3- Crown /root ratio: (2/3)

-.

)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.

2

3

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

3

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.