Biomechanics and FPD problems Flashcards
What is a simple FPD?
–1 or 2 missing teeth
–2 abutments
What is a complex FPD?
–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
What can cause a failure in FPD?
Abnormal stress created by torque and leverage create material
failure and tooth failure
–caries
–gingival inflammation
–poor framework design
–poor occlusion
–material failure
What are the problems caused by deflection (bending) of FPD?
-Fracture of Porcelain
-Connector breakage
-Retainer loosening and subsequent caries
-”Unfavorable” tooth or tissue response
The deflection is directly proportional to the cube of the __________ of its span.
length
- bigger the length of span = bigger the deflection
What is the equation for deflection?
FPD flexure varies directly by x where x is the interabutment __________.
distance
Deflection varies inversely by the cube of its __________________.
height (thickness)
FPD flexure varies inversely by t where t is the occluso-cervical ____________ of the connector
thickness
- bigger the thickness = lower the deflection
1/2t (half the thickness) = _____ times increase in flexture
8
1/3t = ____ times increase in flexure
27
Thickness is ___________ proportional the delfection
inversly
Length of span is ___________ proportional the delfection
directly
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?
- modify preparations to increase retention and resistance form / structural durability
- place boxes / grooves in response to direction of anticipated torque
Occlusal force on pontics can cause _____________ torque.
Mesial-Distal
Forces at an oblique angle on pontics or outside the center of the restoration cause _____ torque
F-L (around M-D axis of rotation)
Grooves / boxes _____________ resistance to dislodgement
increase
Where should you place grooves and boxes in a prep?
-Place boxes / grooves in response to direction of anticipated torque.
-Perpendicular to the movement around the axis of rotation
What should you consider to increase retention/resistance?
-Wall length / occlusal convergence / geometric resistance form
-Consider adhesive cement (bonding) for ↑ retention
Where would you place axial grooves to gain the additional resistance in preps to the right?
- Buccal or Lingual
_____ Grooves offset dislodgement in response to forces outside the inter-abutment axis
F-L
How do you know what’s adequate O-C thickness for a pontic/connector?
Wax up and complete abutment evaluation to see what connector measurements are possible
What are the connector requirements for O-C height x F-L width?
-Minimum Height x Width (Metal): 3-4mm x 3-4 mm
-Minimum Height x Width (ceramic/Zirconia): 4mm x 4mm
What are the different types of connector design?
rigid (typical bridge) vs non-rigid
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
Canine replacement stress is more favorable in the _____________ arch
mandibular
Why is canine replacement better in the mandibular arch?
Mand = Forces outside arch act to seat the pontic (strong – compression)
Max = Forces inside arch push the pontic buccally (weak - tension)
Canine pontic lies __________ the inter-abutment axis
outside
Why is a canine replacement an issue with abutments?
Adjacent teeth (Lateral and 1st premolar) are weak abutments
Should not replace more than one additional tooth when replacing a ____________
canine
Canine plus 2 contiguous teeth =
poor prognosis restore with implants if possible
Double abutments help stabilize the prosthesis by…
distributing forces over more teeth (do not necessarily increase retention)
What are the criteria necessary for secondary abutments?
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
What is a pier abutment?
An edentulous space on both sides of a lone free-standing abutment
Cause of failure in Peir Abutments is most often a…
loosened retainer
What are the reasons for a pier abutment failure?
- 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)
What does a non-rigid connector for a pier abutment do?
-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
What are the indications for a non-rigid connector?
-Pier abutment FPD
-Long span FPD with multiple abutments
-Non-parallel abutments (Tipped molar)
-Planning for failure
What are the contraindications for a non-rigid connector?
-Long span FPD with two abutments
-Excessively mobile teeth
-Unopposed teeth
When are non-rigid connectors with pier abutments primarily used?
-Long span FDP with multiple abutments
-Combined anterior and posterior FDPs
Long axes of abutment teeth should converge by no more than _________ degrees (maximum angle of tilting) if FPD is made.
25 - 30
What is the main issue with tilted molar abutments?
-Discrepancy between long axis of molar and premolar abutments
What are the characteristics of tilted molars as abutments?
-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)
True/False: On titled molars, occlusal reduction is not always the same as clearance needed.
True!
Remove only enough tooth structure to provide necessary space for the restoration
What should you be aware of with tilted molars and occlusal forces?
-Non-axial loading (horizontal) often leads to proximal crestal bone loss
What does molar uprighting (orthodontic movement) do?
-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.
What does a non-rigid connector do for a tilted molar abutment?
-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
What is a proximal half crown?
does not involve distal wall
-3/4 crown rotated 90 degrees
What are the requirements for a proximal half crown?
-Caries-free distal surface
-Low incidence of caries
-Even marginal ridge height
-Short span length
Why would you do a coping and a crown on a tilted molar?
- Design for failure
- Protect tooth (reduced fracture of crown of tooth with bridge stresses)
Must __________ molar to accommodate the thickness of coping and crown for tilted molar.
over-reduce
Cantilievers are only for patients who are contraindicated for…
implants
What is the criteria for a cantiliever
-Replace only 1 tooth, and have at least 2 abutments
Why are cantilevers not recommended for most patients?
-Removing tooth structure on two teeth.
-Occlusal forces create guarded prognosis
What is the criteria for a cantiliever abutment teeth?
-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
Occlusal forces on standard FPD direct forces along ____________ of abutment teeth.
long axis
Cantilever pontic without abutment root directs occlusal forces apically putting both ____________ forces on abutments
vertical and lateral
Why are implant-supported cantilevers good?
implants are embedded in the bone so they make good abutments
Only the __________ should be used as a solo abutment for a cantilever
canine
- long Root, esthetics easier, occlusal forces more lateral and therefore protective of pontic
When using a cantilever (on canine) to replace a lateral incisor what can you use for more stability?
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.
How can you use a cantilever to fix a 1st premolar?
-Use full veneer retainers on the 2nd premolar and 1st molar.
-Limit occlusion on the pontic.
What is the biggest problem with cantilevers on premolars?
OCCLUSION
What is the biggest problem with using a cantilever that has a rest (occlusal rest) on another tooth?
caries!
What is the prognosis for replacing a 1st molar with a cantilever?
-Unfavorable prognosis!!
-Extreme leverage forces generated by posterior position
-Occlusal forces place tensile stress
on 2 degree retainer
If you must replace a 1st molar with a cantilever what can you do?
-Pontic size small (premolar)
-Light occlusal contact; no excursive contact
-Pontic and connector need maximum O-G height for rigidity
What is the prognosis for replacing a central incisor with a cantilever?
-Lateral incisor abutment (bad Root shape, occlusal forces)
-Severe vertical overlap
-Repeated de-cementation with this particular case (Occlusal Forces)
not good…
