin porous-to-dense compact bone in the nasal region and thick cortical bone on the palatal aspect. The trabecular bone often is less dense than the mandible.13 Classically, bone quality of edentulous jaws is classified as type I through type IV.14 Type I bone has homogeneous cortical bone with no cancellous bone, whereas type IV bone has an extremely thin compact layer and cancellous bone of reduced density. Type II carries mostly cortical bone and some cancellous bone, while Type III presents cancellous bone surrounded with a 3- or 4-millimeter–thick layer of compact bone. Researchers have reported predominance of Type III bone in the anterior and premolar regions of the maxilla.15

Quantity of available bone. The alveolar bone reacts to dental extraction by remodeling its structures, removing bone at its outer surfaces and depositing bone in the empty sockets. The various factors affecting alveolar bone resorption can be classified as mechanical, biological and anatomical. Five general groups of diverse jaw shapes encountered after extraction are as follows16–18 (Figure 2Go):


Figure 2
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  		Figure 2. Diverse jaw shapes encountered in the maxilla after extraction.

 
– most of the alveolar ridge is present;
– moderate residual ridge resorption has occurred;
– advanced residual ridge resorption has occurred and only basal bone remains;
– some resorption of the basal bone has occurred;
– extreme resorption of the basal bone has taken place.

Tooth-related considerations. The maxillary lateral incisor crown is more slender than the central incisor and may lean more medially.19 The labial surface is more convex than is the central incisor; frequently, the root is bent distally or distolingually near the apex.19

The maxillary permanent canine is at the corner of the dental arch, and its anatomy reflects the beginning transition from anterior to posterior tooth forms.19 The root of the canine is the longest and strongest in the human dentition.19 For an acceptable esthetic outcome, the clinician should keep the crown height and length proportions of implant-supported restorations similar to those of the natural teeth (TableGo).20 Also, the anatomical location of maxillary canines in relation to ipsilateral and contralateral dental arches creates additional challenges for implant placement and for manipulation of the gingiva around implant-supported restorations.


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  		TABLE Various dimensions of teeth in the maxillary anterior sextant.

 
Relationship of lips to teeth and gingiva. In the average smile, the lip is positioned to show 75 to 100 percent of the maxillary central incisor and the interproximal gingiva. A high smile line reveals the total cervical-incisal length of the maxillary anterior teeth and a contiguous band of gingiva. A low smile line displays less than 75 percent of the anterior teeth.21


   ANATOMY AND MORPHOLOGY OF SOFT TISSUES
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Periodontal biotypes. The soft-tissue phenotype (that is, shape and thickness) contouring a crown can be defined as the "periodontal biotype." Researchers22,23 have described two periodontal biotypes: the thick-flat biotype and the thin-scalloped biotype. Different biotypes have a tendency to respond differently to inflammation and surgical injury. In a patient with a thin-scalloped periodontium, the surgical and restorative intervention involved in esthetic implant therapy may result in some degree of soft-tissue recession.24 Also, the thin maxillary buccal plate is predisposed to defect formation secondary to remodeling and resorption of bone after extractions and/or implant therapy. On the other hand, the thick-flat periodontium resists recession and reacts to surgical and restorative therapy with pocket formation.24 This type of tissue is predisposed to forming notches and scars that can jeopardize the final esthetic and functional results. Researchers21,24 have evaluated the peri-implant biotype and categorized it as thick or thin, similar to periodontal biotype.

Biological width concept and dental implants. The physiological dentogingival junction of natural teeth includes the length of the epithelial attachment, the length of the connective-tissue attachment and the depth of the sulcus. This also is known as "the biological width."25 The mean value of the biological width around a natural tooth is 2.73 mm.25 The implant-epithelium junction is similar to that in the natural dentition, except that it is shorter and thinner than the tooth-epithelium junction.26 Because of the absence of a cementum layer around an implant, most connective-tissue fibers in supracrestal region are oriented in a direction parallel to the implant surface.27,28 Furthermore, investigators have observed the presence of an avascular zone, 50 to 100 micrometers wide, of dense circular connective-tissue fibers that are in direct contact with the implant post at the supracrestal area.29 The biological width around implants can have significant influence on the character of soft tissues and depends on a variety of characteristics that include implant design, presence of adjacent teeth and quality of soft tissue. For example, one-piece implant designs have been implicated in more closely mimicking the biological width around natural teeth.30,31 Similarly, platform switching (as in controlling the dimension of the abutment) during the period of osseointegration affects biological width by altering the position of the microgap and controlling circumferential bone loss around dental implants.30 In addition, a scalloped implant platform is available that follows the osseous structure of the maxillary anterior teeth and may prevent interproximal crestal bone resorption during healing.32 These results may have important implications when dealing with esthetic implant-borne restorations, considering that long-term esthetic survival depends on soft-tissue dimensions that remain healthy and vertically constant over time.

Anatomical basis for optimal implant positioning. The extent of alveolar bone resorption that follows tooth extraction depends on a number of factors, including existing periodontal disease, trauma, time after the extraction and the quality of alveolar bone.33 The reduction in width of the maxillary alveolar ridge after tooth extraction is greater than the loss in height.34 Severity of resorption also depends on whether the patient is wearing a removable denture.35 A transformation from skeletal Class I or II to a Class III relation between maxillary and mandibular jaws routinely can be seen in totally edentulous patients34 and can affect the maxillary esthetic result owing to implant angulation in the upper jaw.

Guided bone regeneration (GBR) procedures are performed routinely before or during dental implant placement to increase the width and the height of the alveolar ridge.36 Particulate bone graft materials37 in the presence or absence of resorbable or nonresorbable membrane38 can be used to augment bone, whereas bone blocks fixed with miniscrews and a barrier membrane may be indicated in areas with more extensive bone defects.39

Creating alveolar bone height usually is more challenging than creating alveolar bone width, and it almost always requires placement of a bone block.39 This can be obtained from the chin, the mandibular ramus, the rib or the hip of the patient, or it can be purchased from a tissue bank.40 Although controversial, it has been suggested that vertical bone length should be able to support at least a 10-mm implant with a crown-to-root ratio of 1:1.41

One of the main requirements for a GBR surgical approach is the availability of keratinized tissue to cover the wound and to allow biomaterial stability. Scar tissue often makes wound coverage difficult, and onlay- or inlay-type soft-tissue grafts may be required to cover bone graft material.42

Soft-tissue support for papilla reconstruction and preservation. The bony support between a tooth and an implant or between two implants has been shown to be an important criterion in creating or preserving the papilla.43,44 For example, when the measurement from the interproximal coronal contact point to the crest of bone is 5 mm or less, the papilla is present almost 100 percent of the time.45 When the distance is 6 mm or greater, the papilla is present 50 percent of the time or less. Tarnow and colleagues46 reported a mean papillary height between two adjacent implants as 3.4 mm. One difficulty in maintaining or re-forming a papilla between two implants is that the biological width around an implant usually is located apical to the implant abutment connection. In the esthetic zone, the distance from alveolar crest to the adjacent tooth cementoenamel junction should be 3 to 5 mm to achieve ideal implant localization47 and appropriate space for the peri-implant sulcus to form (Figure 3Go). This location places the biological width subcrestally, whereas in a natural tooth, biological width always forms supracrestally.


Figure 3
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  		Figure 3. Distance from alveolar crest to adjacent tooth cementoenamel junction (CEJ). A. Initial presentation. Notice buccal concavity and papilla covering interproximal area. B. Implant location after guided bone regeneration. Implant was placed 3 millimeters apically to the adjacent tooth CEJ without reducing alveolar bone height. C. Final restoration with ideal implant localization and accurate space for peri-implant sulcus to form.

 
The localization of the alveolar crest is important not only for partially edentulous patients but also for those who are totally edentulous. Depending on the type of restoration (fixed bridge, hybrid or bar/ball attachment–supported overdenture) adequate vertical space should be available for different restorative parts to be placed.48–50 Thus, alveolar bone height reduction may be required before implant placement can take place.

Mesial-distal position of implant in bone. A minimum of 1.25 mm of clearance is required between the implant fixture and adjacent teeth for proper osseointegration and decreased risk of damage to adjacent natural teeth (Figure 4Go).51 However, an average creastal bone loss of 1.04 mm has been reported when interimplant space is 3 mm or less compared with 0.45 mm crestal bone loss when this distance is greater than 3 mm.52 When calculating the mesial-distal distance to select the appropriate implant diameter, one also has to consider the space required for the fabrication of contact points between crowns. Thus, a minimum of 1.5 to 2 mm of clearance from the adjacent tooth is recommended to obtain optimum esthetics with appropriate space for prosthetic devices related to various implant designs and also for peri-implant tissue health.50,52


Figure 4
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  		Figure 4. Calculation of mesial-distal distance to select appropriate implant diameter. A. A minimum of 1.5 to 2 millimeters of clearance (d and b) is required between implant and adjacent tooth for proper osseointegration and decreased risk of damage to adjacent teeth. B. A minimum of 3 to 4 mm clearance (a) is required between two adjacent implants to decrease crestal bone loss. This space also is important for the fabrication of contact points between crowns.

 
Buccolingual positioning of implant in bone. Two factors play an important role in clinical decisions regarding buccolingual positioning of an implant: bone thickness with adequate blood supply and the appropriate implant angulation for the proper emergence profile (Figure 5Go). An implant should be surrounded with bone at least 1 mm thick on both the buccal and lingual aspects. When a mean facial bone thickness of 1.8 mm or larger remains after site preparation, the potential for bone loss decreases significantly and bone apposition is more likely to occur (Figure 6Go).53 In addition, the implant body should be aligned with adjacent teeth as well as with the dentition in the opposing arch.54 One researcher has recommended that the implant be oriented 5 degrees palatally and closer to the palatal cortical aspect to minimize buccal angulation and to open up space for screw retention.55 If an implant must be placed palatally, for each millimeter of palatal inclination, the implant should be placed an additional millimeter apically to correct angulation.55 If the buccolingual dimension of the maxillary arch is compromised, GBR should be considered to allow implant placement at the appropriate buccal or lingual position.


Figure 5
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  		Figure 5. Buccolingual positioning of the implant. Bone thickness with adequate blood supply and appropriate implant angulation for the proper emergence profile. A. Implant placed too far to the buccal aspect, leaving thin buccal bone and a poor emergence profile. B. Implant placed too far to the lingual aspect, leaving a more than ideal space on the buccal aspect and jeopardizing the occlusal space.

 

Figure 6
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  		Figure 6. Buccolingual bone walls and immediate implant placement. A. Removing remaining nonrestorable root by using burs. B. Intact buccal and lingual bone walls after root removal. C. Immediate placement, using an implant of a diameter larger than the natural root diameter. D. One-stage healing. E. Final restoration with acceptable esthetic presentation.

 
Trajectory of the implant = emergence profile. The emergence profile of a dental implant depends on both implant body angulation and the existing status of the periodontal tissues. The clinical parameters that have been reported earlier should be considered for an optimal emergence of the implant restoration. In regard to implant angulation, implant bodies should be placed at angles less than 25 degrees since esthetic needs cannot be fulfilled easily with implants placed with wider angles.56,57 The clinician should carefully evaluate the soft-tissue characteristics—including the amount of keratinized tissue, biotype and papilla form—before performing implant surgery. It is important to remember that soft-tissue augmentation is not possible without hard-tissue support.58 Therefore, a ridge deficiency at the implant site should be within 3 mm of its optimal contour to allow the clinician to modify the soft tissues suitably. To have ideal localization, implant placement in bone requires placement of the implant platform 3 to 5 mm from the cementoenamel junction of the adjacent tooth.30,43 Furthermore, both buccal and lingual bone walls should be at least 1 to 2 mm in thickness (Figure 6Go).

Hard- and soft-tissue remodeling during the first year. Up to the mid-1990s, alveolar bone loss at the crest was considered to be a physiological response to healing during the first year after dental implant placement.59 This was thought to occur as a result of mechanical stress caused by the implant body at alveolar crest level and was defined as "saucerization."60 Currently, it is accepted that this phenomenon occurs not only owing to mechanical stress created by the implant body at the crest but also owing to lack of a space for biological width61 and the existence of microgap62 at the alveolar crest level. Cochran and colleagues61 reported that a space of approximately 3 mm in height is required for peri-implant sulcus formation around dental implants without alveolar bone loss. Thus, several implant designs have been modified to allow a polished collar to create biological width at the alveolar crest. The dilemma is that existence of a polished collar may create esthetic problems due to metal color showing through the gingiva. Some dental implant surgeons have recommended embedding the polished collar into alveolar bone, especially at esthetic regions. This should not be considered as a routine practice, since bone will not integrate on a polished titanium surface and the alveolar crest will have a higher risk of resorption. Also, soft-tissue changes can occur; an additional 0.75 mm and 0.9 mm of tissue recession can occur at six months and one year, respectively, after abutment connection.63–65


   CONCLUSION
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 ABSTRACT
 THE ANATOMICAL BASIS FOR...
 ANATOMY AND MORPHOLOGY OF...
 CONCLUSION
REFERENCES
 
Surgical and restorative concepts related to implant dentistry have been modified tremendously through the years. The ultimate goal of implant-supported restorative therapy is to replace a tooth with a structure that will mimic what is lost functionally and esthetically. Several surgical techniques have been developed to regenerate soft and hard tissue with the aim of improving esthetics. These procedures allow the dentist to increase tissue support around dental implants. In addition, several parameters have been developed to control the esthetic outcome of the treatment. The initial trend of case reports and personal communications has been replaced by clinical studies, though there still is a need for well-controlled, longitudinal investigations. General dentists and specialists who would like to include implant dentistry in their practices should be familiar with the current improvements and limitations of this fast-developing discipline.


   FOOTNOTES
 

Dr. Leblebicioglu is an assistant professor,, Section of Periodontology, College of Dentistry, The Ohio State University, Columbus.


Dr. Rawal is assistant professor, Department of Periodontology, College of Dentistry, The University of Tennessee, Memphis.


Dr. Mariotti is the chairperson, Section of Periodontology, College of Dentistry, The Ohio State University, 305 West 12th Ave., Columbus, Ohio 43210, e-mail "mariotti.3{at}osu.edu". Address reprint requests to Dr. Mariotti.


The authors thank Dr. Maria Lavda (University of Illinois, Chicago) and Dr. Selim Pamuk (Istanbul University, Turkey) for allowing them to document the restorative treatment performed by these doctors.


   REFERENCES
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