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BDS2 CLINICAL Prosthodontics > RPD DESIGN - RETENTION > Flashcards

RPD DESIGN - RETENTION Flashcards

1
Q

What is retention defined as

A

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

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2
Q

How can retention be achieved

A

through mechanical, muscular and physical means

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3
Q

How can mechanical retention be achieved

A

through clasps

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4
Q

How can muscular retention be achieved

A

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

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5
Q

How can physical retention be achieved

A

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

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6
Q

How does adhesion result in retention

A

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

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7
Q

How does cohesion result in retention

A

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

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8
Q

How does atmospheric pressure result in retention

A

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

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9
Q

Why is physical retention less applicable to chrome dentures

A

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

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10
Q

Why are pin and post dams done

A

to create an effective seal

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11
Q

What is a post dam

A

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

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12
Q

What is the palatine fovea

A

is often used for locating the palatal seal area

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13
Q

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

A

anterior

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14
Q

For an RPD where is the post dam

A

on the HARD PALATE

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15
Q

What are pin dams

A

much shallower grooves than the post dam

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16
Q

What does pin dams allow

A

○ 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

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17
Q

Where are pin dams generally placed in regards to the GM

A

Pin dams are generally placed 5mm away from the GM

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18
Q

What is direct retention defined as

A

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

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19
Q

What is indirect retention defined as

A

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

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20
Q

Where do we generally want our retention to be

A

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

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21
Q

What are guide planes

A

Guide planes are two parallel surfaces and they provide frictional retention

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22
Q

What are the advantages of guide planes

A

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

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23
Q

What are the disadvantages of guide planes

A

they are hard to find and hard to create

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

24
Q

What is a clasp

A

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

25
Q

Where is the clasp placed for it to be effective

A

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

26
Q

What are the different ways clasps can be made

A

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

27
Q

What are the 2 types of clasps

A

occlusal approaching

gingival approaching

28
Q

What do clasps work most effectively with

A

with a rest

29
Q

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

A

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

30
Q

What different aspects of a clasps design can effect its flexibility

A 
section
length
thickness
curvature 
alloy
31
Q

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

A

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

32
Q

What is the effect of a clasps length on its flexibility

A

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

33
Q

What is the effect of thickness on a clasps flexibility

A

has a profound effect of flexibility

thinner = more flexible

34
Q

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

A

it will be distorted permanently

35
Q

How much of the clasp engages with the undercut

A

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

36
Q

What happens if too much of the clasp engages the undercut

A

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

37
Q

What do the different parts of the gingival approaching clasp engage

A

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

38
Q

What is an advantage of a gingival approaching claps

A

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

39
Q

What is the dimensions of a cobalt chrome clasp

A

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

40
Q

What is the path of insertion

A

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

41
Q

What will the choice of path of insertion be influenced by

A

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

42
Q

How can path of insertion be used for retention

A

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

43
Q

What is the best pattern of retention

A

TRIANGULAR PATTERN OF RETENTION

44
Q

Why is triangular pattern of retention the best one

A

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

45
Q

What is an RPI

A

STRESS RELIEVING clasp system

46
Q

Where is an RPI used

A

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

47
Q

How does an RPI work

A

○ 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

48
Q

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

A

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

49
Q

What does an RPI consist of

A

rest (occlusal)
proximal plate
I bar clasp

50
Q

Where is the rest placed in an RPI

A

Mesial of the tooth

51
Q

Where does the proximal plate sit

A

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

52
Q

What is the function of the proximal plate

A

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

53
Q

When the patient bits what does the RPI do

A

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

54
Q

What happens to the proximal plate when the patient bites

A

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

55
Q

What provides reciprocation in an RPI

A

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

56
Q

Where is an RPI predominantly used

A

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

BDS2 CLINICAL Prosthodontics (16 decks)

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