Physical Factors in Denture Fit

Physical Factors of Retention

• Interfacial surface tension is the resistance to separation possessed by the film of liquid between two well-adapted surfaces, found in the thin film of saliva between the denture base and the mucosa of the basal seat.
• A perfect fit is essential for interfacial surface tension to be effective, and it is directly proportional to the size of the basal seat of the denture.
• Capillary attraction (capillarity) is a force that causes the surface of a liquid to become elevated or depressed when it is in contact with a solid, and it helps to retain the denture when the adaptation of the denture base to the mucosa is close.
• Capillary attraction is directly proportional to the area covered by the denture.
• Atmospheric pressure acts to resist dislodging forces applied to the denture, and it requires a perfect seal around the entire border of the denture to be effective.

Retention, Support, and Stability

• Retention is the resistance to the forces of dislodgment along the path of placement.
• Support is the foundation area on which a dental prosthesis rests.
• Stability is the quality of a prosthesis to be firm, steady, or constant, to resist displacement by functional horizontal or rotational stresses.

Factors of Complete Denture Retention

• Physical factors:
  • Adhesion: the physical attraction of unlike molecules to each other, effective in direct proportion to the close adaptation of the denture base to the supporting tissues and the fluidity of saliva.
  • Cohesion: the physical attraction of like molecules for each other, effective in direct proportion to the area covered by the denture.
• Anatomical factors:
  • Mucosa: firm, compressible mucosa of even thickness is most favorable for denture retention, while thick and flabby mucosa will reduce stability.
  • Saliva: a medium that allows physical factors of retention to act, with thin, scanty saliva leading to poor retention, and thick viscous saliva increasing denture retention.
• Mechanical factors:
  • Shape of the polished denture surface: properly shaped to allow the cheeks and tongue to stabilize the denture.
  • Wide coverage: covering a wide area by the fitting surface of the denture increases the forces of adhesion and cohesion.
  • Placing the teeth in the neutral zone: allowing the oral and facial musculature to supply supplementary retentive forces.
  • Engagement of undercuts: possible to allow mechanical resistance to a direct dislodging force.
  • Denture borders: properly extended borders help peripheral seal and increase denture retention.