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.

36

CONTENTS

INTRODUCTION

HISTORICAL BACKGROUND

BIOENGINEERING PRINCIPLES

FUNCTIONS

IDEAL REQUIREMENTS

OCCLUSAL REST, CINGULUM REST, INCISAL

REST

DISCUSSION

SUMMARY & CONCLUSION

REFERENCES

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