Thoughts on double arch impressions
We spend much time talking about preps and impressions with doctors. Meanwhile, bite registrations are often almost an afterthought since they are a more manageable problem than inadequate preps and impressions.
by Michael DiTolla, DDS, FAGD
We spend much time talking about preps and impressions with doctors. Meanwhile, bite registrations are often almost an afterthought since they are a more manageable problem than inadequate preps and impressions. But when it comes to the time required to seat a restoration, bite registrations play a large part in how long an appointment will be. As a restorative dentist, there is almost no better feeling than to drop a restoration onto a preparation, and have the patient bite down and report that everything is great.
It is always helpful to observe a patient’s maximum intercuspation position prior to administering anesthesia. I always make sure to check the interdigitation of the teeth on the contralateral side. This is the primary way I verify that the patient is in maximum intercuspation when the impression tray is seated in the patient’s mouth. It is also helpful to mark the patient’s occlusion with articulating paper prior to preparation to help understand the patient’s occlusal scheme. This is especially true for the teeth being prepared since we want to replicate this occlusal scheme on the temporary, too.
On several occasions, Dr. Gordon Christensen has stated the most accurate bite registration in dentistry is that of a properly done double arch tray. He also said that this type of bite is more accurate than full arch impressions and a separate bite registration. This is good news for fans of double arch trays, but this also means that a dentist and a laboratory must correctly handle the double arch tray.
Let’s begin by looking at the following steps to see how a dentist should utilize a double arch tray:
Step 1: Select the proper double arch tray. For me, plastic double arch trays have always seemed too flexible to provide enough support for the impression material. A little pressure between the dentist’s fingers can cause the plastic tray to collapse. Meanwhile, even more pressure to a metal tray does not distort it. My choice for a double arch tray is the Quad-Tray from Clinician’s Choice (www.clinicianschoice.com).
Step 2: Verify the fit of the selected double arch tray by placing it in a patient’s mouth without impression material and having the patient bite. We want to make sure that the tray can be placed to clear the maxillary tuberosity and the retromolar pad. There are patients who cannot close down with this type of double arch tray in their mouths. These patients are better suited for full arch impressions. We check this by observing the contralateral side of the arch where we looked previously to observe the interdigitation of the teeth on that side. If a patient is able to bite together without shifting his or her mandible, and the contralateral teeth are in the proper position, the tray is a good fit.
Step 3: Verify that the double arch tray is long enough to capture the arch from the most distal tooth to the cuspid on the same side. If the anterior portion of the tray mesh does not cover the cuspid, you should select a longer double arch tray or consider full arch impressions.
Step 4: Once the double arch tray with the impression material has been placed, have the patient bite together and use the contralateral side to verify maximum intercuspation of these teeth.
Even when following these steps, there are times when the impression is placed in a patient’s mouth and the person simply cannot bite down completely. This usually happens because you have lost orientation when inserting the tray since it is now covered with impression material. If I cannot get the patient into maximum intercuspation after three or four attempts, I have the patient hold the position while the impression material sets. I then take a separate bite registration. When this happens, make sure to note on the lab slip that the technician needs to ignore the bite registered on the double arch tray and should use the separate bite registration.
In addition, the laboratory needs to follow certain steps to ensure that the double arch method is accurate. The lab should pour one side of the double arch tray and let the material set. The lab then flips the impression and pours the other side while the first pour remains in the impression.
With both sides of the impression poured and set, the lab now attaches an articulation hinge. At this stage, the key is that the laboratory attach the hinge before either of the poured models is removed from the impression for the first time. This is important because, once the model is removed from the impression, it is virtually impossible to get the model to reseat completely in the impression.
As long as the articulation hinge is attached before either model is removed from the impression, the most accurate bite registration can be achieved — as Dr. Christensen stated earlier. It is a challenge for a laboratory to do this procedure correctly. Thus, you might hear laboratory personnel say that they do not like double arch trays. But it is difficult not to favor the most accurate bites in dentistry.
Dr. Michael DiTolla is the Director of Clinical Research and Education at Glidewell Laboratories in Newport Beach, Calif. He lectures nationwide on both restorative and cosmetic dentistry. Dr. DiTolla has several free clinical programs available online or on DVD at www.glidewell-lab.com. For more information on this article or his seminars, please contact him at www.drditolla.com.