Medical Engineering & Physics 36 (2014) 962–967

Contents lists available at ScienceDirect

Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy

Technical note

Stress distributions in maxillary central incisors restored with various types of post materials and designs A.A. Madfa a , M.R. Abdul Kadir b , J. Kashani b , S. Saidin b , E. Sulaiman c,d , J. Marhazlinda e , R. Rahbari f , B.J.J. Abdullah g , H. Abdullah c , N.H. Abu Kasim c,d,∗ a

Department of Conservative Dentistry, Faculty of Dentistry, University of Thamar, Dhamar, Yemen Medical Implant Technology Group, Faculty of Biosciences & Medical Engineering, Universiti Teknologi Malaysia, Malaysia c Department of Restorative Dentistry, Faculty of Dentistry, University of Malaya, Kuala Lumpur, Malaysia d Biomaterials Technology Research Group, Faculty of Dentistry, University of Malaya, Kuala Lumpur, Malaysia e Dental Research & Training Unit, Faculty of Dentistry, University of Malaya, Kuala Lumpur, Malaysia f Faculty of Applied Science and Engineering, University of Toronto, Canada g Department of Biomedical Imaging, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia b

a r t i c l e

i n f o

Article history: Received 10 May 2013 Received in revised form 14 January 2014 Accepted 30 March 2014 Keywords: Finite element analysis Endodontic post materials Endodontic post design Stress analysis

a b s t r a c t Different dental post designs and materials affect the stability of restoration of a tooth. This study aimed to analyse and compare the stability of two shapes of dental posts (parallel-sided and tapered) made of five different materials (titanium, zirconia, carbon fibre and glass fibre) by investigating their stress transfer through the finite element (FE) method. Ten three-dimensional (3D) FE models of a maxillary central incisor restored with two different designs and five different materials were constructed. An oblique loading of 100 N was applied to each 3D model. Analyses along the centre of the post, the crowncement/core and the post-cement/dentine interfaces were computed, and the means were calculated. One-way ANOVAs followed by post hoc tests were used to evaluate the effectiveness of the post materials and designs (p = 0.05). For post designs, the tapered posts introduced significantly higher stress compared with the parallel-sided post (p < 0.05), especially along the centre of the post. Of the materials, the highest level of stress was found for stainless steel, followed by zirconia, titanium, glass fibre and carbon fibre posts (p < 0.05). The carbon and glass fibre posts reduced the stress distribution at the middle and apical part of the posts compared with the stainless steel, zirconia and titanium posts. The opposite results were observed at the crown-cement/core interface. © 2014 IPEM. Published by Elsevier Ltd. All rights reserved.

1. Introduction Restoration of maxillary anterior teeth presents a great challenge in the everyday practice of dental clinicians. Despite many developments in materials and techniques, patients’ demand for improved aesthetics, function and longevity of such restoration drives researchers and practitioners to make further developments. This challenge is even greater in cases where there is massive tooth damage, due to caries or trauma, because a damaged tooth possesses less resistance to fracture due to a reduction in the number of cross-linked collagen fibres and a loss of moisture within the tooth [1]. In such cases, there is often a need to compensate for the

∗ Corresponding author at: Department of Restorative Dentistry, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +6 03 79674806; fax: +6 03 79674533. E-mail address: [email protected] (N.H. Abu Kasim). http://dx.doi.org/10.1016/j.medengphy.2014.03.018 1350-4533/© 2014 IPEM. Published by Elsevier Ltd. All rights reserved.

lack of tooth substance by additional restoration, which is achieved by placing a post and core in a root canal of the tooth [2]. A dental post is useful for building up and thereby retaining coronal restoration, but the post does not reinforce the root of the tooth [3]. Moreover, some authors assert that posts may interfere with the mechanical resistance of a root-treated tooth, leading to an increased risk of damage to the remaining tooth structure [4,5]. To date, there is no consensus about the ideal material or technique for restoring root-treated teeth [6–8]. Traditionally, custom-made posts and cores were the system of choice; today, however, prefabricated metal and non-metal posts, combined with resin cores, are considered a viable alternative. The prefabricated post and core system is one of the most popular systems because it requires less chair time [9]. Nevertheless, the most common problems experienced with the prefabricated post and core technique are fracture of the root and loosening of the post and core. Long-term follow-up studies on the post and core technique have shown a range of ‘survival’ rates, confirming that root fractures are a common occurrence in the clinical practice [10–12].

A.A. Madfa et al. / Medical Engineering & Physics 36 (2014) 962–967

Several methods have been used to analyse the effects of the post and core system on stress distribution in dentine and its surrounding structure. Two commonly used methods are the experimental method and the finite element method (FEM). The experimental method includes a tensile test, a shear loading test, and photoelastic analysis [13–15]. Photoelasticity is a two-dimensional (2D) method for analysing the ability of transparent materials to exhibit colourful patterns when viewed under polarised light [13]. It should be noted that three-dimensional (3D) photoelastic stress analysis is seldom used or reported because such experiments are very expensive and difficult to conduct. In contrast, FEM has been presented as the most accurate way for stress analysis of teeth for more than two decades [16–19]. Furthermore, the 3D FE method shows internal stress, allowing predictions to be made about potential failure. Because post designs [20] and post materials [21] play important roles in stress distribution in the dentine and surrounding structures, this study aimed to compare the stress distributions in maxillary central incisor with the 3D FE method. The maxillary central incisors were restored using either smooth parallel-sided posts or tapered posts, each made from any one of the following materials: stainless steel, titanium, zirconia, glass fibre and carbon fibre. 2. Materials and methods 2.1. Model geometry A 3D model of an adult maxillary central incisor was developed using a Computed Tomography (CT) datasets (Medical Ethics No.: DF OS0901/0006(1), Faculty of Dentistry, University of Malaya) consisting of 98 slices with a thickness of 1 mm for each slice. The 3D models were constructed along with their surrounding cortical and cancellous bones using Mimics software (Materialise NV, Belgium) and Hounsfield’s Unit. A periodontal ligament (PDL) was modelled based on the tooth root, with a thickness of 0.25 mm [22], and it was subtracted from the volume of the cortical and cancellous bone. The restoration of the tooth root, including the dental posts (parallel sided and tapered posts), composite cores and cement layers, made of base metals and porcelain, were then modelled based on the geometry of the tooth root using ‘Solid Works’ software (Dassault Systèmes, USA). The size of the parallel-sided post was designed at 1.5 mm, while the tapered post was 1.5 mm at the top and 0.9 mm at the narrowest part. The diameters of the parallel-sided post and the top of the tapered post were standardised at the same dimension to avoid differences in the surface area of the posts. Fig. 1 shows a schematic illustration of the geometric model and the loading conditions of the tetrahedral mesh structure. 2.2. Finite element analysis For stress analysis, this study used four-node first-order linear tetrahedral solid elements (C3D4). These C3D4s used fine mesh to obtain accurate results because the constant stress on the tetrahedral elements exhibited low convergence. The total numbers of elements and nodes for the tetrahedral-mesh model were 634,279 elements and 874,696 nodes. The nodes along the bottom line of the model, referred to as ‘alveolar bone’, were fixed in all degrees of freedom [23]. All components were assumed to be perfectly bonded without any gaps between the components. The contact between each component was set to ‘glue contact’. An oblique load of 100 N, angled at 45◦ , was chosen to simulate the masticatory force, as shown in Fig. 1 [24]. The elastic properties of the materials used in the geometric model are presented in Tables 1 and 2. Any stress that was likely to occur during the endodontic treatment was ignored. The generation of the FE model and the calculation of stress

963

Table 1 Mechanical properties of isotropic materials. Component

Elastic modulus (MPa)

Poisson’s ratio

References

Cortical bone Cancellous bone Dentine PDL Porcelain Gutta-percha Titanium post Composite resin Zinc-oxide phosphate Adhesive cement resin Stainless steel post Metal coping Zirconia post

13,700 1370 18,600 0.069 69,000 140 116,000 12,000 22,000 18,600 210,000 96,600 200,000

0.30 0.30 0.32 0.45 0.28 0.45 0.33 0.33 0.35 0.28 0.30 0.35 0.32

[30] [30] [30] [42] [43] [42] [25] [34] [34] [34] [34] [44] [45]

Table 2 Mechanical properties of orthotropic materials [34]. Properties

Carbon fibre post

Glass fibre post

Ex (MPa) Ey (MPa) Ez (MPa) Vxy Vxz Vyz Gxy Gxz Gyz

11,800 7200 7200 0.27 0.34 0.27 2700 2800 2700

37,000 9500 9500 0.27 0.34 0.27 3100 3500 3100

distributions were performed using Cosmos Works 2009 (Dassault Systèmes, USA). 2.3. Statistical analysis Data for stress distribution along the centre of the post (63 nodes), the crown-cement/interface (2.869 nodes) and the postcement/dentine interface (24,234 nodes) were analysed using SPSS version 18.0 (Chicago, USA). Initially, it was intended that a twoway ANOVA would measure the effects of post design and the use of different materials on the mean stress (p = 0.05) along the centre of the post, crown-cement/core and post-cement/dentine interface. However, due to unmet assumptions of normal distribution of the residual, equality of the variances and significant interaction, an independent t-test, one-way ANOVA and Mann–Whitney or Kruskal–Wallis test were performed, as appropriate, to achieve the various objectives. 3. Results Stress distributions for different post and core systems following oblique loading are presented in Fig. 2. The maximum stress was observed on the buccal aspect of the posts, irrespective of design and material, with an exception for the carbon fibre post. The maximum stress on the root was found with the stainless steel, titanium and zirconia posts, near the cervical palatal root region, while the minimum stress was found with the carbon and glass fibre posts (Table 3). In both carbon fibre posts of both shapes, stress was concentrated at the buccal cervical area of the crown and at the middle buccal part of the dentine (Fig. 2(d)). 3.1. Stress distribution along the centre of the post Statistically significant differences in the stress distribution were observed for the different post designs and materials (Table 4). Generally, the tapered post design introduced significantly higher stress along the centre of the post in comparison with the

964

A.A. Madfa et al. / Medical Engineering & Physics 36 (2014) 962–967

Fig. 1. (a) Schematic illustration of the geometric model and (b) loading conditions for the tetrahedral mesh structure.

Table 3 Maximum value of von Mises stress distribution within the root posts.

Table 6 Effect of post designs and materials on the stress distribution at the crowncement/core interface.

Post design (MPa) Post material

Parallel-sided post

Stainless steel Titanium Zirconia Carbon fibre Glass fibre

Tapered post

24.89 21.13 24.68 19.52 19.21

26.02 20.72 25.46 19.46 18.20

Table 4 Effect of post designs and materials on stress distribution along the centres of the posts. Factor under study Design Parallel-sided post Tapered post Materials Stainless steel post Titanium post Zirconia post Carbon fibre post Glass fibre post

Mean (SD) (MPa)

P-value

11.99 (±7.43) 14.14 (±8.59)

Stress distributions in maxillary central incisors restored with various types of post materials and designs.

Different dental post designs and materials affect the stability of restoration of a tooth. This study aimed to analyse and compare the stability of t...
1MB Sizes 0 Downloads 3 Views