The fabrication of complete removable dentures has evolved noticeably over the last 15 years with the advent of new materials and a better understanding of patients’ expectations. There is general agreement about one aspect of complete denture treatment: an accurate impression of the edentulous alveolar ridges and adjacent functional structures must be obtained before proceeding to fabricate the complete dentures. Without this foundation, there is no hope of providing patients with ideal function, comfort, and aesthetics.
Over a long period of time, 2 useful impression methods1,2 have evolved for the fabrication of dentures. These are the static and functional impression techniques.
THE STATIC (MUCOSTATIC) AND FUNCTIONAL IMPRESSION METHODS
The static impression technique was described in 1938 by Page3 and subsequently by Addison,4 who popularized it. The technique sought to create an extremely accurate impression of undisturbed and uncompressed tissues.
The functional impression method was described in 1931 by Pendelton5 as a “positive pressure technique.” Frank6 later explained how this impression method should apply pressure to certain areas while reducing pressure in other areas that are less tolerant of loading forces. The functional impression technique is based on sound principles7 and is commonly used today.
In summary, the static impression technique does not compress tissue, while the functional impression technique is a selective pressure method capturing all muscular attachments (frena) in their most active position or flexure.
To improve the accuracy as well as the efficiency of a complete denture impression, the authors have developed a method incorporating both approaches into a new impression technique that utilizes a single disposable impression tray (Harry J. Bosworth) or a stock metal tray (Schreinemakers, Accu-Liner Products). This eliminates 2 separate impression appointments and results in an accurate final imprint of the edentulous alveolar ridge. Aspects of both function and rest (stasis) are captured. The new impression system is called the combo functional/static imprinting technique (a trademarked term). This system provides for the impression of all functional muscular attachments (frena) and a static (minimum load) basal seat.
After reviewing all methods described in the literature, the use of a relatively new impression material emerged as the key to this combo technique. With the use of different viscosities of impression material, border-molding can be captured independently of the basal seat imprint. The goal is to utilize selected viscosities to produce an accurate impression of tissues that have different functional activity, character, and mobility.
Furthermore, a new classification system to define tissue character and mobility was developed to facilitate the selection of different viscosities of impression materials. This classification scheme will be defined in this article.
The use of polyvinyl siloxane (PVS) impression materials meets the requirements of this technique. These materials were developed primarily for use in fixed prosthodontic procedures, whereas removable prosthodontic impression procedures generally utilize irreversible hydrocolloid (alginate), polysulfide (rubber base), polyether, or zinc oxide/eugenol materials.8
This article reports on a preliminary clinical evaluation of this technique to determine the accuracy of edentulous impressions made with different viscosities of PVS impression materials. Also described is an evaluation of the working characteristics of PVS materials in this application.
|Figure 1. Edentulous stock metal tray with resin stops (left). Edentulous disposable tray with PVS stops (middle). Edentulous disposable tray with finger supports (right).|
The impressions were made using special stock edentulous impression trays made of disposable resin and autoclavable metal. Tissue tray stops were placed onto the metal tray utilizing a urethane reline material (Triad VLC Hi-Flow, DENTSPLY Trubyte), or a rigid-viscosity PVS impression material (Aquasil Ultra, Rigid, DENTSPLY Caulk) was placed on the resin trays (Figure 1). The urethane resin adhered to the metal tray best, while the PVS adhered to the resin trays more securely.
|Figure 2. Computer measured water-powder ratio for stone mix.|
|Figure 3. Vacuum-mechanical stone mixer.|
|Figure 4. Heat- and light-cured monomer-free urethane denture base curing apparatus (Eclipse, DENTSPLY Trubyte).|
Impressions were poured in die stone, which was measured using an automated, computerized measuring device9 (Smartbox, Amann, Figure 2) and mixed with a mechanical vacuum mixer following the manufacturer’s guidelines (Vacu-ret-Delta, Reitel, USA, Figure 3) to control the expansion and contraction properties of the stone (Marva-Cal Super Die Stone, Accurate Set). Denture bases were fabricated using a polyurethane denture system (Eclipse, DENTSPLY Trubyte, Figure 4).
From a pool of 12 patients, 3 patients participated in this study, designated as X, Y, and Z. At each patient’s first appointment, medical history, photo release, privacy practices, re-search documentation information, and release forms were signed and witnessed.
A thorough examination was then performed and impressions made. At the second appointment, processed denture bases made from these impressions were evaluated for fit and stability.
Patients were classified according to the Massad/Thornton Tissue Character Mobility Test (M/T).10 This records the character of the tissue overlying the alveolar ridge and the degree of tissue displacement. The tissue was then assigned a classification ranging from 1A to 3C, as described below:
The tissue character is classified as (1) coarse and fibrotic, (2) average, or (3) thin and delicate. The tissue attachment or displacement from the residual alveolar ridge is measured from (A) 0 to 0.5 mm, (B) 0.5 to 1.5 mm, or (C) greater than 1.5 mm.
The M/T was performed on each patient as part of the examination. The ends of 2 flat-handle mouth mirrors were placed on the crest of the ridge in both the premaxillary and postmaxillary areas, and pressure was applied bucco-lingually and mesio-distally to determine tissue mobility. (Note: M/T can be used to determine tissue character and mobility in the mandible as well as the maxilla; this article describes its use in the maxilla only.)
Of the 12 patients initially evaluated, one patient representing classifications 1A, 2B, and 3C were chosen, for a total of 3 patients. (These 3 classifications were selected because they represent clinical conditions ranging from the most favorable [1A] to the least favorable [3C].)
Patient X had maxillary ridge classification 1A. This classification indicated that the patient’s tissue appeared coarse and fibrotic and had mobility of less than 0.5 mm. Patient Y had maxillary ridge classification 2B. This classification indicated that the patient’s tissue appeared average and had mobility in the 0.5- to 1.5-mm range. Patient Z had maxillary ridge classification 3C. This classification indicated that the patient’s tissue appeared very thin and fragile and had mobility of more than 1.5 mm.
Three impressions were made on each patient. The impression method used was a functional/static technique, which achieves a final impression in one visit using a stock edentulous tray. The technique was performed as follows:
(1) Three trays of appropriate size were selected for each patient.
|Figure 5. Border-molded impression utilizing a heavy-body PVS material.|
(2) Tissue stops were placed in each tray with a light-cured polyurethane material or a PVS rigid-viscosity material. Four spherical pieces of material were placed in each tray—one in the anterior, one in each posterior ridge area, and one in the palatal area. The tray was then placed in the patient’s mouth and moved into the proper position, ensuring that the handle was centered in reference to both the incisive papilla and the anterior frenum, and that the tray borders were slightly below all flexed frena. The stops were allowed to set in the mouth. These tissue stops ensured proper reseating of the tray and adequate thickness of PVS impression material (Figure 5).
|Figure 6. Five different PVS viscosities: extralight PVS (top), light PVS (left, top), medium PVS (left bottom), heavy PVS (right, bottom), and very heavy PVS (right, top).|
(3) Border-molding of the impressions was achieved using Aquasil Ultra Smart Wetting Impression Material, rigid- and heavy-body viscosities (DENTSPLY Caulk, Figure 6). The material was mixed in an automix gun and dispensed onto the borders of the tray and the postpalatal area. The tray was placed in the mouth, and the patients were asked to pucker their lips outward. This movement activates the muscles of facial expression, capturing their movement onto the vestibular impression border.
Anterior border-molding was performed by physically pulling the anterior frenum downward. The buccal frenum border-molding was performed by pulling the buccal frenum downward and slightly anteriorly. The patient was then instructed to drop the mandible to its maximum opening position, delineating the post-zygomatic space and hamular frenum. The hamular frenum is the superior portion of the pterygomandibular raphae and lies directly behind the tuberosity. This muscle is difficult to capture in an impression. When the mandible drops, the frenum contracts, allowing it to move into the impression. The technician can then accurately record the muscle in the final prosthesis, decreasing dislodgement forces.
After vigorous border-molding, the impression was allowed to set fully, at which time it was removed and examined to determine if additional material should be added or relieved as well as to evaluate the extension and integrity of the border detail.
(4) Wash. For the final wash, 3 impressions were taken on each patient using 3 different viscosities of PVS material: medium, light viscosity, and extralight viscosity. In each case, the impression material was applied to the entire intaglio (tissue) surface.
|Figure 7. Peripheral border displaying the vertical and horizontal components.|
The previously described border-molding procedures were repeated, and the material was allowed to set fully. Each impression was removed; the amount of force necessary for removal was subjectively recorded by 2 evaluators. The final impression was once again examined for detail of the impression and border integrity in both vertical and horizontal directions (Figure 7).
|Figure 8. Red wax applied to rim of impression to reveal a 2-mm peripheral border.||Figure 9. Alginate boxed impressions ready for casting stone.|
Each impression was left undisturbed for approximately 1 hour, allowing for gas release from the PVS material. Each was then boxed for pouring utilizing magnetic boxing strips (Almore International) and alginate material (Kromopan). The alginate was contoured to display a 2-mm roll, measured from the vestibular vertical border extending to the labial/buccal horizontal border (Figure 8). This was accomplished to reflect the correct width of the peripheral border. The stone was measured with a computerized dispensing machine (controlling the water-powder ratio), mixed with a mechanical, timed vacuum mixer, and vibrated into the boxed impression (Figure 9). The stone was allowed to harden according to manufacturer’s recommendations.
|Figure 10. Occlusal rim fabricated to an uneven surface.|
Once the models were trimmed, denture bases were fabricated on each cast using a polyurethane light- and heat-cured system (Eclipse, DENTS-PLY Trubyte) following the manufacturer’s instructions. A polyurethane rim was then attached to each denture base to mimic denture teeth and to act as a holding device during the evaluations. These additional rims were designed to provide uneven occlusion. Each of the rims was made to the same specifications (Figure 10).
The finished denture bases were then evaluated in the patient’s mouth. The evaluations were accomplished independently by 2 clinicians. This was accomplished by wetting the denture base and placing it in the mouth with firm pressure. Each was then evaluated for retention upon manipulation of all facial muscles and activation of frena attachments.
Retention was checked upon biting on the uneven occlusal surfaces.
As previously described, each of the 3 impressions on each patient was evaluated for resistance to removal, impression detail, and border distortion. Further, the fabricated denture bases were evaluated for noticeable rocking upon patient biting on uneven occlusal surfaces.
As noted previously, border-molding impressions were taken both with heavy-body and with rigid-body material. No clinical difference was noted between the two. The clinicians, however, found that the working characteristics of the rigid-body material were more desirable for border-molding impressions. Wash impressions utilized 3 different viscosities of impression material: medium, light, and extralight (Figure 6).
All of the impressions were closely examined after the border-molding and wash stages.
The borders showed excellent detail. None displayed any significant distortion, despite the vigorous functional movements performed during the border-molding procedures. At least one of the 3 denture bases on all 3 patients revealed excellent retention, requiring manipulation (anterior force and/or placing air beneath the flange) for removal. It should be noted that a posterior palatal seal (post dam) was not added to these denture bases. None of the bases dislodged upon the patient occluding on the uneven occlusal surfaces. The denture base retention was subjectively rated (from No. 1 for best retention to No. 3 for least retention) for each patient, and the following results were found:
|Figure 11. (a) Definitive medium-viscosity PVS maxillary wash impression, (b) cast formed from 11a, and (c) denture base and rim formed from 11b.|
(1) Patient X (1A):
•No. 1 retention—base fabricated from medium-wash impression (Figure 11)
•No. 2 retention—base fabricated from extralight-viscosity wash
•No. 3 retention—base fabricated from light-viscosity wash.
|Figure 12. (a) Definitive light-viscosity PVS maxillary wash impression, (b) cast formed from 12a, and (c) denture base and rim formed from 12b.|
(2) Patient Y (2B):
•No. 1 retention—base fabricated from light-viscosity wash impression (Figure 12)
•No. 2 retention—base fabricated from extralight-viscosity wash
•No. 3 retention—base fabricated from medium-viscosity wash.
|Figure 13. (a) Definitive extralight-viscosity PVS maxillary wash impression, (b) cast formed from 13a, and (c) denture base and rim formed from 13b.|
(3) Patient Z (3C):
•No. 1 retention—base fabricated from extralight-viscosity wash impression (Figure 13)
•No. 2 retention—base fabricated from light-viscosity wash
•No. 3 retention—base fabricated from medium-viscosity wash.
All of the impressions exhibited resistance to removal, which is an indication of an accurate impression of an edentulous arch. The viscosities of the washes were viewed differently depending on the character and mobility of the tissue. The looser, more fragile tissue required the lighter-viscosity material to provide detail of the denture base without compressing the loosely attached tissues.
It is reasonable to conclude from this preliminary study that border-molding required a heavier-viscosity impression material than the basal tissue wash imprint, which was best captured with a lighter-viscosity impression material. As described here, the borders of the impression displayed excellent extension and detail. There was no visible distortion of the borders despite the functional manipulations during the border-molding procedure. The heavier-viscosity material acted more like border-molding compound and provided proper resistance to manipulation, resulting in accurate borders.11 Other impression materials, notably alginates, cannot be bor-der-molded aggressively without distortion or thinning to a nonusable knife edge.
This preliminary evaluation has suggested that the difference in retention of the denture may be indicative of the preferred wash materials as related to tissue consistency.
However, further evaluation is needed. In the authors’ opinion, based on the parameters established, all the denture bases exhibited good retention.
There appeared to be no discernable clinical difference between regular-setting and fast-setting PVS materials. Both clinicians, however, preferred the fast set. It can be suggested that the regular-set material be utilized until the clinician is comfortable with the technique. The fast-set material saves time and reduces hand fatigue, since the tray does not have to be kept in place for a long period of time.
Polyvinyl siloxane impressions appear to be very acceptable for soft-tissue impressions. They are particularly well-suited for the functional/static technique described in this article. It can be concluded that when used properly, these materials provide impression detail and accuracy that is comparable to other impression systems. Lastly, it is understood that in addition to accurate impressions, factors including diagnosis, patient management, and correct jaw relationship records must be carefully considered to achieve optimal complete denture success.
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Dr. Massad practices in Tulsa, Okla, as a general practitioner with a focus on removable complete prosthodontics and conducts extensive national and international continuing education programs for the dental profession. He is visiting faculty at The Pankey Institute, an adjunct professor at Tufts University School of Dental Medicine in Boston, and an adjunct associate clinical professor in the department of prosthodontics at the University of Texas Health Science Center at San Antonio Dental School. Dr. Massad is an adjunct associate professor of forensic dentistry at the Oklahoma State University College of Osteopathic Medicine and has published articles in the Journal of Prosthetic Dentistry, International Journal of Periodontal and Restorative Dentistry, Compendium of Continuing Dental Education, Dentistry Today, and others. He can be reached at (918) 749-5600 or by visiting drjoemassad.com.
Disclosure: Dr. Massad participated in a research project at the University of Oklahoma School of Dentistry on “centric relation studies” funded by DENTSPLY International and was an evaluator in a study funded by DENTSPLY for development of a new and improved denture base.
Dr. Lobel is in private practice in Saugus, Mass. He serves as clinical assistant professor, Department of Prosthodontics and Operative Dentistry at Tufts University, Boston. He received the Dean’s Award for Excellence in Clin