As a Dental Professional you already have detailed knowledge about the types of dental materials available today and their properties and clinical preparation procedures to be followed when using them. Today, different types of dental materials are available for the CAD/CAM technique and there can be fundamental differences between the new and the traditional manufacturing methods. Understanding how restorations are being milled is not just the domain of the dental technician, but the potential limitation of the prepared tooth which is controlled by the clinician.
Preparation Techniques – Understanding the differences achieved between four or five axis’s, wet or dry, chairside or laboratory milling systems is very important. Putting this knowledge into daily practice with simplified predictable preparation techniques relates to long lasting planned treatment and patient care.
Surface finish of the tooth – Firstly, an excellent tooth preparation and secondly, the predictability of results are very important success factors when restoring teeth especially in the aesthetic zone. Though in most daily dentistry cases, a simple re-design of the shape and contour of the tooth can be achieved utilising different types of rotary instruments.
There are numerous papers written discussing the effect of surface roughness to the effect of cementation and bonding techniques 1, 2, 3, 4. This will not be discussed but knowledge of cementation types and techniques is vital to the longevity of the restoration. So, consideration should also be given into the tooth surface morphology and surface roughness when considering the milling or grinding strategies of the various milling units currently available in the market.
Tooth Shape: Preparation and Contour Considerations
Preparation (rotary) Instruments – There are multiple manufacturers of rotary instruments such as Intensiv, Komet, Premier, etc. Within these companies there are multiple kits and instruments available for the various techniques that you may subscribe to. Consideration should be given to the size, shape and grit size for the preparation and finishing of the tooth surface. Table 1 identifies the various grit sizes available and the variation within the grit sizes (manufacturing preferences). There will a clear difference in what the tooth surface morphology can be achieved with selective use of shape and grit size.
|Diamond Burs colour coding and grit size|
|Bur Type||Colour||Grit Size||ISO No.|
|Super coarse||Black ring||181µm||544|
|Medium||No ring or Blue||107-126µm||524|
Finishing the prepared tooth with at least a red banded instrument provides for a smoother surface finish but does also provide another benefit in the field of milling units. As a guide the surface finish for most four and five axis milling units around the 20-30µm, +/- 10 µm. Preparations finished with a medium grit bur are in fact rougher than what a milling unit will generally provide for on the fitting surface.
Examples of tooth preparation technique (Fig’s 1, 2, 3, 4, 5) and the importance of understanding the parameter settings influence (internal software rules and guidelines (Fig’s. 6, 7) within each CAD system. Parameters are a series of rules that control how the fitting and external surfaces of the restoration to be milled. Parameters influence the fit of the restorations along with the prepared tooth contour and finish.
Situations exist for Crowns, Inlay’s, Onlay’s, Veneers and Bridges where it is possible to record undercuts with multiple images, but a machine (4 axis) will generally not mill (build) ceramic or wax into the undercut area. A dental laboratory will normally block out this area with wax or in the digital world, block out utilising software in the parameter settings. You need to be able to remove a wax pattern or restoration from a stone die or on the prepared tooth if being completed chairside.
Rectify situations like the one shown in Figure 1 by simply changing the path of insertion in software or adjusting parameter settings are an important factor to ensure successful results when working with all CAD/CAM systems. In Figures 2-5 the milling or grinding burs will not remove or in some cases leave behind material as there is either an undercut, rough surface, ill-defined margin (Fig.2), not enough room for the burs (Fig.3 mesial or distal), a gutter on the margin (Fig.4, J-curve) or a sharp internal line angle (Fig.5) will generally not be able reproduce such an acute angle. Some milling systems offer 0.3mm cutters that maybe able reproduce the shape. Most chairside (wet) systems only utilise 1.0mm grinding burs. So in preparing the tooth the rotary instruments should be similar in size to the milling or grinding burs being utilised.
Milling or Grinding Considerations
The CAM processing involves the milling of various materials whether it be glass ceramics on stub through to Zirconia, Wax, PMMA, CoCr and PEEK materials on a disc in either four or five axis milling and grinding units. The most efficient way of constructing a restoration and the importance of the insertion axis, the design of the preparation margin and contact point design are all taken into consideration when milling. The milling process is the last step in the creation of any type of restoration based on the virtual model. To improve the results, it is important to use the right instruments for milling or grinding. Moreover, the selection of the right material greatly influences clinical success.
Each milling unit whether it be 4 or 5 axis has options for different sizes of either milling or grinding burs (Fig. 16). There are various rotary instruments available for either chairside, bench top laboratory (wet or dry) or manufacturing centres depend on the type unit used. Looking at the big picture there are a multitude of rotary instruments ranging from 0.3 – 20mm. Within this group there are multiple shaft lengths and cutting shapes that will be specific to the 4 or 5 axis wet or dry milling unit. Manufacturers optimise the type of milling or grinding strategies for different materials and burs within their respective CAM programs. Check to see what the manufacturer offers. There is no one size fits all.
The various types of materials and designed restorations that can be milled is dependent on the following;
1 Size of milling chamber. This generally relates to materials that are attached to a milling stub such as the chairside systems of Dentsply Sirona, Planmeca or laboratory bench top or floor mounted units (manufacturing centres) from companies such as Roland DG, Wieland, imes-icore, Amann Girrbach or DMG Ultrasonic 20. These units generally utilise a 98.5mm disc of material.
2 The Milling Axis for 4 and 5 axis units (Fig. 17 5 Axis milling) achieved in milling include X, Y, Z and the A and B axis. These axis are very important especially when considering implant or large span restorative. The B Axis is the critical axis to consider.
a. In 4 axis milling units generally the B axis may only rotate 15°. This will be a limiting factor especially with milling bridges, surgical guides and potentially implant cases. Think undercuts and size restoration as the limitation. These milling units are generally for use in milling single units and up to 3-4 unit bridges.
b. In 5 axis milling units the B axis can range from 20° to 35°. Clinically think of a full arch restorations (implant bars), V- shaped arch or lower mandible where the first molar is missing and the second molar has rotated towards the mesial and the 2nd pre-molar has also moved. In this case the height (Z value) will come into play and if there is not enough rotation in the B axis you may not be able to accommodate a restoration.
3 Milling or Grinding burs length, types and diameter. As a guide 4 or 5 axis milling burs have varying lengths, types and diameters. The length of the bur utilised in the milling unit can be seen as limitation of what can be milled. In essence the more you pay for a milling unit the more materials and types of complex restorations can be fabricated.
a. There is a variety of instrument types and sizes for milling or grinding burs specifically designed to mill PMMA, Wax, Zirconia, Glass Ceramics, Hybrid Ceramics, PEEK and CoCr. So consider carefully when selecting the material to be milled and check with the manufacturer as to what is available for the specific material. It’s not just pressing the button to see what happens.
Step By Step – Simple solutions
So what does all of this mean in the real world when a tooth is being prepared? In some cases software can assess the type of preparation relative what is possible with different types of materials and CAM systems. How can you overcome some of these situations with simple techniques?
1 Firstly review the preparation instruments you are using. Are the diamond or tungsten instruments smaller diameter than what you are using either in chairside milling unit or the milling unit being utilised by the laboratory or manufacturing centre.
2 The tooth surface finish should be smooth with rounded internal line angles and at minimum finished with a red banded diamond instrument. An ideal surface finish should be with either yellow band diamond, or maybe even a white stone.
3 External line angles with inlays and onlays should be at 90° to the extension. The Cavosurface margin should be well defined. Consider the Reciprocating handpiece and respective files (available from multiple manufacturers) with one side for cutting and the other side is safe sided (Fig. 14, 15). The handpiece and blades are also ideal finishing and polishing after cementation to remove an extra cement at the margin inter-proximally.
4 Either chamfer or shoulder margins depending on what material (restorative) and the cement protocol chosen. Potentially utilise preparation diamonds with guide pins. The advantage offered by a guide pin is that offers to protect the gingiva during sub-gingival preparations and an even preparation margin avoiding the gutter effect. These are available from Intensiv and Komet (Fig.13).
5 Ask your dental laboratory what is the standard milling instrument size they use?
Recognising that the dental professional have quite different and distinct needs, DDC offers a consultancy or partnership model which allows you, the dental professional, to extract from DDC the support and advice that is appropriate and most useful for you. More information is available via DDC Professional Development Workshops and lectures. A PDF copy of the article is available via firstname.lastname@example.org. Conversations never hurt and often fantastic results occur when we put our heads together and collaborate!
Lookout for the next article – Digital Dentistry – Perceptions and Misconceptions | Considerations in choosing your CAD/CAM or Digital Solution.
1. Ayad M.F., Rosenstiel S.F., Salama M. – Influence of tooth surface roughness and type of cement on retention of complete cast crowns. J Prosthet Dent 1997;77(2): 116-121.
2. Ogata M., Harada N., Yamaguchi S., Nakajima M., Pereira P.N., Tagami J. – Effects of different burs on dentin bond strengths of self-etching primer bonding systems. Oper Dent. 2001;26(4):375-82.
3. Semeraro S., Mezzanzanica D., Spreafico D., Gagliani M., Re D., Tanaka T., Sidhu S.K., Sano H. -Effect of different bur grinding on the bond strength of self-etching adhesives. Oper Dent. 2006;31(3):317-23.
4. Yiu C.K., Hiraishi N., King N.M., Tay F.R. – Effect of dentinal surface preparation on bond strength of self-etching adhesives. J Adhes Dent. 2008;10(3):173-82.
5. Intensiv show a number of clinical cases utilising various grit sizes and tooth contour on their website http://www.intensiv.ch/cat-casoclinico/restoration/.
6. Fig. 13 courtesy Dr Andreas Kurbad, Viiersen, Germany.
7. Fig. 14 Clinical case from Dr S Giani, Varese, Italy. Interproximal finishing with Intensiv Swingle and Intensiv Proxoshape PS2. http://www.intensiv.ch/prodotti/intensiv-swingle/.
8. Fig. 15 Intensiv Proxoshape PS2. http://www.intensiv.ch/prodotti/intensiv-swingle/.
9. Fig. 16 Milling and grinding burs, are from imes-icore, Planmeca, Dentsply Sirona and VHF.
10. Fig. 17 courtesy Roland DG. 5 Axis diagram.