Biomechanics Flashcards
Simple FPD:
(2)
–1 or 2 missing teeth
–2 abutments
Complex FPD:
(5)
–3 or more missing teeth
–Missing Canine
–1,3, or greater than three abutments
Splinted, pier, cantilever
–Non-parallel abutments
–Combined anterior and posterior FPD
FPD Failure
Abnormal stress created by (2) create material
failure and tooth failure
torque and leverage
”Failure in FPD’s are more due to biomechanical factors like:
(5)
–caries
–gingival inflammation
–poor framework design
–poor occlusion
–material failure.”
Problems caused by bending and deflection
Problems caused by bending and deflection
(4)
Problems caused by bending and deflection
-Fracture of Porcelain
-Connector breakage
-Retainer loosening and subsequent caries
-”Unfavorable” tooth or tissue response
Law of Beams
-Deflection = Load (Length)^3
4e Width (Height)^3
The deflection is directly proportional to the
cube of
the length of its span.
FPD flexure varies directly by — where x is the inter-
abutment distance.
x^3
Deflection varies — by the cube of its
height (thickness).
inversely
Deflection =
Load (Length)^3
4e Width (Height)^3
FPD flexure varies — by t^3 where
t is the occluso-cervical thickness of
the connector
inversely
1/2t = – times increase in flexure
1/3t = – times increase in flexure
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Connector Design – Rigid vs Non-Rigid
Rigid =
Cast or Soldered
Connector requirements – O-C Height vs F-L Width
-Minimum Height (Metal):
-Minimum Height x Width (ceramic/Zirconia):
3-4mm x 3-4 mm
4mm x 4mm
Pontic/Connector design needs adequate O-C thickness
-How do you know what’s adequate?
-Wax up and complete abutment evaluation to see what connector
measurements are possible.
Use a bridge material with high yield strength (Yield strength refers to an
indication of maximum stress that can be developed in a material without
causing plastic deformation.)
-Zirconia with appropriate size or a PFM alloy with high yield strength
If concerns exist with connector strength, consider using
an adhesive cement
(bond) for increased retention on abutments
Abutments and retainers receive — dislodging forces
than a single crown
greater
Modify preparations to increase
retention and resistance
form / structural durability
Place — in response to direction of
anticipated torque
boxes / grooves
Occlusal force on
pontics can cause
Mesial-Distal
torque.
Forces at an oblique
angle or outside the
center of the restoration
cause
F-L torque (around
M-D axis of rotation) .
Grooves / boxes increase resistance to dislodgement.
(2)
-Place boxes / grooves in response to direction of anticipated
torque.
-Perpendicular to the movement around the axis of rotation
skipped
Use retainer with appropriate retention / resistance
(2)
-Wall length / occlusal convergence / geometric resistance
form
-Consider adhesive cement (bonding) for ↑ retention.
Pontics lying outside the inter-abutment axis act as a lever arm →
torqueing movement.
Gain additional resistance in opposite direction from lever arm; How?
-Axial grooves
Where would you place axial grooves to gain the additional resistance in preps to the right?
–Buccal and Lingual
— Grooves offset dislodgement in response to forces
outside the inter-abutment axis
F-L
Canine Replacement (Complex FDP)
(3)
Canine pontic lies outside the inter-abutment axis
Stress is greater and therefore less favorable on maxillary arch
Forces inside arch push the pontic buccally (weak - tension)
Stress more favorable in mandibular arch
Forces outside arch act to seat the pontic (strong – compression)
Canine Replacement FDP
Pontic lies outside the inter-abutment axis
Adjacent teeth (Lateral and 1st premolar) are —
abutments
Should not replace more than one additional tooth
when replacing a —
Canine plus 2 contiguous teeth –
weak
canine
poor prognosis
restore with implants if possible
(Splint central incisors and premolar / molar)
Double abutments help stabilize the prosthesis by
distributing forces over
more teeth (do not necessarily increase retention).
–This applies to especially to periodontally weakened teeth
Criteria for secondary abutments
(3)
-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
Pier Abutment:
An edentulous space on both sides of a lone free-standing abutment
Pier Abutment:
Consider the Physiologic tooth movement
-Direction and amount varies from anterior to posterior
Cause of failure in Peir Abutments is most often a —
(3)
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)
-Stresses can be concentrated around
the abutment teeth
and between retainers and abutment preps
-Slight movement in non-rigid connectors can minimize the
transfer of stress from the particularly loaded segment to the
rest of the FPD
If a non-rigid connector is placed on
the distal side of the retainer on a
middle abutment, movement in a
mesial direction will
seat the key into
the keyway.
If a non-rigid connector is placed
on the mesial side of the middle
abutment, mesially-directed
movement will
un-seat the key.
Non-Rigid Connectors - Indications:
(4)
-Pier abutment FPD
-Long span FPD with multiple abutments
-Non-parallel abutments – Tipped molar
-Planning for failure
Non-Rigid Connectors-Contraindications:
(3)
-Long span FPD with two abutments
-Excessively mobile teeth
-Unopposed teeth
Non-Rigid Connectors:
(2)
-Long span FDP with multiple abutments
-Combined anterior and posterior FDPs
Special Considerations: Tilted Molar Abutments
-Discrepancy between
-Long axes of abutment teeth should converge by no more than
long axis of molar and premolar abutments
25o - 30o (maximum
angle of tilting) if FPD is made.
Special Considerations: Tilted Molar Abutments
-Generally poor —
-Mesial wall must be
-Distal adjacent tooth may intrude on
Mesial surface may need
Consider — uprighting (3rd molar extraction)
abutments
over-reduced / overtapered (↓ resistance)
the path of insertion
re-contouring or restoration or extraction
orthodontic
-Occlusal reduction is not always the same as
clearance needed.
-Remove only enough tooth structure to provide necessary space for the restoration.
-Occlusal reduction is not always the same as clearance needed.
-Remove only enough tooth structure to provide necessary space for the restoration.
-Allows for longer axial wall length because
retention can be compromised with tilted molar
abutments
-An — distributes forces favorably by directing forces in the long axis of the
abutment teeth.
FPD
Well-aligned abutment teeth provide better support than
tipped abutment
teeth.
-Non-axial loading (horizontal) often leads to
proximal crestal bone loss
Molar uprighting (orthodontic movement)
(4)
-Places abutment in better position for preparation
-Distributes forces under loading through long axis of tooth (helps
prevent/eliminate 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. And then maybe an implant would be better? Hmm….
Tilted Molar Abutments - Non-Rigid Connector
-Allows
-Keyway in distal of
premolar to avoid
-Must prepare box in —
of premolar preparation
slight movement -short span
intrusion of molar (mesial
seating action)
distal
Non-Rigid Connectors:
(2)
-Non-parallel abutments –
tipped molar
-Buccal / lingual grooves
parallel with mesial wall
increase resistance form -
molar
Proximal Half Crown –
does not involve
distal wall
-3/4 crown rotated 90o
Proximal Half Crown –
Requirements:
(4)
-Caries-free distal surface
-Low incidence of caries
-Even marginal ridge height
-Short span length
Tilted Molar Abutments - Copings and Crowns
Full crown preparation and
coping with path of insertion in
long axis of tooth.
Tilted Molar Abutments - Copings and Crowns
Full coverage crown
compensates for
discrepancy
in paths of insertion.
Must —reduce molar to
accommodate the thickness of
coping and crown.
over
WHY would you do this?
(2)
Design for failure
Protect tooth (reduced
fracture of crown of
tooth with bridge
stresses
Cantilever Fixed Partial Dentures
Primarily only for patients contraindicated for
implants. Why?
(2)
-Removing tooth structure on two teeth.
-Occlusal forces create guarded
prognosis
Criteria for Cantilever:
(1)
-Replace only 1 tooth, and have at least 2
abutments
Criteria for abutment teeth: Cantilever
(3)
-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
Cantilever Fixed Partial Dentures
(3)
-Occlusal forces on standard FPD direct forces along long axis of abutment teeth.
-Cantilever pontic without abutment root directs occlusal forces apically putting both
vertical and lateral forces on abutments.
-Abutment choice is VERY important!
Replacement of
Lateral Incisor
-Only the — should be used as a solo abutment
(Why?)
canine
Long Root, Esthetics easier, occlusal forces more lateral and therefore protective of
pontic.
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.
Cantilever Fixed Partial Dentures
(2) is necessary
Good clinical crown length / orthodontic position
Cantilever FDP:
Replacement of First Premolar
(2)
-Use full veneer retainers on the 2nd premolar and 1st molar.
-Limit occlusion on the pontic.
Cantilever: Premolars
—- rest used to
support premolar
cantilever pontic
-Either cemented or
bonded.
Mesial
-When using a rest on a cantilever pontic,
place a rest seat in a —
—- can develop due to inadequate
cleansability.
restoration on the
abutment.
Caries
First molar replacement with cantilever
-Unfavorable prognosis!!
(2)
-Extreme leverage forces
generated by posterior position
-Occlusal forces place tensile stress
on 2o retainer
First molar replacement with cantilever
If absolutely necessary:
(5)
-Pontic size small (premolar)
-Light occlusal contact; no excursive
contact
-Pontic and connector need
maximum O-G height for rigidity
-Good crown:root ratio of abutments
-Clinical crowns - maximum
preparation length and
resistance form
Central incisor replacement with cantilever
Pontic
-Unfavorable Prognosis!!:
1. Lateral incisor abutment
-Why is this negative?
2. Severe vertical overlap
-Why is this negative?
3. Repeated de-cementation with
this particular case.
-Why would this keep
happening?
-Root shape, occlusal
forces
-Occlusal forces
-Occlusal forces
What other options could you have
done here to replace #9?
(2)
-Conventional bridge from #8-#10
-Single implant
-What else??
