Prosthodontics: Retention, Support, Stability

Retention

The quality of a denture that resists vertical tissue away movement.
Resistance of the denture to dislodgment.

Support

The quality of the prosthesis to resist vertical tissue-ward force.
Resistance to tissue toward movement.
Transferring and distributing occlusal stresses to the supporting oral structures.

Stability

The quality of the prosthesis to be firm, steady, or constant, to resist displacement by functional horizontal or rotational forces (Tipping - Rocking).
Bracing: Resistance to lateral movement of the denture.

Physical Factors Affecting Retention

Adhesion: Physical attraction between unlike molecules in the presence of water creates a retentive force (mucosa and acrylic denture base in the presence of saliva).
- Retention supplied by adhesion depends on:
  - Area covered by the denture.
  - Close adaptation of the denture to the supporting tissues.
  - Fluidity of saliva
Cohesion: Physical attraction between similar molecules creates a retentive force (The force where molecules of the matter adhere to one another).
- Usually occurs within saliva that is present between the denture base and mucosa.
Interfacial Surface Tension: A combination of adhesion and cohesion.
- Attributed to the attractive forces of the surface molecules of the liquid.
Atmospheric Pressure: Hydrostatic pressure (force/unit area) due to the weight of the atmosphere on the surface.
- When an upper denture is inserted, air is expelled from between it and mucous membrane. In the presence of a perfect seal around its entire border, no air can get in.
- The pressure acting on the fitting surface of the denture is less than that acting on the non-fitting surface.
- The difference between these two pressures gives a positive force holding the denture in place.
Capillary Attraction: Capillarity is what causes a liquid to rise in a capillary tube.
- With intimate contact between the mucosa and denture base, they act as a capillary tube in which the saliva is present, seeking to increase the contact between the denture and tissues.
Gravity: Acts as a retentive force for the mandibular denture and a displacing force for the maxillary one.
- It's insignificant in comparison with other forces acting on CD.
Viscosity: Resistance by one part of a liquid in moving over another part.
- Plays an important role in the sealing of a complete denture.
- Saliva should be of medium viscosity.
  - Thin watery saliva does not seal the denture well and the seal can be easily broken.
  - Thick ropy saliva is likely to cause gagging, but it is better than thin (excellent for sealing).

To Achieve Maximum Benefits from Physical Factors

Good basal adaptation to the underlying tissues.
Wide area of coverage.
Perfect border seal.
Fluidity of saliva.
Direction of displacing forces: Adhesion acts most powerfully at right angles to the surface.
Impression technique, subsequent denture base design and fabrication.
Maxillary versus Mandibular denture

Mechanical Factors Affecting Retention

Undercuts: Means mechanical locking.
- Common sites for undercuts:
  - Maxilla: Distobuccal vestibule and tuberosity.
  - Mandible: Distolingual vestibule.
- If it is bilateral, it's better to insert the denture into the undercut area first, and then rotate the other side into place on the opposite side.
Occlusion: Occlusion in a complete denture to enhance retention and stability.
- Examples:
  - Balanced occlusion.
  - Monoplane occlusion.
Leverage: The height of the occlusal plane should be as near the base as possible.
Contour of Denture Base: Accurate fitting surface achieved by a good impression.
- Highly polished and contoured polished surfaces.
Physiological Factors Affecting Retention
Quality and Quantity of Saliva: Absence of saliva (xerostomia), thick ropy or excessive saliva decrease the retention of a complete denture.
Arch Relationship: Ideal jaw relation provides better retention.
- Severe retrognathic or prognathous ridge relationship can compromise the prosthetic treatment.
Neuromuscular Control: Functional forces exerted by the musculature of the patient that can affect retention (masseter muscle).
- Dentures, when placed for the first time, most muscular actions tend to expel them; they are considered a foreign body.
- Positive muscular control of the denture requires that its design must follow certain criteria:
  - Teeth should be in the neutral zone.
  - Position of the occlusal plane in relation to the tongue.
  - Shape of the polished surface.
Buccinator Muscle in Relation to a Complete Denture:
- The buccinator muscle can affect the stability of the denture, particularly the maxillary denture by pulling it laterally.
- To minimize this, the denture should be well-adapted to the mucosa and have a good border seal.
  - Also, the shape of the denture base should be designed to resist buccinator muscle action.
- A well-designed denture will reduce the effects of the buccinator muscle on the denture.
Tongue: The tongue can influence the stability of the denture, especially the mandibular denture.
- A denture must be designed to allow for normal tongue movements without displacement.
- The shape of the denture base and the placement of the teeth can influence the tongue's movements.
- A well-designed denture will accommodate the tongue's movements while maintaining stability.