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 two missing teeth, they would be a candidate for a:
simple FPD
List the parameters for a complex FPD:
- 3 or more missing teeth
- missing canines
- 1, 3 or greater than 3 abutments
- splinters, pier, or cantilever
- non-parallel abutments
- combined anterior & posterior FPD
Would the following image be classified as simple or complex?
simple
Would the following image be classified as simple or complex?
complex
(replacing more than 3 teeth)
Would the following image be classified as simple or complex?
complex
Would the following image be classified as simple or complex?
complex
Abnormal stress created by torque and average create:
material failure & tooth failure
Failure in FPDs 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 case 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 broke, this 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, _________ is directly proportional to ______
deflection; cube of the length of its span
FPD flexures varies directly by x^3 where x 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; 3p results in a deflection of ____x
8x; 27x
Deflection varies _____ by the cube of its height (thickness)
inversely
Deflection varies inversely by the _____
cube of its height (thickness)
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
27
Achieving the connector height needed can be:
clinically challenging
When achieving the connector height what dimensions must be considered?
- FL
- Occlusal-cervical
A connector may 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 3-4mm
The minimum height x width for 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 has an adequate O-C 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 material with intermediate yield strength
False- high yield strength
If concerns exist with connector strength, considers using an ______ for increased retention on abutments
adhesive cement (bond)
Why might you use an adhesive cement (bond) with the 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 connector design?
In response to direction of anticipated torque
Occlusal forces on pontics can cause:
mesial-distal torque
With a downward force on the middle two units, what results?
MD torque
Forces at oblique angle or out center of rotation as seen in the image below causes _____ around the ____ axis of rotation
FL torque; MD
Grooves/boxes _____ resistance to dislodgement
Increase
When placing boxes and grooves to increase resistance to dislodgment, places them in response to:
direction 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 to gain the additional resistance in the preps to the right?
Buccal & lingual
______ offset dislodgment in response to forces outside the inter-abutment axis
FL grooves
What would the 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 _____
buccally; 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 forced _____ , putting pressure on _____
outward; lateral & premolar
For mandibular canines the occlusal force is being pushed:
inward towards the arch
T/F: For maxillary canine replacement the pontic lies outside the inter-abutment axis
True
For canine replacement the adjacent teeth (lateral & 1st premolar) serve as:
weak abutments
When replacing a canine, you should not replace:
more than one additional tooth
Canine + 2 contiguous teeth =
poor prognosis
Canines + 2 contiguous teeth = poor prognosis, so you should instead try to restoree with:
implants if 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?
Double abutments (splinting)
Double abutments (splinting) help 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 second 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
Cause of fail in pier abutments is most often a:
loosened retainer
List three causes of a loosened retainer in a pier abutment:
- 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)
The following image shows a:
pier abutment bridge
______ in non-rigid connectors can minimize the transfer of stress from the particular loaded segment to the rest of the pier abutment FPD
slight movement
A ____ or ____ can minimize problems often encountered with pier abutments
nonrigid connector; cantilever
What does the key in the image function to do?
It will allow some movement so the bridge isnt taking so much forces 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 keyway
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, medially-directed movement will:
unseat the key
If a non-rigid connector is placed on the _____ side of the middle abutment, medially-directed movement will unseat the key
mesial
What does the arrow indicate in the following image, why might we see this?
pier abutment with decay; pier abutments are hard to clean under
Non-rigid connector indications: (4)
- pier abutment FPD
- long span FPD with multiple abutments
- non-parallel abutments (tipped molar)
- planning for failure
Non-rigid connector contraindications: (3)
- long span FPD with two abutments
- excessively mobile teeth
- unopposed teeth
Why are unopposed teeth a contraindication for non-rigid connectors?
no occlusal so you wouldnt need a key way
The following 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 following 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 & posterior FPDs
These would be reasons to choose a:
Pier abutment with non-rigid connectors
What step is essential in 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 insertion 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, ______ must be over-reduced/overtapered which results in ____
mesial wall; decreased resistance
With a tilted molar as an 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 & adjacent teeth)
- orthodontic uprighting of tilted molar abutment
- mesial surface recontouring 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 the occlusal is not always the same as clearance needed in regard 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 necessary 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 favorable by directing forces in:
the long axis of the abutment teeth
______ abutment teeth provide better support than ____ abutment teeth
well-aligned; tipped
______ (horizontal) often leads to proximal crestal bone loss
non-axial loading
Non-axial loading (horizontal) often leads to:
proximal crestal bone loss
In the following image, what does the color 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 & in what case would this be utilized?
A 3/4 crown that is rotated 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 homecare
When considering copings and crowns on a tilted molar abutments, you would need to:
communicate with the patient that this is NOT an ideal treatment & failure is a risk
Full crown coping with path of insertion in long axis of tooth where the full coverage crown compensates in discrepancies in path of insertion caused by a tilted molar:
Copings & crown on tilted molar abutment
What must be done to the molar in order in order to accommodate the thickness of coping and crown?
over-reduction
Why would you consider coping and 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 create 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 requirements for the abutment teeth with a cantilever FPD?
- Resistance form for preparations
- O-C height for connectors without impinging on interdental papilla
What can be seen in the following image?
Cantilever FPD
What can be seen in the following image?
Tilted molar abutment copings & crowns
Because most of us do not have laterals in occlusion, this makes it one area where a ______ is a more ideal option
cantilever FPD
Occlusal forces on a STANDARD FPD direct forces along the:
long axis of abutment teeth
Cantilever pontic without abutment root directs occlusal forces _____ putting both ______ & ____ forces on abutments
apically; vertical & lateral
What tooth is commonly replaced with a cantilever FPD?
Lateral incisor
Only the ____ should be used as the 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 & 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 2nd premolar and 1st molar instead of a cantilever FPD?
Full veneer retainers
With a cantilever FPD replacing a premolar we must limit:
occlusion on the pontic
When a cantilever is used on premolars, ____ 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 tooth) what is the biggest problem?
caries due to inadequate cleansability
What can be seen in the following image?
Rest seat on cantilever pontic
Describe the prognosis for a first molar replacement with a cantilever:
unfavorable
Why is there an unfavorable prognosis for 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 & connector need maximum O-G 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 a 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 #9 yields 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
