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Multiple-Unit Master Impression
Robert A. Lowe, DDS
Multiple-unit cases can test the limits of even the best elastomeric impression materials, and an accurate impression is essential for successful case completion. The key to a good impression is proper selection of an impression material and systematic application of preparatory procedures before making the impression.
Important properties of an impression material include elastic recovery, tear strength, stability, dimensional change, surface wetting, and working and set times. The elastic recovery of a material is its ability to spring back after distortion and is extremely important for obtaining an accurate impression. Tear strength is a consideration because detail can be lost in critical areas if a material tears when removed from the mouth. The stability of the final impression will provide flexibility as to when models must be poured. Dimensional change should be minimal in order to provide accurate working models. Maximum surface wetting is important for detailed reproduction in impressions and working models. Setting and working times are critical because distortion will result from exceeding a material’s working time or removing an impression from the mouth before its setting time is complete.
This article illustrates proper impression material selection and preparatory procedures when making a final impression for a multiple-unit clinical case.
The patient shown in Figure 1, a 28-year-old woman, was prepared for a multiple-unit esthetic reconstruction, including a single-unit implant in the maxillary left lateral incisor position. Her desire was to have a more beautiful, brighter smile. Orthodontic correction before tooth preparation allowed for the maximum conservation of tooth structure. Her maxillary and mandibular teeth were prepared, a master impression was taken, and provisional restorations were placed. Because it would take about 4 months for the implant in the maxillary left lateral incisor position to integrate, all of the definitive restorations were fabricated and delivered except for teeth Nos. 7 through 11. Although the maxillary left cuspid was fabricated, it was not placed in order to allow for a more stabile provisionalization without the use of a traditional “flipper” appliance while the implant integrated to the bone. Figure 2 is a retracted view of the maxillary anterior segment after osseointegration was completed and the patient presented for a master impression of teeth Nos. 7 through 10. The definitive restoration that was previously fabricated for the maxillary left cuspid was placed on the preparation and removed in the master impression to ensure proper contact to the maxillary left lateral incisor and to act as a “shade tab” for the fabrication of the remaining ceramic restorations.
Aquasil Ultra Xtra (DENTSPLY International, York, PA) (Figure 3) was chosen to make the master impression. This polyvinylsiloxane impression material has all the desirable properties detailed earlier in this article along with an extended working time. It has been this author’s experience that when working with multiple units and an open-tray impression for the implant restoration, additional time is required to locate and remove excess impression material around the screw holding the impression coping so that the impression can be easily removed from the mouth when set. The extended working time of this material was necessary because exceeding the working time of an impression material will result in distortion.
The provisional restoration was removed from the maxillary anterior segment. A gingivectomy was performed using a Waterlase® MD (Biolase Technology, Irvine, CA) around the healing abutment to idealize the cervico-incisal height of the implant restoration as compared with the clinical crown length of the maxillary right lateral incisor (Figure 4).
Figure 5 shows the retraction medium in place as well as the open-tray impression coping that was placed on the implant. The natural teeth were then coated with B4 surface optimizer to help lower the surface tension on the teeth and get a more accurate master impression (Figure 6). Figure 7 shows an incisal view of the prepared teeth after removal of the top (#1) retraction cord. A double-cord technique was utilized for maximum tissue management. The first cord, #00, was left in place and would be removed after the master impression was made. This technique will ensure that not only the margin is captured in the master impression, but also 0.5 mm of tooth surface apical to the restorative margin so that the ceramist can easily identify margins for proper die trimming.1 Aquasil Ultra Xtra light-body material was then syringed into the sulci of the retracted teeth and also around the implant impression coping (Figure 8). During this time, the dental assistant was loading the impression tray (Figure 9) with Aquasil Ultra Xtra tray material and the loaded tray was placed in the patient’s mouth (Figure 10). With a 5-minute, 30-second working time, there is plenty of time to position the tray, locate the opening for the implant impression coping, and uncover the screw so that removal of the tray upon seating will be facilitated. Figure 11 and Figure 12 are views of the completed master impression for the case. The close-up view highlights the exquisite detail that was captured in this multiple-unit impression with Aquasil Ultra Xtra.
In the case presented, a multiple-unit impression was required that included an implant as well as natural abutments. Key to the success of the case was the selection of an appropriate impression material and the application of sound preparatory procedures. When these principles are applied the clinician can predictably expect accurate multiple-unit impressions and properly fitting final restorations.
1. Killian S. Let’s relate impressioning to die trimming. Available at: www.dentistrytoday.com. Online February 2007. Accessed March 2009.
About the Author
Robert A. Lowe, DDS
American Board of Aesthetic Dentistry
Charlotte Center for Cosmetic Dentistry
Charlotte, North Carolina