Biomechanics of removable partial 4

Dentures
Biomechanical Considerations
Possible M ovements of Partial Denture Self-Assessment Aids

emovable partial dentures by design are intended supporting structures for removable partial dentures
R to be removed from and replaced (abutment teeth and residual ridges) are living things and
into the mouth. Because of this, they are not rigidly are subjected to forces. In consideration of maintaining
connected to the teeth or tissues, which means they the health of these structures, the dentist must consider
are subject to movement in response to functional direction, duration, and frequency of force application, as
loads such as those created by mastication. It is well as the magnitude of the force.
important for clinicians providing removable partial In the final analysis it is bone that provides the
denture service to understand the possible movements support for a removable prosthesis, that is, alveolar bone
in response to function and to be able to logically by way of the periodontal ligament and bone of the
design the component parts of the removable partial residual ridge through its soft tissue covering. If
denture to help control these movements. The potentially destructive forces can be minimized, then the
following biomechanical considerations provide a physiologic tolerances of the supporting structures are
background regarding principles of the movement generally capable of withstanding these forces without
potential associated with removable partial dentures, physiologic or pathologic change. To a great extent, the
and the subsequent chapters covering the various forces occurring through a removable prosthesis can be
component parts describe how these components are widely distributed, directed, and minimized by the
designed and how they are used to control the selection, the design, and the location of components of
resultant movements of the prostheses. the removable partial denture and by development of
harmonious occlusion.
Unquestionably, the design of removable partial
dentures requires mechanical and biologic
considerations. M ost dentists are capable of applying
simple mechanical principles to the design of a
BIOM ECHANICAL removable partial denture. For example, the lid of a paint
CONSIDERATIONS can is more easily
As M axfield stated, "Common observation clearly 25
indicates that the ability of living things to tolerate
force is largely dependent upon the magnitude or
intensity of the force." The
 26 M cCracken's removable partial prosthodontics

Fig. 4-2 Lever is simply a rigid bar supported Fig. 4-3 Distal extension removable partial denture will
somewhere between its two ends. It can be used to move rotate when force is directed on denture base. Differences
objects by application of force (weight) much less than in displaceability of periodontal ligament, supporting
weight of object being moved. abutment teeth, and soft tissues covering residual ridge
permit this rotation. It would seem that rotation of denture
is in combination of directions rather than unidirectional.
pried off with a screwdriver than it is with a half dollar!
The longer the handle, the less effort (force) it takes.
This is a simple application of the mechanics of
leverage. By the same token, a lever system represented wedge, screw, wheel and axle, pulley, and inclined plane
by a distal extension removable partial denture can (Fig. 4-1). Of the simple machines, the lever and the
magnify the applied force to the terminal abutments, inclined plane should be avoided in designing removable
which is most undesirable. partial dentures.
Tylman correctly stated, "Great caution and reserve In its simplest form, a lever is a rigid bar supported
are essential whenever an attempt is made to interpret somewhere along its length. It may rest on the support or
biological phenomena entirely by mathematical may be supported from
computation." However, an understanding of simple above. The support point of the lever is called the
machines should enhance our rationalization of the fulcrum, and the lever can move around the fulcrum (Fig.
design of removable partial dentures to accomplish the 4-2).
ob The rotational movement of an extension base type of
jective to preserve oral structures. A removable removable partial denture, when a force is placed on the
partial denture can be, and often is, unknowingly denture base,
designed as a destructive machine. is illustrated in Fig. 4-13. It will rotate in relation to the
M achines may be classified into two general three cranial planes because of differences in the support
categories: simple and complex. Complex machines are characteristics of the abutment teeth and the soft tissues
combinations of many simple machines. The six simple covering the residual ridge (Fig. 4-3). Even though
machines are lever,

0_.,..................... .".' """""'--'-'-__' ......... o_._...c.................... 0.. ......................... _ ................... , _____ .---..-
 Fig. 4-4 There are three classes of levers. Classification is based on location of fulcrum, F;
resistance, R; and direction of effort (force), E. Examples of each class are illustrated.

Effort
--3'-- 6'
IR1 E* M echanical - arm
,1\ advantage - Resistance
R__ Effort arm arm
f3Ol 6
M A=-=2
l!QJ arm 3
\J_lb.
M

Fig. 4-5 Length of lever from fulcrum, F, to resistance, R, is called resistance arm. That portion of
lever from fulcrum to point of application of force, E, is called effort arm. Whenever effort arm is
longer than resistance arm,
d mechanical advantage is in favor of effort arm, proportional to
difference in length of the two arms. In other words, when effort arm is twice the length of
resistance arm, 25-pound weight on effort arm will balance 50-pound weight at end of resistance
arm.

the gross movement of the denture may be small, the 4-6). A cantilever design should be avoided (Fig. 4-7).
potential exists for detrimental_ leverlike forces to be Examples of other leverlike designs, as well as
imposed on abutment teeth, especially when suggestions for alternative designs, to avoid or to
servicing (that is, relining) the prosthesis is neglected minimize their destructive potential are illustrated in Figs.
over a long period. There are three types of levers: 4-8 and 4-9.
first, second, and third class (Fig. 4-4). The potential A tooth is apparently better able to tolerate vertically
of a lever system to relatively magnify a force is directed forces than off-vertical, torquing, or near
illustrated in Fig. 4-5. horizontal forces. This characteristic is observed clinically
A cantilever is a beam supported only at one end and was substantiated many years ago by the work of
and can act as a first-class lever (Fig.
28 M cCracken's removable partial prosthodontics

Fig. 4-6 Cantilever can be described as rigid beam Fig. 4-7 Design often seen for distal extension
supported only at one end. When force is directed removable partial denture. Cast circumferential
against unsupported end of beam, cantilever can act as direct retainer engages mesiobuccal undercut and is
first-class lever. M echanical advantage in this illustration supported by distoclusal rest. This could be
is in favor of effoit arm. considered a cantilever design, and it may impart
detrimental first-class lever force to abutment if
tissue support under extension base allows excessive
vertical movement toward the residual ridge.

Fig. 4-8 Potential for first-class lever action exists in this Class II, modification 1, removable partial
denture framework. If cast circumferential direct retainer with a mesiobuccal undercut on right first
premolar were used, force placed on denture base could impart upward and posteriorly moving
force on premolar, resulting in loss of contact between premolar and canine. Tissue support from
extension base area is most important to minimize lever action of clasp. Retainer design could help
accommodate more of an anteriorly directed force during rotation of the denture base in an attempt
to maintain tooth contact. Other alternatives to first premolar design of direct retainer would be
tapered wrought-wire retentive arm that uses mesiobuccal undercut or just has buccal stabilizing
arm above height of contour.
 Chapter 4 Biomechanics of removable partial dentures 29

F F

A B

Fig. 4-9 Illustration A uses bar type of retainer, minor connector contacting guiding plane on distal
surface of premolar, and mesio-occlusal rest, to reduce cantilever or first-class lever force when
and if denture rotates toward residual ridge. B, Tapered wrought -wire retentive arm, minor
connector contacting guiding plane on distal surface of premolar, and mesio-occlusal rest. This
design is applicable when distobuccal undercut cannot be found or created or when tissue undercut
contraindicates placing bar-type retentive arm. This design would be kinder to periodontal ligament
than would cast, half-round retentive arm. Again, tissue support of extension base is key factor in
reducing lever action of clasp arm. Note: Depending on amount of contact of minor connector
proximal plate with guiding plane, fulcrum point will change.

Box and Synge* of Toronto. It seems rational that

more periodontal fibers are activated to resist the
application of vertical forces to teeth than are
activated to resist the application of off-vertical
forces (Fig. 4-10). t
Again, a distal extension removable partial denture
rotates when forces are applied to the artificial teeth
attached to the extension base. Because it can be
assumed that this rotation must create predominantly
off-vertical forces, location of stabilizing and
retentive components in relation to the horizontal
axis of rotation of the abutment becomes extremely
important. An abutment tooth will better tolerate off-
vertical forces if these forces accrue as near as
possible to the horizontal axis of rotation of the
abutment (Fig. 4-11). The axial surface contours of
abutment teeth must be altered to locate components
of direct retainer assemblies more favorably in
relation to the abutment's horizontal axis (Fig. 4-12).

Fig. 4-10 M ore periodontal fibers are activated to resist

forces directed vertically on tooth than are activated to
resist horizontally (off-vertical) directed force. Horizontal
'Box HK: Experimental traumatogenic occlusion in axis of rotation is located somewhere in root of tooth.
sheep, Oral Health 25:9, 1935.
 30 M cCracken's removable partial prosthodontics

I
.I
:. :
.I
U
A B Lingual

Buccal

Fig. 4-11 A, Fencepost is more readily removed by Fig. 4-12 Abutment has been contoured to allow rather
application of force near its top than by applying same favorable location of retentive and reciprocal-stabilizing
force nearer ground level. B, Retentive (buccal surface) components (mirror view), This is similar to lower figure
and reciprocal (lingual surface) components (mirror in Fig. 4-11, A.
view) of this direct retainer assembly are located much
nearer occlusal surface than they should be. This
tissues, the accuracy and extent of the denture base, and
represents similar effect of force application shown in top
the total functional load applied. A review of prosthesis
figure of illustration A.
rotational movement that is possible around various axes
in the mouth provides some understanding of how
component parts of removable partial dentures should be
prescribed to control 'prosthesis movement.
One movement is rotation about an axis through the
POSSIBLE MOVEMENTS OF PARTIAL
most posterior abutments. This axis may be through
DENTURE occlusal rests or any other rigid portion of a direct retainer
Presuming that direct retainers are functioning to assembly located occlusally or incisally to the height of
minimize vertical displacement, rotational movement will contour of the primary abutments (Fig. 4-13, A). This
occur about some axis as the distal extension base or axis, known as the fulcrum line, is the center of rotation
bases either move toward, away, or horizontally across as the distal extension base moves toward the supporting
the underlying tissues. Unfortunately, these possible tissues when an occlusal load is applied. The axis of
movements do not occur singularly or independently but rotation may shift toward more anteriorly placed com-
tend to be dynamic and all occur at the same time. The ponents, occlusal or incisal to the height of contour of the
greatest movement possible is found in the tooth-tissue- abutment, as the base moves away from the supporting
supported prosthesis because of the reliance on the distal tissues when vertical dislodging forces act on the partial
extension supporting tissue to share the functional loads denture. These dislodging forces result from the vertical
with the teeth. M ovement of a distal extension base
toward the ridge tissues will be proportionate to the
quality of those
 Chapter 4 Biomechanics of removable partial dentures 31

pull of food between opposing tooth surfaces, the

effect of moving border tissues, and the forces of
gravity against a maxillary partial denture. Presuming
that the direct retainers are functional and that the
supportive anterior components remain seated,
rotation rather than total displacement should occur.
Vertical tissueward movement of the denture base is
A
resisted by the tissues of the residual ridge in
proportion to the supporting quality of those tissues,
the accuracy of the fit of the denture base, and the
total amount of occlusal load applied. M ovement of
the base in the opposite direction is resisted by the
action of the retentive clasp arms on terminal abut-
ments and the action of stabilizing minor connectors
in conjunction with seated, vertical support elements
of the framework anterior to the terminal abutments
acting as indirect retainers. Indirect retainers should
be placed as far as possible from the distal extension
base, affording the best possible leverage advantage
against the liftlng of the distal extension base.
A second movement is rotation about a
longitudinal axis as the distal extension basemoves in
a rotary direction about the residual ridge (Fig. 4-13, B
B). This movement is resisted primarily by the
rigidity of the major and minor connectors and their
ability to resist torque. If the connectors are not rigid
or if a stress-breaker exists between the distal
extension base and the major connector, this rotation
about a longitudinal axis either applies undue stress
to the sides of the supporting ridge or causes
horizontal shifting of the denture base.
A third movement is rotation about an imaginary
vertical axis located near the center of the dental arch
(Fig. 4-13, C). This movement occurs under function
as diagonal and horizontal occlusal forces are
brought to bear on the partial denture. It is resisted by
stabilizing components, such as reciprocal clasp arms
and minor connectors that are in contact with vertical c
tooth surfaces. Such stabilizing components are
essential to any partial denture design regardless of
the manner of support and the type of direct retention
employed. Stabilizing components on one side of the
arch act to
stabilize the partial denture against horizontal
Fig.4-13 Three possible movements of distal extension
partial denture. A, Rotation around fulcrum line passing
through the most posterior abutments when denture base
moves vertically toward or away from supporting residual
ridges. B, Rotation around longitudinal axis formed by
crest of residual ridge. C, Rotation around vertical axis
located near center of arch.
32 M cCracken's removable partial prosthodontics

forces applied from the opposite side. It is obvious that ment occurs in all partial dentures; therefore stabilizing
rigid connectors must be used to make this effect components against horizontal movement must be
possible. incorporated into any partial denture design.
Horizontal forces always will exist to some degree For prostheses capable of movement in three planes,
because of lateral stresses occurring during mastication, occlusal rests should only provide occlusal support to
bruxism, clenching, and other patient habits. These forces resist tissueward movement. All movements of the partial
are accentuated by failure to consider the orientation of denture other than
the occlusal plane, the influence of malpositioned teeth in those in a tissueward direction should be
the arch, and the effect of abnormal jaw relationships. resisted by components other than occlusal rests. For the
The magnitude of lateral stress may be minimized by occlusal rest to enter into a stabilizing function would
fabricating an occlusion that is in harmony with the result in a direct transfer of torque to the abutment tooth.
opposing dentition and that is free of lateral interference Because movements around three different axes are
during eccentric jaw movements. possible in a distal extension partial denture, an occlusal
The amount of horizontal movement occurring in the rest for such a partial denture should not have steep
partial denture therefore depends on the magnitude of the vertical walls or locking dovetails, which could possibly
lateral forces that are applied and on the effectiveness of cause horizontal and torquing forces to be applied
the stabilizing components. intracoronally to the abutment tooth.
In a tooth-supported partial denture, movement of the In the tooth-supported denture, the only movements
base toward the edentulous ridge is prevented primarify of any significance are horizontal, and these may be
by the rests on the abutment teeth and to some degree by resisted by the stabilizing effect of components placed on
any rigid portion of the framework located occlusal to the the axial surfaces of the abutments. Therefore in the
height of contour. M ovement away from the edentulous toothsupported denture, the use of intracoronal rests is
ridge is prevented by the action of direct retainers on the permissible. In these instances, the rests provide not only
abutments that are situated at each end of each edentulous occlusal support but also significant horizontal
space and by the rigid, minor connector stabilizing stabilization.
components. Therefore the first of the three possible In contrast, all Class I and Class II partial dentures,
movements can be controlled in the tooth-supported having one or more distal extension bases, are not totally
denture. The second possible movement, which is about a tooth supported; neither are they completely retained by
longitudinal axis, is prevented by the rigid components of bounding
the direct retainers on the abutment teeth, as well as by abutments. Any extensive Class III or Class IV
the ability of the major connector to resist torque. This partial denture that does not have adequate abutment
movement is much less in the toothsupported denture support falls into the same category. These latter
because of the presence of posterior abutments. The third dentures may derive some support from the edentulous
possible move ridge and therefore may have a composite support from
both teeth and ridge tissues.
 Chapter 4 Biomechanics of removable partial dentures 33

SELF-ASSESSMENT 8. Of the simple machines, which two are more likely to

be encountered in the design of removable partial
AIDS
dentures?
1. What elements prevent movement of the base(s) of a 9. What is a lever? A cantilever?
tooth-supported denture toward the basal seats? 10. Name the three classes of levers and give an
2. M ovement of a distal extension base away from example of each.
basal seats will occur as a rotational movement 11. Of the three classes of lever systems, which
or as two are most likely to be encountered in
3. What is the difference between fulcrum line removable partial prosthodontics?
and axis of rotation? 12. Explain how one would figure the mechan
4. Identify the fulcrum line on a Class I ical advantage of a lever system, given
arch; a Class II, modification 1; and a dimensions of effort and resistance arms.
Class Iv. 13. What class lever system is most likely to be
5. In the treatment planning and design phase of encountered with a restoration on a Class II,
partial denture service, the functional movements modification 1, arch when a force is placed on the
of removable partial dentures should be extension base?
considered when de 14. What factor permits a distal extension denture to
signing the individual of rotate when the denture base is forced toward the
the prosthesis. basal seat?
6. Forces are transmitted to abutment teeth' 15. Is an abutment tooth better able to resist a force
and residual ridges by removable partial directed apically or directed horizontally? Why?
dentures. One of the factors of a force is its 16. Where is the horizontal (tipping) axis of an
magnitude. List the other three factors of a force abutment tooth located?
that a dentist must consider in designing 17. Why should components of a direct retainer
removable partial dentures. assembly be located as close to the tipping
7. The design of a removable restoration requires axis of a tooth as possible?
consideration of mechanics as well as biologic
considerations. True or false?