This paper proposes a review of the different opportunities described in literature for quantitative assessment of natural and reconstructed teeth using optical methods. These experimental method are particularly interesting for understanding the behavior of the natural tooth, but even more so for the CAD/CAM (Computerized aided design and computerized aided manufacturing) bonded ceramics reconstructions. Indeed CAD/CAM ceramics are increasingly used as therapeutic options. However, little is known about their mechanical behavior under stress, as the response of the prepared tooth supporting it. In the past years, optical approaches were proposed to get whole field and quantitative measurement of their mechanical properties. This paper discusses the main methods described in literature, as photo elasticity, digital Moiré interferometry, speckle interferometry and digital color holography.
In dentistry, 3D IOS (intra-oral scanners) are gaining an increasing popularity essentially for the production of dental prostheses. Until now, there is no normalized procedure to determine the resolution of IOS. Such a procedure could be a positive parameter for the IOS market and a first step in their normalization. The aim of this study is to present a reproducible methodology to estimate the noise and resolution of any type of IOS. For the noise, we used the IOS Trios 2 (3Shape) and the Carestream 3500 (Carestream) for noise and resolution. As reference, we used an ultra-flat and ultra-smooth alumina. Being perfectly flat, any record of roughness should be interpreted as noise. In this study, the root mean square (RMS) values obtained are ranged between 5.29 and 12.58 micrometers. Significant differences have been found between the central part and the whole mesh. This is due to edge effect: deviation from a flat surface is more important on the edge of meshes than the internal part. To evaluate the resolution, a ceramic tip, well-polished was recorded with the IOS’s and compared to the mesh obtained with micro tomography (5 micrometer resolution). We measured the distance between the two plans of the tip, considered as the small detail recorded. We found a distance from 89 to 121 micrometers with IOS studied and 25 micrometers with micro CT. Those methods, simple and reproducible, could be perfectly suitable to evaluate and compare commercial all types of IOS’s.
Computerized Aided Design (CAD) and Computerized Aided Manufacturing (CAM) ceramic occlusal veneers are increasingly used as therapeutic options. However, little is known about their mechanical behavior under stress, as the response of the prepared tooth that supports it. The aim of this article is to use for the first time 3D color holography to evaluate the behavior of a molar occlusal veneer under stress and the response of the prepared tooth. The occlusal surface of a lower molar is prepared to receive a specific monolithic ceramic reconstruction manufactured with a chairside CAD/CAM system. Longitudinally cut samples are used to get a planar object observation and to “look inside” the tooth. A digital holographic set-up permits to obtain the contact-less and one-shot measurement of the three-dimensional displacement field at the surface of the tooth sample; stain fields are evaluated with low noise-sensitive computation. The results show an excellent behavior of the restored tooth without areas of excessive stress concentrations, but also a significant involvement of the dentin enamel junction. So, we demonstrate that the ceramic occlusal veneer seems to behave in accordance with the biomechanical concepts ensuring the longevity of the reconstituted tooth. It follows that we demonstrate that 3D holography is a highly recommended method for studying dental biomechanics.
New trends in dental prosthodontic interventions tend to preserve the maximum of "body" structure. With the evolution
of CAD-CAM techniques, it is now possible to measure "in mouth" the remaining dental tissues. The prosthetic crown is
then designed using this shape on which it will be glued on, and also by taking into account the contact surface of the
opposite jaw tooth.
Several theories discuss on the glue thickness and formulation, but also on the way to evolve to a more biocompatible
crown and also new biomechanical concepts. In order to validate these new concepts and materials, and to study the
mechanical properties and mechanical integrity of the prosthesis, high resolution optical measurements of the
deformations of the glue and the crown are needed. Samples are two intact premolars extracted for orthodontics reasons.
The reference sample has no modifications on the tooth while the second sample tooth is shaped to receive a feldspathic
ceramic monoblock crown which will be glued.
This crown was manufactured with a chairside CAD-CAM system from an intra-oral optical print. The software allows
to realize a nearly perfect clone of the reference sample. The necessary space for the glue is also entered with ideal
values. This duplication process yields to obtain two samples with identical anatomy for further processing. The glue
joint thickness can also be modified if required.
The purpose is to compare the behaviour of a natural tooth and its prosthetic clone manufactured with "biomechanical" concepts. Vertical cut samples have been used to deal with planar object observation, and also to look "inside" the tooth.
We have developed a complete apparatus enabling the study of the compressive mechanical behaviour of the concerned
tooth by speckle interferometry.
Because in plane displacements are of great interest for orthodontic measurements<sup>1</sup>, an optical fiber in-plane sensitive
interferometer has been designed. The fibers are wrapped around piezoelectric transducers to perform "4-buckets" phase
shifting leading to phase variations during the compression test.
In-plane displacement fields from speckle interferometry already showed very interesting data concerning the
mechanical behaviour of teeth: the dentine-enamel junction (DEJ) and the glue junction have been shown including their
interfacing function. Mechanical action of the tooth surrounding medium will also be discussed.