Biomechanics Flashcards
what is a simple FPD for
-1 or 2 missing teeth
- 2 abutments
what is a complex FPD for
- 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
what are most failures in FPDs are due to
biomechanical factors
what biomechanical factors cause FPD failure
- caries
- gingival inflammation
- poor framework design
- poor occlusion
- material failure
what problems are caused by bending and deflection
- fracture of porcelain
- connector breakage
- retainer loosening and subsequent caries
- unfavorable tooth or tissue response
what is the relationship between deflection and length of a bridge
the deflection is directly proportional to the cube of the length of its span
what is the law of beams
deflection = load (length)^3/ 4e (width)(height)^3
what is the relationship between deflection and height of the bridge
deflection varies inversely by the cube of its height (thickness)
what are the types of rigid connectors
cast or soldered
what is the minimum height requirement for metal connectors
3-4mm x 3-4mm
what is the minimum height x width requirement for ceramic/zirconia connectors
4mm x 4mm
use a bridge material with ______
high yield strength
what is yield strength
an indication of maximum stress that can be developed in a material without causing plastic deformation
if concerns exist with connector strength use:
an adhesive cement for increased retention on abutments
occlusal force on pontics can cause ______ torque
mesial-distal
forces at an oblique angle or outside the center of the restoration cause _____
F-L torque around M-D axis of rotation
grooves/boxes ____ resistance to dislodgement
increase
place boxes/grooves _______ to the movement around the axis of rotation
perpendicular
what things help with retention and resistance in a retainer
wall length/ occlusal convergence / geometric resistance form
pontics lying outside the inter abutment axis act as a _____ causing ____
lever arm; torque movement
how do you gain additional resistance in opposite direction from the lever arm
axial grooves
where would you place axial grooves to gain the additional resistance in preps in the anterior
buccal and lingual
what torquing direction do bridges in the anterior move
out and up
what torquing movement do bridges in the posterior want to move
up and towards each other
is the maxillary canine or mandibular canine more favorable for a canine pontic
mandibular
why are maxillary pontics not favored
forces inside the arch push the pontic buccally and are weak from tension
why are mandibular canine pontics favored
forces outside the arch act to seat the pontic and are strong from compression
how many additional teeth can you replace if you are replacing a canine
none
what should you treatment plan if you have a canine and 2 contiguous teeth
restore with implants
what is another name for double abutments
splinting
what do double abutments do
help stabilize the prosthesis by distributing forces over more teeth but does not necessarily increase retention
what cases are double abutments good for
periodontally weakened teeth
what are the criteria for secondary abutments
-root surface area and crown to root ratio 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
is the physiologic tooth movement direction and amount the same from anterior and posterior
no
cause of failure in pier abutments is most often a:
loosened retainer
what are the causes of 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
where are stresses concentrated in pier abutments
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
where should a non-rigid connector be placed and why
on the distal side of the retainer on a middle abutment because the movement in a mesial direction will seat the key into the keyway
why should the non-rigid connector not be placed on the mesial side of the middle abutment
mesially directed movement will un seat the key
what are the main problems with pier abutments
loosened retainers and decay
what are the indications for non-rigid connectors
- pier abutment FPD
- long span FPD with multiple abutments
- non-parallel abutments- tipped molar
- planning for failure
what are the contraindications for non rigid connectors
- long span FPD with two abutments
- excessively mobile teeth
-unopposed teeth
when are non-rigid connectors used
- long span FPD with multiple abutments
- combined anterior and posterior FPDs
what type of framework must be used in non rigid connectors
metal framework
long axes of abutment teeth should converge by no more than _____
25-30 degrees
tilted molar abutments are generally ____- abutments
poor
how should a tilted molar abutment be prepared and what does this cause
mesial wall must be over-reduced/ overtapered but this decreases resistance
what might be an issue with the distal adjacent tooth with a mesially tipped molar
the distal adjacent tooth may intrude on the path of insertion
how can the distal adjacent tooth intruding on the path of insertion of a FPD insertion be corrected
-mesial surface may need re-contouring or restoration or extraction
- orthodontic uprighting or 3rd molar extraction
why can occlusal reduction not always be the same as clearance needed
it allows for longer axial wall length because retention can be compromised with tilted molar abutments
an FPD distributes forces favorably by:
directing forces in the long axis of the abutment teeth
non-axial loading (horizontal) often leads to:
proximal crestal bone loss
what does molar uprighting do
- places abutment in better position for preparation
- distributes forces under loading through long axis of tooth to help prevent/eliminate mesial bony defects
- enables replacement of optimum occlusion
how long does ortho take prior to bridge placement
1-2 years
describe a proximal half crown
- does not involve the distal wall
-3/4 crown rotated 90 degres
when are proximal half crowns used
-tilted molar abutments
- caries free distal surface
- low incidence of caries
- even marginal ridge height
- short span length
full coverage crown compensates for:
discrepancy in paths of insertion
must ______ molar to accomodate for the thickness of coping and crown
over-reduce
why would you do copings and crowns
- design for failure
- protect tooth
when are cantilevers primarily used
for patients contraindicated for implants
why are cantilevers not used often
- must remove tooth structure on two teeth
- occlusal forces create guarded prognosis
what are the criteria for cantilever
replace only 1 tooth and have at least 2 abutments
what are the criteria for abutment teeth for a cantilever
- long roots with good configuration
- long clinical crown
- resistance form for preparations occlusal-cervical height for connectors without impinging on interdental papilla
- favorable crown: root ratio and healthy periodontium
cantilever pontic without abutment root directs occlusal forces:
apically putting both vertical and lateral forces on abutments
where is it most ideal to do cantilever and why
lateral incisor because it is often not in occlusion
when replacing the lateral incisor with a cantilever what tooth should be used as the solo abutment and why
canine because it has a long root, esthetics are easier, occlusal forces are more lateral and therefore protect the pontic
where can a rest be placed for a lateral incisor cantilever
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
what is necessary for the canine when it is being used as an abutment for a lateral incisor
good clinical crown and good orthodontic position
what should the abutment design be for a cantilever replacement of first premolar
- use full veneer retainers on the 2nd premolar and 1st molar
- limit occlusion on the pontic
- mesial rest used to support premolar cantilever pontic
- either cemented or bonded
whats the biggest problem with cantilever premolars
occlusion
why can caries develop under rest seats
inadequate cleansability
what is the prognosis for first molar replacement with cantilevers and why
- unfavorable prognosis
- extreme leverage forces generated by posterior position
- occlusal forces place tensile stress on secondary retainer
what are the criteria for first molar replacement with cantilevers if absolutely necessary
- pontic size small such as a premolar
- light occlusal contact, no excursive contact
- pontic and connector needed maximum O-G height for rigidity
- good crown: root ratio of abutment
- clinical crowns - maximum preparation length and resistance form
what is the prognosis for central incisor replacement and why
unfavorable
- lateral incisor abutment is a weak abutment because of root shape and occlusal forces
- severe vertical overlap because of occlusal forces
- repeated de-cementation with this case because of occlusal forces
what are the other options to replace a maxillary central incisor
- conventional bridge
- single implant
- RPD
