Planning Precision Attachments

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Removable partial dentures

Stanley F. Lorencki, D.D.S., WA.

University of Califomiu, College of Dentistry, San Francisco, Calif.

1 he prefabricated precision attachment restoration offers intracoronal retention

and a means of fixation or splinting of teeth, and it has the added singular feature

of being a removable prosthesis,

Precision attachment restorations, rather than

fixed partial dentures, are the restorations of choice in unusual conditions that might

include nonparallel abutments, long edentulous spans, distal extension bases, or

residual ridges of peculiar shapes.

When a precision attachment prosthesis is indicated, essential principles must

be precisely followed. Each phase must be analyzed thoroughly before proceeding

to the next phase.

DIAGNOSIS

By means of accurate radiographs, diagnostic casts, a thorough mouth examina-

tion, and a health history, a professional and critical evaluation is made of the

support available for the partial denture in relation to the stresses that the restora-

tion will transmit to the supporting structures .I The ability of the abutment teeth to

resist occlusal stresses without damage to the periodontal membrane is directly pro-

portional to the amount and quality of the investing tissues. These factors, in turn,

are determined by the crown:root ratio and by the number, shape, and size of the

roots. The use of a denture base of maximum size to help transmit stresses to the tis-

sues helps reduce the load on abutment teeth.

Quite often selective grinding of teeth may be required to obtain a correct rela-

tionship between opposing teeth which will minimize traumatic. effects of an in-

harmonious occlusion. Occasionally, judicious splinting is required to of fset shear

forces on abutments and to provide stability. Bilateral support will always provide

additional stability. Not to be overlooked is simplicity of design and precise fi t of

component parts.

With the biomechanical aspects established, the plan for the design of the restora-

tion may be formulated. The dental laboratory technician should be informed of

clinical aspects of the treatment plan which might conceivably af fect laboratory

procedures.

Ass ociate Clinic al Professor, Division of Fixrd Prosthodontics.

506


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Precision attachment restorations 507

EST BLISHING THE P TH OF INSERTION

A duplicate diagnostic cast is mounted on a surveyor table, and the tilt which

will determine the proper path of insertion of the restoration is selected. This angula-

tion will permit placement of the female part of each attachment within the normal

contours of the abutment teeth.

The smallest attachment available always seems too large for the abutment tooth.

Therefore, the selected path of insertion should be one which provides the greatest

conservation of tooth structure and health of the pulp.

Fine lines that are parallel to the long axes of the abutment teeth are drawn on

the sides of the base of the diagnostic cast. A surveyor analyzer rod is placed against

the proximal wall of an abutment to project its long axis. The angle formed by the

intersection of the extensions of these fine lines is bisected, and a mean line is drawn

on the cast. The table of the surveying instrument is adjusted so that the mean line

on the side of the cast is parallel to the analyzer rod of the surveyor, thus providing

the correct anteroposterior tilt . This angle of the cast is an average of the angles

formed by the proximal surfaces of the abutments and the base, and at this tilt,

the average amount of preparation of the involved teeth is required to accom-

modate the attachments. Any other angle would require the removal of an excess

amount of tooth structure from one or the other o f the abutments. The determined

anteroposterior tilt is maintained in all succeeding steps.

A visual examination is necessary to veri fy that the attachments can be placed

over the center of the ridge and still be positioned buccolingually within the contours

of the teeth. This buccolingual tilt of the cast is determined by placing the female

attachment on the mandrel in the surveyor and by placing the attachment against

the proximal surface of one abutment. The lateral tilt of the table is adjusted until

the attachment can be accommodated within the confines of the tooth and still can be

placed over the center of the ridge. Undercuts in the ridge are noted. A fine line is

drawn on the tooth parallel to the female attachment. The same procedure is fol-

lowed with the other abutment, and the lateral til t is adjusted to a harmonious

angle. This angle is usually parallel to the mean of the angle formed by the projec-

tion of lines parallel to the female attachments. Occasionally the tilt o f one or both

abutments is such that recontouring by means of a full veneer crown or reposition-

ing by orthodontic means will be required.

The long axes of the abutments may parallel each other, and there would be no

mean line. If this occurs, an upper cast is slightly tilted either anteriorly or poster-

iorly, and a lower cast is tilted posteriorly. This slight tilt will preclude the possibility

of the path of insertion being perpendicular to the occlusal plane and the subsequent

risk of the restoration being displaced easily by gravi ty, the pull of sticky foods, or

the uplifting push of the tongue.

POSITIONING THE TT CHMENT

When the proper tilt is selected, the surveyor table is locked in position. A man-

drel designed to fi t the attachment is mounted in the surveyor holder, and a female

part of the attachment is placed on the mandrel. The fla t side of the attachment is

positioned against the side of the abutment adjacent to the edentulous space. The

center of the mandrel should be directly over the center of the ridge. The outline of


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508 Lorencki

J. Pros. Dent.

May, 1969

the attachment is scribed on the abutment and will represent the buccolingual width

of the box form on the preparation necessary to house the female part of the attach-

ment .

The box forms are cut in the abutments on the cast to a size that will accom-

modate the female part of each attachment. The attachments must be of maximum

length and should be placed within the natural confines of the teeth.

The preparation of the box forms on the cast helps in formulating a blueprint for

the work to be carried out in the mouth. An acrylic resin, compound, or gutta-

percha matrix of each box is made. The matrix extends onto the occlusal surface

of the tooth and serves as an index to determine the minima1 size of the box when

the preparation is cut in the mouth.

Another type of template is made of gold and covers a great deal of the uncut

stone tooth but does not cover any part of the box form. This gold index casting ex-

tends to the periphery of the box form and merely outlines the box. When placed

on the uncut natural tooth, it will provide an outline of the extent of the structure

to be removed from the tooth to provide the necessary space for the attachment.

DDITION L IMPORT NT CONSIDER TIONS

The following additional factors are important considerations in planning pre-

cision attachment restorations.

1. It is not necessary for the abutments to be parallel to one another. Parallelism

is achieved by paralleling the attachments.

2. Space must be allowed for a thin layer o f gold between the female part of the

attachment and the walls of the prepared box.

3. The female part of the attachment should be located within the natural

contour of the coronal portion of the tooth to minimize stresses and to form proper

patterns for flow o f foods.

84. All the attachments should be approximately the same length to prevent

tipping and possible unseating of the restoration.

SUMM RY

The important aspects for planning precision attachment restorations have been

described. Thorough diagnosis, establishment of the path of insertion, and proper

positioning of attachments are critical to the success of the restoration and must be

carefully planned prior to initiation of cutting procedures.

References

1. Kabc enell, J. L.: Stress Breaking for Partial Dentures, J. A. D. A. 63: 593-602, 1961.

2. Lucia, V. 0.: Modern Gnathological Concepts, St. Louis, 1961, The C. V. Mosby Com-

paw.

3. Wilso n, W. H., and Lang, K. L.: Prac tical Crown and Bridge Prosthodo ntics, New York,

1962, McGraw-Hill Book Company, Inc., pp. 209-211.

4. Cohn, L. A.: The Phys iologic Bas is for Tooth Fixation in Precision-Attach ed Partial

Dentures, J. PROS. DEX T. 6: 220-244, 1956.

~JNIVER SITY OF CALIPORNIA

SAN FRANCISCO MEDICAL CENTER

SAN FRANCISCO, CALIF.

94122

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Planning Precision Attachments