Prosthodontics

Prosthodontics Dentistry Sampler Chapter 3: Definitive Impressions From Application of the Neutral Zone in Prosthodontics. by Joseph J. Massad, David R. Cagna, et al 35 3 Definitive Impressions Preimpression Considerations attempts are designed to record the mucosa in a completely passive, nondisplaced state. A denture impression represents a negative Although it is impractical to achieve this fully, likeness of structures within the edentulous the use of a highly flowable impression material mouth [1]. Inaccurate impressions will result to avoid tissue distortion is widely accepted. in ill‐fitting and unstable dentures. It is para- The selective‐pressure concept considers the mount that denture‐bearing tissues be variable constitution of individual intraoral healthy, unchanging, and free of pathologies, anatomy and attempts to direct greater func- soreness, inflammation, and distortion, prior tional loads to primary stress bearing areas. to making definitive impressions. Associated Customized trays have traditionally been systemic disease, diet, chronic trauma, and fashioned in which more space relief or vent- boney abnormalities should be addressed. ing is provided for the non‐stress‐bearing tis- Complete dentures fabricated on unfit sues. In addition, the perimeters of the trays denture‐bearing mucosa will lead to further are customized to conform to the functional deterioration of tissue health and compro- extents of the vestibules. Presently, variations mise the prosthetic outcome [2]. of this concept are the most widely accepted and practiced [6]. A majority of dental schools currently Background impart to students a multistep, selective, pressure technique, which includes a pri- Complete denture impression techniques mary impression, construction of a primary have enjoyed a rich history in the dental lit- cast on which a custom definitive impres- erature. Three dominant philosophies have sion tray is created, intraoral development of evolved: (i) definitive‐pressure impressions; the tray periphery, and definitive impres- (ii) minimal‐pressure impressions; and (iii) sions made with an appropriately flowable selective‐pressure impressions [3–5]. In the material [7, 8]. first theory, attempts are made to capture the intraoral tissues in a force‐loaded state. Although now seldom used for impression Impression Fundamentals making, modifications of this method do have practical value as a technique for relin- The following objectives for making defini- ing intaglio denture surfaces. The theory of mini- tive edentulous impressions have been sug- mal‐pressure employs the principles of gested [9–11]. These axioms help to provide mucostatics. In contrast to the first theory, support, stability, and retention for the Application of the Neutral Zone in Prosthodontics, First Edition. Joseph J. Massad, David R. Cagna, Charles J. Goodacre, Russell A. Wicks and Swati A. Ahuja. © 2017 John Wiley & Sons, Inc. Published 2017 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/massad/neutral Chapter No.: 1 Title Name: <TITLENAME> c03.indd Comp. by: <USER> Date: 19 May 2017 Time: 06:49:34 AM Stage: <STAGE> WorkFlow:<WORKFLOW> Page Number: 35 36 Application of the Neutral Zone in Prosthodontics eventual prosthesis and preserve health of maxilla, moderate pressure may be applied the biologic tissues: on the horizontal portions of the hard palate, tuberosities, rugae, and the ridge crests (if 1) The impression should cover the entire covered with firm, healthy, nondisplaceable basal seat within the limits of function of mucosa). These are considered to be stress‐ various orofacial muscles. Maximum bearing areas in the maxilla. The median coverage helps dissipate the forces over a palatal raphe and incisal papilla need to be large area, reducing the amount of force relieved and recorded with minimum pres- on each square millimeter. sure. For making an impression of the eden- 2) The impression should have maximum con- tulous mandible, the buccal shelves, ridge tact with denture‐bearing mucosa, to ensure crests, and retromolar pads (if firm) are con- adequate fit of the final prosthesis. sidered stress‐bearing areas and can be 3) The borders of complete dentures should recorded with moderate pressure. Mobile be defined physiologically so that they are mandibular ridge crests and associated lin- in harmony with the anatomic and func- gual extensions need to be recorded with less tional limits of the denture foundation pressure. and adjacent tissues. 4) The impression should be made such that its dimensions and contours replicate the intended contours of the definitive Impression Materials complete denture. In order to make an accurate impression, it Historically, much attention has been given is imperative to understand the anatomy and to the wide variety of materials available for physiology of the individual edentulous making edentulous impressions including mouth and the methods to identify and cap- plaster, modeling plastic impression com- ture dynamic postures. The vestibular extents pound, zinc oxide‐eugenol paste, irreversible of both arches should be relieved and func- hydrocolloid, polysulfide, polyether, and tionally re‐established by associated muscular vinyl polysiloxane. All of these materials may activation within the impression to form the perform adequately in the hands of an expe- eventual denture flange borders. A medium rienced practitioner. that has both additive and subtractive work- More recently, vinyl polysiloxane (VPS) ing properties is preferred to accomplish this. has become a popular material of choice for Care must be taken not to suppress or overex- definitive dental impressions. VPS impres- tend areas of attachment such as the frenulae sion material, an addition reaction silicone, or areas that express physiologic mobility is available in different vi scosities (extra requirements such as the floor of the mouth light, light, medium, heavy, and rigid) and and retromylohyoid region of the mandible with different working times. Subsequent [12]. The postpalatal seal area is defined as the layers of VPS can be added sequentially to soft tissue area at or posterior to the junction existing polymerized material, permitting a of the hard and soft palates, on which pressure layering technique. Heavier bodied materi- within physiologic limits can be applied by a als offer sufficient working times and viscos- maxillary complete denture to aid in its reten- ity to permit physiological molding of the tion. It is important to identify, assess, and borders of the impression tray and primary capture the extent of this region in the maxil- stress‐bearing areas, while more flowable lary impression [13]. materials can capture mobile tissues without It is also crucial to understand where more distortion [15]. Vinyl polysiloxane impres- moderate pressure can be applied and where sion materials are easy to use and disinfect, it should be minimized during the making of are accurate, have favorable tear strength, definitive impressions [14]. In the edentulous and can remain dimensionally stable for an Definitive Impressions 37 extended period. These materials have Specialized thermoplastic stock trays, spe- excellent elasticity and wettability, which cifically designed to meet the requirements enhance detailed recording of oral tissues of edentulous patients, have been produced and complement gypsum cast production for making definitive impressions (Massad [16]. They also offer the benefit of repeated edentulous impression trays). These trays are pourability for the production of multiple constructed of a polystyrene‐based polymer casts from a single impression. and can be customized and molded accord- ing to individual patient anatomy and physiology. They are designed with contoured Edentulous Impression Trays vestibular borders, retention slots, and ergonomic finger rests. The retention slots Physical properties of the impression tray designed in the tray help record the denture‐ and its manipulation constitute important bearing soft tissues with minimum distortion considerations in the impression process. and displacement and also provide a means Historically, the use of custom impression of mechanical retention of impression mate- trays to make definitive edentulous impres- rial. Ergonomic finger‐rest extensions help sions has been considered essential for accu- the molded tray to remain rigid and to dis- rate results. Improvements in impression tribute pressure evenly across the denture materials and impression trays have made it bearing areas. Well‐designed tray handles possible to avoid the necessity for custom are positioned such that they do not interfere impression trays [15]. Today, stock trays are with the border‐molding movements. available that can be molded and shaped to conform to the dimensions and contours suitable to the anatomy of edentulous Technique for Making patients. These can eliminate the need Single Appointment for making primary impressions and for Definitive Impressions construction of custom trays. Stock impres- for Conventional Complete sion trays used for making a definitive impression, should meet the following Dentures requirements [15]: An innovative technique is presented to 1) The tray should be rigid enough to permit simplify the procedure of impression making border‐molding procedures and support and decrease the number of clinical appoint- the impression material. ments [17]. 2) The trays should be available in various sizes for variety of edentulous arches. A Tray Selection and

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