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Fixed 2 > Biomechanics and FPD problems > Flashcards

Biomechanics and FPD problems Flashcards

1
Q

What is a simple FPD?

A

–1 or 2 missing teeth
–2 abutments

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2
Q

What is a complex FPD?

A

–3 or more missing teeth
–Missing Canine
–1,3, or greater than three abutments (splinted, pier, cantilever)
–Non-parallel abutments (tilted molar)
–Combined anterior and posterior FPD

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3
Q

What can cause a failure in FPD?

A

Abnormal stress created by torque and leverage create material
failure and tooth failure
–caries
–gingival inflammation
–poor framework design
–poor occlusion
–material failure

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4
Q

What are the problems caused by deflection (bending) of FPD?

A

-Fracture of Porcelain
-Connector breakage
-Retainer loosening and subsequent caries
-”Unfavorable” tooth or tissue response

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5
Q

The deflection is directly proportional to the cube of the __________ of its span.

A

length
- bigger the length of span = bigger the deflection

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6
Q

What is the equation for deflection?

A
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7
Q

FPD flexure varies directly by x where x is the interabutment __________.

A

distance

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8
Q

Deflection varies inversely by the cube of its __________________.

A

height (thickness)

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9
Q

FPD flexure varies inversely by t where t is the occluso-cervical ____________ of the connector

A

thickness
- bigger the thickness = lower the deflection

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10
Q

1/2t (half the thickness) = _____ times increase in flexture

A

8

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11
Q

1/3t = ____ times increase in flexure

A

27

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12
Q

Thickness is ___________ proportional the delfection

A

inversly

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13
Q

Length of span is ___________ proportional the delfection

A

directly

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14
Q

Abutments and retainers used in a fixed partial denture receive greater dislodging forces than a single crown. How can we help our prosthesis resist these forces?

A
  • modify preparations to increase retention and resistance form / structural durability
  • place boxes / grooves in response to direction of anticipated torque
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15
Q

Occlusal force on pontics can cause _____________ torque.

A

Mesial-Distal

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16
Q

Forces at an oblique angle on pontics or outside the center of the restoration cause _____ torque

A

F-L (around M-D axis of rotation)

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17
Q

Grooves / boxes _____________ resistance to dislodgement

A

increase

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18
Q

Where should you place grooves and boxes in a prep?

A

-Place boxes / grooves in response to direction of anticipated torque.
-Perpendicular to the movement around the axis of rotation

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19
Q

What should you consider to increase retention/resistance?

A

-Wall length / occlusal convergence / geometric resistance form
-Consider adhesive cement (bonding) for ↑ retention

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20
Q

Where would you place axial grooves to gain the additional resistance in preps to the right?

A
  • Buccal or Lingual
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21
Q

_____ Grooves offset dislodgement in response to forces outside the inter-abutment axis

A

F-L

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22
Q

How do you know what’s adequate O-C thickness for a pontic/connector?

A

Wax up and complete abutment evaluation to see what connector measurements are possible

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23
Q

What are the connector requirements for O-C height x F-L width?

A

-Minimum Height x Width (Metal): 3-4mm x 3-4 mm
-Minimum Height x Width (ceramic/Zirconia): 4mm x 4mm

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24
Q

What are the different types of connector design?

A

rigid (typical bridge) vs non-rigid

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25
Q

Use a bridge material with high yield strength. Yield strength refers to…

A

an indication of maximum stress that can be developed in a material without causing plastic deformation

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26
Q

Canine replacement stress is more favorable in the _____________ arch

A

mandibular

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27
Q

Why is canine replacement better in the mandibular arch?

A

Mand = Forces outside arch act to seat the pontic (strong – compression)

Max = Forces inside arch push the pontic buccally (weak - tension)

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28
Q

Canine pontic lies __________ the inter-abutment axis

A

outside

29
Q

Why is a canine replacement an issue with abutments?

A

Adjacent teeth (Lateral and 1st premolar) are weak abutments

30
Q

Should not replace more than one additional tooth when replacing a ____________

A

canine

31
Q

Canine plus 2 contiguous teeth =

A

poor prognosis restore with implants if possible

32
Q

Double abutments help stabilize the prosthesis by…

A

distributing forces over more teeth (do not necessarily increase retention)

33
Q

What are the criteria necessary for secondary abutments?

A

Same as a regular abutment
–Root surface area and C:R must equal to the primary abutments
–Secondary retainers must have greater than or equal to retention of primary retainers
–Long crown length and adequate interproximal space for connectors

34
Q

What is a pier abutment?

A

An edentulous space on both sides of a lone free-standing abutment

35
Q

Cause of failure in Peir Abutments is most often a…

A

loosened retainer

36
Q

What are the reasons for a pier abutment failure?

A
  • Loosened retainer
    -Prosthesis flexure creating movement of teeth
    -Tensile stresses between terminal retainers and abutments; intrusion of abutments under loading
    -Differences in retentive capacities between abutments (relative to size)
37
Q

What does a non-rigid connector for a pier abutment do?

A

-Slight movement in non-rigid connectors can minimize the transfer of stress from the particularly loaded segment to the rest of the FPD

38
Q

If a non-rigid connector is placed on the distal side of the retainer on a middle abutment, movement in a mesial direction will…

A

seat the key into the keyway

39
Q

If a non-rigid connector is placed on the mesial side of the middle abutment, mesially-directed movement will…

A

un-seat the key

40
Q

What are the indications for a non-rigid connector?

A

-Pier abutment FPD
-Long span FPD with multiple abutments
-Non-parallel abutments (Tipped molar)
-Planning for failure

41
Q

What are the contraindications for a non-rigid connector?

A

-Long span FPD with two abutments
-Excessively mobile teeth
-Unopposed teeth

42
Q

When are non-rigid connectors with pier abutments primarily used?

A

-Long span FDP with multiple abutments
-Combined anterior and posterior FDPs

43
Q

Long axes of abutment teeth should converge by no more than _________ degrees (maximum angle of tilting) if FPD is made.

A

25 - 30

44
Q

What is the main issue with tilted molar abutments?

A

-Discrepancy between long axis of molar and premolar abutments

45
Q

What are the characteristics of tilted molars as abutments?

A

-Generally poor abutments
-Mesial wall must be over-reduced / overtapered (↓ resistance)
-Distal adjacent tooth may intrude on the path of insertion (may need re-contouring/restoration)

46
Q

True/False: On titled molars, occlusal reduction is not always the same as clearance needed.

A

True!
Remove only enough tooth structure to provide necessary space for the restoration

47
Q

What should you be aware of with tilted molars and occlusal forces?

A

-Non-axial loading (horizontal) often leads to proximal crestal bone loss

48
Q

What does molar uprighting (orthodontic movement) do?

A

-Places abutment in better position for preparation
-Distributes forces under loading through long axis of tooth (helps prevent mesial bony defects)
-Enables replacement of optimum occlusion
-Requires EXCELLENT communication and treatment planning skills to educate patient. Often extra 1-2 years of ortho prior to bridge placement.

49
Q

What does a non-rigid connector do for a tilted molar abutment?

A

-Allows slight movement (short span)
-Keyway in distal of premolar to avoid intrusion of molar (mesial seating action)
-Must prepare box in distal of premolar preparation

50
Q

What is a proximal half crown?

A

does not involve distal wall
-3/4 crown rotated 90 degrees

51
Q

What are the requirements for a proximal half crown?

A

-Caries-free distal surface
-Low incidence of caries
-Even marginal ridge height
-Short span length

52
Q

Why would you do a coping and a crown on a tilted molar?

A
  • Design for failure
  • Protect tooth (reduced fracture of crown of tooth with bridge stresses)
53
Q

Must __________ molar to accommodate the thickness of coping and crown for tilted molar.

A

over-reduce

54
Q

Cantilievers are only for patients who are contraindicated for…

A

implants

55
Q

What is the criteria for a cantiliever

A

-Replace only 1 tooth, and have at least 2 abutments

56
Q

Why are cantilevers not recommended for most patients?

A

-Removing tooth structure on two teeth.
-Occlusal forces create guarded prognosis

57
Q

What is the criteria for a cantiliever abutment teeth?

A

-Long roots w/ good configuration
-Long clinical crown
—Resistance form for preparations
—O-C height for connectors w/o impinging on interdental papilla
-Favorable crown:root ratio and healthy periodontium

58
Q

Occlusal forces on standard FPD direct forces along ____________ of abutment teeth.

A

long axis

59
Q

Cantilever pontic without abutment root directs occlusal forces apically putting both ____________ forces on abutments

A

vertical and lateral

60
Q

Why are implant-supported cantilevers good?

A

implants are embedded in the bone so they make good abutments

61
Q

Only the __________ should be used as a solo abutment for a cantilever

A

canine
- long Root, esthetics easier, occlusal forces more lateral and therefore protective of pontic

62
Q

When using a cantilever (on canine) to replace a lateral incisor what can you use for more stability?

A

A Rest can be placed on mesial of pontic against a rest prep in a restoration in the distal of the central incisor or slight wrap-around of proximal contact.

63
Q

How can you use a cantilever to fix a 1st premolar?

A

-Use full veneer retainers on the 2nd premolar and 1st molar.
-Limit occlusion on the pontic.

64
Q

What is the biggest problem with cantilevers on premolars?

A

OCCLUSION

65
Q

What is the biggest problem with using a cantilever that has a rest (occlusal rest) on another tooth?

A

caries!

66
Q

What is the prognosis for replacing a 1st molar with a cantilever?

A

-Unfavorable prognosis!!
-Extreme leverage forces generated by posterior position
-Occlusal forces place tensile stress
on 2 degree retainer

67
Q

If you must replace a 1st molar with a cantilever what can you do?

A

-Pontic size small (premolar)
-Light occlusal contact; no excursive contact
-Pontic and connector need maximum O-G height for rigidity

68
Q

What is the prognosis for replacing a central incisor with a cantilever?

A

-Lateral incisor abutment (bad Root shape, occlusal forces)
-Severe vertical overlap
-Repeated de-cementation with
this particular case (Occlusal Forces)

Fixed 2 (3 decks)

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