Dental Data Capture

The first step of every dental CAD/CAM system is digitizing the oral environment of the patient. For the framework application, the input geometry is twofold: the implant positions and the tooth set-up. No existing system can yet acquire this dental data directly in the mouth of the patient with high precision. However, emerging technologies and new research will put emphasis on developing accurate intra-oral scanners (Strub et al., 2006). In this way, manual interventions like taking impressions and making plaster work models could be avoided.

The position and inclination of each implant vary depending on the presence of qualitative and quantitative surrounding bone and on the anatomical location of nerves and blood vessels. The measurement of the position of the implants has to be very accurate with regard to the final fit between framework and each implant. Due to the complexity of the upper part of the implants and because implants are often installed deeply in the tissue, mechanical or optical techniques have difficulties to measure directly the position of the implants. Therefore the use of registration elements is preferred. Figure 7.4(a) shows for example cylindrical registration elements mounted upon the implant replicas of the work model.

These registration elements can be measured by contact digitizers or by optical scanners. There has also been tried to measure them by 3D Micro Computer Tomography (Tomohawk), but the outcome could not fulfill the required precision and this procedure is more relevant when cross-sectional images are wanted.

Contact digitizers are more accurate than optical scanners but the digitizing process is slower and less easy to automate. For a rapid and fully digital procedure, optical scanners are preferred. For one case with five framework-implant connections, the measurement results of five different optical scanning systems have been compared with the outcome of a contact digitizer. As contact measurement method a tactile probe (Renishaw) mounted on a Coordinates Measurement Machine (Mitutoyo) has been operated manually to measure the position and inclination of each registration element with high precision (<10 mm), as shown on Fig. 7.4(b).

Five different optical scanners (Table 7.1) have also measured these five registration elements. Two dedicated dental scanners and three scanners for general purpose were tested. The accuracy stated by the manufacturers varies from 20 to 40 mm for all five types1. An optical scan of the mounted

Fig. 7.4 (a) Registration elements; (b) Tactile probe; (c) Meshed scan of registration elements; (d) Meshed scan of tooth set-up (Source: K.U.Leuven)

Fig. 7.4 (a) Registration elements; (b) Tactile probe; (c) Meshed scan of registration elements; (d) Meshed scan of tooth set-up (Source: K.U.Leuven)

1 The accuracies stated by the manufacturers are not really comparable as they are obtained under different conditions and represent quite different quantities: e.g. 1a, 2a or 3a values, variation on individual measuring points or on geometrical feature size or position, etc.

Table 7.1 The five tested optical scanners with their manufacturer and measurement principle

Optical scanner

Company Measurement principle

Dental 3D Scanner D-250

Dental Laserscan 3D

Atos Small Objects 3D Digitizer

Optical probe LC50 (Coupled to 3D

CMM) 3D Digitizer VI-910

3shape Laser line scanning (2D triangulation) Willytec Laser line scanning (2D triangulation) GOM White light pattern projection (3D

triangulation) Metris Laser line scanning (2D triangulation)

Minolta Laser line scanning (2D triangulation)

registration elements typically leads to a point cloud of more than 300.000 points (Fig. 7.4(c)).

Using numerical algorithms the position and inclination of each registration element were computed. The midpoint coordinates of every implant top plane were compared with the ones obtained by tactile measurement. The maximum deviation in the direction perpendicular to the implant plane was 15 mm and within the plane 35 mm. Consequently, optical scanning can capture the implant positions with sufficient precision.

The measurement of position and inclination of the registration elements provide sufficient information to design the 'lower' part of the framework that fits on the implants. The precision of digitizing the tooth set-up is not very critical, in contrast to the position of the implants, because no major effect is stated with regard to possible biological or mechanical failures. Any optical scanner can digitize the tooth set-up (Fig. 7.4(d)). Since the data of the implant positions will have to be combined with the tooth set-up data, both measurements are mathematically matched with the work model as reference object.

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