Cr-Co RPD

Question 1

Kennedy classifications

Answer 1

Class 1 - bilateral edentulous areas
Class 2 - unilateral edentulous area posterior to remaining teeth
Class 3 - unilateral edentulous area with natural teeth anterior and posterior to it
Class 4 - single edentulous area located anterior to remaining teeth

Question 2

Potential problems with each Kennedy classification

Answer 2

Class 1) absence of distal abutments creates problems of both support + retention
Class 2) both tooth and mucosa supported, mods can be tooth supported
Class 3) opportunity to be entirely tooth supported however clasps on anterior teeth may be unaesthetic –> tooth and tissue
Class 4) aesthetics are key, anterior held by labial flange and add adequate posterior indirect retention

(BASED OF EXAMPLES in lec ^)

Question 3

Surveying instruments are parallel sided with flanges designed to measure 0.25mm, 0.56mm and 0.75mm. Which thickness of undercut CANNOT be used for a Co-Cr RPD and why?

Answer 3

undercuts greater than 0.25mm - if clasp engages more than 0.25mm then likely that its proportional limit will be exceeded when the denture is seated or removed –> clasp thus permanently deformed and therefore non-retentive

(if greater suggest another denture such as acrylic or flex, or suprabulge clasps?)

Question 4

Clinical steps for making RPD (9)

Answer 4

1. Alignate impressions, study models
2. Survey study models and check undercut depths
3. Design RPD (use coloured pencils). Give estimate $
4. Do mouth preparations (occlusal rests seats etc) THEN secondary impressions with PVS or polyether
5. Try in Cr-Co framework with wax rims attached (now is time to reject framework), take jaw relation and shade selection
6. Try in wax
7. Delivery
8. Recall

Question 5

Components of Cr-Co RPD (5)

Answer 5

1. major connectors
2. minor connectors
3. occlusal rest seats
4. retainers or clasps

(2-4 = clasp assembly)

5. reciprocation (each retainer/clasp must have reciprocation
6. indirect retainers
7. denture base + teeth

Question 6

Rules for retentive occlusally approaching clasps in Co-Cr (5)

Answer 6

1. should run from side of tooth with least undercut to side with greatest –> most effective utilisation of available undercut (otherwise undercut might be too little to provide retention of tip of clasp)
2. Only terminal 1/3 of clasp can enter undercut (going the wrong way the tip may be too close to gingival margin and origin so high that could cause an occlusal interference)
3. Engage 0.25mm of undercut
4. Atleast 15mm in length (ring clasp may be longer on molar but increased curvature results in corresponding increase in stiffness so that an undercut of 0.25mm remains maximum that can be engaged safely + a gingivally approaching clasp can be made longer than 15mm)
5. Restricted to molar teeth (in molar ~15mm but in pm or canine will be considerably less)

Question 7

What in mouth preparation might be done for Co-Cr RPD? (3)

Answer 7

• occlusal rest seats
• guiding planes
• crown work

Question 8

Contraindications for Infrabulge / gingivally approaching clasps in Co-Cr (4)

Answer 8

1. contraindicated unless there is a tissue undercut bucally on the alveolus more than 1mm in depth within 3mm of the gingival margin!!! (undercut of these dimensions means the clasp is relieved extensively from attached mucosa –> wont traumatise when being inserted but such relief causes arm of clasp to be prominent and possibly cause irritation of buccal mucosa and food trapping
2. contraindicated if B sulcus less than 4mm in depth, as insufficient depth to accomodate without much of the length of the clasp arm being too close to gingival margin (exception is De Van clasp which is gingivally approaching and runs along border of saddle to engage DB undercut of abutment, it does not enter sulcus area buccal to clasped tooth)
3. Where the survey line is too close to the occlusal surface
4. Where there is a deep cervical tooth undercut

Question 9

What 2 retentive components must a distal extension saddle always have? What are requirements of the retentive parts and why?

Answer 9

1. Retentive I-bar clasp whose tip contacts the most prominent part of the B surface of the abutment tooth mesiodistally → thus when the distal extension saddle sinks under O loads, the tip of the clasp moves mesially out contact and doesnt apply any damaging torque (same for all gingivally approaching I-bar clasps)
2. a UNILATERAL distal extension saddle (Kennedy II) should have 1 clasp as close to saddle as possible and the other as far posteriorly as possible:

• Provides the most efficient direct retention for the mesial end of the saddle
• locates the clasp axis far posteriorly as possible so that the most effective indirect retention can be provided

Question 10

Dimensions of occlusal rest seats (2) and shape (1)

Answer 10

1.5mm deep

2mm wide

shape of teaspoon

Question 11

Classifications of RPD support

Answer 11

tooth supported

tissue supported

tooth and tissue

Question 12

Label clasp components

Answer 12

Question 13

Requirements of major connector (6)

Answer 13

1. rigidity, cross arch stability and force distribution
2. dont substantially alter natural contour of lingual surface of md arch or palatal vault
3. covers no more tissue than necessary
4. don’t interfere/irritate tongue
5. no food trapping
6. biocompatible alloy

Question 14

Design guidelines for major connectors (6)

Answer 14

1. free from movable tissue
2. doesn’t impinge on gingiva
3. doesn’t impinge bony and soft tissue prominences
4. border parallel to gingival margins of remaining teeth
5. if must cross gingival margins, must be at right angles to minimise coverage
6. anterior border must terminate on posterior slope of prominent rugae (to be as symmetrical as possible)

Question 15

Mandibular (6) and maxillary major connectors

Answer 15

Mandibular (1 & 2 most common)

1. lingual bar
2. lingual plate
3. sublingual bar with continous cingulum bar
4. cingulum bar
5. labial bar

Maxillary

1. palatal strap
2. anterior-posterior palatal strap
3. palatal plate
4. U-shaped plate (horse-shoe)
5. palatal bar
6. anterior-posterior bar

Question 16

How to decide between mandibular lingual bar or lingual plate? (2) What are the features of lingual plate? (2)

Answer 16

if 8mm space between gingival tissue and lingual sulcus + number and prognosis of remaining teeth

• knife edge and in contact with cingulum
• closes interproximal spaces to level of contact points → if diastema: interrupted lingual plate

Question 17

What does a minor connector do? Is a saddle a minor connector?

Answer 17

join components (such as rests, components, clasps, saddles) to the major connector

can contribute to bracing and reciprocation

yes, saddle considered minor connector

Question 18

Types of minor connectors (3)

Answer 18

proximal plates

embrasure minor connectors

saddle or retentive framework/gridwork

Question 19

2 types of saddle/retentive framework/gridwork

Answer 19

Mesh type

• flatter
• more rigid
• less retention of acrylic is openings small

Lattice type

• potentially superior retention for acrylic
• might interfere with setting of teeth, if struts are too thick

(both types acceptable if designed correctly)

Question 20

How should retentive framework be at the junction with major connector? (2)

Answer 20

• butt joint with slight undercut in metal
• ⅔ of length from abutment tooth to hamular notch for the maxilla / retromolar pad for md

Question 21

Function of rests (2). Two categories of rests (2). 3 Types of rests (3)

Answer 21

• provide vertical stop
• transfer force to teeth and tissues

1. primary rest (part of retentive clasp assembly)
2. secondary rest or auxillary rest (additional support or indirect retention)

→ occlusal rests, cingulum/lingual rests, incisal rests

Question 22

How much of the tooth should an occlusal rest occupy?

Answer 22

⅓-½ of mesiodistal diameter of abutment

about ⅓ buccolingual width of tooth measured from cusp to cusp

1.5 depth, 2mm width

Question 23

2 types of clasps

Answer 23

suprabulge

infrabulge

Question 24

Where should the arms of a suprabulge clasp be?

Answer 24

retentive arm - only terminal ⅓ engage undercut (⅔ abovesurvey line)

reciprocal arm - never below surveyed line

Question 25

4 guidelines and 4 contraindications of infrabulge clasps

Answer 25

1. approach arm must never impinge on soft tissues
2. approach arm must uniformly taper
3. usually engage 0.25mm undercut
4. should cross gingival margin at 90 deg
5. when deep tooth cervical undercut exists
6. when severe tissue undercut exists
7. buccal sulcus <4mm
8. survey line too close to occlusal surface

Question 26

T/F Tooth surface modification is often indicated to produce good guiding planes and to alter the height of the contour (surveyed line) of the abutment teeth

Answer 26

T

Question 27

Where should an indirect retainer be placed?

Answer 27

first identify fulcrum line, place indirect retainers perpendicular to fulcrum line as far as practical

Question 28

Purposes (3) and Indications (3) of altered cast denture base

Answer 28

Purposes

1. Improve adaptation of denture to ridge to minimise movement during function
2. Equalise stress between ridge and abutment
3. Improve contours of peripheries

Indications

1. distal extension (Kennedy I and II)
2. Extensive edentulous spans
3. Any case where peripheries are distorted and need correction

Question 29

Sequence of design

Answer 29

1. Support: saddles, rests (primary, secondary)
2. Retention: direct retainers (clasps or attachments)
3. Stability (reciprocation, major connector, minor connector, indirect retainer)

Question 30

Alternatives to RPD

Answer 30

• implants (locator attachments): better stability, rigidity, aesthetics, less ridge resorption
• tooth colour clasps: disadvantages - thick, porous, fatigue
• attachments: eliminates need for rests and clasps → aesthetic, better vertical support and horizontal stabilisation
• rotational path RPD (Kennedy class III and IV) eliminates need for clasps on one side by placing rigid element into undercut, rotate the other end into place (clasps) 0

Question 31

What to do after surveying models?

Answer 31

• find retentive areas, how deep are undercuts?
• natural guide plines
• define best path of insertion
• check aesthetics of clasp design and positioning
• tripod model (mark model so it can be reoriented on surveyor)

Question 32

Types of impression trays

Answer 32

• custom trays
• metal -full or quadrant
• plastic - full or quadrant
• non-perforated or perforated

Question 33

What are musts for triple tray/dual arch/closed bite impressions (2)

Answer 33

must always have teeth on other side of prep (e.g. 46 prep, 45 and 47 must be present

take bite reg because can be inaccurate (filament between upper and lower teeth can cause slight distortion in occlusal morphology)

Question 34

3 Advantages if special trays

Answer 34

1. reduced distortion and rebound effect
2. full tray cross arch stabilisation
3. 3-4mm relief around prepared teeth (thicker material → more accurate)

(make sure you at 3 or more stops in non-critical areas)

Question 35

Why is adhesive essential for impression?

Answer 35

without it the impression shrinks towards centre of mass (smaller impression → crown may not seat properly)

(make sure to leave for 10mins after applying so it has time to evaporate)

Question 36

What is the double mix impression technique, Why is it time sensitive?

Answer 36

1. syringe light body material around prepared teeth (dont start injecting until DA comes back with tray)
2. place tray loaded with heavy body into mouth
3. patient should perform no jaw movement at this time
4. hold the tray firmly until thoroughly set, remove, disinfect

TIME SENSITIVE → once light body starts to set, it will not bond to the heavy body → distortion

Question 37

What is the putty wash impression procedure?

Answer 37

1. place putty into tray
2. cover putty with thin mylar or plastic sheet to act as spacer
3. place tray in mouth and seat
4. after putty set, remove spacer, rinse and thoroughly dry impression
5. impression now becomes very rigid custom tray
6. syringe wash material around prepared area
7. syringe additional wash material into putty impression

Question 38

What is essential for gingival tissue management before taking an impression?

Answer 38

haemostasis (for bleeding tissue only)

• aluminium chloride (Hemodent) - disrupts setting of PVS but thorough rinsing resolves this
• ferric sulphate (Astringident) - also “ ”

retraction

• 3M Astringent retraction paste (15% AlCl3 and kaolin clay)
• cord

Question 39

Guidlines for Infrabulge clasps? (4)

Answer 39

1. approach arm Must not impinge on soft tissue
2. approach arm bust uniformly taper
3. usually engages a 0.25mm undercut
4. crosses gingival margin at 90 degrees

Question 40

(more for crowns)

What is the problem with Plane Line articulators? (2)

when is a plane line articulator acceptable to use?

Answer 40

• Articular axis far from the physiological axis → posterior restorations end up high and need adjustment upon insertion
• Lateral jaw movement cannot be stimulated

-ok to use for single posterior crowns w canine guidance