DIRECT RETAINERS

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DIRECT RETAINERS

• Dr. Waseem Bahjat MushtahaSpecialized in
  prosthodontics

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• The removable partial denture must have
  sufficient retention to resist reasonable
  dislodging forces.
• Means of retention
• A-Mechanical means
• 1-Direct retainers.
• 2-The intimate relationship of partial denture
  components with the abutments.
• 3- The denture base engages soft tissue
  undercuts.
• 4-Indirect retainers.
• B-Physiological means
• The same physical means of complete denture
  retention act on the removable partial denture
  bases and maxillary major connectors.

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DIRECT RETAINERS

• Definition
• The component that engages an abutment tooth and
  in so doing resists dislodging forces applied to
  a removable partial denture is called direct
  retainer.

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• Types of direct retainers
• 1-Intracoronal retainers (internal or precision
  attachment)
• It consists of two units ,the first one is a
  female receptacle in a crown or inlay in the
  abutment. The second part is a male insert
  attached to the partial denture. The retention is
  derived by the frictional fit and wedging action
  between the male and female components when a
  dislodging force alters their parallelism.
• Advantages
• 1-Elimination of visible retentive and support
  components.
• 2-Better vertical support than extracoronal
  retainers. since its location directs the force
  along the long axis of the tooth. 3-Horizontal
  stabilization to the dentures is provided.

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• Disadvantages
• 1-It require complicated clinical and laboratory
  procedures.
• 2-Require prepared abutment and casting.
• 3-Frictional retention is progressively lost
  because of its wear.
• 4-It is difficult to repair and replace.
• 5-It is not used in short crowns and large pulp.
• 6-Expensive.
• 7-lt requires stress breaker in distal extension
  cases

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• II-Extracoronal retainers (clasps)
• The clasp is a retainer in which a flexible arm
  engages an external surface of an abutment tooth
  in an undercut area cervical to the greatest
  convexity of the tooth (height of contour)
• Requirements of clasps
• All clasps must be designed so that they satisfy
  the following six basic requirements
  I-RETENTION
• The retentive clasp arm provides resistance to
  vertical dislodging forces (retention).

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• Factors determining the amount of clasp
  retention
• A-Amount of undercuts
• the greater the amount of tooth undercut engaged
  by the clasp, the greater the retention.
• B- The greater the vertical distance
• the retentive clasp terminal is placed in an
  undercut, the greater will be the retention.
• C-The flexibility of the clasp arm
• as the flexibility of the arm is increased, the
  retention decreases. The arm flexibility depends
  on
• 1-The diameter of the arm the smaller the
  diameter of the arm, the greater the flexibility.
• 2- The length of the arm the longer the arm, the
  greater the flexibility.
• 3- The taper of the arm the greater the taper of
  the arm, the greater the flexibility.
• 4-The cross-section of the arm the half round
  clasp arm is less flexible than rounded clasp
  arm, which provides flexibility in all
  directions.
• i-The material of the arm the chromium cobalt
  alloy is less flexible than cast gold alloy, and
  the wrought wire is more flexible than cast
  alloys.

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• II-BRACING (STABILITY)
• Rigid portions of clasps (the proximal two thirds
  of the retentive clasp arm and the entire length
  of the bracing arm) provide resistance to
  horizontal, lateral or torsional components of
  force (stability). III- SUPPORT
• The rest provides resistance to vertical seating
  forces (support).
• IV- RECIPROCATION
• The bracing arm provides resistance to horizontal
  forces exerted on a tooth by the retentive clasp
  terminal during seating and unseating of the
  prosthesis (reciprocation).

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• V- ENCIRCLEMENT
• Adequate encirclement prevents horizontal tooth
  movement within the confines of the clasp. This
  is achieved by engagement of a greater than 180
  degrees (more than half of the tooth
  circumference), or a minimum of three widely
  separated points of contact .
• VI- PASSIVITY
• There should be no active force on the tooth when
  the clasp is in place. Its retentive function
  should be activated only when a dislodging force
  is applied.

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TYPES CLASPS

• I-Occlusally approaching (circumferential or
  subrabulge) clasps
• The retentive clasp arm originates from a minor
  connector or proximal plate, usually near the
  occlusal surface and approaches the undercuts
  from an occlusal direction.
• II-Gingivallv approaching (bar, roach or
  infrabulge clasp The retentive arm originates
  from a major connector or denture base, passing
  adjacent to the soft tissues and approaching the
  tooth from a gingival direction.
• III- Combination
• Cast clasp arm and wrought wire clasp arm.

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I-OCCLUSALLY APPROACHING (CIRCUMFERENTIAL) CLASPS

• Design
• 1-The retentive arm flexible terminal engages a
  measured undercut in the gingival one third of an
  abutment crown.
• 2- The retentive clasp arm terminus should not
  contact the free gingival margin and should end
  at the mesial or distal line angle pointing
  towards the occlusal surface.
• 3-The stabilization (reciprocation) arm and the
  proximal two thirds of the retentive arm should
  be placed no higher than the middle third of the
  tooth, preferably at the junction of the gingival
  and middle third.
• 4- The bracing clasp arm should be slightly
  thicker than the retentive clasp arm to promote
  rigidity.

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• Advantages
• l-Provides excellent support, bracing and
  retention.
• 2- It is the easiest clasp to construct and
  repair.
• 3-Causes less food impaction than the bar clasps.
• Disadvantages
• 1 -Covers more tooth surface than the bar clasp,
  with more caries susceptibility.
• 2-The occlusal approach tends to increase the
  occlusal table.
• 3-lt is not esthetically acceptable in anterior
  region of the mouth
• 4-lt can be adjusted in a buccolingual but not
  in occIusogingival direction.

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TYPES OF OCCLUALLY APPROACING CLASPS

• l-Circlet (Aker's) clasp
• Indication
• It is used for the tooth- supported partial
  dentures.
• Design
• I-It consists of
• a- A minor connector.
• b- Occlusal rest.
• C-Cast circumferential retentive clasp arm.
• d-Cast circumferential bracing clasp arm.
• 2-Engages 0.0 I inch undercut.
• 3-Provides bilateral bracing.

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• 2-C-clasp (hairpin clasp)
• Indication
• When a distofacial undercut exists. It is rarelv
  indicated, since its retentive arm covers a large
  amount of tooth structure.
• Design
• 1-It consists of
• a-Rest.
• b-Minor connector.

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• c-Cast circumferential C- retentive clasp arm it
  crosses the facial surface of the tooth from its
  origin then turns back to engage proximal
  undercut below its point of origin. d-Cast
  circumferential bracing clasp arm.
• 2-lt engage 0.01 inch undercut.
• 3-Provides bilateral bracing.
• Disadvantages
• 1-It covers considerable tooth surface.
• 2-Display too much metal
• 3-Its flexibility is limited.

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• 3-Embrasure clasp
• Indications
• For unmodified class II and class III. in the
  side were no tooth missing.
• Design
• 1-It is essentially two circlet clasps
  originating from common
• minor connector, therefore it is composed of
• a-Two occlusal rests Rests are used to provide
  support and avoid interproximal wedging and food
  impaction. Enough preparation for the occlusal
  rests and extension of the preparation to the
  marginal ridge buccolingual to accommodate the
  clasp arms is necessary. Teeth interproximal
  contact should be maintained during this
  preparation.
• b-One minor connector.
• c- Two cast circumferential retentive clasp arms.
• d- Two circumferential bracing clasp arms.
• 2- Provides bilateral bracing.
• 3-Utilizes 0.01 inch undercut.

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• 4-Multiple Aker's clasp
• Indications
• 1-In tooth borne RPD when additional retention
  and bracing are needed.
• 2-May be used instead of embrasure clasp when the
  undercut areas are adjacent to each other.
• Design
• It is two opposing circlet clasps joined at the
  terminal end of the reciprocal arms

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• 5-RPA-Clasp
• Indication
• Used in tooth mucosa borne partial dentures where
  an RPIclasp cannot be used because of bar clasp
  arm contraindications

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• Design
• I-It consists of
• a-Mesial occlusal rest.
• b- Minor connector, placed into the mesiolingual
  embrasure, but not contacting the adjacent tooth.
• c- Proximal plate A distal guiding plane
  extending from the marginal ridge to the junction
  of the middle and gingival third of the abutment
  is prepared to receive proximal plate. The
  proximal plate contacts only 1 mm of the gingival
  portion of the guiding plane.
• d- Cast circumferential retentive clasp arm
  arises from the proximal plate and engages
  mesiobuccal undercut. The shoulder of the
  retentive arm contacts the tooth at the height of
  contour at the junction of the middle and
  gingival third of the tooth.
• 2- Upon function the proximal plate and the
  retentive tip move mesiogingivally into a deeper
  undercut to decrease stresses falling upon the
  abutment.
• 2-It engage 0.01 inch undercut.
• 3-Provides bilateral bracing.

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• 6-Ring clasp
• Indication
• In tilted single standing posterior molar.

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• Design
• 1-Its component parts are
• a- Rest (s) The main occlusal rest is placed on
  the mesial marginal ridge and an auxiliary rest
  may be placed on the opposite marginal ridge to
  prevent further movement of the mesially inclined
  tooth.
• b Minor connector.
• c-A single cast circumferential retentive clasp
  arm which encircles nearly the entire
  circumference of the tooth.
• On mandibular molar the clasp arm begins on the
  mesiobuccal surface and terminates in an undercut
  area on the mesiolingual surface.
• On the maxillary molar the direction of clasp is
  reverse it begins at the mesiolingual surface
  and terminates on the mesiobuccal surface.
• d- Reinforcing strut arm for retentive clasp arm
  provides rigidity to the clasp arm on the non
  retentive side. This rigidity is necessary for
  reciprocation and stabilization.
• 2-Engages 0.02-0.03 inch undercut.
• 3-Provides unilateral bracing.
• 4-It covers too much tooth surface.

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II- THE GINGIVALLY APPROACHING CLASPS

• Design
• 1-The approach arm of the bar clasp must never
  impinge on soft tissue.
• 2- The tissue surface of the approach arm should
  be smooth and polished with no relief provided.
• 3- The approach arm should taper uniformly.
• 4- The vertical portion of the approach arm
  should cross the gingival margin at 90 degree
  angle.
• 5- The distance between the vertical projection
  arm and other vertical component should be at
  least 5 mm.
• Advantages
• 1-Greater retention than occlusally approaching
  clasps due to the trip action.
• 2-It is more esthetic than the occlusally
  approaching clasps.
• 3-The flexibility of the bar clasp arm can be
  controlled by its taper and length.
• 4-Covers less tooth structure than the occlusally
  approaching clasp (less caries susceptibility).
• Disadvantages
• 1-Greater tendency to collect and hold food
  debris (more gingival irritation) than the
  occlusally approaching clasp.
• 2- The retentive arm does not contribute to
  bracing and stability.

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• Contraindications
• l-When a deep cervical tooth undercut exists. 2-
  When a severe tissue undercut exists.
• 3- When there is a shallow vestibule.
• 4- When there is excessive buccal or lingual tilt
  of the tooth.
• 5- When the height of contour is close to the
  occlusal surface of the tooth.

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TYPE OF THE GINGIVALLY APPROACHING CLASPS

• I-Bar clasp (Roach or vertical projection)
  Indications
• I-On abutment for tooth supported or tooth-mucosa
  supported RPD when there is distobuccal undercut.
• 2-In situations in which esthetics is important.

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• Design
• 1-Component parts It is usually used as a
  combination clasp from
• a-Rest.
• b-Minor connector.
• C A cast bar retentive clasp arm.
• d- A cast circumferential bracing clasp arm.
• 2-Provides unilateral bracing.
• 3 -Engages 0.0 I inch undercut.
• 4-The shape of the retentive terminal as a T,
  modified T, Y, or I classify the bar clasp.
• a-The I and Y retentive clasp arms provide
  tripping action, which is called push type clasp.

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• b- The T and modified T clasps did not have
  tripping action, since the retentive terminal
  actually engage the undercut from an occlusal
  direction. One terminal of the T -bar clasp lies
  above the survey line for bracing and the other
  terminal below the survey line for retention. The
  modified T-bar clasp has only one terminal below
  the survey line.

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• 2-RPI- CLASP
• Indication
• Commonly used for tooth mucosa borne partial
  dentures.
• Design
• 1-It consists of
• a-Mesial rest.
• b-Minor connector, placed into the mesiolingual
  embrasure, but not contacting the adjacent tooth.
• c-Proximal plate contacts approximately 1 mm of
  the gingival portion of guiding plane.
• d-Cast I bar retentive clasp arm located at the
  mesiobuccal prominence of the tooth or mesial to
  it.
• 2-Engages 0.01 inch undercut.
• 3-Provides unilateral bracing.
• 4- The proximal plate and the minor connector
  provide stabilization and reciprocation.
• 5-During function, proximal plate and I-bar clasp
  arm move in a mesiogingival direction-disengaging
  tooth. This distributes more functional load to
  edentulous ridge.

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III- Combination Clasp

• Indication
• Commonly used for tooth mucosa borne RPD, when
  there is mesiobuccal undercut on the abutment
  tooth.
• Design
• 1-Cornponent parts are
• a- Rest
• b- Minor connector
• c- Wrought wire circumferential retentive clasp
  arm.
• d-Cast circumferential bracing clasp arm.
• 2-Engages 0.01- 0.02 inch undercut.
• 3-Provides bilateral bracing, but less than a
  cast circumferential clasp.

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• Advantages
• 1 - Greater flexibility.
• 2-It is more esthetically acceptable because it
  is placed too much gingival.
• 3-lt flex in all directions (round in cross
  section), which allow it to dissipate forces
  exerted on the abutment tooth.
• 4- The retentive arm can be adjusted in all
  directions.
• 5-Makes little tooth contact (line contact.
  rather than the surface contact of the cast clasp
  arm).
• Disadvantages
• 1-It is easily distorted or fractured by careless
  handling by the patient.
• 2-It does not possess the bracing and stabilizing
  qualities of cast clasp arm.
• 3-It involves extra step in fabrication

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• Criteria for clasp selection
• The selection of the clasp depends mainly on the
  type of support, the presence of undercut areas,
  and esthetics.
• 1-For bounded saddle the retentive undercut
  present is used with any acceptable clasp type
  (clasping for convenience), 2-For distal
  extension base a stress releasing clasp that
  equitably distribute the force between the
  abutment and the ridge is used
• a-If a mesiobuccal undercut is available on the
  terminal abutment, a combination clasp with the
  wrought wire, RPI, or RPA clasps are used.
• b-If the retentive undercut is located on the
  distobuccal surface, a bar clasp, reverse circlet
  (the minor connector and the occlusal rests are
  placed mesially) and the C-clasp are used.
• c-If mesiolingual undercut is present a wrought
  wire clasp is used.