Direct retainers for distal-extension removable partial dentures

REMOVABLE

PROSTHODONTICS

SECTION EDITORS

LOUIS BLATTERFEIN

S. HOWARD PAYNE

Direct retainers for distal-extension partial dentures

removable

Richard P. Frank, D.D.S., M.S.D.* University of Washington, School of Dentistry, Seattle, Wash.

T he selection of direct retainers has stimulated general discussion, numerous articles, and considerable interest among dentists for many years. Definite opinions have been formed as to which retainer type is best suited for a distal-extension removable partial denture. The advantages and disadvantages of several frequently used clasp assemblies will be evaluated in an attempt to combine philosophies and place them in perspective. The basis of this evaluation is the dental literature and longitudinal observations of removable partial dentures constructed at the University of Washington.

SURVEYING The guiding planes are identified by survey of the cast. Guiding planes are parallel surfaces, found on the proximal and sometimes the lingual surfaces of teeth, that guide the insertion and removal of a removable partial denture. A retentive undercut exists only if it is related to a definite path of removal. Thus, parallel surfaces must be identified before retentive undercuts. Krol’ suggested that the metal guiding plates on a framework for a distal-extension prosthesis should contact only the gingival portion of proximal guiding planes to avoid the development of an unfavorable fulcrum point when the distal-extension base moves gingivally. However, greater coverage of the proximal guiding planes has been shown to be acceptable if the casting is adjusted to allow some gingival movement of the distalextension base.2

RECIPROCATION Reciprocal elements can be (1) a clasp arm, (2) a lingual plate, or (3) a minor connector (Fig. 1). A lingual clasp arm appears to be the reciprocal element of choice when a distal rest is used because there is less soft tissue coverage. Schwalm et al.3 evaluated the results of removable partial dentures that had been worn for 1 to 2 years. They found that more plaque accumulated on surfaces that were covered by the framework, with significantly more gingivitis in these regions. The

Presented at the Academy of Denture Prosthetics, Kansas City, MO. *Professor, Department of Prosthodontics. 562

same group of patients, after 8 to 9 years of denture wearing were examined by Chandler and Brudvik.4 No differences in plaque levels were found regardless of the design of the prosthesis. However, less gingivitis was found in regions not covered by the removable partial denture. Hence, it seems advisable to avoid covering the gingival tissues with a removable partial denture framework whenever possible. However, results may not be as good as expected when only a small amount of tissue is left exposed. The gingival tissue may swell or hypertrophy due to encroachment by the surrounding framework (Fig. 2). Thus, it seems best to either expose a large section of gingival tissue or cover it completely with the metal framework.

Lingual plate A lingual .plate major connector is used instead of a lingual bar when there is a lack of space or if it is likely that artificial teeth will need to be added to the framework at.a later time. The lingual plate also serves as the reciprocal element. Goodkind illustrated the potential wedging effect of a lingual plate at the distolingual line angle of an abutment tooth when an extension base moves toward tissue. The wedging effect can force the abutment tooth facially as a result of pivoting of the metal framework where it contacts the tooth above the height of contour. During recall appointments, the dentist should be especially careful to examine this region on cast crowns and the corresponding region inside the framework for a shiny facet. The facet indicates. that an undesirable fulcrum point has developed. The framework can be relieved or the denture base relined, or both. Krol’ and EliasorP have proposed a method to avoid an undesirable fulcrum point near the origin of a cast retentive clasp arm. Only the occlusal edge of the rigid portion of the clasp arm contacts the height of contour. The corresponding gingival edge of the clasp arm does not touch the tooth. Thus, the clasp is able to disengage from the tooth during movement of the distal-extension base toward tissue. The same concept should be applied to wrought wire retentive clasp arms near their origins, where they are inflexible, and to clasp arms and plates that serve as reciprocal elements. Lingual plates should NOVEMBER

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Fig. 1. A, Reciprocation is provided by a clasp arm on left molar and premolar and by minor connectors at mesial and distal lingual line angles of right premolar. B, Lingual plating on the canines and first premolars provides

Fig. 2. A and l3, Tissue lingual to first premolar has hypertrophied into small open area of framework. (Photographs courtesy of Dr. James Brudvik. j

reciprocation.

make contact only at the height of contour. It is advisable to critically evaluate the completed framework and to be prepared to make difficult and time-consuming adjustments of the metal if the lingual

plate is overextended

(Fig. 3). Wrought wires that are positioned incorrectly relative to the height of contour should be removed, readapted, and resoldered.

Minor connector The third form of reciprocation is a minor connector supporting a mesial occlusal rest. Reciprocation is obtained from contact by the framework with the tooth at the mesiolingual line angle and the distal surface.7 There is no lingual contact. In the lower jaw, this form requires adequate space between the functional level of the floor of the mouth and the free gingival margin to allow adequate gingival exposure. Use of minor connector reciprocation minimizes tissue coverage and undesirable fulcrum points. However, minimal tooth contact by the framework may result in a lack of retention and stability if the abutment teeth have short clinical crowns or

nonparallel

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reciprocal DENTISTRY

surfaces.

This

Fig. 3. Lingual plate covers most of lingual surfaces. Excessive contact above height of contour can introduce undesirable fulcrum points.

problem becomes acute if the dental technician is overzealous in finishing the framework (Fig. 4).

RETENTION Wrought wire clasp arm A wrought wire clasp arm is preferred for retention when the undercut occurs in the mesial third of the buccal surface of an abutment tooth (Fig. 5). The 563

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Fig. 4. Excessive finishing on the inner surface of minor connectors quickly results in an unstable framework since this design has minimal tooth contact.

Fig. 6. Poor clasp arm form results if height of contour is in occlusal third of tooth near clasp origin.

Fig. 5. This properly adapted wrought wire clasp arm engages an undercut in mesial third of buccal surface.

Fig. 7. Clasp arm form, length, function, and appearance are inadequate. The clasp arm should parallel free gingival margin if height of contour permits.

terminal third of the clasp arm lies gingival to the height of contour while the proximal two thirds lies on the height of contour. The advantages of a wrought wire clasp arm are adjustability, lack of tissue coverage, and repairability. Wrought wire clasp arms have several disadvantages. They may be poorly adapted by the dental technician. However, the dentist may be responsible for poor clasp form by failing to remove an interference at the proximal buccal line angle (Fig. 6). A second problem with wrought wire clasp arms is the loss of adaptation of the wire after a period of time, which may be due to improper placement of the wire. When the wire is directed gingivdly in a straight line toward the undercut, the only portion of the clasp that contacts the height of contour during insertion is the tip of the clasp arm (Fig. 7). This type of contact can cause distortion with repeated insertion and removal. The clasp arm should

cross the height of contour to the desired depth of undercut and then curve occlusally to parallel the height of contour. This allows a significant length of the clasp arm to contact the height of contour during insertion or removal. Proper form and material selection can eliminate the problem of loss of adaptation of a wrought wire clasp arm. Another objection to the use of wrought wire clasp arms is their susceptibility to fracture. However, this problem can be controlled by improved construction techniques and careful selection of material. Brudvik and Wormley* measured the effect of solder joint location on clasp flexibility. More than 40% of the flexibility was lost if the framework was cast to the wire or if the wire was soldered on the distal guiding plate. Only a 6% decrease was noted if the wire was soldered farther back on the framework. Clinical observation of removable partial dentures with clasp solder joints placed in this

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Table I. Relation between wrought diameter and clinical factors

wire

Wire diameter (inch) Clinical factor Periodontal support

Reciprocation

Clasp length

Undercut depth

Retention desired

16 ga Degree

(0.040)

Good Average Poor Poor (divergent surfaces) Average Good (parallel surfaces) Long (9+ mm) Average (7-8 mm) Short (5-6 mm) Shallow (0.010 in) Average (0.015 in) Deep (0.020 in) Good Average Easily removed

19 ga (0.036)

20 ga (0.032)

X X X X X X X X X X X X X X X

location has revealed a dramatic reduction in the number of fractured clasp arms and an increase in their flexibility. Choosing a wire. Many dentists do not take advantage of the many different wrought wires available. Proper material selection is crucial in determining whether wires maintain their adaptation, whether breakage will be a problem, and whether flexibility will be adequate. Guidelines for choosing the appropriate gauge of wire are abutment periodontal support, degree of reciprocation, clasp arm length, undercut depth, and amount of retention desired. Table I summarizes the relation between gauge of wire and these clinical factors. In most instances, the five factors listed for selecting the wire gauge will not be in complete agreement. Judgement is required to determine which of the factors are most important. As a general guide, an 18- or 19-gauge wire is used on natural canines because they usually exhibit good root form, poor reciprocation, and will accept an average or long clasp arm. Natural mandibular premolars usually receive a 19- or 20-gauge wire because of their tapered root form, fair to poor reciprocation, and possibilities of average or shorter clasp-arm length. A 19-gauge wire is most useful for maxillary premolars. A 19- or 20-gauge wire is indicated for abutment teeth with surveyed crowns that have precise reciprocation. Different gauges may be chosen for the

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Fig. 8. Both clasps have been properly placed for this lower removable partial denture. Esthetic advantage of a bar clasp versus a wrought wire clasp for a distalextension prostlhesis is arguable.

DENTISTRY

Fig. 9. This I-bar clasp arm has inadequate contact with tooth because of excessive finishing.

abutment teeth within the same dental arch, if deemed advisable. I-bar clasp arm Abutment teet.h that have an undercut in the midbuccal portion of the tooth can often accept a cast I-bar clasp arm. It establishes minimal tooth contact and thus reduces the potential for caries and the potential of the clasp arm to apply destructive forces to the tooth when the distal-extension base moves. Bar clasp arms resist lifting of the distal-extension base better than do wrought wire clasp arms.9 The type of clasp arm used has a much greater influence on the amount of denture base displacement than does the presence or the location of an indirect re:tainer. Many dentists believe that bar clasp arms are esthetically superior to other clasp arms, but this evaluation can be influenced grcatiy by clasp arm position (Fig. 8).

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when there is a large undercut on the tooth or alveolus. Encompassing more than half of the circumference of the abutment tooth is critical when a bar clasp arm is used, because of minimal tooth-framework contact. Without this amount of contact, the removable partial denture may move distally away from the tooth. Retention and stability will be inadequate even though guiding planes and retentive undercuts have been identified. The position of the right and left mesial lingual portions of the abutment teeth most often determine whether the prosthesis will remain in the intended position. A T-shaped bar clasp arm can be used when these tooth surfaces diverge distally (Fig. 10). The mesial portion of the T does not engage an undercut, but it does wrap sufficiently around the tooth to prevent distal movement of the prosthesis. Krol’ has shown how reciprocation can be obtained with an I-bar clasp arm under the same conditions when it is placed mesial to the point of greatest convexity on the buccal surface.

L-bar clasp arm

Fig. 10. A, Lingual surfaces of first premoiars diverge toward distal. Resistance to distal movement of prosthesis will be minimal. B, A mesial extension on a bar clasp arm can be used to obtain adequate encirclement when shape or position of abutment tooth does not provide predictable reciprocation.

A common problem that occurs when an I-bar clasp arm is used to retain a distal-extension prosthesis is inadequate retention because either too little undercut was originally selected or the casting was slightly overfinished. Adjusting the cast retentive arm to increase the retention is difficult. This objection can be overcome by using a wrought wire in forming the I-bar arm. This method combines the adjustability of the wire with the advantages of the bar clasp arm. Excessive finishing of the tissue side of an I-bar clasp arm can be a problem (Fig. 9). Often, the only portion of the I-bar clasp arm that contacts the tooth is the tip of the arm at the height of contour which results in a lack of retention. Contact with the tooth is desired from the height of contour to the chosen depth of undercut. Another problem that can occur with a bar clasp arm is a large space between the approach part of the arm and the underlying soft tissue. This space can cause trapping of food debris and irritation of the cheek mucosa, The space may be necessary to allow seating of the casting

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The L-bar is another form of bar clasp arm. It is used when an undercut is located in the distal third of the buccal surface of an abutment tooth. From a theoretical standpoint, this clasp may be unable to disengage from the tooth as the distal-extension base moves toward tissue.’ The clasp arm would bind against the tooth and create a mesially directed force on the tooth. However, the clasp arm is quite flexible because of its length, and no ill effects from binding have been noted clinically. The L-bar clasp arm is less likely than the I-bar clasp arm to result in insufficient retention in the completed framework caused by incorrect waxing or finishing techniques because of more tooth contact.

SUMMARY Clinical observations at the University of Washington indicated that the wrought wire clasp assembly, the I-bar clasp assembly, and the L-bar clasp assembly yield similar results. A periodic recall system for patients with removable partial dentures has been in effect since 1976. Several thousand observations have been made by the faculty, and no special trend of problems has been noted that would cause one clasp assembly to be favored over another. These three types of clasp assemblies work satisfactorily when properly chosen and carefully applied. REFERENCES

1. Krol AJ: RPI (rest,proximalplate,I bar) claspretainerand its modifications. Dent Clin North Am 17:631, 1973.

2. KratochvilFJ, CaputoAA: Photoelastic analysisof pressureon teeth and bonesupportingremovablepartial dentures. J PROSTHETD~~~32:52,1974.

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Schwalm CA, Smith DE, Erikson JD: A clinical study of patients 1 to 2 years after placement of removable partial dentures. J PROSTHET DENT 38~380, 1977. Chandler JA, Brudvik JS: Clinical evaluation of patients eight to nine years after placement of removable partial dentures. J PROSTHET DENT .51:736, 1984.

5. Goodkind RJ: The effects of removable partial dentures on abutment tooth mobility: A clinical study. J PROSTHET DENT 30:139, 1973. 0. Eliason CM: RPA clasp design for distal-extension removable partial dentures. J PROSTHET DENT 49:25, 1983. 7. Kratochvil FJ: Influence of occlusal rest position and clasp design on movement of abutment teeth. J PROSTHET DENT 13~114, 1963.

8.

Brudvik JS, Wormley JH: Construction techniques for wroughtwire retentive clasp arms as related to c!asp flexibility. ,J

9.

Frank RP, N&olls JI: An investigation of ihe effectiveness of indirect retainers. J PROSTHET DENT 383494. 1077.

PR~STHET DENT 30~769, 1973.

Reprint requeststo: DR. RICHARD P. FI:ANK UNIVERSITY OF WASHINGTON SCHOLL OF DENTISTRY, SM-52 SEATTLE, WA 98195

A cephalaimetric technique for prosthodontic di *s and treatment planning Spiro J. Chaconas, D.D.S., MS.,* and Demitrios Gonidis, D.D.S.** University Greece

of California,

School of Dentistry,

Los Angeles, Calif.;

A

nthropologists have long studied the ethnographic determination of facial form and pattern. By studying different ethnic, age, and sex groups and by measuring the size of the various parts and recording variations in position and shape of cranial and facial structures, broad standards were devised that describe the human head. As a specialized part of anthropometry, the “measurement of man,” study of the head became known as “craniometry” or “cephalometry.” Development of cephalometric analyses has enabled the orthodontist to study various skeletal and dental relationships that correlate radiographic measurements with clinical observations. Orthodontists now use cephalometric techniques to plan treatment, to monitor the patient during therapy, and to analyze growth and mechanotherapy after active patient care. Through these studies, much information pertinent to prosthodontic diagnosis, treatment planning, and prognosis has become evident. However, most prosthodontists seldom use cephalometric techniques and analyses as major diagnostic entities. This article will review basic cephalometric land-

*Professor of Orthodontics, University of California, Los Angeles, School of Dentistry. **Lecturer in Prosthetics, University of Athens, School of Dentistry.

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University

of Athens, School of Dentistry,

Athens.

marks and analyses and suggest a method of use for complete denture patients.

REVIEW OF THE LITl3RATWRE In 1931 cephalometry was reported in the literature almost simultaneously by Broadbent,’ an orthodontist, and Hofrath,2 a prosthodontist. Broadbent’s objective was mainly to provide a technique to measure craniofacial growth changes, whereas Hofrath’s idea was to evaluate the results of prosthodontic recanstruction. The clinical phase of cephalometry was developed by the contributions of orthodontists such as Downs,3 Riedel,4 Steiner,5 T~eed,~ and Ricketts.’ A few dentists have made attempts to use cephalometry as a diagnostic tool in prosthodontics. In 1956, Ricketts’ discussed variations in facial morphology as observed cephalometrically, and also emphasized the use of laminography in determining temporomandibular joint (TMJ) disturbances. NassiP later related the position of the mandibular central in&or to the lower lip to correctly position artificial anterior teeth. L’Estrange and Vig’O studied the orientation of the occlusal plane in dentulous and edentulous subjects. Their results indicated a close angular relationship between the oc~lusal and maxillary planes for both the dentulous and edentulous groups. DiPietro and Moe@&‘” discussed the significance of the Frankfort mandibular plane angle to

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