Post on 13-Dec-2015
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Dr. Golam Wakil
DIRECT RETAINERS(Part I)
DIRECT RETAINERS(Part I)
CONTENTS
Part I
• Introduction
• Classification
• Parts of clasp assembly
• Principles of clasp design
• Blatterfein & Ney system
• Functions of clasp assembly
• Types of retainers:
- occlusally approaching
clasps
Part II
• Types of retainers:
- gingivally approaching
clasp
- other clasp designs
• Relative merits &
demerits of cast &
wrought wire clasps
• Intracoronal retainers
• Summary
2
Introduction
• Retention is that quality inherent in the removable
partial denture that resists the vertical forces of
dislodgement for e.g., the force of gravity, the
adhesiveness of foods, or the forces associated with
the opening of the jaws.
3
• Direct retainer is any unit of removable partial denture that engages an abutment tooth in such a manner as to resist displacement of the prosthesis away from basal seat tissues by
1. Functional means by engaging a tooth undercut lying cervically to the height of the contour.
2. Wedge principle.
3. Mechanical means
4
DEFINITION:
Direct retainer is that component of a partial removable dental prosthesis used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment (GPT 8)
5
Factors affecting the retention in Removable
Partial Dentures:
I. Primary retention : This is mechanical in
action.
II. Secondary Retention: Achieved by intimate
relationship of the denture base and the rigid
major connector to the underlying soft tissues.
a. Adhesion
b. Cohesion
c. Atmosphere pressure
d. Molding of tissues
e. Effect of gravity
6
Classification of direct retainers
• Direct Retainers are of two types:
• Extra Coronal
• Intra Coronal
7
Extra Coronal Direct Retainers:
Mechanical resistance to displacement.
They are of three types:
a. Manufactured attachment.
Eg. Dalbo, Spring loaded plungers.
b. Flexible clips/Rings.
c. Clasp type retainer.
The flexible area engages a prepared
depression are an undercut area cervical to the
area of greatest convexity of the tooth. 8
• Intra Coronal Retainer:
Which is cast or attached totally within the restored natural contours of an abutment tooth
Regarded as an internal/precision attachment.
• Formulated by Dr.Herman.E.S. Chayes in 1906.
• Cast/Attached within the tooth structure
• Prefabricated
• Frictional resistance.
9
EXTRACORONAL DIRECT RETAINERS• Occlusally
approaching / Suprabulge / Ney Type I clasp / Circumferential
• Gingivally
approaching / Infrabulge/ Bar/ Roach / Ney Type II Clasp
10
The basic parts of a clasp assembly :
• Rest : It is the part of the clasp that lies on the occlusal, lingual or incisal surface of a tooth and resist tissue ward movement of the clasp.
• Body of the clasp : It is the part of the clasp that connects the rest and shoulder of the clasp to the minor connector.
11
• Shoulder : It is the part of the clasp that connects the body to the clasp terminals. It must lie above the height of contour and provide some stabilization against horizontal displacement of the prosthesis.
• Reciprocal arm : A rigid clasp arm placed above the height of contour on the side of the tooth, opposing the retentive clasp arm.
12
• Retentive arm : It is the part of the clasp comprising the shoulder which is not flexible and is located above the height of the contour.
• Retentive terminal : It is the terminal end of the retentive clasp arm. It is the only component of the removable partial denture that lies on the tooth surface cervical to the height of the contour. It possesses a certain degree of flexibility and offers the property of direct retention.
13
• Minor connector : It is the part of the clasp that joins the body of the clasp to the remainder of the framework and must be rigid.
• Approach arm : It is a component of the bar clasp. It is a minor connector that projects from the framework, runs along the mucosa and turns to cross the gingival margin of the abutment tooth to approach the undercut from a gingival direction.
14
15
PRINCIPLES OF CLASP DESIGN• Encirclement:
More than 1800 of greatest circumference of the tooth must be included passing from diverging axial surface to converging axial surface.
This may be in the form of continuous contact when circumferential clasp arms are used.
Bar clasps are used, at least 3 areas of tooth contact must be embracing more than ½ the tooth circumference. These are occlusal rest area, the retentive terminal area and reciprocal terminal area.
16
The surveyed cast clasp embraces about 270° of the abutment tooth. Its parts are : Bracing arm (blue). Retentive arm (red).
Shoulder and rest (yellow). Minor connector (green).
17
• Support:
Property of the clasp that resist the displacement of the clasp in gingival direction.
Primary support units of a clasp are occlusal, lingual or incisal rest.
The occlusal rest must be designed so that cervical movement of the clasp arm is prevented.
The rest should provide only vertical support.
18
• Reciprocation:• Each retentive terminal should be opposed by a
reciprocal arm or element capable of resisting any orthodontic pressures exerted by the retentive arm. Reciprocal and stabilizing elements must be rigidly connected bilaterally.
• This arm is positioned on the opposite side of the tooth from the retentive arm.
19
• In addition to reciprocating stress generated against the tooth by the retentive clasp, it also play an important role in stabilizing the denture against horizontal movement.
20
• Some cases, additional rest is positioned on the opposite side of the tooth and minor connector will provide reciprocation.
• Reciprocal clasp must be rigid, it is not tapered as the retentive clasp. Reciprocal arm should be positioned on the surface of a tooth is reasonally parallel to the denture’s path of insertion and removal.
21
• If it is placed on the surface that is tapered occlusally, a slight movement of denture will cause the clasp to loose contact with the tooth and reciprocation and retention is lost.
• It must be positioned above the height of contour at the junction of the gingival and middle 3rd.
• To reciprocate the forms properly, it should contact the tooth at the same time or before the retentive arm does.
22
23
• Retention:
The path of escapement of each retentive clasp terminal should be other than parallel to the path of removal of the prosthesis.
The amount of retention always should be the minimum necessary to resist reasonable dislodging forces.
Only the terminal third of an occlusally approaching clasp (stippled section) should engage the undercut.
A gingivally approaching clasp contacts the tooth surface only at its tip.
The retentive clasp is divided into 3 parts; each with its arm functional requirement.• The terminal third is flexible and engages the undercut.• The middle third has a limit degree of flexibility and may engage a minimal amount of undercut.• Proximal third, or shoulder, is rigid, and must be positioned above the height of contour.
24
• Stability / Bracing:
All 3 clasp have reciprocal or bracing arm, which provides equal amount of stability.
Stability is the resistance to horizontal displacement of a prosthesis.
All clasp terminal except the retentive clasp terminals contribute to this property in varying degree.
Eg: Cast circumferential clasp exhibits great amount of stability, because its shoulder is rigid and it aids in stabilization.
The wrought wire clasp has flexible shoulder .
Bar clasp does not have a shoulder so both provide less stability.
25
• Passivity:
A clasp in place should be completely passive. The retentive function is activated only when dislodging forces are applied to the partial denture.
26
• Bilateral Opposition:
Unless guide planes will positively control the path of removal, retentive clasp should be bilateral opposed. i.e., buccal retention on the other or lingual on one side opposed by lingual on the other.
In class II situation, the 3rd abutment may have either buccal or lingual retention.
In class III retention may be either bilaterally or dimeterically opposed.
27
• Stress Breaking:
Clasp retainers on abutment teeth adjacent to distal extension bases should be designed so that they will avoid direct transmission of tipping and rotational forces on abutments. In effect they must act as stress breakers either by their design or by their construction.
This is accomplished by proper location of the retentive terminal or by use of a more flexible arm is relation to rotation of the denture under varying direct forces.
28
• Location of components:
The reciprocal element of the clasp assembly should be located at the junction of the gingival and middle 3rd of the crowns of an abutment teeth. The terminal end of the retentive arm is placed in the gingival 3rd of the crown.
29
Criteria for Clasp Selection
1. Surveyline location and degree of undercut.
2. Requirement of retention and stability
depending on whether upper or lower arch
and configuration of edentulous area or areas;
axis of rotation and selection of retainers.
30
31
SURVEY LINE
• Blatterfein in 1951 put forth a
simple and comprehensive
classification of surveyline with
suggestion on clasp selection.
• He divided the buccal and lingual
tooth surface into two halves using a
vertical imaginary line through the
long axis of the tooth. These halves
were described as the nearzone and
farzone depending on its closeness
to the edentulous space.
• He described four kinds of surveyline:
1. Typical surveyline or medium
2. Atypical A or Diagonal
3. Atypical B or High
4. Atypical C or Low
32
33
Typical or medium:
Extends from the occlusogingival midpoint in the
near zone to the junction between the occlusal two
third and cervical one third in the far zone.
Clasps suggested for use where such a survey line
exists include the occlusally approaching and
gingivally approaching clasps.
34
• Atypical A or diagonal:
This runs diagonally across the tooth surface
from a high position in the nearzone to a low
position in the farzone.
- A reverse action or hairpin clasp is
recommended.
- Gingivally approaching clasp may also be used.
35
• Atypical B or High Surveyline:
This type of surveyline is parallel to the occlusal surface and
lies close to it.
A wrought occlusally approaching clasp arm may be used.
If accompanied by a low surveyline on the opposite side of the
tooth, a ring, back action or reverse back action have been
recommended.
36
• Atypical C or Low Surveyline:
The low surveyline is parallel to occlusal surface but has just
above the level of the gingival margin.
This type of surveyline contraindicate the placement of a
retentive clasp arm on the tooth surface concerned, as the
arm would need to be placed too close to the gingival margin
for safe application.
When all the usable surface of a tooth present the survey lines of this type, retention may be obtained by:
1. Placement of a crown on the tooth to artificially develop undercuts.
2. Placement of a class IV gold inlay – a dimple is in the inlay and a ball head on a gingivally approach arm is positioned to engage the dimple.
3. An extended clasp may be used where the tooth offers favourable conditions for retention.
4. Undercut may be developed by recontouring the tooth.
37
Ney System:
• 3 basic surveyline with an appropriate clasp form:
• Class I :
Survey line runs diagonally across the tooth
surface from a low position on the side of the rest
to a high position on the other proximal side.
A cast occlusally approaching arm or its variants,
back action, reverse back action and ring clasps
are recommended called as Ney class I clasps.38
• Class II:
Similar to Blatterfein atypical A or Diagonal
surveyline. Here gingivally approaching is
recommended and termed as Ney class II clasp.
• Class III:
It is the same as the Blatterfein Atypical B or High
surveyline. The wrought wire arm is used and
termed as the Ney class III clasp.
39
Selection of clasp based on requirement of retention & stability
• A large number of edentulous area, bilaterally placed would mean more number of clasps and guide planes, thus the entire prosthesis has greater retention and stability.
• Cast circumferential clasp properly designed shows greater retentive and bracing properties than a bar clasp used in a similar situation.
• Ideal amount of retention is that which will retain the removable partial denture against reasonable dislodging forces without placing the undue strain on the abutment teeth. Thus minimum retention and maximum stability is the ideal. 40
• Support:
Selection based on nature of support whether tooth / tissue or tooth and tissue borne and length of the edentulous span.
In case of long span – class I situation preservation of abutment teeth is important, where a RPI, RPL clasp is used to prevent torque on the abutment.
41
• Root size and form of the tooth:
Clinical conditions of supporting structures - alveolar
bone and periodontal ligament should be considered Abutment teeth with short, conical roots, bone loss, and periodontal
ligament, mild periodontal problem may not be able to withstand
lateral forces that would be within physiologic limits of a healthy
sound tooth. Preservation of such teeth as abutments would necessitate
the use of bar clasps with increased length of clasp arm of round
cross-section, of type IV casting gold alloy or wrought gold wires.
42
• Oral hygiene and caries:
1. Maintaining oral hygiene is pre requisite to
treatment with cast partial denture.
2. High caries index contraindicate the use of clasp
with an unprotected tooth surface as with a ring
clasp.
3. Bar clasp has minimum tooth structure contact and
less interference with natural cleansing action.
Patient education is essential to maintenance of
oral hygiene, proper use of prostheses, its
placement and removal.43
• Esthetics:
Gingivally approaching claps are generally preferred for
esthetic reason but may be more unestheic than
occlusally approaching clasp if the patient has a high lip
line and exposes the excessive gingiva.
• Presence of excessive tissue undercut:
Gingivally approaching clasp is contraindicated in the
presence of excessive tissue undercut.
44
Functions of Clasp Assembly
• Retention
• Support
• Stabilization or Bracing
• Reciprocation
• Indirect retention
45
Retention
• Clasp retention is based on the resistance of metal to deformation. For a clasp to be retentive, it must be placed in an undercut area of the tooth, where it is forced to deform when a vertical dislodging force is applied.
• It is this resistance to deformation that generates retention.
46
Factors affecting retention of a clasp
A. Size of the angle of cervical convergence:
• When the surveyor blade contacts a tooth on the cast at its greatest convexity, a triangle is formed, the apex of which is at the point of contact of the surveyor blade with the tooth, and the base is the area of the cast representing the gingival tissues. The apical angle is called the angle of cervical convergence.
• To be retentive a tooth must have an angle of convergence cervical to the height of contour.
47
48
• Guiding planes determine the path of placement and removal of a partial denture. Therefore without the use of guide planes, clasp retention will either be determined or practically non existent.
The guide plane moves down the proximal surface which is prepared on the distal aspect of the tooth. When the denture is fully seated, the plane contacts the lower parts of that surface.
B. Degree of undercut
• Relative uniformity of retention will depend on the location of the retentive part of the clasp arm, which is not in relation to the height of contour, but in relation to the angle of cervical convergence.
• Retentive clasp arms must be located so that they lie in the same approximate degree of undercut on each abutment tooth, despite the variation in the distance below the height of contour.
• The measurement of the degree of undercut by mechanical means is achieved by the help of an undercut gauge attached to a dental surveyor. 49
The retentive force is dictated by tooth shape and by clasp
design. Though clasps 1 and 2 are in an undercut of
0.25mm, 1 offers more retention than 2.50
C. Flexibility of clasp arms(i) Length of the clasp arm : The longer the clasp arm, the more flexible it is, all other factors being equal. The length of a clasp arm is measured from the point at which a uniform taper begins.
A Co-Cr clasp arm engaging the same degree of undercut will have different flexibility and resistance to distortion on the molar and the premolar because of the difference in length. 51
Circumferential Bar type
Length (inches)
Flexibility (inches)
Length (inches)
Flexibility (inches)
0-0.3 0.004 0-0.7 0.004
0.3-0.6 0.008 0.7-0.9 0.008
0.6-0.8 0.012 0.9-1.0 0.012
52
(ii) Diameter of clasp arm:
• The diameter of a clasp arm is inversely proportional to its flexibility, all other factors being equal.
• The average diameter to be considered is at a point midway between its origin and its terminal end.
• The thickness of the clasp arm in the buccolingual direction is to be considered rather than the width in the occluso-gingival direction.
53
(iii) Cross-sectional form
• Round cross-sectional
form enables the clasp to
be flexible in all
directions whereas the
half-round form limits the
flexibility to only one
direction.
• Cast clasps are half round
in form and they flex
away from the tooth, but
edgewise flexing is
limited.
• If the cross-sectional area of clasp is doubled, the stiffness will be increased 4 times and the flexibility reduced 4 times.
54
(iv) Material used for clasp arm :
Type IV gold alloys and cobalt-chrome alloys
which have different modulus of elasticity.
The modulus of elasticity of cobalt-chrome alloys
is greater than that of cast golds, which have a
higher modulus than wrought gold wires.
55
The retentive terminal has to be flexible and therefore have
low modulus of elasticity. The reciprocal elements have to
be stiff and unyielding and have high modulus of elasticity.
Therefore a clasp of the same cross-section is stiffer in
cobalt-chrome than in cast gold. This can be overcome by
using longer clasps of thinner section, and by doing
contour modification so as to reduce the degree of
undercut. 56
(v) Structure of the alloy
• The alloy may be cast or wrought in nature. Wrought wires have greater flexibility than a cast structure due to its grain structure being fibrous.
• The tensile strength of a wrought structure is at least 29% greater than that of the cast alloy from which it was made.
• Wrought forms can be used in smaller diameters to enhance the flexibility and they offer minimum friction and can have a stress breaking effect.
57
Support
• Support is the property of the clasp which enables it to resist displacement in a gingival direction.
• Primary support is obtained by the occlusal or incisal rest.
• Secondary support is obtained by the rigid components i.e. body and shoulder of the clasp which are placed above the greatest diameter of the tooth.
58
Stabilization or Bracing:• It is the resistance to horizontal components of
masticatory force (GPT 8)• All of the clasp components, with the exception of the
retentive terminal, contribute this property in varying degrees.
• Bracing elements, united by rigid major connector, are capable of distributing horizontal forces throughout the partially edentulous arch.
• The components of the cast circumferential clasp offer better stabilization than either the bar clasp or the wrought wire clasp, because of greater amount of rigidity of the clasp material.
59
Reciprocation
• Reciprocation (GPT 8) may be defined as “the mechanism by which lateral forces generated by a retentive clasp passing over a height of contour are counterbalanced by a reciprocal clasp passing along a reciprocal guiding plane”.
• For effective reciprocation clasps should be planned and designed so that the two arms of the clasps are in balanced.
60
RECIPROCATION
61
Indirect Retention
• The reciprocal arm may behave as an indirect retainer when it rests occlusal to the height of contour on the abutment tooth, lying anterior to the fulcrum line.
• Lifting of a distal extension base away from the tissues is resisted by a rigid arm, which is not displaced cervically
62
TYPES OF CLASP RETAINERS
• Circumferential clasps/ Occlusally approaching• Circumferential clasp• Embrasure clasp.• Ring clasp.• Back action clasp.• Reverse action / hair pin clasp.• Multiple clasps.• Half-and-half clasp.• Combination clasp.• Onlay clasp.
63
Modification
Bar/Roach clasps / Gingivally approaching clasps.
• T-clasp
• Modified T-clasp
• Y-clasp
• I-clasp
• RPI concept.
64
Other clasp designs RPA clasp.
VRHR clasp.
Clasps utilizing proximal undercuts
-
Mesiodistal clasp
-Devan clasp.
Movable arm clasp.
Cingulum clasp
65
CIRCUMFERENTIAL CLASP
Circlet clasp.• It is one of the most commonly used clasps with all tooth-supported
partial dentures because of its retentive and stabilizing ability. The basic design consists of a buccal retentive arm and a lingual reciprocal arm originating from a common body.
The simple circlet clasp• Design:• It consists of an occlusal rest in the distal fossa of an abutment
tooth adjacent to the distal extension base. The terminal third of the retentive arm engages a mesiobuccal undercut and the reciprocal arm is positioned above the survey line on the lingual surface.
66
Circumferential clasp on a molar
67
The reverse approach circlet clasp
• It is most commonly used on the mandibular bicuspids and the retentive terminal engages the distobuccal undercut.
Design:
• It consists of a mesial occlusal rest, a horizontal reciprocal arm, and a retentive arm engaging the distobuccal undercut adjacent to the edentulous area.
68
Advantages:
1) The clasp fulfills the requirement of support, stability
reciprocation, encirclement & passivity better than any
other type of clasp.
2) It is easy to construct.
3) It is simple to repair. 69
Disadvantages:
1. It tends to increase the circumference of the crown.
2. In is not acceptable in the anterior region.
3. It covers more tooth surface then the bar clasp and
prone to caries.
4. Retentive undercuts on some teeth are difficult to reach
with retentive terminal of the clasp 70
EMBRASURE CLASP
• Bonwill clasp
• Rib clasp
• Double a
• Back to back clasp
These clasps are used mainly in the fabrication of unmodified Class II or Class III partial denture situation; when there are no edentulous spaces available on the opposite side of the arch to aid in clasping.
71
Embrasure clasp on a maxillary premolar and molar
72
It should be applied only when the teeth are sound and
retentive areas are available or when multiple restorations
are justified.
Design• The embrasure clasp should have double occlusal rests,
two retentive clasp arms and the two reciprocal clasp arms either bilaterally or diagonally opposed.
• The double occlusal rests prevent interproximal wedging by the prosthesis, which could cause separation of the abutment tooth and result in food impaction and clasp displacement.
• In addition to providing support, occlusal rests also serve to shunt food away from the contact area. Therefore the embrasure clasp should be used with double occlusal rests, even when definite proximal shoulders can be established.
73
• An auxillary rest or a bar clasp arm can be
substituted for a circumferential reciprocal arm as
long as definite reciprocation and stabilization is
achieved.
Contra indication:
1. Short and bulboss crown.
2. Not preferred in teeth with more undercuts.
Disadvantages:
• Needs adequate cleaness in occlusal surface.
• Breakage of inadequate preparation and clearance
• Wedging action. 74
RING CLASP
• It is that form of a circumferential clasp that
encircles nearly all of the tooth from its point of
origin.
• It is usually used when a proximal undercut cannot
be approached by any other means.
75
• This ring clasp permits engagement of this undercut by encircling
almost the entire tooth from its point of origin.
• Mandibular molar, clasp encircles the tooth beginning on the
mesiobuccal surface and terminating in an infra bulge area on the
mesiolingual surface.
76
• Because of the length of the clasp, it must be designed with additional support, usually in the form of an auxillary bracing arm.
• In mandible, bracing arm usually extends from the
acrylic resin retention metal, run across the mucosa and
turns upward to engage the buccal arm of the ring clasp
near the center of the buccal surface.
• This can provide reciprocation and some amount of
stability for the denture.
• The entire clasp except the retentive terminal shall be
placed above the height of the contour.
• An additional occlusal rest can be placed may provide
additional support and prevent mesial migration of tooth.77
• Contra indication:
1. In mandibular molar, where the attachment of
buccinator muscle is so close to the tooth, that the
auxillary bracing arm encroaches on it.
2. When the bracing arm must cross the soft tissue
undercut.
78
• Advantages:
1. Excellent bracing
2. Decreased leverage
3. Less stress to abutment teeth.
• Disadvantages:
1. Needs long crown and enough occlusal clearance.
2. Esthetically objectionable
3. Difficult to repair.79
BACK ACTION CLASP
• It is a modification of the ring clasp.
• It has a rest which is connected to a rigid minor connector.
Indication:
For unilateral and bilateral distal extension partial denture.
80
REVERSE ACTION CLASP OR HAIR PIN• This clasp is essentially a simple circlet clasp in which the
retentive arm after crossing the facial surface of the tooth from
its point of origin loops back in a hairpin turn to engage a
proximal undercut below its point of origin.
• The upper part of the retentive arm must be considered to be
minor connector and should be rigid.
• The lower part of the clasp arm should be tapered. It is the only
flexible part of the clasp arm.
81
• The crown of the abutment tooth must have sufficient occlusogingival height to accommodate this double width of the clasp arm.
• The upper and lower arms of the retentive clasp must also be shaped in such a way that food debris will not be retained between them.
• And there must be enough space between the arms so that the metal may be adequately finished and polished.
82
• Indication:
1. Distal extension partial denture.
2. Mesially inclined posterior.
3. Undercut addition to the edentulous area.
4. If when proximal undercut must be used on a posterior
abutment and when the tissue undercut or high tissue
attachment prevent the use of bar clasp arm, the reverse
action clasp may be preferable.
5. If lingual undercut is present which prevent the
placement of a supporting strut without tongue
interference, hairpin clasp is indicated. 83
• Contra Indication:
1. Tight occlusal contact, increase posterior overbite, short crown,
2. Clasp cover considerable tooth surface and may trap debris.
• Advantages:• Easier to construct
• Adjust
• Disadvantages:• Food entrapment
• Esthetics
• Caries 84
MULTIPLE CLASP
85
Multiple clasp is two opposing simple circlet clasp joined
at the terminal end of the two reciprocal arms.
MULTIPLE CLASP
Indication:
1. When additional retention is needed.
2. Tooth borne partial denture
3. When the partial denture replaces an entire half of the
dental arch.
86
• Advantages:
1. Less metal display
2. Less tooth coverage
3. Leaves room for the mesial portion of the denture
base to a larger extent than it would be otherwise.
4. It braces the abutment on the mesial even if the tooth
is tipped distally.
5. Marginal gingiva can be left uncovered of the
abutment teeth for better partial denture health. 87
Half-and-half clasp
88
This clasp consists of a circumferential retention arm arising from one direction and a reciprocal arm arising from another minor connector.
• This design provides retention , a principle that should be
applied only to a unilateral denture design.
• The buccal arm provides for bracing only.
• The lingual arm utilizes an undercut adjacent to the
edentulous space for retention.
89
• Indication:
• Lingually inclined premolars where lingual undercut
are close to the edentulous space.
• Contra indication:
• Buccal inclined premolars –
If it is used for distal extension RPD a distal
rest should be placed. Mesial rest also can be used in
conjunction with the distal rest.90
COMBINATION CLASP
91
• This type of clasp consists of a wrought wire retentive
clasp
arm and a cast reciprocal clasp arm. • The retentive arm is usually occlusally approaching,
but it may
also be used from a gingivally approaching direction.
Uses:
• It is used on abutment tooth adjacent to a distal extension
base where only a mesiogingival undercut exists on the
abutment or where a large tissue undercut
contraindicates a bar type retainer.
• The tapered wrought wire retentive arm offers greater
flexibility than does the cast clasp arm and therefore
better dissipates functional stresses.
92
Advantages
• Flexibility on account of fibrous grain structure of the
wrought wire retentive arm.
• Adjustability : It can be adjusted later to increase or
decrease the retention without danger of breakage.
• Esthetic appearance: since it is used in smaller diameters
of round cross-section. 93
A minimum of tooth surface is covered because
of its line contact with the tooth, rather than a
surface contact of a cast clasp arm.
Fatigue failures in service are less likely to
occur with the tapered wrought wire retentive arm.
94
Disadvantages
• It involves extra steps in fabrication, particularly when
high fusing chromium alloys are used.
• It may be distorted by careless handling on the part of
the patient.
• Since it is bent by hand, it may be less accurately
adapted and therefore provide less stabilization above
the height of contour.
95
EXTENDED CLASP ARM
• It is similar to the circumferential arm but it covers
two teeth. It remains above the surveyline of the 1st
tooth; crosses the undercut of the adjacent tooth. It
is rarely used as direct retainer.
96
Indications:
97
• Tooth supported RPD.
• Tooth next to edentulous space has no buccal and
lingual
undercut.
• The occlusion in the embrasure area will not allow
passage
of the clasp arm to an undercut on the second tooth
from
the edentulous space.
• The second tooth from edentulous space has a
buccal
undercut available.
• Contra Indication:
1. Distal extension dentures because the retentive lies
forward of the axis rotation. Functional forces will
cause rotation around the rest and upward movement
of clasp tip.
• Advantages:
1. It has splinting and stabilization action.
2. Distribution of lateral loads over two teeth.
98
• Disadvantages:
1. Tooth structure covered.
2. Easily distorts.
3. Breakage of the arm.
4. If made in gold limited to 2 premolar
in Cr. Ch – longer arm can be used.
- 2 molars can be clasped.99
MESIODISTAL CLASP
• Used to clasp canine and central only if little undercut on buccal surface.
Disadvantage:
• Metal displaces alloy made of gold.
If diastema between the lateral incisor and canine, then the space provides a accommodation for the mesial part of the clasp otherwise space reaction with safe side disc, contact point with L.I. is returned when clasp an position. It is similar to inlay. 100
ONLAY CLASP1. Extends from an occlusal onlay into an undercut
located mesio distally.
2. This clasp is an extended occlusal rest with buccal and lingual clasp arms.
3. This clasp may originate from any point on the onlay that will not create any occlusal interference.
4. If the onlay is prepared with chrome cobalt alloy, and is opposed by natural teeth, the occlusal surface should be constructed of acrylic resin or gold, if you use chrome alloy, because of its extreme hardness will cause rapid wear of enamel
101
Indications:
1. When the occlusal surface of the
abutment tooth is the occlusal
plane mesially tilted rotated tooth
molar.
2. Only in caries resistant mouth
unless it is covered by gold
crown.
102
• Advantages:
1. Mesially tilted molars to be used to provide
retention.
2. 3rd molar occlusion may be improved.
• Disadvantages:
1. Difficult to fit clasp to tooth.
2. Increased contact area, accurate impression
and the resultant cast is difficult to achieve.
103
DEVAN CLASP
• Uses proximal undercut and has a small head that bears on tooth entirely below survey line.
• Clasp arises and lies closely against at the periphery of the denture base.
• Denture base is under extended to provide room for the approaching arm.
• It is reciprocated with lingual and palatal strut.
• It gives little bracing effect.
104
Devan clasp
105
• Advantages:
1. Esthetically acceptable, became of interproximal
location, or it is hidden behind the buccal concavity.
2. The distribution of stress during insertion and
removal is minimal.
3. Increased retention without tipping action on the
abutment.
4. Less chance of accidental deformation because it
doesn’t project very far away from the denture base.
106
DALBO EXTRACORONAL DALBO EXTRACORONAL ATTACHMENTATTACHMENT
Components consist of :• A, L-shaped male portion that is attached to an abutment
crown; • B, female sleeve that is placed in artificial tooth adjacent to
abutment, and coiled spring that fits into female portion. • Design permits some vertical movement of denture under Design permits some vertical movement of denture under
force through compression of coiled spring. force through compression of coiled spring.
107
THANK YOU
108
Part II
109
CONTENTS
Part II
• Types of retainers:
- gingivally approaching
clasp
- other clasp designs
• Relative merits &
demerits of cast &
wrought wire clasps
• Intracoronal retainers
• Summary
GINGIVALLY APPROACHING CLASPS• Infra Bulge
• Push Clasp
• Roach Clasp
111
This bar clasp approaches the retentive undercut
in a gingival direction resulting in a push type of
retention. This push type of retention is more
effective than pull type retention characteristic of
circumferential clasp- tripping action.
• This clasp is termed by
F.E. Roach in 1930 and
hence the name Roach
clasp.
• The bar clasp is classified
by shape of the retentive
terminal T, modified T, I,
Y forms, all of which
originate from the denture
base frame work and
approaches the undercut
from gingival direction.112
PARTS OF BAR-CLASP
• A, buccal retentive arm engaging measured undercut
• B, stabilizing (reciprocal) elements; proximal plate minor connector on distal;
• C, lingualiy placed mesial minor connector for occlusal rest, which also serves as a stabilizing (reciprocal) component;
• D, mesially placed supporting occlusal rest.
• Advantages:
1. Minimal tooth contact and minimal distortion of normal tooth contours, leading to improved tissue stimulation and oral hygiene and caries and periodontal problems.
2. Improved esthetics if the approach portion of the arm is not visible as it crosses the gingiva.
3. Increased retention because of tripping action.
4. Decreased torquing forces applied to terminal abutments in extension RPDs.
114
• Disadvantages:
1. Cannot be used in the presence of soft tissue undercuts shallow vestibule and high frenum attachments.
2. Bracing action provided by bar clasp is considerable less than that provided by cast circumferential clasps.
3. The bar clasp will not totally disengage in certain distal extension cases.
4. Appearance may be adversely affected if the smile line is high enough to expose the approach arm as it crosses the gingiva
5. Food entrapment.
6. Difficult to fabricate and adjust.
115
• Indication:
1. Class I and Class II distal extension partial dentures to engage
the distobuccal undercut on abutment. It can be employed on
canines and sometimes even on molars.
2. Where the anterior retention is needed: It is often used on
distobuccal surface of maxillary canines and mandibular
premolar as the retentive arm can be hidden from the vision.
Esthetically this clasp is superior to circumferential clasp, it is
inferior in providing stability because of greater flexibility of the
retentive arm.
116
• Guidelines for use:
1. The approach arm of the bar clasp must not
impinge on the soft tissue. It is not desirable to
provide an area of relief under the arms. But the
tissue side of the approach arm should be
smoothed and polished.
2. Minor connector that attaches the occlusal rest to
the frame work should be strong and rigid to
provide some bracing.
3. The approach arm must always be tapered
uniformly from its attachment at the framework to
the clasp terminal. 117
4. The approach arm positioned over a soft tissue undercut will collect food and irritates lips or cheeks.
5. The approach arm should cross the gingival
margin at a 90 degree angle.
118
6. The bar retentive clasp is used only when the retentive undercut is adjacent to the edentulous from which the approach arm originate.
The approach arm must extend on the abutment tooth to the height of contour.
The retentive terminal leaves the approach at that point and extends into the undercut area.
The other terminal which is away from edentulous area is positioned above the height of contour.
The retentive terminal tip must point towards the occlusal surface never towards the gingiva.
119
• The bar clasp should be placed as low as on the tooth
as possible while engaging the height of contour to
reduce the leverage induced stress to the abutment
tooth.
120
• T Clasp
• Modified T Clasp
• Y clasp
• I clasp
121
TYPES OF BAR CLASP
T-CLASP
• Used in combination with cast
circumferential reciprocal arm.
• The retentive terminal and its opposing
encircling finger projects laterally from the
approach arm to form T.
• The retentive terminal must cross under the
height of contour to engage the retentive
undercut, while the other finger of the T
stays on the suprabulge of the tooth.
122
• The approach arm should taper gradually and
uniformly from its origin to the retentive terminal.
•The approach arm contacts the tooth only at the height
of contour.
123
• Indication:
- Most frequently used in distal extension ridge where the
usable undercuts is on the distobuccal surface of the terminal
abutment tooth.
- It can also be used for tooth supported partial denture when
the retentive undercut is located on the abutment tooth
adjacent to the edentulous space.
124
• Contra Indication:
- Should not be used on a terminal abutment adjacent to a distal
extension base if the usable undercut is located on the side of the
tooth away from the edentulous space.
- The T clasp can never be used if the approach arm is in the soft
tissue undercut.
- This clasp cannot be used when the height of contour lies close to
occlusal surface.
125
MODIFIED T-CLASP
• It is a T clasp with the non retentive
(mesial) finger of the cross bar of the
T- terminal omitted.
• Indication:
- Used on canines or premolar for
esthetic reasons.
- When this type of a clasp is used the
encirclement of the abutment tooth is
sacrificed. 126
Y-CLASP
• It is basically a T-clasp, its
configuration occurs when the height
of contour on the facial surface of the
abutment tooth is high on mesial and
distal line angles but low on the
center of the facial surface.
127
I-CLASP
• Used on distobuccal surface of maxillary canine for
esthetic reason.
• Disadvantage:
- The contact of the retentive clasp with the abutment
tooth is the tip of the clasp an area of 2-3 mm.
Encirclement, horizontal stabilization may be
compromised.
128
RPI System (Rest, Proximal plate, I-bar)
Kratochvil in 1963 developed the early clasp assembly which consisted of three separate units connected to each other only through the framework. They were the mesial occlusal rest, a distal guide plane and an I-bar retainer. He preferred a full length guide plane that is subsequently relieved in the mouth to prevent torque or binding.
129
• His purposes were:
• Elimination of the V-shaped food trap distal to
the tooth.
• A highly polished metal contact with the
marginal gingiva, rather than resin .
• Intimate metal-to-tooth contact to minimize
food impaction
130
• This design had certain basic disadvantages:
• Physiologic relief was required to prevent
impingement of gingival tissues during function.
• Since the proximal plate covers a greater surface
area of the tooth, the functional forces are
directed in the horizontal direction, thus the
tooth is located more than the edentulous ridge.
131
• Krol in 1973 made certain modifications in the design of the
proximal plate and named it the RPI bar clasp design.
• He had a 2-3mm of contact of the tooth with the guide plane, the
section below this point being relieved and he felt that the V
shaped space that is left underneath was not as detrimental as the
possible restriction of rotation.
• This design however, leaves a region occlusal to the proximal
plate where contact between the abutment and denture must be
made by the replacement tooth. 132
OTHER CLASP DESIGNS
RPA CLASPS
• The rest-proximal plate-Aker’s clasp was developed and described by Eliason in 1983.
• It consists of a mesial occlusal rest, proximal plate and a circumferential clasp arm, which arises from the superior portion of the proximal plate and extends around the tooth to engage the mesial undercut.
133
RPL CLASP• The effectiveness of RPI clasp assembly can
be increased if the I-bar is replaced by an L-shaped bar direct retainer.
• The retainer has been described as one-half T-bar or a modified T-bar clasp or R- bar clasp.
• The mesio-occlusal rest and proximal plate are designed as in the RPI clasp assembly.
• The L- bar crosses the gingival margin of the abutment tooth in the shortest possible line, ascends to the survey line and engages the disto-buccal undercut.
VRHR Clasp
• The vertical reciprocal horizontal retentive arm
concept was developed by Grasso in 1980 and is
characterized by:
• A distal occlusal rest supported by a minor connector.
• A lingual vertical reciprocal component originating from the
major connector.
• A horizontal retentive arm attached to either the major
connector or the retention latticework for the denture base.
135
CINGULUM CLASP
• Miller in 1972 designed a clasp to satisfy both the mechanical
and esthetic requirements without the shortcomings of the
internal attachment.
• The cingulum clasp has 2 lingual clasp arms. The use of this
clasp requires that the lingual surface of the abutment tooth be
covered with a gold casting.
• A guiding plane is incorporated into the distal surface of the
crown and the clasp is designed as an integral part of the rigid
metal framework. 136
Advantages• Esthetic• A tooth of short clinical crown can be used.• The young pulp is not imperiled by close proximity to metal
which shows thermal conduction.• Less expensive.
Disadvantage
The clasp arms are vulnerable to breakage.
UseUse
The cingulum clasp can be used as a retainer on cuspid teeth
when other extracoronal retainers are esthetically unacceptable
137
MGR clasp design McCartnay described the mesial groove reciprocation clasp
in an effort to use the distofacial surface of the maxillary canine for retention & esthetic.
Retention is provided by 19 gauges round I-bar and retentive dimple located at distobuccally on the tooth.
Reciprocation is provided by mesial groove or rest and distal proximal plate.
Spring Clasp/Twin Flex Clasp
• This consists of a wire clasp soldered into a channel that is cast in the major connector.
• The ability to adjust this clasp and its conventional path of insertion provides an excellent design option for retention to an adjacent edentulous segment.
Equipoise RPD system• Proposed by J. J. Goodman, it is an esthetic
retentive concept for distal extension situations.
• Rests are placed away from edentulous span.
• Vertical inter-proximal reduction of 1 mm between abutment and adjacent tooth is done.
• The unique Class II lever design protects, preserves and strengthens abutment teeth while directing all forces down the long axis of the abutment tooth.
FLEXIBLE PARTIAL DENTURES• Flexible partial dentures are made of a
flexible type of acrylic, which is a more esthetic option than the metal framework partials.
• They do not have a metal framework, which improves esthetics, but because the clasps are acrylic and not metal, they cannot be adjusted.
• This makes it more difficult to get an ideal fit, and because it cannot be adjusted, it may not stay in as well as one with metal clasps, which can be tightened as needed.
MAGNETIC DENTURE RETAINERS
A new retainer for removable dentures was designed which makes use of the concept of sectional denture and employs a magnetic attachment.
The use of a neodymium-iron-boron magnet enclosed in a laser-welded stainless steel casing is a viable method of retention for removable partial dentures.
OCCLUSALLY AND GINGIVALLY APPROACHING CLASPS:
Relative merits and demerits
Retention : The bar clasp approaches the undercut from
below the height of contour and to resist dislodgement,
the clasp pushes towards the occlusal surface of the
abutment tooth. The circumferential clasp engages the
retentive undercut from above the height of contour
and pulls towards the occlusal surface from the
undercut to resist dislodgement.
143
144
• Bracing : The circumferential clasp is rigid in the upper
two-thirds of the retentive arm and offers some bracing or
stabilization against lateral stresses. On the other hand, the
bar clasp is flexible throughout its length and does not
contribute to stability.
145
• Stress breaking effect : The gingivally approaching
clasp allows a certain degree of functional movement of the
distal extension base which helps to dissipate the stresses
and lessen the load on the abutment. Occlusally approaching
clasps have the potential to torque abutment teeth in distal
extension based partial denture situations.
146
• Contact with tooth structure : The gingivally
approaching clasp contacts minimum tooth structure and has a
minimum interference with natural tooth contour permitting
maximum natural cleansing action, whereas the occlusally
approaching clasps covers more of tooth structure. This
occlusal approach may increase the width of the occlusal
table.
147
• Damage to oral tissues : The area of food lodgement is
at the neck of the tooth, with the cementum in this area being
more likely to be affected by caries than enamel. Trauma to the
gingiva can also occur with bar claps unless sufficiently relieved.
Mishandling of the clasps by the patients during removal of the
prosthesis can result in deformation of the clasp and damage to
soft tissues.
148
• Esthetics : Gingivally approaching clasps are more
esthetic than occlusally approaching clasps except in
instances where large amounts of gingiva is visible on
smiling.
149
Relative merits & demerits of cast and wrought wire clasps
• Flexibility : The wrought wire clasp has a high degree of
flexibility which helps to better dissipate functional stress and
give a stress breaking effect.
• Adjustability : The wrought clasp is adjustable to the required
retention.
• Toughness and resiliency : The fabricating process imparts to the
wrought wire a fibrous structure which accounts for its toughness
and resiliency.
• Stabilization : The cast clasp on account of rigid crystalline
structure offers better stability. 150
• Cost : Cast clasp requires less cost as soldering is not required
• Fit : The adaptation of a cast clasp to the abutment tooth is
accurate and not subject to variation by the clinician’s
adjustment as with wrought wire clasps.
• Strength and durability : A solder joint near the origin of a
wrought wire clasp reduces its flexibility and increases the
likelihood of breakage.
• Apart from these, cast clasps can be formed to act as a bracing
element, can easily include an occlusal rest and be cast as an
integral part of gold alloy or a cobalt-chrome denture base.
151
INTRACORONAL
RETAINERS
152
• The intracoronal retainer is usually regarded as an internal
attachment or precision attachment.
• The Glossary of Prosthodontic Terms - 8 defines intracoronal
attachment as:
any prefabricated attachment for support and retention of a
removable dental prosthesis. The male and female components are
positioned within the normal contour of the abutment tooth.
153
154
GPT-8 Defines precision attachment as: a retainer consisting of
a metal receptacle (matrix) and a closely fitting part (patrix); the
matrix is usually contained within the normal or expanded
contours of the crown on the abutment tooth/dental implant and
the patrix is attached to a pontic or the removable dental
prosthesis framework.
As a direct retainer it must provide :
Support
Retention
Reciprocation
Stabilization
Fixation155
The precision attachment is the only type of intracoronal
attachment that provides for all three functions of a removable
partial denture system.
Lateral force transmission or bracing from the parallel proximal
walls of the rest against the rest seat.
Occlusal force transmission or support from the flat gingival
floor of the result on the rest seat.
Primary retention from the frictional fit between the rest and
rest seat.
156
Advantages of intra coronal retainers:
1. Esthetically acceptable, because not much of metal
display like extracoronal retainers.
2. It is preferred in many of the situation because of its
vertical support through a rest seat located more
favourable to the horizontal axis of the abutment tooth.
157
3. Horizontal stabilization to some extent. Similar to
internal rest, but extracoronal stabilization is
needed.
4. Stimulation to the underlying tissues greater when
internal attachment are used because of the
intermittent vertical massage.
158
Disadvantages of intra coronal retainers:
1. They require preparation of abutment tooth and
casting.
2. Difficult clinical and laboratory procedure.
3. They eventually wear, result in loss of frictional
resistance to denture removal.
159
4. Difficult to repair and replace.
5. They are effective in longer teeth and least effective
in shorter teeth.
6. Difficult to place completely in the abutment teeth.
160
Limitations of intra coronal retainers:
1. Large pulp size which is usually related to the age of
patient.
2. Length of the clinical crown, not used in short or
abraded teeth.
3. Expensive
4. Distal extension denture bases.
161
Classification
Classification by Good Kind and Baker in 1976 :
1)Intra coronal
a. resilient
b. non resilient
2) Extra coronal
a. resilient
b. non resilient
162
Gerardo Becerra et al in 1987 classified precision attachments as :
1) Intra coronal attachments
a. Frictional
- tapered and parallel walled boxes and tubes
- adjustable metal plates
- springs
- studs
- locks
b. Magnets 163
2) Extra coronal attachments
a. Cantilever attachments
- rigid attachments
- movable attachments
b. Bar attachments
164
Tapered And Parallel Walled Boxes And Tubes
• Designed to be used in FPD.
• Plastic pre fabricated patterns.
• Provides vertical support and lateral stabilization.
• Simple pin and tube or rectangular block and boxes.
E.g. : Mc Collum attachments.
165
McCOLLUM ATTACHMENTS
166
Adjustable Metal Plates
• Similar to block and box variety .
• Provided with a narrow slit in the metal block or male portion of the
attachment to increase the friction.
• Provides a simple and effective form of direct retention.
• At least 2.5 mm of tooth height is required.
• E.g.: Crismani attachment.
Mc Collum attachment.
Stern attachment
Chayes or Rley attachment.
167
CHEYES ATTACHMENT
CRISMANI ATTACHMENT
Springs
• Incorporated in the male part to control the friction.
• Spring activates a plunger rod which protrudes from male part
to engage a depression in the female part.
• Approximately 4 – 5 mm of vertical height is required.
• E.g.: Schatzmann attachment.
169
SCHATZMANN ATTACHMENT
170
Studs
• A metallic stud can be soldered to post and core and
cemented into an abutment.
• Direct retention can be obtained by using a stud which
clips into an flexible ring.
• Sufficient clearance is required to arrange the artificial
teeth.
• E.g.: Ceka attachment
Rotherman attachment171
Ceka attachment
172
The Ceka attachments was developed by Karl Cluytens in 1951. In 1968 the original attachment was modified to form the present Ceka attachment system.
The Ceka attachment unit consists of a male pin engaging the centre of a circular retaining element. The male retaining pin is attached to the denture and engages the centre of the retaining element that is joined to the abutment tooth.
Locks
• These lock rigidly into the attachments .
• The vertical height required for this attachment is at
least 6 mm.
• Retained with pins or incorporated in post and core
• E.g.: T - block attachment
173
T-ATTACHMENT
174
NEUROHR SPRING LOCK ATTACHMENT
175
Magnets
• Small metal keeper is attached to the tooth surface,
usually into the root canal and magnet is incorporated
into the resin.
• Alloy in the magnet produces a magnetic force that is
strong .
• Magnets are brittle and corrode unless protected in a
stainless steel shelf.
176
CANTILEVER ATTACHMENTS
Rigid attachments
They are pin and tube joints that use a slit in the pin or
multiple pin tubes and slots to enhance retentive friction
between the parts with the natural teeth on the either side of
the edentulous space.
These attachments offer excellent stability and
retention in tooth supported partial dentures.
e.g.: Scott attachment
Thompson dowel rest system.
177
Movable attachments:
• These allow the prosthesis to rotate around a horizontal axis and transmit occlusal forces to the residual alveolar ridge .
• E.g.: Dalbo attachments
178
Bar attachments
• These can be connected to the cast metal crowns or copings .
• Custom made bars can be cast with a flat upper surface to support the prosthesis and parallel sides that help to stabilize it.
E.g.: Dolder bar.
179
SUMMARY & CONCLUSIONS
180
• Function, esthetics and harmony of the dental structures have become one
entity. To develop an esthetically, technically and functionally adequate
restoration, it is essential to treat and prepare the remaining teeth.
• Provision for retention of a prosthesis in a partially edentulous situation
requires a choice between an extracoronal retainer and an intracoronal
retainer.
181
• Direct retainers for distal extension removable partial
dentures should be chosen after careful evaluation of each
individual situation and consideration of the merits and
contraindications of each clasp design. Once the retainer is
chosen, meticulous adherence to proper designing
principles will ensure a successful distal extension
removable partial denture.
“ The clasp was one of the earlier devices of dentistry; it is still
one of our simplest, and can be made one of our most
reliable devices. In conjunction with a correctly made lug, it
is the retaining appliance that can be used in a great
majority of instances, on natural teeth, with the least harm,
either from operative interference with their structure at the
time of application, or from deleterious influence in the
decades that follow.” Edward Kennedy 1932.
BIBLIOGRAPHY• Applegate O.C. : Text book of Removable Partial Prosthodontics, St. Louis,
CV Mosby Co.
• Becker C.M., Campbell H.C., Williams D.L., “The Thompson Dowel-rest
system modified for chrome-cobalt RPD frame works”. J. Prosthet. Dent.,
1978; 39 : 384-391.
• Brudvik J.S., Morris H.F. “Stress relaxation testing. Part II : Influence of
wire alloys, gauges and lengths of clasp behaviour”. J. Prosthet. Dent., 1981;
46 : 374-379.
• Clayton J.A., Kotowicz W.E. “Precision attachments”. D.C.N.A., 1980; 24 : 1.
• Stewart, Rudd and Kuebker: Clinical removable Partial Prosthodontics ,2nd
edn, EuroAmerica Inc, publishers Tokyo, 1997
184
• Cecconi B.T., Asgar K., Dootz F. “The effect of partial denture clasp
design on abutment tooth movement”. J. Prosthet. Dent., 1971; 25 : 44-55.
• Davenport J.C., Baskar R.M., Heath J.R., Ralph J.P. “A color atlas of
RPD”, Wolfe Medical Publications Ltd., 1988.
• Graber G., Haesler V., Weill P. “Color atlas of dental medicine RPD”.
Vol.2, Medical Publishers Inc., New York, 1988.
• Henderson D., McGivney G.P., Castleberry D.J. : McCracken’s
removable partial prosthodontics, 8th Edn. St. Louis ; CV Mosby Co.
• Krol A.J. “Clasp design for extension base RPD”. J. Prosthet. Dent., 1973;
29 : 408-415.
• Miller E.L., “The cingulum clasp”. J. Prosthet. Dent., 1972; 28: 369-372.
185