Smooth vs rough crown preparations

I mentioned this briefly in a previous post but I thought that I would expand on the point further. The argument is: "is it better to have smooth crown preparations or rough ones." The answer to this is relevant as it dictates what steps we take after rough preparation is done and to what level we take the polishing of our preparations.

I don't think anyone is debating the usefulness for macroscopically rough and bumpy preparations as these are likely to incorporate undercuts in the walls of the prep, lead to thin areas in porcelain which greatly increase the crack potential and generally don't look good. However, it can be argued for a neat preparation that is microscopically unpolished and rough to potentially increase the microscopic retention as well as the surface area of the cement or bonding resin.

In any case, rough bumps and imperfections in your preparation can't be polished out with discs or polishing cusp. The preparation needs to be pretty much the want to want it to end up before you polish the surface smoother. This means your reductions need to be adequate, macroscopically smooth and even before you enter the polishing stage. This is because the polishing armamentarium doesn't remove much tooth structure but only changes the surface texture. To an extent, polishing discs can be used to round off sharp edges that can hinder the flow of impression material, cause difficulty scanning, cause internal stress points and can cause overmilling of restorations in these areas (where the drills aren't fine enough to reproduce the sharp internal corners.

Roughened surface texture of preparations increases the surface tension and makes it more difficult for impression materials and cement to flow over it and adapt to the surface. The benefit that some surface roughness will provide to the bond is unlikely to be greater than the benefit gained from good resistance form, a good impression and a good cementation technique. Most likely to will roughen the surface with sandblasting or pumicing before cementation anyway so it may be of the most benefit to smoothen your preps before impressing or scanning.

Indirect restorations are precision fit restorations and the adaptation of the surface of the restoration and the tooth are very important for cement adhesion. I liken the relationship to the glass slides we played around with in microbiology. The glass slide had a drop of water placed on it and a small glass circle was put on top of this. The smooth nature of both glass plates resulted in good wetting of the surfaces and the glasses adhered together very well purely through the power of surface tension. In a similar way, crowns "adhere" to teeth by being well fitting against the tooth. A cement space is required between the tooth and restoration but this is filled with cement and the close adaptation of the two causes the restoration to retain against the tooth. Resistance form that is present in the form of preparation height and corners results in a lower stress to the cement layer in lateral forces.

To conclude, there is benefit in providing a smooth surface to your preparations before impressing. Ensuring your crown preparation is well reduced and ideally contoured and then proceeding to a polishing stage will ensure that your impression and final restoration have the highest chance of success.