Fig 1-9 Periapical radiograph demonstrating the platform switching approach. Note that the implant abutment is smaller in diameter at the platform level.
Two-piece implants are designed for placement of the implant at the level of the bone crest, followed by connection of the implant abutment. The microscopic space between the parts is known as the microgap or the implant-abutment interface. Proximity to the alveolar bone crest implicates this interface as being responsible for remodeling of bone at the time of abutment connection, inflammation of peri-implant mucosa, and peri-implantitis.12,13,64,65 The implant-abutment interface may favor bacterial colonization. The dimension of the interface space is about 10 µm, while the mean diameter of bacteria is less than 2.0 µm.66,67 This microbial leakage may be the main culprit for the presence of chronic inflammatory infiltrate and crestal bone resorption. Occlusal loading of the abutment may result in additional opening of the interface, increasing the level of bacterial penetration.12,66,68–75 The mere presence and contamination of the microgap by periodontal pathogens, however, may not necessarily result in inflammation of the peri-implant mucosa or peri-implantitis.66,75
Many studies have evaluated the effect of platform switching. Some take place in the form of cohort clinical studies (without controls), while others are in the form of randomized controlled trials. Results of such trials have been inconsistent. Generally speaking, platform switching results in a small but statistically significant difference at 1 year after the restoration delivery compared with implants restored with abutments matching the implant platform diameter. Studies demonstrate a difference ranging from 0.25 to 0.37 mm of bone preserved by platform switching.76,77 However, some studies showed no difference at 1 year.78 In a systematic review, the authors concluded that there may be about 0.5 mm of bone preservation with platform-switched implants.79 In the same review, however, caution was emphasized because heterogeneity of data and publication bias were noted. No systematic reviews showed a difference in implant survival rates between the two types of restorative design.77,79 Enkling et al80 performed a randomized clinical trial and concluded that at the 3-year follow-up there was no significant difference in bone height, with a mean intra-individual difference of 0.05 mm between the platform switching and the traditional implant/abutment approaches. In a 5-year follow-up study, Vigolo and Givani81 showed a similar 1-year difference of 0.3 mm, favoring bone levels on platform-switched abutments. No changes were noted from that point out to 5 years.
The smaller abutment diameter would seem to be unfavorable to load, but this may depend on the design of the implant-abutment interface. In an in vitro study, Leutert et al82 found that internally connected abutments with a smaller diameter than the implant platform showed significantly higher bending moments compared with abutments of the same diameter. The small offset does present an obstacle to probing and periodontal maintenance curetting in cases where there is implant disease. In larger teeth, it may also create a larger size mismatch, negatively affecting tissue emergence profile angle.
Based on the available research evidence, it seems reasonable to conclude that platform switching results in a slightly (approximately 0.4 mm) better maintenance of crestal bone height around the implant at 1 year. This does not appear to make a clinically significant difference in patient outcomes; thus, at present, it seems reasonable to choose either a platform-switched or conventional equal-diameter restorative solution based on other parameters. Such parameters might include contour matched to the proposed restoration, availability of a specific custom abutment modality, and desired placement of restorative margins.83 The exception to this may be when adjacent implants must be placed in close proximity.84,85 It remains to be seen in longer-term studies if there is a difference in incidence of peri-implant disease based on abutment diameter at the implant platform.
Tissue Phenotype
General concepts
In 1969, Ochsenbein and Ross86 reported two main types of gingival morphology: flat and scalloped. The flat gingival morphology was closely related to a square tooth form with a minimal distance between the buccal gingival margin and interdental gingival levels. The scalloped gingival morphology was associated with a tapered tooth form and considerable height—about 5 to 6 mm—between the buccal gingival margin and interdental gingival levels. They also indicated that the gingival contours mimic and follow the contours of the underlying bone. Later, the term periodontal biotype was proposed by Seibert and Lindhe,87 classifying the gingiva into two distinct types of gingival morphology: thin (less than 1 mm) and scalloped, or thick (more than 1 mm) and flat. This term has been frequently used clinically to describe the faciolingual thickness of the facial gingiva around teeth (synonymous with gingival thickness), relating it to papilla height. The thin and scalloped gingival type is associated with a tooth with tapered crown form, subtle cervical convexity, small proximal contact areas located near the incisal edge of the tooth, shallow probing depths, a narrow zone of keratinized gingiva, and thin alveolar bone. The thick and flat type corresponds to a tooth with squared facial form, distinct cervical convexity, relatively large and more apically located contact areas, a wide zone of keratinized gingiva, and thicker alveolar bone88 (Table 1-1). In a study assessing different morphologic characteristics of the gingiva, Müller and Eger94 proposed the use of the term phenotype instead of biotype as the correct term in describing features of the marginal periodontium that are influenced by both genetic and environmental factors.
Table 1-1 Features of thin and thick gingival phenotypes*
| Thin phenotype | Thick phenotype | |
|---|---|---|
| Gingival thickness | < 1 mm | ≥ 1 mm |
| Gingival tissue | Delicate and friable | Dense and fibrotic |
| Keratinized gingiva | Narrow band | Wide band |
| Soft tissue architecture | Scalloped | Flat |
| Underlying bony architecture | Thin or minimal | Thick |
| Crown form | Tapered | Square |
| Cervical convexity | Subtle | Distinct |
| Proximal contact area | Located near the incisal edge | Located more apically |
*Data compiled from multiple studies.86–93
However, not all patients will fit the molds of thin scalloped or thick flat gingival morphology. There are certainly variations and combinations of the morphologic characteristics from the basic two gingival phenotypes. De Rouck et al89 identified and confirmed through a cluster analysis the existence of three different gingival types