Abstract
A dual-arch impression technique is reported that enables the dentist to make accurate final impressions of subgingival margins without use of retraction cord, gingival excision, or application of hemostatic agents. The technique relies on specific physical properties of the chosen materials, sequential use of high- and low-viscosity impression materials, and the application of hydraulic force using a standardized method of material application. The procedure is described as the Hydraulic and Hydrophobic Impression Technique (H&H Technique).
Clinical Challenges
The ultimate goal in producing final impressions for crown and bridge restorations is to make accurate impressions that can capture subgingival margins. Conventional one-step impression techniques referred to in this report assume the sequential use of a low- or medium-viscosity impression material and a high viscosity material. In conventional practice, the low-viscosity material is first extruded into the sulcus surrounding the prepared teeth. The impression tray is then filled with a high-viscosity material and placed over the prepared teeth and dental arch. Four clinical challenges frequently hinder the replication of subgingival margins when using the described conventional impression technique. To understand these challenges, it is essential to have a working knowledge of key physical properties of the impression materials selected for use. These physical properties include viscosity, durometer hardness, and resistance to fluid absorption. The first challenge in capturing final impressions of subgingival margins is the use of retraction cord to displace gingival tissue. Low- or medium-viscosity materials typically have a durometer hardness (40 to 50) that is insufficient to displace soft tissue, blood, or saliva. Therefore, it is necessary to mechanically retract the gingiva in order to allow impression materials to passively enter the sulcus and produce an accurate impression of the finish line. The insertion and use of gingival retraction cord is time consuming and uncomfortable for the patient. In certain quadrants, access to the working area is difficult and may require multiple injections of anesthesia, adding to the patient's discomfort and chairtime. In anterior areas where aesthetics are critical, use of retraction cord may damage the interproximal gingiva or the col area, and subsequently result in an unaesthetic appearance. A second challenge with the described conventional impression technique is the necessity to maintain a dry field. The chairside dental team must work together quickly to assure a dry sulcus upon delivery of the low-viscosity material. Assuming the gingiva has been properly retracted, the presence of blood or saliva will still act to deflect the low-viscosity material and result in an inaccurate replication of the subgingival margin. Although the majority of vinyl polysiloxane impression materials resist water absorption, the durometer hardness of low-viscosity materials is insufficient to displace blood or saliva. Following the preparation of subgingival margins, hemorrhaging occurs in the majority of clinical cases. This presents the third clinical challenge of the conventional impression technique. Although hemorrhaging can be controlled with hemostatic agents, it often produces rapid clotting in several areas. If all blood clots are not removed prior to final impressions, the low viscosity, low durometer impression material can be deflected, resulting in an inaccurate replication of the subgingival margins. There are several implicit difficulties with use of hemostatic agents prior to impression taking. Following application of the hemostatic agent, the tooth and sulcus must be rinsed. This causes hemorrhaging to resume, requiring reapplication of the hemostatic agent. The procedure must be repeated until hemorrhaging is controlled. This decreases patient comfort and increases chairtime. Another difficulty can arise with use of hemostatic agents that contain sulfur, which is known to interfere with the setting times of vinyl polysiloxane impression materials. In addition, hemostatic agents that contain epinephrine can be a medical concern for patients with cardiovascular conditions. A fourth clinical challenge with the use of the described conventional impression technique is the potential for damage to the periodontium if electrosurgery is used to remove tissue to create a sulcus. Although electrosurgery can be used to sculpt the gingival tissue and create a free channel for the flow of impression material, it may result in gingival recession, or have an irreversible effect on the health of the periodontium. Electrosurgery can also cause hemorrhaging, in which case retraction cord or hemostatic agents may be necessary anyway.
Hydraulic Force and Durometer Hardness
A Hydraulic and Hydrophobic (H&H) impression technique is presented that allows the dentist to proceed from tooth preparation involving subgingival margins directly to final impression without use of retraction cord and without the requirement of homeostasis or a dry field. This technique relies on use of an impression material with a high durometer, which is used with a specific impression-taking method that generates hydraulic pressure to propel a low durometer material into the sulcus, thereby capturing the details of the subgingival margin. Throughout the description of this technique, the physical properties of selected materials will be referenced with respect to their impact on the accuracy of the final impression.
Materials and Clinical Preparation
The vinyl polysiloxane materials selected for the H&H impression technique necessitate specific physical properties. Since the technique involves two steps, the materials are referred to as first-step and second-step materials. The first-step material must have a high durometer hardness (no less than 85); exhibit a whipped consistency with a high viscosity; and maintain placement without running or slumping after the material is extruded and before it is set (Table I).
As a result of its high durometer, the first step material will exhibit a plaster-like rigidity after it is set. This characteristic is critical to the success of the technique, as it does not permit rebound of the material after setting. The rigidity of the high durometer material creates the hydraulic force needed to move the lower durometer material into the sulcus and displace the gingival tissue. The second-step material must have a durometer hardness of 40-50; exhibit a low viscosity; yet maintain placement without running or slumping after the material is extruded and before it is set (Table II).
The high- and low-durometer materials must both exhibit: a.) hydrophobic properties, that is, resist fluid absorption; and b.) thixotropic behavior, or flow under applied pressure and not slump when placed in the impression tray. These materials will not absorb blood and saliva, because they are dimensionally stable and highly accurate. As with all impression procedures, the working and setting times are determined by the clinician's preference.