1

Ronni A. Schnell, DMD, MAGDBoston University Henry M Goldman School of Dental Medicine

February 8, 2014

University of AlabamaSchool of Dentistry Alumni Association

ALUMNI WEEKEND

Ronni A. Schnell, DMD, MAGDBoston University Henry M Goldman School of Dental Medicine

February 8, 2014

Retention or Rotation? The Art & Science of

Removable Cast Partial Denture Design © 2014

University of AlabamaSchool of Dentistry Alumni Association

ALUMNI WEEKEND 2014

Re-Introduction to

Cast Partial Dentures

Indications for Removable Partials

1. Distal Extensions2. Long Tooth Borne Spans3. Severe Bone Loss4. Recent Extractions5. High Caries Index6. Poor Crown/Root Ratio7. Cost

3

2

1

Kennedy Classificationof Partially Edentulous Arches

• Classified by:The most posterior spaceto be restored by the partial

• Modified by:The number of remaining spacesto be restored by the partial

Vertical Rotationalmovement movement

Distal ExtensionTooth Borne

Partial Denture Movement?

ROTATI

ON

?

Rotation & Classification

• All Class I partials rotate• All Class II partials rotate• Some Class III partials rotate• Some Class IV partials rotate

2

Important Facts #1-4

1. Rotation Causes Torque2.Torque causes tooth loss3.Tipped teeth are more likely to

torque4.Lone standing teeth are more likely

to torque

Teeth most likely to tipIncisors

Lone-Standingteeth

NeighboringTeeth haveIndependentRoot systems

Parts of a cast partial framework

1. Major connector2. Minor connector3. Meshwork4. Finish line5. Clasp6. Rest7. Tissue stop

6

3

41

2

5

5

14

3

2

6

7 7

A/P Palatal Bars Broad Palatal Strap Full Palatal Coverage

Horseshoe Lingual Bar Lingual Blanket

Major Connectors

A

P

Anterior/Posterior Palatal Bars

• Most common• Most rigid• Anterior bar:

– broad & thin– between rugae– 6mm from gingival margin

• Posterior bar:– narrow & thick– ½ tear shaped– thickest at posterior– ant to jx of hard & soft palates

• No right < between bars• Bars ┴ to median suture

Broad Palatal Strap

• Thickness for rigidity may cause speech problem

• Anterior border as far posterior as possible

• Posterior border ┴ to median suture

• Used only when few teeth missing

Full Palatal Coverage• Ideal for Kennedy Class I• Includes Post Palatal Seal• May be all metal:

– good thermal conductor– decr bulk– no Post Dam– not adjustable

Horseshoe

• Rotates horizontally• Best with tooth borne• Best with many guide

planes• Gaggers last resort• Inoperable torus @ jx of

hard & soft palates• Thickness for rigidity may

cause speech problems

Lingual Bar• Most common• Relieved from tissue• ½ pear shape w/ bulk inferiorly• Superior margin 3mm from FGM• Bar is 3mm wide• Requires a min of 6mm fold depth

during tongue fx

3

Lingual Blanket• Indications include:

– High lingual frenum– Perio splinting– Past experience

• Contacts teeth, but not gingiva• Requires rest seats on all lower

anterior• Inferior border at depth compatible

with tongue movement

Major Connectors

1. A/P Palatal Bars: most rigid, 1st choice-max2. Broad Palatal Strap: thick, small case3. Full Palatal Coverage: upper Class I4. Horseshoe: inoperable torus @ H/S palate5. Lingual Bar: most comfortable, 1st choice-mand6. Lingual Blanket: shallow floor, splinting

A/P Palatal Bars Broad Palatal Strap Full Palatal Coverage

Horseshoe

TT

T

T

Metal must never covera palatal torus

Lingual Bar Lingual Blanket

TT T T

Metal must never cover Mandibular tori……if they cannot be avoided they must be removed

STEELE’S FACING

•Acrylic facing•Metal backing•Cemented or cured

Recommended for small anterior space

•Metal backing•Slot projection

STEELE’S FACING

TUBE TOOTH

•Single Posterior•Hollow ground tooth•Nail head projection

Recommended for small posterior space

Abutments

• Definition: any tooth which contacts the metal framework

• Primary (1º) abutment: has a clasp• Secondary (2º) abutment: has no clasp

The Direct Retainer (Clasp)

1. Rest2. Retainer3. Bracer

1

2

2

1

3

1

3

4

Type I vs Type II• Occlusal origin• From minor connector• Crosses bulge• 1/3rd retentive• B or L retention

• Gingival origin• From meshwork• Doesn’t cross bulge• All retentive• B retention only

“Suprabulge” “Infrabulge”

approach arm

Type I Direct Retainers Cast “C” clasp Wire “C” clasp (Combo)

Wireis 3Xmore flexiblethancast

XOther “Overwrought” Options

Likely to break

Likely to wedge

Likely to wedge

Two rests req’d

Too rigid

Extra-bracing

Type II Direct Retainers

approach arm

tube tooth

Note that both approach arm & tube tooth can occupy the same space.

Narrower spaces require longer approach arms

2mm

3mm Approach Arms

3mm 3mm 3mm 3mm

OK OK OK N/A N/A

Tooth = tissue Tissue undercut Tissue ledge Tissue undercut Severe tiltbelow 3mm above 3mm

Acceptable locations Unacceptable locations Severe tipping

Severe tissue undercut

Shallowfold

Approach Arms

5

Approach arm misuse

Reverse Skipping teeth Molars

Approach Arm Misuse

NO LINGUAL or PALATALapproach arm arms

NO approach arms positioned DISTALLY

NO DOUBLEapproach arms or SKIPPING of teeth

Inappropriate Uses

For Approach Arms

xx

NO approach arms on MOLARS

NO DOUBLEapproach arms or SKIPPING of teeth

Inappropriate Uses

For Approach Arms

xx

NO approach arms approaching from the mesial

Inappropriate Uses

For Approach ArmsImportant Fact #5

ClaspsCauseTorque

Clasp LocationTo reduce rotational or tipping forces,place clasps as gingivally as possible

Occlusal 1/3rd

Gingival 1/3rd

Effect of height of contour on Clasp location

Occlusal 1/3rd

Gingival 1/3rd

Effect of height of contour on Clasp location Important Fact #6

Lowering Clasps ReducesTorque

6

Bracing preventstooth movement

as clasp crosses bulge

Bracing (Reciprocation)

Absence of Bracingcauses tooth movement

Important Fact #7

BracingReducesTorque

2

1

3

Bracing Options1. Arm2. Blanket3. 2 Minor

Connectors

Bracing effectiveness must be evaluated from the occlusal

3 Bracing Options

Bracer shouldbe directlyoppositeRetainer

Heights of contour may be mismatched

idealideal

Bracing effectiveness must also be evaluated from the proximal

Important Fact #8

Lowering Bracer ReducesTorque

Rests

Cingulum rests

Occlusal rests

Functions of Rests

• Preventing cervical movement• Preventing rotational movement• Maintaining or establishing occlusion• Maintaining relationship of clasp to abutment

Rests may cause tooth movement

7

Important Fact #9

RestsCauseTorque

Rest Seats

Cingulum

MLMO

DO

Occlusal

Functions of Rest Seats

• Directing occlusal force along tooth long axiseven without opposing tooth present

• Decreasing torque on abutment• Preventing hyperocclusion of rest

Important Fact #10

Rest Seats ReduceTorque

During rotation Direct retainers = LeversThe Rest determines the type of lever

Important Fact #11

LeversCauseTorque

E E E

F F F

R R R

R R RE E E

Clasps as 1st Class Levers

C Bar RPI

E E E

F F F

F F F

R R R

R R RE E E

Clasps as 2nd Class LeversC Bar RPI

UP DOWN UP DOWN UP DOWN

DOWN DOWN DOWN DOWN DOWN DOWN

E E E

F F F

F F F

R R R

R R RE E E

1st Class Lever

2nd Class Lever

8

Important Fact #12

2nd Class Levers ReduceTorque

The rest closest to the rotation = the lever

One rest or two?The Indirect RetainerDuring rotationIndirect retainers seat &prevent lifting of the distal extension

seatingprevents lifting

SeatingPrevents liftingX

Axis of Rotation

During rotationIndirect retainers= “Anti-levers”

Lifting Lifting Lifting

Seating Seating Seating Seating

Indirect Retention

As the denture base lifts away from tissue,an opposing rest seats against tooth

lifting

seating

The effectiveness of indirect retention decreasesas the distance from the axis of rotation decreases

Indirect Retention helps keep the distal extension base down:1. Locate the most posterior rests on each side2. Connect them to form the axis of rotation3. Locate the midpoint of the axis4. Locate the tooth perpendicular to the midpoint5. Place a rest on that tooth or move laterally

to the nearest acceptable tooth

Important Fact #13

Indirect Retention ReducesTorque

O-R-A-C-G-U-B-TA partial denture design system

Occlusal analysis

Rotation analysis

Abutment selection

Clasp selection

Guide plane survey

Undercut survey

Bracing survey

Tripodization

Our“hypothesis”

Testing our“hypothesis”

9

OCCLUSAL ANALYSIS

1. Plane Correction?2. Extrusion Potential?3. Overbite Problem?4. Spaces Selected for PD?

O-R-A-C-G-U-B-T

1. Plane correction?2. Extrusion potential?3. Overbite problem?4. Spaces selected for PD?

2/3 RMP

Plane correction?

• √ same arch• √ opposing arch• √ retromolar pad• √ radiograph

?

Overbite problem?

Steele’s FacingSteele’s Facingsolution

Anterior overbite may result in Incisal Guidance

ROTATION ANALYSIS

1. Spaces to be restored by the partial?2. Size of those spaces?3. Support of those spaces?

O-R-A-C-G-U-B-T

Spaces selected for PD? Spaces selected for PD?

Kennedy Classification

1. Bilateral Distal Extension (+/- Mods)2. Unilateral Distal Extension (+/- Mods)3. Unilateral Tooth Borne (+/- Mods)4. Bilateral Tooth Borne (No Mods,X Midline)

O-R-A-C-G-U-B-T

10

X

X

+/-

+/-

Tooth borne

Tooth borne

Distal ext.

Distal ext.

Tooth borne

Tooth borne

Tooth borne

Tooth borne

ABUTMENT SELECTION1. As many guide planes as possible2. As far apart as possible3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

O-R-A-C-G-U-B-T

ABUTMENT SELECTION1. As many guide planes as possible2. As far apart as possible3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

O-R-A-C-G-U-B-T

1. As many guide planes as possible…exposed proximal surfaces2. As far apart as possible3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

Adjacent to edentulous

spaces

NB- Guide Planes &Anterior Teeth:

Incisors and themesials of Caninesare not usedbecause of esthetic reasons

1. As many guide planes as possible…exposed proximal surfaces2. As far apart as possible3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

1. As many guide planes as possible2. As far apart as possible…for stability3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

X

1. As many guide planes as possible2. As far apart as possible…for stability3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

1. As many guide planes as possible2. As far apart as possible3. Symmetry where possible…for comfort4. No incisors5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

1. As many guide planes as possible2. As far apart as possible3. Symmetry where possible4. No incisors…to prevent tipping & speech problems5. No lone-standing premolars as 1º abut.6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK

12

1. As many guide planes as possible2. As far apart as possible3. Symmetry where possible4. No incisors5. No lone-standing premolars as 1º6. No lone canine/molar if prox. contact poss.7. Extrusion prevention 8. Indirect retention9. Esthetics OK if design OK…evaluate “high smile line”

CLASP SELECTION

1. Type I vs. Type II Clasps 2. Rotating vs. Non-rotating Partials3. Class I vs. Class II Levers

O-R-A-C-G-U-B-T

Non-rotating PD vs. Rotating PD

• Moves occluso-gingivally• Does not cause torque• Uses simplest clasp

• Moves antero-posteriorly• Causes torque• Must protect all abutments

Non-rotating Partial:C = clasp of choice2nd choice = any

Rotating Partial:RPI = clasp of choice2nd choice = Combo

Rotating Partial:RPI = clasp of choice2nd choice = Combo

wire

Class I vs. Class II Levers

CHANGING LEVER

CHANGING EFFECT ON TOOTH

UP DOWN UP DOWN UP DOWN

DOWN DOWN DOWN DOWN DOWN DOWN

E E E

F F F

F F F

R R R

R R RE E E

Rotating Clasps in Function:

Biting vs. Lifting36 psi vs. gravity

13

●

CDO RESTMB TIP

BITE

●

CDO RESTMB TIP

LIFT●

CDO RESTMB TIP

●

Protects tooth at ridge’s expense by “disengaging” from tooth

RPIMO RESTMidB TIP

●

RPIMO RESTMidB TIP

●

RPACombo*MO RESTMB TIP

*Wire clasp is up to 3 times more flexible than a cast clasp

●

Protects tooth at its own expense by flexing

*Wire clasp is up to 3 times more flexible than a cast clasp

RPACombo*MO RESTMB TIP

●

RPACombo*MO RESTMB TIP

●

BARMO RESTDB TIP

14

●

Protects tooth by having less contact than a C

BARMO RESTDB TIP

●

BARMO RESTDB TIP

●

RPAC*MO RESTMB TIP

* Cast clasp is up to 3 times less flexible than a wire clasp

●

Protects only by using a Class II lever

RPAC*MO RESTMB TIP ●

RPAC*MO RESTMB TIP ●

RPIMO RESTMidB TIP

1st Choice: has potential to disengage from tooth

●

RPAComboMO RESTMB TIP

2nd Choice: if wire, is more flexible than a cast clasp

●

BARMO RESTDB TIP

3rd Choice: is smaller than C, so reduces force on tooth

●

RPACMO RESTMB TIP

4th Choice: if cast, is less flexible than a wire clasp & is larger than a bar clasp

15

RPI

RPA

RPI vsRPA (Combo)

Breaks contact butuses Ridge as Fulcrum

Remains in contact butWire absorbs Stress

CHANGING CLASP SIZE

CHANGING LEVERCHANGING LEVER

CLASP SELECTION

1. Non-rotating? C = Clasp of Choice2nd Choice = any

2. Rotating? RPI = Clasp of Choice2nd Choice = Combo

3. Lever? 2nd Class Lever

O-R-A-C-G-U-B-T

IV.IV.

Non-rotating vs. Rotating PD• Moves occluso-gingivally• Does not cause torque• “C” is simplest,

but any may be usedif “C” cannot

• Moves antero-posteriorly• Causes torque• Must protect all abutments

@ ridge’s expense… RPI@ clasp’s expense… Combo

w/ 2nd class lever

Rotation requires protection!

Design Priorities

#1: Protect Bone if teeth strong#2: Protect Teeth if bone strong#3: If Teeth weak, treat as DE w/ RPI#4: If Bone weak, treat as DE w/ Combo

GUIDE PLANE SURVEY

1. Proximal Surfaces2. Occlusal 1/3rd

3. Disk Parallel to Path

O-R-A-C-G-U-B-T

16

incorrect

correct

Anterior tissue undercut…

…requires “Tail Down” tilt

↑ Tail down tilt mayrequire diskingof teethto create a newGuide Plane

Guide Plane

Guide “Point”

Important Fact #14

Guide Planes ReduceTorque

UNDERCUT SURVEY

1. Facial or Lingual Surfaces (+/- tissue)2. Gingival 1/3rd

3. Dimple Flat Surface → .01 (.02 → .03)4. Disk Non-parallel to lower Type I clasp5. Disk Parallel to lower Type II clasp

O-R-A-C-G-U-B-TClasp Location

To reduce rotational or tipping forces,place clasps as gingivally as possible

↑ Tail down tilt mayrequire “dimpling”of teeth to createa new undercut

Guide Plane

Guide “Point”

Loss of undercut

Creating an Undercut

To create a .01 undercut, dimple a flat area by .01

To create a .02 undercut, dimple a .01 undercut by .01

To create a .03 undercut, dimple a 02 undercut by .01

17

Lowering a Clasp orDecreasing Bulge

• Disk Non-Parallelto lower a Type I clasp

• Disk Parallelto lower a Type II clasp

Type I vs Type II

Lowering a Type I Clasp – Disk Non-Parallel

Lowering a Type I Clasp – Disk Parallel

Lowering a Type II Clasp – Disk Parallel

If a bulbous tooth createsa tissue undercut thatcannot be eliminated bychanging the tilt,the tooth may need

enamoplasty

Disk Parallel for a Type II clasp w/ a straight bur

?

•Path on proximal•View from proximal•Cut only on facial•Use tapered bur forType I clasp

Disk Non-parallel for a Type Iclasp w/ a tapered bur

BRACING SURVEY

1. Arm, Blanket or 2 Minor connector(more than 180º around circumference)

2. Gingival 1/3rd

3. Disk Non-parallel to Lower Ht. of Contour

O-R-A-C-G-U-B-T

3 Bracing Types:

●

BRACINGARM

May torque the tooth

●

18

● ●

BRACINGMINORCONNECTORS

Reduce torque – with less metal contact

●

●

Bracer shouldbe directlyoppositeRetainerideal

ideal

Teeth may be tilted or plastied to lowerthe height of contour.

Disk non-parallel to pathto lower the height of contour.

TRIPODIZATION

1. On 3 sides of cast2. Records A/P & Lateral Tilts3. Must be included in Crowned abutments

O-R-A-C-G-U-B-T

Tripodization

Partials by the Numbers1. Guide Planes:

as ____ as possible2. Clasps:

as ___ * as possibleas ____ as possibleas ____ as possible

3. Rests: as ____ as possible

4. Major connectors: as ____ as possible

* Ante’s Rule

References• Kratochvil, F.J.: Influence of occlusal rest position and clasp design on 

movement of abutment teeth. J. Prosth. Dent. 13:114, 1963.

• Krol A.J. Clasp Design for Extension Base Removable Partial Dentures. J Prosthet Dent 29:408‐415, 1973.

• Berg, T., Jr.: Symposium on Common Failures in Removable Partial Prosthodontics, I‐Bar Myth and Countermyth, Dental Clinics of NA, Vol. 23, No. 1 Jan 1979 & Vol. 28 No. 2 April 1984

• Argerakis, G. P.,: Symposium on Semiprecision Attachments in Removable Partial Dentures, Functional Forces with Removable Partial Dentures, Dental Clinics of NA, Vol. 29, No. 1, Jan 1985.

• Carr, McGivney and Brown McCracken’s Removable Partial Prosthodontics, 12th ed. C.V. Mosby (2011)

• Brien, N.: Conception et trace des Prostheses Partielles Amovibles, 1st ed. Prostho (1996).