We spend a lot of time talking with doctors about preps and impressions. Often, bite registrations are almost an afterthought since they are more manageable than inadequate preps and impressions. But when it comes to the time required to seat a restoration, bite registrations play a large role in the duration of an appointment. There is almost no better feeling than to drop a restoration onto a preparation, have the patient bite down, and report that there are no problems.
It is helpful to observe the patient’s maximum intercuspation position prior to administering anesthesia. I always make sure to observe the interdigitation of the teeth on the contralateral side since this will be the primary way that I verify the patient is in maximum intercuspation when the impression tray is seated in the patient’s mouth. It also is 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 would like to replicate this occlusal scheme on the temporary, too.
On several occasions, Dr. Gordon Christensen has said that the most accurate bite registration in dentistry is a double-arch tray that has been done properly. He also says that this type of bite is even 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 the dentist and the laboratory must handle the double-arch tray correctly. Let’s begin by looking at how the dentist should utilize the 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 your fingers can collapse a plastic tray completely. With a metal tray, you can apply even more pressure, and it won’t distort. The Quad-Tray from Clinician’s Choice (www.clinicianschoice.com) is such a tray. This is my daily choice for a double-arch tray.
Step 2: Verify fit of the selected double-arch tray by placing it in the patient’s mouth without impression material and having the patient bite together. You want to make sure the tray is placed to clear the maxillary tuberosity and the retromolar pad. Some patients cannot close down with this type of double-arch tray in their mouths. These patients are better suited for full-arch impressions. You can check this by observing the arch’s contralateral side where you looked previously when observing the interdigitation of the teeth on that side. If the 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 of sufficient length 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, 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 the patient’s mouth and he or she simply is not able to bite down completely. This usually happens because you have lost orientation when inserting the tray since it is now covered with impression material. After three or four attempts, if I can’t get the patient into maximum intercuspation, I will have the patient hold that position while the impression material sets and then take a separate bite registration. When this happens, make sure to note on the lab slip that the technician should ignore the bite registered on the double-arch tray and use the separate bite registration.
The lab needs to follow certain steps as well to ensure that the double-arch method is as accurate as Dr. Christensen says it can be. The lab technician should pour one side of the double-arch tray, let the material set, then flip the impression and pour the other side while the first pour remains in the impression. Then, with both sides of the impression poured and set, the technician should attach the articulation hinge. The critical part is that the hinge is attached before either of the poured models is removed from the impression for the first time. This is critical because once the model is removed from the impression, it is virtually impossible to get it to reseat completely in the impression. As long as the articulation hinge is attached before either model is removed from the impression, we can achieve the most accurate bite registration to which Dr. Christensen refers. Doing this procedure correctly is more difficult for the lab. You will hear technicians say that they do not like double-arch trays, but it is hard not to be in favor of 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 clinical programs available on DVD through Glidewell. For more information on this article, or to receive a free copy of one of Dr. DiTolla’s clinical DVDs, e-mail him at [email protected].
