Postrestorative reductions in crestal bone height around endosseous dental implants have long been acknowledged to be a normal consequence of implant therapy involving two-stage hexed implants.^1-4 Such remodeling does not typically occur as long as the implant remains completely submerged, but rather develops when an abutment is connected during second-stage surgery, when a two-stage implant is placed and connected to an abutment in a one-stage procedure, or when an implant is prematurely exposed to the oral environment and bacteria.⁵

Research by Hermann, et al demonstrated that crestal bone loss typically occurs approximately 2 mm apical to the implant-abutment junction (IAJ).⁶ This position appears to be constant, regardless of where the IAJ is situated relative to the original level of the bony crest.⁶ The researchers also demonstrated that the addition of a textured, bone-holding surface within 0.5 mm of the IAJ fails to prevent bone resorption within 2 mm apical to the IAJ.⁶

Investigations by various researchers have shed light on why the presence of the IAJ appears to trigger resorption in the adjacent bone. Ericsson, et al found histologic evidence of inflammatory cell infiltrate associated with a 1-mm– to 1.5-mm–tall zone adjacent to the IAJ.⁷ Berglundh and Lindhe concluded that approximately 3 mm of peri-implant mucosa is required to create a mucosal barrier around a dental implant.⁸ This suggests that crestal bone remodeling may occur to create space when inadequate soft-tissue height is present so that a biological seal can be established, isolating the crestal bone and protecting it from the oral environment.

These investigations have focused on implant systems in which the diameter of the implant-seating surface matches that of the abutment. This ubiquitous design positions the abutment inflammatory cell infiltrate in direct approximation to the bone at the time of abutment connection.

Platform Switching

The term “platform switching” refers to the use of a smaller-diameter abutment on a larger-diameter implant collar; this connection shifts the perimeter of the IAJ inward toward the central axis (ie, the middle) of the implant.⁵ Lazzara and Porter theorize that the inward movement of the IAJ in this manner also shifts the inflammatory cell infiltrate inward and away from the adjacent crestal bone, limiting the bone change that occurs around the coronal aspect.⁵ Crestal bone preservation has been observed on other commercially available implant designs, purportedly attributed to microthreads at the coronal aspect of the implant, connection designs, occlusal schemes, or combinations thereof.⁹

In 1991, 5-mm– and 6-mm–diameter implants were introduced with seating surfaces (ie, restorative platforms) of the same dimensions. These large-diameter implants, with a larger surface area, were intended to increase the amount of bone-to-implant contact when placing shorter implants in areas of limited bone height, such as under the maxillary sinus or above the inferior alveolar canal. The ability to increase the bone-to-implant contact by the use of wide-diameter implants also enhanced the likelihood of achieving primary stability in areas of poor-quality bone. At the time of the wide-diameter implants’ introduction, no matching, wide-diameter prosthetic components were available. Hence, clinicians restored them with standard 4-mm abutments.

After a five-year period, the typical pattern of crestal bone resorption was not observed radiographically in which platform switching was utilized. The authors theorize that this occurred because shifting the IAJ inward also repositioned the inflammatory cell infiltrate and confined it within a 90-degree area that was not directly adjacent to the crestal bone.

The ability to reduce or eliminate crestal bone loss can result in significant aesthetic and clinical benefits. In order to facilitate the practice of platform switching, specific implant systems have been developed.

This expanded collar of such implants can provide better engagement of the bone crest, better sealing of extraction sockets, and better primary stability. Restoring, for example, a 4.8-mm implant collar with the corresponding 4.1-mm prosthetic component shifts the IAJ inward, moving the inflammatory infiltrate away from the surrounding bone. To achieve this effect and maintain adequate soft-tissue depth, the implant should be placed crestally if sufficient soft-tissue height and interocclusal space is present, or subcrestally if insufficient soft tissue height and interocclusal space is present.

Case Presentation

A 28-year-old male patient presented with nonrestorable maxillary central incisors that had previously been treated endodontically before being fractured by trauma (Figures 1 and 2). The teeth were carefully extracted and, with the aid of a surgical guide, two 5.0-mm x 13-mm implants were placed in a single-stage protocol (Figures 3-4-5-6-7). The specific implant diameters and lengths were selected by the clinician based on the size and shape of the individual sockets. The implants were placed in a flapless manner in order to protect the buccal cortical plate from injury to the vascular supply, which is often associated with a full-thickness flap. Moreover, great care was taken to avoid touching the buccal plate of the sockets during implant site preparation.

Healing abutments with 5-mm emergence profiles and 4.1-mm restorative platforms were immediately placed (Figures 8 and 9). The patient was then discharged with antibiotic and anti-inflammatory prescriptions.

After three days, two 4.1-mm customizable abutments, prepared by the dental technician on the master cast were inserted into the internal interface of the implants and torqued to 20 Ncm (Figures 10-11-12-13). These titanium abutments had a gold-nitride coating that would prevent graying of the marginal gingival tissue. Two acrylic provisional crowns were then luted to the abutments with temporary cement and adjusted out-of-occlusal contact, following the protocol of immediate nonocclusal loading (Figure 14).¹⁰ An intraoral radiograph was taken (Figure 15), and the patient was instructed to avoid loading the crowns for any purpose for at least eight weeks. Gentle brushing with a toothpaste containing chlorhexidine, however, was recommended.

Following a two-month healing period (Figure 16), clinical osseointegration was confirmed, and two metal-ceramic crowns were placed. The prognosis for maintenance of the interdental papillae was excellent. The definitive crowns were constructed on duplicate abutments made from the surgical index at the time of implant placement.

No additional implant-level impression procedure was required due to the technical prosthetic protocol, which allowed the construction of the definitive crowns on a duplicated model and their subsequent delivery chairside (Figures 17 and 18).

Discussion

Clinical observation of the bone-preserving effects of “platform switching” has been ongoing for more than a decade. This procedure has been used by a number of clinicians successfully around the world.

The procedure requires that the “switch” be in place from the day the implant is uncovered or exposed to the oral cavity in either a one- or two-stage approach. It cannot be utilized after the establishment of the biologic width around a conventional implant-abutment interface configuration to regain crestal bone height. Potential applications include situations where a larger implant is desirable but the prosthetic space is limited, in the aesthetic zone where preservation of the crestal bone can lead to improved aesthetics and where shorter implants must be utilized.

It is important to note that sufficient tissue depth (approximately 3 mm or more) must be available to accommodate an adequate biologic width. In the absence of sufficient soft tissue, bone resorption will result, regardless of the implant geometry.^11-14 This sometimes requires that the implant platform be placed below the bone crest to obtain adequate tissue depth. Additionally, sufficient ridge width (ie, minimum of 6.8 mm) must be present to accommodate the flared 4.8-mm implant collar. Case selection and management, however, may influence the clinical outcome and radiographic evidence of crestal bone preservation.

While bone preservation has been observed for some time as a result of the use of a standard-diameter abutment on a wider-diameter implant, the potential for confusion has existed for clinicians who have attempted to employ this strategy while using standard components. Dental laboratories and restorative dentists are accustomed to working with matching-diameter implants and abutments.

Conclusion

Preliminary evidence suggests that the anticipated bone loss that occurs around two-stage hexed implants may be reduced or eliminated when implants are restored with smaller-diameter abutments, a practice termed platform-switching.⁵ A new implant design has been developed that facilitates this practice, and initial clinical observations indicate the preservation of crestal bone results. Definitive clinical trials are currently underway.

*Clinical Professor, Department of Periodontics, University of Pennsylvania, School of Dental Medicine, Philadelphia, PA; private practice, Philadelphia, PA

†Private practice, Verona, Italy

‡Assistant Clinical Professor and Head of the Section of Implant Dentistry and Oral Rehabilitation, Department of Odontology, Galeazzi Institute, Milan, Italy; private practive, Como, Italy

§Diplomate, American Academy of Periodontology; Associate Clinical Professor, Department of Implant Dentistry, New York University College of Dentistry, New York, NY; private practice, Voorhees, NJ

||Private practice, West Palm Beach, FL

References:  

1. Albrektsson T, Zarb G, Worthington P, Eriksson AR. The long-term efficacy of currently used dental implants: A review and proposed criteria of success. Int J Oral Maxillofac Impl 1986;1(1):11-25.
2. Smith DE, Zarb GA. Criteria for success of osseointegrated endosseous implants. J Prosthet Dent 1989;62(5):567-572.
3. Bengazi F, Wennestrom JL, Lekholm U. Recession of the soft tissue margin at oral implants: A 2-year longitudinal prospective study. Clin Oral Implants Res 1996;7(4):303-310.
4. Morris HF, Ochi S. The influence of implant design, application, and site on clinical performance and crestal bone: A multicenter, multidisciplinary clinical study. Dental Implant Clinical Research Group (Planning Committee). Implant Dent 1992;1(1):49-55.
5. Lazzara RJ, Porter SS. Platform switching: A new concept in implant dentistry for controlling post-restorative bone levels. Accepted for Publication, 2006 Int J Perio Rest Dent.
6. Hermann JS, Schoolfield JD, Nummikoski PV, et al. Crestal bone changes around titanium implants: A methodologic study comparing linear radiographic with histometric measurements. Int J Oral Maxollofac Impl 2001;16(4):475-485.
7. Ericsson I, Persson LG, Berglundh T, et al. Different types of inflammatory reactions in peri-implant soft tissues. J Clin Periodontol 1995;22(3):255-261.
8. Berglundh T, Lindhe J. Dimension of the periimplant mucosa. Biologic width revisited. J Clin Periodontol 1996;23(10):971-973.
9. Morris HF, Winkler S, Ochi S, Kanaan A. A new implant designed to maximize contact with trabecular bone: Survival to 18 months. J Oral Implantol 2001;27(4):164-173.
10. Cocchetto R, Vincenzi G. Delayed and immediate loading of implants in the aesthetic zone: A review of treatment options. Pract Proced Aesthet Dent 2003;15(9):691-698.
11. Todescan FF, Pustiglioni FE, Imbronito AV, et al. Influence of the microgap in the peri-implant hard and soft tissues: A histomorphometric study in dogs. Int J Oral Maxillofac Impl 2002:17(4):467-472.
12. Hermann JS, Cochran DL, Nummikoski PV, Buser D. Crestal bone changes around titanium implants. A radiographic evaluation of unloaded nonsubmerged and submerged implants in the canine mandible. J Periodontol 1997;68(11):1117-1130.
13. Herman J, Buser D, Schenk R, et al. Biologic width around one-and two-piece titanium implants. A histomorphometric evaluation of unloaded nonsubmerged and submerged implants in the canine mandible. Clin Oral Impl Res 2001;12:559-571.
14. Gaucher H, Bentley K, Roy S, et al. A multi-centre study of Osseotite implants supporting mandibular restorations: A 3-year report. J Can Dent Assoc 2001;67(9):528-533.