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International Journal of Computerized Dentistry 2/2005, P. 169-178.

The Prefabricated Anatomical Polychrome CAD/CAM Crown for the inLab System

Ulrich R. Mönkmeyer, Frank Poerschke, Andreas Kurbad, Kurt Reichel, Volker Scharl

 
Keywords
prefabricated crown blanks, semi-finished product, CAD/CAM crown, plastic jacket crown, layering-induced esthetic effect, Cerec, inLab.

Summary

An innovative extension of performance in the area of single crown production is presented. Prefabricated crown blanks with color layering are inserted in the row of teeth with specially developed software and adapted apically to the stump with a CAM milling program.


Introduction

The first CAD/CAM-produced crown was presented by Francois Duret1 in 1988. In 1990, Jef van der Zel2 presented with the Cicero system the idea of a layering technique, which sintered the layers on in individual steps for mechanical processing. Various authors see the present status of CAD/CAM technology in the production of copings, which are machined and veneered by traditional methods.3-5 The greatest advantage consists in the employment of new materials, especially zirconium oxide.6,7 The standardization of production processes and their cost effectiveness are accentuated differently by different authors for the different systems, but play a secondary role.3-7
The few documents and publications of cases in which complete restorations, especially crowns, were produced with CAD/CAM methods show that this indeed is and remains the exception, apart from the large number of documented inlays and partial crowns produced with the Cerec system.8 The entire process of crown production also remains highly interactive and thus cost-intensive with CAD/CAM methods.9


Basic principle of the artegral crown

The artegral ImCrown developed by Merz Dental (Lütjenburg, Germany) is a prefabricated, anatomically formed, color-layered and characterized crown made from a plastic material (Fig 1). It is not milled out from the block. It merely requires apical adaptation to the stump.

Fig. 1: The anatomical, polychrome crown is dimensioned sufficiently in the marginal region and available in five different sizes for the left and right incisors as well as different sizes for the canines for bilateral use.

The crown is inserted and harmoniously aligned digitally in the row of teeth and with the occlusion with the aid of the software for the Cerec 3D and inLab (Sirona, Bensheim, Germany).
The suitable size is selected by the software from the five predetermined crown sizes, and the dimensional changes made by the user are implemented. The minimum material thickness as well as the esthetic effect induced by layering are taken into account. Virtual modeling is not required, since the form of the crown is already prefabricated.
The artegral ImCrown has a natural labial and palatinal surface design. It can be inserted immediately after polishing. The canine crown blanks for bilateral use as well as different left and right incisal crown blanks make up a small assortment, so that one has at hand the correct solution for the restoration of the maxillary anterior teeth. Because of the prefabricated surface contour, interacting with the polychrome layering, the artegral ImCrown is endowed natural esthetics and refraction of light, which moreover can be modified manually with little effort and can be further individualized.

Step-by-step procedure – CAD/CAM methods

A duplicate model (scan model) of the master model produced by the customary method is produced from scannable plaster. This also reproduces the neighboring teeth apart from the stumps to be processed. Scanning is in the "Crown preparation"” mode. The inEOS scanner can be used as an alternative. In this case, the master model can be scanned directly.
After scanning, the three-dimensional image of the model segment can be viewed on the monitor (Fig 2). The insertion axis is determined first. This step must be given the greatest attention, because the internal fit of the crown can be influenced by it. The neighboring teeth are then trimmed virtually. In this way, we obtain an unobstructed view of the working stumps (Fig 3). The bottom line is determined semi-automatically on the working stump by tracing it with the cursor. The best possible tooth is proposed independently by the Cerec 3D software and inLab, and inserted (Fig 4). A few design steps are then sufficient to integrate the crown in the row of teeth with regard to alignment and size (Figs 5 to 10).

Fig. 2: The scanned model can be turned by 360 degrees. Fig. 3: The preparation margin is marked semi-automatically in the usual manner.
Fig. 4:The software proposes the crown in the correct size.
Fig. 5: The crown axis is aligned from the labial view. Fig. 6: The crown is shifted mesial-distally, so that a physiological, proximal contact can be created.
Fig. 7: The crown is not yet aligned in the dental arch. Fig. 8: From the incisal view the crown can be integrated into the dental arch; in this case, one can also tilt it in the direction of the labial view.
Fig. 9: From the lateral aspect, both the correct inclination as well as the correct position of the color layering can be corrected by shifting in incisal or cervical direction. Fig. 10: After releasing, one can assess whether the proximal contact is sufficient

Fitting

Following the milling process (Figs 11 and 12), the crown is finished manually and purely subtractively with regard to its fit. As a rule, the fit is perfect after finishing. The proximal contacts are adjusted working from the cervical and incisal, and a contact foil should be used for this purpose.

Fig. 11 and 12: View of the crown directly after the milling process.

Adapting the functional surface

The incisal edge is best shortened to the approximate length using a silicone rubber wheel (Fig 13). The palatinal functional surface is produced concavely with a large round bur underneath the abrasion surface. The marginal ridges are also created in this way (Fig 14). The occlusion is checked and ground in by centric and excursion movements with a white silicone rubber wheel and using occlusion foil. The ridges are then drawn on the neighboring tooth, and the desired ridges are transferred to the crown. The desired ridges are then ground with a thin diamond conical bur or a correspondingly finely cross-toothed cutter (Fig 15). The labial concavities are then drawn and created with the same tool. The surface texture is individualized with a sharp-edged stone or a diamond grinder.

Fig. 13: The incisal edge length is corrected with a rubber wheel. Fig. 14: The palatinal functional surface is individualized with a few corrections.
Fig. 15: The "face" of the tooth is determined by the correct application of the ridges.

Surface finish

The surface is leveled with a plastic brush (Figs 16 and 17). This is followed by minimally abrasive rubber polishing of the abrasion surfaces and smoothing exposed places (eg, with a "Brownie"” [Shofu, Ratingen Germany]). Preliminary and high-gloss polishing is performed best of all with a goat-hair brush and polishing paste. The final result is achieved within a few seconds (Fig 18).

Fig. 16: The surface texture aimed for is drawn and "copied"”. Fig. 17: The surface is very quickly given the required microstructure with a plastic brush.
Fig. 18: The finished work on the model shows a result which can be achieved with no other method in such a short time

Cementing

The artegral ImCrown can be provisionally cemented with all customary provisional adhesive materials, but preferably with artegral T-Cem. Final adhesion is accomplished with the enamel/dentin adhesive technique. For this purpose, the one-component product artegral One is used on the moist tooth surface after light curing as a basis for the dual-curing cement artegral Cem.

Case report

A 57-year-old male patient came to the practice because of symptoms and increasing loosening of teeth 21 and 22. The radiograph displayed a transverse root fracture in both teeth. The cause given in the case history was a sports accident six weeks previously (Fig 19). The fractured teeth were removed carefully (Fig 20) and an immediate implantation with Straumann TE implants (Straumann, Basel, Switzerland) was performed. Thanks to the high primary stability achieved because of the good bone structure and the special design of these implants, delayed immediate loading was possible. An impression was taken and a model produced one week after the implantation.
This was modified to achieve an optimum emergence profile. Two RN syn- Octa abutments for temporary restorations (Straumann) were modified corresponding to the anatomical conditions (Fig 21). After the model situation was scanned, two artegral crown blanks were fitted virtually by means of the Cerec inLab CAD/CAM program (Fig 22). The crowns were adjusted manually and polished after milling (Fig 23).
Only 24 hours after taking the impression, it was possible to insert the restorations for delayed immediate loading, thanks to the fast CAD/CAM production (Fig 24).

Fig. 19: Initial situation: Condition after a sports accident 6 weeks previously in a 57-year-old male patient. Fig. 20: Fractured teeth 21 and 22 were carefully extracted.
Fig. 21: RN synOcta abutments for temporary restorations (Straumann, Basel,Switzerland) are modified corresponding to the anatomical conditions for delayed immediate loading. Fig. 22: Artegral blanks can be adapted basally to the implant situation with the aid of the Cerec CAD program (Sirona,Bensheim).
Fig. 23: Apart from the exact seat on the implant shoulder, modeling an adequate emergence profile is important. Fig. 24: The finally adapted and polished artegral crowns are ready for insertion.

For this purpose, the abutments were first screwed in and then the crowns were bonded with a provisional cement. The restoration (Fig 25) is designed so that it is not functionally subjected to large loads, but shows a very good esthetic effect (Fig 26).
The advantage of the procedure with CAD/CAM-supported use of artegral crowns is that the crowns can be adapted perfectly to the clinical situation. Production can be undertaken very quickly. In this case, in contrast to previously produced provisional shell restorations, no inconvenient modifications or relining with poorly tissuecompatible temporary plastics have to be performed. On the other hand, taking an impression of the optimum emergence profile early on makes it possible to conserve and support the soft tissue. This means a considerable advantage for the later definitive restoration. The patient enjoys the best possible comfort, good periodontal hygienic conditions and a very good esthetic effect by dispensing with an interim prosthesis.

Fig. 25: After already largely completed wound healing, the RN synOcta abutments are screwed in. Fig. 26: After cementing with a provisional adhesive material, the patient has a functional restoration with optimum esthetics for the entire duration of the healing phase.

Discussion

Today, the plastic jacket crown produced in the laboratory is scarcely considered the standard restoration because of its material disadvantages – hydrolytic degradation, insufficient abrasion properties etc. The dental material IPN used in the artegral crown is plaque resistant, abrasion proof, and temperature resistant, and is characterized by its ability to be machined very well. In this way, as well as due to quality-assured, industrial processing methods, the relevant physical parameters of the material and the adhesive bond between crown and tooth are clearly improved, which may have a positive influence on the long-term prognosis.
Since the working method for Cerec 3D and especially for Cerec inLab users is very economical, it is possible that any potential disadvantages concerning the dwell period is compensated. Various studies already in progress evaluate the economic advantages (time requirements), material property advantages, and product quality advantages.
Currently, crown blanks are available for teeth 13 to 23 (#6–11). Work is being done on experimentally extending the range to all teeth and on using further materials.
Experiments with an innovative material containing nanofluorapatite, which contains an enamel and crystal structure identical to nature in a polymer network, are very promising.
Experimental work is also being done on ceramic blanks which can also be processed additively. Necessary studies for the use of these materials have not yet been completed. The experiments show that their use in the CAD/CAM production of crowns would bring considerable economic advantages. Crowns produced in such a way would certainly not worsen the average quality standard of crown restorations.

References

  1. Duret, F. et al.: CAD/CAM in Dentistry. J Amer Dent Assoc 117, 715-720 (1998).
  2. Zel, J. M. v. d.: Elephant Symposium, Zandfort (1990).
  3. Witkowski S. Computer Integrated Manufacturing als Konzept für das zahntechnische Labor. Quintessenz Zahntech 2002; 28:374-386.
  4. Luthardt R, et al. Aktuelles CAD-CAM Systeme zur Herstellung von keramischem Zahnersatz. Teil 1. ZWR 2001;110:747- 754; Teil 2. ZWR 2001;110:797-802.
  5. Kern M, et al. Neue Perspektive in der Zahnheilkunde? Quintessenz Zahntech 2002;28:1244.1250.
  6. Sudhoff C. Zirkonoxid als Werkstoff für die Implantat- und Kombitechnik. Quintessenz Zahntech 2003;29:730-740.
  7. Langschwager A. Die zirkuläre durch Geschiebe verbundene cercon-Unterkieferbrücke, Quintessenz Zahntech 2003; 29:262-271.
  8. Jedynakiewicz N. Something of a Paradox. Int J Comp Dent 2004;7:223-224.
  9. Kerschbaum Th. Behandlungsbedarf mit Zahnersatz bis 2020. Quintessenz Zahntech 2001;27:810-815.

Address

Ulrich R. Mönkmeyer
C./Sa Cantera 11
07660 Cala D’or
Mallorca, Spain
Phone: +34 971 648 157
E-Mail: d.c.moenkmeyer@terra.es