10. Loads and Levers Flashcards
Overview
Partial dentures subject to many forces
____-vertical, lateral
____ foods- lifting
Actions of tongue, lips & cheeks
Need to review basic elementary school physics of loads and levers
If you understand how a PRDP works you know how to approach each design challenge.
chewing
sticky
Loads \_\_\_\_ support Directly to \_\_\_\_ crown Between two supports fixed bridge or tooth supported PRDP
tooth
single
Loads How is the load applied? \_\_\_\_ vs unequal \_\_\_\_ bridge PRDP with only \_\_\_\_
equal
fixed
rests
Loads
Tissue support
Prosthesis must be designed so that supporting tissues withstand the ____
____ PRDP need support from teeth and tissues
Tissues are ____
Load is ____ the support (free end saddle) must understand lever system to minimize torsional stresses
loads
large
compressible
outside
Levers
Class I
Load (L) weight or force to be acted upon
Effort (E) weight or force required to cause action
Fulcrum (F) pivot point
Perfect system is static:
E x distance to F = L x distance to F
no ____ advantage
L nearer F & L greater E
mechanical ____
L further F & E greater L
mechanical ____
mecahnical
advantage
advantage
Levers
The greater distance of the effort from the fulcrum the ____ the mechanical advantage
bigger
Levers
Class II
Load ____ the fulcrum and the effort
Mechanical advantage as load always ____ fulcrum
between
nearer
Levers
Class III
Effort lies ____ the fulcrum and load
Mechanical disadvantage as the effort is always ____ to the fulcrum
between
nearer
Loads and Levers
Applied load is the ____ force on the denture teeth in the case of support or the displacing force on the denture teeth on opening when chewing sticky food
Fulcrum is the point about which the denture tries to ____
Effort is the restraining component usually the ____
biting
rotate
clasps
Support
Healthy tooth can withstand its own occlusal loading plus ____ of a similar tooth
Soft tissue support should cover the greatest possible ____
____ of occlusal area will reduce the load required to penetrate a bolus of food
1.5
area
reduction
Tooth supported prosthesis \_\_\_\_ supply the support Scenario 1 Short saddle even load on \_\_\_\_ and molars
rests
premolars
Tooth supported prosthesis Scenario 2 Move \_\_\_\_ support further away from the load Prevent \_\_\_\_ of the premolar
anterior
overload
Tooth supported prosthesis
Scenario 3
Anterior support moved even further from the ____,
molar load ____
load
increases
Tooth supported prosthesis
Scenario 4
Multiple ____, load shared by the ____ and premolar
support
canine
Tooth supported prosthesis
Scenario 5
Moving the molar rest to the ____ will result in the ____ and canine sharing the load equally
distal
molar
Tooth &Tissue Supported Prosthesis
Under masticatory load tooth may be intruded ____ mm whereas the mucosa can be compressed up to ____ mm
In Kennedy Class I and II situations the tooth and tissue support is partly unyielding and yielding which can lead to prosthesis ____
Mixing the two forms
of support is not easy,
rocking will occur about the ____ of the most ____ substance
0.1 2 rocking edge unyielding
Tooth &Tissue Supported Prosthesis
The prosthesis will rock about the occlusal rest nearest the ____ part of the prosthesis when the tissue supported part is ____
The second premolar takes too much load
and torque, mechanically and clinically bad
tissue
loaded
Tooth & Tissue Supported Prosthesis
Move the occlusal rest further from the ____
Better, less load & torque to the first premolar, more
compressive support from the soft tissue
Common practice to place rest on ____ side of premolar adjacent to the saddle
saddle
mesial
Tissue & Tooth Supported Prosthesis
Are more rests mechanically useful?
____, adding an additional ____ rest moves the fulcrum to that rest making any rest anterior useless for support
no
distal
Reciprocal Action(reciprocation)
Reciprocal arm lies on the ____ but not in the undercut area.
Prevents ____ of a tooth by a direct retainer
A lone clasp arm without reciprocation will place unacceptable lateral ____ during insertion, removal & function creating and intermittent orthodontic appliance
survey line
displacement
stresses
Bracing
Counteract ____ displacing forces applied to the PRDP
Resistance to ____ components of force applied to the prosthesis as a whole
The ____ arm or denture base provides bracing action
lateral
horizontal
reciprocal
Indirect Retention
Prevents the ____ of a saddle which is cantilevered out from a direct retainer, such as free- end saddle or curved anterior saddle.
Indirect retention is always a Class ____ lever system with ____ mechanical advantage
displacement
III
no
Where would you put the nail (E)?
As a Class III system the ____ E is to the lifting load L the better.
nearer
Fulcrum cannot move must be on ____
Effort is where the ____ (retentive tips) resist ____ movement
teeth
upward
Indirect Retention Class \_\_\_\_ lever system Fulcrum axis cannot \_\_\_\_ (on teeth F-F) Effort (E-E) where clasps (retentive tips) resist \_\_\_\_ movement Load is force trying to displace \_\_\_\_ Posterior part of denture is retained by the \_\_\_\_ lever system
III move upward saddles indirect
Indirect Retention
Indirect retention of a saddle cannot be achieved by ____ or rests acting on their own. Indirect retention can only be achieved through a ____ of clasps and rests to form a lever system to retain the free part of the denture.
clasps
combination
Indirect Retention
Changing the lengths of the ____ alters the efficiency of the system and the degree of indirect retention possible.
The ____ the distance between the rests and the tips of this clasps the better, as it ____ the retaining lever.
lever arms
greater
lengthens
Indirect Retention
The retaining lever is so short that it would be useless against the leverage of the displacing force unless the clasp was so strong that
it would apply an ____ force on the tooth with relatively little ____ force applied to the saddle
extracting
displacing
Indirect Retention
The retaining lever must be a ____ as possible
Fulcrum to Effort
long
Indirect Retention
Indirect retainer for a saddle is a ____ element the other side of a ____ joining the tips of the direct retaining elements.
supporting
line
Indirect retention
- ____ arms
(cingulum rests) - ____ extension of the denture base
cummer
backward
Indirect Retention
Indirect retention is never a Class ____ or Class ____ lever system, which presupposes that the clasp(s) is the fulcrum
The fulcrum is on the ____ (which do not move) and the effort is the tip of the ____ (which can move)
The displacing load (sticky food) is applied to, for example, a ____ saddle as the mouth opens
The prosthesis will try to displace by ____ about the fulcrum created by the most ____ supporting component of the denture, perhaps a cingulum rest
Resistance to movement, the effort , will be provided by the ____(s) placed as far ____ from the fulcrum as possible
I II teeth clasps free-end rotation distant clasp away
Indirect or Cantilevered Support
Indirect support provides support for a saddle which is ____ out beyond the area of available tooth support.
The action is one of a Class ____ lever system and can therefore be very effective or very poor, depending on how the load, effort and fulcrum are distributed.
cantilevered
I
Indirect or Cantilevered Support
In a Class I lever system it is obvious to the user that the greater distance the ____ is from the fulcrum the better (less effort needed to lever up the lid of a tin by using a long screwdriver). Likewise the nearer the ____ to the Load the greater the load that can be moved or balanced (same reasoning).
effort
fulcrum
Indirect or Cantilevered Support
Do not confuse support with ____.
All stages of design are interrelated, whenever you work with rests and clasps you must think of the effect of the lever systems you are creating.
retention
Is this a good design?
No, as the saddle sinks the denture will ____ around the fulcrum and the clasp on the canine will be displaced ____. Too much retention on the canine would cause extracting leverage and mobility.
This is a better design, by moving the rests ____ the rock is reduced and tooth damage is avoided.
rotate
upward
forward
RPI Clasp
The anterior of a free end saddle must be ____ supported otherwise functional load will displace (compress) tissues causing damage.
An occlusal or cingulum ____ is required and should be placed ____ to avoid tilting forces.
tooth
required
rest
mesially
RPI Clasp
____ Rest, ____ Plate, ____ Bar
mesial
distal
I
RPI Clasp
• The denture is going to rotate about the ____ edge of the mesial occlusal rest.
• The I bar is placed on that axis and the active tip will show ____ movement (further into the undercut).
• The distal plate will only touch the ____ survey line so rotational movement will be into ____ of the distal surface without applying torque to the ____.
distal minimal vertical undercut tooth
