10. Loads and Levers Flashcards by J L

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

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Loads
 \_\_\_\_ support
 Directly to \_\_\_\_ crown 
 Between two supports
fixed bridge or tooth supported PRDP

tooth

single

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Loads
 How is the load applied? 
 \_\_\_\_ vs unequal
 \_\_\_\_ bridge
 PRDP with only \_\_\_\_

equal
fixed
rests

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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

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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

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Levers

 The greater distance of the effort from the fulcrum the ____ the mechanical advantage

bigger

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Levers
 Class II
 Load ____ the fulcrum and the effort
 Mechanical advantage as load always ____ fulcrum

between

nearer

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Levers
 Class III
 Effort lies ____ the fulcrum and load
 Mechanical disadvantage as the effort is always ____ to the fulcrum

between

nearer

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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

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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

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Tooth supported prosthesis
 \_\_\_\_ supply the support 
 Scenario 1
 Short saddle even load
on \_\_\_\_ and molars

rests

premolars

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Tooth supported prosthesis
 Scenario 2
 Move \_\_\_\_ support further away from the load 
 Prevent \_\_\_\_ of the
premolar

overload

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Tooth supported prosthesis
 Scenario 3
 Anterior support moved even further from the ____,
molar load ____

load

increases

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Tooth supported prosthesis
 Scenario 4
 Multiple ____, load shared by the ____ and premolar

support

canine

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Tooth supported prosthesis
 Scenario 5
 Moving the molar rest to the ____ will result in the ____ and canine sharing the load equally

distal

molar

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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

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 ```