Proper incisocervical, occlusocervical, and faciolingual positioning of implants promotes the development of restoration contours that transition from the round form of the implant to the desired tooth form. These transitional contours allow the artificial replacement to posses a normal profile when it emerges from the soft tissue. Proper implant positioning also promotes the development of normal soft tissue form and location.
Location For Single Crowns
The implant should be centered mesiodistally in the edentulous space for esthetic reasons (Figure 1A). Centering the implant facilitates the development of normal proximal emergence profile, permits better morphologic replication of the contralateral tooth, and prevents the replacement tooth from appearing tilted in the arch (Figures 16A, 16B).
Faciolingually, the implant should be located at the center of the edentulous space or be located slightly to the facial. A position slightly to the facial can sometimes be advantageous when the existing bone dimensions permit (Figure 1A) since centering the implant may produce a crown with a deficient cervical contour (Figures 17A, 17B) or a crown where porcelain must overlap the facial soft tissue to create the desired cervical crown morphology (Figures 18A, 18B). Overlapping the soft tissue makes oral hygiene more difficult, and it presents an esthetic liability should the soft tissue position recede apically.
When an implant is placed lingual to the faciolingual center of the adjacent teeth, it may be necessary to have the crown substantially overlap the facial soft tissue (like it occurs with certain fixed partial denture pontics) to achieve a normal cervical form. Alternately, a horizontally submerged ovate form (similar to an ovate pontic) can be developed that supports the soft tissue in a more esthetic fashion (Figures 19A, 19B, 19C).
Placing an implant too far facially creates serious esthetic challenges that often cannot be overcome (Figures 20A, 20B). The implant may have to be removed, bone fill allowed to occur, and another implant subsequently placed in a more favorable position.
The incisocervical/occlusocervical location of the implant is largely determined by the location of existing bone since the top of the implant is surgically located at about the level of the bone or slightly apical to the bone crest. The incisocervical/occlusocervical location is also determined by the esthetic need to transition from the round form and smaller diameter of the implant to the larger diameter of the crown that needs to possess a specific cross-sectional geometric form. Typically, implants have been placed apical to the cementoenamel junction of adjacent natural teeth to permit the required changes in morphology to occur somewhat gradually. Initially, it was recommended that implants located in the esthetic zone be placed 4 millimeters or more apical to the cementoenamel junctions of adjacent teeth.8 However, this location places the implants far below the bony crest and bone healing subsequent to implant placement can then produce interproximal bone form changes that negatively affect the soft tissue height and may prevent the soft tissue from filling cervical embrasure spaces. Therefore, when the adjacent bone level is located at a normal incisocervical level, a distance of about 2 mm apical to the cementoenamel junction of adjacent teeth is recommended (Figure 21).
In the anterior maxilla, it is often necessary to fabricate custom abutments to create an angular transition between the long axis alignment of the implant and that desired for the crown. When this need is present, it is important that the implant(s) be positioned incisocervically so a custom metal abutment can be used and it will be not visible. Alternately, ceramic abutments may be needed to provide an acceptable soft tissue color.
It should be remembered that the existing bone morphology may prevent implants from being placed in the desired locations, and bone grafting or distraction osteogenesis may be required before acceptable implant positioning can be achieved.
Location For Fixed Partial Dentures
When multiple implants are placed into the esthetic zone for the support and retention of fixed partial dentures, the implants should be mesiodistally located so they will be centered beneath the individual units of the prosthesis. When the implants are not centered beneath the prosthesis units, the cervical crown forms and embrasures will not appear normal in form. Should this abnormal form be visible during talking or smiling, there will be an esthetic liability (Figures 22A, 22B).
Faciolingually, the implants should be located at the center of the edentulous space or slightly to the facial when the existing bone dimensions permit. Centering the implants may produce prosthesis with deficient cervical contours, or it may be necessary to have porcelain overlap the facial soft tissue to create the desired cervical morphology. As stated with single crowns, overlapping the soft tissue makes oral hygiene more difficult, and it presents an esthetic liability should the soft tissue position recede apically.
When the implants are placed lingual to the faciolingual center of adjacent teeth, the prosthesis units may have to overlap the facial soft tissue (like occurs with natural tooth fixed partial denture pontics) to achieve a normal cervical form. Alternately, a horizontally submerged ovate form (similar to an ovate pontic) can be developed that supports the soft tissue in a more esthetic fashion.
For biomechanical reasons, implants are sometimes placed in a slightly staggered (offset) relationship to one another. The facial or lingual offset of one implant improves the capacity of the prosthesis to resist faciolingual forces. This type of alignment is more often used with posterior prostheses where the leverage forces are greater and the esthetic challenges are less critical. However, offset implants should still be contained without the normal peripheral contours of the overlying units of the prosthesis.
Placing the implants too far facially creates serious esthetic challenges that may result in the need to remove one or more of the implants, allow bone fill to occur, and then subsequently place the implants in a more favorable location.
As with single crowns, it should be remembered that the existing bone morphology may prevent implants from being placed in the desired locations and bone grafting or distraction osteogenesis may be required before acceptable implant positioning can be achieved. Additionally, when bone grafting or distraction is not employed or cannot create the desired result, it may be necessary to recreate the missing soft tissues through the use of gingivally-colored ceramic materials at the cervical aspect of the prosthesis.
The mesiodistal distance between adjacent implants can affect the presence of an interdental papilla. When radiographic measurements of crestal bone loss were made in 36 patients who had two adjacent implants, the average crestal bone loss between implants with more than 3 millimeters of separation was 0.45 millimeters.9 When the implants were separated by 3 millimeters or less, the average crestal bone loss was 1.04 millimeters. Therefore, it has been proposed that 3 millimeters or more of bone should be retained between adjacent implants to minimize crestal bone loss, particularly in esthetic zones.
Location For Complete Arch Prostheses
Implants that support/retain overdentures are commonly located in the canine and premolar areas of the mouth and they should be centered beneath the prosthetic teeth or slightly lingual to the center of the prosthetic teeth. When the implants are located anterior to the teeth (Figures 23A, 23B) or substantially posterior to the teeth, the denture base has to be enlarged to encompass the implant and retentive mechanism. The enlarged base dimensions prolong the time it takes for a patient to adapt to the new prosthesis and can make the adaptation challenging.
There is another negative aspect of placing implants too far facially or lingually. With malaligned implants, efforts are commonly made to reduce the amount of resin base overcontouring, and this process frequently leaves only thin areas of resin over the retentive mechanisms. The thin resin is more prone to fracture.
With fixed complete dentures (complete arch prostheses affixed to multiple implants with screws), the implants should be placed beneath the desired locations for the prosthetic teeth (Figure 5A). These locations permit the development of a normal prosthesis form with cervical contours that support the facial soft tissues and appear appropriately esthetic. The long axes of the implants should ideally be aligned with each other and the long axes should emerge through the lingual aspect of the anterior denture teeth and the occlusal surfaces of posterior teeth. These locations permit the screw access holes (required to affix the prosthesis to the implants) to be located in areas where they have minimal impact on the esthetic result, and the holes can be filled with esthetically colored composite resin fillings that have a minimal impact on the esthetics of the denture teeth. For biomechanical reasons, the implants should also be placed in accordance with the desired arch curvature so there is at least a 10 millimeter anteroposterior dimension to the curved alignment of the implants (Figure 24).