L2- Biomechanics and FPD Problems Flashcards
What are the two types of FPDs?
Simple FPD & Complex FPD
List the parameters for a simple FPD:
- 1 or 2 missing teeth
- 2 abutments
If someone has two abutments they would be a candidate for a:
simple FPD
If someone has only 2 missing teeth, they would be a candidate for a:
simple FPD
List the parameters for a complex FPD:
- 3 or more missing teeth
- missing canine
- 1, 3, or greater than 3 abutments
- splinted, pier, cantilever
- non-parallel abutments
- combined anterior and posterior FPD
Would the following image be classified as simple or complex?
simple FPD
Would the following image be classified as simple or complex?
complex FPD
(replacing more than 3 teeth)
Would the following image be classified as simple or complex?
complex FPD
Would the following image be classified as simple or complex?
complex FPD
Abnormal stress created by tongue and leverage create:
material failure and tooth failure
Failure in FPD’s are more due to ____ factors such as:
Biomechanical
- caries
- gingival inflammation
- poor framework design
- poor occlusion
- material failure
Bending or deflection of the FPD is a result of:
abnormal stress
List the problems caused by bending and deflection:
- fracture of porcelain
- connector breakage
- retainer loosening and subsequent caries
- “unfavorable” tooth or tissue response
In this image we see that the connector is broken, this is likely due to:
flexure
The law of beams states that:
Deflection = Load (length) ^3 / 4e Width (Height) ^3
According to the law of beams, the deflection is ____ proportional to the cube of the length of its span
directly
According to the law of beams, the ___ is directly proportional to the cube of the length of its span
deflection
FPD flexure varies directly by x^3 where x is the:
inter abutment distance
Increased pontics = increased length -
increased deflection
If 1p results in a deflection of x; 2p results in a deflection of __x, and 3p results in a deflection of __x
8x; 27x
Deflection varies ___ by the cube of its height (thickness)
inversely
____ varies inversely by the cube of its height
defelction
FPD flexure varies inversely by t^3 where t is the:
occlusal-cervical thickness of the connector
1/2t= ___ times increase in flexure
1/3t= ___times increase in flexure
8 times
27 times
Achieving the connector height needed can be:
clinically challenging
When achieving the connector height, what dimensions must be considered?
- FL
- occlusal cervical
A connector can be:
rigid or non-rigid
A rigid connector is:
cast or soldered
Connector requirements include:
- OC height
- FL width
The minimum height for a metal (rigid) connector =
3-4mm x 3-4 mm
The minimum height x width for a ceramic/zirconia (rigid) connector =
4mm x 4mm
The Pontic/connector design needs an adequate ___ thickness
O-C
How can you determine if a Pontic/connector design has an adequate OC thickness?
wax up and complete abutment evaluation to see what connector measurements are possible
Refers to an indication of maximum stress that can be developed in a material without causing plastic deformation:
yield strength
Yield strength refers to an indication of ____ that can be developed in a material without causing plastic deformation
maximum stress
T/F: You should use a bridge lateral with intermediate yield strength
False-high yield strength
If concerns exist with connector strength, considering using an ____ for increased retention on abutments
adhesive cement (bond)
Why might you use an adhesive cement (bond) with a connector?
for increased retention on abutments
T/F: Abutments and retainers receive greater dislodging forces than a single crown
True
Where should boxes and grooves be placed in regards to the connector design?
in response to direction of anticipated torque
Occlusal forces on pontics can cause:
medial-distal torque
With a downward force on the middle two units, what results?
MD Torque
Forces at an oblique angle or outside the center of the restoration as seen in the image below causes ____ around the ____ axis of rotation
FL torque; MD
Grooves/Boxes ____ resistance to enlargement
increase
When placing boxes & groove to increase resistance to dislodgment, place them in response to:
director of anticipated torque
Ideally you would place boxes and grooves in response to direction of anticipated torque, this is located:
perpendicular to the movement around the axis of rotation
What features aid in the appropriate retention and resistance of a retainer?
- wall length
- occlusal convergence
- geometric resistance form
Adhesive cement (bonding) can be considered for increased:
retention
Pontics lying outside the inter-abutment axis act as a ____ resulting in ____
lever arm; torquing movement
How might you gain additional resistance in opposite direction from lever arm?
axial grooves
Where would you place axial grooves to gain the additional resistance in the preps to the right?
buccal & lingual
____ offset dislodgment in response to forces outside the inter-abutment axis
facial & lingual grooves
What would this grooves in this image offset?
dislodgment in response to forces outside the inter-abutment axis
If a canine has to be replaced, this is considered:
complex FPD
Canine pontics lie:
outside the inter abutment axis
Because canine pontics lie outside the inter abutment axis, ____ is greater and therefore:
stress; less favorable on maxillary arch
Forces inside the arch push the canine pontic ___ resulting in ___
bucally; weak-tension
Is a canine Pontic more favorable in the maxillary or mandibular arch?
mandibular
Why is a canine Pontic more favorable in the mandibular arch?
Because forces outside the arch act to seat the Pontic (strong- compression)
(stress more favorable in mandibular arch)
Canines are considered to be complex because when you bite on the maxillary canine, this tooth wants to be forces ____, putting pressure on ____
outward; lateral & premolar
For mandibular canines, the occlusal force is being pushed:
inward towards the arch
T/F: For a maxillary canine replacement, the Pontic lies outside the inter-abutment axis
true
For canine replacement, the adjacent teeth (lateral and premolar) serve as:
weak abutments
When replacing a canine, you should not replace:
more than one additional tooth
Canine + 2 contiguous teeth=
poor prognosis
Canine + 2 contiguous teeth = poor prognosis so you should instead try to restore with:
implants when possible
(splint central incisors and premolar/molar)
Double abutments may also be referred to as:
splinting
What can help problems caused by poor crown/root ratio and long spans?
splinting (double abutments)
Double abutments (splinting) helps stabilize the prosthesis by:
distributing forces over more teeth (does not necessarily increase retention)
Double abutments (splinting) helps to stabilize the prosthesis by distributing forces over more teeth (though it does not necessarily increase retention). This applies especially to:
periodontally weakened teeth
List the criteria for secondary abutments:
- 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
When we have a tooth in between two pontics:
pier abutment
An edentulous space on both sides of a lone free-standing abutment
pier abutment
When considering the physiologic tooth movement, direction and amount varies from:
anterior to posterior
The cause of failure in Pier abutments is most often a:
loosened retainer
List three causes of a loosened retainer in a pier abutment:
- PROSTHESIS FLEXURE creatine movement of teeth
- TENSILE STRESSES between terminal retainers and abutments (intrusion of abutments under loading)
- DIFFERENCE IN RETENTIVE CAPACITIES between abutments (relative to size)
The following image shows:
pier abutment bridge
_____ in non-rigid connectors can minimize the transfer of stress from the particularly loaded segments to the rest of the pier abutment FPD
slight movement
A ____ or ____ can minimize problems often encountered with pier abutments
nonrigid connector or cantilever
What does the key in the image function to do?
it will allow some movement so the bridge isnt taking so much force, as well as removes stress
When making a pier abutment with a NON-RIGID connector, the key is put on the ____ of the pier abutment
distal
If a non-rigid connector is placed on the distal side of the retainer on a middle abutment, movement in the mesial direction will:
seat the key into the key way
If a non-rigid connector is placed on the ____ side of the retainer on a middle abutment, movement in the mesial direction will seat the key into the keyway
distal
If a non-rigid connector is placed on the mesial side of the middle abutment, mesially-directed movement will:
un-seat the key
If a non-rigid connector is placed on the ____ side of the middle abutment, mesially-directed movement will un-seat the key
mesial
What does the arrow indicate in the following image?
pier abutment with decay; pier abutments are hard to clean under
Non-rigid connector indications include:
- pier abutment FPD
- long span FPD with multiple abutments
- non-parallel abutments (tipped molar)
- planning for failure
Non-rigid connector contraindications include:
- long span FPD with two abutments
- excessively mobile teeth
- unopposed teeth
Why are unopposed teeth a contraindication for non-rigid connectors?
no occlusal forces so you wouldn’t need a key way
The follow are reasons you (should/should not) choose a NON-RIGID connector:
- long span FPD with two abutments
- excessively mobile teeth
- unopposed teeth
should not!
The follow are reasons you (should/should not) choose a NON-RIGID connector:
- pier abutment FPD
- long span FPD with multiple abutments
- non-parallel abutments (tipped molar)
- planning for failure
should!
If you have to bridge 18-20 and only have two abutments , what option would be CONTRAINDICATED?
non-rigid connector
- Long span FPD with multiple abutments
- Combined anterior and posterior FPDs
These would be reasons to choose a:
pier abutments with non-rigid connectors
What step is essential I the case of a pier abutment with a non-rigid connector?
metal framework try in
What is seen in the following image?
Pier-abutment
What is seen in the following image?
pier-abutment with non-rigid connector
What can be noted in this case?
pretty significant difference in path of insertions (due to tipped molar)
Long axes of abutment teeth should converge by NO MORE than: _____ (maximum angle of tilting) if FPD is made
25-30 degrees
Tilted molars are generally ____ abutments
poor
With a tilted molar as an abutment tooth, the ___ must be over-reduced/ overtapered which results in ___.
mesial wall; decreased resistance
With a tipped molar abutment tooth, ____ may intrude on the path of insertion and require mesial surface re-contouring or extraction
adjacent tooth
With a tilted molar as an abutment tooth, if the distal adjacent tooth intrudes on the path of insertion, what might some options be?
(consider tilted molar and adjacent tooth)
- orthodontic uprighting of tilted molar
- mesial surface re-contouring of distal adjacent tooth
- extraction of adjacent tooth (esp 3rd molars)
T/F: Occlusal reduction is always the same as clearance needed in regards to tilted molar abutments
False- occlusal reduction is NOT always the same as clearance needed
Because occlusal reduction is NOT always the same as clearance needed in regards to tilted molar abutments, you should remove only enough tooth structure to:
provide necessary space for the restoration
By removing only enough tooth structure to provide the necessary space for the restoration on a tilted molar abutment, this allows for ___ because retention can be compromised with tilted molar abutments
longer axial wall length
What can be compromised with tilted molar abutments?
retention
An FPD distributes forces favorably by directed forces in the:
long axis of abutment teeth
_____ abutment teeth provide better support than ___ abutment teeth
well-aligned; tipped
_____ (horizontal) often leads to proximal crystal bone loss
non-axial loading
non-axial loading (horizontal) often leads to:
proximal crystal bone loss
In the following image, what does the colored area indicate?
bone loss associated with tilted molar
An orthodontic movement to correct tilted molars (so they are better abutments)
molar uprighting
Distributes forces under loading through long axis of tooth (helps prevent/eliminate mesial bony defects)
molar uprighting
____ enables replacement of optimum occlusion in the case of a tilted molar
molar uprighting
What is a proximal half crown and in what case would this be utilized?
3/4 crown that is rotated to 90 degrees that does not involve the distal wall of the TIPPED molar abutment
What are the requirements for a proximal half crown?
- caries-free distal surface
- low incidence of caries
- even marginal ridge height
- short span length
When might you use a proximal half crown?
On a tipped molar in a patient with optimal home care
When considering copings and crowns on a tilted molar abutment, you would need to:
communicate with the patient that this is NOT an ideal treatment & there is a failure risk
Full crown coping with path of insertion in longs of tooth where the full coverage crown compensates for discrepancies in path of insertion caused by a tilted molar:
Copings and crowns on tilted molar abutment
What must be done to the molar in order to accommodate the thickness of a coping & crown?
over-reduction
Why would you consider coping & crown on a tilted molar abutment?
- design for failure
- protect tooth (reduced fracture of crown with tooth and bridge stresses)
What is primarily only for patients contraindicated for implants?
Cantilever FPD
Why is a cantilever FPD primarily ONLY an option for patients contraindicated for implants?
- removing tooth structure on TWO teeth
- Occlusal forces created GUARDED PROGNOSIS
Criteria for cantilever?
Replace only 1 tooth, and have at least 2 abutments
The criteria for abutment teeth with a cantilever FPD:
- long roots with good configuration
- long clinical crown
- favorable crown root ratio
- healthy periodontium
Why is a long clinical crown a requirement for the abutment teeth with a cantilever FPD?
- resistance form for preparations
- O-C height for connectors without imprinting on interdental papilla
What can be seen in the following image?
cantilever FPD
What can be seen in the following image?
coping and crown
Because most of us don’t have laterals in occlusion, this makes it one area where a ____ is a more ideal option:
cantilever FPD
Occlusal forces on a STANDARD FPD directs forces along the:
long axis of abutment teeth
Cantilever pontic without abutment root directs occlusal forces ___ putting both -__ and ___ forces on abutments
apically; vertical & lateral
What tooth is commonly replaced with a cantilever FPD?
lateral incisor
Only the ___ should be used as a solo abutment with a cantilever FPD
canine
Why should the only solo abutment used with a cantilever FPD be a canine?
- long root
- easier esthetics
- occlusal forces more lateral and therefore protective of pontic
In order for a cantilever FPD to replace a first premolar, there should be:
super light occlusion
What should be used on the 2nd premolar and 1st molar instead of a cantilever?
full veneer retainers
With a cantilever FPD replacing a premolar, we MUST limit:
occlusion on the Pontic
When a cantilever is used on premolar, ___ is used to support the premolar cantilever pontic
mesial rest
When using a rest on a cantilever pontic (by placing a rest seat in a restoration on the abutment), what is the biggest problem?
caries due to inadequate cleanse ability
What can be seen in the following image?
rest seat on cantilever Pontic
Describe the prognosis of a first molar replacement with a cantilever:
unfavorable
Why is there an unfavorable prognosis with a first molar replacement with a cantilever?
-extreme leverage forces generated by posterior position
- occlusal forces place tensile stress on secondary retainer
If a first molar must be replaced with a cantilever FPD, it is absolutely necessary that:
- pontic size is small (premolar size)
- light occlusal contact with no excursive contact
- pontic and connector need maximum OG height for rigidity
- good C:R ratio of abutments
- clinical crowns have maximum preparation length and resistance form
Describe the prognosis for a central incisor replacement with a cantilever FPD:
unfavorable
Why is there an unfavorable prognosis for central incisor replacement with a cantilever FPD?
- lateral incisor abutment
- severe vertical overlap
Why is a lateral incisor abutment for a central incisor replacement with a cantilever FPD a bad idea?
Due to the root shape of the lateral incisor and occlusal forces
A cantilever FPD for replacement of a central incisor #9yields a poor prognosis. This is partially due to severe vertical overlap which causes heavy occlusal forces. What are some other options for replacement of #9
- conventional bridge from #8-#10
- single implant
