Transcript
-.
)3/2: (Crown /root ratio -3
It is the ratio between the linear
length of that part of tooth above the
level of alveolar crest of bone to that
part of root embedded in the bone.
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If the ratio is 1:1 the tooth may be used
as an abutment in the following cases:
1- Favorable opposing occlusion.
2- Highly motivated patient.
3- Normal occlusal pattern.
4- Good oral hygiene.
5- Favorable root configuration
configuration:Root -4 - Broader roots labio-lingually are preferable than those with rounded cross section.
Broader cross section 1Rounded cross section
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configuration:Root
Multi-rooted widely separated roots
provide better support than converging
fusing roots.
surface area:Periodontal -5
Ante’s law
In 1926 Ante suggested that the root surface area of the abutment teeth should be equal to or surpass that of the tooth or teeth to be replaced.
- The concept of Ante’s law should be
considered as a clinical guideline in bridge
design as there are many factors modifying
this concept which are;
a- Highly motivated patients in plaque
control.
b-Opposing occlusion either sound dentition
or removable prosthesis.
c-Decrease in mesiodistal width of edentulous
span due to bodily movement of teeth.
Length ■ It has been reported that all FPD, long or short possesses a certain degree of bending or flexing when subjected to a load, the longer the span , the greater the flexing.
■ Bending varies directly with the cube of the length & inversely with the cube of the occluso-gingival thickness of the pontic.
Thus considering other factors being equal
If a single tooth pontic span is deflected a certain amount, a
two tooth pontic span will bend 8 times as much, & the
three tooth pontic span will bend 27times as much.
Length On the other hand , a pontic with a given occluso-gingival dimension will bend a certain amount. If its thickness is
decreased by ½ , it will bend 8 times as much.
Clinical sequelae of bridge flexing
Excessive bending may lead to failure of a long span FPD being manifested clinically as:
- Fracture of porcelain veneer.
- Connector breakage
- Retainer loosening
- An unfavorable soft tissue response.
Biomechanical considerations
Clinical implication of the principle of bridge flexing in treatment planning:
1- Replacing 3 posterior teeth with a FPD usually exhibit unfavorable Prognosis esp. in the mandibular arch.
2- long span FPD on short mandibular teeth (i.e. short pontic occluso-gingivally) is expected to have a very disappointing prognosis.
Alternative ttt. Modalities:
- Implant supported FPD.
- RPD.
TO ↓ bending in long span bridges, it is advisable to construct
pontics & connectors of greater oocluso-gingival dimension + use
alloy of ↑ yield strength.
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