Factors Involved in Single Implants
Single implants can replace any tooth in the dental arch. For purposes of single implant placement, the various areas of the oral cavity are broadly classified as comprising the esthetic zone (i.e., the central, lateral, canine, and first premolar areas in the maxilla) and the nonesthetic zone (i.e., the posterior maxilla, posterior mandible, and anterior mandible). This allows the characteristics of each area to be explained separately.
As mentioned, the esthetic zone in the oral cavity consists of the central, lateral, canine, and first premolar areas in the maxilla. These areas are very important because of their role in the esthetic appearance of the patient. A large number of articles have been published on the subject from surgical and prosthetic viewpoints. The principles of implant surgery and osseous and soft tissue considerations in these areas are different from those in other areas of the oral cavity.
The nonesthetic zone of the oral cavity consists of the remaining areas of the two arches, which are classified as the posterior maxilla, posterior mandible, and anterior mandible. Each of these regions has specific characteristics and anatomic features that should be taken into account during the surgical procedure. For example, the maxillary sinuses in the posterior maxilla and the inferior alveolar nerve in the posterior mandible are two important anatomic structures in the nonesthetic zone; if they are ignored during surgery, irreparable injury to the patient may result.
In addition, the spaces between the incisors in the mandible are very small, and the possibility of damaging the adjacent teeth is an important consideration during surgery for single implants. In the following sections, these problems are discussed further, and the surgical techniques for each area are explained.
The fact that the process of bone resorption slows down after tooth extraction has been well-established. The amount of bone resorbed during the first year after tooth extraction is much greater than that during the following years.1 A complex osseous situation exists when bone volume is diminished and the quality of bone is not uniform in different regions of the jaws. These two important factors, the quality and quantity of bone, play an important role in determining implant location and position. In 1985, Zarb and Lekholm created classification systems for the quality and quantity of jaw bones. They classified bone quality as type I to type IV and bone quantity as type A to type E (Figure 3-1).
Figure 3-1 Classification of the quality and quantity of jaw bone. (From Lekholm U, Zarb GA: Patient selection and preparation. In Brånemark PI, Zarb GA, Albrektsson T, editors: Tissue integrated prostheses: osseointegration in clinical dentistry, Chicago, 1985, Quintessence.)
From a quantitative viewpoint, type A and type B bones are ideal; however, more problems are encountered with an increase in bone resorption. First, bone height is determined through radiographic evaluation of eligible jaw areas. Panoramic radiography is the method most commonly used to evaluate bone height.
Generally, the prognosis for the implant improves as the implant’s length increases. However, implant lengths exceeding 13 to 14 mm currently are not recommended. Implants less than 8 mm in length belong to the short implant category; the prognosis for these implants is less favorable than that for long implants. Therefore, if bone height is 8 to 14 mm and no impingement is made on anatomic structures, the condition is ideal for implant placement.
It should be noted that a distance of at least 2 mm should exist between the apex of the implant and the roof of the mandibular canal. However, contact of the apex of the implant with the floor of the maxillary sinus or its perforation does not cause problems if the mucous membrane of the sinus is not ruptured.
Another important factor, which is crucial to the longevity of the implant, is bone width. Implants with a diameter of 4 mm require a minimum of 6 mm of bone width; with a bone width of 7 mm, the long-term prognosis is much better. If thick implants with a diameter of 5 mm are to be used, a bone diameter of 7 to 8 mm is required.
If the remaining bone in the buccal aspect of the implant is less than 1 mm, the area should be reinforced with the guided bone regeneration (GBR) technique. This is more important in the anterior areas of the maxilla, because a thin buccal bone in this area leads to resorption of bone and subsequent gingival recession and exposure of the metallic margin of the implant, compromising the patient’s esthetic appearance. To prevent such problems, all surgeries for single implants in the anterior area of the maxilla should be augmented with bone.2
Similar to bone, which is an important determining factor for the long-term maintenance and success of an implant, keratinized soft tissue around the implant can play an important role in the longevity of the implant and in prevention of peri-implantitis. Considerable research has been dedicated to this issue. Some studies have shown that implants are durable even without keratinized gingiva, and no problems are encountered. Other studies have emphasized that attached keratinized gingiva is favorable and in fact necessary for implants.3 Therefore, to prevent subsequent problems, the logical course is to provide an environment for implant placement in which sufficient keratinized gingiva is present. This environment can be provided during implant placement or subsequent to it. Some advantages of keratinized gingiva around implants are noted in Box 3-1.
During treatment planning for placement of implants, the presence of attached keratinized gingiva, which is very important, should be taken into account. This gingiva should be reconstructed during implant placement or after it if no keratinized gingiva is present. It has been empirically shown that at least 2 mm of attached keratinized gingiva around an implant is sufficient, and the prognosis improves with an increase to more than 2 mm. However, some authors believe that the need for keratinized gingiva is patient specific.4 Therefore, during treatment planning, the amount of attached keratinized gingiva can be measured. If insufficient keratinized gingiva is present, measures can be taken to provide it. If sufficient keratinized gingiva is present, plans should be made so that this gingiva is located in its proper place around the implant.
The apically positioned flap is commonly used in the maxilla, because the palate is predominantly covered with keratinized gingiva. When attached gingiva on the buccal aspect of the implant is insufficient, a palatally inclined incision can be used to direct some keratinized gingiva from the palatal side to the buccal side. In the mandible, if the amount of keratinized gingiva is sufficient on the lingual aspect, the same procedure can be carried out (Figure 3-2).
The free gingival graft was introduced by Bjorn in 1963. For this graft, a split-thickness flap is made in the recipient site at the mucogingival junction (MGL). The periosteum is preserved on the bone, and a segment of the keratinized mucous membrane, approximately the size of the recipient site, is removed from the palatal mucosa or the edentulous ridge and placed in the recipient site (Figure 3-3). The success of this technique has been reported to be very high in the attached keratinized gingiva.
The free connective tissue graft, which is commonly removed from the palate, not only is used to provide attached keratinized gingiva, but also can be used to treat ridge defects. According to a classification system proposed by Seibert, ridge defects are divided into three classes. In class I defects, tissue is lost in the buccolingual dimension; in class II defects, it is lost in the apicocoronal dimension; and in class III defects, both dimensions are involved. Connective tissue can be used as either free or pedicled. Because this technique provides a better gingival color in the recipient area, its use has been recommended in the esthetic zone. To make the graft, a rectangular window is created in the palate; then, a sharp incision is made to elevate a maximum amount of supraperiosteal connective tissue from the palate. The connective tissue graft is placed and sutured in the recipient site of the subepithelial area. Different techniques are used for a connective tissue graft in the attached keratinized gingiva; these are comprehensively explained in reference books.
With bone-level implants, the platform is placed at the level of the jaw bones. These implants are used in the regions of the esthetic zone; they are placed deep into bone so that no metallic surfaces are visible (Figure 3-4). Current changes in implant surfaces and microthreads on the upper areas adjacent to the bone have resulted in assumptions that crestal bone may be more stable (Figure 3-5).
Figure 3-4 Two types of bone-level implants.
Figure 3-5 Implant with microthreads on top.
Tissue-level implants usually have a collar with a smooth titanium surface (Figure 3-6). The platforms of the implants are usually located 1.5 to 3 mm above the bone level, and the titanium collar is a proper location for the attachment of the gingival soft tissue. An advantage of these implants is a decrease in resorption of bone at the crest, because formation of the biologic width does not require resorption of the crestal bone. A disadvantage of these implants is the visibility of the metallic collar of the implant, resulting in an unesthetic appearance when the gingival tissue is thin. However, because the posterior regions of both arches are not located in the esthetic zone and are not visible during speaking and smiling, these implants can be used in the nonesthetic zone.
Figure 3-6 Two types of tissue-level implants.
Another advantage of tissue-level implants is that they can be placed with single-stage surgery, without any need for a second surgical procedure. This provides the peri-implant soft tissues with more time and opportunity for growth, development, and stabil/>