CHAPTER 7: DIRECT RETAINERS 6.

Amount of retention
PRIMARY RETENTION • should always be the minimum necessary to resist
dislodging forces.
• Accomplished mechanically by placing retaining
elements (direct retainers on abutment teeth 7. Reciprocal elements
SECONDARY RETENTION • Should be located at junction of gingival & middle
thirds of the crown of abutment.
• Provided by the intimate relationship of minor
• The terminal end of retentive arm is optimally
connector contact w/ guiding planes, denture
placed in gingival third of crown.
bases, & major connectors (maxillary) w/
underlying tissue TYPES OF DIRECT RETAINERS:
• Similar to retention of complete denture
1. INTRACORONAL RETAINER (PRECISION
• Proportionate to accuracy of impression
ATTACHMENT)
registration, accuracy of fit, total involved area of
contact • Principle of internal attachment first formulated
DIRECT RETAINERS by Dr. Herman E.S. Chayes, 1906
• May be cast/ attached totally within restored
• A unit of partial denture that engages an natural contours of abutment tooth
abutment tooth to resist displacement of • Composed of prefabricated machined key &
prosthesis away from basal seat tissue keyway
BASIC PRINCIPLES OF CLASP DESIGN: 2. EXTRACORONAL RETAINER
1. PRINCIPLE OF ENCIRCLEMENT • Most commonly used; uses mechanical resistance
to displacement through components placed on or
• More than 180° in greatest circumference of tooth
attached to the external surfaces of abutment
must be engaged by clasp assembly
tooth
• Continuous contact – ex: circumferential clasp
• Manufactured attachment: uses flexible clips or
• Discontinuous contact – ex: bar clasp rings
➢ 3 areas encircling the tooth: TYPES OF CLASP ASSEMBLIES:
1. Occlusal rest area
2. Retentive clasp terminal area 1. Clasps designed to accommodate distal extension
3. Reciprocal clasp terminal area functional movement
2. Clasps designed w/out movement accommodation
2. VERTICAL SUPPORT • SUPRABULGE RETAINERS – above HOC/ survey line
• INFRABULGE RETAINERS – below HOC/ survey line
• occlusal rests
3. RECIPROCATION
1. CLASPS DESIGNED TO ACCOMMODATE DISTAL
• retentive terminal (buccal) opposed by reciprocal/ EXTENSION FUNCTIONAL MOVEMENT
stabilizing components (lingual) Two strategies may be adopted to:
4. CLASP RETAINERS I. Change the fulcrum location & resistance arm
• Should be designed so that they avoid direct engaging effect (mesial rest concept)
transmission of tipping & rotational forces II. Minimize the effect of lever through the use of
• Must act as stress-breakers flexible arm (wrought-wire retentive arm)

5. RETENTIVE CLASPS 1. RPI clasp

2. Bar clasp
• should be bilaterally opposed
3. Infrabulge clasp
4. Combination clasp
5. RPA/Akers clasp
 • Cast reciprocal arm: usually circumferential, but
1. RPI CLASP may be in the form of bar clasp
• Rest (mesial), Proximal plate (distal), I-bar (buccal)
Advantages:
3 basic approaches of RPI system:
a. Flexibility
a. Guiding plane & proximal plate should extend b. Adjustability
entire length w/ tissue relief c. Esthetic appearance of wrought-wire retentive
b. Guiding plane & proximal plate should extend arm
from marginal ridge to the junction of middle d. Minimum coverage of tooth surface as result
& gingival thirds of its line contact w/ tooth
c. Proximal plate contacts approximately 1 mm e. Less likely occurrence of fatigue failures
of gingival portion of guiding plane
Disadvantages:
2. BAR CLASP a. Involves extra steps in fabrication; when high-
• Approaches the undercut from BELOW the height fusing chromium alloys are used
of contour or GINGIVAL direction b. May be distorted by careless handling on the
part of patient
Indications:
c. Since it is bent by hand, may be less accurately
a. When a small degree of undercut (0.01 in) adapted to tooth, therefore less stabilization
exists in suprabulge portion
b. For tooth-supported partial dentures/ d. May distort w/ function & not engage tooth
modification areas 5. RPA CLASP (AKERS CLASP)
c. Distal extension base situations • Similar to RPI, except wrought wire circumferential
d. Esthetic consideration must be clasp (Akers) is used
accommodated & cast clasp is indicated
2. CLASPS DESIGNED WITHOUT MOVEMENT
Contraindications: ACCOMODATION
a. Severe tooth/ tissue undercuts 1. CIRCUMFERENTIAL CLASP
b. Shallow buccal vestibules • Approaches the undercut from ABOVE the height
c. Severe buccal/ lingual tilts of abutment of contour or OCCLUSAL direction

3. INFRABULGE CLASP Disadvantages:

Advantages: a. More tooth surface covered due to its occlusal

origin
a. Interproximal location esthetic advantage b. Buccal surface of mandibular teeth & lingual
b. Increased retention w/out tipping action on surface of maxillary teeth, its occlusal
abutment approach may increase width of occlusal tooth
c. Less chance of accidental distortion resulting surface
from its proximity to denture border c. More metal display in mandibular arch
d. Its half-round form prevents adjustment to
4. COMBINATION CLASP increase/ decrease retention. True adjustment
• Another strategy to reduce effect of Class I level in is impossible with most cast clasps
distal extension situations is to use a flexible 2. RING CLASP
component in the resistance arm • Clasp, 2 occlusal rest, supporting strut
• Consists of a wrought-wire retentive clasp arm • Encircles nearly all of a tooth from its point of
(tapered, round, 18-gauge wrought wire) & a cast origin
reciprocal arm • Used when a proximal undercut cannot be
• Retentive arm: almost always circumferential, but approached by other means
also may be used in the manner of a bar, • Should never be used as an unsupported ring
originating gingivally from denture base
• Should be used w/ supporting strut on non-
retentive side, w/ or w/out auxiliary occlusal rest
opposite marginal ridge
• Should be used on protected abutments since it
covers such large area of surface
3. EMBRASURE CLASP
• Used in unmodified Class II/ Class III where no
edentulous spaces on opposite side of the arch aid
in clasping
• Should be used w/ double occlusal rests to avoid
interproximal wedging
• Should have 2 retentive, 2 reciprocal clasps arms,
2 occlusal rests
• Multiple clasp, half-and-half clasp, reverse action
clasp
4. HALF & HALF CLASP
✓ 2 occlusal rests, 2 retentive arm

FACTORS AFFECTING THE AMOUNT OF RETENTION:

1. Size of and distance into the angle of cervical
convergence
✓ Height of contour = area of greatest convexity
2. Length of clasp arm
✓ Longer = more flexible
3. Diameter of clasp arm
✓ Greater average diameter = less flexible
4. Cross-sectional form of the clasp arm
✓ The only universally flexible form is the round
form, impossible to obtain
5. Clasp material used
6. Relative uniformity of retention
7. Stabilizing-reciprocal clast clasp arm

CHROMIUM-COBALT ALLOYS:

• Low density
• Low material cost
• Low flexibility
• High modulus of elasticity
• Resistance to tarnish
INDIRECT RETAINERS – composed of a rest & is located
on the opposite side of fulcrum line away from DEB