CAD/CAM Technology in Implant Abutment Design

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The Transgingival Section
Various designs have been used over the years for the section of the abutment extending from the top of the implant through the soft tissue. Early root-form endosseous implant systems advocated a straight-walled design through the gingiva.1,2 However, this resulted in ridge-lap crown designs that presented hygienic and esthetic challenges.

In the 1990s, abutment systems were introduced to create an "emergence profile."3-6 In this scenario, the base of the abutment flares as it rises coronally from the top of the implant. The premise of this design is that the restoration emerges through the soft tissue in a similar way to the natural clinical crown. While providing an esthetic, cleansable result, this design requires that clinical precautions be taken to avoid excessive pressure on the labial tissue, which can lead to recession.7

Research has been conducted and some products brought to market with concave transgingival sections.8 The concept behind this design is that creating a thicker band of gingiva around the abutment will stabilize the soft tissue and mask the gray color of the titanium abutment at the gingival margin. A potential challenge with this design is that removing or replacing the abutment would likely necessitate anesthetizing of the patient. The narrow neck design could also lead to thin, potentially weak abutment walls.

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When seating the abutment, a critical factor is what is placed on the implant. A properly fabricated provisional restoration is ideal to help shape and sculpt the soft tissue contours. If a stock healing abutment is chosen, one with a flare similar to the tooth being replaced should be used. The soft tissue and/or the abutment should be adjusted if blanching occurs upon delivery and does not dissipate within 10 minutes, as excessive pressure on the tissue can lead to recession. A periapical radiograph should be taken to verify complete seating of the abutment.

Setting the Margin
For cemented implant restorations, the location of the margin is even more critical than on natural preps. Several studies have emphasized the detrimental effect of excess cement on peri-implant bone and soft tissue.9,10 Using a scan of the model, the margin can be set approximately 1 mm below the crest of the soft tissue in esthetic areas. Typically, it will be placed at or 0.5 mm below the crest interproximally and lingually/palatally, which provides an esthetic result and easy access for the complete removal of excess cement. The shoulder of the margin can also be planned based on the intended crown type.

Angle Correction
If the trajectory of the implant diverges from the position of the crown of the tooth to be replaced, the abutment can be designed to correct this angle.11 Custom abutments can be manufactured to correct divergence; however, it is not recommended to exceed 20 degrees of angle correction.

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Crown Support and Retention
Virtual design of the abutment allows the technician to create the ideal base to support the crown. Retention of the crown is based on the height and taper of the abutment.12 CAD software allows the abutment and crown to be visualized to determine the optimal abutment base width and height. A six-degree taper is typically provided for retention. The height of the abutment should be maximized, with consideration given to providing adequate occlusal/incisal space for the crown. Finally, the cement space for the crown can be precisely adjusted.

Anterior Case Example
Figure 1: Using 3Shape CAD software, the margin of the abutment is set following the gingival contours. The grid can be used as a measurement aid. The transgingival section of the abutment fills the sulcus and creates the base for the restoration. NOTE: Glidewell Laboratories will virtually blanch the tissue no more than 0.5 mm, unless specified otherwise on the lab prescription.
Figure 2: A full-contour crown is placed over the abutment and the interproximal contacts are adjusted. Because implants have no periodontal ligament and therefore no give, long, broad, light interproximal contacts are designed. Once the crown is designed, the supragingival section of the abutment is adjusted to create adequate space, support and retention for the restoration.
Figure 3: The height of the abutment is adjusted in relation to the crown.
Figure 4: The mesial-distal width of the abutment is adjusted.
Figure 5: Adjustments are made to the palatal contours.
Figure 6: The labial contours and spacing are adjusted.

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Posterior Case Example
Figure 1: Using the same CAD software as in the previous case, the margin and transgingival section are set.
Figure 2: A full-contour crown is placed over the abutment, and the occlusal and interproximal contacts are adjusted (red circles).
Figure 3: The height and taper of the abutment is adjusted.
Figure 4: Adjustments are made to the labial, lingual/palatal and interproximal contours.
Figure 5: Once the abutment design is completed, the file is transferred to a 5-axis milling machine and the abutment is fabricated.

Conclusion
The use of CAD/CAM technology to design and fabricate custom abutments provides numerous clinical advantages. This method allows the clinician to specify design criteria and material choice on the lab prescription. The abutment is then custom-designed for the specific case and precision-milled, resulting in a superior restoration.