BENEFITS OF USING CAD/CAM TECHNOLOGY IN DENTAL PROSTHETICS

Headnote

Abstract

The purpose of this mini-review was to describe the CAD/CAM systems, their principles and their equipment, to highlight the major historical milestones in the development stages of dedicated dental CAD/CAM systems, and to identify the advantages/ disadvantages of using optical impressions compared to conventional impressions. This technique provides benefits in both directions of the designing and manufacturing process of a prosthetic restoration, i.e. in clinical and technological sectors. The main clinical benefits are strength, aesthetics, comfort, while the technical ones assure an easier, faster and more accurate fabrication process.

Keywords: CAD/CAM technology, processable materials, marginal accuracy, optical impressions.

1.INTRODUCTION

Along its history, dental medicine has gone through several stages of development that undoubtedly marked its evolution, such as the moment when the conventional fabrication of analogous restorations was changed by the use of computer technology. Nowadays, computerassisted manufacturing is extensively involved in restorative dentistry, being associated with high accuracy, accelerated production speed and reduced manual labour [1,2].

The development strategy of Computer Aided Designing - Computer Aided Manufacturing (CAD/CAM) includes automating the production process and optimizing the quality of restorations by using new biocompatible materials, especially high performance ceramics, such as zirconia and lithium disilicate [3]. The involvement of engineering and computer science in dental technology has led to the development of digital data collection systems (digital impression or scanning of conventional casts or impressions), designing systems based on the collected digital data, milling systems for various materials based on the performed design. Due to the high performance equipment available, both the transfer of data to the dental laboratory and the fabrication process are easier, while the time for the execution of dental restorations is shorter.

2.OVERVIEW

Currently applied in the field of dentistry are three main manufacturing techniques, namely formative, subtractive and additive techniques.

Formative techniques are based on the action of heat or of a positive or negative pressure, and on the way in which objects are made by pouring/ pressing into molds, stamping, vacuum aspiration, or by the lost wax casting technique.

Additive manufacturing is also called "3D printing", a process that involves building layer upon layer of material based on the CAD design, extensively used in the field of prosthodontics.

Subtractive manufacturing is characterized by building a three-dimensional physical model of the future object, based on a digital model, through selective mechanical or laser removal from a block of material. This is the most widely used technology in dental medicine. Subtractive manufacturing is based on CNC (computer numerical control) systems. CAD/CAM technology has become an increasingly popular part of dentistry over the past 25 years [4].

In its common acception, subtractive manufacturing is synonymous to CAD/CAM systems, being based on an integrated process that combines the use of computers for designing activities with their use in manufacturing procedures. CAD systems consist of a computer with one or more terminals containing video monitors and interactive graphics input devices. CAM systems involve using high performance numerically controlled machine tools, and programmable industrial robots. The elaborated designs obtained during the CAD process are directly converted to commands for the production machines by the use of CAM software, which will guide the milling machine in the manufacturing of the desired object.

This technique provides benefits in both designing and manufacturing of a prosthetic restoration, i.e. in clinical and technological sectors. The main clinical benefits are resistance, aesthetics, comfort, while the technical ones assure an easier, faster and more accurate fabrication process.

Several decisive personalities contributed to the development of CAD/CAM technology. Bruce Altschuler introduced the concept of holodontography in 1973 [5], based on the principles of holography enunciated by Gabor. Altschuler describes an experimental holographic system that uses two low-power He-Ne laser systems coupled for scanning a prosthetic field. Also based on the principle of holography, Francois Duret presents in his doctoral dissertation, defended in 1973 at the Claude Bernard University in Lyon, the concept of optical impressions [6]. In the 10 chapters of the thesis, dr. Duret exposes the disadvantages of chemical impressions, describes the components that will make up the future system of capturing and recording of the dental morphology, the type of laser used and its interaction with the tissues of the oral cavity, how to convert holography into electronic signals that can be processed by computers, computer type and software used to design the future prosthetic restoration, the mode of transmission of the design to the milling system and the process of milling and materials for use.

Following conceptualization of opto-electronic acquisition systems, John Young, together with Bruce Altschuler, manufactured in 1977 the first milling of an occlusal surface of a lower first molar [7]. A CNC milling machine was used for milling, which made the occlusal surface of the molar from a metal block, at various scales. The experimental CNC machine was under development at the School of Aeronautical Medicine of the Air Force Base in Brooks, Texas, USA.

Two years later, Paul Heitlinger and Fritz Rodder developed an intraoral optoelectronic impression system that could be coupled to a milling machine, through which they obtained a milled working model. On this model, a dental technician fabricated an inlay, a crown and a bridge, using conventional methods. These optoelectronic impression systems, together with the milling system, were patented a year later [8].

In the early 1980's, Werner Mormann from the University of Zurich and Marco Bradestini from Brandestini Instruments in Zurich conceived a system that allowed restoring posterior teeth with tooth-colored materials through CAD/CAM milling. At that time, direct fillings showed unsatisfactory results due to polymerization and thermal shrinkage, high water absorption, mechanical stress and dimensional changes induced by the tooth structure, which caused the formation of a defective marginal closure and, consequently, failure of restoration [9]. Based on his own in vitro and in vivo studies with pressed and hot polymerized composite resin inlays, Mörmann developed the hypothesis that ceramic inlays can be adhesively aggregated with a composite resin as a cementing agent [10]. The concept of adhesive cementation was later confirmed by in vitro [11] and in vivo studies [12]. Mörmann developed the clinical concept of adhesive aggregate ceramic inlays and, at the same time, raised the issue of the rapid manufacture of this type of restoration. Thus, Mormann and Bradestini developed a prototype CAD/CAM system for dental practice, to allow the production of ceramic inlays in a single treatment session. The prototype system, called CEREC (CEramic REConstruction), consisted of a mobile CAD/CAM unit, which integrated a computer, keyboard, trackball, pedal, an intraoral optoelectronic fingerprinting system as input devices, a monitor and a milling compartment as output devices [12]. Using this prototype and a block of feldspar ceramics produced by VITA, the inner face of an inlay is milled, based on a single image of the cavity.

In the year 1987, there were three commercially available CAD/CAM systems on the market: Mormann in collaboration with Siemens launches the CEREC 1 system, dr. Duret in collaboration with Henson launches the DURET system, and dr. Matts Andersson in collaboration with Nobel Biocare launches the PROCERA system. In the prototype phase, there was a fourth system, called Minnesota, developed by the team of the Minnesota School of Dentistry, led by dr. Dianne Rekow [13,14].

The subsequent evolution of the CEREC CAD/CAM was marked by the moment of transition from two-dimensional to threedimensional software (Table 1).

Currently, there are a multitude of CAD/ CAM systems, manufactured by various companies. Regardless of the type of system used in dental clinics or laboratories, they necessarily include 4 components: an image capturing system, a design software, a milling system and a communication system. Dental office systems include an intraoral scanner with or without the possibility of coupling this device to a design software and milling system, which enables manufacturing dental restoration in office. Laboratory systems present the following components: a scanner for the impression/ model, a software for design, a CAM software and a milling/ 3D printing unit.

3.PROCESSABLE MATERIALS

The evolution of materials, particularly ceramics, flourished with the CAD-CAM technology. By definition, ceramics represents the inclusion of crystals into the glass matrix. Glass is translucent, but also fragile. The larger and more numerous the crystals, the better the mechanical properties they confer to the ceramics, to the detriment of optical properties.

Several types of ceramics are associated with CAD/CAM technology: feldspathic ceramics, lithium disilicate ceramics, hybrid ceramics, zirconium oxide. Tooth-colored CAD/CAM restorative materials have been successfully documented over the last two decades, with promising performance [2,15,16].

Initially, only zirconia or alumina were available for processing abutments, but manufacturers introduced other processable materials, including glass ceramics and lithium disilicate as multilayer blocks, for a significant improvement of aesthetic appearance [8]. The possibility of using a wide range of materials is an essential advantage of subtractive manufacturing, and allows making an object with highest accuracy and best finish of surfaces.

Depending on the CAD / CAM system, the list of materials that can be processed by this can be more or less extensive. Some materials require milling in a wet environment, others in a dry or dry-wet environment. Generally, wax, zirconium dioxide and polymethyl methacrylate used for provisional restorations are milled in a dry environment. The last two can be also milled in a wet environment. Wet milling is compulsory for ceramics based on lithium disilicate, feldspathic ceramics and composite resins.

Feldspathic ceramics has been considered a golden standard, due to its tooth-like appearance, based on light transmission and natural effect [17,18]. However, the low mechanical properties, for instance, fracture strength, was a limiting factor for its application and stimulated further advancements of glass ceramics reinforced with different fillers [18-22].

Hence, lithium disilicate (LiS2) ceramics - as a high-strength and highly aesthetic material and polymer-infiltrated ceramics - with hardness and elastic modulus similar to the dental structures, higher fracture toughness and reduced brittleness were developed [23-25].

In the dental clinic, where quick fabrication is necessary, the lithium disilicate ceramic material is preferred because it requires less time to mill and crystallize [26]. The lithium disilicate ceramic crown is superior to a zirconia crown in terms of better aesthetics, faster fabrication, and easier post-processing [27,28].

The reported survival rate of CAD/CAMfabricated polymer-infiltrated ceramic inlays is 97.4 and 95.6% for partial coverage restorations after 3 years [29]. By comparison, the mean survival rate of CAD/CAM processed LiS2 veneers reaches 99% with 96.4% success after 5 years. However, the high clinical survival and success rates of CAD/CAM single restorations are based not only on the novel materials, but are strongly determined by the strength and durability of the bond formed between the restorative material, luting cement and substrate [30,31].

The performance of a CAD-CAM system relative to marginal adaptation is influenced by the type of restorative material used [32]. Theoretically, 50 to 100 pm in vitro fitting accuracy appeared to be achievable, as confirmed in a later study [33]. Most of the CAD-CAM restorations/ infrastructures ranged within the clinically acceptable marginal discrepancy [MD] domain [32].

Compared with CAD-CAM restorations, most of the heat-pressed lithium disilicate crowns displayed equal or lower marginal discrepancy values. Slip-casting crowns exhibited similar or better marginal accuracy than those fabricated with CAD-CAM. Cobalt-chromium and titanium implant infrastructures produced using a CADCAM system elicited lower marginal discrepancy values than zirconia [34]. The majority of cobaltchromium restorations/ infrastructures produced by (selective laser melting) displayed better marginal accuracy than those fabricated with the casting technique. Compared with copy milling, the majority of zirconia restorations/ infrastructures produced by CAD-CAM milling exhibited better marginal adaptation [32,35-37].

4.PRINCIPLES AND METHODS

To overcome the drawbacks of the conventional impression methods applied in dentistry, digital models can be created by indirect or direct approaches, where the indirect method uses laser optical scanning or computed tomography imaging of conventional impressions or plaster casts to produce digital virtual models [38-41].

Direct digital impressions overcome the disadvantages of the commonly used elastomeric impression materials, including technique sensitivity, patient discomfort, dimensional changes, and laboratory errors. With the development of the chairside CAD/CAM technology, intraoral scanners have been widely used and advanced with reliable accuracy [42].

Optical scanners operate according to the principle of three-dimensional data collection by active triangulation. The source of light and the receptor are positioned at a well-defined angle from one another, and the computer calculates the three-dimensional data from the image resulting from this mutual position.

Direct digital impression methods using intraoral scanners have many advantages over conventional impressions [43,44] with precise in vivo results, but the variability in scanner output is still large, with error levels somewhat around or below the conventional impression method [45]. Also, the digital impression may be associated with potential distortions caused by limitations in the scanning technology and accumulation of datasets while scanning a longer arch [46,47].

To overcome these shortcomings, devices based on various non-contact optical technologies, such as confocal microscopy, active stereovision, and triangulation are under development or have already been introduced in the dental market [48,49].

5.DISCUSSIONS

The study of Lee and Gallucci, conducted at Medipol University in Istanbul, illustrates the differences between conventional and digital impression, the results being by far in favor of digital impression [48]. The conventional process requires time for the preparation of impression trays and the setting time of impression materials is standardized. Furthermore, patients prefer the intraoral scan rather than the conventional impression method [38,49].

The accuracy of conventional impressions is subject to a potential risk of error, related to a series of external factors, such as: the human factor, the material, the type of impression tray used and the impression technique [50-54]. Also, this technique is difficult in terms of material manipulation and in creating a sense of comfort for the patient because of the insertion technique, as represented in Figure 1.

By comparison, digital impressions offer speed, efficiency, ability of storing captured information indefinitely and transferring digital images between the dental office and the laboratory. The advantages of digital impressions and scanning systems are improved patient acceptance, reduced distortion of impression materials, 3D pre-visualization of tooth preparations, potential cost- and timeeffectiveness [55,56] and improved patient comfort, as shown in Figure 2.

An intraoral scanner is an essential tool in the chairside CAD/CAM system, because it allows acquisition of a virtual cast directly from patient's oral cavity, without the need for any additional work process [37]. With the exception of zirconia, which takes 6-8 h for the postmilling process, feldspathic, leucite-reinforced, lithium disilicate ceramics, and composite resins are considered for chairside CAD/CAM restorations [57].

Optical impression is a powerful tool for patient communication. With optical impressions, patients feel more involved in their treatment and it is possible to establish a more effective communication with them; this emotional involvement may have a positive impact on the overall treatment [58]. Several studies indicate that the efficiency outcomes of the digital impression technique were higher than those of the conventional impression technique, with respect to the necessary treatment time and perceptions of the subjects [59].

6.CONCLUSIONS

In recent years, dentistry is dominated by the trend of technology present in all branches of activity. The CAD/CAM technology is appreciated both by doctors, technicians, and by patients, as a reliable, efficient and convenient method of performing prosthetic treatments. These systems are also considered a major advantage of digital technology, offering a higher degree of precision and aesthetic appearance of prosthetic restorations.

References

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