Principles of Clasp Assemblies
Importance
- Essential for the design of removable partial denture (RPD)
- Key to success of the final prosthesis
- Comprised of 4 components: direct retainer, rest, reciprocal component, and minor connector
Components
Direct Retainer
- Engages the undercut below the height of contour
- Prevents displacement of the prosthesis in an occlusal direction
- Can be buccal or lingual
Rest
- Placed in a prepared rest seat on an abutment tooth
- Limits movement in the cervical direction
- Transmits functional forces to the abutment
Reciprocal Component
- Resists horizontal stresses produced by the retentive arm
- Stabilizes the tooth to prevent movement
- Can be an arm or plating
Minor Connector
- Joins the major connector with other components (rest, direct retainers, base)
- Example: guide plate
Retention
Types
- Primary Retention: Via direct retainers on abutment teeth
- Secondary Retention: Close contact with tooth guide planes, denture bases, and major connectors
Direct Retainers
- Two types: intercoronal (within the crown) and extracoronal (outside the crown)
- Precision attachments: milled with precision instruments, cast to the framework or picked up in acrylic resin
- Components: matrix (female part within the crown) and patrix (male part on the RPD)
Extra Coronal Retainers
- Mechanical resistance, placed on external surface
- Types: cast circumferential clasp, plastic patterns (semi-precision with nylon male)
Internal vs External Attachments
Internal Attachments
- Aesthetic, favorable force direction
- Complicated clinic and lab protocols
- Require cast restorations, difficult to repair/replace, costly
External Attachments
- Stiffer, durable (less wear), less torque on the tooth
- Chrome cobalt and nickel chromium stiffer but less flexible than gold or wrought wire
RPD Design Considerations
- Dentist must determine path of insertion (using a dental surveyor)
- Direct retainer must flex over the height of contour into the undercut and become passive upon seating
- Specific paths provide necessary undercuts and non-retentive areas
Clasp Types
- Cast Circumferential (Akers)
- Super bulge approach
- Divided into segments: shoulder (A), terminus (C), midsection (B)
- Vertical Projection (Roach)
- Infra bulge approach
- Divided into: approach arm (horizontal), terminus (vertical)
Factors Affecting Clasp Function
- Angle of cervical convergence: determines depth of undercut and clasp placement
- Flexibility influenced by arm length, shape, diameter, taper, material
- Cast circumferential less flexible, wrought wire more flexible
Clasp Encirclement
- Must encompass more than 180 degrees of the tooth
- Ensures tooth retention during occlusal forces, insertion, and removal
Passive Clasp Assembly
- No adverse forces when fully seated
- Activates only under dislodging forces
- Ensures tooth stability and comfort
Rest Functions
- Provides vertical support, maintains component positions, prevents soft tissue impingement, distributes occlusal forces
- Types: occlusal rests, lingual/cingulum rests, incisal rests
- Must prevent cervical movement and maintain occlusal relationships
End of Part 1. Continue with Clasp Assemblies Lecture Part 2 for continuation.