Since the introduction of osseointegrated implants for the restoration of missing or lost teeth, treatment options for the partially or fully edentulous patient have expanded. Removable prostheses are no longer the only available option to treat the edentulous state. In fact, this option is no longer the first to be considered. Furthermore, removal of healthy tooth structure in order to provide support for a tooth-borne fixed prosthesis now can often be avoided.
Implant therapy provides improved function, preservation of crestal bone, an improved long-term outcome, and better quality of life.1 Outcome surveys of fixed prostheses on natural teeth reveal a 10-year survival rate of approximately 75%.2 Success rates for endosseous implants over that time have been shown to be greater than 90%.3,4
Despite the clinical success of dental implants, maintaining crestal bone after implant placement remains a challenge. Undesirable osseous and soft-tissue changes can take place subsequent to implant placement. This article reviews the literature related to the dynamics that are in effect when an implant is placed in bone. A technique will be presented to limit adverse osseous and soft-tissue changes.
BONE REMODELING SUBSEQUENT TO ABUTMENT CONNECTION
As for all clinical modalities, the placement and restoration of implants have many variables that must be carefully controlled. One particular aspect of implant therapy that can be particularly challenging is the maintenance of crestal bone levels around dental implants. A comprehensive understanding of factors that affect crestal bone changes around endosseous implants, and subsequent consequences of those osseous changes, is essential for successful implant therapy.
Figure 1. Illustrates 1.5 to 2 mm of crestal bone loss from implant-abutment interface.
Herman, et al investigated the crestal bone changes around titanium implants by radiographic evaluation of implants placed in the mandibles of dogs. From this study and a subsequent study,5,6 the following can be concluded:
(1) The rough/smooth border on 1-piece nonsubmerged implants determines the initial bone-to-implant contact.
(2) In 2-piece implants, the first bone-to-implant contact was located 1.5 to 2 mm apical to the microgap between the 2 pieces (Figure 1).7
(3) Osseous changes occur after the creation of a microgap at the implant-abutment interface.
Figure 2. Horizontal component of bone loss after abutment connection.
Crestal bone changes around implants from the microgap to the bone crest have both horizontal and vertical components. Following abutment connection, crestal bone will move laterally 1.3 to 1.4 mm from the microgap (Figure 2).8 Understanding the consequences of these bony changes will influence decision making as it pertains to implant location, size, and design.
CONSEQUENCES OF CRESTAL BONE CHANGES
The dentogingival and peri-implant complexes are similar in their cellular composition, consisting of keratinized oral epithelium and nonkeratinized junctional epithelium.9 Histologically, differences are observed in the attachment to teeth or implants. While the attachment of the junctional epithelium to teeth is mediated by glycoproteins, a pseudo-attachment via hemidesmosomes exists around endosseous implants. In addition, connective tissue fibers mechanically insert into root cementum. This is not seen with fixtures; instead, a tight cuff is formed around titanium implants.10
Figure 3. The components of biologic width.
In their study of the periodontium, Gargiulo, et al determined the average histological dimension of the epithelial attachment (0.097 mm) and connective tissue attachment (1.07 mm) (Figure 3).11 Known as the biologic width, this dimension is a key determinant of aesthetics, particularly in the anterior region of the mouth. The biologic width found around teeth is also characteristic of the peri-implant complex. The dimensions are similar to that of teeth and are stable even after loading.5 With this information, the position of the gingival attachment in relation to the osseous crest can be determined.
The importance of interdental papillae to the aesthetic result has been extensively discussed.12,13 The interproximal height of bone (IHB) has a distinct influence on interdental papillae. Garber, et al demonstrated that a predictable papilla length is achievable and maintainable in the maxillary anterior sextant, as measured from the most coronal interproximal height of bone immediately adjacent to a tooth or implant fixture. The mean papilla length for an implant-tooth relationship was found to be 6.5 mm; for an implant-implant relationship the mean papilla length was 4.5 mm.14 In another study, the papilla length was determined to be 3.4 mm between adjacent implants.15
Soft-tissue architecture is critically important. In the interproximal region, inadequate soft tissue will result in a “black triangle,” which can be seen in aged patients and/or those with periodontal disease.16 Facial soft-tissue deficiencies can create the appearance of a tooth/implant that is longer than desired. Adequate interdental soft tissue depends on the height of the interproximal bone and its relation to the contact point. When the distance from the contact point to the interproximal height of bone is greater than 5 mm, avoiding the “black triangle” is difficult.17
This information is critical to achieving patients’ aesthetic expectations. Circumferential bone loss increases the distance from the contact point to the interproximal height of bone. Consequently, predictable papilla formation is more difficult. In addition, an increase in crestal bone loss will cause the facial soft tissue to recede as the new biologic width is formed.
Figure 4. Implant proximity limitations are obeyed. Maintenance of IHB.
Figure 5. Infringement of proximity limitations, resulting in loss of IHB.
For a 2-piece implant placed at or below the crest of bone, certain changes are noted after connection of the abutment.7 Bone resorption of approximately 1.5 to 2 mm from the implant-abutment interface should occur circumferentially.5 As a result of this osseous change, the soft-tissue architecture will be affected as biologic width is reestablished.18 The result will be most evident on the facial aspect, where recession may be noted at the gingival crest. If proximity guidelines were not obeyed when placing the implant next to an adjacent implant (3 mm) or tooth (1.5 mm), then interproximal bone will be insufficient to support papilla formation (Figures 4 and 5), and a “black triangle” may be evident.14
Figure 6. Radiograph illustrating crestal bone loss in the posterior maxilla.
Sinus augmentation procedures have become commonplace when implants are to be placed in the posterior maxilla.19 Often, implants must be placed in areas where there is little autogenous bone.19 After grafting, the fixture is placed in bone grafted from an autogenous site, from allogeneic bone, or from a synthetic source. Although these procedures have shown to be predictable, maintenance of the patient’s native bone is important.19 For example, if the patient has 3 mm of bone inferior to the floor of the sinus, an augmentation procedure may be performed that will allow a 10-mm implant to be placed in this region. Following osseointegration and abutment connection, approximately 1.5 mm of bone will resorb if the implant is placed at the crest of bone (Figure 6).7 This results in 1.5 mm of remaining native bone for integration. What is important is the fact that this 10-mm implant is now in 8.5 mm of bone, of which more than 80% is grafted.
CONTROLLING CRESTAL BONE LOSS
Figure 7. An illustration of “platform switching.”
Prevention of bone loss following abutment connection may allow for more predictable long-term results when implants are placed in the cosmetic zone or in augmented bone. Implant companies and clinicians have expended considerable effort to find ways to control crestal bone loss after abutment connection.20 Use of nonsubmerged implants to eliminate bone loss is a proven way to accomplish this.5 A scalloped implant platform has been developed to follow the osseous architecture and eliminate crestal bone loss by maintaining the microgap in a supracrestal position.20 Another method is altering the horizontal position of the microgap.21 As stated previously, crestal bone loss following abutment connection has both horizontal and vertical components. The horizontal component consists of the 1.3 to 1.4 mm of remodeling from the microgap to the crest of bone.8 If the horizontal component can be controlled or decreased, then crestal bone loss can also be decreased.21 One way to decrease the horizontal component is by medializing the position of the microgap, ie, using a smaller abutment. For example, if a 5-mm implant platform is placed at or below the crest of bone, a 4.1-mm abutment is utilized in order to medialize the position of the microgap, which would be 0.45 mm circumferentially (Figure 7). This technique can be termed “platform switching.”
Figure 8. Radiograph illustrating medialization of microgap, maintenance of crestal bone.
Figure 9. Radiograph illustrating medialization of microgap, maintenance of IHB.
Figure 10. Implant restoration of No. 8. Note maintenance of soft-tissue contours.
Figure 11. Radiograph illustrating platform switching in posterior maxilla. Note maintenance of bone.
By medializing the position of the microgap using the technique of platform switching, crestal bone changes will be minimized if not nullified, because the horizontal component of crestal bone loss will be eliminated (Figure 8).21 The result is maintenance of both interproximal height of bone and buccal-lingual dimension (Figures 9 and 10).21 In the posterior maxilla where sinus augmentation has occurred, maintenance of crestal bone will result in an increase in the volume of bone in contact with the implant (Figure 11).21
Platform switching is a simple and effective way to control circumferential bone loss around dental implants. By altering the horizontal position of the microgap, the horizontal component of bone loss after abutment connection can be reduced.21 However, this technique has its limitations. Switching can only be utilized with components that have similar designs. In addition, a sufficient amount of vertical height is still needed to develop a proper emergence profile.
Maintaining crestal bone after implant placement remains a challenge. Undesirable osseous and soft-tissue changes can take place subsequent to implant placement. A technique for controlling circumferential bone loss around dental implants—“platform switching”—has been described. Additional questions remain. Can implants be placed closer than 3 mm from an adjacent implant while still maintaining interproximal height of bone?14 Can implants be placed less than 1.5 mm from an adjacent tooth and still maintain interproximal bone?14 Can implants be placed at or below the osseous crest and avoid bone loss to the first thread after abutment connection?5 Can implant aesthetics be improved with the use of platform switching?
The answers to these questions may greatly influence future implant and abutment designs to preserve critical support around dental implants.
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6. King GN, Hermann JS, Schoolfield JD, et al. Influence of the size of the microgap on crestal bone levels in non-submerged dental implants: a radiographic study in the canine mandible. J Periodontol. 2002;73:1111-1117.
7. Hermann JS, Cochran DL, Nummikoski PV, et al. Crestal bone changes around titanium implants. A radiographic evaluation of unloaded nonsubmerged and submerged implants in the canine mandible. J Periodontol. 1997;68:1117-1130.
8. Tarnow DP, Cho SC, Wallace SS. The effect of inter-implant distance on the height of inter-implant bone crest. J Periodontol. 2000;71:546-549.
9. Lindhe J. Textbook of Clinical Periodontology. 1st ed. Philadelphia, Pa: WB Saunders; 1983:19-66.
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12. Langer B. Soft tissue complications: the esthetic dilemma. Int J Oral Maxillofac Implants. 2003;18:767-768.
13. Kois JC. Predictable single-tooth peri-implant esthetics: five diagnostic keys. Compend Contin Educ Dent. 2004;25:895-900.
14. Salama H, Salama MA, Garber D, et al. The interproximal height of bone: a guidepost to predictable aesthetic strategies and soft tissue contours in anterior tooth replacement. Pract Periodontics Aesthet Dent. 1998;10:1131-1141.
16. Hartmann R, Muller F. Clinical studies on the appearance of natural anterior teeth in young and old adults. Gerodontology. 2004; 21(1):10-16.
17. Tarnow DP, Magner AW, Fletcher P. The effect of the distance from the contact point to the crest of bone on the presence or absence of the interproximal dental papilla. J Periodontol. 1992;63:995-996.
18. Gargiulo A, Krajewski J, Gargiulo M. Defining biologic width in crown lengthening. CDS Rev. 1995;88:20-23.
19. Wallace SS, Froum SJ. Effect of maxillary sinus augmentation on the survival of endosseous dental implants. A systematic review. Ann Periodontol. 2003;8:328-343.
20. Wohrle PS. Nobel Perfect esthetic scalloped implant: rationale for a new design. Clin Implant Dent Relat Res. 2003;5 suppl 1:64-73.
Dr. Gardner completed a general practice residency and a 2-year fellowship in advanced prosthetics and implant dentistry at North Shore University Hospital (NSUH). He remains on staff at NSUH, where he teaches in the department of implant dentistry. Dr. Gardner maintains a private practice in Roslyn Heights, NY, and can be reached at (516) 484-6394.