Investigation of Biomechanical Aspects of the Screw- Retained Restoration Secured by Conical Head Abutment Screw: A Finite Element and Mathematical Analysis

Special Article – Dental Implants

J Dent App. 2016; 3(3): 340-346.

# Investigation of Biomechanical Aspects of the Screw- Retained Restoration Secured by Conical Head Abutment Screw: A Finite Element and Mathematical Analysis

Taghavi H¹, Arabi Z², Rahmani Z³, Alikhasi M4, Bulaqi HA5, Paknejad M6 and Safari H7*

¹Department of Prosthodontics, School of Dentistry, Qom University of Medical Sciences, Qom, Iran

²Department of Computer Engineering, Payame Noor University, Tehran, Iran

³Oral and Maxillofacial Research Center, Dentistry Faculty of Tabriz University of Medical Sciences, Tabriz, Iran

4Department of Prosthodontics, Faculty of Dentistry, Tehran University of Medical Sciences, Tehran, Iran

5Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran

6Department of Periodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran

7Department of Periodontics, School of Dentistry, Qom University of Medical Sciences, Qom, Iran

*Corresponding author: Hamed Safari, Department of Periodontics, School of Dentistry, Qom University of Medical Sciences, Qom, Iran

Received: October 18, 2016; Accepted: November 16, 2016; Published: November 18, 2016

## Abstract

Introduction: The purpose of this study was to investigate the biomechanical aspects of screw-retained restorations secured by conical-head abutment screws.

Materials and Methods: A Three-dimensional finite element models of a Straumann implant, a cast-to gold abutment, a conical-head abutment screw, and prostheses with screw-retained fixation method were constructed. ABAQUS software was used to simulate the one-step process of screw tightening. Additionally, to separately recognize the behavior of each influential parameter, mathematical method was used to reduce the computational costs. Distribution of stress and radial displacement obtained by FE analysis were presented, and predicted values of the stresses in cross-sectional areas of the prostheses, obtained by mathematical method, were presented in regard to influential parameters (ceramic thickness and cone angle of conical-head abutment screw).

Results: During tightening, due to the conical nature of the abutment screw, lateral pressure was developed in the internal conical region of abutment, and thus caused stresses within and around the prostheses. The distribution of tangential stress along the Metal-Ceramic thickness was not uniform. Cone angle of conical head abutment screw (up to 30 degrees) has a reverse effect on stresses created in the metal-ceramic interface, in contrast to the ceramic thickness.

Conclusion: Metal ceramic interface bonding acts as a stress concentration point in screw-retained restorations due to the differences in material properties. In the cross-sectional areas of the prostheses, lateral pressure arising from conical-head abutment screw tightening can propagate the existing microcracks in metal-ceramic interface and thus can lead to porcelain fracture or chipping of the veneer.

Keywords: Conical-Head Abutment Screw; Screw-Retained Restoration; Metal-Ceramic; Finite Element Analysis; Mathematical Analysis

## Introduction

Implant-supported restorations either cement retained or screwretained, are a common treatment modality for missing teeth [1-5]. Although both retention types have advantages and disadvantage [1- 6], the choice of connection type is based on the clinician’s preference [4,7]. According to various studies, while screw-retained restorations offer retrievability, higher stability and security [8,9], cement-retained restorations have the potential for complete passivity as their primary advantage [3,10,11].

Considering implant survival and failure rates, there are no significant differences between screw- and cement-retained restorations [1,2,12,13]. In a systematic review, the estimated 5-year restoration survival rate was 96.03% for screw-retained restorations and 95.55% for cement-retained restorations [1]. However, some authors reported statistically significant differences in technical and biologic complications between screw- and cement-retained restorations [1,2,13]. Sailer et al [13], indicated that technical complications occurred primarily with screw-retained restorations and that biologic complications occurred primarily with cementretained restorations.

Some investigators reported that porcelain fracture or chipping of the veneer in implant-supported restorations was more prevalent in screw-retained restorations than cement-retained restorations, as a technical complication [1,2,14-16]. In an in-vitro study performed by Torrado et al [14], the fracture resistance of porcelain was demonstrated to be significantly higher in cement-retained restorations than in screw-retained restorations. In another in-vitro study, Karl et al [15], indicated that weak points in the ceramic layer are caused by the presence of screw-access holes in screw-retained restorations. Nissan et al [16], observed that the incidence of porcelain fracture was 38% for screw-retained restorations and 4% for cementretained restorations.

Screw-retained restoration could be directly connected to the implant using cast-to or castable abutments. The design of the abutment-abutment screw head interface can be either conical-head or flat-head [17]. Conical-head joints provide increased stability and seal performance due to a friction-locking mechanism between the two mating parts [17,18]. Coppede et al [17], concluded that the shape of the abutment screw head significantly affected the resistance to screw loosening. Budynas and Nisbett presented a lateral pressure on the conical region under axial force (preload) [19]. This pressure leads to the generation of radial stress in the cross-section of the conical region. In a systematic review, Wittneben et al [1], reported that the existence of radial stress may interrupt the integrity of the metal frame and the porcelain veneer at their interface during tightening of the assembly.

The application of metal-ceramic (MC) restorations is common in prosthetic treatment [20]. Several previous studies discussed the causes of porcelain fracture in the context of MC restoration failures [21-23]. Although, the ceramic materials used in MC restorations have the advantages of high biocompatibility and esthetic, the primary disadvantage of these materials is sensitivity to microscopic cracks due to their brittle nature [24]. Furthermore, cracks can be created during the process of fusing and cooling porcelain on the metal frame due to differences in bulk modulus and thermal coefficient of expansion between the materials [23,25].

Porcelain fracture and delamination of metal-ceramic bonding have been implicitly reported based on clinical and experimental comparative studies of screw- and cement-retained restorations. A comprehensive study of the reason causing this phenomena has not been conducted. Moreover, few studies have considered the reason why screw retained-restorations have low fracture resistance.

This gap in the literature may be caused by the restrictions of the conventional experimental methods. However, the finite element method facilitates an accurate study of the stress and strain distribution within the models by overcoming the problems of the conventional methods such as stress measuring within the models and time consuming.

Therefore, the aim of this study was to investigate the biomechanical aspects of the technical problems such as porcelain fracture or chipping of the veneer associated with screw-retained restorations secured by conical-head abutment screws, and also the reason of high possibility of their fracture.

## Finite element modeling

The primary form of the second premolar tooth was used for computer-aided design (CAD) modeling of the implant-supported single restoration with screw-retained fixation method. In the CAD model, the metal frame and the porcelain were modeled on the abutment to ensure that they acted as one piece, and the abutment hole was continued to the occlusal surface to provide accessibility to the abutment screw and the ability to tighten the screw. Additionally, lingual ledge for the metal framework was considered. The threedimensional CAD model of the screw-retained restoration is presented in Figure 1A.