Journal of Indian Society of Pedodontics and Preventive Dentistry
Journal of Indian Society of Pedodontics and Preventive Dentistry
                                                   Official journal of the Indian Society of Pedodontics and Preventive Dentistry                           
Year : 2015  |  Volume : 33  |  Issue : 3  |  Page : 183--191

Finite Element Stress Analysis of Stainless Steel Crowns


Attiguppe R Prabhakar, Chandrashekar M Yavagal, Amrita Chakraborty, S Sugandhan 
 Department of Pediatric Dentistry, Bapuji Dental College and Hospital, Davangere, Karnataka, India

Correspondence Address:
Dr. Attiguppe R Prabhakar
Department of Pediatric Dentistry, Bapuji Dental College and Hospital, Davangere - 577 004, Karnataka
India

Abstract

Background: Though stainless steel crowns (SSCs) have often been stated as the best restorative modality, there are limited studies demonstrating its efficacy in restoring the functional integrity of the primary dentition. Hence has arisen, the necessity to establish the supremacy of SSCs. Aim: Evaluation of the efficacy of SSC to with stand compressive (0°), shearing (90°), and torsional (45°) stress when used as a restorative material. Settings and Design: The study design employed four finite element models, each with differing amounts of tooth structure, which were exported to ANSYS software and subjected to an average simulated bite force of 245N. Materials and Methods: Four maxillary deciduous primary molars restored with SSCs (3M ESPE) were subjected to spiral computed tomography (CT) in order to obtain three-dimensional (3D) images, which were then converted into finite element models. They were each subjected to forces along the long axis of the tooth and at 45°and 90°. Results: The maximal equivalent von Mises stress was demonstrated in the SSCs of all the models with only a minimal amount observed in the underlying dentine. In all situations, the maximal equivalent von Mises stress was well below the ultimate tensile strength values of stainless steel and dentine. Conclusion: Even at maximal physiologic masticatory force levels, a grossly destructed tooth restored with SSC is able to resist deformation.



How to cite this article:
Prabhakar AR, Yavagal CM, Chakraborty A, Sugandhan S. Finite Element Stress Analysis of Stainless Steel Crowns.J Indian Soc Pedod Prev Dent 2015;33:183-191


How to cite this URL:
Prabhakar AR, Yavagal CM, Chakraborty A, Sugandhan S. Finite Element Stress Analysis of Stainless Steel Crowns. J Indian Soc Pedod Prev Dent [serial online] 2015 [cited 2021 Dec 8 ];33:183-191
Available from: https://www.jisppd.com/text.asp?2015/33/3/183/160352


Full Text

 Introduction



Clinical trials, retrospective studies, prospective studies, reviews, and meta-analysis conducted over time [1] have established the efficacy of stainless steel crowns (SSCs) as a semipermanent restorative therapy for primary teeth affected with rampant caries, hypoplasia, following pulp therapy and for those being used as anchorage for interceptive orthodontic appliances. [2] However, in spite of 52 outcome-related reports that have been published, there are no evidence-based, well-designed randomized controlled trials to establish the durability of the SSC. [1] Moreover, the existing studies [3] on the reaction of SSCs to stress are all 'in vitro' and an extrapolation of their results to an in vivo setting is obviously impractical. When a structure is subjected to a load, stress is induced in the structure, which may lead to deformation of the latter. Through finite element analysis, evidence can be gathered on the stress concentration areas along with the study of a single variable in a complex structure. The advent of finite element analysis has made it possible to demonstrate the propagation of stress through each part of a tooth and its restoration. Since then, numerous finite element studies have been conducted to demonstrate the distribution of stress through normal and restored teeth. [4] An analysis of teeth restored with SSC using the finite element model would provide perceptive data on the physical response of the crown-tooth system to masticatory stresses.

Hence, the objective of this study was to demonstrate the efficacy of SSCs to withstand masticatory forces through the reliable model of finite element analysis. Hence, this study has been undertaken to demonstrate the mechanical behavior of the SSC under masticatory stress when used to restore differing amounts of tooth structure through a finite element analysis.

 Materials and Methods



This study utilizes four models of primary maxillary second molar each with differing amounts of tooth structure.

For the model preparation, 169L crown preparation bur was used by a single operator to eliminate interoperator bias. The crowns were prepared according to the guidelines of 3M ESPE.

A gradual, sequential circumferential tooth reduction was performed from model 1 to 4 such that [Table 1]; {Table 1}

Model 1: Only superficial reduction as required to place the crown.

Model 2: A little more than half the enamel is reduced (56%) leaving behind intact dentine.

Model 3: All the enamel is reduced along with a little less than half (37%) the dentine.

Model 4: All the enamel with most of the dentine is reduced. Only 30% of the dentine is left behind.

After crown preparation, SSCs (3M ESPE) were luted using glass ionomer cement (GC Fuji Type I).

For the generation of the finite element models, the four models of primary second molar were subjected first to spiral computed tomography (CT) in order to obtain a three-dimensional (3D) image of the prepared models. Horizontal sections of 0.5mm were taken of each model and fed to ANSYS (ANSYS v.12; ANSYS Inc, Canonsburg, PA, USA) software. The scans were converted into cloud data points, subsequent to which they were connected, thus forming the surface model of each prepared tooth.

The SSC was considered to be of uniform width of 0.13 mm [5] and the cement lining progressively increased from model 1 to 4 with model 1 having a cement thickness of 200 µm.

A previous study has shown that the biting forces in primary dentition fall in the range of 161-330N; [6] and hence, in the present study an average force of 245N was applied at four different angulations to each model in order to simulate the various physiologic masticatory conditions. In one scenario, the force was in the axial direction simulating maximum bite forces. [7] The models were loaded on six points on the occlusal surface; two points on the inner inclines of the buccal cusps, two points on the inner inclines of the palatal cusp, and two points on the outer inclines of the palatal cusps. In the second scenario, angulated forces were applied on the palatal inclines of the buccal cusps to simulate lateral mastication. Forces at 0°, 45°, and 90° to the long axis of the tooth were applied.

The Finite Element Analysis is done considering the amount of tooth structure present and the mechanical properties of the material through, which stress is being propagated [Table 2].{Table 2}

The stress values and patterns due to load application were calculated based on the von Mises dimensional criterion, which always yields positive results. The equation used is:

σe = ½ ((σ1 - σ2 ) 2 + (σ2 - σ3) 2 + (σ3 - σ1) 2) 1/2

Where σ1 , σ2 , and σ3 represent the principal stresses within the material. [8]

 Results



The images obtained from the Finite Element Analysis are all color graded such that dark blue represents areas experiencing minimal von Mises stress and red represents areas experiencing maximal von Mises stress.

Fracture occurs when the von Mises stress generated within either the SSC or luting cement or dentine are above their respective ultimate tensile strengths [Table 3].{Table 3}

As the data is studied from model 1 to 4, for all types of forces, the von Mises stresses are seen to increase. Whether the forces are along the long axis of the tooth [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12] at an angle [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22], [Figure 23], [Figure 24], [Figure 25], [Figure 26], [Figure 27], [Figure 28], [Figure 29], [Figure 30], [Figure 31], [Figure 32], [Figure 33], [Figure 34], [Figure 35], [Figure 36], they increase with decreasing amount of tooth structure.{Figure 1}{Figure 2}{Figure 3}{Figure 4}{Figure 5}{Figure 6}{Figure 7}{Figure 8}{Figure 9}{Figure 10}{Figure 11}{Figure 12}{Figure 13}{Figure 14}{Figure 15}{Figure 16}{Figure 17}{Figure 18}{Figure 19}{Figure 20}{Figure 21}{Figure 22}{Figure 23}{Figure 24}{Figure 25}{Figure 26}{Figure 27}{Figure 28}{Figure 29}{Figure 30}{Figure 31}{Figure 32}{Figure 33}{Figure 34}{Figure 35}{Figure 36}

The maximal von Mises stresses are [Table 3] and [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 21], [Figure 22], [Figure 23], [Figure 24] and [Figure 29], [Figure 30], [Figure 31], [Figure 32] generated within the SSC at axial as well as lateral loading. However, the values obtained are well below the ultimate tensile strength of SSC.

Lateral forces applied at 90° [Table 3] and [Figure 29], [Figure 30], [Figure 31], [Figure 32], [Figure 33], [Figure 34], [Figure 35], [Figure 36] generate the maximal von Mises stresses within both the SSC as well as dentine. However, even in this scenario, the stress values are still well below the ultimate tensile strength of both crown and dentine.

Von Mises stresses are also observed within the luting cement, glass ionomer cement (GC Fuji Type I). However, the stresses are well above the ultimate tensile strength of glass ionomer cement (GC Fuji Type I) at 24 h though the ultimate tensile strength gradually increases to above the Von Mises stresses after 30 days [Table 3] and [Figure 9], [Figure 10], [Figure 11], [Figure 12].

In all the finite element models, the pulp is seen to react the most to stress as is demonstrated by the red color of the pulp chamber in the finite element images [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 25], [Figure 26], [Figure 27], [Figure 28] and [Figure 33], [Figure 34], [Figure 35], [Figure 36].

 Discussion



Numerous studies have already been conducted on SSCs and the author feels this study not only adds but also substantiates the claims of the previous studies. Hutcheson et al., [9] compared multi surface composite restoration with SSC restorations in a randomized controlled trial involving 40 molars observed for 1 year and concluded the teeth restored with composite were not as durable nor considered an esthetic alternative to the SSC. In a literature review conducted by Attari and Roberts, [10] it was concluded that preformed metal crowns were indicated for the restoration of badly broken down primary molars and their success rate was superior to all other restorative materials. However, there was an obvious lack of prospective, well-controlled studies and more research is needed. Though finite element analysis to demonstrate the physical behavior of normal and restored teeth [11] as well as of all ceramic crowns on posterior teeth [12] have been conducted, there is no finite element analysis performed on SSCs to demonstrate their behavior under masticatory forces. This study effectively demonstrates how the SSC when used to restore even a grossly destructed primary tooth would prevent its fracture during mastication by absorbing most of the forces and allowing minimal forces to reach the dentine, thus ensuring the resultant dentinal stresses are well below its ultimate tensile strength [Table 3] and [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 21], [Figure 22], [Figure 23], [Figure 24], and [Figure 29], [Figure 30], [Figure 31], [Figure 32].

Conventional glass ionomer cement has long been considered an appropriate cementing agent for SSCs as concluded by Garcia-Godoy and Landry, [13] Khinda and Grewal, [14] and Yilmaz et al., [15] in their various studies. However, even glass ionomer cement is susceptible to fracture upto 24 h after mixing and cementing. Hence, it becomes imperative to warn the caretaker as well as the child to refrain from biting on the crown for at least 1 day. The main aim of any dental treatment is the prevention of degeneration and necrosis of the dental pulp; and this study clearly demonstrates that even when restored with a SSC, the pulp is the most susceptible to masticatory forces. Thus, adequate protection of the pulp through sufficient thickness of luting cement and dentinal structure is an essentiality.

One of the drawbacks of this study is that the base of all the models was completely fixed at the level of the cervical constriction of the crown. This was an assumption made for the sake of simplicity. A more realistic modeling would require the inclusion of root length, surrounding bone, and periodontal structure. To ensure consistency, the materials are assumed to be homogenous, isotropic, and linearly elastic; which in reality they are not. Taking these factors into consideration, more biologically designed experiments need to be conducted. This would ensure an improvement upon the evidence laid down by the present study in establishing the efficacy of SSC to withstand stress.

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