The role of dental implants is to replace natural teeth in edentulous spaces. To be functionally useful, implants have to pierce the oral mucosa and enter the oral cavity, thus establishing a transmucosal connection between the external environment and the inner parts of the body. In one-piece implants, the transmucosal component comprises part of the implant, while in two-piece implants the transmucosal component is a separate part from the implant. A one-piece direct implant system is a one-piece implant, where the pillar for crown placement is part of the implant, instead of being a separate screwed-in component (Figure 1).
Historically, most implants have been two-piece implants in order to have them sheltered submucosally during initial healing, so as to avoid the influence of the oral flora and of mechanical stresses. This submerged approach, ad modum Brånemark, has long been considered as a mandatory condition for safely obtaining osseointegration. It was later demonstrated in animal studies1-3 and in clinical human studies,4 however, that nonsubmerged implants placed following a one-stage surgical approach reach comparable levels of success and were as safe as submerged implants placed according to a two-stage surgical approach. These results allowed the broader use of one-piece implants.
With two-piece implants, the transmucosal component (ie, the abutment) can be disconnected from the implant body. As the soft tissues are adherent to this abutment, this disconnection will destroy the soft tissue/implant component interface. If this trauma is infrequently repeated,5 there will be no negative effect on the crestal bone level. Alternately, if several disconnections are performed,6 or if the loosening is undetected and leads to lasting abutment mobility,7 then an apical migration of the connective tissue adhesion and of the junctional epithelium will occur, leading to marginal bone loss. Similarly, when nonbiocompatible materials are placed at the transmucosal level, soft tissues will not be able to adhere and will migrate apically until they reach a biocompatible surface at the implant level. On the contrary, with one-piece implants, the transmucosal surface is always biocompatible and cannot loosen, which is why they are considered as a safer option with regards to the soft tissue interface.
Nevertheless, it has been demonstrated that one-piece implants can also lead to higher marginal bone remodelling when the machined transmucosal neck, devoid of bone-interlocking elements, is placed at an infrabony level.8 The reason for this is probably biomechanical, as it has been shown that retention elements at the neck of the implant will counteract marginal bone resorption.9 Shear forces are deleterious for the crestal bone, whereas compressive forces are beneficial and stimulating for the latter. The first thread, or groove, is the first zone of three-dimensional bone interlocking where stresses are compressive, promoting bone stabilization.
Thus, the vertical position of the first thread will generally determine the level to which the marginal bone will remodel; the deeper infrabony that the first thread will be located, the more bone resorption that will occur.10 This can sometimes be a problem with flapless surgical approaches, as the bone level cannot be directly observed. As a consequence, the thickness of the soft tissues must be carefully determined with a periodontal probe before implant placement in order to avoid submerging the first thread too deep. Consequently, an error in the vertical positioning of the implant can ruin the benefit of using a one-piece implant and of using a flapless approach, which causes surgical trauma to the marginal bone.
The same consequence can occur if, horizontally, the implant does not respect the biological thickness of crestal bone. In cases where the implant is placed in an excessively thin bone crest, or when the positioning thins one of the cortical plates, the result will also be crestal bone loss.
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As Sharpey’s fibers solidly anchored on teeth inside a cementum layer do not exist on implants, the connective tissue located between the bone level and the junctional epithelium is only adherent to the transmucosal implant component. This interface is very brittle, which means that a nonmobile, fibrous soft tissue environment is essential around implants in order to limit repeated tearing of the soft tissue interface (ie, the biological seal) due to soft tissue mobility. Consequently, a “punch” technique for accessing the crestal bone without reflecting a flap can sometimes be extremely detrimental when performed in insufficient amounts of fibrous mucosa.
It is critical for the clinician to understand that the width of keratinized epithelium visible from the outside of the mucosa is not the appropriate criteria for determining if a punch approach can be performed. The connective fibers underneath the keratinized epithelium are orientated perpendicular to the bone crest: A punch approach preserving a 2–mm-wide external keratinized epithelium can nearly remove the inner fibrous tissue and leave mobile muscular insertions at the implant neck.
When implants are immediately loaded with cemented crowns, extreme care must be taken to avoid the subgingival injection of resin or of provisional cementation material, which can easily occur since the soft tissue/implant interface is not yet healed and the mucosal seal not yet reformed. The injection of these nonbiocompatible materials would definitively impair the formation of the biological width and oblige the soft tissue interface to migrate apically in order to find a biocompatible surface to adhere to, ultimately leading to a 3-mm loss of bone and to the formation of a periodontal pocket (Figures 2-3-4).
Given that these four basic conditions are respected, one-piece implants placed according to a flapless approach and immediately loaded with cemented restorations are safe and effective.
*Professor and Head, Department of Periodontology and Dental Surgery, University of Liege, Liege, Belgium.
†Editor-in-Chief, Practical Procedures & Aesthetic Dentistry; private practice, Paris, France.
- Ericsson I, Nilner K, Klinge B, Glantz PO. Radiographical and histological characteristics of submerged and nonsubmerged titanium implants. An experimental study in Labrador dog. Clin Oral Implants Res 1996;7(1):20-26.
- Abrahamsson I, Berglundh T, Moon IS, Lindhe J. Peri-implant tissues at submerged and non-submerged titanium implants. J Clin Periodontol 1999;26(9):600-607.
- Weber HP, Buser D, Donath K, et al. Comparison of healed tissues adjacent to submerged and non-submerged unloaded titanium dental implants. A histometric study in beagle dogs. Clin Oral Implants Res 1996;7(1):11-19.
- Petersson A, Rangert B, Randow K, Ericsson I. Marginal bone resorption at different treatment concepts using Brånemark dental implants in anterior mandibles. Clin Implant Dent Relat Res 2001;3(3):142-147.
- Abrahamsson I, Berglundh T, Lindhe J. The mucosal barrier following abutment dis/reconnection. An experimental study in dogs. J Clin Periodontal 1997:24(8):568-572.
- Abrahamsson I, Berglundh T, Sekino S, Lindhe J. Tissue reactions to abutment shift: An experimental study in dogs. Clin Implant Dent Relat Res 2003;5(2):82-88.
- Hammerle CH, Bragger U, Burgin W, Lang NP. The effect of subcrestal placement of the polished surface of ITI implants on marginal soft and hard tissues. Clin Oral Implants Res 1996;7(2):111-119.
- Hermann JS, Buser D, Schenk RK, et al. Biologic width around one- and two-piece titanium implants. Clin Oral Implants Res 2001;12(6):559-571.
- Hansson S. The implant neck: Smooth or provided with retention elements. A biomechanical approach. Clin Oral Implants Res 1999:10(5):394-405.
- Rompen E, Touati B, Van Dooren E. Factors influencing marginal tissue remodeling around implants. Pract Proced Aesthet Dent 2003;15(10):754-761.