Objectives of impression making

OBJECTIVES OF IMPRESSION MAKINGRETENTIONSTABILITYSUPPORTESTHETICSPRESERVATION OF REMAINING STRUCTURES RETENTION Retention is defined as that state of a denture wherein functional forces are unable to destroy the attachment existing between the denture and the mucoperiosteum. Retention resists the adhesiveness of food, the force of gravity, & the forces associated with the opening of jaws. Retention begins with the impression. It depends upon factors that produce attachment of the denture to the mucosa. 81Factors affecting Retention Anatomical factors Physiological factors Physical factors Mechanical factors Muscular factors 82Anatomical factors Size of the denture bearing area Quality of the denture bearing area. Factors affecting Retention 83Physiological factors Saliva and its quality Factors affecting Retention 84Physical factors Adhesion Cohesion Interfacial surface tension Capallarity and capillary attraction Atmospheric pressure and peripheral seal Factors affecting Retention Adhesion :-It is the physical attraction of unlike molecules It acts when saliva sticks to the denture base & to the mucous membrane of basal seat . Adhesion is achievied by ionic forces between charged salivary glycoproteins & surface epithelium or acrylic resin.Quality of adhesion depends on :- The most adhesive saliva is thin serous but contains some mucous components.Thick & ropy salivais very adhesive but tends to build up so that it is too thick in palatal area & interferes with oral adaptation .In this situation patient should rinse out the ropy saliva every two to three hours The amount of retention provided by adhesion is directly proportional to the area covered by denture.Mandibular dentures cover less surface area than maxillary prosthesis & therefore are subject to a lower magnitude of adhesive retentive forces.Similarly patients with small jaws or very flat alveolar ridges cannot expect retention to be as great as can patients with large jaws or prominent alveoli. Cohesion:-it is the physical attraction of like molecules for each other .it occurs within the layer of fluid (usually saliva ) that is present between the denture base & the mucosa. Normal saliva is not very cohesive , therefore most of the retentive forces of denture –mucosa interface comes from adhesive & interfacial surface tension factors. Interfacial surface tension :-it is the resistance to separation of two parallel surfaces that is imparted by a film of liquid between them .It is dependent on the ability of the fluid to wet the rigid surrounding material . If the surrounding material has low surface tension , as oral mucosa does ,fluid will maximize its contact with the material, thereby wetting it readily & spreading out in a thin film.If the material has high surface tension ,fluid will minimize its contact with the material , resulting in formation of beads on the material surface. All denture base material have higher surface tension than oral mucosa ,but once coated by salivary pellicle ,their surface tension is reduced ,which promotes maximizing the surface area between liquid & base.Role of surface tension is through capillary attraction or capillarity.When the adaptation of denture base to mucosa is sufficiently close ,the space filled with a thin film of saliva act like a capillary tube in that the liquid seeks to increase its contact with both denture & mucosal surface. It plays a major role in retention of maxillary denture. It is totally dependent on presence of air at the margin of liquid & solid contact (liquid air interface).As there is excess saliva along the lower border of mandibular denture, Surface tension is lost in mandibular denture due to loss of liquid air interface at denture border . Mucostatics dismiss adhesion and cohesion as factors in retention, the entire phenomenon being attributed to interfacial surface tension. But an analysis has proved that if it was not for the forces of adhesion and cohesion, the forces of interfacial surface tension wont exist. Attachment of a denture is possible because both tissue and denture base material can become wet which means its molecule will adhere to water molecules. Oral & facial musculature :-supplement retentive forces , provided :-a)Teeth are positioned in “neutral zone “between the cheeks & tongueb)The polished surface of the denture are properly shaped.If the buccal flange of maxillary denture slope up & out of occlusal surface of teeth & the buccal flange of mandibular denture slope down & out from the occlusal plane, the contraction of buccinator will tend to retain both denture on basal seat. Atmospheric pressure:-Act to resist dislodging forces applied to the denture ,if the denture have an effective seal around their borders.Retention due to atmospheric pressure is directly proportional to the area covered by the denture base. In function, atmospheric pressure is superior to interfacial surface tension as a retentive force, for forces horizontal as well as parallel to the mean of mucosal plane are resisted. Interfacial surface tension will resist only forces perpendicular to the axis of surface tension forces. Factors affecting RetentionMechanical factors Undercuts Retentive springs Magnetic forces Denture adhesive Suction chambers and suction discs 99 100Muscular factors The muscles apply supplementary retentive forces on the denture. It is most effective in the neutral zone. Factors affecting Retention 101STABILITYThe quality of a denture to be firm, steady, or constant, to resist displacement by functional stresses and not to be subject to change of position when force is applied. It is the ability of the denture to withstand horizontal forces. 102Factors Affecting Stability Vertical height of the residual ridge. Quality of soft tissue covering the ridge. Occlusal plane Quality of the impression. Teeth arrangement. Contour of the polished surfaces. SUPPORTIt is the resistance to vertical forces of mastication & to occlusal or other forces applied in a direction toward the basal seat .When the natural teeth are missing ,the alveolar ridge & their covering of mucosal tissue become the supporting elements. Unfortunately , they were never meant to endure the forces of mastication & other constant occlusal pressure that result from swallowing , clenching ,or bruxing.To make the best of bad situation , it is necessary to enhance the available support by utilizing maximum coverageof all usable ridge bearing areas. Areas of support are divided into:- Primary support area:- area of edentulous ridge that are at right angle to occlusal forces & usually do not resorb easily . Maxillary:- a)posterior ridge b) flat areas of the palate Mandibular:- a)buccal shelf areab)Posterior ridge c)pear shaped pad Secondary supporting area:- area of edentulous ridge that are greater than at right angle to occlusal forces ; also the area of dentulous ridge that are at right angle to occlusal forces but tend to resorb under load. Maxillary :- anterior ridge ,rugae & all ridge slopes Mandibular:- anterior ridge & all ridge slopes 108ESTHETICS The thickness of the denture flanges is one of the important factors that govern esthetics. Thicker denture flanges are preferred in long-term edentulous patients to give required labial fullness. Impression should perfectly reproduce the width and height of the entire sulcus for the proper fabrication of the flanges.

Objectives of impression making

by Archana Logishetty

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