In previous articles I've covered the topic of mounted models and their importance in treating clinical cases accurately and efficiently. Once you have completed the process of producing the patient's centric relation bite records, it's now time to use the records to actually mount the models.
Since these are diagnostic models you will most likely be completing this process in-house by either yourself as the clinician mounting the models or delegating it to one of your staff members. This process is not as easy as just taking the records, sticking the models on the articulator and throwing stone; there are a few steps that you need to take first.
Step 1: Evaluate and clean up models. I personally take all of my diagnostic impressions with alginate, although you can use silicone or an alginate substitute as well. Prior to mounting make sure that there aren't any bubbles present on the occlusal surfaces of the models. If you don't pay attention to the bubbles, the records will not accurately fit on the models leading to inaccurate mountings. To clean the models, use a cleoid discoid or a carver with magnification.
Step 2: Trim the silicone record. If you used wax to create your CR record, the wax is generally ready to be used for mounting with no adjustment necessary. However, if you used silicone to make the CR bite records, there are a few extra steps that you need to take prior to mounting. Silicone is an excellent material for taking bite records; the problem is that it's actually too accurate compared to the accuracy of the stone models. Prior to mounting, we need to remove some of the accuracy of the record. The first thing to do is trim off any part of the bite record that touches soft tissue. Generally, I will trim the distal of the record through the distal cusp of the last tooth.
The next step usually involves using a scalpel blade to cut the thickness of the material down until I have cusp indentations of 1-1.5mm. The purpose of this is to visually allow you to confirm that the model is seated completely in the record.
The last step in the trimming process is to use an acrylic burr to remove the primary and secondary anatomy from the record as well as the occlusal embrasures as these areas on the model will not be as accurate as the record. The goal for a properly trimmed record is the have the model seated in the record only touching the cusp tips. How do you know when you have trimmed enough? The only way to know is to try it back on the model and make sure there isn't any "bounce" between the upper and lower models. If there is, go back to the models and record, evaluate them both, and adjust accordingly.
Step 3: Mount models. Once the records are trimmed, the maxillary model is mounted first using the facebow. Once the upper is mounted, you'll mount the lower model using centric relation bite record. Since the bite record is taken at a more open vertical dimension, be sure to open the pin on the articulator about 3-4mm to accommodate for the thickness of the record. This allows the pin to approximate zero when the models are mounted. It is okay if the pin isn't exactly at zero, you just want it to be close.