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ORIGINAL ARTICLE
Year : 2015  |  Volume : 33  |  Issue : 3  |  Page : 183-191
 

Finite Element Stress Analysis of Stainless Steel Crowns


Department of Pediatric Dentistry, Bapuji Dental College and Hospital, Davangere, Karnataka, India

Date of Web Publication9-Jul-2015

Correspondence Address:
Dr. Attiguppe R Prabhakar
Department of Pediatric Dentistry, Bapuji Dental College and Hospital, Davangere - 577 004, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-4388.160352

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   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.


Keywords: Deciduous molar, finite element analysis, fracture resistance, stainless steel crowns


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;33:183-91

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 2019 Nov 13];33:183-91. Available from: http://www.jisppd.com/text.asp?2015/33/3/183/160352



   Introduction Top


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 Top


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: Number of elements of nodes of each finite element mesh model along with the extent of tooth reduction


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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: Mechanical properties of the materials and teeth used for the study


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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 Top


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: Tabulation of the magnitude of stress in each case and the ultimate tensile strength of the corresponding materials


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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: Stress pattern on axial loading of stainless steel crown in model 1


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Figure 2: Stress pattern on axial loading of stainless steel crown in model 2


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Figure 3: Stress pattern on axial loading of stainless steel crown in model 3


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Figure 4: Stress pattern on axial loading of stainless steel crown in model 4


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Figure 5: Stress pattern on axial loading of dentine in model 1


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Figure 6: Stress pattern on axial loading of dentine in model 2


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Figure 7: Stress pattern on axial loading of dentine in model 3


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Figure 8: Stress pattern on axial loading of dentine in model 4


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Figure 9: Stress pattern on axial loading of luting cement in model 1


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Figure 10: Stress pattern on axial loading of luting cement in model 2


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Figure 11: Stress pattern on axial loading of luting cement in model 3


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Figure 12: Stress pattern on axial loading of luting cement in model 4


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Figure 13: Stress pattern on lateral loading (0°) of stainless steel crown in model 1


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Figure 14: Stress pattern on lateral loading (0°) of stainless steel crown in model 2


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Figure 15: Stress pattern on lateral loading (0°) of stainless steel crown in model 3


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Figure 16: Stress pattern on lateral loading (0°) of stainless steel crown in model 4


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Figure 17: Stress pattern on lateral loading (0°) of dentine in model 1


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Figure 18: Stress pattern on lateral loading (0°) of dentine in model 2


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Figure 19: Stress pattern on lateral loading (0°) of dentine in model 3


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Figure 20: Stress pattern on lateral loading (0°) of dentine in model 4


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Figure 21: Stress pattern on lateral loading (45°) of stainless steel crown in model 1


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Figure 22: Stress pattern on lateral loading (45°) of stainless steel crown in model 2


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Figure 23: Stress pattern on lateral loading (45°) of SSC in model 3


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Figure 24: Stress pattern on lateral loading (45°) of SSC in model 4


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Figure 25: Stress pattern on lateral loading (45°) of dentine in model 1


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Figure 26: Stress pattern on lateral loading (45°) of dentine in model 2


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Figure 27: Stress pattern on lateral loading (45°) of dentine in model 3


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Figure 28: Stress pattern on lateral loading (45°) of dentine in model 4


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Figure 29: Stress pattern on lateral loading (90°) of SSC in model 1


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Figure 30: Stress pattern on lateral loading (90°) of SSC in model 2


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Figure 31: Stress pattern on lateral loading (90°) of SSC in model 3


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Figure 32: Stress pattern on lateral loading (90°) of SSC in model 4


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Figure 33: Stress pattern on lateral loading (90°) of dentine in model 1


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Figure 34: Stress pattern on lateral loading (90°) of dentine in model 2


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Figure 35: Stress pattern on lateral loading (90°) of dentine in model 3


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Figure 36: Stress pattern on lateral loading (90°) of dentine in model 4


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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 Top


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.

 
   References Top

1.
Rock WP. British Society of Paediatric Dentistry. UK National Clinical Guidelines in Paediatric Dentistry. Extraction of primary teeth - balance and compensation. Int J Pediatr Dent 2002;12:151-3.  Back to cited text no. 1
    
2.
Clinical Affairs Committee-Restorative Dentistry subcommittee, Council on Clinical Affairs. Guideline on Pediatric Restorative Dentistry 2012. Reference Manual 34:12-3.  Back to cited text no. 2
    
3.
Beattie S, Taskonak B, Jones J, Chin J, Sanders B, Tomlin A, et al. Fracture resistance of 3 types of primary esthetic stainless steel crowns. J Can Dent Assoc 2011;77:b90.  Back to cited text no. 3
    
4.
Humphrey WP. Chrome alloy in children′s dentistry. St. Louis Dent Scoc 1950;21:15-6.  Back to cited text no. 4
    
5.
3M ESPE. A comprehensive guide to achieving the best results with 3M ESPE Prefabricated crowns. Weblog. [Online] Available from: http://multimedia.3m.com/mws/mediawebserver?mwsId=66666UF6EVsSyXTtn8TyLxF6EVtQEVs6EVs6EVs6E666666 - [Last accessed on 2013 Sept 10].  Back to cited text no. 5
    
6.
Rentes AM, Gaviao MB, Amaral JR. Bite force determination in children with primary dentition. J Oral Rehabil 2002;29:1174-80.  Back to cited text no. 6
    
7.
Bakke M, Holm B, Jensen BL, Micher L, Moller E. Unilateral, isometric bite force in 8-68-year-old women and men related to occlusal factors. Scand J Dent Res 1990;98:149-58.  Back to cited text no. 7
    
8.
Gubruz T, Sengul F, Altun C. Finite element stress analysis of short-post core and over restorations prepared with different restorative materials. Dent Mater J 2008;27:499-507.  Back to cited text no. 8
    
9.
Hutcheson C, Seale NS, McWhorter A, Kerins C, Wright J. Multi-surface composite vs stainless steel crown restorations after mineral trioxide aggregate pulpotomy: A randomized controlled trial. Pediatr Dent 2012;34:460-7.  Back to cited text no. 9
    
10.
Attari N, Roberts JF. Restoration of primary teeth with crowns: A systematic review of the literature. Eur Arch Paediatr Dent 2006;7:58-62.  Back to cited text no. 10
    
11.
Yettram AL, Wright KW, Pickard HM. Finite element stress analysis of the crowns of normal and restored teeth. J Dent Res 1976;55:1004-11.  Back to cited text no. 11
[PUBMED]    
12.
Proos KA, Swain MV, Ironside J, Steven GP. Finite element analysis studies of a metal-ceramic crown on a first premolar tooth. Int J Prosthodont 2002;15:521-7.  Back to cited text no. 12
    
13.
Garcia-Godoy F, Landry JK. Evaluation of stainless steel crowns luted with a glass ionomer cement. J Pedod 1989 Summer;13:328-30.  Back to cited text no. 13
    
14.
Khinda VI, Grewal N. Retentive [correction of Preventive] efficacy of glass ionomer, zinc phosphate and zinc polycarboxylate luting cements in preformed stainless steel crowns: A comparative clinical study. J Indian Soc Pedod Prev Dent 2002;20:41-6.  Back to cited text no. 14
[PUBMED]    
15.
Yilmaz Y, Simsek S, Dalmis A, Gurbuz T, Kocogullari ME. Evaluation of stainless steel crowns cemented with glass-ionomer and resin-modified glass-ionomer luting cements. Am J Dent 2006;19:106-10.  Back to cited text no. 15
    


    Figures

  [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]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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