Removable partial dentures

The rationale of resilient hinge-action stressbreakers

Merrill C. Mensor, Jr., A.B., D.D.S.*

San Mateo, Calif.

T he use of stressbreaker joints in tissue-borne partial dentures is always a con- troversial subject. Gnathological and periodontal gpproaches of ideal fun&ion and splinting of groups of teeth have contributed to the preservation of the oral struc- tures. These factors can be enhanced by the recognition and employment of sound, scientific methods of stress direction that are presented in this article.

Stress is a function of force per unit area; it has cofactors of duration, intensity, and frequency exhibiting themselves as vector quantities of shear, torque, and mo- ment. The reluctance of prosthodontists to use stressbreakers has been largely due to excessive wear and frequent breakage or to excessive and nonbiologic movement which causes premature resorption of the bearing area. The vertically resilient hinge-action stressbreakers provide a selected area of clinical function. They serve as the connector elements in partial dentures, they promote hygiene and esthetics, and they contribute to the preservation of the teeth and their supporting structures.

The need for stressbreakers on free-end removable partial dentures has been recognized on the basis that the resilience or displaceability of the mucosal tissue ranges between 0.4 and 2 mm.: while the vertical resilience of a normal, healthy tooth in its socket is approximately 0.1 mrn.tl, e This tissue resilience differential of 4 to 20 times the axial displaceability of a normal tooth in its socket dictates the necessity for some form of stress direction in the partial denture design.

ORIGINAL DATA

Some of the earlier research on tissue movement was done with the Gerber at- tachments at the University of Zirich ( 1954 and 1956) under the guidance of pro- fessor Dr. Albert Gerber and W. Fener.8 Most of the work on the Dalbo attachment

Read before the American Prosthodontic Society in Las Vegas, Nev.

*Instructor, Prosthetic Dentistry--Crown and Bridge, School of Dentistry, College of Physicians and Surgeons, University of the Pacific, San Francisco, Calif.

tHans Dalla Bona, Biel, Switzerland: Personal communication, June, 1962.

$A. Gerber, University of Ziirich: Personal communication, July, 1965.

y&urirK~ ‘3” ”

Resilient hinge-action stressbreakers 205

was carried out under the direction of Professor Dr. H. Rehm in the Prosthetic De- partment of the University of Freiburg in Breisgau, West Germany (1958 and 1962) .3-5 The research report” presented by Dr. Korber of the University of Frei- burg on soft and hard tissue movement is the most significant as an aid to the logi- cal evaluation of the concepts to be considered.*

The figures and legends should be studied to achieve thought in the fourth dimension. Korber divided tooth movement into its many component factors as shown by his diagrams and graphs, He started with the known motion differences between an anchor tooth and the free-end denture base, and he made an attempt to differentiate the entire movement complex of mastication into its individual com- ponents. The single movement components were called movement processes of (1) the anchor tooth, (2) p eriodontal, distal movement capacity, (3) bending the anchor tooth in its longitudinal axis, (4) periodontal intrusion, and (5) compressing the tooth itself, as well as the biostatic reaction of the mucous membrane. Measured function components were studied with modern procedures by technical measure- ment, and the original diagrams of the directly recorded measurement values re- vealed ranges of individual movement components which permit a number of statements to be made relative to the tilting movement of the anchor teeth under different supporting and fixing elements.

Schilli4 reported on the Dalbo attachment on eleven patients comparing its various function potentials with those of the standard free-end partial denture. A more elaborate study was carried out by Kohl3 who used the Dalbo attachment to evaluate resilient silicone as a denture base material. These were parallel studies on 8 different patients employing a telescopic crown carrying a free-end partial denture and employing the same type of telescopic element to carry the Dalbo attachment in its various applications. The findings of these studies are illustrated by two graphs showing the pressure curves (Figs. 1 and 2). The highest values were obtained by the spring-loaded attachments and by the vertical resilience without a spring, i.e., the tissue-borne, vertically resilient attachment doubled the effectiveness of the prosthesis. Also, the resilient bases showed an over-all average 8 to 10 per cent im- provement in chewing function in both the spring-loaded and nonspring-loaded partial dentures. All the curves in the second graph (Fig. 2) show a characteristic tendency to drop off at the distal end of the denture base which tends to support the concept of maximum base extension and shortened occlusal table in an antero- posterior direction to obtain the “snowshoe” effect of support.

ENGINEERING BASIS

The influence diagram with its associated curve for each condition of shear and moment (Fig. 3) graphically illustrates the fallacies of the many dogmas of concept and design now in use. Two definitions are necessary to understand the influence diagram61 7: Shear is a stress resulting from applied forces that cause two contiguous parts of a body to slide, relative to each other, in a direction parallel to their place of contact. Moment is any force that will create rotation about a given point. Look-

*The material was presented in German and has never been published in the English language; the parts of the text utilized here have been translated by this writer.

100

90

80

70

60

++--+--I

/

---- CASE1 ----- CASES

-- CASE 2 ++++cf*~ CASE6

CASE 3 --+-+- CASE7

. . . . . . . . . . . . . . . C-E 4 *-cc CASEB

I NORMAL FREE-END PROSTHESIS, TISSUE-BORNE

II DALBD - ATTACHMENT PROSTHESIS, HARD BASE, WITHOUT SPRING

Ill DALBO - ATTACHMENT PROSTHESIS, SOFT BASE, WITHOUT SPRING

IV DALBD - ATTACHMENT PROSTHESIS, HARD BASE, WITH SPRtNG

V DALBO - ATTACHMENT PROSTHESIS, SOFT BASE, WITH SPRtNG

Fig. 1. Average pressure curves of 8 different patients for the 5 partial denture situations. The ordinate represents percentages with 100 being the arbitrary maximum, and the abscissa represents the 5 different partial denturr situations (represented by reman numerals). (After Kiihl, University of Freiburg.)

Volume 20 Number 3 Re.rilient hinge-action stressbreakers 207

1. BICUSPID 2. BICUSPID I. MOLAR 2. MOLAR

---.I- DALBO - ATTACHMENT PROSTHESIS, SOFT BASE, WITH SPRING

DALBO - ATTACHMENT PROSTHESIS, HARD BASE, WITH SPRING

-m--m- DALBO - ATTACHMENT PROSTHESIS, SOFT BASE, WITHOUT SPRING

m--m DALBO - ATTACHMENT PROSTHESIS, HARD BASE, WITHOUT SPRING

-m--M NORMAL FREE-END PROSTHESIS, TISSUE-BORNE

Fig. 2. Average pressure curves of each partial denture situation for the 8 patients. The ordinate represents the average forces applied in kilograms, and the abscissa represents the position of the applied forces on the denture base of the prosthesis. (After K6h1, University of Freiburg.)

208 Mensor J. Pros. Dent. Srptember, 1968

I Rigid

Fig. 3. Influence Diagram: S, shear; M, moment; F, a forre applied at any given point on the occlusal table; Rigid, the standard frw-end removable partial drnture; Hinge, hinge-type par- tial dentures; Vu.-Res., the vertically resilient hinge-action stress directors.

Can+* t

Con+* - Fig. 4. Types of Beams: Cant., cantilever: Sup., support; Cont., continu~lrrs

ing at the diagrams in Fig. 3, we observe that, by definition, the standard free-end partial denture has both shear and moment, the hinge type partial denture has shear only, and the vertically resilient hinge action partial denture has neither of the quantities within the area of function.

The objective of both the “mucostatic” techniques and the vertically resilient

hinge-action stressbreakers is to develop the denture base into a continuously sup- ported beam represented by the lower figure in the beam system diagram (Fig. 4) .6 These are static representations of a biologically active system. Since all free-end

Volume 20 Number 3 Resilient hinge-action stressbreakers 209

denture base connecting elements are analogous to an eccentrically loaded beam, the analogies will be true for the intracoronal as well as the extracoronal connectors.

The planes of function of the vertically resilient hinge-action stressbreakers can be divided into vertical, horizontal, and sagittal planes.8 Let us assume for discussion that the prosthesis was independent of the anchor tooth; we can then see that there would be multidirectional forces applied to the denture base of the pros- thesis. The function of the attachment is to select and direct those forces which would be beneficial to the gnathostomatic system while ruling out the other com- ponent forces through proper design and engineering.

PHYSIOLOGIC BASIS

Wolff9 stated that “every change in the use or static relations of bone leads not only to a change in its internal structure and architecture, but also to a change in its external form and function.” According to Boucher,lO “stress is force or pressure exceeding that produced in normal functions. Stress exerted against the teeth and their attachment apparatuses by occlusal forces may be within the adaptive capacities of the tissues or the tissues may not be capable of compensation and adaptation, and the result is tissue destruction.” As stated earlier, stress is a function of force per unit area and has cofactors of duration, intensity, and frequency ex- hibiting themselves as vector quantities of shear, torque, and moment. BoucheP states that “bone in stress with applications of excessive pressure stimuli to bone, adaptation may occur by the formation of thicker and more numerous trabeculae; or, if tissue components cannot compensate for excessive stress, bone resorption will occur.” A stressbreaker is defined3 as “a device that is incorporated in a removable partial denture to relieve the abutment teeth of part or all the induced load to insure their loading being kept within safe tolerance limits.”

Most of us accept the fact that forces are best tolerated down the long axis of the tooth and over the crest of the ridges in the denture-bearing area. There are some philosophies of rehabilitation that dictate a narrow buccolingual width of the occlusal table in balanced occlusion, and there are gnathologic approaches which dictate normal tooth contour and dimension on the occlusal table.* It is not the purpose of this article to advocate any of these philosophies but rather to point them out in the over-all consideration of the design of the prosthesis.

THE DALBO SYSTEM

According to the classification system of Nally,‘l the Dalbo attachments fall into the classification subdivision of extracoronal stressbreaker conjunctors. The simplicity of use, assembly, application, and esthetics in partial denture prosthesis and full-mouth reconstruction allows a wide employment of the Dalbo attachment especially in Applegate’s Class I and II patients. I2 In some instances, the lateral

stability in the attachment design for unilateral partial dentures permits the use of a design without cross-arch stabilization. This has to be determined for each

individual patient, and the problem of cross-arch stabilization will follow.

*C. Stuart, San Francisco, Calif.: Personal communications, 1965.

210 Metuor J. Pros. Dent. Srptember, 1968

The design of the Dalbo 667 is based on the Roach system of channel conjunc- tors13 (Fig. 5) which consists of a T-shaped beam with a ball-shaped appendix that receives the boxing. It is closed on one end and open on the other, and it extends into two flanges which are machined to engage the “T” beam as a guide rail. The base of the hull is grooved to form the lamellae which engages the ball-shaped appendix in a spring clasp action. The hull itself holds a small rebound spring of a stainless steel alloy which permits the attachment to ha1.e a maximum vertical translation of 1 mm.

Dalbo stressbreakers are divided into four, standard size groups and four Ceram- icor size groups with integral flanges for castimg. In function: the Dalbo attachment is a vertically resilient hinge that has no shear or moment within the envelope of motion. It is passive at rest and displaces tissue only in function. The postoperative roentgenographic evaluations demonstrate the lack of breakdown of the periodontal membrane and underlying bone, and clinically, ljaticnts verify this by their lack of awareness of the prosthesis in the mouth. It must be pointed out that the spring load of the Dalbo attachment maintains the denture base at a passive rest position when not in use. In function, it actively permits the denture base to have a transla- tion of the hinge action, vertical resilience. and a combination of these basic actions throughout the entire rnasticatory stroke. This deli\.ers a physiologically normal stimulation to the underlying tissue and qrcatly reduces or eliminates one of the major causes of pressure atrophy of the ridge. At the same time it eliminates the Class I lever action of nonrnovable partial dentures or the shear and moment effect of the hinge-action or vertical slot-type of partial dentures on the anchor tooth and its underlying structures.

The Dalbo attachment was developed by Hans Dalla Bona of Biel, Switzerland.

z%eT”, ‘3” Resilient hinge-action stressbreakers 211

It was first distributed for general usage in 1954.” In 1962, the Dalbo attachment underwent a major design change for retention purposes. In 1964, the design was further improved to include a flange on the male component to facilitate the direct casting in the Ceramicor version for the porcelain bonded to metal technique.

The Dalbo attachment is an extracoronal attachment and it can be used for patients of all ages. The abutment design assumes that the dentist has a clear under- standing of margin control, reduction for retention, and box form crown design for three-quarter crowns to prevent the “pulling” of the margin near the attachment. Full coverage is always preferred for the primary anchor tooth in attachment prothesis design.

CROSS-ARCH STABILIZATION

The need for cross-arch stabilization can be best understood by visualizing a multidirectional movement potential. The denture base can have a “fishtail” or “yawing” action. It can have a bodily movement or an axial rotation “roll” (rotation around the vertical axis of the abutment tooth). Cross-arch stabilization will elimi- nate these movements if it is able to anchor the prosthesis, if it has available move- ment so as to allow for the full potential of the working attachment side, and if it is rigid enough to provide lateral stability for the partial denture in function. The lack of cross-arch components in the partial denture design has resulted in a great number of failures in otherwise sound partial denture construction.t6

The design of a unilateral partial denture with a free-end denture base must incorporate a double abutment for lateral stability and protection of the abutment teeth. The partial denture design should incorporate cross-arch stabilization if (1) the denture base carries more than two teeth, (2) the residual alveolar ridge area is relatively flat or absent, (3) the abutment teeth are periodontally involved, (4) the “transtrusion” movements create excessive torque, (5) the denture base surface area coverage is inadequate, or (6) the abutments involve anterior teeth.

In bilateral partial dentures, the cross-arch stabilization is supplied by the lingual and palatal bars, and it is not necessary to use the double abutments for the reasons mentioned in the unilateral application.

The minimal amount of movement required for the cross-arch stabilizer is shown in Fig. 6. Assume that the attachment should move 1 mm. and that the distance between the stabilizer and the compensated attachment is 40 mm. The sine of the angle of lo multiplied by the arch width will indicate a movement potential of greater than 2 mm. Therefore, any cross-arch attachment that will meet the de- mands, as outlined, and have less than lo of movement without creating a de- structive force on the compensating side can be employed. For this purpose, the Huser Hookz4t I3 has been quite successful, as has been the Roach attachment and the B-M-B Lock.§ Cross-arch stabilization must be employed to permit the orienta-

*Hans Dalla Bona, Biel, Switzerland: Personal communication, July, 1962.

+E. H. Dyer, San Francisco, Calif.: Personal communications, April, 1964.

SCendres & Mktaux S.A., Biel/Bienne, Switzerland.

§MCtaux Prdcieux S.A., Neuchatel, Switzerland.

212 Mensor J. Pros. Dent. September, 1968

given 40mm.

C.A. 2mm.

Max. Attachment is l+mm.

k- 40mm -j

Fig. 6. The resultant movement of the sine of the angle times thr cross-arch width proves that the cross-arch stabilizer must have less than 1” of freedom in order to be effective. Sine < 1’ = sine of the angle of lo ; C..4. 2 mm. = the resultant movement of the sine times the arch width (40 mm. = average width of arch) ; Max. Attachment, = the maximum movement possible with the Dalbo attachment.

tion of the attachments so that maximal use of the vertically resilient hinge can be realized without producing destructive forces within the envelope of closing movc- ments. The rectilinear direction of the attachment system and its maximum surface area contact is enhanced by relating the attachment position to a line running through the central fossae of the occlusion (Fig. 7). This cannot be accomplished on arch forms whose ridge crest parallelism exceeds approximately 5’ from the median sagittal line without the help of a universal joint stabilizer. The interrupted line that courses from the anchor tooth through the central fossae of the occlusal table determines the alignment orientation of the attachment so that the ultimate line of force will travel through the long axis and down the ridge crest to effectively direct the forces into the most favorable area.

DALBO-M ATTACHMENT

The last major design change of the Dalbo stress directors is incorporated in the Dalbo-M series of attachments. This new design has all the desirable features of the Dalbo system plus a positional guidance lock to aid in the fabrication of the prosthesis and the refinement of the occlusion.

The general description of the Dalbo stress director applies to the Dalbo-M with the addition of a bilateral groove and a bilateral tapped flange with the threaded hole coincidental with the groove. The function of the slot is to permit the insertion and removal of the appliance with the screw in the locked position. The groo\Te receives the tip of the screw to maintain concentricity of the attach- ment. \;t’ith the screw in the locked position, the balancing or idling interferences can be eliminated, the occlusion can be refined and positively related to the fixed components, and the prosthesis may be quickly and inexpensively relined. A “muco- static” base can be developed on a two-stage technique, and a functionally generated path technique can be employed with the locked attachment. The Dalbo-M has two screws. One is long for processing, and the other is the functional locking screw.

Resilient hinge-action stressbreakers 213

Fig. 7. The orientation of the attachment relative to the ridge crest and the occlusal table.

The locking screw is not threaded externally, but it has locking notches for fixation in the acrylic resin saddle. When the locking screw is backed out of the groove, the full function potential of the standard Dalbo attachment comes into play. This at- tachment can serve as a nonresilient connector, and two locking screws with a plug of acrylic resin can be substituted for the spring in the attachment. The Dalbo-M can be employed for fixed removable partial dentures as well as for free-end partial denture construction.

SUMMARY

European studies with the Dalbo attachments were reviewed. These were sup- ported by the influence diagrams together with the principles of engineering and

214 Mensor J. Pros. l)ent. Stptrmber, 1968

Fig. 8. Occlusal, latrral, and oblique views of the Dalbo-M attachnrrnt.

physioloLgy to establish the biomechanical rationale in the selection of the vertically resilient hinge-action stressbreakers. Cross-arch stabilization was discussed. The Dalbo-M attachment was presented for the first time as a fully controllable system. The results of the experiments at the IJniversity of Freiburg as well as the clinical experience with patients shows that the stress-directed partial denture “conditions” the mucous membrane and the underlying osseous structure to a point where the stresses of mastication can be tolerated.

References

1. Steiger, A. A., and Boitcl, R. H.: Precision Work for Partial Dentures, Ziirich, Switzer- land, 1959, Berichthaus, pp. 143-144, 157, 20.5.

2. Rehm, H., Kijrber, E., and KGrber, K. H.: Biophsikalischer Beitrag zur Problematik starr abgestutzter Freiendprothesen, Deutsch. Zahnaerztl Z. 17: pp. 963-1075, 1962.

Volume 20 Number 3 Resilient hinge-action stressbreakers 215

3. K6h1, K.: Untersuchungen iiber die Belastbarkeit des zahnlosen Alveolarfortsatzes unter verschiedenen prothetischen Bedingungen mit besonderer Bcrucksichtigung von weich- bleibendem Kunststoff, Dissertation, University of Freiburg in Breisgau, West Germany, 1961.

4. Schilli, G.: Ein Beitrag zur Versorgung einseitiger Freiendliicken mit Hilfe des Dalbo- Gelenkes. Dissertation, University of Freiburg in Breisgau, West Germany, 1959.

5. Schreiber, S.: Klinische Untersuchungen und Messung tiber die Belastbarkeit einseitiger Freiendprothesen, Deutsch. Zahnarztbl. 14: 5-l 1, 1960.

6. Eshbach, 0. W.: Hand Book of Engineering Fundamentals, ed. 7, New York, 1958, John Wiley & Sons, Inc., pp. 502, 532-534.

7. Webster’s Third New International Dictionary, Springfield, Mass., 1961, G. & C. Mer- riam Company.

8. Tabet, G.: Classification CinCmatiques des Attachements Rupteurs de Forces: Indication en Prothtse DCcolletCe, Rev. Fran+ Odonto-Stomatol. 8: 6, 1961.

9. Blakiston’s New Gould Medical Dictionary, Philadelphia, 1949, Blakiston Company, p. 1143.

10. Boucher, C. 0.: Current Clinical Dental Terminology, St. Louis, 1963, The C. V. Mosby Company.

11. Nally, J. N.: The Use of Prefabricated Precision Attachments, Internat. D. J. 11: 192-216, 1961.

12. Applegate, 0. C.: Essentials of Removable Denture Prosthesis, Philadelphia, 1954, W. B. Saunders Company.

13. Mensor, M. C., Jr.: PrincipIes and Application of the Vertically Resilient Hinge Action Stressbreakers in Partial Denture Construction, NADL Operational Text Material, vol. 1, No. 15, pp. 1 and 3, 1964.

14. Mensor, M. C., Jr.: Principles and Application of the Vertically Resilient Hinge Action Stressbreakers in Partial Denture Construction, NADL Operational Text Material, vol. 1, No. 16, p. 7, 1965.

15. Ibid.: p. 9. 16. Ibid.: p. 16.

MILLS SQUARE TOWER 100 S. ELLSWORTH AVE. SAN MATEO, CALIF. 94401