Lab 10: Retention (Clasps) and Support (Rests) - For removable dentures

What is a major and minor connector (for RPDs)?

Major connector

• Main body of a denture which provides rigidity and unites all the denture parts

Minor connector

• Connects all other parts of the denture to the major connector

Define: Direct retainer parts

Direct retainer

• Parts of a denture that prevents it from dropping out of the mouth. Clasps are often used as a direct retainer

Define: Reciprocation/Bracing

Reciprocation/Bracing

• Retentive factors preventing the denture moving horizontally

Define: Indirect retainer

Indirect retainer

• Part of the denture preventing it rocking or sifting sideways or rotating during eating and talking e.g. Rests, connectors and acrylic

• They prevent lever action of the fulcrum point

Define: Support (RPDs)

Support/ Rests in RPDs

• Prevents the denture from sinking towards the soft tissues

• Rest is a small projection of metal from the main frame

What is a reciprocating arm?

Reciprocating arm is something to provide opposite pressure from the clasp

Support on RPDs normally uses teeth as rests. However on edentulous you use mucosal support, what can this result in?

Mucosal support normally results faster resorption rate of the ridge

Tooth support in RPDs is often advised to share across 2 teeth or more, why?

To share the load between two teeth

What are the 4 main functions of the rests?

Rest functions

• Transmit axial loads to natgural teeth (vertical)

• Deflect food away (rounded)

• Indirect retainer and support for major connector (limits soft tissues)

• Main clasp-to-tooth relationship

What are the design criteria for a rest?

Rest criteria for design

• Rigid

• Don’t interfere with occlusion

• Rounded to permit movement in function

• Angle of less that 90o with minor connector (transmit vertical load to long axis)

What are the 3 types of rest design (placement)?

• Occlusal rest

• Cingulum rest

• Incisal rest

What are the keys to a successful Retention/Clasps selection?

Select a direct retainer that will control tipping and torquing forces to prevent dislodging

What are the materials used for clasps?

• Chrome cobalt (0.25mm)

• Gold (0.5mm)

• Stainless steel wire (0.75mm)

• Tooth colour resins

• Flexible denture base

What is the gold standard material for RPDs clasps?

RPDs gold standard is Cast chrome cobalt

• Good tensile strength, durability and accuracy

• Either Occlusally or gingivally approaching

What are the reasons you would use stainless steel wires for clasps?

Stainless steel wire is used because

• It’s the easiest and cheapest material to use

• Well tolerated

• Not aesthetically great

What are the reasons you would use Gold wires for clasps?

Gold clasps

• Well tolerated and some like the aesthetic

• Costly

What are the mm distances for flexible dentures and why there’s a safety issue?

• Flexible dentures are Nylon based (Monomer free)

• Major connector thickness: 1.5mm (allows flexibility)

• Flexible clasps are 1mm above gingival necks

• These clasps are close to the tissues and therefore a sharp safety risk

What is known about tooth shaded materials for claps?

• Aesthetics meet patients demands

• Scarce information on staining an stability

Other than clasps what are other forms of gaining retention?

• Adhesion (e.g Fixident) / Cohesion (attraction between molecules and surfaces

• Capillary attraction (space between denture and basal tissues)

• Fluid viscosity

• Post dam