Influence of the Microgap in the Peri-implant Hard and Soft Tissues: A Histomorphometric Study in Dogs

Purpose: The purpose of this study was to examine the dimensions and relationships of the periimplant tissues surrounding osseointegrated 2-stage implants placed at different depths in bone. Materials and Methods: Twenty-four implants were placed in the mandibles of 4 mongrel dogs. A modification of the surgical protocol was introduced so that in group I, implants remained 1 mm above the bone crest; in group II, implants were placed level with the bone crest; and group III implants were countersunk to approximately 1 mm below the bone crest. After 3 months, abutment operations were carried out with the placement of 3-mm standard abutments. Following a healing period of 3 months the dogs were sacrificed. A total of 20 implants were available for histometric analysis. Non-decalcified sections were evaluated for the dimensions of the junctional epithelium, connective tissue band, marginal bone level, and bone-to-metal contact. Results: Histologic observations showed a mucosal barrier consisting of keratinized oral epithelium continuous with a thin junctional epithelium facing the implant and abutment surface. Junctional epithelium showed a mean of 1.67 mm for group I, 1.93 mm for group II, and 2.78 mm for group III. These values were not statistically different. The band of connective tissue had a mean of 1.13 mm for group I, 0.92 mm for group II, and 1.60 mm for group III. These values were not statistically different, except for group II versus group III. Bone level had a mean of 2.50 mm for group I, 2.30 mm for group II, and 1.60 mm for group III. These differences were significant between groups I and III. The surface of bone contact along the implant (BMC%) showed mean values of 46.8% in group I, 53.7% in group II, and 49.0% in group III (no significant differences among the 3 groups). Discussion: There was a clear tendency of the epithelium and connective tissue to be longer the deeper the implants were placed, although those differences were not statistically significant. Bone loss was smaller for group III (countersink group). This is not in accordance with recent articles which have stated that bone will maintain its biologic width. Conclusions: When the microgap between implants and abutments was placed deeper in the bone, additional bone loss did not result.