ملخص المحاضرة الثانية
Retention,stability and support
🟢Definition
🔸Retention is defined as the quality of a denture that resists movement away from the basal tissue.
🟢Surfaces of a Complete Denture
🔸A complete denture consists of three primary surfaces:
•Occlusal Surface: The portion that makes contact or near-contact with the opposing denture or dentition.
•Polished Surface: The polished part of the denture base (including labial, buccal, and lingual tooth surfaces) in contact with the lips, cheeks, and tongue. Proper contouring harmonizes with muscle function (e.g., buccinator action) to keep the denture seated.
•Impression Surface: The portion shaped directly by the impression, including the borders. Lingual flanges are sloped toward the center of the mouth to allow the tongue to fit against them and maintain the border seal.
🟢Factors Affecting Retention
🔴Anatomical Factors
🔸Ridge Form:
•High & Flat Crest: Good for recent extractions, though tooth setting space may be limited.
•Flat Ridge: Poor retention/stability; requires impression extension beyond the mylohyoid area.
•Undercuts: Common in upper ridges; handled via surgery or path-of-insertion changes.
•Knife-edge Ridge: Causes pain/laceration; requires relief.
•Flabby Ridge: Movable tissue causing poor seal; managed by modified impression techniques or surgery.
🔸Vault Form:
•U-shaped provides optimal retention and stability
•V-shaped offers retention without stability
•flat vaults lack sufficient depth for either
🔸Arch Form: Square arches are best because they offer four contact points and resist lateral forces better than ovoid or tapered forms.
🔸Arch Relationship: Class III is common due to resorption patterns; Class II is unfavorable due to smaller surface area and contact difficulty.
🔸Interarch Distance & Mucosa: Smaller interarch space yields better retention. Ideal mucosa is firm, compressible, and of even thickness.
🔸Tongue: An abnormally large tongue can dislodge dentures.
🔴Physical Factors
🔸Adhesion: Attraction between unlike molecules (saliva to mucosa and denture base). Higher with thin serous saliva and larger coverage area
🔸Cohesion: Attraction between like molecules (saliva to saliva). Enhanced by thin salivary films and close denture adaptation.
🔸Interfacial Surface Tension & Capillary Attraction: Enhanced by thin, even saliva, max surface coverage, and precise base adaptation.
🔸Atmospheric Pressure & Seals: Distracting forces create negative pressure underneath if a good peripheral seal is present.
🔸Posterior Palatal Seal (PPS): Maintains contact during function, prevents gagging, prevents food accumulation, and compensates for polymerization shrinkage. Classified via House classification (Class I, II, and III) based on vault depth.
🔸Other Factors: Gravity aids mandibular dentures but displaces maxillary ones. High viscosity or poor wettability (high contact angle) reduces retention.
🔴Mechanical Factors
🔸Undercuts: Unilateral undercuts can aid retention, while bilateral undercuts cause insertion interference.
🔸Magnets: Intramucosal or attachment magnets assist retention in severely resorbed ridges or overdentures.
🔸Denture Adhesives:
•Indications: Well-made dentures needing extra security, xerostomia, neurological conditions, or post-oncology surgical cases.
🔸Contraindications: Ill-fitting or worn-out dentures, substitute for relining, or temporary/immediate dentures.
🔸Side Effects: High zinc levels, paresthesia/nerve damage, anemia, and bone marrow issues.
🔸Vacuum Devices (Suction Chambers/Discs): Obsolete due to their tendency to cause palatal tissue hyperplasia.
🔴Muscular Factors
🔸Relies on setting teeth within the neutral zone (equally balanced between cheek/lip and tongue forces).
🔸 Requires correct occlusal plane height, accurate border extension, and contoured polished surfaces.
🔸Primary muscles involved include the buccinators, orbicularis oris, and tongue musculature.
🔴Surgical Factors
Surgical procedures
-vestibuloplasty
-tuberoplasty
-ridge augmentation
