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:
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 physi- ology. They are designed with contoured Edentulous Impression Trays vestibular borders, retention slots, and ergo- nomic 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 Tray Adaptation large tray used on a small arch or small tray used on a large arch will distort the As a preliminary step, before engaging the tissues. mouth, it is recommended to clean and 3) The trays should permit both subtrac- hydrate the denture‐bearing soft tissues prior tive and additive modification of den- to impression making. This can be accom- ture borders to permit recording the plished by asking the patient to take a few vestibular extensions physiologically sips of warm water, to hold it in the mouth without distorting or displacing soft and then to use a rotary oscillating or pulsat- tissues. ing toothbrush to massage the denture bear- 4) The tray design should provide retention ing soft tissues. Care should be taken not to for the impression material. allow excess water to leak out of the mouth 5) The tray handle should not interfere with prior to rinsing. This procedure will not only border‐molding movements. help massage and hydrate the tissues but will 38 Application of the Neutral Zone in Prosthodontics
Figure 3.1 Maxillary thermoplastic tray immersed in Figure 3.2 Patient performing various movements water bath. to physiologically mold thermoplastic tray.
also help decouple residual plaque and any Fabrication of Tray Stops residual denture adhesive that may compro- Making a definitive impression requires mise the tray or impression procedures. multiple placements of the impression tray The featured trays are available in five in the oral cavity. Formation of tray stops is maxillary sizes and five mandibular sizes. critical to this technique so that consistent The appropriately sized stock impression and repeatable tray placements position tray is selected by placing each end of a can be achieved. Tray stops help accom- measuring caliper on the posterior third plish several objectives as described in (maxillary tuberosity for the maxilla and Box 3.1 [15]. retromolar pad for the mandible) of the Nickel‐sized circles of rigid-viscosity VPS alveolar ridge. This measurement should impression material are injected into the correlate with measured ridge size. The maxillary tray in four locations (incisor, selected tray is tried into the mouth and molar and mid-palate) and the tray is verified. The entire tray can be molded by carried into the patient’s mouth. For the heat to improve its conformation to the mandibular tray, stops are formed in three patient’s arch by placing into a controlled locations (incisor and bimolar). Trays heated water bath at 165 °F for approxi- are centered and seated on the respective mately 20–30 s or until the tray material sof- tens (Figure 3.1). Cheek retractors can be used to permit easy access to the oral cavity. After softening, the tray is immediately Box 3.1 Objectives of tray stops
reinserted into the mouth and the ● To provide a constant path of tray patient guided to perform border‐molding insertion. movements (Figure 3.2). ● To provide appropriate space for impres- The tray material hardens in 5–10 s, after sion material. which it can be removed from the mouth and ● To help stabilize the tray in a centered posi- analyzed for accuracy. The tray and tray bor- tion and prevent rotational movement of ders can be modified using laboratory rotary the tray during impression making. instruments and direct flame heat molding if ● To help stabilize the tray during patient necessary. Ideally, the tray borders should be generated border molding procedures. positioned in the middle of the vestibules ● To prevent overseating of the tray while and be wide enough not to create tissue making a definitive impression. impingement. Definitive Impressions 39
Figure 3.3 Tray stops formed in maxillary and mandibular trays.
arch and kept 2–3 mm away from the vestibular sulcular extents. Impression material is allowed to polymerize for 2 min- utes. The tray is removed, evaluated and modified as needed before proceeding. The tray stops are trimmed with a sharp blade to reduce areas of excessive tissue contact (Figure 3.3).
Border Molding the Impression Tray
Border molding is the process of “shaping of the periphery of an impression tray, by func- tional or manual manipulation of the adja- Figure 3.4 VPS material dispensed along peripheral cent tissue, to duplicate the contour and size extent of maxillary tray. of the vestibule.” The border‐molding proce- dure aids in defining the appropriate length and dimension of denture border, which is manipulations for physiologically molding critical to establish a peripheral seal. the borders of the mandibular tray are listed Overextended denture borders may encroach in Box 3.3. on muscle attachments leading to soreness, Following complete polymerization of the ulceration of the tissues, and denture dis- impression material, the impression tray is lodgement during function. Rigid viscosity removed from the mouth and the borders VPS is dispensed along the peripheral extents evaluated to ensure recording of all the ana- of the tray (Figure 3.4), placed in the mouth tomic and functional detail. Areas of tray and centered over the ridge with the aid of show through should be adjusted with labo- the tray stops. ratory rotary instruments or a sharp blade The dynamic manipulations for physiolog- by 1–2 mm. Additional relief should be ically molding the borders of the maxillary provided for non-pressure bearing tissues. tray are listed in Box 3.2. The tissue All the borders are then relieved by 1–2 mm 40 Application of the Neutral Zone in Prosthodontics
Box 3.2 Dynamic manipulations for molding the borders of the maxillary tray 1) The philtrum is grasped close to the lip line 4) Instruct the patient to move the mandible and pulled downward to record the labial from side to side and to drop down to frenum. opening position. Both these actions help 2) The patient is asked to purse the lips out- record the disto‐buccal vestibular sulci and ward with a sucking action (Figure 3.5) to hamular frena (Figure 3.6). register the labial vestibular extension. 5) Occlude the patient’s nostrils and ask them 3) The corners of the mouth and cheek are to exhale through the nose only (Figure 3.7) grasped with forefingers and thumb and (Valsalva maneuver). This will lead to migra- pulled downward and forward to record tion of the soft palate downwards to its the buccal frena and buccal vestibular functional position thereby forming the extensions. postpalatal border.
Figure 3.5 Patient purses lips outward with a Figure 3.6 Patient moves the mandible from side sucking action to register the labial vestibular to side, drops it down to opening position, to record extension. distobuccal vestibular sulci and hamular frena.
Figure 3.7 Patient’s nostrils occluded and patient instructed to exhale only through nose to aid in forming postpalatal border. Definitive Impressions 41
Box 3.3 The tissue manipulations for physiologically molding the borders of the mandibular tray 1) The patient is asked to place the tip of the 4) The corners of mouth are grasped with tongue straight out and forward, then side to the forefingers and thumb and pulled side, then back touching the roof of the mouth, downward and forward to record the and then to swallow. These movements help buccal frena and distobuccal vestibular record the lingual vestibule and retromylohy- extensions. oid fossae without overextension of borders. 5) The patient is asked to open wide and then 2) The lower lip is grasped at the lip line, then close the lower jaw on the practitioner’s pulled upward and forward; this registers the finger while the practitioner exerts a down- labial frenum and labial vestibular extensions. ward force on the tray (Figure 3.9). The activa- 3) The tray is secured by placing two fingers on tion of the masseter muscle causes bulging the tray finger support and the thumb under of the buccinator, leading to the develop- the patient’s chin (Figure 3.8). The patient is ment of the massetric notch in the impres- asked to purse the lips out and suck. This sion. This also helps define the posterior records the buccal and labial vestibules. extent and retromolar pad areas.
Figure 3.8 Impression tray is secured and patient is Figure 3.9 The patient asked to open wide and asked to purse lips out and suck to record the then close the lower jaw while the practitioner buccal and labial vestibules. exerts a downward force on tray to define posterior extent and retromolar pad areas.
prior to making definitive impressions viscosity VPS is re commended for firm, (Figure 3.10). nondisplaceable tissues. For mobile, flabby or redundant tissues, relief can be provided Final (Definitive) Impression by removing adjacent set impression materi- als from the tray and extra‐light viscosity The appropriate viscosity of VPS for the final VPS can be used. A combination of different impression is chosen based on the condition viscosities can be used simultaneously, of the existing denture‐bearing tissues ( tissue depending on the nature and character of character and mobility.) Medium / light denture‐bearing tissues. 42 Application of the Neutral Zone in Prosthodontics
Figure 3.10 Border molded and relieved maxillary and mandibular impression tray.
The tray is loaded with the chosen vis- cosities of the VPS impression material (Figure 3.11), and is placed and centered in the mouth until tactile resistance is felt once the established tray stops are appr oximated. The manipulations listed previously for the respective arches are repeated to again mold and record all borders. Following complete polymerization of the VPS material, the impression is removed from the mouth and each evaluated for anatomic, functional and surface detail (Figure 3.12). Excess material can be trimmed, if necessary, with Figure 3.11 Maxillary impression tray loaded with sharp scissors / blade. Patients may be asked two different viscosities of VPS material. to remove the impression from the mouth
Figure 3.12 Maxillary and mandibular definitive edentulous impressions. Definitive Impressions 43
themselves so that they can truly perceive Implant‐Retained the retention of their new prosthesis. If Overdentures patients seem to be dissatisfied with the retention of the impression, they should be A variety of attachment systems have advised to consider an implant‐retained been developed and marketed for use with prosthesis. implant‐retained overdentures. Some systems are designed to pick up retentive components directly in the finished denture. Others use an Techniques for Making indirect approach, which include an impres- Single Appointment sion with transfer copings (Figure 3.13). Definitive Impressions A master cast is created that displays all of the for Implant‐Assisted anatomy used to construct an overdenture and Complete Dentures also contains analogs to which retentive com- and Immediate Dentures ponents are attached and processed into the denture base. Impressions for implant retained, free‐standing, attachment systems are generally The use of endosseous dental implants to made using a “closed‐tray” technique. assist in the support, stability, and retention of removable prostheses is considered an effec- Tray Selection tive treatment modality for edentulous patients. Individuals wearing implant‐assisted Carefully examine the dimensions of the den- overdentures typically report improved oral tal arch and select the appropriate stock comfort and function when compared with impression tray. The thermoplastic impression conventional, mucosa‐suppor ted prosthesis trays illustrated (strong‐Massad dentate and [18–23]. Vinyl polysiloxane (VPS) impression implant trays – Figure 3.14) are constructed materials are well suited to address both the from a polystyrene‐based polymer and are accurate registration of denture‐bearing tis- available in three maxillary sizes and three sues and peripheral anatomy, and the stable mandibular sizes: small, medium, and large. recording of dental implant positions and indi- These trays are clear to permit see‐through vidual implant trajectories [24]. visibility to assist in size selection, and heat Implant overdenture VPS impression tech- moldable to aid in adapting the tray. Sufficient niques involve overdenture attachment selec- room between the tray and all implant attach- tion, tray selection and adaptation, tray stops, ments and impression components must be border molding, and the definitive impression. provided.
Attachment Selection
A distinction should be made regarding the number of implants and configuration of the attachment systems recommended for the individual edentulous patient. The num- ber of implants could range from two to four (accompanied by free‐standing abutments, which retain the denture but still rely on the mucosa contacting the denture base to help support the prosthesis), to four or more (using a bar constructed to connect the abut- ments to provide both retention and support Figure 3.13 Implant attachment transfer copings for the prosthesis). connected to each overdenture abutment. 44 Application of the Neutral Zone in Prosthodontics
Figure 3.14 Clear thermoplastic stock impression Figure 3.15 Tray stops formed with rigid VPS tray for implant / dentate impressions. impression material.
Impression Technique
To achieve consistently repeatable tray placements, tray stops are developed as described before using a rigid‐viscosity material (Figure 3.15). Border molding using a rigid‐viscosity material is accomplished using the listed protocol (Figure 3.16). Once the material has set, the tray is removed from the mouth and the borders are adjusted and relieved 1–2 mm, in preparation of the definitive impression. Care should be taken to also relieve any set material that has Figure 3.16 Tray borders molded and trimmed prior engaged the implant attachment transfer to making final impression. components. Both low and high viscosity VPS materi- als are placed in the developed tray. High viscosity VPS is placed in the area of the abutment impression copings. Lower vis- cosity materials are used elsewhere in the tray. Low‐viscosity VPS material is injected around the implant attachment transfers in the mouth. The tray is placed into the mouth using tray stops and tray borders to guide tray placement. The border molding protocol is repeated. Once the material has polymerized, the tray is removed and the impression examined for correct anatomic, functional and surface detail (Figure 3.17). Any required corrections can be made Figure 3.17 Mandibular closed tray definitive using the subtractive and additive methods impression for implant retained overdenture described above. fabrication. Definitive Impressions 45
Implant‐Retained and Border molding is accomplished as before Supported Overdentures with rigid‐viscosity material. Low‐viscosity material is injected around each attached cor- Support and retention for a complete rect spacing viscosity material placed into the implant overdenture is possible with enough tray except around the implants where a rigid‐ adequately dispersed free‐standing abut- viscosity material is injected in the tray. Once ments. However, this section will discuss properly seated in the mouth, excess material impression procedures unique to bar‐sup- extruding through the holes created in the tray ported prostheses [25]. should be cleared quickly to visualize and A variety of manufacturers offer transfer access the attachment screws easily. Upon copings, which are attached by screws to the polymerization of the VPS material, loosen intraoral components, for use during impres- each of the attachment screws and then sion procedures. Transfers can be made remove the impression, including the transfer directly from the implant interfaces or from the copings, from the mouth. The definitive interfaces of intervening transmucosal abut- impression should be inspected for proper ments. Resultant master casts will contain containment of the copings and appropriate analogs of the construction platforms for a anatomic, functional, and surface details connecting bar, which will be complemented (Figure 3.20). Analogs for the construction by retentive components processed within the overdenture. Impressions for implant‐bar retained and supported overdentures are generally made using an “open‐tray” technique. Previously mentioned edentulous or dentate thermoplastic trays can be used. Direct trans- fer copings are connected intraorally to each implant or abutment interface (Figure 3.18). Positions are estimated and holes placed in the tray with an acrylic bur to allow passage of the transfers through the tray when seated. Tray stops are accomplished as before, using rigid viscosity VPS material. It is important to relieve the areas of any set material adjacent to Figure 3.19 Tray stops formed and tray is relieved in the transfer copings (Figure 3.19). areas adjacent to transfer copings.
Figure 3.20 Open tray definitive impression for Figure 3.18 Direct transfer copings connected to fabrication of implant supported and retained all implants. overdenture. 46 Application of the Neutral Zone in Prosthodontics
(a) (b)
Figure 3.21 (a) Implant analogs attached to transfer copings and soft tissue material placed appropriately prior to cast fabrication; (b) definitive implant level soft tissue cast.
platforms for the bar are attached to the trans- fers within the impression and surrounding soft tissue material placed prior to cast pro- duction (Figures 3.21a and 3.21b).
Immediate Dentures
Combined with the many advantages of VPS impression material, a convenient definitive immediate denture impression technique can be accomplished in a single appointment [26]. The appropriate size dentate, clear, Figure 3.22 Tray stops formed for immediate thermoplastic tray is selected and adapted to denture definitive impression. the partially dentate arch. Tray stops are created for the existing residual teeth and ridge using rigid‐bodied VPS (Figure 3.22). Rigid‐viscosity materials are placed on the perimeter and the adapted tray is seated using the stops as guides. Border molding is accomplished using the methods previously described (Figure 3.23). For partially dentate patients, using an elastic material for this process allows the impression to be removed from the mouth despite engaging potentially confounding tis- sue undercuts. Upon removal, the borders are defined and relieved. The definitive impres- sion is made using low viscosity VPS material Figure 3.23 Border molded impression. Note: in the edentulous areas and injecting extra‐ borders are trimmed by 1–2 mm prior to making low viscosity material around all residual teeth definitive impressions. Definitive Impressions 47 using manual syringes (Figure 3.24). Extra‐low Master Cast Production viscosity VPS material possesses lower tear strength, permitting easier recovery of the Once satisfied with the definitive impr essions, cured impression from the patient’s mouth. Its recommended disinfection procedures are use also minimizes the chances of breakage performed. Although VPS materials possess of teeth (especially those that are prone to excellent dimensional stability and can breakage, which exhibit long clinical crowns generally be transported dependably, some due to advanced periodontal disease) from practitioners prefer to verify quality by the cast, while recovering the cast from the creating master casts in office. Definitive impression. casts should be accurate, void free and cre- The loaded tray is placed into the mouth ated with a properly formed base [27]. A using the stops as guides. All border‐mold- base former can be used with a bulk sili- ing manipulations are repeated. Upon cone product to customize (bead and box) polymerization of the VPS, remove and the outline and thickness of the cast base, inspect the impression for appropriate adapted to the appropriate dimensions. An anatomic, functional, and surface details approved, ADA standard 25, type‐III stone (Figure 3.25). may be used for construction (Figure 3.26). The process of mixing these powdered gypsum products with the appropriate water is critical to the cast outcome. Automatic dispensing and mixing machines (Figure 3.27) are useful to control consist- ent production. Vacuum mixing at reduced atmospheric pressure helps to prevent void formation. The resulting cast should require little trimming or surface pre- paration (Figure 3.28).
Figure 3.24 Extra low viscosity impression material injected around all teeth.
Figure 3.25 Definitive impression for fabrication of Figure 3.26 Fluid dental stone poured in boxed and maxillary immediate denture. beaded impression for definitive cast fabrication. 48 Application of the Neutral Zone in Prosthodontics
Figure 3.27 Automatic dispensing unit and automatic vacuum mixing unit for dental stone.
Summary
This chapter describes a novel technique of making a definitive impression in a single appointment eliminating the need of custom tray fabrication. Contemporary materials and trays have facilitated the development of this technique. This technique is simple, easy, accurate, and can be incorporated into any dental practice. Quality master Figure 3.28 Definitive mandibular cast (top) and casts can also be created successfully in the definitive mandibular impression (bottom). dental office.
References
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