Biomechanics of Removable Partial Dentures Flashcards
What is biomechanics?
- Application of mechanical engineering principles in the living organism
- Mechanical forces exerted on RPD during functional & parafunctional mandibular movements should be properly directed to the supporting tissue to elicit the most favorable response
Alveolar bone provides support to the RPD via:
– Abutment teeth & periodontal ligaments
– Residual ridges through soft tissue covering
Why are teeth better able to tolerate vertical forces down the long-axis?
More PDL fibers activated to resist force
What forces on a tooth are deleterious?
off-axis
torque
horizontal
Excess for on the alveolar area may result in…
– Mucosal ulcerations
– Bone resorption
Whatare the forces acting on an RPD?
- Vertical (Dislodging)
- Horizontal (Lateral)
- Vertical (Seating)
What are the requirements of an RPD?
- Retention: Resistance to dislodging
- Stability: Resistance to horizontal
- Support: Resistance to vertical seating
What is retention?
Resistance to Vertical dislodging forces
What are the retention RPD components?
– Direct retainer: Retentive clasp
– Indirect retainer
– Proximal plates (Friction)
What is stability?
Resistance to horizontal, lateral, or torsional forces
What are the stability RPD components?
– Minor connectors
– Proximal plates
– Lingual plates
– Denture bases
What is support?
Resistance to vertical seating force and this is most important to oral health.
What are the support RPD components?
– Rests
– Major connectors: Maxillary tooth-tissue supported RPD
– Denture bases
What is reciprocation?
the means by which one part of a restoration is made to counter the effects created by another part
RPD’s true reciprocation can only be achieved if the reciprocating element touches the tooth _________ the retentive clasp
before
What are the biomechanical classificaiton of RPDs?
- Tooth-supported RPD
– Abutment teeth support RPD
– Class III & IV RPD - Tooth-tissue supported RPD
– Denture base is supported
by both teeth & residual ridge
– Class I, II, long-span IV RPDs
Class ____ RPDs best resist functional forces
III
What are the biomechanics of tooth-supported RPDs?
- With occlusal loading, vertical seating forces directed down tooth long axis
*Limited vertical dislodgement
*Limited horizontal forces on Class III RPD - Class III RPDs best resist functional forces
What limits the vertical dislodgement of a tooth-supported RPD?
retentive clasp & proximal plates
What are the biomechanics of tooth-tissue supported RPDs?
– Subject to greater stress
– Combination of tooth & soft tissue support
– RPD extension movement permits rotational movement around fulcrums in 3 planes
How does rotation in Sagittal Plane around Horizontal Plane Fulcrum happen?
- Fulcrum through rests closest to edentulous areas
- Inferior-superior denture base movement of the distal end
What occurs when there is rotation in sagittal plane around horizontal plane fulcrum?
– Vertical seating force
– Vertical dislodgement force
What concepts allow for resistance to rotation around the horizontal fulcrum?
- Retention: resistance to rotation away from ridge (Vertical dislodgement)
- Support: resistance to rotation toward ridge (Vertical seating)
What parts of the RPD allow for resistance to rotation around the horizontal fulcrum?
- Retention - Direct retainer, Indirect retainer, Proximal plates
- Support - Rests, Major connector (Maxillary), Denture base
What occurs when there is rotation in vertical plane around longitudinal/sagittal fulcrum?
- Fulcrum through crest of ridge
- Rocking or side-to-side movement over the crest of the ridge
What concepts allow for resistance to rotation in vertical plane around longitudinal/sagittal fulcrum?
- Stability: resistance to rotation around ridge crest
What parts of the RPD allow for resistance to rotation in vertical plane around longitudinal/sagittal fulcrum?
- Rigid connectors
- Clasps
- Denture base
Tooth-supported RPD forces
What occurs when there is rotation in the horizontal plane around vertical fulcrum?
- Fulcrum at center of dental arch
- Horizontal twisting results in buccolingual movement of RPD
Where is the fulcrum line through?
rest seats
What concepts allow for resistance to rotation in the horizontal plane around vertical fulcrum?
- Stability: resistance to horizontal movement
What parts of the RPD allow for resistance to rotation in the horizontal plane around vertical fulcrum?
- Minor connectors
- Proximal plates
- Rigid portions of clasps
- Lingual plates
- Denture base
When extension RPD rotates through horizontal fulcrum it acts as a…
lever
If the RPD acts as a level (horizontal fulcrum) when is the result?
deleterious effects on teeth (magnified loading force)
______ system can magnify force through mechanical advantage
Lever
Class I & II levers encountered in what kind of RPDs?
tooth-tissue supported (Extension)
Fulcrums & levers are not usually a factor in what kind of RPDs?
tooth-supported RPDs
Class __ lever occurs during rotation around horizontal fulcrum line through terminal rests
I
Retentive clasp should or should not be anterior to terminal rest fulcrum line?
SHOULD NOT
What can happen if the retentive clasp is anterior to terminal rest fulcrum line?
Detrimental torquing forces applied to abutment
What clasp is a class I lever type?
- Distal rest/Distal Guide Plate/Distal extension RPD:
– Circumferential clasp tip anterior to rest/ fulcrum.
– Circumferential clasp moves occlusally during function.
– Directs detrimental distal torquing force to abutment
What clasp is a class II level type?
- Mesial rest/Distal Guide Plate/Distal extension RPD:
– Circumferential clasp tip slightly posterior to rest/fulcrum.
– Circumferential clasp moves more mesially during function.
– Clasp tip tends to move into deeper undercut - Produces less leverage on abutment than with distal rest.
What are the alternatives to class I lever clasps?
- No clasp
- Clasp in less undercut
- Non-retentive clasp (tip not in undercut)
- Wrought wire clasp
_____________ occurs when abutment with retentive clasp anterior to horizontal fulcrum line/axis of rotation (AR) in Class II arch with posterior modification space
Class I Lever
slide 27-29
Class I Lever occurs with _______ dislodgement forces
Vertical
What are the disadvantages of a class I lever?
- Allows greater vertical dislodgement to occur
- Potential for tissue impingement under mandibular major connector
- No indirect retention
What are the advantages of a class II lever?
- Vertical dislodgement limited
- Potential for tissue impingement by major connector reduced
- Indirect retainer present
What kind of RPD has no fulcrum or lever?
Class III RPD
What kind of RPDs have fulcrums and potential levers?
Class I, II & long-span IV RPDs (Extension RPDs)
What are the characteristics of RPD horizontal fulcrums?
- Longer modification space
- More complicated
- 2 fulcrums
What are factors Influencing Magnitude of Stress Transmitted to Abutment Teeth?
- Length of extension span
- Quality of Support Ridge
- Flexural qualities of clasp
- Clasp design
- Abutment tooth surface
- Occlusal Harmony
The length of extension span correlates to the length of the…
lever effort arm
What type of ridge is better?
Broad ridge better support than thin, knife-edge ridge
* Better resistance to both vertical & horizontal force
If a clasp is more flexible, _____ stress transmitted to abutments
less
If a clasp is more flexible, ____ horizontal stability
less
If a clasp is more flexible, _____ stress transmitted to residual ridge
more
Flexural qualities of a clasp are determined by…
- Clasp length & diameter
- Clasp material
A disharmonious occlusion may generate ________ forces
horizontal
When magnified by the factor of RPD leverage, horizontal forces
may be destructive to…
abutment teeth & residual ridges
What is stress control by RPD design?
Forces exerted through RPD can be widely distributed, directed, & minimized by the selection, the design, & the location of the RPD components, in conjunction with the development of harmonious occlusion
