As an advisor in the General Practice Residency Program at Loyola Hospital in Chicago and member of Spear Resident Faculty, I receive questions from students and participants regarding various techniques and processes related to restorative dentistry.
I was recently confronted with these questions and concerns:
“Hey, Dr. Bonk, I am frustrated and having difficulty getting my crowns to fit consistently. I am using a triple-tray with polyvinyl siloxane. I don't know what the problem is. Is the dental laboratory doing something wrong? Can you help me figure it out or give me some insight?”
In 1983, Wilson and Werrin first described the dual arch impression technique, which is also known as the double arch impression technique or the “triple-tray” technique. This impression technique involved the use of a special tray that registers the impression of the opposing segments of the dentition.
It simultaneously records the occlusal relationship of the opposing segments. The patient closes into a plastic or metal tray that has a mesh separating the opposing dentition. The tooth is prepared and then the tray is loaded with the impression material.
The patient is instructed to “bite down” or put their teeth together until the material is set. The impression is then poured and mounted on an “articulator.” The final restoration is fabricated from these triple-tray obtained models.
The use of the triple-tray impression technique is widespread. Glidwell Lab reported in 2016 that 85% of the restorative impressions received were triple-tray impressions. The most common use for this technique is for single-unit posterior restorations. While multiple tooth restorations (quadrants) and anterior tooth dual arch impressions are also common, McCracken surveyed 1,700 dentists and found that polyvinyl siloxane is the most common impression material utilized (77%). Polyether use was utilized 12% of the time and only 9% of dentists utilized digital/optical scanning.
Accurate impressions are required for consistent crown fabrication. There are many factors that control or are related to this accuracy. Some considerations are in the control of the dentist. The dental lab affects some aspects and, lastly, the patient can affect the final impression accuracy.
Some dentist-controlled impression factors include visible defects such as incomplete margin detail, air bubbles, voids, pulls, unset impression material, blood and saliva, trapped cords and cotton rolls, and improper union of the material. The dental lab can affect the accuracy, as well.
Improper measurement of water and stone mixing, and unsupported impression trays during stone pouring, can cause tray flexure and inaccurate articulator mounting of the opposing models. Additionally, the patient may affect the ultimate accuracy and fit of the final restoration.
Moisture control, amount and position of bite forces during material setting, mandibular flexure and tongue movement are potential hazards the final fit of the crown or restoration. Dental team member education and understanding of impression material handling, in addition to patient management, are also crucial to the accuracy and predictability of the restoration.
These factors and others potentially affect the final restorative outcome when impressing for definitive restorations using a dual arch technique.
Distortion is a three-dimensional effect inherent in the steps involved in fabrication of an indirect restoration. Impression material recoil is an important consideration, as well. Impression material must resist the forces that are generated upon removal from the mouth.
If the material is not fully set, due to a variety of factors, plastic deformation may occur and affect the final fit of the restoration. Distortion may also occur from a detachment of impression material from the tray, especially during impression removal. These defects and distortions may be very difficult to see at the time of impression.
Many articles have been written about the accuracy of the triple-tray impression technique. Metal trays appear to provide greater accuracy than plastic trays. One study found that triple-trays were as accurate as custom single arch tray impressions. Another study found that gypsum dies measured from a plastic impression tray were 30 microns smaller in mesial-distal dimension than the controls.
There is wide variability in the literature regarding the accuracy of the triple-tray technique, but the general recommendation is that only single unit restorations should be fabricated.
Based upon the literature review of the dual-arch impression technique, there are some key messages you can take home to your practice. While this technique is widely utilized and accepted by practicing dentists, the variation in the literature suggests caution with its use.
There is the possibility of significant inaccuracy due to many factors. These factors include:
- Most dual arch impression trays offer minimal lateral support for material. This lack of support may allow for distortion in a buccal-lingual direction.
- Plastic dual trays are very flexible compared to metal. These plastic trays can distort more easily.
- Impression materials with higher filler content and rigidity are preferred over more flexible materials.
- Triple-tray techniques are best suited for single-unit posterior restorations. Multiple units and anterior tooth restoration impressions should be obtained with full arch custom trays.
- Patient biting variability into the tray may create significant distortion during the critical material setting time.
- Dental lab pouring and handling of the dual tray impression can affect the stone model accuracy.
- The unilateral dental arch capture does not allow the dental lab an opportunity to evaluate the contralateral teeth for shape, contour and functional wear.
- Articulator mounting of a quadrant arch does not adequately replicate the patient's functional movements and does not allow for centric relation/centric occlusion evaluation.
- Triple tray techniques are time efficient and convenient. But short- and long-term predictability of the final restoration may be compromised due to improper fit and function.
We perform accurate restoration impressions daily. It is important that we apply proper technique based on sound principles. As dentists, we need to evaluate our patients and our impression techniques to determine the best process for obtaining efficient, accurate and predictable restorations.
I hope these insights into the science and the process of dual tray impressions help you to self-evaluate the technique and its application in your practice.
Jeffrey Bonk, D.D.S., is a member of Spear Resident Faculty.
Withrow, D. “By the Numbers”. Chairside Magazine. 2017;11(4)
Kraus, Richard F. “Dental articulator.” U.S. Patent No. 5,586,884. 24 Dec. 1996.
Breeding, Larry C., and Donna L. Dixon. “Accuracy of casts generated from dual-arch impressions.” Journal of Prosthetic Dentistry 84.4 (2000): 403-407.
Wilson, Edmund G., and S. Rand Werrin. “Double arch impressions for simplified restorative dentistry." The Journal of prosthetic dentistry 49.2 (1983): 198-202.
Ceyhan, Jeffrey A., Glen H. Johnson, and Xavier Lepe. “The effect of tray selection, viscosity of impression material, and sequence of pour on the accuracy of dies made from dual-arch impressions.” Journal of Prosthetic Dentistry 90.2 (2003): 143-149.
De Lima, Luciana Martinelli Santayana, et al. “In vivo study of the accuracy of dual-arch impressions.” Journal of international oral health: JIOH 6.3 (2014): 50.
McCracken, Michael S., et al. “Impression Techniques Used for Single-Unit Crowns: Findings from the National Dental Practice-Based Research Network.” Journal of Prosthodontics (2017).