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On 29 June 2015, the International Organization for Standardization, ISO, published a revised version of ISO 12836, the standard for assessing the precision of the scanning devices used in dentistry to create digital impressions for prosthetics. By precision we mean that a scanning device delivers the same digitized geometrical model of a test object every time within a given uncertainty in dimensions. Precision is the manufacturer’s responsibility.

The revision to ISO 12836 seems minor, but was necessary in order to restrict its scope to laboratory scanners and to exclude intra-oral scanners. Attempts to apply the previous version of the standard to intra-oral scanners in several laboratories, including NIOM’s partners in the Department for Prosthetic Dentistry at the University of Oslo, showed that the prescribed test objects were unsuitable. NIOM’s John Tibballs led the work to revise the standard.

There are now moves afoot to develop a standard for intra-oral scanners. The basic tenets of ISO 12836 for determining measurement precision are sound, so one possible approach is to expand its range of test objects to include full-arch models and materials with tooth-like translucency. However, ISO 12836 is restricted to defining test methods, placing no requirements on the scanner itself.

In working towards a standard for scanning devices, the ISO process has to decide whether the market is ready to develop a product standard. A product standard specifies minimum performance capabilities and the test methods for assessing them, and mandates the information that a manufacturer shall supply with the product.

The quantification of potential scanning inaccuracy could be such a performance capability.

Accuracy is how well any measurement actually agrees with reality, a factor not fully addressed in ISO 12836. The confines of the oral environment can lead to “dilution of precision” (DOP), a term from GPS navigation, which involves computations similar to those for a dental scanning device. Ultimately, of course, the accuracy of the digital impression is assessed when the dentist comes to place the prosthesis. Any inaccuracy discovered at this late stage is, of course, costly.

DOP quantifies the increased likelihood of inaccuracy in the digital impression. The clinician, who has responsibility for an impression’s accuracy, must counter DOP, either by performing extra scanning or by measuring key dimensions independently.

Intra-oral scanners are based on a variety of technologies that combine pattern recognition with photogrammetry, with interferometry or with confocal microscopy. Proprietary graphical interfaces are a key marketing feature. Manufacturers have largely mastered the techniques for achieving suitable geometrical precision, although scanning for complete-arch constructions remains a challenge.

A proposal to develop a new standard for inter-oral scanners applicable to all technologies is currently being voted on among ISO participants.

 


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