6.1 Introduction

Due to the advancement of digital technology, conventional dental impressions are rapidly being replaced by digital impressions using intraoral scanners (IOS) and may be totally replaced within the next decade. Intraoral scanners (IOS) are devices for capturing direct optical impressions in dentistry. They project a light source onto the dental arches, and prepared tooth surfaces with the images being captured by imaging sensors.

Many dental laboratories scan casts or impressions and design their restorations digitally using computers, ensuring an efficient and cost-effective workflow.

This chapter attempts to address all these concerns. In addition, the benefits of digital impressions along with its limitations are also discussed.

6.2 History

Digital impression systems can be further be divided into either direct digitalization (intraoral scanners) or indirect digitalization (extraoral scanners) depending on the requirement. Intraoral scanners were introduced as stand-alone devices that capture a digital impression and send the file to a dental laboratory for prosthesis fabrication. They were originally a part of CAD/CAM systems which produce a digital impression of prepared teeth.

When using extraoral scanners, there is no need for any change to the clinical workflow for taking conventional impressions. Either the impression itself is scanned, the impression is poured with stone and scanned, or a model is printed and scanned, to create a digital 3D file.

6.3 Comparison between Conventional and Digital Impressions

A meta-analysis and systematic review found most of the studies included in their study to be in vitro studies that gained better marginal and internal fit of fixed restorations with digital rather than conventional impression techniques [1]. They also concluded that restorations produced from digital dies had smaller marginal and internal discrepancy compared to stereolithography apparatus (SLA) polyurethane dies.

Recently, less deviation for short span restorations were observed using current IOS compared to conventional impressions, but this does not apply for long span restorations where conventional impression techniques still provide the lowest deviation [2, 3] Consequently, a recent systematic review investigating accuracy of digital impressions in fixed prosthodontics concluded that conventional impressions performed using high-precision impression materials showed greater accuracy than digital impressions [4].

6.4 Technology of Digital Impression Systems

Intraoral scanning systems use different imaging technologies (either a laser or a video) to capture their 3D images. Some systems such as CEREC base its imaging on triangulation, a technique in which a light source is reflected off an object. Light triangulation limits accuracy when scanning curved surfaces especially those that do not reflect light evenly, such as teeth with amalgam restorations. Therefore, some systems require the use of titanium dioxide powder as a contrast medium to correct the problem of light triangulation, whereas others do not.

Current systems use different light source technologies, including laser, structured (striped) light, or LED illumination.

Digital impression data transfer systems using IOS can be classified in different ways. If the IOS allows the digital impression to be sent directly via export of source files such as STL (Standard Tessellation Language or “send-to-lab”), PLY (Polygon File Format), and OBJ (Object File Format) to different laboratory units giving the desired flexibility [5], it is known as an open system, and if not, it is a closed system.

Open systems allow practitioners to work with different laboratories and maximize the potential of their investment with options. The STL file format is simple and small, thus making its processing faster, but without representation of color or texture. On the other hand, OBJ and PLY formats can store properties such as color and texture and benefit from improved 3D printers.

In a closed system, the digital impressions are sent to the manufacturing company, at a fee. The advantage is that since the configuration, collection, and manipulation of the data is by the same manufacturer, it provides security and a single place for delivery. Some scanners allow only acquisition of data, which is then sent to the laboratory for further processing and manufacturing. On the other hand, there are scanners that besides acquisition are able to mill or print same day, thereby allowing the patient to have a dental restoration in a single sitting.

Data collection methods, transfer of images, strategies for tracking, and size of scanner head may vary between different types of scanners, but each procedure produces a digital model of the patient’s dentition.

6.5 Advantages of Digital Impressions

Advantages of digital impressions include eliminating the need of materials for making impressions, leading to better patient comfort [6, 7], with little or no gagging. This avoids the pouring of casts, and adjustments are easier due to the positive image of tooth preparation being visible on the computer screen. Similarly, it does not require temporization or retraction-cord packing, the rubber dam can be used with digital impressions, does not need to be disinfected except for the tip, is easier to store, and most importantly, the dental restoration can be delivered on the same day of digital scanning using CAD/ CAM technology.

It also allows for better communication with patients as well as dental technicians. Scanners can capture the prepared teeth, the neighboring teeth, and at times, the whole arch. Opposing arch scans and simulation movements in a virtual articulator are also possible [8].

6.6 Disadvantages of Digital Impressions

As with every novel technology, there is a learning curve, and nonexperts might require additional time to take the digital images. Other disadvantages include the inability to detect subgingivally prepared margins of teeth and the high initial investment cost of IOS.

6.7 Clinical Indications and Contraindications of Digital Impressions and IOS

In Prosthodontics, digital impressions can be used to design and mill single tooth crowns, endodontic crowns, resin onlays and inlays, veneers, fixed partial dentures, removable partial denture frameworks, implant bridges post and cores, temporary restorations, and digital smile design. Contraindications include long span fixed partial dentures, long span implant supported fixed partial dentures, and complete removable dentures.

In Orthodontics, they can be used for diagnosis and treatment planning, to fabricate orthodontic aligners, custom-made devices and retainers. They can also be used for guided implant surgery.

General contraindications include inability of the patient to sit still and restricted access to area as in case of the head of the intraoral scanner being too large or if there are interferences by the tongue or an orthodontic appliance [9]. It is important to control bleeding prior to scanning to obtain an acceptable image.

6.8 Accuracy of Intraoral Scanners

A variety of IOSs are used by dentists these days (Figs. 6.1 and 6.2), and with it researchers started reviewing the literature on the use of these devices [10]. The important question to ask is if the accuracy of digital dental models generated from an IOS is equivalent to that of dental plaster models [11]. Accuracy of the device encompasses “trueness (closeness of agreement between the experimental dataset and the real object) and precision (closeness of agreement among different scans performed by the same scanner)” [12] (Table 6.2). An important limitation of studies investigating the accuracy of scans is their conduction in vitro, because it has been observed that intraoral conditions can influence their accuracy [9].

Table 6.2

Ideal characteristics of an intraoral scanner

Feature	Requirement
Trueness	High
Precision	High
Resolution	High
Need for powder	Not needed
Scanning speed	Fast
Tip of the scanner	Small
Colored images	Yes
System	Open

A systematic review [13] investigating the mean accuracy of digital technologies including intraoral tissues observed that scanning of a dentate arch by laboratory and IOS ranged between 17 and 378 μm. For prepared teeth, the minimal accuracy was 23 μm, but when the complete arch was scanned it was 60 μm. Scanning of single tooth preparations showed an accuracy between 20 and 40 μm, and that of digital implant scanning had an accuracy of 19–112 μm. Accuracy for partially and completely edentulous arches ranged between 30–220 μm while for completely edentulous arches alone the range was between 44 and 591 μm. Authors of the review concluded that current digital technologies are accurate for specific applications but the scanning of edentulous arches still represents a challenge [13]. Other researchers similarly commented this to be due to the mobility of the tissues and a lack of reference landmarks in edentulous arches [14]. Overall, there are variable outcomes among the different IOS systems [15].

6.9 Comparison of Accuracy between Intraoral Scanners and Extraoral Scanners

When comparing accuracy of IOS with extraoral scanners (EOS), there have been many conflicting reports [9, 16–18]. It has been suggested that intraoral conditions such as saliva and limited spacing can contribute to the inaccuracy of a scan. It is noteworthy to mention that EOS, due to the many steps required to manufacture a restoration, can also have error introduced in the stages of impression taking, gypsum or SLA polyurethane model-making, in addition to the error resulting from digitization [19].

6.10 Common Commercially Available Intraoral Scanning Systems (Table 6.3)

The following intraoral scanners are not ranked in any specific order.

Table 6.3

Comparison of different types of IOS

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