1 - Mechanical behaviour | 2 - Sealing or bonding | 3 - Silanation | 4 - Radiopacity | 5 - Technical data


MECHANICAL BEHAVIOUR


THE FINITE ELEMENTS ANALYSIS MADE BY THE CETIM


This analysis does not claim to reproduce the whole mechanical behaviour of a tooth in function because it is impossible, since too many factors known and unknown are involved.

The purpose of this finite elements analysis is to compare the behaviour of different kinds of posts in the same situation in a surrounding as close as possible to a tooth in function, comprising bone, dentin, ligament, cement and core composite.

This analysis of the mechanical behaviour of posts has permitted to demonstrate that the influencing parameter for posts is their axial elasticity modulus. This analysis was done through the study of the general static behaviour of the post.

The study by finite elements allowed us to calculate and analyze the stress in a corono-radicular reconstitution in composite material held at the root by a steel post, a ceramic post, a carbon post, a SNOWPOST® composite post or by a cast core.

The calculation reveals the zones in the root where the stress is significant. These different restorations reacted under different angles of load. The calculations for the different restorations were then compared.

We also compared the results obtained from the tests done on a healthy tooth under the same conditions.

The method of fixation of the posts in the root was analyzed. We compared in particular the stress transmitted to the tooth in the case of a post sealed in the root and in the case of a bonded post.


WHAT HAPPENS IN A HEALTHY TOOTH ?


In the healthy tooth, the stress is distributed in “strips”. The variation is progressive between the different strips. There is no concentration of stress in the tooth.

Two cases are studied: compression loading and loading at a 20° angle.  


Compression loading
Loading at a 20° angle
Located in the coronal part of the tooth, from there to the apex this stress is gradually diluted along the root: there is no stress concentration. The more important stress is located along the alveolar bone and in the part of the root in contact with the bone. The distribution of the stress in vertical « strips » is typical of a beam in flexion . The stress values do not indicate significant differences in the different parts of the tooth, these values decrease very progressively.


COMPARING HIGH RIGIDITY POSTS & SNOWPOST®


For the study, the post is considered to be sealed (contact in friction between the post and the root canal), not bonded.

Axial E-modulus (= Young modulus) = Ex of the different posts

Rigid posts
Glass fiber posts
Steel
200 GPa
Snowpost®
45 GPa
Ceramic posts
200 GPa
Snowlight®
49 GPa
Carbon posts
140 GPa
 
Titanium posts
120 GPa
Dentin
20 GPa


Loading at a 20° angle
In the healthy tooth, the stress is distributed in vertical strips. The variation between the different strips is gradual, there is no stress concentration in the tooth.

Steel post: as in the case of the compression loading, the post creates a disturbance in the sharing of the stress and induces an overstress at the apex.

SNOWPOST®: when these composite posts are compared with a steel post, the stress is lower and there is a better dilution of the load.



Compression loading
In the case of steel or carbon posts, due to their high rigidity, overstressed zones are visible. The red areas reflect the stress distribution: in the healthy tooth, this area is widely developed, showing that the stress is widely shared. After, the dilution of the stress occurs very regularly and progressively all along the tooth: there is no dark red area close to blue unstressed areas.
The more the post is rigid (the more the E modulus is high), the more the zone where the stress is concentrated is small. This creates sites able to induce cracks which could progress in the future to failure or fracture.
The higher the rigidity of the post, the higher the trauma on the apex and the core composite.
These results can obviously be extended to other rigid posts such as ceramic posts and titanium posts.

With SNOWPOST® composite posts, the stress dilution occurs regularly and progressively all along the tooth, such as in the healthy tooth: that is why SNOWPOST® posts are said to be physiological posts.


ABOUT FATIGUE RESISTANCE


The lifetime of unidirectional fiber reinforced composite posts such as SNOWPOST® & SNOWLIGHT® bonded into the root canal as indicated in the instructions for use is UNLIMITED.

The classical three-points bending test in vitro is suitable for fishing rods evaluation, but not for posts.
A simulation by the finite elements analysis of a SNOWPOST® post's fatigue strength, IN FUNCTION, placed into the tooth with a bonding cement and a core composite, demonstrates that SNOWPOST® composite posts are not concerned by fatigue if the average masticatory force on tooth is not over 250 kgf!
Common values admitted for mastication forces are in the range of 10 kgf for incisors to 45 kgf for molars.

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