Many studying the effects of crown contour on tissue health have concluded that undercontouring is equal to a natural contour or an improvement in terms of gingival health. This has been evaluated in animal models and in humans, discrediting the idea of a protective function of a supragingival bulge. In fact, more plaque accumulates with a larger contour, whereas flatter contours favor gingival health.45–48
Ovate Pontic Sites
Ovate pontics are considered optimal for esthetics and for cleansability. Being smooth and convex, such pontics support the edentulous space while providing appropriate facial and interproximal contours (Figs 1-5 to 1-7). Procedures to enhance the tissue ridge to accommodate such pontics have been proposed.49 The design and fabrication for close adaptation of the pontic to the pontic site require special techniques,50 which are discussed in the following chapters.
Fig 1-5 Removed restoration with a pontic that is inadequately designed. Note the concave ridge lap design and the rough intaglio surface, which may adversely affect the ability of the patient to clean as well as subsequently compromise the health of the pontic site.
Fig 1-6 (a) View of the intaglio surface of an ovate pontic. (b) Being smooth and convex, such pontics support the edentulous space while providing appropriate facial and interproximal contours and allowing the patient to maintain adequate oral hygiene. (Ceramics: Masayuki Saito.)
Fig 1-7 Facial view of the try-in of a ceramic zirconia-based fixed dental prosthesis with an ovate pontic. A cautious and delicate approach is applied during the provisional phase to facilitate an adequate design and a close adaptation of the pontic to the pontic site.
The question arises as to tissue response to materials intimately adapted to these structures. There are many materials that can be used for restorations in contact with tissue. Ovate pontics provide an opportunity to assess tissue response to various metal alloys, polymers (ie, acrylic resins and composite resins), and ceramics. With proper home care and a daily flossing regimen, no real differences are seen among a wide variety of materials.14 This can perhaps be explained by the repeated and frequent separation of the epithelial interface from the restorative material. Flossing this area completely is principally possible under such convex pontics. During initial healing following placement of the pontics, tissue responses may indeed vary. Low-fusing ceramic may be favored over acrylic resins.51 Edentulous spaces in contact with ovate pontics do not show clinical and histologic signs of inflammation in the presence of adequate oral hygiene measures (Fig 1-8). Histologically, this style of pontic is associated with a thinner keratin layer and mild changes in the composition of the adjacent connective tissue, in comparison with tissues not in contact with the pontic.52 Despite a lack of specific research, the tissue is presumed healed by 6 weeks and mature by 12 weeks.
Fig 1-8 Occlusal view of an ovate pontic site demonstrating no clinical signs of inflammation.
When plaque accumulation and severity of gingival inflammation were evaluated underneath pontic sites fabricated with different dental materials (gold alloys, silver-palladium, cobalt-chromium, nickel-chromium, feldspathic porcelain, and composite resin), high levels of plaque control were associated with healthy gingival tissue irrespective of the pontic material used. Inadequate plaque control was associated with the development of gingival inflammation and increases in the amount of bacterial plaque underneath and around the pontic site.14,15
Biologic Width and Implants
General concepts
Lessons learned from the biologic width of natural teeth have parallels when working with dental implants. Like teeth, implants also display a zone of epithelial attachment and a zone of connective tissue apical to that. The connective tissue has different characteristics but occupies the same location as around natural teeth. Unlike teeth with perpendicularly oriented Sharpey’s fibers, with implants, connective tissue fibers display an orientation mostly parallel or oblique to the implant surface.53,54 When titanium abutments are placed at the time of surgery and are never removed, epithelium and connective tissue can attach themselves to the abutment itself.55 This has been evidenced in human cadavers.56 The result is a wider band of total attachment than that seen on natural teeth or when implant components are removed and replaced. In a dog model, when similar abutments were removed and replaced repeatedly, epithelial attachment was found to be positioned more apically on the abutment. More bone resorption was also observed when the zone of connective tissue was more apically positioned. It would appear that bone resorbs to allow for an appropriate reestablishment of biologic width even around implants.57 The degree of resorption was similar in a human study comparing abutments placed at the time of surgery with abutments removed and replaced twice.58
Similarly to what is seen with crown lengthening procedures when the tissue is positioned very close to the bone crest, implants also appear to need enough tissue height to form the biologic width to avoid bone resorption. When implants are placed with excessively thinned flap tissues, biologic width has to be re-formed. Loss of crestal bone height is seen in both animal59 and human60 models. It is recommended that flaps placed around implants be coronally positioned adequately to allow for over 2 mm of vertical tissue thickness in order to ensure that biologic width is established without bone resorption.
From a structural point of view, Moon et al61 described the nature of the biologic attachment around dental implants in a beagle dog model. In evaluating the connective tissue zone, they observed one zone of 40 µm close to the implant surface and one more distant zone of 160 µm. While the zones were continuous with one another, they had different characteristics. The zone against the implant was characterized by an absence of blood vessels and an abundance of fibroblasts. The fibroblasts were interposed between thin collagen fibers. The zone farther from the implant contained fewer fibroblasts but more collagen fibers and blood vessels. They speculated that the more fibroblast-rich region plays a role in maintenance of a proper seal to the titanium between the oral environment and the peri-implant bone.
In another beagle dog study, Abrahamsson et al62 examined the nature of attachment to different dental materials used on implant abutments. In observations on titanium, densely sintered high-purity alumina, gold alloy, and feldspathic porcelain (for metal-ceramic restorations), they found that titanium and alumina allowed formation of attachment that included a normal epithelial and connective tissue band totaling approximately 3.5 mm. In contrast, when gold alloy or feldspathic porcelain was used, no attachment was formed on the abutments. The free gingival margin receded and bone resorption occurred, causing the biologic width to reform apical to the materials. This pattern has been repeated in a dog model in which zirconia and titanium maintained attachment while gold alloy lost attachment and adjacent bone over 5 months.63
Biologic width and implant platform switching
A proposed way to preserve marginal bone around implants is to utilize an implant abutment that is smaller in diameter