Retention, Stability and Supportin Complete Denture

• retention is the resistance to removal in a

direction opposite that of insertion.

• Stability is the quality of the denture to be

firm, steady and constant to displacement by

functional, horizontal or rotational stresses.

• Support is the resistance to vertical movement

of the denture base, towards the ridges.

Factors affecting retention of Dentures:

• A-Interfacial Surface Tension

It is the resistance to separation developed between two well

adapted surfaces by a thin film of fluid.

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 and spreading out in a

thin film.

• B-Adhesion

• It is the physical force of attraction between unlike molecules between the surfaces in close contact.

• Adhesion of saliva to the mucous membrane & the denture base achieved through ionic forces between charged salivary glycoproteins & surface epithelium or acrylic resin.

• Xerostomia-ethanol free rinse containing aloe or lanolin or a water soluble lubricating jelly.

• oral pilocarpine 3times daily 5to 10 mg

• It acts when saliva sticks to the denture base and to the mucous membrane of the basal seat. It depends on:

• Close adaptation of the denture base

• Size of denture bearing area

• Type of saliva

• C-Cohesion

• It is the physical attraction

of like molecules for each other.

Cohesion is the physical factor

of electromagnetic force acting between molecules of the same material.

D -Atmospheric Pressure

It is the physical factor of hydrostatic pressure due to weight of the atmosphere on the earth’s surface. When a dislodging force is applied on the denture having good border seal, a negative pressure develops in the space created between

the denture base and the mucous membrane.

Factors affecting retention of denture depend on:

-Shape of the palate

Flat palate increases retention and decrease stability while V- shaped palate decreases retention and increase stability.

Surface area

The force of adhesion is proportional to the area of the surfaces in contact. Denture bearing of maxilla is 22.96cm²and mandible is 12.255cm².

- Closeness of the denture adaptation

Adhesion depends on the closeness of the denture to the tissues. Fit of the denture depends on the accuracy for denture construction.

-Direction of displacing forces

Adhesion acts almost powerfully at right angles to the surface.

Factors relating to denture periphery:

• Atmospheric pressure

When an upper denture is inserted air between it and the mucous membrane is expelled and when the fit of the periphery to the tissues is good no air can get in because there is an efficient seal. This means that the pressure acting on the fitting surface of the denture is less than that acting on the non-fitting surface (atmospheric pressure).

Intra oral factors and biomechanical considerations:

• Undercut

• Jaw size

• Disharmony in jaw sizes

• Arch shape

Square, ovoid, or tapered

• Ridge form

high ridge with flat crest and parallel sides

• The vault form

A flat vault resists vertical displacement but it provides little resistance to lateral displacement. The narrow V- shaped vault is also unfavorable for the retention of dentures.

• The soft palate

The width of the post dam depends upon the

curvature and functional activity of the soft palate.

Aids of retention

• Certain aids have been used to improve retention of complete dentures:

• Vestibuloplasty.

• Ridge Augmentation Procedure

• Implant

• Over denture

• denture adhesives

denture adhesives

• nontoxic, soluble material available either in form as powder or cream that is applied to the tissue surface of the denture to enhance denture retention, stability, & performance

It augment the retention through

1-increasing the adhesive & cohesive properties & viscosity of the medium lying between the denture & the basal seat

2-eliminating the voids between the denture base & the basal seat.

3-because adhesives are more cohesive than saliva, intrinsic physical forces of adhesive resist the pull more than would similar forces within saliva.

Indications:

1-improve the stability of the denture for patient with new denture.

2-pateint with xerostomia

3-provide psychological sense

4-pateints with resection surgery.

5-pateint with poor muscle tone such as Parkinson's disease

6-patient with neurological disorder i.e. cerbrovascular accident.

Contraindications:

1-they are not indicated for retention of improperly fabricated or poorly fitting prosthesis.

2-should not be used as a substitute to a relining materials or tissue conditioner.

Stability

• Denture stability depends on:-

1-the size & form of the basal ridge (loss of

vertical high of the ridge & increase flabby

movable tissue decrease stability). Maximum

coverage of denture bearing area within the

physiological limit

2-the quality of final impression.

3-the form of polished surface.

• 4-the proper location & arrangement of the artificial

teeth. should be located at the neutral zone so the teeth

are subjected to equal and opposite forces from

surrounding muscles. also correct occlusal plane should

be establishing of if too high occlusal plane will force

the tongue into new position i.e. higher position leads

to loss of tongue accuracy causes raise of floor of

mouth & pressure on the border of the lingual flange

cause partial loss of border seal. If the occlusal planetoo low causes destabilizing the denture.

• 5-pateint education regarding tongue position & diethabit

•

support

• Effective support is realized when:

1. Denture is extended to cover maximum surface area

Snow Shoe principle: Increased surface area decreases the stress/unit area leading to decreased tissue displacement which in turn causes denture movement.

2. Those tissues most capable of resisting resorption are selectively loaded during function.

3. Compensation is made for varying tissue resiliency to provide for uniform denture base under function and maintain harmonious occlusal relationship.