direct retainers in removable partial dentures
DESCRIPTION
direct retainers in removable partial denturesTRANSCRIPT
DIRECT RETAINERS
By
Dr Shebin Abraham
Dept. of Prosthodontics
Contents
Introduction
Definitions.
Classification.
Basic part of clasp assembly.
Analysis of tooth contour.
Basic Principles of clasp design.
Cast circumferential clasp.
Bar clasp.
Other types of clasp.
Intracoronal retainers.
Conclusion.
Introduction
Types of retention
Primary retention.
1. By clasps
Secondary retention.
2. Acting through
polished surface of the
denture.
3. Tissue coverage.
Definition:-
Direct retainer:-
“That component of a partial removable dentalprosthesis used to retain and prevent dislodgment,consisting of a clasp assembly or precisionattachment” GPT-8
Direct retention :
“Retention obtained in a partial removable dental prosthesis by the use of clasps or attachments that resist removal from the abutment teeth” –GPT-8
Classification Of Direct Retainers.
Intra coronal Extra coronal
Precision attachment
Semi precision
attachment
Retentive clasp
assemblies
Attachments
Supra bulge clasp
Infra bulge clasp
Clinical removable prosthodontics:- STEWART’S 3rd edition
Intracoronal
Proposed by Dr Herman E S Chayes in 1906.
Cast or attached within the contours of natural teeth(abutment).
Keyway and key…..Opposing vertical walls provides retention.
Retention is achieved by frictional resistance.
1. Precision attachment manufactured by high precision technique andinstruments
2. Semi precision attachment less intimate contact between matrix andmatrix component. Unlike precision attachment they consist oftapering walls and are casted from wax or plastic pattern.
Extracoronal attachment.
First proposed by Henry H Boos 1900 later modified byEwing F Roach in 1908.
Located outside the teeth.
Retention frommechanical resistance.
Permit vertical movement during vertical loading.
Minimize potentially damaging forces to abutment Stressbreaking or stress directing effects.
Occlusally approaching / Suprabulge / Ney Type I clasp / Circumferential:
• Approaches the tooth
undercut from an occlusal
direction
• It is attached to metal
framework above the
height of contour.
Gingivally approaching / Infrabulge/ Bar/ Roach / Ney
Type II Clasp:
• The retentive arm originates
from the metal base or
denture framework
traverses soft tissue and
• Approaches the tooth
undercut area from a
gingival direction.
EXTRACORONAL DIRECT RETAINERS
The basic parts of a clasp assembly include the following:
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 by ensuring
that the retentive terminals of the clasp remain fixed in the desired or
planned depth of undercut.
RES
T
Body of the clasp
It is the part of the clasp that connects the rest and shoulder of the clasp
to the minor connector.
It must be rigid.
Above the height of contour.
Shoulder
It is the part of the clasp that connects the body to the clasp
terminals.
The shoulder 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.
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
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.
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.
Analysis of tooth contour:-
Before moving on to principles of clasp design its vitallyimportant to consider how tooth contour & RPD componentsmust be related to allow a stable prosthetic function.
What is path of insertion??? path of removal ??and height ofcontour???
Point of maximum convexity or the term height of contour Dr Edward Kennedy in 1985.
This critical area of an abutment that provide for retention &stabilization can only be identified with the use of dental castsurveyor.
Prothero’s concept
Proposed “cone theory” of clinicalcrown in 1916.
Provided conceptual basis of mechanicalretention.
Contours of clinical crown resembles twocones sharing a common base.
The line formed at the junction of thisbase represents the greatest diameter ofthe tooth.
This greatest diameter is called height ofcontour or point of maximum convexity.
Devan [1955] referred to thesurface occlusal to the height ofcontour as suprabulge, & thesurface inclining cervically asinfrabulge.
suprabulge
infrabulge
Angle of convergence
o When the surveyor blade contacts a tooth on the cast at itsgreatest convexity, a triangle is formed.
o The apex of this triangle is at the point of contact of surveyorblade with the tooth and base is towards the gingival tissues.
o This apical angle is called angle of cervical convergence. Theimportance of this angle lies in its relationship to the amountof retention.
Basic Principles of clasp design
According to Stewart basic principles are:-
1. Retention.
2. Stability.
3. Support.
4. Reciprocation.
5. Encirclement.
6. Passivity.
Retention
“Retention is the inherent quality of the claspassembly that resists forces acting to dislodgecomponents away from the tooth structure.”
No single component of a clasp assembly is solely responsiblefor prosthesis retention.
Rather, it is effective design and accurate construction thatmake the removable partial denture retentive.
The amount of retention designed into a removable partialdenture should always be the minimum necessary to resistreasonable dislodging forces.
A rigid clasp flexing over the height of contour may transferharmful stresses to an abutment during insertion, removal, andfunctional movement of the prosthesis.
An only a minimum area of contact should be seen.
Amount of retention
Factors that effect retention can be divided into -
1. Tooth factors
Size of angle of cervical convergence
How far clasp is placed in angle of cervical convergence.
2. Prosthesis factors
Clasp length
Clasp diameter
Clasp cross-sectional form[ shape]
Material used for making clasp[ alloy]
Prosthesis factors:-
Length of clasp arm-
Longer the clasp arm the more flexible it will be.
Circumferential clasps more retentive than bar clasp for a given clasp length.
The clasp arm should taper from the point of origin to provide its flexibility.
Diameter of the clasp:
The greater the diameter of a clasp arm the less flexible itwill be.(only in uniform taper)
If its taper is absolutely uniform ,the avg diameter will be ata point midway between its origin & its terminal end.
But if taper is not uniform a point of flexure –therefore apoint of weakness will exist.
Cross-sectional form: Flexibility may exist in any form, but is limited to only one direction in the case
of the half-round form
The only universally flexible form is the round form.
Clasp arm should only flex away from tooth so half round is used.
Round shaped clasp arm used only in distal extension denture bases so that itcan flex in all directions during functional movement and minimize stresses.
Material used for construction:
Gold alloy greater flexibility than chrome alloys ,
Disadvantage of cast gold alloys bulk of the prosthesiscostly.
Chrome alloys have a higher modulus of elasticity & thereforeless flexibility.
So in less undercut areas CoCr alloy can be used but in casesof deep undercut wrought metal can be used.
Relative uniformity of
retention:
1. Retention on all principal abutments should be as equal aspossible.
2. Retentive clasp arms should be located so that they lie in thesame approx.. Degree of undercut on each abutment tooth.
3. Retentive clasp positioning should also be same on thecontralateral side of arch.(atleast for one teeth).
Support
“Support is the quality of a clasp assembly that resistsdisplacement of a prosthesis in an apical direction.”
Provided by occlusal rest.
1. A properly prepared rest seat and corresponding rest serve to resistdisplacement of the prosthesis toward the supporting teeth and softtissues, thereby ensuring that the clasp assembly maintains its intendedrelation to the abutment, and
2. Transmit functional forces parallel to the long axes of the abutments.
Stabilization.
“Stability is the quality of a clasp assembly that resistsdisplacement of a prosthesis in a horizontal direction.”
It helps the denture be steady constant firm and resistdisplacement due to function stresses and also preventchange in position of the denture.
It is provided by :-
1. Reciprocal element.
2. The shoulder(s) of a cast circumferential retentive clasp.
3. Vertically oriented minor connectors.
Reciprocation
“Reciprocation is the quality of a clasp assembly that counteracts lateral displacement of an abutment when the retentive clasp terminus passes over the height of contour.”
As the retentive arm passes over the height of contour it flexes creating lateral forces damage to the tooth.
The reciprocal element may be a
1. Retentive arm of clasp
2. Lingual plating,
3. Combination of mesial and distal minor connectors.
Points to be remembered while
providing reciprocation
1. To optimize reciprocation, the axial surface of an abutmentshould be prepared parallel to the path of insertion and removal.
2. It should be placed above the height of contour.
3. To provide true reciprocation, the reciprocal clasp arm must bein contact during the entire period of retentive claspdeformation.
Encirclement.
“Encirclement is the characteristic of a clasp assembly that prevents
movement of an abutment away from the associated clasp assembly”.
Clasp assembly 180 degrees contact.
The engagement can be in the form of continuous contact, such
as circumferential clasp, or discontinuous contact, such as Bar
clasp.
Both provide tooth contact in at least 3 areas encircling the
tooth-
Passivity.
“Passivity is the quality of a clasp assembly thatprevents the transmission of adverse forces to theassociated abutment when the prosthesis iscompletely seated.”
When fully seated it should be passive.
Should be activated only when dislodging forces are applied.
Other principles
1. Retentive clasp element should always be placed on facial surface of tooth.(mainly in premolars)
2. Only one retentive element should be used per tooth opposed by a reciprocal element.
3. Clasp retainers on the abutment adjacent to distal extension bases should be designed so they will avoid direct transmission of tipping & rotational forces to the abutment.
4. The path of escapement for each retentive clasp terminal must be other than parallel to the path of removal for the prosthesis to require clasp engagement with resistance to deformation.
CRITERIA FOR CLASP SELECTION
Survey line
Requirements of retention and stability depending on the number, configuration of edentulous areas.
Nature of support
Root size and form
Esthetics
Presence of excessive tissue undercut
Oral hygiene and patient awareness
Cast circumferential clasp.
First proposed by Dr N B Nebbit. Later modified by Dr Polk E Aker.
Simple and easy to construct
Large amount of tooth surface that is covered by the clasp assembly leads to enamel decalcification.
Design rules..
1. The clasp should arise from the main body of the claspassembly above the height of contour. The retentive armshould extend cervically and circumferentially in a gentlyarcing manner.
2. All the components of the C clasp should be present abovethe height of contour except the retentive tip.
3. The retentive terminus should always be directed towardsthe occlusal surface never towards the gingiva.
4. It should always terminate at the mesial or distal line anglenever at midfacial or midlingual surface.
5. The retentive arm should be positioned as far apically on theabutment as is practical.(not impinging the gingiva)
6. Special considerations in case of distal extension case:-
A cast circumferential clasp should not be used to engage (a) themesiofacial surface of an abutment adjacent to a posterior edentulousspace or
the distofacial surface of an abutment adjacent to an extensive anterioredentulous space.
Simple circlet clasp.
It is one of the most commonly used clasps retentive and stabilizing ability.
The basic design consist of
1. buccal retentive arm and
2. a lingual reciprocal arm originating from a common body.
3. Occlusal rest.
With retentive arm projecting away from the edentulous space.
Advantages:-
1. Fulfils the design requirements of support, stability, reciprocation, encirclement, and passivity.
2. Its uncomplicated design features make it easy to construct and relatively simple to repair.
Disadvantages:-
1. If used in distal extension base due to fulcrum rotation of the clasp it can cause damage to teeth.
2. Can increase the circumference of teeth and lead to food accumulation and decalcification.
Reverse circlet clasp
Used when undercut is located at the facial distoangle adjacent to an edentulous space.
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.
Advantages:-
Decreases the harmful stresses to teeth.
Disadvantages:-
Week clasp if sufficient preparation is not done.
Poor aesthetics' if used in premolars and cuspids.
Multiple circlet clasp:
A multiple circlet clasp design involves two simple circletclasps joined at the terminal aspects of their reciprocalelements.
Used in periodontally weekend teeth to splint them
Disadvantages are same as circlet clasp.
Embrasure clasp
Also known as the Bonwill clasp.
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.
Design:-
Double occlusal rests, two retentive clasp arms and the two reciprocal clasp arms either bilaterally or diagonally opposed.
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.
Disadvantages:-
Improper clearance can give rise thin section of clasp leading to breakage.
Ring clasp
Circumferential clasp encircles nearlyall of the tooth from its point of origin.
It is usually used when a proximalundercut cannot be approached by anyother means.
In case of tilted molars
A support strut is provided on the nonretentive arm.
Advantages:-
Provides adequate encirclement.
excellent retention with adequate flexibility due to increased length of clasp arm
Disadvantages:-
Decalcification of teeth
Increased occlusal table.
Poor structure of clasp.
Hairpin clasp or reverse action or
fish hook clasp
A simple circlet clasp in which the retentive arm loops back to engage an undercut apical to the point of origin.
Used when a distofacial undercut is present adjacent to the edentulous space.
Retentive arm has two horizontal components.
1. The occlusal portion minor connector and must be rigid.
2. The apical portion pass over the height of contour to engage the desired undercut.
Consideration:-
Sufficient clinical crown height.
Space between occlusal and apical arm.
Occlusal arm shouldn’t interfere within the occlusion
This clasp is indicated when the
1. soft tissue contour precludes use of a bar-type clasp and
2. when the reverse circlet cannot be considered becauseof a lack of occlusal clearance.
Half and half clasp
It consists of a circumferential retentive arm arising from one direction and a reciprocal arm arising from another.
used only for unilateral partial denture design.
Combination clasp
Given by O C Applegate.
This type of clasp consists of a wrought wire retentive clasp arm and a cast reciprocal clasp arm.
Wrought retentive arm is circular in cross section.
Used in deep undercut case.
Onlay clasp:-
Indicated when the occlusal surface of the abutmentlies noticeably apical to the occlusal plane.
occlusal surfaces of the abutments are covered withcrowns clasp arms arises
It establishes the occlusal plane.
Indicated in caries free individual.
Occlusal surface should be restored with gold acrylicinserts.
Infrabulge clasp/bar clasp/roach clasp.
Popularized by Ewing Roach in 1930 called it the Bar Clasp.
An infrabulge clasp approaches the undercut region of an abutment from an apical direction.
Push type retention.
Flexibility of clasp from length and taper.
More aesthetic than c clasp.
Ex :- y clasp, t clasp, I clasp.
Design rules…
1. The approach arm of an infrabulge clasp must not impinge onthe soft tissues adjacent to the abutment.
2. The approach arm should cross perpendicular to the freegingival margin. It shouldn’t impinge the underlying gingiva.
3. Shouldn’t be used in area of tissue undercut.
4. Uniform length and adequate taper should be given forsufficient flexibility.
5. The clasp terminus tip should be placed as apical as possibleon the abutment teeth.
T clasp
Name is from the shape of the retentiveterminal.
Used in class 1 and class 2 situation.
distofacial undercut is seen.
The retentive terminal consist of horizontaltwo projection the one on the distal sideengages the undercut and the one on themesial side is above the height of contour.
T clasp is contraindicated when the height ofcontour is at the occlusal one thirds.
In modified t clasp the non retentive arm isabsent.
Y clasp is similar to t clasp with the approacharm ending cervical to the retentive arm.
I clasp lack the horizontal retentive arms butonly a horizontal retentive tip.
Only the retentive tip contacts the abutmentsurface only at the undercut region.
The amount of contact is about 2 to 3mm inheight and 1.5 mm in width.
Other types of clasp philosophies.
RPI,
RPA
DeVan’s Clasp
VRHR clasp or Grasso clasp
Euipose clasp
RPI concept.
RPI stands for Rest Proximal plate I bar.
Introduced by Kratochvil in 1963 it consisted of threedifferent parts connected to the metal framework.
Mesial occlusal rest, a distal guide plan, and an I barretainer.
The guide plan contacts the full length of the of theproximal surface of the tooth.
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.
Krol in 1973 made certain modifications in the design
under the “minimal coverage criteria”
Rest preparations are less extensive in the RPIsystem.
Rests extend only into the triangular fossa, even inmolar preparations, and canine rest
2-3mm guide plane in which only 1mm contact wasseen from the guide plate.
Arthur Krol JPD 1973;23;408-415
BASIC PRINCIPLES OF RPI
CONCEPT
The mesiobuccal rest with the minor connector is placed
into the mesiolingual embrasure, but not contacting the
adjacent tooth.
A distal guiding plane, extending from the marginal ridge
to the junction of the middle and gingival thirds of the
abutment tooth, is prepared to receive a proximal plate
The proximal plate in conjunction with the mesial occlusal
rest and minor connector provides the stabilizing and
reciprocal aspects of the clasp assembly.
The I-bar contributes to the retentive aspect and should be
located in the gingival third of the buccal or labial surface of
the abutment in 0.01 inch undercut.
Three different approaches
to RPI clasp
These approaches are based on the location of proximal plate ,location of the I bar
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.
De VAN CLASP
Also called as the mirror view clasp
Two occlusal rest on the lingual side of the teeth.
M.M.DeVan JPD 1955;5,208-14
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.
Joseph Grasso JPD 1980,43;618-21
Equipoise clasp
Proposed by J. J. Goodman in 1990, 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.
Optional bucco-lingual retentive groove at mid and gingival
third junction on distal surface of abutment tooth is provided.
Quintessence Int. 1996
May;27(5):333-40.
Metal free-
1. Natural-flex
2. Optiflex
3. Proflex clear wire
4. Valplast
5. Cu-sil
Occlusally and gingivally approaching clasps:
Relative merits and demerits
Bar claspThe bar clasp approaches the
undercut from below the height of contour pushes towards the occlusal surface abutment tooth
• Easier to seat and more difficult to remove
Circumferential clasp
• Above the height of contour • Pulls towards the occlusal
surface from the undercut to resist dislodgement.
retention
Bar clasp
It is more flexible because of which it provides less bracing or stability against lateral stresses.
Circumferential clasp
• Because of its rigidity it provides very good stability or
bracing
bracing
Bar clasp
It allows a certain degree of functional movement of the distal
extension base dissipate the stresses and lessen the load on
the abutment
Circumferential clasp
Potential to torque abutment teeth in distal extension based
partial denture situations.
Stress breaking effect
Bar clasp
Minimal tooth contact and less damage to tooth
Circumferential clasp
More tooth contact leading to food accumulation
Contact with tooth structure
Bar clasp
Minimum relief can lead to tissue damage to mucosa under the approach arm
Circumferential clasp
Damage to gingiva can take place during improper
removal of clasp
Damage to oral tissues
Bar clasp
Very less metal display so high aesthetics
Circumferential clasp
Increased metal display so poor Esthetics.
Esthetics
INTRACORONAL RETAINERS/
precision attachments
The intracoronal retainer is usually regarded as an internalattachment or precision attachment.
Definition:-
“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 and the patrix is attached to a pontic or the removable
partial denture framework.”
INDICATIONS
To provide movable joint in Removable Bridgework, fixed removable bridges.
To stabilize unilateral saddles.
Pier abutments.
Titled molars. F.P.D's in severely misaligned abutments.
Use in Over dentures (different forms of retainer are bar, telescopic, use of auxillary attachments).
Fixed removable implant restorations.
LIMITATIONS
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.(6mm crown)
3. Expensive
4. Distal extension denture bases.
ADVANTAGES
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 favorable
to the horizontal axis of the abutment tooth.
3. Stimulation to the underlying tissues greater when internal
attachment are used because of the intermittent vertical
massage.
4. It permits proper tooth form to be maintained and allows
for control of vertical, mesiodistal and buccolingual
displacement of the prosthesis.
5. It provides for excellent retention of the prosthesis on
account of frictional resistance between opposing
parallel vertical walls that serve to limit movement and
resist removal of the partial denture.
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.
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.
MATERIAL OF CHOICE
Pt, Iridoplatinum, Gold and Pt, Gold and Pd.
Type III and IV type of gold is to be used for crown castings.
Base metal alloys are also used now a days as low cost
2 alternate ways to construct crown and rest seat.
Rest seat may be cast against the full coverage restoration
Rest seat may be soldered into place.
Classification
Classification by Good Kind and Baker in 1976 :
1)Intra coronal
a. resilient
b. non resilient
Intracoronal retainers
Frictional resistance.
Tapered and parallel-walled boxed and tubes.
Adjustable metal plates
Springs
Studs:
Locks
Magnetic resistance.
How To Choose An Attachment?
It is the length of attachment, not its width that is main criteria inchoosing attachments.
For each length there are 3 different sizes (width) of precision attachments(anterior, bicuspid, molar.)
Width is measured from one side of rest to other.
Full length of a precision attachment is 8 mm for full benefit of bracing,support and retention a minimal of 5mm height is must.
This means that the clinical length of crown must be at least 7 mm so as toaccommodate an attachment of 5mm and in addition a minimum of 2mmbetween the gingival floor of attachment and gingival margin.
Otherwise a periodontal problem may be created.
Precision attachment
selection
Kennedy’s class I and class II partially edentulous
arches
The most difficult type of treatment plan.
Some practitioners advocate non rigid and resilient attachments and some advocate
resilient attachment in distal extension to minimize rotation and torquing of the
abutment tooth, when the components of an attachment are rigidly connected.
Int J Prosthodont. 1990 Mar-Apr;3(2):169-74
Another philosophy , known as the stable base precision attachment
RPD concept or floating denture base concept recommends
incorporation of rigid internal attachments and a cast metal base
made from mucostatic impression of the residual ridge.
The male portion of the attachment is connected to the denture
base , allowing the complete seating within the abutment.
Kennedy’s class III partially edentulous arch.
• Rigid internal attachments are recommended .
• Provides good retention, support and brazing because of its rigid
interlocking components.
• If the posterior abutment prognosis is questionable then a resilient
type of attachments are recommended with anterior abutment.
Kennedy’s class IV partially edentulous arch
The ideal RPD design for such situation
involves the use of a tissue bar placed close to
the edentulous ridge and connected as a fixed
unit to the abutment teeth on either side of
the space using crowns.
Conclusion
Keep the prosthesis design as simple as possible…
Make RPD more comfortable, more efficient andaffordable to the patient.
References
1. Clinical removable prosthodontics:- STEWART’S 3rd edition
2. Mc cracken removable partial denture prosthodontics – 12th edition.
3. Davenport J.C., Baskar R.M., Heath J.R., Ralph J.P. “A color atlas of RPD”, Wolfe Medical Publications Ltd., 1988.
4. Krol A.J. “Clasp design for extension base RPD”. J. Prosthet. Dent., 1973; 29 : 408-415.
5. M.M.DeVan JPD 1955;5,208-14
6. Joseph Grasso JPD 1980,43;618-21
7. Quintessence Int. 1996 May;27(5):333-40.
8. Burns DR, Ward JE. Int J Prosthodont. 1990 Mar-Apr;3(2):169-74
Thank you