Int Poster J Dent Oral Med 2007, Vol 9 No 02, Poster 365
Effekt of Design Modifikations on Lingual Bars Rigidity
Lecturer Dr. Liliana Sandu, Professor Cristina Bortun, Assistant Professor Florin Topala,
"Victor Babes" University of Medicine and Pharmacy, University School of Dentistry, Timisoara, Romania
June 28th - July 1st
84th Conference of IADR (International Association of Dental Research)
It has been widely accepted that rigidity is one of the desirable characteristics of removable partial dentures major connectors. Failure of the major connectors to provide rigidity may damage the supporting oral structures like abutment teeth, residual ridges and underlying tissues.
The basic forms of mandibular major connectors are half-pear and half-oval. The dentist and the dental technician are responsible for the appropriate design and achievement of the major connectors depending on each clinical situation. The relative height of the floor of the mouth is very important.
The aim of the research was to investigate the effect of lingual bar major connector design on flexing and torque resistance by means of three-dimensional finite element analysis.
Material and Methods
Eighteen designs of lingual bars of different cross-sectional shapes and dimensions (4-5 mm height, 1.5-3 mm thickness, 0.3-0.75 thickness/height ratio) were developed using finite element analysis modeling. The geometrical models of these were generated using three-dimensional elements. A three-dimensional finite element analysis software (Cosmos/M, version 2.5; Structural Research and Analysis, Santa Monica, California) was used for the study of structural simulations. The finite element method was selected because it is known as an established theoretical technique for engineering problems.
Two groups of half-pear and half-oval shapes with different cross-sections (5.88-10.59 mm²) were constructed for comparison. The analysis required the creation of a computer simulated model. The basic procedure was to consider the complete structure as an assemblage of individual elements. Therefore, each model was divided or meshed into 4000 individual, finite elements. Eight-node three-dimensional elements were used. Adjacently elements were connected to 5324 nodes on their common boundaries.
In building the finite element model, the characteristics of the Co-Cr alloy (Wironium®LA; Bego, Bremen, Germany) used for the framework were entered into the computer programme. The characteristics included were: tensile strength (Rm) of: 940 MPa, ductile yield (Rp0.2) of: 640 MPa, elasticity modulus (E) of: 2.2x105 MPa, Vickers hardness (HV) of: 360, and Poisson's ratio (v) of: 0.3. Vertical and horizontal forces of 30 N were applied to one end of the bars, while the opposite side was fixed in all directions. Considering that the load values used were those that appear normally during mastication, it was sufficient to limit the study at this interval.
The rigidity of the experimental Co-Cr major connectors was evaluated by measuring relative displacements and von Mises stresses generated under simulated torsional and compressive loads for both groups. Generated Von Misses equivalent stresses and displacements were calculated numerically and plotted graphically. Results were displayed as coloured stress contour plots to identify regions of different stress concentrations.
For each load case, its own legend was attached, where stresses are reproduced in MPa and displacements in mm. Figures 1-4 display the Von Mises equivalent stress which was evaluated for all situations considered, under different loading values.
|Fig. 1. Results of the finite element analysis of a half-pear lingual bar (dimensions 5 mm/2 mm): a. stresses under compressive load, b. displacements under compressive load, c. stresses under torsional load, d. displacements under torsional load.
|Fig. 2. Results of the finite element analysis of a half-pear lingual bar (dimensions 4 mm/2.5 mm): a. stresses under compressive load, b. displacements under compressive load, c. stresses under torsional load, d. displacements under torsional load.
|Fig. 3. Results of the finite element analysis of a half-oval lingual bar (dimensions 5 mm/2 mm): a. stresses under compressive load, b. displacements under compressive load, c. stresses under torsional load, d. displacements under torsional load.
|Fig. 4. Results of the finite element analysis of a half-oval lingual bar (dimensions 4 mm/ 2.5mm): a. stresses under compressive load, b. displacements under compressive load, c. stresses under torsional load, d. Displacements under torsional load.
Stresses and displacements under compression loading simulating vertical forces were lower than those obtained for torsional loading simulating horizontal forces (61-63% for stresses and 41-74% for displacements). The displacements for compressive loading became closer to them for torsional loads with the decrease of the thickness/height ratio. Resulted displacements and stresses were smaller for bars with an increased thickness/height ratio. Values measured for half-oval designs were not significantly higher than those for the half-pear shapes (table 1).
|Table 1. Stresses and displacements under compression and torsional loading for all load cases.|
||Section area (mm²)
||Stress under compressive load (MPa)
||Displacement under compressive load (mm)
||Stress under torsional load (MPa)
||Displacement under torsional load (mm)
The results of this in vitro study suggest that the thickness of the lingual bar major connector should be increased to improve the rigidity of the framework to torsional and compressive loads.
Cross-section shapes of the lingual bars have a lesser effect on rigidity from biomechanical point of view.
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This Poster was submitted by Lecturer Dr. Liliana Sandu.
Lecturer Dr. Liliana Sandu
University School of Dentistry
Specialization Dental Technology
9 Revolutiei 1989