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Retention (Clasps (Requirements of Clasp Assembly (1) Retention resists…

- - - - Advantages
        
        Provides retention, support and bracing
        
        Does not impinge on vestibular tissue
        
        More rigid
        
        Can be used in cases where undercits are more severe
        
        Good retention
        
        Disadvantages
        
        More tooth coverage than roaches
        
        Poor aesthetics in the anterior region
        
        Adjustments are difficult
        
        May need to trim more of coronal structure if occlusion is tight (less space for C clasp to go through)
        
        Create 'pump-handle' action on abutment teeth in distal extension
    - - Advantages
        
        More efficient retention due to trip action
        
        Less modification of crown required
        
        Less tooth contact (only retentive part touches tooth)
        
        Cleaner (covers less tooth surface)
        
        Less prone to caries
        
        Aesthetically superior
        
        Greater adjustability
      - Disadvantages
        
        More bothersome to vestibular tissues
        
        Too flexible for effective bracing
        
        Unaesthetic in patients with high lip line
        
        Not suitable where abutment teeth demonstrate sever buccal or lingual tilting
        
        Guide planes are advocated to enhance the effectiveness of the clasp
        
        Tendancy to collect and hold food debris
      - Contraindications
        
        Deep cervical tooth undercuts --> food traps
        
        Severe soft tissue or bony undercut
        
        Insufficient vestibular depth for approach arm (need at least 4mm:3mm from ginvival margin, 1mm for thickness of approach arm)
        
        Pronounced frenal attachments in the area -- impiingement
        
        Excessive buccal/ lingual tilt of the tooth
        
        Very high survery line on abutment tooth
  - - - Flexibility inversely proportional to bulk
      - Co-Cr higher modulus of elasticity (more rigid) and lower proportional limit (liability to deform) compared to gold -- retentive clasp arm must be thinner and engage less undercut
      - Wrought wire clasps have greater tensile strength than cast clasps - can be used in smaller diameters to provide flexibility without fracture or fatigue
      - Amount of undercut required: 0.5mm for wrought and 0.25mm for cast
  - - - P: proximal plate (on guide plane on distal of tooth)
      - I: I bar (retentive arm)
      - Usually used for free-end saddles (KCl I and II)
      - R: rest (always mesial)
    - - Rest: on mesio-occlusal surface; minor connector on mesio-lingual embrasure
      - Proximal plate: on GP on distal surface of tooth; extends lingually so that distance between minor connector and proximal plate < mesio-distal width of tooth (proximal plate and rest act as reciprocators for I bar)
      - I bar (cast): on buccal surface; bend in bar should be 3mm from gingival margin-- prevent food trap and length required for flexibility
    - - Rest: on mesiolingual surface
      - I bar: located on mesio-buccal surface, so that it's directly reciprocated by proximal plate
    - - Insufficient depth of vestibule (inferior border of I bar must be at least 4mm from gingival margin)
      - No labial/ buccal undercut
      - Severe soft tissue undercut
      - Only disto-buccal undercut available
- - - - Surface forces associated with adhesion of saliva to denture and mucosa
        
        Adhesion depends on
        
        Close fit of denture
        
        Surface area covered
        
        Viscocity of saliva
        
        Adhesion is inversely proportional to
        
        Saliva film thickness (distance between tissue and denture)
        
        Gravity-- retains /L and displaces /U
        
        Can be maximised by closeness of fit and increased area of contact
      - Fluid forces e.g. surface tension of saliva, cohesive forces within saliva film, viscocity of saliva
      - Atmospheric pressure dependant on border seal (for more complete dentures)
  - - - Held in place by tongue pressing against palate
      - Adhesion between denture/ saliva and salva/ mucosa
  - - - Guide planes
      - Extensions into undercuts
      - Prefabricated attachments
      - Implants
      - Clasps