Dual arch impression techniques are the most common way to prepare indirect restorations for one to three posterior teeth. It is faster, more efficient, and more economical than separate full arch impressions. However, a significant compromise in the restoration outcome can occur if the last tooth in the arch is prepared, and there isn't a tooth that acts as a vertical stop at the terminal tooth in the arch for the simple hinge articulator used for articulation.
The key to a successful restoration's fit and function is for the dental laboratory technician, the clinician, and the staff understanding and accepting the expectations.
During the restoration's try-in, reasonable expectations are:
- Restoration Morphology: The restoration morphology will not match the contra-lateral tooth unless the technician has a full arch diagnostic cast, which can be used as a guide for the anatomic form and morphology details.
- Occlusal Consideration: Expect to make more occlusal adjustments than full arch impressions because it is difficult to confirm that the patient has entirely closed into the maximum intercuspal position.
Dual arch impressions require a fiber mesh between the upper and lower impression material on the tray. Depending on the material thickness of this fiber mesh, it may negatively affect the bite by shifting it when the patient closes into the impression material.
In addition, the detail of the tooth may be lost where the opposing teeth are in direct contact. It is possible to bite through the fiber mesh, making direct tooth-to-tooth contact. When the technician pours the impression, the stone passes through the hole, which negatively affects the occlusal contacts and can negatively affect the morphology of the opposing tooth.
Here, the optimal dual arch impression technique involves pouring in die stone and mounting on a simple hinge articulator. These articulators do not represent the anatomic norms from the temporomandibular joint (TMJ) to the teeth. Frequently there are no anterior teeth impressed. Therefore, no lateral or protrusive movements can be evaluated on these simple hinge articulators. Only maximum intercuspal contacts can be evaluated, and all eccentric contacts must be evaluated and adjusted intraorally.
Impression Trays and Materials
Tray Type: The literature supports the requirement of using a ridged tray, optimally metal. Flexible trays can distort in the mouth when the patient bites into the impression material and subsequently can rebound when taking it out of the mouth. This will distort the impression, and it will be impossible to determine if distortion has occurred.
Impression Material: The literature supports using a heavy body impression material as a tray replacement. Vinyl polysiloxane materials tend to be more accurate than polyether materials. The heavy body material helps to displace the cheek and tongue and supports the die stone when the impression is poured in the laboratory because it sets up rigid.
Lighter body material is used over all the teeth to be impressed in addition to the prepared tooth or teeth. There must be intimate integration of these two viscosities of material. Ideally, it should set intraorally relatively quickly, approximately three minutes. The longer it takes to set, the greater the likelihood of patient jaw movement with resulting distortion.
Dual Arch Impression Techniques for Improved Accuracy
The following techniques will help improve the accuracy of impressions:
- Before taking the impression, check the tooth contacts in maximum intercuspal position on the contralateral side to be impressed. Check the contacts with and without the unloaded tray to ensure the tray does not shift the occlusion. Make a mental note of the contacts so they can be confirmed.
- When taking the impression, have the patient bite into the impression material until their teeth touch. Confirm the proper occlusion on the contralateral side. Have the patient squeeze firmly and put their fist under their chin with applied upward pressure to help remind themselves not to move their jaw or open it.
- Take a bite registration over the prepared tooth/teeth only so the lab can confirm the accuracy of the bite into the impression material. Remember to trim and check the bite registration intraorally to verify its accuracy. Do not assume it is correct.
- The laboratory should make two pours of the impression. The first pour-in die stone should only be of the prepared teeth and used as the master die. The second pour should create the solid cast, opposing teeth, and articulations.
Following the recommendations for dual arch impression techniques, materials and processes outlined in this article create the most realistic and accurate expectations to pass on to your dental lab and result in the best outcome for your patient.
Robert Winter, D.D.S., is a member of Spear Resident Faculty.
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- Broilo JR, Ghiggi PC, Burges GA, Burnett Jr LH, Sophr AM. Accuracy of the second pour casts using dual-arch trays. Stomatology Baltic Dental and Facial Journal 2011; 13(1): 15-18.
- Carrotte PV, Johnson A, Winstanley RB. The influence of the impression tray and the accuracy of impressions for crown and bridge work- an investigation and review. Br Dent J 1998 Dec 12-26; 185(11-12): 580-585.