Management of loading forces on mandibular

distal-extension prostheses. Part II: Classification for

matching modalities to clinical situations
Brian D. Monteith, M.Ch.D.*
Medical University of Southern Africa, Faculty of Dentistry, Medunsa, Republic of South Africa

I n 1929 Wright 1 stated that " . . . the present and denture design seems more sensible than an arbitrary
future problem in the field of prosthesis pertains to the and indiscriminate adherence to a particular technique
preservation of the denture supporting structures." In for all patients. A n example is the "stressbreaker school"
1977, almost 50 years into that future, Hickey2 urged described by Miller? "Those that advocate that the
that patients be made to understand tlaat gums and bone residual ridges bear the brunt of the burden, and that the
were not intended to support dentures, and that teeth be spared all the stress possible, e m p l o y . . , a
" . . . there are changes.., caused by the loss of teeth stressbreaker.., so that the denture base can move
and the loads that dentures place upon the gums." independently Of the clasp, t h u s . . , sheltering the abut-
While both remarks were probably made primarily ment teeth almost entirely from the stress created by the
with complete dentures in mind, they are doubly perti- movement of the base." This implies total disregard for
nent when applied to distal-extension removable partial the resistance potential to stress of the two structures
dentures. The disparity of support that exists between involved. The application of such a treatment approach
tooth and alveolar ridge structures has given rise to for a patient with abutment teeth resistant to periodontal
numerous design philosophies. Unfortunately, some of breakdown in conjunction with severely resorbed residu-
these are claimed to be universal solutions to all remov- al ridges would be grossly irresponsible. The fact that
able partial denture problems; the claims being corrobo- such practices pass unchallenged serves to highlight the
rated by impressive numbers of successfully treated need for a greater awareness of how important it is that
patients. However, scant attention is given to the possi- the resistance potential of the supporting structures be
bility of tissue damage that might result from the assessed for each patient before the imposition of pros-
injudicious application of such methods to all patients. thesis stresses.
Certainly, when a periodontal text comments on the lack The purpose of this article is to present a simple
of experimental evidence available to assess the advan- classification by means of which the total resistance
tages of one removable partial denture design over potential of the supporting structures of a distal-
another and suggests that a choice be based on " . . . clin- extension jaw can be evaluated. This is achieved first by
ical judgments and inferences gained from clinical expe- identifying the ridge resistance and abutment resistance
rience, ''3 there seems to be room for a more reasoned of the jaw and then by awarding each of these either a
approach. positive or a negative score. Thus the jaw is classified as
falling into one of four possible categories (Table I). It is
CLASSIFICATION RATIONALE hoped that such a classification will increase practitioner
The response of the supporting structures to a pros- awareness that the supporting structures should dictate
thesis frequently varies greatly; some mouths appear to the choice of treatment modality. For instance, an
remain relatively healthy even in the presence of multi- indiscriminate approach to a Class IV jaw with negative
ple causative factors of deterioration. Dawson 4 ascribes ridge and abutment resistance factors will likely prove
this phenomenon to differences in host resistance less successful than if the same approach were applied to
brought about by the multifactorial interplay of caus- a Class I jaw, where both factors are positive.
ative and contributing factors. His plea is that the type of
response each patient presents to these factors should be DETERMINING RIDGE RESISTANCE
considered when the treatment plan is designed. The It has long been recognized that the rate of resorption
application of such an approach to removable partial of the residual ridge tissues varies among individuals and
within an individual at different times. 6 Much work has
been devoted to identifying the factors that act for or
*Professorand Head, Department of ProstheticDentistry. against this variation. By grouping them into (1) aria-
832 DECEMBER 1984 V O L U M E 52 NUMBER 6
CLASSIFICATION FOR PROSTHESIS DESIGN

Table I. Jaw classifications

Resistance Ridge Abutment
potential resistance resistance

Class I + +
C l a s s II + -
Class Ill - +
Class IV - -

tomic, (2) biologic, and (3) mechanical factors, Atwood 6

instilled an orderliness of approach that is of practical
value in the clinical context. 7 Anatomic factors include
the size and shape of the residual ridge, the type of bone,
and the nature of the periosteum. Biologic factors involve
age, sex, hormonal balance, dietary factors, and the
presence or absence of osteoporosis. Mechanical factors
feature two subgroups: functional factors, which have to
do with frequency, direction, and amount of force, and
prosthetic factors, which involve the denture base, the Fig. 1. Atwood's trefoil. A diagrammatic representa-
tion of how interaction of anatomic, biologic, and
form and type of teeth used, and the interocclusal
mechanical factors determine residual ridge resorption.
distance. Atwood 7 furthermore claims that the presence Extent is depicted by intensity of cross-hatching; the
of one or more of the three main categories will have a greater the influence of each group of factors, the more
directly proportional effect on the amount of residual the representative circles will overlap. (Redrawn from
ridge resorption that will be characteristic for an individ- Atwood, D.A.: Reduction of residual ridges. J PROSTHET
DENT 26:266, 1971.)
ual; a concept that is illustrated by his familiar trefoil
(Fig. 1).
In investigating ridge resistance, some authorities use
laboratory analyses of a patient's blood and stool, ~ possesses minimal resorptive potential; this is the only
particularly in relation to biologic factors such as diet type that warrants a positive score. The buccolingual
and hormonal balance. The simplest method to assess the resorption pattern that gives rise to the type II ridge
resistance of an individual's tissues to prosthodontic configuration, combined with the vertical bone loss of
stresses is to examine the amount of breakdown that has types III and IV, presupposes a lack of resistance of the
already taken place, because future trends may be fairly bone to load. Such load may result from denture pressure
reliably read in the tissue's past performance. When or, in mouths where atrophy has occurred in the absence
evaluating ridge resistance in this manner, it is helpful to of a prosthesis, to the pressures exerted by an atrophying
use a ridge c,lassification that reflects the sequential mucosa seeking a reduced area. TM In either event, residual
events of progressive alveolar resorption. Such a classifi- ridges with type II, III, or IV configurations suggest that
cation can be derived from that of Gershkoff and they are susceptible to breakdown, and consequently
Goldberg 9 and used to group residual ridges into four should be scored as having a negative ridge resistance.
categories:
DETERMINING ABUTMENT RESISTANCE
Type I: Characterized by high, well-rounded bone profiles,
indicative of minimal alveolar resorption In the search for some indication as to the possible
Type II: Buccolingual resorption has resulted m a high but behavior of the periodontal structures of abutment teeth
narrow knife-edge ridge that might be bony; however, under stress, evidence of previous tissue breakdown can
further vertical reduction of the alveolar bone results in be used in much the same way as when determining
the ridge becoming little more than a flabby, unsupported, ridge resistance.
fibrous renmant
Type III: Typified by a rounded but considerably lower Apical migration of junctional epithelium
residual ridge, resulting from the sequential disappear-
ance of the knife-edge of type II When evaluating periodontal integrity to assign an
Type IV: Reflects the terminal stage of residual ridge abutment resistance score, one of the two major features
resorption, the atrophying process having bypassed such to look for is evidence of apical migration of the
bony landraarks as the mylohyoid ridge and the external junctional epithelium of the projected abutment teeth.
oblique line This manifests as an increase in pocket depth with an
When a score is awarded to one of the above residual accompanying loss of bony support. Increased mobility
ridge types, it should be obvious that type I alone may or may not be present.
THE JOURNAL OF PROSTHETIC DENTISTRY 833
 MONTEITH

TOTAL RESISTANCE
CLASS I C L A S S II C L A S S III CLASS IV
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . i. . . . . . . . . . |

Ug FLEXIBLE DISTAL EXTENSION .................................................................

i
k- PROSTHESIS .............................................................. :"
i
.J

¢3 FLOATING DISTAt. EXTENSION ................................................................

O PROSTHESIS (HINDELS) :::::::::::::]:::::]:::::::::::::::::::::::::::::::::::::::::::::::]::::::]::::::::::::::::::::]::::::::::::::::
.;:::::::: ', . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.... i ............................................ i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a
a~
IAL;ERED CAST
L::: ::::::;: ::::::1:; ;; :;:11 ;:;:!:;11
(APPLEGATE)
:::;11 :;:;::;:l :;:11;;:;-'::.'1;:1:11 ::11:1:::1:: : : : 1 : : ' . : : : ',:: ,.r:; ,. : ; ; : : : _.: : : ; ; : ,. ; : 1 : : : ; : : 1:;1

Fig. 2. Matching removable partial denture modalities according to four classes of

supporting structures resistance. Note that stressbreaker designs (flexible denture bases)
and designs based on floating denture base concept should not be used in Class III or
Class IV jaws, which have a negative bone factor.

The significance of this first feature lies in the direction cannot occur. In recognition of its major
reduction of periodontal support, which results from the etiologic role, the presence of bacterial plaque constitutes
loss of much of the investing alveolar bone. It would the other major feature to observe in assessing abutment
certainly be irresponsible to impose prosthesis load on a resistance of a partially edentulous jaw.
tooth that, because of reduced periodontal support, is The actual etiologic mechanisms are complex but in
only able to keep itself standing. Secondary occlusal essence involve the immunologic response of the tissues
trauma describes such a condition; that is, one in which to the ingress of endotoxins that derive from the plaque
the tissues are in such a weakened state because of organisms. 12Bone resorption itself is thought to occur in
previous breakdown that they are unable to resist forces response to the release of various lymphokines as part of
that, when applied to normal tissues, will cause no the immunologic reaction. 13 The speed with which this
damage. To impose the additional load of a removable happens, as well as the fact that it happens at all, is
partial denture on teeth that exhibit secondary occlusal determined by various contributing factors. A contribut-
trauma will amount to an unacceptable occlusal insult. ing factor, according to Dawson's 4 definition, is some-
It should be stressed, however, that although occlusal thing that cannot by itself cause disease, but which acts
stresses can cause damage, they cannot on their own by lowering the resistance of the host to a separate
cause apical migration of the junctional epithelium. causative factor. In the present instance the latter is
Poison and Zander" achieved damage that amounted to bacterial plaque.
a 35% loss of alveolar bone around teeth by subjecting In this context, occlusal trauma acts as a contributing
them to experimental stresses. The bone showed a factor by siding with the forces of periodontal breakdown
remarkable degree of regeneration when the stresses against those of attempted repair. Therefore, if loss of
were removed; while the periodontal ligaments, which periodontal support has occurred and plaque is present,
had been reduced to an amorphous mass, exhibited a even if the condition appears to have stabilized, the
swift recovery of normal fiber orientation and cellularity. introduction of occlusal stresses by a removable partial
Most significant, however, was the observation that the denture may so decrease the resistance of the periodon-
junctional epithelium had steadfastly maintained its tium to the bacterial factors of plaque that swift and
original level throughout the experiment. relentless breakdown can result. Such a situation should
be scored as having a negative abutment resistance.
Bacterial plaque On the other hand, with no evidence of epithelial
It is clear that there must be some other cause for the migration and no signs of vertical bone loss, a jaw should
kind of breakdown that results in a change in the be rated as having a positive abutment resistance even if
epithelial level. This is attributed to the effects of plaque is present. Obviously, such a patient should be
bacterial plaque. Without the presence of bacterial motivated to improve the plaque index; however, the fact
plaque, weakening of periodontal support in a vertical that the bacterial plaque has not led to periodontal

8,34 DECEMBER 1984 V O L U M E 52 NUMBER 6

CLASSIFICATION FOR PROSTHESIS DESIGN

breakdown (particularly if the patient is elderly) sug- A prime consideration when negative abutment resis-
gests the presence of abutment resistance sufficiently tance is present is the removal of the chief etiologic factor
strong as to render the introduction of occlusal stresses in periodontal breakdown. Whether the cooperation of a
by a prosthesi,; a justifiable risk. periodontist is deemed necessary or not, it should be a
rule that no treatment for a patient of this class should be
APPROACH TO TOTAL RESISTANCE attempted before a total commitment to plaque control
POTENTIAL has been established by the patient.
Once the respective ridge and abutment resistance
scores have been determined in a specific situation,
Class Ill: Ridge resistance negative, abutment
reference to the classification will give an immediate
resistance positive
indication as to the most biologically suitable design The management of unfavorable ridges in terms of
modality (Fig. 2). Atwood's trefoil (Fig. 1) presumes an inadequate set of
anatomic factors and a commensurate need for improve-
Class I: Ridge resistance positive, abutment ment of the other two groups of factors. Improvement of
resistance positive the biologic group of factors will be seen as an attempt to
Obtaining tile greatest possible denture base coverage improve a defective state of resistance and will involve a
is a welt-established prosthodontic principle and one dietary regimen such as that prescribed by Barone. 16
which is in total accord with the "square law" relating Mechanical factors, on the other hand, are intimately
tissue displacement to area. 14 The ridge resistance of the bound up with prosthesis design and should fulfill the
type I alveolar ridge is vested in the fact that with requirements of a minimal load transmission to the
minimal alveolar resorption, its area remains largely insufficiently resistant tissues. The presence of resistant
undiminished. This is also true of its endowment of abutment teeth in combination with unfavorable residu-
keratinized epithelium, which according to Fleisch and al ridges contraindicates the use of stressbreakers, float-
Austin ~Sis the oral covering best able to resist stress. Any ing denture base designs, or any other modality that
design modality can, therefore, be used for this class of spares the teeth by directing loading forces toward the
patient with a high expectation for success. ridges. The altered cast principle appears to provide the
best chance of success because it provides the only means
Class II: Ridge resistance positive, abutment by which the ridge tissue can be loaded without exceed-
resistance negative ing the load-bearing capacity of any constituent part of
This class is the only one in which the approach of the the total ridge.
stressbreaker school mentioned above may be justified.
Any design modality that seeks to direct functional loads Class IV: Ridge resistance negative, abutment
away from the teeth toward the ridge will be applicable. resistance negative
Design considerations should be aimed at protecting the When both tooth and ridge tissues lack resistance, the
teeth from torsional forces. Extracoronal rigid attach- proposed regimens for meeting inadequacies in one or
ments should be avoided and special attention given to the other might be tried in combination, provided that
clasp design and the location of rest seats. Crowns on shielding the one does not compromise the other. Plaque
abutment teeth permit the use of splinting as a means of control and building up resistance by dietary means
periodontal stabilization and minimize less than ideal would be mandatory; however, the minimization of
crown/root ratios through the use of intracoronal attach- mechanical insult presents one of the most difficult of
ments or deep-seated rests. It was postulated 14 that prosthodontic problems.
floating denture base prostheses remain essentially tis-
sue-borne. This property makes them suited to this SUMMARY
category, particularly in variations that involve the use of When a mandibular distal-extension removable par-
intracoronal attachments where the male portion is tial denture is planned, consideration should be given to
effectively prevented from contacting its seat. This the capacity of the supporting structures to accept the
technical modification, however, does not preclude the functional loading of the prosthesis. To prescribe a
use of the "altered cast" principles14; for although the treatment modality that will not cause their breakdown,
male portion in this situation is encouraged to make an analysis should be made of the relative strength and
seating contact, the event is timed to occur at the moment weakness of the ridges and abutment teeth. This judg-
of optimal residual ridge displacement. In this manner, ment can be visualized by classifying partially edentu-
most of the loading force will have been taken up by the lous jaws into one of the four categories that are possible
resistant ridge before the point where the abutments when the two key factors of ridge resistance and abut-
become loaded. ment teeth are combined.

THE JOURNAL OF PROSTHETIC DENTISTRY 835

 MONTEITH

REFERENCES
Lundeen, H. C., and Gibbs, C. H., editors: Advances in
1. Wright, W. H.: The importance of tissue changes under artificial Occlusion. Boston, 1982, John Wright & Sons, pp 143-148.
dentures. J Am Dent Assoc 16:1027, 1929_ 12. Hausmann, E.: Potential pathways for bone resorption in human
2. Hickey, J. C.: Denturism--We must make an ethical choice. J periodontal disease. J Periodontol 45:338, 1974.
PROSTHETDENT 38:360, 1977. 13. Horton, J. E., Oppenheim, J. J_, and Mergenhagen, S. E.: A role
3. Carranza, F. A.: Glickman's Clinical Periodontology, ed 5. for cell-mediated immunity in the pathogenesis of periodontal
Philadelphia, 1979, W. B. Saunders Co., p 1017_ disease. J Periodontol 45:351, 1974.
4. Dawson, P. E.: Evaluation, Diagnosis and Treatment of Occlusal 14. Monteith, B. D.: Management of loading forces on mandibular
Problems. St. Louis, 1974, The C- V. Mosby Co., pp 1-10. distal-extension prostheses. Part I: An evaluation of concepts for
5. Miller, E. L.: Removable Partial Prosthodontics. Baltimore, design. J PROSTHETDENT 52:673, 1984.
1972, The Williams & Wilkins Co., pp 141-142. 15. Fleisch, L., and Austin, J. G.: A histological study of the response
6. Atwood, D. A.: Some clinical factors related to rate of resorption of masticatory and lining mucosa to mechanical loading in the
of residual ridges. J PROSTHETDENT 12:441, 1962. vervet monkey. J PROSTHETDENT 39:211, 1978.
7. Atwood, D. A.: Reduction of residual ridges. J PROSTHETDENT 16. Barone, J. V.: Nutrition--Phase one of the edentulous patient.
26:266, 1971. J PROSTHETDENT 40:122, 1978.
8. Page, M. E.: Systemic and prosthodontic treatment to prevent
bone resorption in edentulous patients. J PROSTHET DENT Reprint requests to:
33:483, 1975_ DR. BRIAND MONTEITH
9. Gershk0ff, A., and Goldberg, N. I." Implant Dentures. London, MEDICALUNIVERSITYOF SOUTHAFRICA
1957, Pitman Medical Publishing Co. Ltd., pp 56-58. FACULTYOF DENTISTRY
10. Lammie, G. A.: Aging changes and the complete lower denture. J P.O. MEDDNSA0204
PROSTHETDENT 6:450, 1956. REPUBLICOF SOUTHAFRICA
11. Poison, A. M., and Zander, H. A.: Occlusal traumatism. In

Observations on the histologic features of the

human edentulous ridge. Part III: Bone
D. D. Krajicek, D.D.S.,* J o h n D o o n e r , D.D.S.,** and Kendall Porter, D.D.S.***
Topeka, Kan., and University of Missouri-Kansas City, School of Dentistry, Kansas City, Mo.

Study of the bone of the edentulous residual ridge -: y

has included the n a t u r e of the periosteum; the type of "*
bone, trabeculae, and m a r r o w present; the presence of ~ ~.
bone resorption and apposition; and the presence of ,i ~ -~:~
bone "defects" and i n f l a m m a t i o n . 1-2 T h e objective of ~': ~ : ~

Presented at the International Association for Dental Research

meeting, Chicago, Ill.
Supported by research funds from the Veterans Administration.
*Private practice; Visiting Professor, Howard University, School of
Dentistry, Washington, D.C_; formerly, Chief, Dental Service,
Leavenworth Veterans Administration Medical Center, Leaven-
worth, Kan.
**Chief, Dental Services, Leavenworth Veterans Administration
Medical Center, Leavenworth, Kan.; Associate Professor, Univer-
sity of Missouri-Kansas City, School of Dentistry_ Fig. 1. R i d g e crest area i l l u s t r a t i n g o s t e o g e n i c perios-
***Professor of Dentistry, Department of Histology, University of teal layer (arrow). ( H e m a t o x y l i n a n d eosin. O r i g i n a l
Missouri-Kansas City, School of Dentistry. m a g n i f i c a t i o n x450.)

836 DECEMBER1984 VOLUME52 NUMBER6