
Int Poster J Dent Oral Med 9 (2007), Nr. 3 15. Sep. 2007
Int Poster J Dent Oral Med 2007, Vol 9 No 03, Poster 372
finite element analysis of thermal stresses in circumferential cast clasps of removable partial dentures
Language: English
Authors:
Lecturer Dr. Liliana Sandu, Professor Dr. Cristina Bortun, Assist. Prof. Sorin Porojan
"Victor Babes" University of Medicine and Pharmacy Timisoara
Professor Eng. Nicolae Faur, Politehnica University Timisoara
Date/Event/Venue:
September, 2123, 2006
Fourth International Conference on Materials and Manufacturing Technologies
ClujNapoca, Romania
Introduction
The oral cavity is subjected to thermal aggressions induced by hot/cold
liquids and food every day. In case of the patients wearing removable
partial dentures, these thermal changes produce alterations in this
prostheses, as a result of heat transfer. It is known that restorative
materials expand when warmed by hot liquids or food and contract when
exposed to cold substances. Exposing removable partial dentures to a range
of temperatures induces stress in all their components [1]. Several studies
evaluated the removable partial dentures by the finite element analysis, but
none of these evaluated the thermal stress [2, 3].
Objectives
The purpose of the study was to explore the influence of thermal oral
changes on the circumferential cast clasps, popular retainers of the
removable partial dentures. Only these critical components were selected
because high stresses in the dentures are the main causes of deformation and
fracture of the clasps [4, 5].
Material and Methods
The object of study was represented by real cast circumferential clasps:
Akers clasp and back action clasp. They were selected for a comparative
analysis. The Akers clasp is a rigid circumferential clasp, with a rigid
minor connector and the back action clasp is a clasp with high elasticity
and with a long and elastic minor connector. A 3dimensional finite element
method (Cosmos/M, version 2.5; Structural Research and Analysis, Santa
Monica, Calif) was used to explore the temperature distribution, thermal
stress and the influence of thermal changes on stress and displacements of
the clasps during functions. The geometric models of the clasps were
generated by manual measurement of the 3dimensional coordinates of the
clasp surfaces. The finite element models were developed by dividing the
model of Akers clasp into 1017 elements, connected at 607 nodes and of the
back action clasp into 1772 elements, connected at 1263 nodes. Shell 3
elements (triangular thin shell) were used. In making the finite element
models, the mechanical, elastic and thermal characteristics of the CoCr
alloy (WironiumLA; Bego, Bremen, Germany) used for the framework were
entered into the computer program [6, 7]: tensile strength Rm = 1000 MPa;
ductile Yield Rp 0.2 = 700 MPa; modulus of elasticity E = 2.2 x 10^{5}
MPa; Poisson's ratio = 0.3; Vickers hardness (HV 10) = 340; thermal
conductivity 15 J/smK; thermal expansion coefficient 14.3 x 10^{6}
K^{1}. The finite element analysis [8] was used in order to assess stress
distribution and displacements resulting the simulation of static and
thermal charges. At the end, both of them were cumulated to evaluate the
importance of considering thermal variations for stress analysis of the cast
clasps.


Fig. 1. Von Misses equivalent stress resulting the simulation of a static charge (average force of 20N) in the Akers clasp. 
Fig. 2. Von Misses equivalent stress resulting the simulation of a static charge (average force of 20N) in the back action clasp. 


Fig. 3. Temperature distribution for the Akers clasp in case of thermal variations from 37°C to 0°C. 
Fig. 4. Temperature distribution for the back action clasp in case of thermal variations from 37°C to 60°C. 


Fig. 5. Temperature distribution for the back action clasp in case of thermal variations from 37°C to 0°C. 
Fig. 6. Temperature distribution for the back action clasp in case of thermal variations from 37°C to 60°C. 
Results
Generated stresses and displacements were calculated numerically and
plotted graphically. Fig. 1 and 2 illustrate the Von Misses equivalent
stress resulting the simulation of a static charge (average force of 20 N),
which was evaluated for the two models. The thermal analysis was conducted
with external stimuli of 0°C and 60°C. Fig. 3 and 4 show the temperature
distribution in case of thermal variations from 37°C to 0°C and 60°C, for
both of the clasps. The analysis of stress induced by ingestion of cold
liquids revealed high stresses in the retentive clasp arms. For the Akers
clasp the highest value was 154.58 MPa and for the back action clasp more
lower, only 62.46 MPa. Similar was calculated the stress resulting from the
ingestion of hot liquids. High stresses were present in the occlusal rests
of the clasps and for the back action clasp also in the minor connector, at
the end where it connects with the major connector. At the end, both of the
static and thermal charges were cumulated to assess stress distribution and
displacements, simulating the ingestion of cold and hot food. This complex
situation was evaluated to estimate the importance of considering thermal
variations for stress analysis of the cast clasps. For the ingestion of
could food simulation the maximal stress values were not very different from
the cases where the charges were evaluated separate and were closer to them
resulting from the thermal stress analysis (Fig. 7). The result is not the
sum of the individual maximal values because of the stress distribution
alteration. In case of warm food ingestion simulation, the distribution of
the equivalent stress was similar to that resulted at the thermal analysis
for hot liquids. The maximal values were close to them and were even little
smaller (Fig. 8). In this case the magnitude of the displacements is
different for the analyzed clasps. The displacements are greater for the
more elastic clasp, the back action clasp. For both the maximal values are
located at the tip of the retentive arms. Reproducing the displacements is
suggestive for the direction of the clasp movements during functions.


Fig. 7. Equivalent stress distribution when static (20N) and thermal charges (0°C) were cumulated:
a) Akers clasp. b) back action clasp. 


Fig. 8. Equivalent stress and displacement distribution when static (20N) and thermal (60°C ) charges were cumulated: a) Akers clasp. b) back action clasp. 
Conclusions
 Thermal variations induce stress in dental clasps, high temperatures having more aggressive effect than lower one.
 The displacement analysis is suggestive for the direction of the clasp movements during functions.
 Considering thermal variations is very important for stress analysis of the cast clasps.
Literature
 D.S. Palmer, M.T. Barco, E.J. Billy: Temperature extremes produced orally by hot and cold liquids. J Prosthet Dent, 67 (1992), pp.325327.
 L. Sandu, N. Faur, C. Bortun: Investigation of Stress Distribution in Circumferential Cast Clasps using the Finite Element Analysis. Proceedings of the 6th International Confernce on Boundary and Finite Element, 1619 October, 2003 (editors: I. Dobre, N. Faur), Ed. Politehnica, Timişoara (2003), pp. 183188.
 Y. Sato, K. Tsuga, Y. Abe, Y. Akagawa: Finite element analysis of the effect of vertical curvature on halfoval cast clasps, J Oral Rehabil, 26 (1999) pp. 554558,.
 C. Bortun, M. Leretter, L. Sandu: Tehnologia protezelor partale mobilizabile, vol. I and II, Ed. Eurobit, Timisoara (2002).
 L. Sandu, C. Borţun, N. Faur, M. Negruţiu: Croşetele dentare turnare  alegere, proiectare şi analiză numerică, Ed. Eurobit, Timişoara (2004).
 L. Sandu, N. Faur, C. Borţun: Efectul variaţiilor termice asupra croşetelor dentare turnate analiză cu metoda elementelor finite. Revista Naţională de Stomatologie, volumul I, nr. 3 (2003), pp. 3135.
 L. Sandu, N. Faur, C. Borţun: Spannungsanalyse der Klammern von Modellgussprothesen durch die Methode der finiten Elemente, Quintessenz Zahntech 32:4 (2006), pp. 372381.
 N. Faur: Elemente finite fundamente. Ed. Politehnica, Timişoara (2002).
This Poster was submitted by Dr. Liliana Sandu.
Correspondence address:
Lecturer Dr. Liliana Sandu
"Victor Babes" University of Medicine and Pharmacy
9 Revolutiei 1989 Blv.
300041 Timisoara
Romania
