A modified direct retainer design for distal-extension removable partial dentures

CURTIS

9.

10. 11.

A modified direct retainer distal-extension removable Z. Ben-& Tel

12.

abnormal jaw relations: maxillary protrusion and wider upper arch. J PROSTHET DENT 1974;32:107-11. Wright WH. Selection and arrangement of artificial teeth for complete prosthetic dentures. J Am Dent Assoc 1936;23:22912307. Payne SH. Selective occlusion. J PROSTHET DENT 1955;s: 301-4. Kelly E. Changes caused by a mandibular removable partial denture opposing a maxillary complete denture. J PROSTHET DENT 1972;27:140-50.

Aviv

D.M.D.,+ University,

The

I. Aviv, Maurice

D.M.D.,** and Gabriela

Saunders TR, Gillis RE, Desjardins RP. The maxillary complete denture opposing the mandibuIar bilateral distal-extension partial denture: Treatment considerations. J PROSTHET DENT 1979;41:124-8.

Reprint requests to: DR. THOMAS A. CURTIS UNIVERSITY OF CALIFORNIA, SAN FRANCISCO SCHOOL OF DENTISTRY Box 0758 SAN FRANCISCO. CA 94143-0758

design for partial dentures

and H. S. Cardash, B.D.S., L.D.S.+** Goldschleger

V

arious extracoronal clasp assemblies have been designed for abutment teeth adjacent to a distalextension ridge to allow rotation of a removable partial denture base toward tissue without torquing the clasped tooth.’ Kratochvil* popularized the proximal plate/ mesio-occlusal rest/I-bar (RPI) design. Krol’ modified the proximal plate, naming the system “The RPI bar clasp design.” Proponents of the RPI system claim that (1) it is more esthetically acceptable, (2) tooth coverage is minimal, (3) the retainer disengages when unfavorable forces are applied, and (4) adequate retention is available with minimum undercuts. The effectiveness of this assembly can be increased if the I-bar is replaced by an L-shaped bar direct retainer. The retainer has been described as one half T-bar or a modified T-bar clasp.4 The mesio-occlusal rest and proximal plate are designed as in the RPI clasp assembly. However, the I-bar is replaced by an L-shaped direct retainer arising from the framework distobuccal to the abutment tooth. The L-bar crosses the gingival margin of the abutment tooth in the shortest possible line, ascends to the survey line, and engages the distobuccal undercut (Fig. 1).

*Clinical Lecturer, Department of Oral Rehabilitation. **Instructor, Department of Oral Rehabilitation. ***Clinical Senior Lecturer, Department of Oral Rehabilitation 342

ET AL

School

of Dental

Medicine,

Tel

Aviv,

Israel

RATIONALE OF THE L-BAR CLASP Location of the retentive clasp tip The retentive tip of a clasp should be placed on the abutment tooth so as to allow it to disengage as the distal-extension base moves toward tissue and thus avoid creating a class II lever effect on the abutment. A retentive clasp tip placed distal to the horizontal axis of rotation (through the occlusal rests) will meet this demand.*%3 For the purpose of considering retainer movement around an occlusal rest axis of rotation, the retentive surface of the tooth can be divided into (1) a region where vertical movement only occurs, (2) a region of horizontal movement, and (3) an intermediate zone in which both vertical and horizontal movements take place (Fig. 2). A retentive clasp tip placed on the most distal part of the buccal surface at the same horizontal level as the axis of rotation (rests) will undergo a downward vertical movement and disengage as the distal-extension base moves toward tissue (Fig. 1). If a retentive clasp tip (an I-bar) is placed more apical to or nearer to the axis of rotation, a more anterior movement will occur (Fig. 3). A clasp assembly designed with a distally placed clasp tip will reduce this anterior movement of the tip to negligible proportions.

Length

of the distal-extension

base

The longer the distal-extension base, the smaller the effect of the movement of the retentive clasp toward SEPTEMBER

1988

VOLUME

60

NUMBER

3

MODIFIED

DIRECT

RETAINER

DESIGN

Fig. 1. L-bar direct retainer. Semilunar lines represent different radii of rotation of retainer tip around fulcrum F (rest).

Fig. 3. I-bar placed more apical to or nearer rest(F) will move anteriorly as denture base moves toward tissue.

Fig. 4. Effect of length of denture base on clasp movement. AB = Distance moved by denture base; CD = distance of clasp tip to fulcrum; x = distance clasp tip moves. Fig. 2. Radii of rotation around rest F. Three retentive surfaces. (A), Vertical movement zone, (B) horizontal movement zone, (C) intermediate zone.

reducesthe amount of vertical movementof the clasptip, permitting

tissue (Fig. 4). The vertical movement of a clasp tip x on the abutment relatlive to the length of the distal-extension

basesmay be deducedmathematically.: AB

X

Tana=BD=G

xx-----

AB

CD BD

When the most distal abutment is a first premolar, the length of the distal-extension basehas beencalculated at approximately 48 mm.5A 2 mm movementtoward tissue at the distal end of the extension basewill result in 0.25 mm vertical movement of the retentive clasp tip. A retentive clasp placedin the region of the abutment tooth designatedfor vertical movement will move downward and will disengage.whenocclusalforcesare placedon the denture base.

Height

of the ab,utment

tooth

The greater the height of the clinical crown of the abutment tooth, the more difficult it is to place the retentive clasp tip at or near the samehorizontal level as the occlusal rest. Esthetics is compromisedand a long undesirable flexible bar is required. This is especially true when a mandibular canine is used as a distal abutment. However, the long distal-extension base THE

JOURNAL

OF PROSTHETIC

DENTISTRY

Depth

it to be placed in a more apical

position.

of the rest seat

The deepestpart of the rest seat preparation will act as the axis of rotation of the distal-extension removable partial denture. A minimum depth of 1 to 1.5 mm has beensuggested.’The more apical the fulcrum, the easier it is to place the clasptip at the samehorizontal plane as the occlusalrest and in the vertical movement region of the retentive surface.

Advantages

of the L-bar

The I-bar is free to disengagefrom the tooth when rotation of the denture occurs around the mesio-occlusal resk6 The L-bar when placed near or at the same horizontal level as the occlusal rest alsofrees itself from the abutment tooth. Becauseit is locatedmore distally on the buccal surface, the L-bar is more esthetically acceptable. The absenceof undercuts on the buccal surface of a mandibular premolar or a canine necessitatestoothrecontouring if an I-bar is used. This recontouring may have to be extended anteriorly to provide freedom for disengagementof the I-bar as the extension basemoves toward tissue. A distobuccal undercut is invariably present for use of an L-bar. A distal path of insertion may be usedwithout stressingthe abutment tooth. The L-bar is easier to grasp, facilitating removal of the prosthesis. 343

BEN-UR,

SUMMARY

3.

The rationale for designing a direct retainer for a distal-extension removable partial denture is described. The advantages of an L-bar clasp arm over the I-bar clasp arm are discussed. The retentive surface of an abutment tooth is divided into three zones according to the clasp tip movement and the importance of placing the retentive tip in the zone of vertical movement is emphasized. REFERENCES 1.

2.

Henderson D, McGivney GP, Castleberry DJ. McCracken’s removable partial prosthodontics. 7th ed. St. Louis: The CV Mosby Co, 1985;107. Kratochvil FJ. Influence of occlusal rest position on movement of abutment teeth. J PROSTHET DENT 1963;13:114-23.

An alternate centric relation a distal-extension removable M. J. Edge, D.M.D.,*

and James M. Podnar,

4. 5. 6.

AVIV,

AND

CARDASH

Krol AJ. Clasp design for extension-base removable partial dentures. J PR~STHET DENT 1973;29:408-15. Boucher LJ, Renner RP. Treatment of partially edentulous patients. St Louis: The CV Mosby Co, 1982;26, 147. Kraus B, Jordan RE, Abrams L. Dental anatomy and occlusion. 6th ed. Baltimore: The Williams & Wilkins Co, 1964;6-116. Demer WJ. An analysis of mesial rest-I-bar clasp designs. J PROSTHET DENT 1976;36:243-53.

Reprint requests to: DR. Z. BEN-UR TEL AVIV UNIVERSITY THE MAURICE AND GABRIELA DENTAL TEL AVIV

GOLDSCHLEGER

SCHWL

OF

MEDICINE

ISRAEL

recording technique partial denture

for

D.D.S.**

United States Army Dental Command, Fort Gordon, Ga.

Various authors have advocated the using of acrylic resin record bases to record centric relation when making distal-extension removable partial dentures.‘s2 These The opinions or assertions contained herein are the private views of the authors and are not be to construed as official or as reflecting the views of the Department of the Army or Department of Defense. *Colonel, U.S. Army, Dental Corps; Chief, Fixed Prosthodontics. **Major, U.S. Army, Dental Corps; U.S. Army DENTAC, Fort Eustis, Va.

record bases are accurate, reliable, and indicated, especially when a corrected impression technique has been used. However, in many instances where the distalextension base is small and/or unilateral, accurate and dependable records may also be obtained by other means. This article describes an alternate technique for obtaining centric relation records in these instances by using a kneadable silicone impression putty instead of acrylic resin record bases.

Fig. 1. Framework in place showing putty through retentive mesh.

Fig. 2. Patient closed in centric relation with putty interposed between framework mesh and opposing occlusion.

344

extruding

SEPTEMBER

1988

VOLUME

60

NUMBER

3