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INTRODUCTION The recognition, understanding, and incorporation of certain mechanical, biologic, and physical factors are necessary to ensure optimal complete denture treatment These factors are the determinants that promote the properties of retention, stability, and support in the finished prosthesis Bohannan ’ appropriately noted that “technique itself is merely the practical application of principles, and if the principles are unsound, the most elaborate and painstaking technique certainly is doomed to failure.”

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Success of prosthesis depends upon

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Boucher describes retention as the most spectacular yet probably t he least important of all complete denture objectives. This property may indeed be least important; however, it provides psychological comfort According to Hartwell  R etention is too often given more consideration if all other objectives are achieved retention will be adequate


DEFINITION Resistance of denture to vertical movement away from tissue --- prosthetic dentistry glossary Quality inherent in the dental prosthesis acting to resist the forces of dislodgment along the path of placement— GPT 8 TH ed Retention its resistance to removal in direction opposite to that of its insertion--- Bouchers 12 th ed Resistance of movement of denture from its basal seat especially in vertical direction--- Winkler 2 nd ed




ANATOMICAL FACTORS Determine the extent of denture base. More the surface area more will be the retention. Denture bearing area of maxillae is 22.96 cm2 and mandible is 12.255cm2 ( Boucher 12 th ed ) Tyson (1967) with series of experiment showed retention is primarily obtained through effect of soft tissue impaction against denture base, creating a seal which prevents air getting between denture and mucosa

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[ A] Denture on a flat ridge that has insufficient buccal extension is like a loose piston which can be easily withdrawn [B] correct buccal extension give firm retention like a closely fitting piston (Tyson 1967)


MAXILLARY DENTURE BASE RETENTION Incorporation of posterior palatal seal Posterior palatal seal maintains tissue contact during base movement or soft palate function Buccal space or retrozygomatic space must be filled to avoid ingress of air beneath the denture


MANDIBULAR DENTURE BASE RETENTION Difficult to achieve retention due to Movable floor of mouth Lack of ideal ridge height Special attention given to Triangularis muscle--- buccal frenum Mentalis muscle--- labial flange The denture base should cover the posterior extension of the firmly bound, keratinized tissue of the pear-shaped pad. Lammie and Krol suggest beading this region at the junction of the pear-shaped and retromolar pad to ensure peripheral seal


LINGUAL MANDIBULAR ANATOMIC INFLUENCE Retromylohyoid fossa Adequate seal obtained by gentle compressing tissue lingual to retromolar pad cont …

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Medial pterygoid -influences distolingual flange causing bulge in retromylohyoid space Floor of mouth-depends on mylohyoid muscle when muscle is active seal occurs at border of lingual flange Tongue occupies entire space superior to floor of mouth it promotes seal and enhances retention by contacting lingual border of denture Overextension of lingual border can be suspected if a groove formed by mylohyoid ridge crosses it anterior to 2 nd molar region


ANTERIOR LINGUAL ANATOMIC INFLUENCE Most difficult to obtain seal Muscle acting genioglossus - --raises lingual frenum Mylohyoid – raises floor of mouth anteriorly as well as posteriorly

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Methods to establish anterior border seal First lingual flange is extended inferiorly to contact the highest level of the floor of the mouth The flange can then be extended posteriorly to contact the sublingual folds and thereby establish a seal when the tongue is at rest and the floor of the mouth drops

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S light displacement of the mucosa anteriorly to provides a seal when the muscular floor of the mouth is at rest. This is accomplished by adding a slight additional amount of softened border- molding material to the inner surface of the previously molded anterior lingual area and reseating the custom tray


NEUROMUSCULAR FACTORS As patient experiences wearing of denture neuromuscular factor become major determinant in complete denture retention Patient learn to alter muscular function to harmonize with prosthesis Orofacial musculature supply supplementary retentive forces Example of neuromuscular control-incising of food by anterior teeth by raising tongue up at back and presses against distal border of maxillary denture

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Orofacial musculature to be effective in retention:- Denture base must be properly extended to maximum area possible without interfering health and function of structure that surrounds the denture Occlusal plane must be at correct level Arch form should be in neutral zone Polished surface of denture properly shaped

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When dentures are first fitted, muscular control takes some time to develop and is therefore likely to be inefficient. Thus, it is during this initial learning period that the physical forces of retention are particularly important. The stronger these forces are, the smaller will be the demand on the patient’s skill in controlling the dentures In difficult cases it may be helpful to advise the use of a denture fixative, as improved retention and stability will give the patient confidence during the period of adaptation (Grasso et al. 1994 ) Cineradiographic studies-loss of physical retention during functional movement, muscular control is important during this period




ADHESION Physical attraction of unlike molecules to one another –GPT Adhesion is achieved through ionic forces between charged salivary glycoprotein and surface epithelium or acrylic resin ( Bouchers 12 edtn ) Denture adhesion is increased with closeness of adaptation to oral mucosa

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A ttempts have been made to improve the wettability of denture base material by O Brien and Ryge 1965 Hargreaves and Foster 1976 Winkler et,al 1975.Not effective as after two weeks as surface energy is reduced by contamination Adhesion is chemical interaction across the interface of two solid. This concept is at molecular level .No such known mechanism has been known for direct acrylic mucosa reaction (B.W Darvell and R.K.F Clark (British dental journal 189,05 sep 2009) Adhesion in denture retention plays a part only in connection with the wetting of the capillary space, Even in it, however, there is no relationship of direct proportion. ( Blahova I, Neuman M. Physical factors in retention of complete dentures. J Prosthet Dent 1971; 25 : 230-235)


COHESION Molecular attraction by which the particles of a body are united throughout their mass.-->GPT 8 TH edition Occurs with in the layers of saliva Layer of saliva should be thin It is also not considered important factor .Cohesion is internal strength of material. Cohesive failure means separation of molecules with in body. Such strength for saliva is very high, it has never claimed that loss of retention due to cohesive failure (B.W Darvell and R.K.F Clark British dental journal 189,05 sep 2009 )


INTERFACIAL SURFACE TENSION The resistance to separation of two parallel surfaces that is imparted by film of liquid between them. ( Bouchers 12 th ed ) It is found in thin film of saliva between denture base and mucosa of basal seat therefore furnishes retentive forces by maximizing contact with both surfaces It is quite similar to action of cohesion and capillarity One of its requirements for its retentive value is minimal distortion or displacement of soft tissue by impression This acts with the air-liquid interface acting between two surfaces where a thin film of liquid holds the surfaces on the either sides

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Pa atmospheric pressure, F separating force exerted on two glass slab, Pf fluid pressure with in film

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Plays a major role in maxillary denture retention—as there is liquid air interface at denture border by a concave meniscus the pressure within the fluid is less than that of the surrounding medium The size of this pressure differential is inversely related to the diameter of the meniscus, i.e. the closer the fit of the denture to the tissues the stronger the retentive force attributable to surface tension.

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It’s like if we take two glass slab dip them in water and take out and put one glass slab over other, it will be difficult to separate them ,its due to interfacial surface tension but if we put them back in water they are easily separated as there is no liquid air interface.. This phenomenon is noticeable in mandibular denture when there is excess of saliva, there is minimum interfacial surface tension as there is no liquid air interface

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According to Stefan(1879) F interfacial surface tension, K-viscosity , R-denture area , H-width of space between denture base and mucosa, V velocity of displacing force F decreases with increase in H . F increases with increase in R To obtain maximum interfacial surface tension Saliva should be thin and even Perfect adaptation should be present between tissue and denture base Denture base should cover large area


VISCOSITY A very crucial role in denture retention As a denture is pulled away from the tissues, saliva is drawn into the space being created beneath the denture. A retentive force is generated by a resistance to this flow of saliva, resulting from the viscous properties of the saliva and the dimensions of the channel through which it flows Narrower the channel and the greater the viscosity of the saliva, the more effective should be the retention But if the saliva is very viscous, saliva result in thick and discontinuous film, Any discontinuities, such as air bubbles, in the saliva film reduce retention dramatically because air flows infinitely more readily than saliva and therefore offers very little resistance to denture displacement

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relationship between width of wall of buccal sulcus and resistance to saliva flow

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Denture displaced->pressure with in saliva film drops->mucosa is drawn tightly against denture-> channel becomes narrow->resistance to flow of saliva->increased retention Incidentally , this will also increase retention due to surface tension, because narrowing of the channel between denture and mucosa will reduce the diameter of the meniscus, and therefore increase the pressure differential between the saliva film and air . For this phenomenon denture border should not be too thin as there will be no impaction of buccal mucosa

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Fig- Drop in pressure of the saliva film beneath the denture causing impaction of the buccal mucosa and greatly increased retention*


CAPILLARITY That quality or state which, because of surface tension, causes elevation or depression of the surface of a liquid that is in contact with the solid walls of a vessel  GPT 8 th edt FACTORS Close adaptation of denture base to soft tissue Greater surface of denture bearing area Thin film of saliva must be present


ATMOSPHERIC PRESSURE AND PERIPHERAL SEAL Emergency retentive force Some called it SUCTION It operates when dislodging force applied to denture Average value 14.7lbs/in2 Cause of retention is the pressure difference under the denture and outside it. For atmospheric pressure to act there should be vacuum beneath the denture or a negative pressure but it is only possible if “true seal” is present ,which is not possible(B.W Darvell and R.K.F Clark British dental journal 189,05 sep 2009)


PERIPHERAL SEAL For atmospheric pressure to be effective denture must have perfect seal around its entire border Peripheral seal should be thought of an area of contact between surrounding mobile mucosa and denture surface D epends upon Area of contact  should be continuous all around the denture Pressure between denture and tissue Seal should be developed at buccal and lingual surfaces rather than at depth of sulcus

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Coronal section through buccal sulcus showing three different peripheral seal. High and narrow seal will encroach action of buccinators which will pull denture down on other hand wide peripheral seal will reposition buccinators in original place i.e. outward ,it will push denture inward rather than outward which favours retention

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IMPORTANCE OF FACIAL SEAL As depth of sulcus varies during function, so to prevent displacement of denture ,flanges of denture are laterally extended so that they contact and slightly displace the buccal and labial mucosa to produce facial seal, thus achieving constant seal

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Lateral extension of the buccal flange to produce a facial seal

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Right: denture poorly retained because the thin flanges failed to create a facial seal


PHYSIOLOGICAL FACTORS SALIVA When there is adequate peripheral seal, method by which saliva can influence the retention is by rate at which it can pass across periphery of denture when subjected to displacing forces Copious watery saliva excreted from buccal and palatal glands will reduce negative pressure between denture and mucosa, by its fast rate of flow and denture will displace easily Thick ropy saliva secreted by palatal glands accumulates between tissue surface of denture and palate leads to loss of retention


MECHENICAL FACTORS Undercuts, rotational insertion paths, and parallel walls Effect of gravity Retentive devices Denture adhesives


UNDERCUTS ROTATIONAL INSERTION PATHS AND PARALLEL WALLS The resiliency of the mucosa and submucosa overlying basal bone allows for the existence of modest undercuts that can enhance retention Less severe undercuts of the lateral tuberosities , maxillary premolar areas, distolingual areas, and lingual mandibular mid body areas can be extremely helpful to the retention of the prosthesis If the undercut area is seated first and the remainder of the denture base can be brought into proximity with the basal seat on rotation of the prosthesis around the undercut part that is already seated, this “ rotational path” will provide resistance to vertical displacement.

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EXAMPLE Area inferior to the retromolar pad, into which the distolingual extensions of the mandibular base can be introduced from the superior and posterior prior to rotating the anterior segment of the denture down over the alveolar process The opposite sequence is common in the maxilla, where a prominent or even undercut anterior alveolus may dictate an insertion path that begins with seating the anterior in a posterior and superior direction and ends with rotation of the posterior border over the backs of the tuberosities

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Selection of path of insertion to improve retention by utilising undercuts: (a) single path of insertion to engage labial undercut; (b) dual path of insertion to engage unilateral undercut.

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Prominent alveolar ridges with parallel buccal and lingual walls may also provide significant retention by increasing the surface area between denture and mucosa and thereby maximizing interfacial and atmospheric forces Very flat ridges display strong resistance to displacement perpendicular to the basal seat but they are very susceptible to movement parallel to the basal seat


EFFECT OF GRAVITY Weight of denture base*


RETENTIVE DEVICES Suction chamber and suction devices* Intra oral magnets Implant supported dentures


DENTURE ADHESIVE “Denture adhesive” is used to refer to a commercially available, nontoxic, soluble material (powder, cream, or liquid) that is applied to the tissue surface of the denture to enhance denture retention, stability, and performance ( Bouchers 12th ed ) MECHANISM I ncreasing the adhesive and cohesive properties and viscosity of the medium lying between the denture and its basal seat Eliminating voids between the denture base and its basal seat.

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Denture adhesive


PSYCHOLOGICAL FACTORS Intelligence Expectations Apprehension Gagging reflex Previous denture experience


SUMMARY Thus retention is an important objective of impression making but if all other objectives are achieved it will be fairly present. Above all importance of technique of impression making and knowledge of anatomy of oral structures is the key in success of making proper retentive prosthesis



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