Advances in Dental Impressions

Several recent improvements have led to the ability to achieve more accurate impressions.

By Gregori M. Kurtzman, DDS

Dental impressions form the backbone of the treatment we provide, whether to help plan indirect restorations or models are needed for fabrication of clear-tray orthodontic appliances. Yet, many practitioners do not pay enough attention to their impressions. Dental laboratories consistently complain that the quality of impressions tends to be low, making their task more difficult in fabricating the desired restorations. The Samet study reported that 89% of impressions sent to laboratories have visible errors.¹

This article will address several advances in the impression realm that can provide better and more consistent impression results, thus providing the laboratory with better impressions and allowing them to fabricate more accurate restorations that reduce chairside delivery times.

A New Implant Tray

Open-tray implant impressions traditionally required either a custom tray to be fabricated or the use of a stock tray. Either way, holes needed to be cut on the occlusal surface of the tray for the long pins of the impression heads to protrude. This presented some challenges in creating the openings in the best positions without over removal of the tray resin, which can weaken the trays' rigidity. Should a custom tray be selected, additional time and cost is involved, and requires a preliminary impression and model to be poured followed by fabrication of the tray in the laboratory.

The Miratray^® Implant Advanced Tray (Hager Worldwide, www.hagerworldwide.com) simplifies the process of taking open-tray implant impressions. The tray is provided in three maxillary sizes and three mandibular sizes and is unique in its design. The occlusal surface is covered by a transparent foil to allow for easy identification of the pin head intraorally. Retention slots and an internal rim provide mechanical retention to retain the impression material within the tray. Should the practitioner chose to supplement the retention with a polyvinyl siloxane (PVS) adhesive, it is recommended that it not be applied to the foil surface because this may obscure visualization of the pins when inserting the tray to proper depth. It should be noted that PVS adhesive does not adhere to putty PVS materials and do not, therefore, increase retention of the impression material to the interior of the tray. The PVS adhesive does adhere to all other PVS viscosities.

The technique involves selection of an appropriate sized tray (Figure 1 and Figure 2), then filling the tray with an appropriate impression material; the author recommends either a universal body PVS or a tray or putty PVS. The tray is then inserted over the open-tray impression heads and pressed down crestally until the top of the impression pins are visible through the transparent foil. The practitioner then presses the tray further until the pins puncture the foil and are visible through the foil (Figure 3). This contains the impression material within the tray without the problem often seen with custom or modified stock trays of the impression material obscuring the tops of the pins. Upon setting, the pins are rotated in a counterclockwise direction and removed from the impression, which is then removed intraorally. Because of the tray's design, it can be used in all implant impression situations whether the arch is partially dentate or fully edentulous (Figure 4, Figure 5 and Figure 6).

Denture Impression Trays

Capturing good impressions of the fully edentulous arch can be challenging. The stock trays used for crown-and-bridge purposes are designed for the dentate arch. To use these for edentulous impressions requires large amounts of impression material. As impression materials are most accurate in uniform thicknesses, this potentially can lead to less accuracy in the models created from these impressions. Traditionally, the impression process involved taking a preliminary impression from which a model was cast and a custom tray was then fabricated for the final impression tray. Unfortunately, this requires an additional clinical visit plus time to fabricate the custom tray. Various edentulous trays have been introduced over the years, such as the Accu-Dent^® trays (Ivoclar Vivadent, www.ivoclarvivadent.com) which were plastic trays available in several sizes for both the maxilla and mandible that were shaped to capture the anatomy required for the fabrication of full dentures. These trays were matched with a two-part irreversible hydrocolloid (alginate) that had one part placed into the tray and the other syringed into the vestibule and then seated together. Metal versions of these trays were introduced by other companies but had the same negative qualities associated with the original Accu-Dent trays, namely, the use of an impression material that required immediate pouring, an inability to modify the trays to fit individual anatomical variations (due to the cost of the individual trays), and the potential for the trays to be lost at the laboratory.

Dr. Joseph Massad, looking at both the positive and negative aspects of these edentulous stock trays, designed a series of disposable modifiable edentulous trays with the help of DENTSPLY Caulk (www.caulk.com). The Massad™ Edentulous Impression Tray is provided in five maxillary and five mandibular sizes. The trays are fabricated from a thermoplastic allowing for chairside modification to ensure the best fit for any patient. Additionally, the trays are anatomically shaped to capture the anatomy that is required for fabrication of a good edentulous cast, so that the resulting denture fits ideally (Figure 7 and Figure 8). Additionally, recognizing that most dental offices do not wish to pour impressions and irreversible hydrocolloids require immediate pouring, the technique for the trays uses PVS impression materials (Figure 9 and Figure 10). This allows the impression to be sent to the laboratory to pour the cast, eliminating the sense of urgency.²

The technique recommends selecting the tray that best fits the arch. It is better to have a tray that is slightly too large then one that is too small. The tray can then be heated using an alcohol torch or hot water bath to make the tray moldable. The tray is then taken to the mouth and adapted to the existing arch. After customizing the tray, a heavy-body PVS is placed in the tray to act as tissue stops, and inserted intraorally. Upon setting, the stops are trimmed and a rigid- or heavy-body PVS is applied to the tray's flanges and the tray is reinserted into the arch for border molding. The border molding is trimmed to reduce areas that are too thick, and the frenums are modified so as not to impinge on the tissue in these areas. A wash impression is taken with a monophase/medium-body PVS, or may also be supplemented with a light-body in the premaxilla area when the tissue in this area is very flaccid. The resulting impressions capture all of the necessary anatomical details including: vestibular depth, hamular notches, frenum, retro molar pads, and mylohyoid ridges.

Pre-Impression Surface Optimizer

Working in a wet environment, even when the tooth's surface is dried, there is still a level of moisture present in the dentin of the prepared tooth. This may hamper flow of the impression material and decrease the accuracy of the impression. Early PVS impression materials tended to be hydrophobic and required extremely dry environments to capture the needed details. Manufacturers have worked with the chemistry to make these materials hydrophilic in nature and better able to flow over the teeth being captured.

Current PVS impression materials have measured contact angles of approximately 7º, significantly improving their hydrophilic properties. Yet contact angles of > 70º have been reported for dehydrated dentin surfaces.³ This disparity in contact angle between the impression material and tooth surface can hamper the flow of the impression material in wetting the preparation's surface and flow into the sulcus subgingivally to capture the preparation's margins.

Impression material flow can be improved with the use of surface modifiers to the preparation prior to application of the impression material. The surface modifiers act as surfactants, increasing wettability of the surface by decreasing the contact angle. The better the impression material wets the surface, the less chance of voids and failure to capture the details of the preparation and margins.⁴

B4^® Pre-Impression Surface Optimizer (DENTSPLY Caulk) is a pre-impression surface modifier that has been reported by the manufacturer to improve wettability of the dentin and improve the contact angles of dentin to the impression material. At this time, B4 is only being offered with DENTSPLY Caulk's hydrophilic vinyl polysiloxane (Aquasil Ultra Smart Wetting^® Impression Material) and is not available as a separate component. When applied to substrates including sulcular tissue, tooth preparations, ceramic, composite, or metal core build-up material or implant abutments, it is reported to provide a uniform surface condition with reduced surface tension prior to the application of the impression material. This optimized surface with its reduced contact angle on all substrates allows the capture of fine preparation detail both supra- and subgingivally.⁵

Conclusion

Impressions can be a challenging aspect of dental practice. Good impressions are the key to providing great dental restorations; inaccuracies at the beginning of the process multiply their effects on the result as the process progresses. For this reason, practitioners need to capture the most accurate impressions at the start of the process that will be concluded at the laboratory. Selection of proper trays and impression techniques can improve the resulting impressions. Additionally, the new surface optimizers assist in overcoming the wettability of the substrates clinicians are trying to capture, providing greater degrees of surface detail and accuracy in the resulting impression.

References

1. Samet N, Shofat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. J Prosthet Dent. 2005;94(2):112-117.

2. Massad JJ, Cagna DR. Vinyl polysiloxane impression material in removable prosthodontics. Part 1: edentulous impressions. Compend Contin Educ Dent. 2007;28(8):452-459.

3. Rosales JI, Marshall GW, Marshall SJ, et al. Acid-etching and hydration influence on dentin roughness and wettability. J Dent Res. 1999;78(9):1554-1559.

4. Millar BJ, Dunne SM, Robinson PB. The effect of a surface wetting agent on void formation in impressions. J Prosthet Dent. 1997;77(1):54-56.

5. Boghosian AA. Clinical and Material Factors in Achieving the Ideal Impression. Dental Economics. Available at: www.ineedce.com.