Post on 08-May-2017
I. INTRODUCTION
Partial Denture Rest - A rigid extension of a fixed or 'removable partial
denture that prevents movement towards the mucosa and transmits
functional forces to the teeth.
According to glossary of Prosthodontic terms -
1.Rest - is a projection or attachment, usually on the side of an object.
2 .Rest seat - is the prepared recess in a tooth or restoration created to
receive the occlusal, incisal, cingulum or lingual rest.
According to Earnest L. Miller -
I .Rest - is a projection of the clasp which lies in a prepared recess of the
abutment tooth and acts to support and stabilize the removable partial
denture
2. Rest seat - The prepared recess in a tooth created to receive the occlusal,
incisal or lingual rest.
According to Mccrackens -
1 .Rest - Any unit of a partial denture that rests on a tooth surface to
provide vertical support.
2 .Rest seat - The prepared surface of an abutment to receive the rest.
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II. HISTORICAL RIVIEW
1) It was in 1899 that Bonwill recommended the use of rests on
removable partial denture which have been universally
considered inviolate.
2) Henderson D, Steffel V Miller and McCracken stated that the
principle function of the rest is to transfer vertical occlusal
stresses to the abutment teeth and to prevent movement of the
removable partial denture in a cervical direction.
3) Mac Gregor and Stewart stated that rests maintain clasps in
their correct position prevent food impaction and prevent the
denture from sinking into or causing over displacement of the
soft tissues.
4) Thayer and KrateSchvil stated that when all forces are borne by
the removing bone bone and soft tissues more rapid bone
resorption will occur. And rest reduces the bone resorption.
III. ENGINEERING THE REST PREPARATION
In order for tooth support to be optimally effective, it must be
provided by sturdy rests, placed in properly engineered recesses, in the
surfaces of the teeth. The planning and preparation of the recess should.
be carried out in consonance with well established bioengineering
principles. A fundamental fact is that periodontal ligament is not designed
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by nature to provide a cushioning effect for the tooth but on the contrary, is
a suspensory ligament by means of which the tooth is suspended in its
alveolus.
Thus it may be seen that a horizontal stress applied against the tooth
will be resisted by fewer than half of the periodontal membrane fibres,
whereas a vertical stress will be resisted by all of the fibres with the
exception of those at the apex.
The forces which act on the tooth in a direction along its long axis-
are transferred by the periodontal ligament to the bone as tension, which is
tolerated quite well. In contrast to this, the transverse or torsional stresses
that are transmitted to the tooth are transferred to the periodontal ligament
and to the bone as pressure, which will not be well tolerated. Depending
on the magnitude and the duration of the stress, the result may be crushing
of the periodontal ligament or even necrosis and bone resorption. Thus
applying this principle to the foundation for the rests, it becomes apparent
that the recess should be prepared within the confines of the greatest tooth
mass, so that force directed against the tooth will be resisted by the greatest
number of periodontal fibres. Another important point to be noted is that
the floor of the recess should be perpendicular to the long axis of the tooth,
so that stress will be directed axially and so that torsional stresses are
reduced to a minimum.
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IV. FUNCTIONS 07 THE RESTS
1) The primary purpose of the rest is to provide vertical support for the
partial denture and thus resist the movement in a cervical direction. '
2) In doing so it also maintains components in their planned positions
maintains established occlusal relationships by preventing
setting of the denture
3) Prevents impingement of soft tissues
4) Directs and distributes occlusal loads to the abutment teeth
It transmits vertical load as well as the lateral load to the tooth.
I. TRANSMISSION OF VERTICAL LOAD
When vertical load is applied to a bounded saddle which is
supported at both ends by occlusal rests, pressure is exerted against the
rested teeth. This results in a stretching of the periodontal fibres and an
eventual stressing of the bone that surrounds the teeth.
The proportion of the magnitude of load that will be transmitted to
the teeth on which the rests are placed can be varied -which was discussed
by chick.
For e.g. rests placed at equal distances from the middle of the
saddle'-will, assuming the load to be applied to the middle of the saddle,
transmit an equal load to the teeth on which they are placed. According to
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this, the load on the premolar can be reduced and that on the molar
increased by moving the rest to the mesial aspect of the premolar and this
may be desirable as molar teeth can accept greater loads than premolars.
Thus, it may be stated as a general principle that if it is necessary to
decrease the magnitude of the load on a tooth, then the rest should be
moved away from the saddle concurrently this will increase the load on the
other tooth by an equal amount.
Thus transmission of the vertical load to the tooth with the help of
rests, preserves bone o£ the edentulous ridge and also helps to prevent -
1) Breakdown of the periodontal membrane
2) Loss of correct occlusal relationships
3) Loss of correct position of clasp arms
II. TRANSMISSION OF LATERAL LOAD
When a saddle carries occlusal rests at each end they may aid in the
distribution of lateral load to the abutment teeth. If the occlusal surface of
the abutment is flat, then no lateral stress is communicated to it, whereas if
the rest fits accurately into a box shaped preparation the lateral load is
transmitted entirely to the tooth. When the occlual rest is placed in
contact with the sloping walls of the cusps of the tooth, an intermediate
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condition exists and some of the lateral force is conveyed as lateral stress to
the teeth.
It is more usual to provide saucer shaped rest seat
preparations which allow a very slight lateral movement of the rests over
the. teeth. This allows the flanges of the saddles to compress the mucosal
tissues and transfer a proportion of stress to the underlying bone, thus
sharing the load between the ridge and the abutment teeth.
III. PREVENTS IMPINGEMENT OF SOFT TISSUE
If rests are not placed - there may be bone resorption and as the
saddle sinks, damage to the related gingival margin and periodontal
membrane ensues. The pressure of the saddle on the gingival margin has
two effects.
1) Gingivitis is initiated which is liable to develop into a
periodontal involvement of the abutment tooth and hasten its loss
2) As the saddle sinks it effects a mechanical stripping of the gingiva
and periodontal membrane from its tooth attachment and later tooth
loss. Because of these two processes a partial denture which has
ho occlusal rests in its design has become known as "Gum stripper".
Thus a rest is a controlling factor in the triad of prosthesis -
tooth - periodontium.
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IV. It maintain the components especially clasps in their position on
the tooth so as to maintain a desired tooth clasp and tooth base relationship.
It provides reciprocation and bracing. Bracing is the provision of resistance
to lateral displacement of the denture and is a function performed jointly by
the rest and by the clasp arms in a clasp unit. Reciprocation in anterior
tooth by a rest. This is seen most commonly where a cingulum rest on a
canine acts as reciprocation for a gingival ly approaching retentive clasp
arm placed on the labial surface of the tooth.
It prevents a spreading of the clasp arms, with subsequent
displacement of the clasp and the prosthesis.
V. Deflection of the food
The occlusal rest covers a space which might otherwise exist
between the saddle and abutment.
A free end lower saddle lightly clasped to its abutment tends, in
function, to be displaced backwards, opening up the potential apace
between saddle and tooth, into which food is liable to be packed. This may
because of caries on the proximal surface of the tooth or more often
result in a traumatic gingivitis. When an occlusal rest covers-this space,
entrance of food is prevented since it is deflected buccally or lingually.
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VI. Improves the occlusion
An occlusal rest can be shaped to improve the existing occlusion. A
common example of this is when a rest placed on a tilted tooth is built up
to give larger contact with opposing teeth.
VII. A rest may serve to prevent supra eruption of the tooth on which it is
placed where no opposing tooth is present to perform this function.
VIII. A rest may act as an Indirect retainer
All types of rests, (occlusal, cingulum, incisal) may function as
indirect retainers when placed on the opposite side of the fulcrum axis to
that on which the displacing saddle is situated A.A.Grant/Johnson.
An example of this application is seen in the design of partial
denture for kennedy class iv upper dentition.
During the mastication of sticky foods, there will be tendency for
the saddle to move away from the underlying tissues, the denture rotating
about an axis joining the rests placed for saddle support. This tendency
for rotation can be resisted by placing occlusal rests bilaterally on the
posterior aspects of the last standing teeth. The posterior rests are joined to
the saddle by means of anteroposteriorly directed bars lying on the palate.
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AUXILLARY REST
The most frequently used indirect retainer is an auxiliary rest
located on an occlusal surface as far away from the distal extension base as
possible. In case of Kennedy class I arch, they are placed on mesial
marginal ridge of first premolar though the longest perpendicular to the
fulcrum line would be in the vicinity of central incisors which are too
weak. Bilateral rests on first premolars' are quite effective, even though
located closer to the axis of rotation.
In class II partial dentures they are placed on the opposite first
premolar tooth of the distal extension base.
AMINE extensions from occlusal rests
Such an extension is used to effect indirect retention by
increasing the distance of resisting element from the fulcrum line.
Canine rests
When mesial marginal ridge of the first premolar is too close to the
fulcrum line then a rest on adjacent canine tooth may be used.
Rests can be discussed under two headings - Anterior and
Posterior rests.
The anterior rest design has the basic support area as close to the
centre of the tooth as possible.
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1. The ideal anterior rest fulfils the following requirements:
1) The centre is deeper than the surrounding rest surface
2) It is rounded in all aspects (no sharp line angles)
3) There is easy access for impression making and cleaning
4) There are no undercuts
5) It is placed as close to the gingiva and bone as' possible to reduce
leverage
6) There is no interference with planned occlusion
7) It is contoured so that when increased force is applied to the
prosthesis the rest will engage more securely to prevent separation.
8) It is positioned in line with the residual ridge
2. The ideal posterior rest fulfils the following requirements
1) It provides rigid support
2) It extends to the centre of the tooth-in tooth supported situations
3) All aspects are rounded, with no sharp angles (for ease of cleaning
and making impressions and to prevent tooth fracture)
4) The end of the rest is slightly deeper and rounded.
5) There are no undercuts in path of insertion
6) It restores the occlusal plane
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7) It provides reciprocation
OCCLUSAl. REST
Is a rigid extension of a removable partial denture that contacts the
occlusal surface of a tooth or restoration, the occlusal surface of which
may have been prepared to receive it.
FORM OF OCCLUSAL REST AND REST SEAT
General configuration.
1) The outline form of an occlusal rest seat should be a
"rounded" triangular shape with the apex towards the centre of the
occlusal surface
2) It should be as long as it is wide and the ideal width for the occlusal
recess is approximately one-half the measurement between buccal
and lingual cusp tips of the bicuspids and slightly less for molars
3) When the tooth is in normal alignment, the recess should be
centered over the rest of the residual ridge although it is not possible
when the teeth are markedly rotated or tipped.
4) It is important that the proximo occlusal line angle of the
preparation not be sharp but have a smoothly flowing contour of a
waterfall.
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The reasons for this:
a) If the angle is left sharp, the rest may be too thin at this
point, hence vulnerable to fracture.
b) The resulting short, unprotected enamel rods are
susceptible to breakage.
c) Because the distal extension base will inevitably move to
some extent in function, the entire bearing area of the recess
should be formed So as to allow the clasp a slight degree of
movement without transmitting a torsional stress to the tooth.
Thus there is no place in a properly prepared, recess for sharp
angles.
5) Depth of the recess
1) The base o£ the triangular shape should be at least 2.5 mm for both
molars and premolars. Rest seats of smaller dimensions do not
provide for an adequate' bulk of metal for rests, especially if the rest
is contoured to restore the occlusal morphology of the abutment
tooth.
2) The marginal ridge of the abutment tooth at the site of the rest seat
must be lowered to permit a sufficient bulk of metal for - strength
and rigidity of the rest and minor connector.
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This then means that a reduction of the marginal ridge of
approximately 1.5 mm is usually necessary.
3) The floor of the occlusal rest seat should be apical to the
marginal ridge and occluaal surface and should be concave or spoon
shaped.
4) The angle formed by the occlusal rest and the vertical minor
connector from which it originates should be less than 90° only in
this way can the occlusal forces be directed along the long axis of
the abutment tooth. An angle greater than 900 fails to transmit
occlusal forces along the supporting axis of the abutment tooth.
It also permits slippage of the prosthesis away from the abutment
and causes orthodontic forces applied to an inclined plane.
5) If the recess is being prepared in an uncovered tooth surface, it
should not be so deep as to penetrate the enamel. If clearing the
opposing occlusion require a depth of the recess that risks
penetration of the enamel into the underlying dentin a gold
restoration should be placed in the tooth. If the tooth cannot be
restored in gold then the height of the opposing cusp in order to gain
inter occlusal clearance.
6) Rest seats are preferably not prepared opposite supporting cusps,
as the opposing teeth will change position and the space provided by
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rest seats will be obliterated by the time denture is fitted. If it is
necessary then .place them a little to one side, or flatten the tip of a
supporting cusp to provide enough room for occlusal rest but yet
allow the sides of the cusp to contact the- opposing tooth at the
edges of the rest seat.
7) If the penetration of the enamel does occur during the
preparation of the recess despite due precautions, a small cavity is
made and filled with amalgam or gold foil.
8) Fissures and grooves which lie adjacent to the boundaries of the
recess should blend smoothly with the margins of the
preparation especially in premolars when rest seats prepared cross
the central fissure of the tooth and may produce a food trap
between the apex of the occlusal rest and the fissure.
Rounding the end of the rest in a conventional manner leaves a
pyramidal hollow between it and the fissure where as a slight pointed
extension towards the fissure reduces this hollow and may decrease the risk
of caries.
CHOICE OF THE FOUNDATION SURFACE
Tooth surfaces, upon which the recess for an occlusal rest must be
prepared, differ in both contour and thickness of enamel from tooth to
tooth and from mouth to mouth. The nearest thing to an ideal surface upon
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which to place an occlusal rest is gold alloy. Natural tooth is next- in order
followed by fused porcelain and last the silver amalgam.
The rest should never be placed on a silicate cement nor on one of
acyclic resin, because they cannot sustain the vigorous stresses to which it
will be exposed under the partial denture clasp unit.
The Rotated tooth
When it is necessary to clasp a tooth which has rotated out of its
normal position, the preferred treatment is to cover the crown with a
restoration which realigns the surface of the tooth. The other alternative if
possible alter its axial surfaces sufficiently to render it more suitable for
clasping and to place the occlusal rest in the mesial or distal fossae,
eventhough they be situated buccal or lingual to alveolar ridge. If it is not
practical to place in either fossae, it should be remembered that the
occlusal rest may be placed any where on the surface of the tooth where a
properly designed recess can be prepared to support it -
MANDIBULAR BICUSPID WITH RUDIMENTARY CUSP
It is difficult to prepare occlusal rest in such a situation the solution
is to cover such a crown with gold restoration, building a quasi-cingulum
rest into the wax pattern similar to the type employed for the mandibular
cuspids.
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THE INDIRECT RETENTION REST
The rest which is used for this purpose should be located as far
anterior to the fulcrum line as mouth condition permits. Occlusal rest are
pef erred to anterior rest. The mesial fossa of the bicuspid is prepared in
conventional saucer shape. The rest for indirect retainer is then designed to
get into the prepared recess on the bicuspid and tOi extend over onto the
lingual surface of the cuspid. The lingual surface of the cuspid requires
no preparation. Such a design exploits the excellent indirect retention
which the cuspid is capable of supplying and combines it with the equally
excellent vertical support offered by the bicuspid.
THE REST RECESS IN THB ABRADED TOOTH
The preparation" of the rest seat in enamel which has already been
worn is questionable and covering the tooth with a cast crown is the
preferred treatment.
THE MESIALLY INCLINED MOLARS
When the molar abutment teeth are severely tilted, it is usually
supposed that the occlusal rest should be placed on the part of the tooth
furthest from the deepest undercut in the belief that this will prevent further
tilting of the tooth. Quite frequently occlusal rest placed in this position
requires quite a deep rest seat to accommodate it, since it lies on the part of
the occlusal surface that is in contact with the opposing teeth, whereas
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there ia space on the side of the tooth to which it is tilted. The rest may be
prepared on the side to which the tooth is tilted provided the seat of the rest
is at right angles to the long axis of the tooth and provided that the saddle
abuts on the tooth in such a way that it cannot tilt forward while the saddle
i.e. in place. The favourable form of the rest is a tooth of rounded contour
where the connector for the rest lies against a sloping mesial surface
preventing mesial rotation of the tooth.
THE EMBRASURE REST (Inter proxiltial occlusal rest seats)
1) The design of direct retainer assembly may require that
interproximal occlulaal rests be used. The rest seats are prepared as
adjoining occlusal rest seats with the exception that the preparations
must be extended farther lingually than conventional preparation.
Adjacent rests rather than a single rest are used to avoid inter
proximal wedging -by the framework and also they shunt the food away
from contact points.
In preparing such rest seats, care must be exercised to avoid
eliminating contact -point of abutment teeth. Yet Sufficient tooth structure
must be removed to allow for adequate bulk of the component to be so
shaped that occlusion will not be altered. The lingual interproximal area
requires only a modicum of preparation and creation of vertical groove
must be avoided to prevent torquing effect on the abutments by the minor
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connector. Analysis of the diagnostic casts is mandatory to assess inter
occlusal contact areas where rests are to be placed.
THE ONLAY REST
On the abutment tooth which has an occlusal surface below the
plane of occlusion an onlay rest is preferred. This should be done only in
caries resistant mouth as there is danger -of enamel de calcification.
Better tooth be covered with gold crown. Patients hygiene maintained and
prosthesis worn only 8 hrs a day fissures be widened and shallowed for self
cleansing purpose.
INTERNAL OCCLUSAL RESTS
A partial denture that is totally tooth supported by means of cast
retainers on all abutment teeth may use internal occlusal rests for tooth
occlusal support and horizontal stabilization.
An internal occlusal rest is not an internal attachment occlusal
support is derived from the floor of the rest' seat and from an additional
occlusal bevel if such is provided.
Horizontal stabilization is derived from the rest should be parallel to
the path of placement slightly tapered occlusally and slightly dovetailed to
prevent dislodgement proximally.
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The advantage of this rest is it facilitates the elimination of visible
clasp arm buccally and permits the location of the rest seat in a more
favourable position in relation to the "tipping" axis of the abutment.
Retention provided by lingual clasp arm lying in a natural or prepared
infrabulge area on the abutment tooth.
RING RESTS
Occlusal rests on isolated posterior teeth present a particular
problem. Many of these teeth are in firm occlusion with their opponents
and any modification of their occlusal surfaces allows them to change
position either by tilting or by errupting further. Ring rests are usually best
for isolated teeth provided there is sufficient area of tooth above the survey
line to accommodate them. An additional advantage of ring rests is that
they provide a better distribution of load to the abutment Geddes 1958
made a variety of cobalt chromium castings with different type of
occlusal rests, he found that the biting load on tooth borne saddles was
closely related to the kind of occlusal rest used
1) The average maximum load on the natural premolar tooth 16 kg
and molar 21 kg. So theoritically the combined load 37 kg the
patient is able to bite but where short occlusal rests were used only
27 kg exerted.
2) Long oeclusal rests - 32 kgs
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3) Where ring rests surrounding the tooth above the survey line were
used, load exceeded by I kg (38 kgs. )
THE EFFECT OF OCCLUSAL REST ON A TOOTH
For purpose of discussion - a tooth may be considered to be a
section of cone with a curved surface. and two parallel flat circular
surfaces. '' The circular surface of greater size may be taken as occlusal
surface and that the smaller area the apical area of the tooth. (Exceptions
are upper 1st and 2nd molars they have divergent roots).
1) If vertical pressure is applied over the whole of the occlusal surface
an uncomplicated downward movement of the tooth takes place into
its socket.
2) If, however, a vertical pressure is applied at the periphery of 'the
occlusal surface the downward movement is complicated by a
torque being placed on the tooth and a rotatory effect is introduced.
Such a rotatory effect should be minimised and three methods
available for this purpose are:
a) A large mesio distal coverage of the tooth may be used,
the tip of the rest preferably extending to the centre of
the mesiodistal. fissure.
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b) A second is by. reciprocation, 2 shorter rests placed
diametrically opposite one on the mesial part of the' occlusal
surface and one on distal.
c) The third to place the rest on the surface of tooth further
removed from the saddle so that contiguous standing teeth
help to resist the rotatory, movement.
In majority of the cases this torque can be resisted by the bony
support of the natural tooth, if the tooth is vertically placed in the same axis
as the force under consideration. However, if the tooth is tilted and its long
axis lies at an angle to vertical force complication may arise. Frank-J.
Kratochvil, Commander - demonstrated the influence of occlusal rest
position on movement .of abutment teeth.
A training aid was used to demonstrate the effect of the rest
placement. This aid had a changeable rotation pin, and the movement of a
partial denture base can be shown.
The direction of movement of denture base with a distal rest is
shown.
The gingival part of the denture base adjacent to the posterior
abutment moves in an arc almost parallel to the mucosa. This results in
little or no support from the mucosa near the tooth. Also, the soft tissue
adjacent to the tooth may be pinched, with resultant tissue strangulation.
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As the denture is followed posteriorly, the arc of the movement becomes
nearly perpendicular to the surface of the mucosa.
By the movement of the rotation point (occlusal rest) to the mesial
surface of the most posterior tooth. The arc of movement of the denture
base is changed. The direction of movement and force applicatin is more
nearly perpendicular to the surface of the mueosa in each region under the
base.
The mesial fulcrum will increase the support provided by the soft
tissues. Also the direction of movement at the gingival region adjacent to
the tooth is less likely to cause punching or strangulation of the gingival
tissues.
The placement of an occlusal rest distally to the central axis of the
posterior abutment tooth will tend to tip the teeth posteriorly. If the rest
is placed on the mesio-occlusal surface, it will tend to tip the tooth
mesially so it will receive support and bracing assistance from the teeth
anterior to it.
When movement of the edentulous base occurs the force exerted on
the tooth can be compared to that of a precision-constructed and fitted
'wrench'. which tends to tip or pull the tooth backward. This can result
in tooth mobility, bone loss, and tooth and denture movement bodily,
with resulting occlusal disharmony.
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As a result of transfering the rest to the mesial surface, the denture
movement will force the tooth anteriorly and reverse the wrench effect.
All remaining teeth will continue to help withstand this forward force.
REST SEAT PREPARATION
I. Sound Enamel
In most instances, preparation of proximal tooth surfaces is
necessary to provide proximalguiding planes and to eliminate undesirable
undercuts that rigid parts of the frame work must pass during its placement
and removal.
The preparation of occlusal rest seats always must follow proximal
preparation, never precede it. Only after the alteration is completed the
location of the occlusal rest seat in relation to the marginal ridge be
determined. When proximal preparation follows occlusal rest seat
preparation the inevitable consequence is that the marginal ridge is too low
arid two sharp with the centre of the floor of the rest seat too close to the
marginal ridge. So it is impossible to correct the rest preparation
without making it too deep and then irreparable damage has been done.
Occlusal rest seats in sound .enamel may be prepared with diamond
points of approximately the size of no.6 and 8 round burs or with carbide
burs. The larger of the two diamonds is used first to lower the marginal
ridge and to establish the outline form of the rest seat. The resulting
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occlusal rest seat is then complete except that the floor is not sufficiently
concave.
The smaller diamond point is then used to deepen the floor of the
occlusal rest seat, at the same time forming th desired spoon shape inside
the lowered marginal ridge.
Smoothing the enamel rods by the planing action of a round bar of
suitable size revolving at moderate speed, followed by use of an abrasive
rubber point, is usually the only polishing needed.
When a small enamel defect is encountered in the preparation of an
occlusal rest seat, ignore it at-a point until the rest preparation is
completed and then with small burs preparing the remaining defect to
receive a small gold foil restoration. A fluoride gel should be applied to
abutment teeth following enamel recontouring. Application should be done
after impression are made for the cast on which frame work is fabricated as
fluoride gel and irreversible hydrocolloids are incompatible.
II. OCCLUSAL REST SEAT PREPARATIOIM IN EXISTING
RESTORATION
It is same as in enamel. Proximal preparation first and then rest
seat should be placed.
There is always' a possibility that an existing restoration may be
perforated in the process of making an ideal occlusal rest seat. Though
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some compromise is permissible the basic principles of rest seat
preparation should not be violated when perforation occurs it may be filled
with gold foil.
III. OCCLUSAL REST SEATS IN NEW RESTORATION
They should be placed in the wax pattern. The location of the
occlusal rest should be shown when the tooth is prepared for a crown or an
inlay so that sufficient clearance may be provided in the preparation for the
rest.
IV. OCCLUSAL REST SEATS IN CROWN AND INLAYS
These are generally made larger and deeper than the enamel. Those
made in abutment crowns supporting tooth borne dentures may be slightly
deeper than those in abutments supporting a distal extension base
Internal rests - are also created first in wax, either with suitable burs
in a handpiece holder or by waxing around a lubricated mandrel held in
the surveyor.
ANTERIOR RESTS
1) Cingulum rest
2) Incisal rest
Cingulum rest: Is a portion of partial denture that contacts the prepared or
natural cingulum of the tooth.
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Incisal rest : A rigid extension of a removable partial denture that contacts
the tooth at the incisal edge.
Lingual rest: A metallic extension of a removable partial denture frame
work that fits into a prepared depression within an abutment tooth's
lingual surface.
Anterior tooth do not naturally bend themselves to either clasping or
to accommodation of rest due to their morphology.
Root form, root length, inclination of the tooth and ratio of length
of clinical crown to the alveolar support must be considered in
determining the site and form of rests placed in anterior teeth.
Where morphology of the tooth is such that the horizontal surface is
present at the superior aspect of cingulum rest may be placed directly
without any preparation.
Many problems are encountered where direct placement is used
without preparation of the tooth.
Firstly
1) The rest presents a positive built-up on the tooth surface in an area
of high tongue activity and so may give rise to tongue irritation.
2) Unless the surface on which the rest is placed is at right
angles to the long axis of the tooth (which is a rare finding) a
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horizontal- force component will arise by inclined plane action
when the rest applies load to the tooth. This may cause
horizontal tooth movement, which is undesirable.
3) Inadequate apace may be present for a rest where the anterior tooth
relationship is close.
Cingulum Rest: The cingulum rest on an anterior tooth has an important
advantage over incisal rest, in that it is closer to the centre of rotation of
the tooth and hence will exert less leverage.
It has two additional advantages:
1) It is discretely hidden from view
2) It tends to be less bothersome to a curious tongue
If the maxillary cuspid and incisor that may be considered for this
rest.
In incisors the' enamel is thin and so they should be first covered
with metal before rest.
THE PREPARATION OF THE ANTERIOR TOOTH TO RECEIVE
THE REST
1) A slightly rounded 'v' ia prepared on the lingual surface at the
junction of the gingival and middle third of the tooth. The apex of
the 'v' is directed incisally. The preparation is started by using an
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inverted cone shaped diamond stone and progressing to smaller,
tapered stones with round ends to complete the preparation. All
line angles must be eliminated and enamel seat must be polished.
The floor of the rest seat should be toward the cingulum rather than
the axial wall. Care should be taken not to create an enamel
undercut, which would interfere with placement of the denture.
2) A rest may also be placed on the lingual surface of a cast veneer
crown, three quarter crown or of inlay.
1) RESTS IN INLAYS AND ONLAYS
A basic inlay with parallel pins can provide a positive rest when the
remainder of the tooth does not require restoring.
The advent of acid etching and bounding has introduced a
completely new, time saving an non-invasive procedure that shows great
promise. The rests can be fabricated of metal and bounded to the enamel
with minimum preparation of the tooth surface.
2) COMPOSITE LIGHT CURES
In some instances, it is possible to build up the lingual cervical
portion of the tooth with light cured plastic to provide an adequate rest if
the support part of the rest is contoured in enamel and if the plastic only
reinforces the lateral side of the rest form. This can be done only in tooth
supported cases.
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INCISAL RESTS
The ideal incisal rest fulfils the following requirements
1) It provides a positive seat by extending over the incisal edge onto
the labial surface of the tooth.
2) It restores anterior anatomy as required
3) It stabilizes mobile teeth
Indications for use of the incisal 'rest are:
1) The need to provide a positive rest
2) The need to provide stabilization.
3) The need to restore anterior guidance
4) Geriatric considerations
Incisal rests can be placed on any anterior tooth. They may take various
forms
Form 1: An extension of metal work on the lingual or palatal surface of an
anterior tooth to provide a thin cover over the whole incisal surface of the
tooth. This provides a very positive resting action, but presents an
unesthetic show of metal -
Form 2: Incisal rest at incisal angles on prepared rest seats. These are
used as auxiliary rests or an indirect retainers. The incisal rest seat is
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prepared in the form of a rounded notch at an incisal angle or on an incisal
edge, with the deepest portion of the preparation apical to incisal edge.
The notch should be beveled both lingually and labially. A incisal rest seat
should be approximately 2.5 mm wide and 1.5mm deep so that the rest will
be strong without leaving to exceed the natural contours of the incisal edge.
Form 3: Full incisal rests. I use of such placement depends on following
factors:
1. They may take the advantage of natural incisal facetating.
2. Tooth morphology may not permit other designs.
3. Such rests can restore defective or abraded tooth anatomy.
4. Incisal rests provide stabilization
5. Full incisal rests may restore or provide guidance.
DISCUSSION
1) WIDENING THE SUPPORT BASE
As large (wider) as possible can be easily achieved in class III
without modification.
Class IV adjacent to edentulous area and bilaterally as distal as possible.
Class II adjacent to the edentulous area as well as adjacent to modification
areas or embrassure clasps making it triangular.
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Class I - adjacent to the edentulous area moving mesially and additional
rests anteriorly to minimize vertical load or soft tissue.
In class III, IV - the support area is usually quadrangular
In class II - triangular
In class I – linear or narrow zone
Prior abutments - mesial or distal
In case of anterior modifications cingulum or cingulum rests.
II. CARE DURING THE PREPARATION
Care should be taken while preparing the rest seat namely
1) The speed and coolant for the preservation of the vitality of the pulp
2) The preparation should extend only in enamel in a natural tooth
3) There should not be any sharp angles
4) The seat should be prepared in a manner that the forces are
directed along the long axis of the tooth
5) Accuracy of the fit of the rest in their rest seats
Albert Seiden - 1958 gave reasons for ill fiting rests
a) Poor impression and duplicating technique
IV. THE EFFECT OF REST DESIGN ON TRANSMISSION OF
LOAD TO ABUTMENT TOOTH
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Mesial rest and distal rest or combination.
A laboratory study by Bert T. Cecooni, determined the effects of several
types of partial denture rests on abutment tooth movement. It was
found that, as related to the transmission of abutment teeth, the 'depth of
the 'rest (in a gingival direction) is more significant factor than the type of
rest and thus they decrease abutment tooth movement, it was also
stated that bilateral loading of a removable partial, denture causes less
abutment tooth movement than does unilateral loading.
V. SO CALLED INDIRECT RETENTION RESTS
Kenedy class IV.
The example given by A. A. Grant/Johnson said the posteriorly
placed occlusal rest as indirect retainer.
But according to definition of indirect detention - It is only in cases
of distal extension cases.
These posteriorly placed occlusal rests only prevent the tissue ward
movement of the the denture base and the retentive portion of the clasp arm
helps preventing movement away from the tissue.
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VI. INCISAL RESTS
The incisal rest seat preparation at the incisal angles is far away
from the axis of rotation as already mentioned. So if the incisal rest. seat
preparation may be done on the marginal ridge close to the cervical
portion it may serve two purposes.
1) It is close to the axis of rotation so may be less chances of tilting
movement of the abutment tooth
2) More esthetic.
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SUMMARY
Rests and rest seats deserve special consideration in removable
partial denture construction. Proper understanding of the functions, design
and placement of the rests is necessary in partial denture treatment and also
in providing foundation to maintain health of remaining structures
supporting partial denture prosthesis.
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REFERENCES
1. FRANK J.KRATOCHVIL,COMANDER (DC) USN: Influence of
occlusal rest position and clasp design on movement of abutment
teeth. J.P.D. 1961.
2. Byron P. Sansom, Robert J. Flinton, Vincent J. Paiks: Rest designs
for inclined posterior abutments. A photoelastic comparison. J.P.D.
July, 1987.
3. Mitchell A Stem, James S. Bruidvik and Richard P. Frank: Clinical
evaluation of removable partial denture rest seat adaptation. J.P.D.
1985.
4. Bert T Cecooni : Effect of rest design on transmission of forces to
abutment teeth. J.P.D. 1974.
5. Douglas A. Meinig : Removable partial denture without rests.
J.P.D. 1994.
6. Albert Seiden : Occlusal rests and rest seats. J.P.D. 1958.
7. Bradley, Angleo, Caputo: Photoelastic analysis of stress in resin
bonded cigulum rest seats. J.P.D. 1986.
8. Me Cracken's : Removable partial denture prosthodontics. 8th
edition.
35
9. Erenst L. Miller, Joseph E. Grasso : Removable partial
prosthodontic 2nd edition.
10. John Osborne, George Alexander Lammie : Partial dentures, 4th
edition.
11. A.A. Grant / W. Johnson : Removable denture prosthodontics. 2nd
edition.
12. Kratochvil : Partial removable prosthodontics.
13. David M. Watt, A Roy Mac Gregor : Designing partial dentures.
14. D.J. Neill, J.D. Walter : Partial dentures. 2nd edition.
15. John F. Bates : Removable partial denture construction. 2nd edition.
16. Russell J. Stratton & Frank J. Wiebelt : An atlas of removable
partial dentures design.
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CONTENTS
INTRODUCTION
HISTORICAL BACKGROUND
BIOENGINEERING PRINCIPLES
FUNCTIONS
IDEAL REQUIREMENTS
OCCLUSAL REST, CINGULUM REST, INCISAL
REST
DISCUSSION
SUMMARY & CONCLUSION
REFERENCES
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