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PRINCIPLES OF RETENTION, SUPPORT AND STABILITY IN COMPLETE DENTURES.
MWANGI S.K
NGURUNGU W.N
02/09/2014
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“Hii meno yako daktari inatoka nikikula na nikiongea”
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RETENTION
Definition
Types
Factors affecting retention
Primary, secondary and mechanical retention
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Retention in complete dentures refer to the ability of that denture to resist displacement in a direction opposite the path of insertion
Vertical forces are involved in retention e.g. sticky food gravity chewing action and oral musculature tongue cheeks etc.
Retention in complete dentures can be primary secondary.
Primary retention involve both physical and mechanical means.
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Primary retention
Physical forces
Cohesion
Adhesion
Interfacial surface tension
Capillarity
Atmospheric pressure
Gravity
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Adhesion
Force of attraction between two unlike molecules
Denture-saliva-mucosa
Cohesion Force of attraction between two molecules of the same type
Molecules of saliva; force between them maintain its continuity enhancing retention.
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Interfacial surface tension Force between two parallel surfaces separated by a thin film of liquid.
Saliva act as a separating media between denture flanges and the mucosa. A concave meniscus is formed between saliva and air reducing their pressure on that side. When denture sits on that side an intimate contact is formed. This contribute to the retention of the denture
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Capillarity action When the space between the denture and mucosa is narrow enough it
act as a tube that sucks saliva and distributes it throughout the denture bearing area increasing retention.
Gravity Beneficial for mandibular denture but may be detrimental for the upper
denture
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Atmospheric pressure Suction effect. When a force is exerted perpendicular and away from
fully seated denture the p between the denture and the mucosa falls below the ambient pressure resisting displacement. Proportional to the size of the denture bearing area
Good fit ,bolder molding and good impression must be ensured.
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Muscular forces harmony must be created between the denture and the oral musculature. Tongue frenal relieve mylohyoid, buccinators masticatory muscles play a role. Denture must be well polished. Depend on the ability of the patient to adapt to the new prosthesis.
Secondary retention Indirect retention obtained when stability and support of the denture is
optimum i.e. when other forces which tend to unseat the denture are eliminated.
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Factors affecting physical forces
Saliva
Quantity and quality:xerostomia? excessive salivation? type of saliva?
Surface area
The greater the surface area the better the retention maxillary denture mandibular denture
Intimacy of contact the closer the denture and the denture bearing area are the better the retention
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Peripheral seal
Good peripheral seal improves retention; good bolder molding posterior seal and peripheral seal prevent air entry into the denture and denture bearing area.
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MECHANICAL RETENTION
Modifications made on complete dentures to enhance retention
They include:
Undercuts
Mucosal inserts
Over denture attachments
Rubber suction discs
Magnets
Implants
Denture adhesives and soft tissue liners
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STABILITY
The ability of a prosthesis to resist displacement by functional horizontal or rotational forces.
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Stability is affected by;
Residual ridge size and contour
Residual ridge quality
Palatal vault
Neutral zone and surrounding musculature
Abnormal ridge relationships
Occlusal factors
Intimate contact
Direct bone anchorage
Flange shape and contour
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SUPPORT
Resistance to vertical movement or displacement of the dentures towards the basal seat area
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Sources of support
All denture bearing areas
Maxilla – hard palate, maxillary tuberosity, residual alveolar ridge
Mandible – buccal shelf, residual alveolar ridge.
Thus impressions must cover as much area as possible, with relief of the limiting structures.
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Factors that enhance support
Basal bone resistant to resorption
Firm, resilient and keratinized tissues well attached to the underlying bone
Tissues of uniform thickness
Support should be at right angle or near right angle to the occlusal forces
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Factors affecting support
Bone support- implants/abutments vs mucosal support
Surface area
Impression technique
Accuracy of fit
Nature of the supporting mucosa
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