Evaluation of modern post and core systems by testing the mechanical fatigue resistance.
Vitalariu, Anca ; Comaneci, Radu
1. INTRODUCTION
The post and core systems (PCS) represent an important issue in
dental practice because they are the only chance to save the teeth with
large coronal destructions and to avoid their extraction. Treatment
failures caused by mechanical deficiencies of post and cores represent a
problem of clinical significance and justify the researches orientated
through the increasing of their performances and clinical longevity.
In the latest years there have been developed new techniques and
materials for PCS: ceramic posts (cast or prefabricated), and composite
posts reinforced with different kind of fibers: carbon, glass, quartz,
silica, in order to replace the metallic posts wich have a lot of
disadvances: poor esthetics and biocompatibility (the migration of
metallic ions from the post's alloy in the oral tissues is well
known, high Young modulus that increase the risk of catastrophic
(irreversible) root fractures, followed by the tooth's extraction
(Rosentrit et al., 2000).
Ceramic posts offer mechanical properties very close to those of
dental alloys, an excellent esthetics and biocompatibility, because
eliminate the risk of corrosion, bimetallism and allergic reactions.
Ceramic posts are made of zirconium dioxide partially stabilized by the
addition of yttrium ([Y.sub.2][O.sub.3]) (Hochman & Zalkind, 1999).
The usage of fiber reinforced post is based on harmony between Young
modulus of the dentinal root, the post, luting cement and composite core
material, and on the chemical connection between them. This result in a
system that behaves like a mono-block under the[degrees]Cclusal forces
(Bateman et al.,2003). Even the fiber reinforced posts have many
advantages: proven biocompatibility, non corrosive, an equilibrate transmission of[degrees]Cclusal forces, no interferences with modern
techniques of investigation (RMN, scanner), easily removable to permit
endodontic re-treatment, their clinical reliability must be demonstrate
by passing time. Even this "in vivo" tests are the most
reliable, they have the disadvantage that are time consuming because are
requesting long clinical observation periods. To overcome the
difficulties of clinical studies, numerous "in vitro" (in
laboratory) methods were developed to address specific properties of
post and core restorations. Most of them, have been focused on the
determination of the mechanical resistance to a simple stress, being
static tests (Maccari et al.,2003). But the experience proved that the
fatigue of the restorative materials is a primordial factor in clinical
failures. The fatigue resistance tests represent an essential research
instrument because they simulate the repetitive cyclic pattern of
the[degrees]Cclusal forces and replace the clinical tests that are time
consuming and thus contribute to decrease of clinical evaluation costs
and time (Heydecke et al., 2002).
2. MATERIAL AND METHOD
2.1 Preparing of the samples for the test
For this experiment we used forty human maxillary incisors having
similar dimensions and without cracks, decay, or abrasion. The teeth
were endondontically treated and mounted in acrylic resin blocks with a
simulated periodontal ligament. The samples were divided into three
experimental groups and one control group:
* group C--teeth restored with prefabricated carbon fiber
reinforced posts
* group Z--teeth restored with prefabricated zirconia posts
* group T--teeth restored with prefabricated titanium posts
* group M (control)--endodontically treated teeth but with no posts
The teeth from control group were covered with cast metallic
crowns. In experimental groups (C, Z and T) the anatomic crowns of the
teeth were sectioned perpendicular to the long axis, and the root canals
were prepared for the posts. Gutta-percha was removed from the roots
leaving 3 mm of root canal filling in the apical portion to ensure the
apical seal of the endodontic space.
The posts were cemented with a dual-cure resin cement (Panavia F)
and the core build-up was performed with a light curing composite resin.
For the mechanical test the teeth were mounted in stainless steel
cylinders filled with self-polymerizing resin. In order to simulate the
periodontal ligament, a thin layer of silicone (0,2 mm) was applied
along to the root surfaces. All teeth were covered with cast metallic
crowns. On the palatal surface of each metal crown, right above the
cingulum, a 0.3mm deep and 1mm-wide notch was made, in order to place
there the compressive load during the test. The load was applied using a
chisel-shaped steel pin, corresponding to the shape and dimension of the
incisal edge of a lower incisor.
2.2 Experimental fatigue tests of post and core systems
Each test specimen was intermittently loaded to a maximum level of
25 daN at an angulation of 45 degrees to the long axis of the tooth
(Figure 1), at a frequency of 1.3 Hz until the failure.
The testing device was stopped after 400 000 cyclic loads if no
failure had[degrees]Ccurred. Root fractures, post fractures and post and
crown decementation were considered causes of failure. The survival
rates of the groups were statistically compared with a Kaplan-Meier
analysis.
[FIGURE 1 OMITTED]
3. DEVICES AND EQUIPMENT
Measurement system assisted by computer consists of load cell with
power supply, signal conditioner, data acquisition (DAQ) device and
virtual instrument software, (Figure 2).
--Load cell type Gefran CulKM--Italy with nominal load of 10kN and
350Qm strain gauges in full bridge configuration with 0.2% accuracy is
connected to a stabilized 10VDC power supply;
--Signal Conditioner type SC 2043 SG National Instruments USA
provides both the nominal excitation voltage for load cell and the
router of the conditioned strain gauge bridge signal to DAQ card;
--DAQ Card type PCI 6023E National Instruments--USA take the load
cell signal (with a sampling rate of 200 kS/s) via signal conditioner
and displays both the graphic and numerical evolution in real time;
--Virtual instrument VI is the LabView application which provides
the communication between DAQ device and the computer.
[FIGURE 2 OMITTED]
4. RESULTS
The results obtained after 400.000 impacts are shown in Table 1 and
Figure 3. Only one fracture was observed in the control group (teeth
with no post) and three in carbon fiber group while in zirconia and
titanium groups were recorded six fractures in each group. The survival
rates recorded in the experimental groups (C, Z and T) were lower than
that of the control group. The differences between the experimental
groups were significant, the teeth reconstructed with posts reinforced
with carbon fibers had a higher survival rates than the teeth
reconstructed with titanium and zirconia posts.
5. CONCLUSION
Teeth reconstructed with fiber reinforced posts exhibit a higher
fatigue resistance than teeth reconstructed with rigid posts (titanium
or zirconia posts). Fiber posts reduce the risk of root fracture and are
the best choice in reconstruction of endodontically treated teeth.
Knowing the advantages and disadvantages of different type of post, the
dentist will be able to avoid a post that predispose to irreparable
fractures.
6. REFERENCES
Bateman, G.; Ricketts, D.N.J.& Saunders, W.P. (2003).
Fibre-based post systems: a review of the literature. Br Dent J, No.195,
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Heydecke, G.; Butz, F.; Hussein, A.& Strub, J.R. (2002).
Fracture strength after dynamic loading of endodontically treated teeth
restored with different post-and-core systems. J Prosthet Dent , No.87,
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Hochman, N.& Zalkind, M.(1999). New all-ceramic indirect
post-and-core system. J Prosthet Dent, Vol.81, 1999, pp. 625-629
Maccari, P.C.; Conceicao, E.N.& Nunes, M.F. (2003). Fracture
resistance of endodontically treated teeth restored with three different
prefabricated esthetic posts. J Esthet Restor Dent, Vol.15, No.1,
(2003), pp.25-30
Rosentritt, M.; Furer, C.; Behr, M.; Lang, R.& Handel, G.
(2000). Comparison of in vitro fracture strength of metallic and
tooth-coloured posts and cores. J Oral Rehabil, Vol.27, No.7, (2000),
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