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Periodontics/Removing 5 mm (in height) bone in crown lengthening: is that too much ?


QUESTION: My lower L. molar at the back (#7) was badly damaged, had root canal treatment and filling and was then fitted with a crown in 2009. In 2012 this crown started to come off very often. Each time it came off it was recemented, but the last time on 24/12/2014 it was swallowed by mistake while eating.

This molar is very very short, only ~ 1 mm above the gum line. I have been thinking about having crown lengthening surgery before installing a new crown.  I saw a periodontist and he said that he would have to remove 5 mm (4 mm ? I don't remember) depth of bone surrounding this tooth in this surgery. Without using any special instrument, I estimate that my lower jaw bone beneath this molar is about 1 inch (2.54 cm) high only.

Doctor, is this periodontist correct about the depth of bone to remove ? How did he arrive at this number (5 mm) ? Is half a cm bone loss too much ? I am 60, female, Chinese. What are the consequences of losing this much bone ?

Also, what factors affect the thickness of bone to be removed ? Is the material of the new crown a factor ?

I am terrified and worried.

ANSWER: Hi Dorothy,

To properly restore a tooth you need 2mm of 'ferrule' (allows crown to properly 'grab' the tooth and biologic width of 3mm (1mm sulcus+1mm Connective Tissue+ 1mm Junctional Epithelium);Hence 5mm.  Doesn't mean you have to remove 5mm of bone.

So you need minimally 5mm from top of the tooth to the top of bone if you don't have proper 'ferrule' for the crown margin .  Material of crown is a factor if the dentist is trying to 'hide' the margin of a Porcelain-fused-metal crown in the sulcus b/c otherwise the margin will show as gray of the metal.  If you used an all ceremic crown or all gold, there would be less need to go deeper within the sulcus of the tooth.

Hope that makes sense...


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QUESTION: Thank you very much for your prompt reply.

Please correct me if I am wrong:
A minimum of 5 mm from top of the tooth to the top of bone is required to maintain the biologic width and hold the crown firmly. In my case, only ~1 mm of tooth is above the gum line, so the periodontist will have to remove 5 mm depth of (gum + bone) i.e. ~3 mm of gum (~1 mm to allow room for the sulcus, ~1 mm for the junctional epithelium and ~1 mm for the connective tissue), and ~2 mm bone. If he is trying to 'hide' the margin of a Porcelain-fused-metal crown in the sulcus, he will have to remove more bone (an additional 1 mm ??)

How about the thickness (not depth) of gum/bone to be removed ? Does this depend on the material of the new crown also ?

What are the consequences of removing 5 mm+ of (gum + bone) ?  Will the tooth be loosened ?  Will the roots be exposed and difficult to clean and cause problems ? Will the adjacent tooth/teeth be affected ? Will the lower jaw be affected ?  The new gum line of this tooth will be lower than the gum line of the other teeth. Will there be a problem when eating ?

I suppose the removed bone will be lost forever. How about the gum ? Will it grow back (to original ~1 mm junctional epithelium and ~1 mm connective tissue) ?

Hi Dorothy,

Very good questions.  If your dentist is going to 'hide' the margin of the tooth sub-gingivally, he will need more than the 5mm.  The 5mm (ferrule + biologic width) I mentioned is the minimum space needed.  However, a lot of the tooth broken near the gumline will have either caries that need to be removed or unsupported tooth structure which need to be removed before you start the 2mm ferrule so it all depends on where sound tooth structure starts circumferentially.

The type of crown will also make a difference as well. For example, if you use an all ceramic crown like e.Max or Zirconia, there is little need to place the margin sub-gingival.  The thickness of the gum also makes a difference because thin gums will tend to recede more than thick gums after surgery and after crown placement.

Tooth loosening as you put it is always a possibility with hard/soft tissue crown lengthening b/c you are removing supporting bone; however, so long as you have enough root length, this will not be a problem.  More of the root will be exposed, but it will be covered with a crown so sensitivity should not be an issue.  Crown lengthening is a very predictable dental procedure and the tooth should functional normally a new crown has been placed over the tooth.  The gum may grow back 1mm after surgery as 'creeping attachment.'

-Dr. John Kong


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John Kong, DDS,


I am a board-certified periodontist and an expert on dental implants. I am currently in private practice at Better Living through Dentistry. I can answer questions related to the treatment of periodontitis (gum disease), dental implants, sinus lifts, bone grafts, gum grafts, surgical exposure of teeth, and minor orthodontics.


I have been practicing periodontics and placing dental implants since 2006 on the Upper West Side, Manhattan, NY and in Forest Hills, Queens, NY at Better Living through Dentistry. I am currently an attending and Chief of Implantology at Flushing Hospital Medical Center.

American Board of Periodontology, American Academy of Periodontics, American Dental Association, New York State Dental Association

Wesleyan University / Amherst College, BA; SUNY Stony Brook Dental School, DDS; University of Medicine and Dentistry of New Jersey, Certificate in Periodontics; New York University, 1-year program in Full-Mouth Reconstruction; Certified in Invisalign

Awards and Honors
Northeast Society of Periodontists Achievement Award

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