The mandibular posterior fixed partial denture

The mandibular posterior fixed partial denture

FIXED PROSTHODONTICS OPERATIVE DENTISTRY DAVID E. BEAUDREAU, SAMUEL E. GUYER, WILLIAM LEFKOWITZ, Section editors The mandibular posterior fixed ptiiu...

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FIXED PROSTHODONTICS OPERATIVE DENTISTRY DAVID E. BEAUDREAU, SAMUEL E. GUYER, WILLIAM LEFKOWITZ, Section editors

The mandibular posterior fixed ptiiul Donald A. Behrend, M.D.Sc., B.Sc., F.R.A.C.D.S., Melbourne,

In

Ph.D.

Australia

1936, Hirschfeldl

delivered

a classic paper in which he listed 80 abnormalities

that may rest&, from the loss of a mandibular and those of innumerable the most common consecutive

denture

of all major

patients

first molar.

In spite of his warnings

authors before and since, the loss of this tooth is probably dental defects.

undergoing

In the author’s

oral rehabilitation

practice,

80 out of 100

were missing one or both man-

dibular first molars. In most patients, obvious gotten

the esthetic

and the reduction Consequently, of technical

efficiency

with these difficulties

denture

than are associated

and the risk of failure,

an apathetic

patient

is accepted

the replacement.

of a fixed partial

difficulties

trying to persuade

of the loss of a mandibular

few patients demand

of view, the construction number

effect

of masticatory

molar

is not

and eventually

for-

From the dentist’s point

in this region presents

a greater

with other fixed prostheses.

Faced

the busy dentist may see little point in

to accept

a treatment

the benefits of which

are not obvious. This

situation

is unfortunate

sence of a mandibular a progressive

posterior

deterioration

the fields of periodontal mandibular

joint

serious problems If a dentist as a momentary of replacing

function,

INDICATIONS

out, the continued

with consequent

caries susceptibility,

and facial esthetics.

problems

masticatory

The widespread

ab-

leads to

extending

into

efficiency,

temporo-

existence

of discon-

posterior arch represents one of the most fundamental

and

in public dental health. views each patient’s dentition not as a static set of conditions but phase in a continuing process, he will tell the patient the importance

AND

CONTUAlNDlCATlONS

POSTERIOR

absence

of temporomandibular 622

pointed

such missing teeth.

A MANDIBULAR Although

of the occlusion, health,

tinuities of the mandibular

for, as Hirschfeld’

tooth other than the third molar generally

FIXED

of third molar

FOR

PARTIAL occlusal

THE

CONSTRUCTKW

OF

DENTURE support has been adduced

as a cause

joint disorders, there appears to be no evidence to support this

Volume Number

37 6

Mandibular

posterior

fixed

Fig. 1. The first molar space has been reduced by bodily migration lengths are short. A fixed partial denture is not indicated.

partial

dentures

623

of adjacent teeth. Crown

claim.* In patients of European descent, it is rare for the mandible to be large enough to hold 16 erupted teeth with the third molar crowns fully exposed. Ext&ction of the third molars when all other teeth are present enables most patients to maintain better oral hygiene in the posterior part of the mouth. Indeed, absence of tioth second and third molars does not seem to be a significant disability and a mandibular arch complete to the first molar on one or both sides provides a sufficient, although not optimal, masticatory surface for most patients. If a mandibular second molar is missing and both the first and third molars are present, a fixed partial denture will probably be difficult to clean with littIe compensating benefit. Consideration should be given to the orthodontic repositioning of the third molar against the distal part of the first molar. In a few other situations, orthodontic treatment or autotransplantation may be used to eliminate spaces in the mandibular posterior segment. Apart from these patients and those in whom the over-all state of the dentition is too poor to justify treatment, prosthetic replacement of missing premolars and first molars is nearly always indicated. The choice between fixed and removable prostheses is based mainly on the length of span in relation to the total root support of the abutments. The rationale for choosing a removable prosthesis to restore a long span is that bilateral bracing can be provided more easily and that the restoration will not fail because of flexion. In practice, these considerations are favorable for removable prostheses only in the most extreme cases. Given reasonable root structure and support, fixed partial dentures restoring edentulous spaces of up to about 22 mm. in length can be made with a favorable prognosis. The disadvantages of removable partial dentures in regard to gingival irritation, food retention, discomfort, and inconvenience are well known. The fixed partial denture should be recommended to the patient whenever the mechanical criteria are favorable. Unfortunately, in the true borderline decisions, even extensive experience and sound judgment cannot provide a definitive prognosis, and the situation amounts to a form of gambling decision.” If the fixed prosthesis succeeds, its advantages over the removable alternative are considerable, but if it fails, there is more to be lost. With an intelligent patient, it is often best to explain the situation frankly, give him some estimate of the odds, and leave the final decision to him.

624

J. l’rosthet. Dent June, 1977

Behrend

Fig. 2. A typical smile, with no exposure of lower teeth. Fig. 3. The patient in Fig. 2 laughing. The lower teeth are partly

Figs. 4 and 5. The

model

shows

the

typical

amount

exposed

of visibility

of lower

teeth.

Infrequently, teeth migrate into an edentulous space without tilting. In this situation, replacement of the missing tooth is much less imperative. For example, with an arch where the first molar space has been reduced by bodily migration and the crown lengths are short, insertion of a pontic would complicate the maintenance of oral hygiene (Fig. 1) , Unless the patient is prepared to have the premolars repositioned orthodontically first, there is probably more to be lost than gained with such a replacement. DESIGN

FOR MANDJBULAR

FIXED

PARTJAL

DENTURE

Choice of materials. In choosing the materials and design for retainers and pontics, the factor most often misunderstood is esthetics. The average patient when smiling naturally exposes little or nothing of the mandibular teeth (Fig. 2). The greatest normal exposure occurs during laughing (Fig. 3) _ There is, of course, great variation between individuals, but typically, during laughter, about half the labial surface of the mandibular incisors and canines becomes visible as well as about one quarter of the buccal surface of the premolars and most of their occlusal surface. The area of visiblity may or may not include a small part of the mesial cusps of the first molar. If this visibility pattern (Figs. 4 and 5) is compared with the fixed partial

Fig. 6. A fixed pin facing, and

partial a full

denture is constructed gold crown. Compare

Fig. 7. Ceramic-metal restorations with is separate from the fixed partial denture.

wide

of a three-quarter with Fig. 5. gold

collars.

The

crown,

a pontic

crown

on the

with first

a longpremolar

denture (Fig. 6)) all that will be seen of this restoration and its supporting teeth is gold. This point is made not to criticize that which has been successful, but to show that it would not satisfy the esthetic demands of a patient who expects a natural appearance. Provided that the lingual cusp of a mandibular premolar is sufficiently long to allow a retentive preparation to be made, the use of a three-quarter crown as an abutment has many advantages, but the fixed partial denture will not be an esthetic one, and the patient’s acceptance of this must be determined before the restoration is made. Similarly, the use of ceramic-metal crown retainers with gold occlusal surfaces results in little esthetic improvement over complete gold crowns. The only design for a mandibular posterior fixed partial denture that achieves natural appearance is one involving full porcelain coverage of both the occlusal surfaces and the upper part of the buccal surfaces of the involved teeth, at least back to the mesial half of the first molar (see Figs. 12 and 36). The most common objection raised to the use of porcelain coverage of an OCclusal surface is that the preparation must be more radical to accommodate the thickness of porcelain and underlying gold. However, if gold alone is used on the occlusal surface, the minimum thickness of material required is about 1 mm., while if ceramicmetal is used, the minimum is about 0.5 mm. of metal covered by 1 mm. of porcelain. Thus, only an extra 0.5 mm. of tooth structure need be removed to accommodate the porcelain, and there are few teeth which cannot safely afford this reduction. .\ second objection raised to this design is that correct occlusion is more difficult to establish. This problem can be overcome in all but the most complex restorations by constructing the occlusal surface of the fixed partial denture to ideal anatomic form and adjusting the occluding surfaces of the maxillary teeth at the time of insertion. A third objection is that the porcelain surface will cause excessive wear of the opposing maxillary teeth. While it is true that in vitro tests show porcelain to be relatively abrasive of enamel or gold, this abrasiveness diminishes after the initial wear has taken place.” However, the problem hardly arises in the case of a mandibular

626

1. P~oatkt. Dent. June. 1977

Behkend

Fig. 8. Ceramic-metal restorations Fig. 9. A fixed partial denture between

the two pontics

with

supragingival

replaces the second serves no useful purpose.

margins. premolar

and

first

molar.

The

embrasurc

posterior fixed partial denture. In most occlusal schemes, teeth posterior to the first premolar contact only in centric occlusion, but do not participate in gliding contacts. Therefore, they are not exposed to rapid wear, regardless of moderate disparities in the hardness of the opposing materials. The visibility pattern in this quadrant has further implications. When complete ceramic-metal crowns are used on a mandibular premolar with sharply tapered form, it is difficult to provide adequate thickness of both porcelain and metal near the buccal gingival margin without producing an excessive convexity. In these instances, the porcelain-metal junction should be placed several millimeters above the gingival margin, where the tooth can be adequately reduced to accommodate both materials (Fig. 7). The wide gold collar will not be visible. Further, if the clinical crown of a mandibular premolar is fairly long, the buccal surface intact, and caries control adequate, there is no reason why the crown rnargin should not be placed well above the buccal gingiva (Fig. 8). The advantages of this design are obvious. With regard to the materials used for pontics, reference to the visibility pattern again gives a logical basis for choice. If an esthetic first molar pontic is important. the occlusal surface must be covered by porcelain. The buccal surface is never seen. Thus, the use of long-pin porcelain facings on the buccal aspects of mandibular molar pontics is irrational. Pontic construction is complicated by their use, and failure of these facings is common. The choice of materials should be between plain gold and ceramic-metal with a porcelain occlusal surface. Pontic form. As the lower half of a mandibular pontic will not be seen, the shape of its inferior surface shoud be determined solely bv hygienic considerations. This fact was recognized by the designers of the so-called “hygienic pontic” which was popular some years ago. While this form has definite advantages, it causes problems of tongue irritation to many patients and is seldom used. The ideal pontic tissue surface, as has long been recognized, should be convex in all directions; and in the mandibular posterior region, this principle leads to the now generally accepted spheroidal design. Such a pontic, egg-shaped in its lower half and separated from the crest of the ridge by 0.5 to 1 mm., is relatively easy to clean with dental floss. As demonstrated by Stein5 any pontic in contact with the tissue on which plaque is allowed to remain may cause chronic ulceration.

Mandibular

postc

Figs. 10 and 11. Buccal and lingual views of a fixed partial in their gingival halves to eliminate interpontic embrasures.

Fig.

12. The

same

Fig. 13. Diagnostic lar first molar.

fixed

partial

casts show

denture the typical

seen in Figs. arch

10 and

irregularities

al dentures

denture

with

11 has been resulting

from

three

pontics

627

“fused”

inserted. loss of the mandibu-

Multiple pontics present a special situation. Consider the fixed partial denture illustrated in Fig. 9. The gingival embrasure mesial to the second premolar pontic is essential to provide space for the papilla and access for cleaning the distal surface of the first premolar. The embrasure distal to the first molar pontic serves a similar essential purpose. But what is the use of the gingival embrasure between the two pontics? It weakens the bridge span mechanically, is a nidus for food and plaque collection, and interferes with the passage of dental floss mesiodistally under the pontics in their ginfixed partial denture. It seems more rational to “fuse” multiple gival halves, eliminating the interpontic embrasures but retaining the appearance of individual teeth on the occlusal surface (Figs. 10 to 12). MECHANICAL

PROBLEMS

ARISING

FROM

TOOTH

MIGRATION

The classical picture of irregularity of the dental arch resulting after some years from the loss of the mandibular first molar (Fig. 13) presents three particular mechanical obstacles to fixed partial denture construction: ( 1) the mesial inclination of the mandibular second molar, (2) the mesial inclination of the mandibular third molar over the distal gingival margin of the second molar, and (3) the overeruption of the maxillary first molar. Let us consider how each of these obstacles may be dealt with during the construction of the fixed partial denture.

628

J. Prosthet. Deni. June, 1917

Behrend

Fig. 14. A radiograph of a tilted mandibular the line drawn on the film, upright preparation at this early age.

second molar of a 19-year-old girl. As shown by of the tooth would not expose the pulp, even

A

Fig. 15. Upright preparation of a premolar and tilted second molar as abutments: A, teeth before preparation; B, the mesial surface of the molar has been prepared correctly, but the remaining surfaces have been aligned to the existing long axis, resulting in an overtapered preparation; C, correct preparation of the molar and premolar; D, the completed fixed partial denture. The mesial cusps of the molar have been raised, the distal cusps have been lowered, and the correct occlusal plane has been restored.

Mesial

inclination

of the mandibular

second

molar

This is a problem familiar to all dentists who construct posterior fixed partial dentures.” There are four principal techniques for dealing with it : ( 1) “prepare the tooth upright,” i.e., shape the tooth to receive a crown on a vertical path of insertion, (2) use a telescope crown for the molar abutment, (3) make the restoration in two sections joined by an attachment, and (4) upright the molar orthodontically before constructing the fixed partial denture. Upright preparation of an inclined molar. For most inclined molar abutments, this is the simplest and most appropriate technique. The usual fear of an inexperienced dentist is that, in reducing the mesial surface to parallel the premolar preparation, he will expose the pulp. Examination of the radiograph usually shows this fear to be unfounded (Fig. 14).

Mandibular

Fig. 16. An electrosurgical tissue distal to the terminal

instrument molar prior

posterior

fixed

(with fine wire tip) is used to abutment preparation.

partial

dentures

629

to remove

a wedge

of soft

It is best to prepare the premolar abutment first, for this tooth allows little latitude in alignment. Then, the mesial surface of the molar is prepared with the same path of insertion. Parallelism must be checked using a mirror large enough to see both teeth. Having prepared the mesial surface, a common tendency is to forget the inclination of the tooth and to prepare the remaining surfaces in line with its existing long axis. This leads to an overtapered preparation (Fig. 15, A and B) . What is needed is a clear picture of the final form of the preparation either in the operator’s mind or on a previously prepared diagnostic cast. In relation to the ideal occlusal plane, the mesial cusps are usually too low and the distal cusps too high. Thus, the distal portion of the occlusal surface requires severe reduction, often 3 mm. or more, while the mesial portion may often be left untouched. To provide retentive opposition to the mesial surface, the distal surface must be cut sharply gingivally, producing a gingival margin in the form of a deep, beveled chamfer. To provide sufficient length for this surface, the gingival margin must be extended deep into the gingival crevice (Fig. 15, C) . When the abutment molar is the terminal tooth in the arch, exposure of this deep distal gingival margin for preparation and impression making requires the removal of a wedge of soft tissue which is accomplished with an electrosurgical instrument (Fig. 16). The buccal and lingual surfaces also usually require realignment, for the molar with a mesial inclination usually has a lingual inclination as well. Again, a common tendency is for the dentist to prepare these surfaces in line with the existing long axis (Fig. 17, A and B) . Examination of the preparations in a large mirror then shows the lingual surface of the molar to have an undercut in relation to the axis of the premolar. If the lingual surface is now reprepared (Fig. 17, C) , an overtapered form results, and if, to correct this, the buccal surface is cut in more gingivally, the pulp may be exposed. Again, the preparation must be planned carefully before starting, when it can be seen that the occlusal part of the buccal surface requires little or no reduction (Fig. 17, D) . In teeth with severe inclinations, the margin is frequently placed supragingivally near the mesiolingual line angle. To avoid distortion of the impression by the under-

630

J. Prosthet. Dent. June, 1977

Behrend

B

Fig. 17. aration; has an surface,

Buccolingual view of the preparation of a tilted molar abutment: A, tooth before prepB, the tooth has been prepared incorrectly, in line with its long axis; the lingual surface undercut relative to the premolar; C, the undercut has been eliminated from the lingual but the preparation is now overtapered; D, correct preparation of the tooth.

Fig. 18. The undercut area on the mesiolingual aspect of the margin is filled with calcium hydroxide liner to prevent distortion the removal of a wedge of tissue distal to the tooth. Fig. 19. A prepared a cross pin is inserted

tilted molar with during cementation

very short distal wall. at the point marked.

To

molar below the preparation of the impression. Note also overcome

lack

of retention,

cut tooth surface below this margin, the undercut is filled with calcium hydroxide liner to a level just below the margin prior to making impressions (Fig. 18). In all these instances, diagnostic casts are essential before and during the preparation, and if uncertainty exists, trial preparations should be done on a duplicate cast. Occasionally, despite deepening of the distal gingival crevice, the necessary reduction of the occlusal surface results in a preparation with a short distal wall. Retention should be supplemented by cross-pinning at the time of insertion. The technique for this procedure has been described by Courtade and Timmermansi and Fusilier.* The cross pin * is most easily placed in the mesial part of the buccal surface (Figs. 19 and 20). *TMS

pins,

regular,

Whaledent

Inc.,

New

York,

N. Y.

Mandibular

Fig. 20. Placement Fig. 21. A severely

posterior

fixed

partial

631

of the cross pin. tilted

molar.

Fig. 22. The inner gold coping is in place on the model. Several pinholes the thicker parts of the gold without penetrating the interior surface. Fig. cap

dentures

23. The assembled with pins. It will

fixed partial denture. be cemented to the

The

molar

restoration

have

is completed

coping at the time of insertion.

been

drilled

in

by an occlusal

The pontic design

shown would not be used. The telescope crown retainer. If the inclination of the molar is too severe for uprighting in preparation and orthodontic treatment is not acceptable, a telescope crown may be indicated. The design for this retainer, in which the outer crown completely envelopes the inner coping, suffers from the defects of a double margin and excessive bulk in the gingival region. These defects are eliminated with a design developed by Wark.” The tooth is prepared with some degree of uprighting from its long axis. The coping is made with the axial contours of a normal crown but with a flat occlusal surface of minimal thickness. A number of pinholes are drilled parallel to the anterior abutment into the bulkier parts of the coping, usually on the mesial and buccal aspects. These holes do not penetrate the interior surface of the coping. The superstructure comprises the occlusal part of the crown, with a short peripheral bevel, and the pins. First, the coping is cemented to the molar preparation, followed by cementation of the superstructure to the premolar preparation and the coping (Figs.

The

21 to 23).

two-part

fixed

partial

denture

with

precision

attachment.

The

indications

632

.I. Prosrhet. Dent. June, 1977

Behrend

Figs. 24 and 25. Two-part the premolar

abutment.

The

Figs. 26 arrd 27. Diagnostic upright

the tilted

mandibular

fixed partial denture with precision attachment crown on the first premolar is separate.

casts indicate the molar. The patient

“before” and “after” is 49 years of age.

in the distal

orthodontic

part

treatment

of

to

for use of this design are the same as those for the telescope crown retainer, i.e., a molar inclined too severely for uprighting in preparation and a patient unwilling or unable to accept orthodontic treatment. Because of the need to resist further mesial tilting of the molar, the matrix of the attachment is placed in the distal surface of the premolar retainer. Again, some uprighting of the molar should be achieved in preparation, but since the molar preparation and hence the attachment will still be mesially inclined, it is difficult to accommodate the attachment within a normally contoured premolar crown. This restricts the use of the technique to patients in whom the premolar abutment has been endodontically treated (Figs. 24 and 25). A number of small dovetail, H-section or cylindrical attachments are available for this application. If the span is long, it is advisable to lock the patrix and matrix together by means of a cross pin or screw. Orthodontic uprighting of the inclined molar. Orthodontic treatment provides an elegant solution to the problem of the more severely inclined molar. Indeed, there are some patients in whom the tooth, although erupted, is virtually horizontal, with the opposmg maxillary molar occluding against its distal surface, and construction of a satisfactory fixed restoration would be impossible without preliminary orthodontic treatment (Figs. 26 to 29). A number of orthodontic techniques are available for this procedure,l’. I1 and

Figs.

Fig.

28 and

29. Radiographs

30. Orthodontic

Fig. 31. The diagram the bands. The force the roots mesially.

bands

and wire

shows tending

indicate

“‘before”

are used to upright

and

“after”

a tilted

orthodontic

treatment.

molar.

how an initially straight wire is deformed to fit the brackets to straighten the wire moves the crown of the molar distally

on and

while the treatment may best be carried out by an orthodontist, it is important for the prosthodontist to understand the principle of the technique to be used so that he will anticipate the type of movement which can be expected. A simple method is to band the molar and two or three teeth anterior to the space (Fig. 30). An initially straight wire, which requires deformation, is attached to the brackets so that forces are set up in the wire tending to return it to its straight condition (Fig. 31). This will result in rotation of the molar about a point somewhere near the center of the tooth. The crown will move distally and the roots mesially. This procedure should be considered when planning treatment. It is not necessary to effect complete uprighting of the tooth. Most severely tilted teeth in middle-aged patients can be brought to a satisfactory position within 4 months. Following completion of the orthodontic treatment, the tooth should be held in retention for some months before preparation to receive the fixed partiaI denture. The wire is made passive, and the brackets are covered with self-curing acrylic resin for strength, comfort, and ease of cleaning (Fig. 32).

634

Behrend

J. Prosthet. Dent. June, 1977

Fig. 32. The during

wire the retention

Fig. 33. Orthodontic part of the second of age.

is made passive, period.

and

the brackets

are covered

with

uprighting of tilted molars. A 7 mm. periodontal molar was reduced to 2 mm. by the tooth movement.

self-curing

acrylic

resin

pocket at the mesial The patient is 52 years

Figs. 34 and 35. Radiographs of the same teeth before orthodontic treatment and during preparation for the fixed partial denture after completion of orthodontic treatment. The angle of incidence of the bone to the mesial surface of the root of the second molar has changed from acute to obtuse.

.. ..-.. Fig. 36. The fixed partial Fig. 37. The diagnostic of the second

molar

denture is inserted. cast shows tilted second and third molars. The is covered by the mesial surface of the third molar.

distal

gingival

line

Volume Number

Mandibular

37 6

Fig. 38. The diagram shows the second molar is exposed.

Fig.

39. OcclusaI

Fig.

40. The

fixed

view

how

of working

partial

the

teeth

may

cast of the prepared

denture

and

third

molar

posterior

be prepared

fixed

partial

so that

the

dentures

gingival

margin

635

of

teeth. crown

have

been

inserted.

A further indication for orthodontic uprighting, often combined with that of extreme inclination, is the presence of a periodontal pocket at the mesial surface of the tooth. In the absence of inflammation, application of orthodontic force to a tooth may be expected to stimulate bone deposition in the region of periodontal fiber tension .I2 In this situation, it appears that the epithelial attachment on the mesial surface of the molar remains in a constant position on the tooth and is elevated to the level of the alveolar crest as the tooth is uprighted.13 The pocket, in other words, is turned inside out (Figs. 33 to 36). Mesial

inclination

of

the

mandibular

third

molar

:2 further problem arises when both second and third mandibular molars are present and both are inclined mesially. In this situation, the distal cusps of both teeth are above the occlusal plane and the distal gingival line of the second molar is covered in the vertical direction by the mesial surface of the third molar (Fig. 37). In an extreme case, it will be necessary to extract the third molar and upright the second molar orthodontically. Generally, however, if the third molar is functional, it can be retained and realigned to the correct occlusal plane by “upright preparation” for a complete crown in a manner similar to that used for the second molar. At the same time, a fairly radical reduction of its mesial surface will expose the distal gingival margin of the second molar preparation (Figs. 38 and 39). The fixed partial denture can then be inserted on a vertical path, followed by the third molar crown (Fig. 40).

636

J. Prosthet. Dent. June, 1977

Behrend

Fig.

41. All

Fig. and

42. A cantilever fixed partial the first and second premolars.

right-side

Overeruption

molars

are missing. denture

of the maxillary

is retained

by full

crowns

on the mandibular

canine

first molar

The third mechanical obstacle is the extruded maxillary first molar. Construction of a fixed partial denture without reducing the length of this tooth perpetuates the deformity of the occlusal plane and usually produces occlusal interferences in protrusive and nonworking lateral mandibular movements. Although it is possible to shorten the maxillary molar at the time of tooth preparation, it is generally better to prepare the mandibular teeth almost without reference to the opposing maxillary teeth. The prepared occlusal surfaces are aligned, with allowance for the thickness of restorative material to be used, to an imaginary ideal mandibular occlusal plane. This can best be judged by viewing from the front of the patient with the mouth at approximately eye level. It is wise to tell the patient that the overerupted tooth will need to be shortened later. When the working casts are mounted on the articulator, it is easier to see what corrections must be made in the maxillary arch. These can now be done on the stone cast. Frequently, the cusps of other maxillary teeth opposing the fixed partial denture will also require shortening. When the fixed partial denture is about to be tried in the mouth, the patient should be told that it will be “too high” and why all corrections will be made in the maxillary arch. Before placing the restoration in the mouth, the nearest pair of opposing natural teeth should be tested for contact with a thin metal shim.* Then, with the fixed partial denture seated, the overerupted maxillary tooth or teeth are reduced until contact

is restored

as before.

In most patients, although some dentin is exposed, no treatment other than application of desensitizing solution is required for the reduced molar. However, if the reduction has been extensive, it will be necessary to restore the tooth with a gold onlay. The need for this can usually be assessed at the time of treatment planning. Pulp exposure necessitating endodontic treatment is rare, but is justified if a satisfactory occlusal plane cannot otherwise be achieved. *Occlusal

registration

strips,

Artus

Corporation,

Englewood,

N. J.

Mandibular

A

posterior

fixed partial

B

dentures

C

Fig. 43. Preparation

of a naturally tapered lower canine: A, the tooth before the tooth has been prepared by removing an even amount of tooth substance preparation coronal surface; the preparation is overtapered; C, a “terraced” on the lingual surface providing retentive opposition to the labial surface.

Fig. 44. A four-unit retainer

supplemented

cantilever fixed by a cingulum

partial pin.

denture

Fig. 45. A horizontal

cross

pin is inserted

through

THE CANTILEVER

FIXED

PARTIAL

DENTURE

637

with

the

the canine

retention crown

preparation; B, over the whole with two steps

of the during

canine

crown

cementation.

A mandibular quadrant of six contiguous teeth provides adequate masticatory surface area for most patients. In fact, five and a half teeth are generally sufficient. If natural teeth are present back to and including the second premolar, a premolar-sized pontic cantilevered at the end of the arch is nearly always the most suitable prosthesis, even when the situation is bilateral. The maxillary first molar is prevented from overerupting by the cantilevered pontic, and the maxillary second molar, if present, may be stabilized by an extension from the distal part of the first molar. A cantilevered pontic in this region is subject to considerable forces and should be supported by three sound teeth. The safest design includes complete crowns on the mandibular canine and the first and second premolars (Figs. 41 and 42). THE MANDIBULAR

CANINE

ABUTMENT

When a crown on the mandibular canine is used as a retainer for a mandibular posterior fixed partial denture (e.g., for the cantilevered type), the tapered form of

638

J. Prosthet. Dent. June, 1977

Be/wend

this tooth often raises problems of retention. If an equal amount of tooth substance is removed from the coronal surface, an overtapered preparation may result (Fig. 43, ‘4 and B). To overcome this problem, a “terraced” preparation can be used. This involves preparing the lingual surface of the tooth in two steps, or terraces, each of which has an axial wall almost parallel to the labial surface (Fig. 43, C ! In more critical situations, the retention of the crown on the canine may be supplemented by a vertical pin in the cingulum (Fig. 44) or by a horizontal cross pin inserted from the lingual surface at the time of cementation (Fig. 45) SUMMARY Absence of a mandibular posterior tooth, particularly the first molar, is the basic common denominator in a wide range of dental disorders. Despite their awareness of this fact, many dentists are deterred from the routine construction of mandibular posterior fixed partial dentures because of several technical difficulties. An attempt has been made to show how these difficulties may be overcome to give a predictable and satisfactory result. References 1.

Hirschfeld, J.: The Individual Missing Tooth: A Factor in Dental and Periodontal Disease, J. Am. Dent. Assoc. 24: 67-82, 1937. 2. Schweitzer, J. M.: Oral Rehabilitation, St. Louis, 1951, The C. V. Mosby Company, chap. 18. 3. Behrend, D. A.: Decision-making in Oral Rehabilitation, Amt. Dent. J. 15: 383-388, 1970. 4. Monasky, G. E., and Taylor, D. F.: Studies on the Wear of Porcelain, Enamel and Gold, J. PROSTHET. DENT. 25: 299-306, 1971. 5. Stein, R. S.: Pontic-Residual Ridge Relationship: A Research Report, J. PROSTHET. DENT. 16: 251-285, 1966. 6. McAllister, H. H.: The Tilted Molar Abutment, Dent. Clin. North Am., March, 1965, pp. 25-32. 7. Courtade, G. L., and Timmermans, J. J.: Pins in Restorative Dentistry, St. Louis, 1971, The C. V. Mosby Company, p. 267. 8. Fusilier, C. N.: Cross-pinning for Added Retention, J. PROSTHET. DENT. 31: 397-402, 1974. 9. Wark, B. G.: Personal Communication, 1967. 10. Abelson, M. N.: Molar Uprighting, Dent. Clin. North Am., July, 1971, pp. 539-548. 11. Khouw, F. E., and Norton L. A.: The Mechanism of Fixed Molar Uprighting Appliances, J. PROSTHET. DENT. 27: 381-389, 1972. 12. Ingber, J. A.: Forced Eruption. 1. A Method of Treating Isolated One and Two Wall Infrabony Osseous Defects--Rationale and Case Report, J. Periodontol. 45: 199-206, 1974. 13. Brown, I. S.: The Effect of Orthodontic Therapy on Certain Types of Periodontal Defects. 1. Clinical Findings, J. Periodontol. 44: 742-756, 1973. 147 COLLINS ST. MELBOURNE,VXCTORIA, AUSTRALIA

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