As discussed in Part I of this series, an easy way to transfer the esthetics and occlusion from the diagnostic wax-up to the mouth is to use the "eggshell" provisional technique. As I explained, rather than lightly prepping the model, I inject the provisional material into the putty itself. Once the material that has been injected into the putty is set, the next step is to trim and hollow the shell. This is done in two distinct steps and can take a little bit of time to do.
Step 1: Trim the provisional shell to the free gingival margin. In this step you're essentially trimming back to the gingival margin, or more accurately, back to where the bulk-out wax ended on the gingival margin. If the wax margin is not well delineated it will be very difficult to determine how much needs to be trimmed off the shell. This first step in the trimming process should be done by orienting the acrylic bur perpendicularly to he axial wall along all the buccal and palatal surfaces.
Step 2: "Hollow" out the internal of the shell to allow it to freely seat over the preparations without binding. Once your first step is completed you can use a rounded acrylic burr or a #6 round burr to hollow out the internal of the shell. Upon completion, the provisional should be very thin once the internal is thinned out – which is exactly why this technique is called an "eggshell" provisional. The palatal and buccal walls of the provisionals usually measure around .5mm thick.
On the incisal edges and occlusal table, the walls can be ~1mm thick, since more preparation depth is anticipated in these areas. At this point, you will also want to verify that you aren't leaving too much material interproximally which could impinge on the interproximal soft tissue and prevent complete seating. Lastly, a disc is used to open up the facial and palatal gingival embrasures to help ensure that the shell doesn't impinge on the papillae.
At this point, the shell is ready to take to the mouth. After prepping the teeth, you'll want to try the shell in the mouth. It should be noted that when trying the shell over the preps, it should not bind on the teeth at all. It's will actually be the soft tissue that helps determine the seating of the shell. This trying in and fitting the shell properly is the most important step when using the shell technique. Rushing this step can affect the seating of the provisional and cause problems both with the occlusion and esthetics.
The best way to know if the shell is seated correctly (not over-seated or under-seated) is to use calipers to measure from incisal edge to gingival margin on the diagnostic wax-up, and verify the measurement in the mouth when fitting the shell. This verification can be done on multiple teeth to ensure that it isn't binding somewhere or being over seated. This is typically done after the prep is essentially completed, but before cords are packed on the anterior teeth and the margins dropped.
The reason is that once cord is placed, your reference position for the gingival margin is no longer accurate. To make the sure the shell will be repositioned accurately after packing cord, place some Triad material in the incisal 1/3 of the centrals, seat and manipulate the shell until its seated correctly (using measurements and the opposing occlusion) and cure the Triad. Now you can pack your cords and refine the preparation without losing the ability to re-seat the provisional accurately.