Abutment Evaluation&Biomechanics in f.p.d

8/3/2019 Abutment Evaluation&Biomechanics in f.p.d.

1/75

Abutment selection in fixed

partial denture

GUIDED BY:-DR. A S KAULDEPT. OF PROSTHODONTICS

K.D DENTAL COLLEGE

MATHURA

PRESENTED BY:

Preety dagarDate :-1stau ust 1


2/75

Contents :- Definition

Introduction

Factors influencing abutment selection

Root and their supporting structure

Biological consideration

Special problems

Summary

Bibliography


3/75

Definition :-

ABUTMENT

A tooth, a portion of a tooth, or thatportion of a dental implant that serves

to support and/or retain a prosthesis(GPT-8)


4/75

Introduction

Every restoration must be able to withstand

the constant occlusal forces to which it issubjected.

This is of particular significance whendesigning and fabricating a fixed partialdenture,

since the forces that would normally be absorbedby the missing tooth are transmitted, through thepontic, connectors, and retainers, to the abutmentteeth.


5/75

Abutment teeth are therefore called uponto withstand the forces normally directedto the-missing teeth, in addition to those

usually applied to the abutments.

h h h b d b


6/75

Whether a tooth can be used as an abutment or

not?????

Whenever possible, an abutment should be avital

tooth.Vitality of tooth

it should be asymptomatic, with radiographicevidence of good seal and complete obturation ofthe canal, to be used as an abutment.

If it isendodontically

treated

should not be used as an FPD abutmentunless they are endodontically treated.

Pulp capped teeth

Less coronal structure

Mobility Normally, abutment teeth should not exhibit anymobility, since they will be carrying an extra load

The tooth must have some sound, surviving coronal tooth structure toinsure longevity. However, some compensation can be made through

the use of a dowel core, or a pin retained amalgam or acomposite core.


7/75

The supporting tissues surrounding theabutment teeth must be healthy and free from

inflammation before any prosthesis can becontemplated..


8/75

The roots and the supporting tissues of theabutment teeth should be evaluated for threefactors:

Periodontalligament area.

Rootconfiguration.

Crown-root ratio.

i


9/75

Crown-root ratio:

Definition:- The physical relationship between theportion of the tooth within alveolar bone compared with

the portion not within the alveolar bone, as determinedby radiograph . (GPT -8)

As the level of alveolar bone moves apically, the leverarm of that portion out of bone increases, and thechances for harmful lateral forces is increased.


10/75

Different crown-root ratio for a toothto be utilized as a FPD abutment

(Reynolds JM in 1968) :- Ideal crown-root ratio :-1:2

Optimum crown-root ratio :-2:3

Minimum acceptable ratio :-1:1

under normal circumstances


11/75

However , there are situations in which acrown root ratio greater than 1:1 might be

considered adequate.

If the occlusion opposing a FPD is

composed of artificial teeth,occlusal forcewill be diminished, with less stress on theabutment teeth

ff O ( ) h


12/75

Klaffenbach AO (1936): reported that theocclusal force exerted against prostheticappliances has been shown to be considerably

less than that against natural teeth

Force(lb)


13/75

Penny RE, Kraal JH (1979): reported that anabutment tooth with a less than desirablecrown-root ratio is more likely to successfully

support a fixed partial denture ifthe opposingocclusion is composed of mobile, periodontallyinvolved teeth than if the opposing teeth areperiodontally sound.

The crown-root ratio alone is not anadequate criteria for evaluating a

prospective abutment tooth.


14/75

Root configuration :-

This is an important point in the assessment of

an abutments suitability from a periodontalstandpoint.

Roots that arebroader labiolinguallythan they

are mesiodistally are preferable to roots that areround in cross-section.


15/75

Multirooted posterior teeth with widelyseparated roots will offer better periodontalsupport than roots that converge, fuse,or generallypresent a conical configuration.

The tooth with conical roots can be used as anabutment for a short span FPD if all other factors

are optimal.


16/75

A single rooted tooth with evidence ofirregular configuration or with somecurvature in the apical third of the root is

preferable to the tooth that has a nearlyperfect taper.


17/75

Periodontal ligament area :-

Larger teeth have a greater surface area and

are better able to bear added stress.

The areas of the root surfaces of thevarious teeth have been reported byJepsen (1963).


18/75

Comparative root surface areas of maxillary teeth.


19/75

Comparative root surface areas of mandibular teeth.

Wh i b h b l b f


20/75

When supporting bone has been lost because ofperiodontal disease, the involved teeth have alessened capacity to serve as abutments.

Kalkwarf KL, Kreici RF, (1986): reportedthat millimeter per millimeter, the loss ofperiodontal support from root resorption isonly one-third to one-half as critical as the lossof alveolar crestal bone.

The length of the pontic span that can besuccessfully restored is limited, in part, by theabutment teeth and their ability to accept theadditional load.


21/75

Here we must take intoaccountsAntes law, - IrwinH Antewhich states, in fixeddental prosthodontics, thecombined pericemental area of allabutment teeth supporting a fixeddental prosthesis should be equalto or greater than thepericemental area of the tooth or

teeth to be replaced .(GPT 8 )


22/75

Ante`s law is used

to asses the prognosisof fixed partial denture


23/75

According to this premise, one missingtooth can be successfully replaced if the

abutment teeth are healthy.

The combined root surface area of the secondpremolar and the second molar (A2p+A2m) isgreater than that of the first molar being

replaced (A1m).


24/75

The combined root surface area of the first premolarand the second molar abutment (A1p+A2m) isapproximately equal to that of the teeth being

replaced (A2p+A1m)

If two teeth are missing, a fixed partial

denture probably can replace the missing teeth,but the limit is being approached .


25/75

The combined root surface area of the canine andthe second molar (Ac+A2m) is exceeded by that ofthe teeth being replaced (A1p+A2p+A1m)

When the root surface area of the teeth to bereplaced by pontics surpasses that of the abutment

teeth, a generally unacceptable situation exists


26/75

It is possible for fixed partial dentures toreplace more than two teeth, the mostcommon examples being anterior fixed partial

dentures replacing the four incisors.

Canine to second molar fixed partial dentures

also are possible (if all other conditions areideal) in the maxillary arch, but not as often inthe mandibular arch.

However, any fixed prosthesis replacing morethan two teeth should be considered a highrisk.


27/75

Biomechanical

ConsiderationsBending or deflection

Secondary abutment


28/75

In addition to the increased load placed on theperiodontal ligament by a long span fixed partialdenture, longer spans are less rigid.

Bending or deflection varies directly with thecube of the length and inversely with the cube ofthe occlusogingival thickness of the pontic.

Bending = ( length )

(O-G thickness)

There is one unit of deflection (X) for a given


29/75

There is one unit of deflection (X) for a givenspan length (p).

Compared with a fixed partial denture havinga single tooth pontic span, a

two tooth pontic span will

bend 8-times as much.


30/75

A three tooth pontic span bends 27-times as

much as a single pontic.

There is one unit of deflection (X) for a span


31/75

A pontic with a given occlusogingivaldimension will bend eight times as much if thepontic thickness is halved.

There is one unit of deflection (X) for a spanwith a given thickness (t).


32/75

A long-span fixed partial denture on short

mandibular teeth could have disappointingresults.

Longer pontic spans also have the potential

for producingmore torquing forceson the

fixed partial denture.


33/75

To minimize flexing caused by longand/or thin spans

Pontic designs with a greaterocclusogingival dimension should be

selected.


34/75

All fixed partial dentures, long or short, flexto some extent.

Because of the forces being applied throughthe pontics to the abutment teeth,

the forces on castings serving as retainers forfixed partial dentures are different in magnitude

and direction from those applied to singlerestorations.


35/75

The dislodging forces on a fixed partial dentureretainer tend to act in a mesiodistal direction,

Single restoration :- buccolingual direction offorces

Preparations should be modified accordingly toproduce greater resistance and structuraldurability.

Multiple grooves, including some on the buccaland lingual surfaces, are commonly employedfor this purpose .


36/75

The walls of facial and lingual groovescounteract mesiodistal torque resulting from force

applied to the pontic.

Double abutments are sometimes used as a


37/75

Double abutments are sometimes used as ameans of overcoming problems created byunfavorable crown-root ratios and long spans.

There are several criteria that must be met if asecondary abutment is to strengthen the fixedpartial denture and not became a problemitself.

A secondary abutment must have at least as

much root surface area and as favorable acrown-root ratio as the primary abutmentit isintended to bolster.


38/75

As an example, a canine can be used as asecondary abutment, but it would be unwiseto use a lateral incisor as a secondary

abutment to a canine primary abutment.

The retainers on secondary abutments must

be at least as retentive as the retainers on theprimary abutments .


39/75

When thepontic

flexes

tensile forces

will be applied to the

retainers on the

secondary abutments

With primaryabutmentacting asfulcrums


40/75

There also must be sufficient crown lengthand space between adjacent abutments toprevent impingement on the gingiva under

the connector.


41/75

Arch curvature

has its effect on thestresses occurring in afixed partial denture.

When pontics lie outside theinter abutment axis line, the

pontics act as a lever arm,

which can produce a torquingmovement.

This is a common problemin replacing all four

maxillary incisors with afixed partial denture

Some measure mustbe taken to offset the

torque. This can bestbe accomplished by

Gaining additionalretention in the opposite

direction from the lever arm

and at a distance from theinter abutment axis equal tothe length of the lever arm


42/75

Special ProblemsPier abutment

Tilted molar abutment

Canine replacement fixedpartial denture

Cantilever fixed partial denture

Pier Abutments:-


43/75

Pier Abutments:-

An edentulous space can

occur on both sides of a tooth,

creating alone,freestanding

pier abutment.

Physiologic tooth

movement, arch position of the

abutments, and a disparity in

the retentive capacity of the

retainers can make a rigid five-

unit fixed partial denture a less

than ideal plan of treatment.

Parfitt GJ (1960): studies that


44/75

Parfitt GJ (1960): studies that

periodontometry had shown that the

FACIOLINGUAL MOVEMENT:-56 to108um, and

INTRUSION :- 28 UM.

Teeth in different segments of the


45/75

Teeth in different segments of the

arch move in different directions.

Because of the curvature of the arch,the faciolingual movement of an

anterior tooth occurs at a considerable

angle to the faciolingual movement of amolar.

It has been found that:-


46/75

It has been found that:

As a result of the middle abutment

acting as a fulcrum

forces are transmitted to the terminal retainers

causing failure of the weaker retainer


47/75

Stand lee JR , Caputo AA (1988): suggested that

tension between the terminal retainers and their

respective abutments, rather than a pier fulcrum, isthe mechanism of failure.

Intrusion of the abutments under the loading could

lead to failure between any retainer and its

respective abutment.

The loosened casting will leak around the

margin caries is likely to become

extensive before discovery .

The retention on a smaller anterior tooth is


48/75

The retention on a smaller anterior tooth is

usually less than that of a posterior tooth

because of its generally smaller dimensions.

Since there are limits to increasing a retainers

capacity to withstand displac ing forces ,

some means must be used to neutrali ze the

effects of those forces

The use of a NONRIGID CONNECTORhas

been recommended to reduce this hazard.

In spite of an apparently close fit, the movement


49/75

p pp y ,

in a nonrigid connector is enough to prevent the

transfer of stress from the segment being loaded

to the rest of the fixed partial denture.

The nonrigid connector is a broken-stress

mechanical union of retainer and pontic .


50/75

The most commonly used nonrigid

design consists :-

T-shaped key :-attached to

the pontic

Keyway :- placed within a

retainer.

S th l d JK H ll d GA & Sl d TB


51/75

Sutherland JK, Holland GA & Sluder TB

(1980): reported that a nonrigid fixed partial

denture transfers shear stress to supporting bone

rather than concentrating it in the connectors .

It appears to minimize mesiodistal torquing

of the abutments while permit ting them to move

independently.

Landry KE, Johnson PR ,Parks VJ, Pelleu GB

(1987): reported that a rigid fixed part ial

denture distr ibutes the load more evenly than anonrigiddesign, making it preferable for teeth

with decreased periodontal attachment.


52/75

The location of the stress-breaking device in the

five-unit pier-abutment restoration is Important.

It usually is placed onthe middle abutment,

since placement of it on either of the terminal

abutments

could result in the pontic acting as a lever arm.


53/75

Key way of the connector withinthe normal distal contours of the pier

abutment

Key mesial side of the distal

pontic.

Nearly 98% of posterior teeth tilt mesially


54/75

when subjected to occlusal forces.

If the key way of the connector is placed on

the distal side of the pier abutment

Mesial movement seats the key into the key

way more solidly.

Placement of the keyway on the mesial side


55/75

f y y

Causes the key to be unseated during its mesialmovements.

this could produce apathologic mobility in the

canine or failure of the canine retainer.

Tilted Molar Abutment:-


56/75

Tilted Molar Abutment:

A common problem that occurs with somefrequency is the mandibular second molar

abutment that has tilted mesially into the

space formerly occupied by the first molar.

It is impossible to prepare the abutment


57/75

p p p

teeth for a fixed partial denture along

the long axes of the respective teeth and

achieve a common path of insertion.

When a mandibular molar tilts mesially , there is discrepancybetween the long axis of molar and of premolar

There is further complication if


58/75

the third molar is present. Itwill usually have drifted and

tilted with the second molar.

Because the path of insertionfor the fixed partial denture willbe dictated by the smallerpremolar abutment,

i t is probable that the path ofinsertion will be nearly parallel to the former long axis of themolar abu tment before it tilt edmesially.

As a result , the mesial surfaceof the t ipped third molar wil l encroach upon the path of insert ion of the f ixed part ial denture, the reby preven ting i t

from seating completely .

R t i


59/75

Restoring orrecontouring the mesialsurface of the third

molar.

If theencroachment

is slight

Uprighting of the molarby orthodontic treatment. If the tiltingis severe

If orthodontic correction is not possible, or if it is possible to achieve only a partial correction, a

fixed partial denture can stil l be made. It hasbeen suggested that the long axis of the

prospective abutments should converge by no

more than 25 to 30 degrees.

Aproximal half crown sometimes can be


60/75

used as a retainer on the distal abutment.

If there is a severe marginal ridge height

discrepancy between the distal of the second molar

and the mesial of the third molar as a result of

tipping, the proximal half crown is contraindicated.

Atelescope crownand coping can also be used as a


61/75

retainer on the distal abutment. A full crown preparation

with heavy reduction is made to follow the long axis of

the tilted molar.

An inner coping ismade to fit the tooth

preparation(The marginal adaptation for this restoration

is provided by the coping).

Proximal half crown serve asthe retainer

which is fitted over the coping.


62/75

This restoration allows for total coverage ofthe clinical crown while compensating for the

discrepancy between the paths of insertion

of the abutments.

The nonrigid connector is another solution


63/75

to the problem of the ti lted fixed partial

denture abutment.

Afull crown preparation is done on the

molar, with its path of insertion parallel with

the long axis of that t ilted tooth.

Abox form is placed in the distal surface of

the premolar to accommodate a keyway in the

distal of the premolar crown.


64/75

It is tempt ing to place the connector on themesial aspect of the tipped molar, but this

could leadto even greater tipping of the

tooth.

A nonrigid connector for the tipped molar

abutment is most useful when the molar

exhibits a marked lingual as well as mesial

inclination.


65/75

Canine-Replacement

Fixed Partial Dentures

Fixed partial denturesreplacing canines can


66/75

replacing canines canbe difficult becausethe canine often liesoutside the

interabutment axis.

The prospectiveabutments ar e the later alincisor, usually the

weakest tooth in theentire arch, and the f irstpremolar, the weakestposteri or toot h.

A f ixed part ial denture

replacing a maxi l lary canine i ssubjected to m ore s t resse s thanthat replac ing a mandibularcanine, s inc e forces ar et ransmi t ted outward ( labia l ly)on the maxi l lary arch

On the mandibular canine the forces are directed


67/75

On the mandibular canine the forces are directed

inward (lingually).

Any fixed partial denture replacing a canineshould be considered a complex fixed partial

denture. No fixed partial denture replacing a canine

should replace more than one additional tooth.


68/75

Cantilever Fixed

Partial Dentures


69/75

A cantilever fixed partialdenture is one that has an

abutment or abutments atone end only, with the otherend of the pontic remaining

unattached.

This is a potentially

destructive design with thelever arm created by thepontic, and it is frequently

misused.


70/75

When a cantilever pontic is employed to replace a

missing tooth,forces applied to the pontic have an

entirely different effect on the abutment tooth.Thepontic acts as a leverthat tends to bedepressed

under forces with a strong occlusal vector.

Ewing JE (1957): suggested that abutment teeth


71/75

for cantilever fixed partial dentures should be

evaluated with an eye toward lengthy roots with a

favorable conf iguration, long clinical crowns,

crown-root ratios, and healthy periodontium.

Wright (1986): reported tha t cantilever fixed

partial dentures should replace only one tooth andhave at l east two abutments.

A cantilever can be used for replacing a


72/75

p g

maxillary lateral incisor. There should be no

occlusal contact on the pontic in either centric or

lateral excursions.

The canine must be used as an abutment, and it

can serve in the role of solo abutment only if it

has a long root and good bone support.

There should be a rest on the mesial of the

pontic against a rest preparation in an inlay or

other metallic restoration on the distal of the

central incisor to prevent rotation of the ponticand abutment.

A cantilever pontic can also be used to replace a missing first


73/75

premolar. This scheme will work best if occlusal contact is

limited to the distal fossa.

Full veneer retainers are required on both the second

premolar and first molar. These teeth must exhibit excellentbone support. This design is attractive if the canine is

unmarred and if a full veneer restoration is required for the

first molar in any event.

SUMMARY :


74/75

SUMMARY :-

When planning and treating cases involving fixedprosthodontic restorations, it is important that all theapplicable parameters are taken into account.

The prosthodontist must not focus too much on the

finer details of constructing a perfect restoration, orrisk creating a failure because proper engineeringprinciples was not used.

If success is to be attained the prosthodontist musttake into account the opposing occlusion, periodontal

bone loss, attachment apparatus, length of span, crown

root ratio and inclination of abutment teeth.

Bibliography:-


75/75

g p y Shillingburg HT et al. Fundamentals of fixed

prosthodontics. 3st ed.

Rosenstiel SF, Land MF, Fujimoto J.contemporary fixed prosthodontics. 3rd ed.

Reynold JM. Abutment selection for fixedprosthodontics. J Prosthet Dent. 1968;19:483-488

Penny RE, Kraal JH. Crown-to-root ratio: Itssignificance in restorative Dentistry J ProsthetDent.1979;42: 34-38

Documents

Abutment Evaluation&Biomechanics in f.p.d