Environmental morphology is affected by the amount of available bone (bone volume) and its form and location; the amount of soft tissue present coupled with its type, form, and location; and the coronal morphology of the natural teeth.
Bone Volume, Form and Location
The occlusocervical height of the residual ridge should be sufficient to accommodate the shortest implants (7 millimeters). In the posterior mandible, at least 2 millimeters of additional bone is required to maintain a safe distance from the inferior alveolar canal. Therefore, the minimal occlusocervical bone height in the posterior mandible is 9 millimeters whereas 7 millimeters is the minimal dimension in the anterior mandible. However, it should be remembered that short implants (7–10 millimeters) have higher failure rates than longer implants. Recently, implants that are shorter than 7 millimeters have been introduced for use when the available bone dimensions are very limited but there is currently a lack of long-term clinical data from a broad range of practitioners.
In the maxilla, 7 millimeters of bone height is sufficient to accommodate short implants. However, the use of 7–10 mm long implants is a greater concern in the maxilla than the mandible because the implant failure rate is higher in the maxilla. Therefore, 13 mm is the recommended minimum occlusocervical bone dimension in the maxilla.
Occasionally, there may be excessive vertical bone height that limits the space available for ideal prosthesis fabrication. There needs to be adequate space available into which a crown or prosthesis can be placed that possesses the required esthetic and structural forms. The distance from the occlusal plane to the edentulous mucosa at the crest of the ridge should meet the following criteria:
The required faciolingual bone thickness is related to the diameter of the implant being placed. Since 3.25 mm diameter implants are the smallest available, the bone thickness must be sufficient to accommodate an implant of this dimension. It is generally felt that 6 millimeters of faciolingual dimension is the minimal thickness into which 3.25 and 4 mm diameter implants can be placed. Larger diameter implants (5 and 6 mm) require minimal faciolingual bone dimensions of 7 and 8 mm, respectively. These minimal dimensions make implant placement very critical in order to avoid bone fenestration/dehiscence during placement of the implant. It should also be noted that minimal faciolingual dimensions do not permit changing the implant angulation in the bone since the existing faciolingual angulation of the bone determines the long-axis orientation of the implant.
Bone Form After Resorption
In the anterior maxilla, bone resorption usually produces an edentulous ridge where the bone is located lingual to the desired position for the teeth. This relationship places the implants lingual to the facial surfaces of fixed partial dentures, creating esthetic challenges in the cervical aspect of the prostheses (Figure 6). In addition, there is often a flared form to the residual bone that may prevent implants from being ideally aligned with each other1 (Figure 7).
With facially inclined implants, the internal implant threads used to attach other components to the implants flare away from each other, requiring the use of prefabricated angled abutments or custom abutments so prostheses can simultaneously attach to all the flared implants. This process often requires increased time and associated laboratory costs. In addition to the fabrications challenges, there may be esthetic and phonetic challenges with the overlying prostheses. The implant alignment may also make it difficult to avoid open cervical embrasures with single crowns (Figures 8A, 8B) or the units of a fixed partial denture (Figure 9).
Bone Grafting/Distraction Osteogenesis
When there is a deficiency in the occlusocervical bone dimension, the faciolingual dimension, or the bone form, it may be necessary to enhance the existing bone morphology through bone grafting or the use of distraction osteogenesis.
Soft Tissue Volume, Form, Type, and Location
Achieving ideal soft tissue form and interdental papilla height can be a challenge when placing implants into highly visible edentulous areas. Interdental dark spaces may be present (Figures 10,c11A, 11B), the marginal tissue may be thicker than the gingival margin present around adjacent teeth (Figure 12), the apical location of the soft tissue margin may not be at the same height as adjacent or contralateral natural teeth, interdental papillae may not possess the most desirable form or height (Figure 11B), and recession of the soft tissue may lead to crown length variations and/or exposed metal (Figures 13A, 13B).
When bone is present at the proper height interproximally, the soft tissue will usually fill cervical embrasure spaces (Figures 14A, 14B). The distance from the soft tissue crest to the bone is important in maintaining the presence of papillae between natural teeth and implants. One study evaluated 52 anterior maxillary papillae in 26 patients to determine the effect of the proximal bone crest on the presence or absence of papillae between an implant and an adjacent tooth.2 A papilla was present 100% of the time when the distance from the proximal contact to the bone was 5 millimeters or less. The papilla was only present 50% of the time when the distance from the contact point to the bone was equal to or greater than 6 millimeters.
Measurements have also been made of the distance from the soft tissue crest to the underlying bone crest adjacent to implants and also the distance from the gingival margin to the bone on the natural teeth adjacent to the implants. The measurements from the proximal bone to the crest of the peri-implant mucosa were 5–7 millimeters most of the time. When the distance between the bone and the desired interproximal height of the soft tissue adjacent to implants is greater than 5–7 mm, there will likely be dark cervical embrasure spaces adjacent to the implants.3 The measurements between the mid-facial soft tissue crest and the facial bone of implants was usually in the 3–4 millimeter range. When the distance between the mid-facial bone and the desired facial location of the soft tissue is greater than 3–4 millimeters, it is not likely that the soft tissue will be located at the desired position on the facial surface of the implant restoration. This information is helpful in determining the likelihood of interdental papillae filling the cervical embrasures adjacent to implant prostheses and the facial soft tissue being located at the desired apical position.
The effect of the periodontal biotype (thick versus thin soft tissue) on the peri-implant mucosa has also been assessed.3 Individuals with thick mucosa had greater distances between the underlying bone and the margin of the peri-implant mucosa than patients with thin mucosa. As a result of this data, the authors propose that papillae adjacent to implants can seldom be recreated when the distance is more than 4 millimeters between the bone crest and the desired height of the interdental papillae when treating patients with thin peri-implant mucosa.
Natural Tooth Crown Morphology
When the facial crown outline form of natural teeth is triangular as opposed to being square, there is a much greater difference between the height of the interdental papilla and the mid-facial gingival crest (Figures 15A & 15B). This relationship produces a gingival architecture that is quite scalloped in form. The underlying bone follows the gingival morphology. As a result, esthetic challenges are magnified following the loss of teeth in patients with highly scalloped soft tissue. The bone remodeling associated with post-extraction healing results in recontouring of the relatively tall interproximal bone and that often leads to open cervical embrasure spaces.
Additionally, teeth that are triangular in form are more often associated with thin, delicate gingiva that is more susceptible to recession.