Biology and Pathology of Peri-implant Soft Tissues

Teeth are anatomically unique because they are the only structures of the body that penetrate a lining or covering epithelium. Thus, teeth and dental implants are two isolated examples of structures that pierce the integument. While proper anchorage of an implant in the bone (osseo-integration) is a prerequisite for its stability, long-term retention of an implant seems to depend on the epithelial and connective tissue attachment to the titanium surface, ie, a complete soft tissue seal protecting the bone from the oral environment.

Several animal and in vitro experiments have stressed that similarities obviously exist between the gingiva and the peri-implant mucosa with respect to both structures and connective tissue components. The importance of the soft tissue seal at implant sites with respect to its "functional success" has not yet been completely evaluated. However, well-controlled experimental studies in this field have been performed by a research team at the Department of Periodontology.

Berglundh et al compared clinically healthy peri-implant mucosa and free marginal gingiva in the beagle dog with respect to structure and composition. Histological analysis revealed that the two soft tissue units both had a keratinized oral epithelium and a junctional epithelium with a length of approximately 2 mm. The height of the gingival supracrestal connective tissue portion was approximately 1 mm where the orientation of the collagen fiber bundles were fan-shaped, with the acellular root cementum serving as a center. Titanium implants, however, lack root cementum, and therefore the collagen fiber bundles in the peri-implant mucosa run mainly parallel with the implant surface and originate from the bone surface.

The supracrestal implant surface facing the connective tissue, defined coronally by the apical cells of the junctional epithelium and apically by the bone surface, was found to be approximately 2 mm in height, ie, two times that of the gingival supracrestal connective tissue attachment. Within this zone that is free from epithelium, a "reaction" must have occurred between the connective tissue and the titanium dioxide abutment surface. This "reaction" will, in one way or another, limit the proliferation possibility of the junctional epithelium. Evidently, this "zone of interaction" is not apprehended as a wound surface. This must have a significant effect on adhesion of the peri-implant mucosa to the titanium abutment, and thus also on the soft tissue defense against an exogenous irritation. The qualitative analysis of the connective tissue portion subjacent to the junctional epithelium and within the supracrestal region revealed that the peri-implant mucosa contained significantly more collagen and fewer fibroblasts than corresponding gingival connective tissue portions.

Berglundh et al studied the vascular topography of the periodontium and the peri-implant soft and hard tissues using the beagle-dog model. The authors observed that the gingiva and the supracrestal connective tissue at teeth are supplied by supraperiosteal vessels lateral to the alveolar process and vessels from the periodontal ligament. The peri-implant mucosa, on the other hand, was found to be supplied by terminal branches of larger vessels originating from the periosteum of the bone at the implant site. In both situations, the blood vessels built a characteristic "crevicular plexus" lateral to the junctional epithelium. At teeth, the supracrestal connective tissue portion demonstrated a rich vascularization, while at the corresponding implant sites very few, if any, vessels were observed. These observations support the suggestion made by Buser et al that the peri-implant soft tissue may have an impaired defense capacity against exogenous irritation.

Ericsson and Lindhe, using the beagle-dog model, examined the resistance to mechanical probing offered by clinically healthy gingival tissues and peri-implant mucosa at titanium dental implants. The authors reported that the probe penetration was more advanced at implants than at teeth (- 2.0 mm and ~ 0.7 mm, respectively). Thus, at the implant sites the probe tip displaced the junctional epithelium as well as the connective tissue portion facing the abutment surface in the lateral direction and stopped close to the bone crest. The tip of the probe thus stopped within the supracrestal connective tissue portion, and occasional rupture of some blood vessels resulted in bleeding. At the tooth sites, however, the tip of the probe consistently terminated coronally to the apical portion of the junctional epithelium, thus roughly identifying the bottom of the gingival pocket. Bleeding on probing is an important tool to properly diagnose the condition of the apical portion of the periodontal soft tissues. In this study, however, bleeding on probing was sometimes observed at implants, but rarely at teeth. Based on current knowledge, the importance of such an observation at implants is doubtful.