How to use bur-diameter dimensions as guides to calibrate ceramic crown prep [VIDEO]

Dramatic advancements in dental ceramics have enhanced the esthetic, functional, and retentive potential of ceramic crowns to a point where metal alloys may be circumvented in esthetic dentistry.

Tooth preparation for ceramic restorations has likewise evolved to complement the unique properties of these innovative materials. Sharp line angles that are acceptable with cast metal-alloy based crowns result in stress points and compromised seating of ceramic crowns. Smooth and rounded contours should lead to a more precise fit of stacked, pressed or computer-milled ceramic crowns.

Lithium disilicate as thin as 0.3 mm, for example, has demonstrated acceptable compressive strength; therefore, tooth preparation can be more conservative. Al-Fouzan and Tashkandi have reported that as much 65% of coronal tooth structure by volume may be removed during conventional crown preparation.⁷ Edelhoff and Sorensen reported similar results: 63% to 72% coronal tooth-structure removal by weight.⁸

More conservative tooth-preparation depth needed for ceramic crowns can potentially further reduce the loss of tooth structure. Because it is etchable, lithium disilicate may be bonded to tooth structure, which is a benefit for short or compromised teeth.

Komet® Expert Kit 4573ST

Developed to meet the specific requirements of all-ceramic crowns produced with conventional or CAD/CAM techniques, the Komet® Expert Kit 4573ST contains a coordinated selection of matched diamond instruments to produce optimal tooth removal and ideal crown preparations.

Incorporating a tapered, round tip and coarse-grit diamonds, the kit’s key instrument (6856.FG.018) is designed to produce a distinct chamfer with rounded internal line angles. The instrument’s large radius and diameter (1.8 mm) help prevent lipping the preparation chamfer and yield smooth surfaces. In addition, its 2° taper allows an optimal preparation angle without tilting the handpiece.

In addition to the 6856.FG.018 diamond and the 6856.FG.016 diamond (for smaller teeth), the kit contains the coarse-grit, smaller diameter 6856.FG.012 diamond as well as the 6837KR.FG.012 coarse-grit diamond for initial shoulder creation; the 6836KR.FG.014 coarse-grit instrument for incisal reduction; and the 6379.FG.023 coarse-grit diamond for palatal reduction.

The kit also contains three fine-grit, finishing diamonds to refine the final preparation.

Anterior Crown Prep with Expert Kit 4573ST

1. A 30-year old female patient presented with a fractured maxillary right central incisor that had previously been restored with a composite-resin restoration.

2. Upon clinical and radiographic evaluation, a ceramic crown restoration was selected for retreatment.

3. Prior to tooth preparation, shade selection was accomplished and communicated to the laboratory through scripted digital images.

4. The Noritake New Color Guide (Kuraray Noritake Dental, Inc., Tokyo, Japan) was used to customize the shade.

5. Proximal tooth surfaces were first prepared with the thin diamond bur (6856.FG.012).

6. Care was taken to avoid inadvertent damage to adjacent tooth structure.

7. Instrument 6837KR.FG.012 was used to prepare a uniform shoulder just above the gingival margin.

8. Following the curve of the facial contour, the labial surface of the tooth was reduced between 0.5 and 1.0 mm, increasing the depth approaching the incisal edge.

9. Relatively equal gingival circumferential tooth reduction is seen in an occlusal image.

10. Incisal reduction between 1.5 and 2 mm was completed with the same 6837KR.RG.012 instrument.

11. Following initial preparation, a Natural Die Shade (IPS Natural Die Material Shade Guide, Ivoclar Vivadent Inc., Amherst, NY) was selected and photographed.

12. Palatal reduction of approximately 1 mm was completed with the egg-shaped diamond instrument 6379.FG.023.

13. Knit-Pak™ size 0 retraction cord (Premier Dental Products Co., Plymouth Meeting, PA) soaked in aluminum chloride was placed to protect the gingiva, and a 0.8 mm margin was carried down to the top of the cord with the 6856.FG.021 diamond.

14. The final preparation was refined with matching finishing diamonds; in this case, the 8856.FG.018 diamond bur was used.

15. With the egg-shaped, fine-grit instrument (8379.FG.023), the palatal surface was finished.

16. Adequate and even tooth reduction was checked and confirmed with a silicone matrix.

17. To complete the preparation, all edges and line angles were rounded with Komet® flexible polishing discs.

18. A smooth and rounded preparation was completed with no damage to adjacent teeth and no gingival trauma.

19. An IPS e.max® lithium-disilicate crown (Ivoclar Vivadent Inc., Amherst, NY) was completed.

20. The new crown was seated on the maxillary right central incisor, and the composite-resin restoration on the mesio-incisal edge of the left central incisor was replaced for a more natural color transition.

21. A precise fit was confirmed on a post-seating radiograph.

Conclusions

Bur kits designed for calibrated tooth reduction and innovative ceramics with strength in thinner layers aid clinicians in meeting their responsibility of conservative tooth preparation that maintains the structural and biological integrity of the prepared teeth.

These burs also aid in producing rounded line angles, optimal preparation convergence and well-defined chamfer margins that are ideal for both zirconia and lithium-disilicate crowns.

References

1. Christensen GJ. Is the rush to all-ceramic crowns justified? J Amer Dent Assoc 2014:145(2):192-194.
2. Valenti M, Valenti A. Retrospective survival analysis of 261 lithium disilicate crowns in a private general practice. Quintessence Int 2009;40(7):573-579.
3. Dhima M, Paulusova V, Carr AB, Rieck KL, Lohse C, Salinas TJ. Practice-based clinical evaluation of ceramic single crowns after at least five years. J Prosthet Dent 2014;111(3):124-130.
4. Dhima M, Paulusova V, Carr AB, Rieck KL, Lohse C, Salinas TJ. Practice-based clinical evaluation of ceramic single crowns after at least five years. J Prosthet Dent 2014;111(3):124-130.
5. Ozer F, Mante FK. Chiche G, Saleh N, Takeichi T, Blatz MB. A retrospective survey on long-term survival of posterior zirconia and porcelain-fused-to-metal crowns in private practice. Quintessence Int 2014;45(1):31-38.
6. Fabbri G, Zarone F, Dellificorelli G, Cannistraro G, De Lorenzi M, Mosca A, Sorrentino R. Clinical evaluation of 860 anterior and posterior lithium disilicate restorations: retrospective study with a mean follow-up of 3 years and a maximum observational period of 6 years. Int J Periodont Restorative Dent 2014;34:165-177.
7. Al-Fouzan AF, Tashkandi DA. Volumetric measurements of removed tooth structure associated with various preparation designs. Int J Prosthodont 2013;26(6):545-548.
8. Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87(5):503-509.