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Digital registration systems III

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Preparation for contact point analysis

Tailoring of relevant information

Registration borders and scanned reference bows along with other interfering areas are not needed for occlusion analysis so they are removed. Only the occlusal surfaces of the teeth are left. The software program has different tools that allow data to be tailored and processed by mouse click.
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Contact points and gaps

Using the heights stored in the grid nodes, which represent the heightmaps of the maxilla and mandible, the difference can be determined and converted to the distance. In this way, the distance can be shown as a gap between the contact points.
Thus, not only the actual and direct contact points are given as contacts but even an interval of distance or approximation is given as early identification of a contact point. To a certain extent, this therefore involves contact point spaces or an area of increasing to maximum occlusal approximation.
The distances are color coded, with red or red-orange indicated the region of greatest approximation or smallest distance. Gaps or large interocclusal distances are marked green to blue.

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Gradient direction of the contact areas

Color coding of the gradient direction is also possible. It shows the direction in which the surface inclines. This allows simplified visual depiction of the lines of the surface.

Cusp inclination in contact areas

The gradient magnitude or inclination of the structures can also be displayed.

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Dynamic contact point analysis

Based on the contacts obtained in static recording (habitual intercuspation), all other contacts that occur with function are recorded by sensors. This shows the observer directly where and to what degree the antagonist pairs occlude dynamically (Ruge 2013).
To distinguish between the static contacts already recorded and the newly added dynamic contacts, static ones appear green and dynamic ones red (Ruge 2013).

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Envelope surfaces

"Envelope surfaces" arise from and characterize the spatial motion process of the mandible. Due to the three-dimensional offset of the mandible, a patient-specific surface envelope develops, which reflects just those motion sequences by means of the tooth structures. If the information obtained from the virtual envelope surfaces is compared with the tooth-guided motion patterns that are simulated on a mechanical articulator, it can be recognized that much more extensive envelope surfaces are demanded for the movements of protrusion and laterotrusion; that is, a lot of space is required for the movements but physiological masticatory sequences were recorded much more precisely and smaller differences were detected. Movements of the jaw and movements during mastication are not identical from the occlusal point of view (Ruge 2013).
As the number of mastication cycles increases, the envelope region expands until about half of the masticatory process has concluded. Additions to the envelope surface region occur in the further course of mastication until it is finished. When food is crushed, sweeping chewing movements can be observed in the early phase of mastication. With increasing crushing of the food, the chewing movements shift centrally again. No or extremely slight occlusal approximation took place until about halfway through the act of mastication because of the size of the piece of food.
From the recording, a patient's preferred side for mastication can be identified. If the patient favors a certain side, greater surface areas can be recognized on the ipsilateral working side. When the masticatory phase is nearing conclusion, when the food is very largely crushed, the masticatory movements again take up less space. Conversely, the teeth move more and more into occlusion.

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Accuracy of the 3D procedure

Conversion of the triangular facets to heightmaps, that is, conversion of a two-dimensional image to a virtual three-dimensional structure, is usually prone to errors and is termed discretization. An attempt is made to keep the error as small as possible and under control by selecting the highest possible resolution for the conversion process.
If the intrinsic motion of the teeth or torsion of the mandibular brace is being queried, the triangular facets and meshes should be left (Ruge 2013).
The possibility of determining volume difference is an important aspect of the use of virtual articulators. The volume measurement method offers numerous advantages throughout the dentistry field. For example, conclusions can be drawn regarding bone atrophy or increase (prior treatment, planning and follow-up in implantology).
Using heightmaps, analysis and measurement of a volume difference is relatively simple. The surface integral can be calculated in succession for each grid node affected by it. The accuracy of the difference measurement depends on several factors:
  • Accuracy of the 3D scanner
  • Accuracy of matching
  • Extent of the discretization error
  • The greater the difference between the situations being compared, the lower the precision after matching
The terms "accuracy" and "resolution" must be distinguished. It must be questioned critically if the manufacturer of a product reports an accuracy of 20 µm but the lengths of the edges of the triangles are a multiple of this distance. To what extent can or should corners be interpolated?
In addition, the quantity of data is sometimes so great that it simply has to be "slimmed down" so that it can be sent via the internet.
To examine the accuracy of a method, it is advisable to check the finished product, that is, the dental prosthesis. Tests of this kind were already performed and analyzed by intraoral scanner or with conventional methods.

Significance and prospects

The advantage of the virtual articulators compared with their mechanical equivalent is the possibility of directly recording and coupling dynamic motion sequences.
This method is used despite a lack of knowledge about the position of the joints. Nevertheless, it is important and useful to define reference structures and use them for orientation.
Recording the envelope curves, which represent functional occlusal approximation graphically, is an important diagnostic criterion for the dental technician's design of prosthetic occlusal surfaces. These aspects can be applied especially with regard to CAD/CAM restorations or in the treatment of CMD patients. The great advantage is that masticatory movements are recorded realistically with accurate details. At the same time, both the masticatory forces and the direction of mastication are also recorded not only topographically but also over time. These possibilities are not available to the conventional mechanical articulator.
Interest therefore focuses on the occlusal design of dental prostheses but the usefulness of a virtual articulator can be extended in some other ways. It is already possible now to record corresponding muscle activity using electromyography in parallel with jaw movement. Muscle voltage potentials can be recorded through electrodes attaches to the skin, which in turn permit conclusions about the activity of the temporalis or masseter muscles, for instance. This makes it possible to place muscle activity and dynamic occlusion into a single context. Combining high-resolution dental scans with radiographic data regarding bone structure into an overall picture has already been implemented in practice. This is put into effect, for example, in the area of implantology, by means of digital volume tomography and 3D images.
Another advantage concerns the access to information regarding bone structure. A familiar problem with conventional impression procedure and also with wide mouth opening during an intraoral scan is the intrinsic mobility of the teeth on the one hand and, on the other hand, deformation of the mandibular brace. If bending of the mandible has taken place, it will no longer fit exactly with the opposite jaw, which is noticeable especially in the molar region. These situations could be simulated on the computer and deviations could be corrected, which ultimately helps to improve the quality of a dental prosthesis.


sources

  • Brawek PK, Wolfart S, Endres L (2013) The clinical accuracy of single crowns exclusively fabricated by digital workflow - The comparison of two systems. Clin Oral Invest 17:2119-2125
  • Ender A, Mehl A (2011) Full arch scans: conventional versus digital impressions - An in- vitro study. Int J Comput Dent 14:11-21
  • Luthardt RG, Loos R, Quaas S (2005) Accuracy of intraoral data aquisition in comparison to the conventional impression. Int J Comput Dent 8:283.294 PMID:16689029
  • Richter EJ (1999) In-vivo- Messungen zur Unterkieferdeformation und Konsequenzen für implantatverankerte Suprastrukturen. Schweiz Monatsschr Zahnmed 109:116-126
  • Ruge S (2013) Zur Quantifizierung der funktionellen Okklusion – Entwicklung spezieller Analyseverfahren für den Einsatz digitaler Technologien in der Zahnmedizin. Dissertationsschrift der Ernst-Moritz-Arndt Universität, Greifswald
  • Seelbach P, Brueckel C, Wöstmann B (2012) Accuracy of digital and conventional impression techniques and workflow. Clin Oral Invest 17:1759-1764