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The base of a complete denture (CD) should be hygienic, biocompatible, and esthetic in its form.1 It also should provide a rigid and intimate fit against the mucosa of ridge to resist masticatory forces while preserving the remaining structures.2-4 Under function, the supporting form of the ridge may differ slightly than when at rest because of the viscoelastic property of mucosa.5
The impression making should conform to the dynamic nature of the ridge mucosa and ensure an adequate extension of the base flange to resist lateral forces developed in the mouth.6 Thus, pressure control appears to be a critical consideration in the design of the denture base, and this has been a focus of research as well as a clinical challenge when making an impression. The lack of a consensus continues to lead to uncertainty in the application of the basic concepts of accurate impressions.
This article discusses the basic principles of both nonpressure mucostatic and selective-pressure functional impression concepts and provides a theoretical and scientific basis for the design of a CD base. A MEDLINE/PubMed search for keywords (complete denture, metal base, acrylic base, nonpressure mucostatic impression, selective-pressure functional impression) was supplemented with a hand search to identify relevant peer-reviewed articles published in English up to 2015.
Mucostatic Concept
The concept of nonpressure mucostatic impressioning (MC) is based on Pascal’s law of fluid mechanics, which states that a contained fluid under pressure will result in an equal distribution of pressure in all directions.7,8 This principle suggests that the mucosa with its high water content must be confined and immobilized under the base. It also stresses the role of the interfacial surface tension developed by salivary film.7-9 The mechanism of denture base retention and stability is based on the principles of adhesion and cohesion, where the molecules of saliva coalesce with each other and adhere to the surface layer of both the denture base and mucosa of the residual ridge.2 The mucosa, therefore, is conceptualized to keep its form under function while maintaining an intimate fit during the masticatory cycle.
The mucostatic impression has traditionally been made with a special formula of zinc oxide eugenol paste to register an accurate negative of the ridge tissue in an undisplaced or passive form.8-10 The tray is formulated to ensure the fit against the ridge and demonstrate rigidity that prevents distortion of the impression. Neither a vent hole nor finger pressure is used when making the impression. The border molding procedure is eliminated to limit the extension of the base to the stress-bearing area of the ridge, and the lingual flange is shortened only to resist the lateral displacement.9,11
The metal base is customarily fabricated using a type IV gold alloy to maximize the fit and provide the rigidity necessary for uniform distribution of forces against the ridge.8,9,12 The intaglio surface of the base is left unpolished to encourage a thin layer of saliva to wet the irregular surface of the base.2 This increases the surface energy of the base due to the increased surface area and details of ridge mucosa.13 The post dam is thus eliminated in the maxillary CD.8,11
Movement of the denture base is minimal in the gingival direction under masticatory forces because the base contains the tissue fluid present in the firmly attached ridge mucosa.8,9,14 The attached tissue is integumentary and incompressible, allowing the uniform dissipation of pressure without displacement during mastication.8 The rigid denture base must fit precisely against the ridge to comply with the fluid mechanics of equal increase of pressure at every other point in the stress-bearing mucosa.
Functional Concept
The concept of selective-pressure functional impressioning (FC) utilizes the various thicknesses and dynamic viscoelasticity of oral mucosa.5 While some areas of mucosa are fairly firm, others are prone to displacement under pressure. The bony structure of the ridge presents different trabecular patterns and morphology in areas of muscle attachments.6,15,16 To conform to these anatomical and functional characteristics of the ridge, pressure is controlled to selectively displace the mucosa when making an impression. The mucosa is displaced within the elastic range of tissue compression to allow an instantaneous rebound upon the release of masticatory forces.17 The goal is to provide adequate functional stimuli and resist the forces of mastication without contributing to the resorptive process of the ridge driven by the base.6,17,18
The buccal shelf presents a loosely attached mucosa,6,16 whereas the crest of the ridge usually consists of a firmly attached mucosa unless the ridge demonstrates an excessive resorptive pattern (Figure 1). The buccal shelf, meanwhile, presents a thick cortical bone and is part of the basal bone receiving tensile stimulation through the actions of muscular attachments. The anatomy and trabecular patterns also are structured to resist the forces of mastication better than the crest of ridge, although the mucosa is less keratinized and prone to displacement under pressure.5,6
The goal of the FC technique is to ensure uniform equilibration of support for the denture by loading the entire stress-bearing area. To meet this objective, the pressure is selectively applied to distinguish the different tissue characteristics and maximize the support by displacing the loosely attached mucosa. Thus, the base is designed to fit against the functional form of ridge contour and rebound to allow the ridge to return to its anatomic form at rest.6,18
The impression is made with a low-viscosity impression material to distinguish the different viscoelastic characteristics of the mucosa.6,18-21 The custom tray may also present a relief and/or vent holes to relieve hydraulic pressure during the making of an impression.22 The border molding procedure is emphasized to maximize the border extension along the vestibules and increase the surface area of the base (Figure 2).6,23 Pressure is controlled using the viscosity of the impression material loaded in the tray, and a post dam is created to ensure the postpalatal seal of the maxillary CD.18,24
Reduced pressure is created underneath the base when the prosthesis is about to lose retention.2,4 Although it is not a vacuum underneath the base, the pressure difference within the oral cavity can create a suction effect and prevent the dislodgment of the base. However, the effect of a border seal is often less than ideal with mandibular dentures because of the increased length of borders and the reduced surface area for creating the pressure difference.
Acrylic resin is usually selected as a material for the base because of its adjustability traits.19-21 The periphery of the base may need to be adjusted intraorally because of its extension along the displaceable lining tissue of the vestibule.25 Although the tissue side of the base is designed to demonstrate the surface details of mucosa, the base usually presents a smoother surface for better control of plaque.26
Discussion
The metal base of MC may present an advantage with regard to fit against the passive and anatomic ridge form due to less dimensional change of gold alloy. The rigidity of the base is also greater with a higher elastic modulus of metal compared to acrylic resin.27,28 Thus, the ridge may be subject to uniform stress under masticatory function and demonstrate a reduced rate of resorption.29 However, the load distribution of a metal base can be less than ideal when considering the uneven thickness of the varied viscoelastic mucosa.5 In addition, contemporary impression materials lack hydrophilicity and have limited capacity to reproduce the surface details of ridge mucosa as conceptualized with zinc oxide eugenol paste.
The application of Pascal’s law of fluid mechanics is suspect because of the incapacity of a CD base to contain tissue fluid.4,11 The attached mucosa of the ridge crest is also compressible under pressure, although the displacement can be less than that of the loosely attached mucosa of the buccal shelf.4 Thus, the mucosa of the ridge crest may be subject to a higher stress than that of the buccal shelf. This uneven pattern of load distribution may continue until it reaches the equilibrium of tissue dynamics under occlusal loading of mastication and muscular actions.4,6
The salivary flow rate should be sufficient to ensure the function of interfacial surface tension and a border seal.2 The mucosa also needs to be wetted by the serous type of saliva to promote the capillary action of liquid membrane.30 Thus, the denture may demonstrate a lack of retention in patients with xerostomia. These patients may need to rely on a denture adhesive, particularly for a maxillary CD because of its function against gravity.31
The tissue health of an edentulous mouth with an acrylic base may be suboptimal compared to such a mouth with a metal base because of the higher porosity of resin.12,27 Nanotechnology and the sustained release of antimicrobial agents appear to be promising developments in regard to modification of an acrylic base to provide antifungal properties.32-34 However, the clinical application of technologies incorporating silver nanoparticles has yet to occur because of the problems of decreased color stability and mechanical strength.
Friction is the resistance to motion of one material over another.27 This property is the measure of restraining force against a motion, and is proportional to the coefficient of friction (COF). Although it characterizes a function of two materials in contact, the COF can vary depending on the condition of surface finish and lubrication.35,36 Dry mucosa can be susceptible to irritation because of an increase in friction between the denture base and mucosa.25,26 Thus, the tissue surface of the base may need to be smoother to reduce the COF and enhance comfort.
The moment of inertia is a measure of resistance against the displacement of an object.37 It usually describes the stability of an object making a circular motion. The higher the inertia, the more stable the object. It has a functional relationship to the mass of an object. The greater the mass, the higher the inertia. Thus, an acrylic base of low density may demonstrate reduced stability against the dislodging forces under clinical function compared to a metal base.8,9 The high density of a metal base for a maxillary CD must resist the force of gravity for retention.4,11
The loss of retention can occur in the vertical direction away from the ridge.2 However, the base may experience a loss of stability with a rotational movement along the crest of the ridge prior to separating from the mucosa of the ridge. Meanwhile, the moment of inertia increases with a square rule to the length of the base extension.37 In fact, the distance measured from the ridge crest to the periphery of the base can be longer with the acrylic base because of the border molding procedure required to meet the objective of FC (Figure 3).6,18-21 Thus, the acrylic base may compensate for the loss of inertia resulting from the reduction of mass compared to the metal base of MC. Meanwhile, the value of product can differ with various densities of metal alloy and degrees of the base extension.6,27,37
The mandibular base should be designed to cover the buccal shelf where masticatory force is strong.16 The border of the lingual flange is less discernable to the tactile sensation of the tongue when the flange extension reaches the lingual vestibule.6 Meanwhile, the extension of an acrylic base is limited when the edentulous mandible demonstrates a narrow buccal shelf and excessive resorption of ridge.16 However, a metal framework can be incorporated into the acrylic base to add weight,38,39 though the efficacy of this procedure to increase inertia and stability has yet to be determined.
The limitation of an acrylic base of FC does not appear to negatively affect the clinical outcomes of patient satisfaction.19-21 No clinical evidence is available to support the superiority of a metal base of MC. The CD procedure can be facilitated with the acrylic base of FC when the clinician is confronted with a compromised ridge anatomy, higher level of muscular attachment, absence of attached mucosa, or xerostomia. Thus, the base design requires clinical judgment that takes into account the functional anatomy of ridge resistance and the oral health of the edentulous mouth, as well as the patient’s comfort regarding base extension.
Conclusion
The impression making of an edentulous arch is critical for the design of a CD base. Neither the nonpressure mucostatic nor the selective-pressure functional impression technique has been proven to be superior to the other. Therefore, the design of a CD base requires a clinical decision that is dependent on the patient’s presentation. Factors that impact this decision include the functional anatomy of ridge resistance, the oral health of the edentulous mouth, and the patient’s comfort when considering base extension.
About the Authors
Won-suk Oh, DDS, MS
Clinical Professor
Department of Biologic & Materials Sciences
University of Michigan School of Dentistry
Ann Arbor, Michigan
Harold F. Morris, DDS, MS
Clinical Assistant Professor
Department of Biologic & Materials Sciences
University of Michigan School of Dentistry
Ann Arbor, Michigan
Former Staff Prosthodontist
Ann Arbor VA Medical Center
Ann Arbor, Michigan
Queries to the authors regarding this course may be submitted to authorqueries@aegiscomm.com.
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