In the field of materials, the end of the twentieth century has been marked by the development of composite materials. Because of their steadily increasing performance, these modern materials are now replacing metal in all sectors. Today, it is impossible to imagine a racing bicycle, a pair of skis or helicopter rotor blades in anything other than composite materials, except of course for antique collector items.
Technological advances in the industry of composites have offered new possibilities to dentistry: not only the development of diverse tooth fillings and core composites, but also of high-performing bonding agents. These composites are auto or photo curing resins, the performance of which being improved by adding a charge of microscopic particles of varying form and composition.
In 1985 in Lyon, France, Professeur BOIS and his research team invented and described endobuccal prosthetic devices, whereby dental posts, made of composite materials with fiber-reinforced resin matrix.
THE FIRST COMPOSITE POSTS: RESIN AND CARBON FIBER |
The idea was to obtain dental posts that are as resistant as metal while avoiding certain inconveniences: corrosion, electro-galvanic reactions, the need for mechanical retention, and the heterogeneity of the post and the core composite. The prevailing idea at the time was that the post must be rigid.
The first composite posts, made of unidirectional longitudinal carbon fibers embedded in a resin matrix were eventually produced and marketed. In spite of their black color and their non X-ray opacity, some pioneering practitioners began to use them because, at the same time, the new concept of the variation of the elasticity modulus in relation to the angle of the load was becoming known.
Indeed, materials are either: |
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isotropic , which means that their E modulus (Young modulus) stays the same no matter what the angle of load (this is the case of homogeneous materials, such as metal), |
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or |
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anisotropic , which is the case of unidirectional composites (called UD composites). These composites present both an axial modulus Ex (in the longitudinal direction of the fibers), and a transversal modulus Ey (perpendicular to the fibers).
As an example: wood is composed of cellulose fibers embedded in a ligneous matrix. |
| The concept of the variation of the elasticity modulus says that when a load is applied at an angle of 20°, 30°, or 45°, the elasticity modulus varies and is a result of both the Ex and Ey moduli. Thus, during function, when a load is applied on a UD carbon fiber post (having a high axial Ex modulus and a low Ey modulus) with an angle of 30°, the E Modulus of this post is believed to be close to that of the dentin, which is, according to researchers, between 18.6 and 22 Gpa (gigapascals). Therefore, carbon fiber posts, being as resistant as steel, are supposed to be gentle for the tooth, thereby avoiding root fracture or failure. |
However, experimental studies have shown different results: carbon posts appear to be just as rigid as metal posts no matter what angle of force is applied (PURTON et al., 1996, TORBJÖRNER et al., 1996, ASMUSSEN et al., 1999).
Furthermore, a finite elements computer study carried out by the CETIM in Nantes, France (SRAC COSMOS/M program, version 2.0), comparing the mechanical behavior of different posts in a restored tooth (i.e. steel, glass fiber -Snowpost- or carbon fiber reinforced), shows identical results for both isotropic material posts and anisotropic UD carbon fiber composite posts. |
The explanation of these results has been provided by competent authorities (Institut des Matériaux Composites of Bordeaux, Ecole Centrale of Lyon, Département Calcul du CETIM of Nantes), and is based upon the geometry of beams.
A beam is an elongated geometric form, having a high length/diameter ratio: such as a dental post.
It is a fact that for a plate of UD fiber composite material (high surface/thickness ratio) there is a variation of the elasticity modulus which is dependant on the angle of load and the axis of the fibers.
But, it is known that in the case of a beam, only the axial Ex modulus is concerned (at 95%) no matter what the angle of the load.
When this angle approaches 90°, i.e perpendicular to the fibers, the Young modulus no longer applies, but instead, the shear strength comes into account (GAY, REYNE, TIMOSHENKO).
BEAM & PLATE |
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A plate is a mechanical pattern with one dimension (thickness) being very low in comparison with the other two (length and width).
In the case of an unidirectional fiber reinforced composite plate, there are two elasticity moduli: the Ex longitudinal modulus, in the axial direction of fibers and the Ey transversal modulus, perpendicular to the fibers. The rigidity of an UD composite plate varies with the angle of the load with regard to the axis of the fibers.
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A beam is a mechanical pattern with one dimension (length) being much higher than the section (in the case of a rod, the diameter). The UD reinforced composite post has the mechanical behavior of a beam.
When the load is applied in the axial direction of the fibers, in traction/compression, the Ex modulus comes into play.
When the load is applied at an intermediary angle, there is a combination of traction and flexion but it is again the Ex modulus which comes into play.
Nevertheless, the resin matrix of a composite post maintains its shock-absorbing qualities in spite of its high rigidity, and therefore, fewer fractures of the tooth occur (ISIDOR et al., 1996, CRA, 1998). |
SNOWPOST® & SNOWLIGHT® COMPOSITE POSTS, THE PHYSIOLOGICAL POSTS |
It is known today that a post does not reinforce the tooth; its function is to maintain the core reconstitution material by unifying it with the root. |
Mechanically, a tooth during function behaves like an elastic beam,
or more precisely, like a beam fixed at one end such as a cantilever. |
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The ideal root canal post must be sufficiently elastic to accompany the flexural movements of the tooth, something that a very rigid post cannot do:
a rigid post works against the natural function of the tooth, it creates zones of tension both in the dentin and at the contact points of the core composite (having an elasticity modulus very close to that of the dentine).
These tensions can cause cracks or fractures both in the tooth and the core composite. |
A comparative study carried out by the CRA in 1998 demonstrated the fact that only those teeth that are reconstituted by the Ribbond procedure, one might as well say without posts, presented no fractures at all.
However, this study did not take into account the latest generation of composite posts, which are not reinforced by carbon fibers, but rather by glass fibers.
A more recent study on esthetic posts published in the May 2004 CRA issue compares the strength of glass fiber posts and metal posts:
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First by measuring the force required to break posts only, giving relative strengths of posts alone, |
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Second by measuring the force required to break posts & core cemented within a tooth, giving indication of clinical performance. |
The summary of strength tests says: "Although differences in post strength were evident when posts were tested alone, strength of posts & core build-up systems were statistically similar.
These data suggest glass fiber posts could be used clinically wherever metal posts have been used." (CRA NEWSLETTER, May 2004 Esthetic posts).
SNOWPOST® & SNOWLIGHT® composite posts have a longitudinal elasticity modulus of 45 GPa and 49 GPa respectively, twice that of the dentin, compared with ten times for steel, seven times for carbon and six times for titanium.
SNOWPOST® & SNOWLIGHT® composite posts are strong enough to unify the core composite and the root canal, but they do not interfere with the natural behavior of the teeth, and the progressive and physiological dilution of the stress all along the dentin, like during the function of the healthy tooth. Furthermore, their shear resistance is equal to that of carbon fiber posts.
Comparison of steel, titanium, carbon epoxy and our esthetic posts |
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Steel |
Titanium |
Carbon |
Snowpost® |
Snowlight® |
Young modulus Ex |
200 GPa |
120 GPa |
141 GPa |
45 GPa |
49 GPa |
Shear resistance |
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-- |
23.9 MPa |
23.1 MPa |
22 MPa |
BIBLIOGRAPHY
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