Biomechanics of Removable Partial Dentures

Question 1

What is biomechanics?

Answer 1

• 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

Question 2

Alveolar bone provides support to the RPD via:

Answer 2

– Abutment teeth & periodontal ligaments
– Residual ridges through soft tissue covering

Question 3

Why are teeth better able to tolerate vertical forces down the long-axis?

Answer 3

More PDL fibers activated to resist force

Question 4

What forces on a tooth are deleterious?

Answer 4

off-axis
torque
horizontal

Question 5

Excess for on the alveolar area may result in…

Answer 5

– Mucosal ulcerations
– Bone resorption

Question 6

Whatare the forces acting on an RPD?

Answer 6

• Vertical (Dislodging)
• Horizontal (Lateral)
• Vertical (Seating)

Question 7

What are the requirements of an RPD?

Answer 7

• Retention: Resistance to dislodging
• Stability: Resistance to horizontal
• Support: Resistance to vertical seating

Question 8

What is retention?

Answer 8

Resistance to Vertical dislodging forces

Question 9

What are the retention RPD components?

Answer 9

– Direct retainer: Retentive clasp
– Indirect retainer
– Proximal plates (Friction)

Question 10

What is stability?

Answer 10

Resistance to horizontal, lateral, or torsional forces

Question 11

What are the stability RPD components?

Answer 11

– Minor connectors
– Proximal plates
– Lingual plates
– Denture bases

Question 12

What is support?

Answer 12

Resistance to vertical seating force and this is most important to oral health.

Question 13

What are the support RPD components?

Answer 13

– Rests
– Major connectors: Maxillary tooth-tissue supported RPD
– Denture bases

Question 14

What is reciprocation?

Answer 14

the means by which one part of a restoration is made to counter the effects created by another part

Question 15

RPD’s true reciprocation can only be achieved if the reciprocating element touches the tooth _________ the retentive clasp

Answer 15

before

Question 16

What are the biomechanical classificaiton of RPDs?

Answer 16

• 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

Question 17

Class ____ RPDs best resist functional forces

Answer 17

III

Question 18

What are the biomechanics of tooth-supported RPDs?

Answer 18

• 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

Question 19

What limits the vertical dislodgement of a tooth-supported RPD?

Answer 19

retentive clasp & proximal plates

Question 20

What are the biomechanics of tooth-tissue supported RPDs?

Answer 20

– Subject to greater stress
– Combination of tooth & soft tissue support
– RPD extension movement permits rotational movement around fulcrums in 3 planes

Question 21

How does rotation in Sagittal Plane around Horizontal Plane Fulcrum happen?

Answer 21

• Fulcrum through rests closest to edentulous areas
• Inferior-superior denture base movement of the distal end

Question 22

What occurs when there is rotation in sagittal plane around horizontal plane fulcrum?

Answer 22

– Vertical seating force
– Vertical dislodgement force

Question 23

What concepts allow for resistance to rotation around the horizontal fulcrum?

Answer 23

• Retention: resistance to rotation away from ridge (Vertical dislodgement)
• Support: resistance to rotation toward ridge (Vertical seating)

Question 24

What parts of the RPD allow for resistance to rotation around the horizontal fulcrum?

Answer 24

• Retention - Direct retainer, Indirect retainer, Proximal plates
• Support - Rests, Major connector (Maxillary), Denture base

Question 25

What occurs when there is rotation in vertical plane around longitudinal/sagittal fulcrum?

Answer 25

* Fulcrum through crest of ridge * Rocking or side-to-side movement over the crest of the ridge

Question 26

What concepts allow for resistance to rotation in vertical plane around longitudinal/sagittal fulcrum?

Answer 26

* Stability: resistance to rotation around ridge crest

Question 27

What parts of the RPD allow for resistance to rotation in vertical plane around longitudinal/sagittal fulcrum?

Answer 27

- Rigid connectors - Clasps - Denture base

Question 28

Answer 28

Tooth-supported RPD forces

Question 29

What occurs when there is rotation in the horizontal plane around vertical fulcrum?

Answer 29

* Fulcrum at center of dental arch * Horizontal twisting results in buccolingual movement of RPD

Question 30

Where is the fulcrum line through?

Answer 30

rest seats

Question 31

What concepts allow for resistance to rotation in the horizontal plane around vertical fulcrum?

Answer 31

* Stability: resistance to horizontal movement

Question 32

What parts of the RPD allow for resistance to rotation in the horizontal plane around vertical fulcrum?

Answer 32

- Minor connectors - Proximal plates - Rigid portions of clasps - Lingual plates - Denture base

Question 33

When extension RPD rotates through horizontal fulcrum it acts as a...

Answer 33

lever

Question 34

If the RPD acts as a level (horizontal fulcrum) when is the result?

Answer 34

deleterious effects on teeth (magnified loading force)

Question 35

______ system can magnify force through mechanical advantage

Answer 35

Lever

Question 36

Class I & II levers encountered in what kind of RPDs?

Answer 36

tooth-tissue supported (Extension)

Question 37

Fulcrums & levers are not usually a factor in what kind of RPDs?

Answer 37

tooth-supported RPDs

Question 38

Class __ lever occurs during rotation around horizontal fulcrum line through terminal rests

Answer 38

I

Question 39

Retentive clasp should or should not be anterior to terminal rest fulcrum line?

Answer 39

SHOULD NOT

Question 40

What can happen if the retentive clasp is anterior to terminal rest fulcrum line?

Answer 40

Detrimental torquing forces applied to abutment

Question 41

What clasp is a class I lever type?

Answer 41

* 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

Question 42

What clasp is a class II level type?

Answer 42

* 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.

Question 43

What are the alternatives to class I lever clasps?

Answer 43

1. No clasp 2. Clasp in less undercut 3. Non-retentive clasp (tip not in undercut) 4. Wrought wire clasp

Question 44

_____________ occurs when abutment with retentive clasp anterior to horizontal fulcrum line/axis of rotation (AR) in Class II arch with posterior modification space

Answer 44

Class I Lever

Question 45

slide 27-29

Question 46

Class I Lever occurs with _______ dislodgement forces

Answer 46

Vertical

Question 47

What are the disadvantages of a class I lever?

Answer 47

* Allows greater vertical dislodgement to occur * Potential for tissue impingement under mandibular major connector * No indirect retention

Question 48

What are the advantages of a class II lever?

Answer 48

* Vertical dislodgement limited * Potential for tissue impingement by major connector reduced * Indirect retainer present

Question 49

What kind of RPD has no fulcrum or lever?

Answer 49

Class III RPD

Question 50

What kind of RPDs have fulcrums and potential levers?

Answer 50

Class I, II & long-span IV RPDs (Extension RPDs)

Question 51

What are the characteristics of RPD horizontal fulcrums?

Answer 51

- Longer modification space - More complicated - 2 fulcrums

Question 52

What are factors Influencing Magnitude of Stress Transmitted to Abutment Teeth?

Answer 52

1. Length of extension span 2. Quality of Support Ridge 3. Flexural qualities of clasp 4. Clasp design 5. Abutment tooth surface 6. Occlusal Harmony

Question 53

The length of extension span correlates to the length of the...

Answer 53

lever effort arm

Question 54

What type of ridge is better?

Answer 54

Broad ridge better support than thin, knife-edge ridge * Better resistance to both vertical & horizontal force

Question 55

If a clasp is more flexible, _____ stress transmitted to abutments

Answer 55

less

Question 56

If a clasp is more flexible, ____ horizontal stability

Answer 56

less

Question 57

If a clasp is more flexible, _____ stress transmitted to residual ridge

Answer 57

more

Question 58

Flexural qualities of a clasp are determined by...

Answer 58

* Clasp length & diameter * Clasp material

Question 59

A disharmonious occlusion may generate ________ forces

Answer 59

horizontal

Question 60

When magnified by the factor of RPD leverage, horizontal forces may be destructive to...

Answer 60

abutment teeth & residual ridges

Question 61

What is stress control by RPD design?

Answer 61

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