If you happen to be creating provisionals for an exceptionally large case, such as a full arch, an easy way to transfer the esthetics and occlusion from the diagnostic wax-up to the mouth is to use the "eggshell" provisional technique. This concept utilizes a prefabricated shell in the desired tooth form (made from the diagnostic wax-up).
Although this technique is typically used with larger cases, it is versatile enough that it can be used in any situation, all the way from a single unit to a three-unit bridge, or even a quadrant. This series will concentrate on using the "shell" provisional in a full arch situation.
To make provisionals in a full arch case while using a matrix (putty or clear matrix) requires the presence of hard tissue stops. If you are leaving parts of teeth untouched (ie. anterior veneer preparations or posterior partial coverage restorations) using a matrix is easy and predictable. If you don't have enough hard tissue stops remaining to secure the matrix, the matrix will then have to rely on the soft tissue to determine the proper "seating" position. The problem when using soft tissue only to seat the matrix is that it is movable/resilient and pressing too hard can compress the soft tissue and hence produce a distorted provisional with regard to esthetics and occlusion. If you don't have enough tooth structure to key the matrix because everything will be full coverage, or if you are opening the VDO, using the eggshell provisional technique can efficiently help transfer the esthetics, occlusion and vertical dimension from the diagnostic wax-up back to the patient in a very efficient and predictable manner.
Create Diagnostic Wax-Up
The first step is to create a diagnostic wax-up on the mounted models. Once this is complete, you have two options to fabricate the shell. The first option is to have the lab fabricate the shell for you. These days this would be done with a CAD/CAM approach. Scan the wax-up, create a very light prep on the model, scan the prepped model, and then have the shell "milled" via a CAD/CAM technique. This can be a very quick and efficient way to have a shell fabricated. The only downside with this approach is that there will be a lab fee for milling the shell that could be quite significant.
Another way to fabricate the shell is to have the technician complete the diagnostic wax-up and then fabricate the shell using more traditional techniques: using your hands! With this, you will first need to duplicate the diagnostic wax-up, so as to leave the original wax-up undisturbed. The easiest way to duplicate a wax-up is to soak the model in very soapy water until it is saturated, then make a traditional alginate impression of the model and pour the impression in stone. This stone model of the diagnostic wax-up will be used to add "bulk-out" wax to help account for the shrinkage of the provisional material.
When working with provisional materials, it's important to remember that virtually all of them shrink. I typically make my shells out of a BisAcryl type of provisional material, although any material can be used. If we don't account for the shrinkage of the material, the shell may shrink to such a point that it won't fit around the margins of the preparation. In order to overcome any shrinkage, a small layer of wax is applied over the cervical 1/3 on both the facial and palatal surfaces of the teeth on the duplicated model of the wax-up. The thickest portion of the wax (~1/2 mm) is started on the gingival margin and then feathers down to nothing in the junction of the cervical and middle 1/3s.
End on the Gingival Margin
It should be noted that the gingival portion must not end on the tooth but rather on the gingival margin itself. This will enable the cervical 1/3 to be over-contoured compared to the original wax-up and in the process, help accommodate any shrinkage of the material. Doing this on a stone model of the wax-up better allows you to visualize where the bulk out wax is applied and how much is placed. Once completed, a putty impression of the "bulked out" model can be taken.
At this point, rather than lightly prepping the model and using the putty as a matrix carrying the material back to the model, I will inject the provisional material into the putty itself. Essentially, the provisional shell will start out in more of a "block form" and then have to be trimmed and hollowed into "shell form" after it sets. Why do I choose to do this rather than lightly prep the model and seat the matrix over the preps? Because BisAcryl tends be a rather brittle when it is very thin and will typically end up breaking when you first try to remove it from the model. In the next article in this series I discuss trimming the eggshell provisional.