4.5 metal cast clasps

what is direct retainer?

• component of partial removable dental prosthesis used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment

what is clasp assembly?

• part of RPD that act as a direct retainer and/or stabiliser for the prosthesis by partially encompassing or contacting an abutment tooth

what are components of clasp assembly?

• clasp

• reciprocal clasp

• cingulum

• incisal or occlusal rest

• minor connector

what is a clasp?

• component of clasp assembly

• engages a portion of the tooth surface and either enters an undercut for retention or remains entirely above the height of contour to act as reciprocating element

what is the component of clasp assembly called that frequently enters an undercut for retention?

• retentive clasp arm

what is the component of clasp assembly called that provides reciprocation by engaging reciprocal guiding planes?

reciprocal clasp arm

(provides action of clasp during removal and insertion of partial removable dental prosthesis)

what is occlusal rest?

• rigid extension of a partial removable dental prosthesis that contacts the occlusal surface of a tooth or restoration

• occlusal surface which may have been prepared to receive it

• modifications - incisal, lingual and cingulum rest

what is a rest seat?

• prepared recess in a tooth or restoration created to receive the occlusal, incisal, cingulum or lingual rest

what is occlusal rest arm?

• minor connector used to attach an occlusal rest to a major part of a removable partial denture

what is an undercut?

portion of surface of an object that is below the hight of contour (biggest contour) in relationship to path of insertion

what is a surveyor?

• instrument used in construction of a dental prosthesis to locate and delineate the contour and relative positions of abutment teeth and associated structures

what is survey line?

• line produced on cast by surveyor marking the greatest prominence of contour in relation to the planned path or placement of a restoration (prosthetic equator)

what is path of placement?

• specific direction in which the prosthesis is placed on the abutment teeth or dental implant(s) (also called path or insertion or withdrawal)

what are the requirements of direct retainers in order to function effectively and not harm abutment teeth or tissue of denture foundation area?

• support

• retention

• cross-tooth reciprocation

• fixation

• passivity

how does a clasp assembly provide support?

• positive contact of rest (component of clasp assembly) with tooth prep

• Proper contact between clasp assembly and abutment tooth ensures retention

distinguish clasp assembly and attachment

• clasp assembly: provides retention by engaging undercuts of teeth

• attachment: connect removable prostheses to natural teeth or implants

How does attachment provide support?

• Through contact between a rigid element on denture and rigid element on abutment teeth

How does clasp assembly provide retention?

• by resistance of flexure by the retentive clasp engaging height of contour of tooth as RPD moves occlusally/incisally

• Frictional contact of rigid portions (proximal minor connectors and reciprocal component

• Tooth surfaces parallel to path of insertion can also contribute to retention

how does attachment provide retention?

• Frictional contact of long parallel walls of attachment

• Mechanical locking devices

• Resistance to deformation of resilient locking materials

• Retentive clasp arms on lingual surface of abutment can frequently provide or supplement retention for attachments

what is reciprocation?

• occurs when lateral forces from a retentive clasp are counterbalanced by a reciprocal component

how does clasp assembly provide reciprocation?

• reciprocal component contacts the tooth on opposite side of retentive clasp arm, stabilising the tooth and negating the force

how does attachment provide cross-tooth reciprocation?

• precise fit of long parallel walls or supplemental reciprocal components

what is cross-tooth recirpocation?

• In RPD, every force exerted by direct retainer on one side of arch is counterbalanced by an equal and opposite force on an abutment tooth on opposite side

• ensures balance and stability in RPD

what is fixation?

• resistance to movement of abutment tooth away from prosthesis and vice versa

how does clasp assembly provide fixation?

• encompass or encircle at least 180 degrees of abutment tooths circumference

• also called encirclement

how does attachment provide fixation?

• dovetail or circular design of component parts

• supplemental clasp arms on lingual or abutment tooth

what is Passivity in direct retainers?

• DR should not exert forces on abutment teeth when RPD is seated

• should be passive during RPD seating, forces should only occur during insertion ro removal

what is the importance of passivity?

• ensures that clasp is completely seated and retention clasp arm is not active

• Helps maintain stability and comfort of RPD

what are the types of Direct retainers?

• intracoronal retainer (in crown)

• Extracoronal retainer (outside of crown, most commonly used)

Intracoronal retainer

• cast or attached totally within restored natural contours of abutment tooth

• which is composed of prefabricated machine key and keyway with opposing vertical walls

• (which serve to limit movement and resist removal of partial denture through friction)

• Usually regarded as internal/precision attachment

• Principle of internal attachment first formulated by Dr Herman E.S Chayes in 1906

Extracoronal retainer

• uses mechanical resistance to displacement through components on external surface of abutments

• Three principle forms:

  • clasp-type retainer

  • manufactured attachment

  • dental implant

• Clasp-type retainer is most common, retains through flexible clasp arm

• Manufactured attachment uses flexible clips or rings, engaging rigid component on abutments

• Dental implants used for retention, eliminating visible clasps

what should be considered when designing extracoronal retainer?

• function and location

• retainer should provide:

  • support

  • stabilisation

  • reciprocation

  • retention

• Retainer-clasp assembly components can be physically attached or originate from major and minor connectors

what are the 4 component parts of clasp assembly?

• minor connector

• principle rest

• retentive arm

• non-retentive arm

minor connector

• one or more minor connector should be present , from which clasp components originate

principle rest

• should be designed to direct stress along long axis of tooth

retentive arm

• engage tooth undercut

• for most clasps, retentive region is only at its terminus (at the end of clasps)

non-retentive arm

• should be present on opposite side of tooth compared to retentive arm

• Provides stabilisation and reciprocation against horizontal movement of prosthesis

• Rigidity of this clasp arm is essential

what are the different types of clasp assemblies and what do they stand for?

• RPI - rest, proximal plate (minor connector), I bar (ideal class II lever system)

• RPA - rest proximal plate (minor connector), Akers clasp

• RPC - rest, proximal plate (minor connector), circumferential clasp (same as one above)

How do RPI,RPA and bar class mesial rest concept clasps accommodate movement?

• by changing fulcrum location

• Commonly used are RPI and RPA clasps

which type of clasp arms are commonly used for RPI and RPA clasps?

• T and Y

• It is not necessary to have the full area of T or Y terminal for adequate clasp retention.

• Only portion of clasp engaging the undercut area

• Large area of contact provides greater frictional resistance, does not guarantee true clasp retention

• It is not necessary to have the full area of T or Y terminal for adequate clasp retention.

• Only portion of clasp engaging the undercut area

• Large area of contact provides greater frictional resistance, does not guarantee true clasp retention

• It is not necessary to have the full area of T or Y terminal for adequate clasp retention.

• Only portion of clasp engaging the undercut area

• Large area of contact provides greater frictional resistance, does not guarantee true clasp retention

remainder of clasp arms (T and Y ) are superfluous (excessive) unless?

• it is needed to encircle abutment tooth by more than 180 degrees at its greatest circumference

types of bar clasp?

• T bar

• Modified T bar

If the bar arm is flexible (for retentive purpose), why would any portion of clasp above height of contour provide only limited stabilisation?

• because it is also part of flexible arm

• Therefore, in many instances, this suprabulge portion of T or Y clasp arm may be removed (part not engaging undercut for retention) and retentive terminal (end of T or Y bar clasps) should be designed to be biologically and mechanically sound rather than to conform to any alphabetical configuration

what is combination clasps?

• made up of two components:

  • wrought wire retentive clasp arm

  • cast reciprocal clasp arm

• It offers balance between retention and stability

• Allows for secure fit of RPD

• ideal for abutments adjacent to distal extension bases

describe retentive clasp arm of combination clasps

• tapered, round wrought-wire

• provides retention by flexing and gripping onto tooth or abutment

• Typically attached to cast framework by casting or soldering (added on, not part of one unit of claps assembly)

describe reciprocal clasp assembly of combination clasps

• made of cast metal

• serves as counterpart to retentive clasp arm

• engages with undercut area of abutment tooth

• Provides stability and support to RPDs

Type A clasp

• distobuccal undercut engaged by one-half T-type bar clasp

• Portion of clasp arm on and above height of contour offers some stabilisation against horizontal rotation of denture bases

what type of clasp are bar clasps?

• infrabulge

• approaches undercut from below the survey line

• examples - T type clasp, I type clasp

Type B clasps

• I bar placed in undercut (infrabulge) of buccal surface

• Retainer contacts tooth only at its tips

• Guiding plane on distal aspect of abutment is contacted by the metal or denture framework

• Mesial rest is used

Guiding planes are parallel surfaces on abutment teeth that help direct the placement and removal of a removable partial denture

Type C clasps

• Interproximal ring clasp engaging the distobuccal undercuts

• Bar-type retainer cannot be used cause of tissue undercuts inferior to buccal surface of abutment

• Interproximal refers to the area between adjacent teeth

Type D clasp

• Round, uniformly tapered 18-gauge wrought wire circumferential retainer arm engaging the mesiobuccal under cut

• Wrought wire used instead of cast arm because of ability of wrought wire to flex omnidirectionally

• omnidirectionally refers to movement occurring in all directions simultaneously

Type E clasp

• Hair pin (back action clasp) and interproximal ring clasp (embrasure clasp) are least desirable of the alphabet classifications of clasp types

• Hair pin is useful when undercut is located below point where clasp arm starts

• Both hairpin and interproximal ring clasps can engage distobuccal undercuts of terminal abutment teeth (posterior abutment teeth) in distal extension cases

• If no significant tissue undercut near gumline, use bar-type clasp instead of engaging distobuccal undercut

• Basically: Use a hairpin or ring clasp when necessary, but opt for a bar clasp if there's no large tissue obstruction

Type F clasps

• Lingual view shows use of double occlusal rest (connected to lingual bar by minor connector)

• Design eliminates need for lingual clasp arm

• Places fulcrum line anteriorly to make better use of residual ridge for support and provide stabilisation against horizontal rotation of denture base

what are circumferential clasps?

• clasps designed without movement accommodation

describe circumferential clasp

• often used with tooth-supported partial dentures

• provides retention and stabilisation

• considered a logical choice due to effectiveness

what does the basic form of circumferential clasp consist of?

• consists of buccal and lingual arm originating from common body

• most logical clasp choice for tooth-supported partial denture due to its retentive and stabilising ability

Improper use of circumferential clasp

• when two retentive clasp arms originate from body and occlusal rest areas, approaching bilateral retentive areas on opposite side of tooth from point of origin

what is the correct form of circumferential clasp?

• one retentive clasp arm, opposed by non-retentive reciprocal arm on opposite side

how are disadvantages of circumferential clasps minimised?

• proper mouth preparation

what does mouth preparation entail for circumferential clasps?

• adequate prep allows for proper placement of clasp’s point of origin below the occlusal surface

• Helps avoid poor aesthetics and increased tooth dimensions

what are the types of ring clasps?

• ring clasp

• embrasure clasp

• embrasure and hairpin clasp (interproximal and hair pin clasp)

Ring clasp

• encircles nearly all of tooth from its point of origin

• Used when a proximal undercut cannot be approached by other means

• proximal undercut refers to a retentive area located on the proximal surface of a tooth—meaning the surface that faces an adjacent tooth. They provide mechanical retention for clasps and other retentive components.

Embrasure clasp

• In Class II and III partial denture, no edentulous space available on opposite side to help in clasping

• This is a disadvantage mechanically

• Embrasure clasp can be used if:

  • teeth and sound

  • retentive areas are available

  • multiple restorations are justified

Embrasure and Hairpin clasp

• Sufficient space between abutment teeth in occlusal third for common body of embrasure clasp

• Historically, embrasure class shows high levels of fractures due to inadequate tooth prep in contact area

• Embrasure should always be used with double occlusal rest, to avoid interproximal wedging by prosthesis (can cause separation of abutments leading to food entrapment and clasp displacement)

• Occlusal rests shunt food away from contact areas, so should always be used when food impaction is possible

• Embrasure clasps have tow retentive clasp arms and two reciprocal clasp arms that are bilaterally and diagonally opposed

what are the types of clasp modifications?

• back action clasp

• multiple clasp

• half-and-half clasp

• reverse-action clasp

back-action clasp

• modification of ring clasp

• same disadvantages and no apparent advantages

• Use is difficult to justify

• - limited flexibility, potential plaque accumulation, stress on abutment tooth

• + good retention, stable support, ideal for engaging undercuts on same side of tooth

what should an occlusal rest ALWAYS be attached to?

• some minor connect and should never be supported by clasp arm alone

• If occlusal rest is part of flexible assembly, it cannot function adequately as an occlusal rest

Multiple clasp

• consists of two opposing circumferential clasps joined at terminal end of two reciprocal arms

• Used when additional retention and stabilisation needed

• May be used for multiple clasping in instances in which partial denture replaces an entire half of dental arch

• May be used rather than embrasure clasp when the only available retentive areas re adjacent to each other

• - two embrasure approaches are necessary rather than a single common embrasure for both clasps

Half-and-half clasp

• consists of circumferential retentive arm arising from one direction and reciprocal arm arising from another

• Second arm must arise from minor connector, this arm used with or without auxiliary occlusal rest

• Reciprocation arising from second minor connector usually accomplished with short bar or auxiliary occlusal rest , thereby avoiding so much tooth coverage

• Little use of half and half clasp in bilateral extension ad it was originally designed to be used for unilateral partial denture design

reverse-action clasp

• also called hairpin clasp arm

• designed to allow proximal undercut to be engaged from an occlusal approach

what is the principle pf design of direct retainers by Ney system?

what are general characterisations of Ney clasp system?

• all have occlusal arms, transferring occlusal forces to so called “physiological way”

• support and reciprocal arms are rigid, nonelastic and situated in suprabulge

• retention arms are elastic situated in infrabulge

• clasp assembly is monolithically connected to denture framework, they are planned, modeled and casted simulatensouly

• All clasps united together, through metal framework and acts and one complex, concerning both: denture, dentition and soft tissues

what is the purpose of surveying?

• identify most favorable tilt

• retentive undercuts

• interferences

• path of insertion

• recoridng position of cast

• measuring retention

• recoridng prosthetic equator

what is resistance arm?

• length of lever from fulcrum to resistance

what is effort arm?

• portion of lever from fulcrum to point of application of force

• longer than resistance arm = advantage