RPD DESIGN - RETENTION

Question 1

What is retention defined as

Answer 1

resistance of a denture being lifted away from the tissues (vertical dislodging forces)

Question 2

How can retention be achieved

Answer 2

through mechanical, muscular and physical means

Question 3

How can mechanical retention be achieved

Answer 3

through clasps

Question 4

How can muscular retention be achieved

Answer 4

Harnessing the patient’s muscular control acting through the polished surface of the denture

Question 5

How can physical retention be achieved

Answer 5

Using existing forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture

Question 6

How does adhesion result in retention

Answer 6

□ Adhesion between saliva and mucosa, adhesion between saliva and denture (defined as the action or process of adhering to a surface or object)

Question 7

How does cohesion result in retention

Answer 7

□ Cohesion is the sticking together of particles of the same substance (saliva particles sticking together)

Question 8

How does atmospheric pressure result in retention

Answer 8

□ Negative pressure - peripheral seal to provide suction to keep the denture in place

Question 9

Why is physical retention less applicable to chrome dentures

Answer 9

More relevant to acrylic, cobalt chrome tends to have more of a skeletal type of design and are heavier so harder to achieve this

Question 10

Why are pin and post dams done

Answer 10

to create an effective seal

Question 11

What is a post dam

Answer 11

A post dam is the posterior periphery of an upper denture in the palate and in FRONT of the palatine fovea

Question 12

What is the palatine fovea

Answer 12

is often used for locating the palatal seal area

Question 13

Where is the palatine fovea located in regards to the vibrating line

Answer 13

anterior

Question 14

For an RPD where is the post dam

Answer 14

on the HARD PALATE

Question 15

What are pin dams

Answer 15

much shallower grooves than the post dam

Question 16

What does pin dams allow

Answer 16

○ The anterior aspect of the finished edge of an RPD aids in ensuring flush fitting and deflection of food material not slipping under the denture

Question 17

Where are pin dams generally placed in regards to the GM

Answer 17

Pin dams are generally placed 5mm away from the GM

Question 18

What is direct retention defined as

Answer 18

Direct retention is defined as the resistance to VERTICAL displacement of the denture

Question 19

What is indirect retention defined as

Answer 19

Indirect retention is defined as resistance to rotational displacement of the denture

Question 20

Where do we generally want our retention to be

Answer 20

as close to the saddle as possible (similar to support) - want clasp tips as close to the saddle

Question 21

What are guide planes

Answer 21

Guide planes are two parallel surfaces and they provide frictional retention

Question 22

What are the advantages of guide planes

Answer 22

provide long term retention as long as the teeth don’t drift
unlike clasps which deform

Question 23

What are the disadvantages of guide planes

Answer 23

they are hard to find and hard to create

may have to change the path of insertion to utilize guide planes

Question 24

What is a clasp

Answer 24

Clasp is a metal arm that, when in position, contacts the tooth preventing removal of the denture base

Question 25

Where is the clasp placed for it to be effective

Answer 25

○ To be effective as a retainer, the clasp is placed below the undercut

Question 26

What are the different ways clasps can be made

Answer 26

○ They can be made in wrought metal and incorporated into the denture base or included as part of a cast denture base

Question 27

What are the 2 types of clasps

Answer 27

occlusal approaching | gingival approaching

Question 28

What do clasps work most effectively with

Answer 28

with a rest

Question 29

What is the condition for clasps that mean that a denture will be retained successfully

Answer 29

only as long as the force required to flex the clasps over the maximum bulbosities of the teeth is greater than the force which is attempting to dislodge the denture

Question 30

What different aspects of a clasps design can effect its flexibility

Answer 30

``` section length thickness curvature alloy ```

Question 31

What is the effect of a section of a clasp on its flexibility

Answer 31

A round section clasp will flex equally in all directions, whereas a half round clasp will flex more readily in the horizontal than vertical plane

Question 32

What is the effect of a clasps length on its flexibility

Answer 32

The longer the clasp arm the more flexible it is Thus an occlusally approaching clasp on a molar tooth will be more flexibly than one on a premolar

Question 33

What is the effect of thickness on a clasps flexibility

Answer 33

has a profound effect of flexibility | thinner = more flexible

Question 34

If a clasp is stressed beyond its proportional limit what will happen

Answer 34

it will be distorted permanently

Question 35

How much of the clasp engages with the undercut

Answer 35

Only the terminal 1/3 of the clasp should be below the undercut

Question 36

What happens if too much of the clasp engages the undercut

Answer 36

then the high force required to move it over the maximum bulbosity will put a considerable strain on the fibres of the periodontal ligament and is likely to exceed the proportional limit of the alloy, thus distorting the clasp

Question 37

What do the different parts of the gingival approaching clasp engage

Answer 37

w Gingivally approaching clasp contacts the tooth surface only at its tip - the remainder is free of contact with the mucosa of the sulcus and the gingival margin

Question 38

What is an advantage of a gingival approaching claps

Answer 38

○ The length of gingivally approaching clasps, unlike occlusally approaching clasps, are not restricted by the dimensions of the clasped tooth

Question 39

What is the dimensions of a cobalt chrome clasp

Answer 39

Clasp should be 15mm long (occlusally approaching) and placed in a horizontal undercut of 0.25mm

Question 40

What is the path of insertion

Answer 40

the path followed by the denture from its first contact with the teeth until it is fully seated can coincide with the path of withdrawal and may or may not coincide with the path of displacement

Question 41

What will the choice of path of insertion be influenced by

Answer 41

The need to utilise guiding surfaces to achieve a pleasing appearance The need to avoid interference by the teeth or ridges with correct positioning of denture components The need to utilise guide surfaces for retention

Question 42

How can path of insertion be used for retention

Answer 42

If you utilise soft tissue undercuts by changing path of insertion then this is a form of retention as it resists withdrawal in the common path of displacement

Question 43

What is the best pattern of retention

Answer 43

TRIANGULAR PATTERN OF RETENTION

Question 44

Why is triangular pattern of retention the best one

Answer 44

○ You have always got one clasp that is going to help dislodging along the plane of two others

Question 45

What is an RPI

Answer 45

STRESS RELIEVING clasp system

Question 46

Where is an RPI used

Answer 46

in a free end saddle and is designed to prevent stress on the last abutment tooth

Question 47

How does an RPI work

Answer 47

○ As the saddle is pressed into the denture bearing mucosa, the denture rotates about a point close to the mesial rest ○ Both the distal guide plate and the I bar disengage from the tooth surface so that potentially harmful torque is avoided

Question 48

Why should a free end saddle not be rigidly attached to the abutment tooth by a combination of stiff clasps and long guide plates

Answer 48

if these are incorporated into the occlusal loads falling on the saddle, which is in effect a long cantilever arm, are likely to result in the RPD acting like extraction forceps, with consequent damage to the supporting structures of the tooth

Question 49

What does an RPI consist of

Answer 49

rest (occlusal) proximal plate I bar clasp

Question 50

Where is the rest placed in an RPI

Answer 50

Mesial of the tooth

Question 51

Where does the proximal plate sit

Answer 51

Doesn't engage undercut - sits on or above survey lin

Question 52

What is the function of the proximal plate

Answer 52

§ Plate helps stabilise the denture and give some degree of BRACING of the denture during function

Question 53

When the patient bits what does the RPI do

Answer 53

§ When patient bites, the saddle sinks towards tissue and clasp and proximal plate DISENGAGE

Question 54

What happens to the proximal plate when the patient bites

Answer 54

Proximal plate moves into undercut that lies below the survey line - not applying any pressure to the tooth

Question 55

What provides reciprocation in an RPI

Answer 55

Reciprocation is provided by the minor connector of rest and the proximal plate - the distance between the two needs to be less than the width of the tooth so that the tooth CANNOT move lingually

Question 56

Where is an RPI predominantly used

Answer 56

for a mandibular free end saddle case but can be utilised in the maxillary arch also