Where should the gingival margin of a crown be placed?

Porcelain Jackets on Maxillary Centrals after over ten years in the mouth.

This seems to be a hotly debated topic among dentists. There is no simple, one size fits all answer. It depends on the strength of the tooth structure remaining after tooth preparation, the type and design of the crown, the oral hygiene of the patient and degree of caries risk.

In a perfect situation crown margins should be placed either supragingivally or at or very slightly below the gingival margin (.5 mm or less). Well fitted crown that don't go deeper tend to cause minimal gingival irritation and often demonstrate no bleeding on periodontal probing. That being the case, there are reasons to advocate a more subgingival margin. Patients with higher caries risk ( high sweet consumption, acid reflux, bulimia, dry mouth, poor brushing technique) can experience recurrent decay around exposed crown margins.  All things being equal, margins placed more subgingivally are probably at less risk for recurrent decay, since they are not exposed to as much plaque that can cause decay.

Another reason for placing margins subgingivally is the presence of a metal collar at the terminus of a ceramometal crown that uses a shoulder bevel combination. Exposed metal collars are just plain ugly and no dentists wants to insert a brand new crown with a grey line visible at the gingival margin.

Traditionally, shoulder bevel preparations have been employed partly to help ensure of an intimately fitted margin with minimal gap between the casting and the tooth. This was considered important because most traditionally employed dental cements ( acid-base salt reactions) were slightly water soluble and if a gap between a crown and the tooth was too big crowns experience cement washout and a higher risk for recurrent decay. Now with the increase use of glass ionomer resin modified crown cements , small differences in the marginal gap are somewhat less important, since these cements are less water soluble and not prone to wash out when exposed to the oral environment.

Many dental schools routinely teach chamfer finish line for crowns ( gingival margin or a half mm supragingival. ) This prep provides "one size fits all" finish line, good for all types of crowns made from all sorts of  materials (ceramometal, emax, full cast metal, and zirconium).  . I guess its easier teaching students one standard preparation and then have them use either a composite or resin modified composite cement for luting.

 Optical  scanners are  being employed to digitally create impressions for crowns and it is recommended that finish lines to be located close to the gingival margin since optical scanners must be able to see the finish line to create an accurate impression. Retraction cord can be placed prior to help  the scanner capture the finish line in the digital impression. 

That being said, the truth is that some teeth really benefit having a crown with a bevel. When there is a need to go deeper than .5 mm below the gingiva, bevels can help preserve the remaining tooth structure since they don't require as much tooth preparation. Also the chamfer or shoulder when used in conjunction with a bevel, can be placed approximately at the gingival margin even when the bevel is placed well below. This can be helpful in restoring teeth that have vertical cracks and need to have subgingival margins placed interproximally to help keep the crack from propagating further. Of course a chamfer can also be placed deeper subgingivally but it should not be a "deep chamfer" and should involve tooth reduction of no more than .5 mm ( my opinion) in order to better preserve the remaining tooth structure .

Another reason to venture subgingivally during a crown preparation is to both increase tooth ferule and also improve crown retention for short tooth preparations. This often the case for some posterior molar preparations since these teeth can be short and can benefit from the addition length for their crown preparations that subgingival preparations afford.

It should be kept in mind that subgingival preparations require a more exacting approach to taking impression and the proper use of retraction cord and lack of bleeding during the impression are indicated. The results of taking an inadequate impression are often visible on post op radiographs and show either over or under extended crown margins. If a sub gingival preparation is chosen then the clinician must leave enough time to obtain an adequate impression that shows a clear and definite finish line. With the proper use of hemostatic agents and a double cord technique for retraction, this can be predictably accomplished by a skilled clinician.

To sum up this discussion, the position of the gingival finish line of dental crowns depends on a number of factors and each case should be evaluated based on what is best for the patient and tooth involved. Tooth position, need for esthetics, length of the tooth, and caries risk should be factors that need to be considered. While for many a finish line at or slightly short of the gingiva is ideal, some teeth may benefit when margins are place more subgingival.

from Ask Dr. Spindel - http://lspindelnycdds.blogspot.com/2019/03/where-should-gingival-margin-of-crown.html - http://lspindelnycdds.blogspot.com/