Evaluation of crack propagation in dental composites by optical coherence tomography

Abstract

Objectives

The purpose of this study was to image the sites of fracture initiation and slow crack propagation in a fiber reinforced composite, using the optical coherence tomography (OCT) technique.

Methods

Bar specimens (2 mm × 3 mm × 25 mm) of fiber reinforced composite were mechanically and thermally cycled to emulate oral conditions. The interior of these samples was analyzed prior to and after loading, using OCT. The device used was a home-built Fourier domain OCT setup working at 800 nm with 6 μm spatial resolution.

Results

Intact specimens after load cycling were analyzed. It was clearly seen that OCT images provide an insight into crack propagation, which is not seen by the naked eye.

Significance

By using OCT the possibility of analyzing the fracture propagation quantitatively, and in depth, was added, opening up possibilities to quantitative studies.

Introduction

The longevity of dental restorations is dependent upon many factors, including operator skill, materials employed and techniques, replacement, the oral environment and its contribution to patient susceptibility to caries, as well as patient compliance with oral hygiene advice [1]. Fatigue in dental restoratives can lead to fractures, which can be influenced by water absorption of the resin matrix [2], [3], [4], by thermal cycling and cyclic loading, including cycling masticatory forces [2], [3], [4], [5], [6], [7], [8]. Although recognized as being important issues, relatively few studies have thoroughly investigated these factors.

Contemporary approaches to fatigue principles consider a fracture process in three phases: crack initiation, slow crack growth, and fast fracture. The latter phase is very short in duration and therefore the time of crack initiation and of slow crack growth account for the useful fatigue resistance of a material. Crack initiation nucleates at heterogeneities like surface and subsurface microcracks, porosities, filler particles or crazes within the materials, not necessarily induced by external loading [9]. External cycling loading is able to originate or further develop a crack, called slow crack growth. Measuring crack propagation is not trivial because identification of the crack tip positions during the fracture process is difficult considering the small size of specimens [10].

There are three characteristic regions surrounding the fracture origin on the fracture surface of a brittle material. The first region is generally a relatively smooth (mirror) region, the second is a slightly stippled (mist) region and the third is a very coarse (hackle) region. This last region leads to macroscopic crack branching generating the bifurcation of the main crack. The boundaries at which mist, hackle and crack branching occurs may or may not be symmetric about the fracture origin. The formation of these three regions occurs at a constant characteristic stress intensity factor [11]. Fig. 1 is a pictorial representation of the above mentioned regions.

The purpose of this in vitro study was to analyze the fracture propagation on a fiber reinforced composite (FRC), which had been subjected to thermal and mechanical cycling, by imaging the sample interior in a non-invasive and non-destructive manner using optical coherence tomography (OCT). OCT is a well-established low-coherence interferometric technique that performs high resolution, non-invasive, cross-sectional tomographic imaging of tissue microstructures [12], [13]. It can be seen as analogous to ultrasound, in the sense that the reflected wave from the tissue carrying the information is analyzed, but using light instead of sound waves. The basic technique employs a broadband light source, an interferometer and a photodetector. The photodetected information is fed to software, which generates a 2D or 3D image. With the use of broadband sources, such as those generated by ultra short laser pulses or super luminescent diodes, OCT images of biological tissue can achieve spatial axial resolution of a few micrometers [14]. Clinically, OCT systems have been widely used for diagnoses in ophthalmology, although several other areas of health care have made use of OCT for image purposes [13]. In dentistry, a series of reports describing OCT applications first appeared in 1998 [15], [16], [17], with imaging of both hard and soft oral tissues. This led to several diagnostics of oral diseases, including periodontal diseases and early caries [18], [19], among others.To the best of the authors’ knowledge no report of OCT performed on dental material is available. By using OCT the possibility of analyzing the fracture propagation at depth and before reaching critical length is added, as well as opening up the possibility for quantitative studies.