biomechanics of design Flashcards
Biomechanics
*def?
* RPD implication?
- 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.
RPD Supporting Structures
alveolar bone
teeth/PDL
mucosa
- Alveolar bone provides support via:
– Abutment teeth & periodontal ligaments
– Residual ridges through soft tissue covering
- Teeth better able to tolerate vertical forces:
down long-axis
– More PDL fibers activated
to resist force
- Deleterious forces on teeth:
– off-axis, torque, horizontal
Excess force on alveolar support may result in:
– Mucosal ulcerations
– Bone resorption
Forces Acting on RPD
- Vertical (Dislodging)
- Horizontal (Lateral)
- Vertical (Seating)
what forces do these resist?
Requirements of RPD
- Retention:
–Resistance to dislodging (vertical) - Stability:
–Resistance to horizontal forces - Support:
–Resistance to vertical seating (vertical)
- Retention
Resistance to Vertical
dislodging forces
- RPD components involved with retention
– Direct retainer: Retentive clasp
– Indirect retainer
– Proximal plates (Friction)
- Stability:
- Stability: Resistance to horizontal, lateral,
or torsional forces
- RPD components involved with stability
– Minor connectors
– Proximal plates
– Lingual plates
– Denture bases
importance?
- Support:
- Support: Resistance to vertical seating force and this is most important to oral health.
RPD components involved with support
– Rests
– Major connectors: Maxillary tooth-tissue supported RPD
– Denture bases
Reciprocation:
- Reciprocation: is 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 how?
- RPD’s true reciprocation can only be achieved if the reciprocating element touches the tooth before the retentive clasp.
- 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 III RPDs best resist _______ forces
- Class III RPDs best resist functional force
where are rests?
occlusal loading of class 3 cases
- With occlusal loading, vertical seating forcesdirected down tooth long axis
– Occlusal rests adjacent to edentulous area
counteracted with?
vertical dislodgement of class 3
*Limited vertical dislodgement
–Counteract through retentive clasp & proximal plates
horizontal forces of class 3
*Limited horizontal forces on Class III RPD
- Class I, II, & long-span IV RPDs
–Subject to greater?
–support from?
–rotational movement?
- Class I, II, & long-span IV RPDs
–Subject to greater stress
–Combination of tooth & soft tissue
support
–RPD extension movement permits
rotational movement around fulcrums in
3 planes
Rotation in Sagittal Plane around
Horizontal Plane Fulcrum
- Fulcrum through rests closest to
edentulous areas - Inferior-superior denture base
movement of the distal end - Occurs with:
– Vertical seating force
– Vertical dislodgement
force
components involved?
Resistance to Rotation Around
Horizontal Fulcrum
done with retention (Direct retainer, Indirect retainer, Proximal plates) and support (Rests, Major connector (Maxillary), Denture base)
Rotation in Vertical Plane through
Longitudinal/Sagittal Fulcrum
- Fulcrum through crest of ridge
- Rocking or side-to-side movement over the crest of the ridge
components involved?
Resistance to Rotation Around
Longitudinal Fulcrum
- Stability: resistance to rotation around
ridge crest
–RPD components involved: Rigid connectors, Clasps, Denture base
Rotation in Horizontal Plane through
Vertical Fulcrum
- Fulcrum at center of dental arch
- Horizontal twisting results in buccolingual movement of RPD
involved components?
Resistance to Rotation Around
Vertical Fulcrum
- Stability: resistance to horizontal
movement
–RPD components involved - Minor connectors, Proximal plates, Rigid portions of clasps, Lingual plates, Denture base
Extension RPD Rotation through
Horizontal Fulcrum: RPD functions as a _______
* Can result in? why?
- RPD functions as a lever
- Can result in deleterious effects on
teeth–Magnified loading forces
what lever classes do we see with RPDs
1 and 2, 2 is more desireable/ less destructive
usually not a factor with tooth supported RPDs
Effect of Rest/Retentive Clasp Positions on lever action
- Class I lever occurs during rotation around horizontal fulcrum line through terminal rests. (Rotation toward ridge)
- Retentive clasp should NOT be anterior to terminal rest fulcrum line.
– Detrimental torquing forces applied to
abutment.
- Distal rest/Distal Guide Plate/Distal extension RPD:
NO GOOD CLASS I LEVER
– Circumferential clasp tip anterior to rest/ fulcrum.
– Circumferential clasp moves occlusally during function.
– Directs detrimental distal torquing force to abutment
- Mesial rest/Distal Guide Plate/Distal extension RPD:
CLASS II LEVER, BETTER
– 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.
SOLUTIONS?
Class II arch with posterior modification space.
Class I Lever occurs when abutment with retentive clasp anterior to horizontal fulcrum line/axis of rotation (AR) in
During RPD rotation toward ridge,
anterior clasp produces detrimental
torquing force on this abutment.
solutions:
1. No clasp
2. Clasp in less undercut
3. Non-retentive clasp (tip not in undercut)
4. Wrought wire clasp
potentila disadvantages
without indirect retention what occurs
- NO Indirect retention
– Class I Lever occurs with Vertical dislodgement forces. (Rotation at horizontal fulcrum)
– Disadvantages
* Allows greater vertical dislodgement to occur.
* Potential for tissue impingement under mandibular major connector.
advantages
Indirect Retainer present
– Class II Lever occurs with Limited Vertical
dislodgement
– Advantages
* Vertical dislodgement limited
* Potential for tissue impingement by major connector reduced
- Class III RPD
–fulcrums or levers?
– Design by?
– No fulcrums or levers
– Design by convenience
- Class I, II & long-span IV RPDs (Extension RPDs)
– Consideration of?
– Potential Levers?
– Consideration of rotation toward & away from residual ridge at Horizontal fulcrum
– Potential Class I & II Levers
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
Factors Influencing Magnitude of Stress
Transmitted to Abutment Teeth
1. Length of extension span
– Correlates to length of lever effort arm
Factors Influencing Magnitude of Stress
Transmitted to Abutment Teeth: quality of ridge support
– Broad ridge better support than thin, knife-edge ridge
* Better resistance to both vertical & horizontal force
Factors Influencing Magnitude of Stress
Transmitted to Abutment Teeth
3. Flexural qualities of clasp
– More flexible, less stress transmitted to abutments
– More flexible, less horizontal stability
– More flexible, more stress transmitted to residual ridge
– Flexural qualities determined by:
* Clasp length & diameter
* Clasp material
Factors Influencing Magnitude of Stress
Transmitted to Abutment Teeth
4. Clasp design
– Clasp passive when completely seated
– Reciprocal arm contacts tooth before
retentive tip passes height of contour
* Neutralize stress from retentive tip
Factors Influencing Magnitude of Stress
Transmitted to Abutment Teeth
5. Abutment tooth surface
– Gold greater frictional resistance to clasp arm movement than enamel
* Greater stress on gold restored tooth
Factors Influencing Magnitude of Stress
Transmitted to Abutment Teeth: occlusal harmony
– A disharmonious occlusion may generate horizontal forces
– When magnified by the factor of RPD leverage, these forces may be destructive to abutment teeth & residual ridges
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.
