15.1 Introduction

Once the machine is purchased for low cost, nowadays, short acquisition time and non-invasiveness are some of the advantages [1]. Following the ALARA (as low as reasonably achievable) principle, radiation for 3D acquisition should be avoided whenever possible. Radiation reduction is essential because the radiation dosage accumulates throughout a subject’s lifetime. Publications, where dental X-rays and increased risk of intracranial meningioma [2] or computed topography (CT) imaging with possible influence to develop brain tumours or leukaemia [3, 4] were controversially discussed, lead to insecurity in patients. Magnetic resonance (MR) imaging would be another alternative to depict soft tissue, but the disadvantages prevail: long acquisition times and expensive and in the majority of magnetic resonance tomographs (MRT), the patient has to lie without moving. Therefore, due to gravidity, the soft tissue might be different to a sitting/standing position. Being able to offer a tool that allows 3D imaging without radiation is very helpful. A 3D laser scanning imaging system, using a nonhazardous laser, with a precise texture reproduction and in some models with colour capturing [5] can replace 3D imaging with radiation in large number of cases.

15.2 The Method of Laser Scanning

The laser scanning can be briefly described as follows: a laser beam is sent through a cylinder lens and a horizontal light beam is generated. This beam will be due to a Galvano mirror projected onto the object which should be scanned. The Galvano mirror enables to capture the object completely, and the reflected light beam will be captured with a CCD (charge-coupled device) sensor (Fig. 15.1).

Distances will be triangulated between the reflecting laser beam and the scanned surface, as shown in Fig. 15.2.

When two points with a known distance to the laser exist, it is possible to calculate the angle of a triangle due to angle measurements between the points and based on this the distance to the unknown point. The laser scanner can detect length, width and the depth of the scanned object [6]. Depending on whether the laser is a handheld, mobile or nonmobile device, a tracking camera is necessary, too. In Fig. 15.3, you see exemplarily the setup of the mobile (handheld) T-scan^®, (Steinbichler, Carl Zeiss Optotechnik GmbH, Neubeuern, Germany). It uses a laser with 670 nm (class II). The handheld device has 29 infrared markers. Three of these markers are used for the precise spatial position when using an optical tracking system. The technical data as published by the manufacture [7] are as follows: resolution of distance measurement, 1 mm; accuracy of distance measurement þ/e, 30 mm; and sensor weight, <1500 g. The tracking system seen in Fig. 15.3 is an Optotrak Pro 1000 (Northern Digital Inc., Ontario, Canada) [8]. The advantage of the T-scan is that the scanner does not have to struggle with shadowing effects due to undercut as in other non-handheld devices. Sometimes multiple images from different angles must be acquired to compensate for missing surface parts. It is desirable to have complex areas as ear, nostril or cleft region complete without large data holes [9]. In Fig. 15.4, a screenshot is shown, as it is visible in real time on the PC monitor. The scan can be manually rotated to ensure that the desirable facial areas are captured completely.

Below, the different usage of 3D laser scanning will be introduced: the scanning of plaster cast models and facial impressions, oral surgical interventions [11], orthognathic treatment and surgery [12], malformation, epithetic, aesthetics and facial movements [13–15]. Overlapping themes will be discussed depending on the aspect.

15.3 The Laser Scanning of Plaster Models, Impressions and Skull Models

The reliability of laser scanners in reproducing 3D objects has already been described [6], the accuracy and reproducibility having been tested on geometrical models and (facial/dental) plaster casts [6]. The comparison of laser scanner and stereophotogrammetry for nasal plaster casts showed no significant differences between these two different techniques [1], whereas measurements on stone casts were somewhat smaller than values obtained directly from subjects (differences between −0.05 and −1.58 mm) [16]. These findings were also detected by Holberg et al. [17]. In their study, the accuracy of facial plaster casts and suitability for 3D mapping was investigated. From 15 adult volunteers, a facial alginate impression was taken. These impressions were poured with plaster resulting in facial plaster casts. The plaster casts and the probands’ faces were captured using a 3D laser scanner operating with structured light. They found – depending on the area of the face – deviations between 0.95 and 3.55 mm. Especially, soft tissue areas as lips, cheeks or tip of the nose as well as the lower face area showed marked differences. Areas with less soft tissue mass which could be shifted showed less deviations, e.g. forehead (1 mm) or nasal base (1.4 mm) [17].

An even better scanning quality can be achieved if a robot arm is used (Fig. 15.5).

In Fig. 15.6, both plaster cast scans (manual and robot arm) seem to be smooth, but looking more precisely, the ear of the manually scanned patient is not captured in its complexity and has to be recaptured.

15.4 The Laser Scanning for Oral Surgical Planning and for the Assessment of Facial Swelling After Oral Surgery

Jung et al. compared four intraoral scanners. One of them was a laser scanner (iTero; Cadent, Align Technology, San Jose, CA, USA). They compared the accuracy using models with buccal brackets and orthodontic wires where they found that the scanning results acquired by iTero were among the most reliable ones.

Buccal brackets and orthodontic wires were not clinically significant on the 3D image [18]. The same scanner was used in a study, comparing different intraoral scanners as well as a lab scanner regarding the accuracy of scanbodies on dental implants. If computer-aided design or a computer-assisted manufacture process for implant prosthetics is requested, the digitization of the scanbodies is necessary. The precision of the intraoral scanners decreased with increasing distance between the scanbodies in contrast to the dental lab scanner. Independent of the increasing distance, a continuous precision was observed [19]. All studies mentioned above were conducted with models. Lee et al. described the use of an intraoral laser scanner (iTero) for capturing the spatial position, surrounding tissue and a special scanning abutment of tooth implants in 36 patients. The acquired data were stored in STL format and used to produce a custom computer-aided design abutment and crown. Adjustments of contact and occlusion were required in six and seven patients, respectively, but the time for the adjustments was below 15 min [20]. Another study, evaluating data from 100 patients, was published by San José V et al. [21]. They were interested in comparing the reliability and accuracy of direct and indirect dental measurements using intraoral laser scanning (iTero), segmented cone beam computed tomography (CBCT) and a scanner for capturing plaster cast models. These results showed that both the iTero and the models segmented from CBCT are usable and accurate enough for dental measurements [21].

A different field is the evaluation of facial swelling after wisdom tooth removal. Before laser scanner and stereophotogrammetry became widely available , tape measurements were performed. Schultze-Mosgau et al. in their study about ibuprofen and methylprednisolone described how to measure the facial swelling by tape and ultrasound. They took distances from the lateral corner of the eye to the mandibular angle, from tragus to the outer corner of the mouth and from the tragus to the pogonion which they marked with a water-resistant pen [22]. The use of an optical 3D scanner (FaceScan3D; 3D-Shape GmbH, Erlangen, Germany) for volumetric measurements of facial swelling is described by Rana et al. [23]. They assessed the swelling after the use of different cooling mechanisms. For calculation of the volume, the patients were photographed while looking into a mirror with standard horizontal and vertical lines simulating a red cross marked on it. Harrison et al. [11] described the use of a laser scanner for the assessment of facial scanning. They evaluated a handheld laser surface scanner (FastSCANTM; Bruker Corporation, MA, USA) for the assessment of postoperative facial swelling. In their study, 20 patients were scanned before and 2 days after wisdom tooth removal. They detected the variability of the position as a source of inaccuracy. Otherwise, they found that the FastSCANTM is a simple, accurate and non-invasive method for the measurement of soft tissue volume changes.

15.5 The Laser Scanning of Malformations

Depending on the kind of malformation, a huge variety of 3D deviations to “normal” anatomy is seen. Just looking at cleft lip and palate patients, the cleft can vary widely. Lips with strong pouting and strong vermillion are seen, but it is also possible that the columella is short in combination with a severely contorted alar wing. Lip length differences also show a broad spectrum of variation, and so does the philtrum. For all of these variations, the non-cleft side (in unilateral clefts) is taken as the normal anatomy. The surgeon aims to establish anatomical symmetry as far as possible [24]. The method of choice to perform the closure of the cleft varies between surgeons; therefore, a 3D tool for assessment of the surgical outcome is of high relevance in this field of maxillofacial surgery. It is also necessary to use standardized, reliable facial landmarks so a comparison is easier achievable. Literature about 3D facial landmarks is available [25, 26]. The identifications of landmarks need a certain amount of practice, but taking the same landmarks will show a learning effect and therefore higher accuracy in less time. For measurements, the patient’s presence is not necessary which is an advantage towards measurements taken directly/manually on the patient’s face.

Since operations in the field of facial malformation are also performed at an early age, the compliance of children is not always given. Mori et al. published in their laser scanning study that, from the age of 5 years, most children showed enough compliance for the scanning procedure [27]. Usable data sets of noncompliant babies can therefore only be captured by using sedation. A sedation for this purpose on its own cannot be justified. Scanning the babies just before the surgical procedure (see Fig. 15.1) is possible with minimal extra amount of time. The setup of the scanner can be done in the absence of the baby so that there is no loss of time regarding this aspect. In Fig. 15.7, you see the preoperative and 7 days postoperative pictures of a patient with a unilateral cleft lip. Figure 15.8 shows the belonging 3D laser scans. For these scans, the Vivid 900 (Konika Minolta Co., Tokyo, Japan) was used. In this case, only one scanner was built up. If only one scanner is available, for a complete 3D view, two separate 3D data sets, one from each side, have to be captured and fused. The Vivid 900 parameters are as follows: resolution, x = 0.17 mm, y = 0.17 mm and z = 0.047 mm; input time, 0.3–2.5 s; and data size, 1.6–2.4 MB, mobile system (11 kg).

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