What causes black cracks in teeth?

Q. I noticed cracks in my molar teeth. Should I wear a mouthguard?

Answered by
Dr. Tooba Qazi
and medically reviewed by iCliniq medical review team.
This is a premium question & answer published on Jul 29, 2015 and last reviewed on: Oct 19, 2019

Hello doctor,

While looking at my teeth the other day, I noticed what looked like cracks in my molar teeth  (2nd from the back) on either side. Recently, I went to the dentist for a cleaning and also was complaining about sensitive teeth to both hot and cold. On both these teeth, I have a bit of receding gums. Although the dentist did not find any cavities, he could not be sure if I needed a root canal treatment and referred me to an endodontist. The endodontist determined that there was not any infection in my gums or teeth and said I did not need a root canal treatment. But he did suggested me to wear a mouthguard at night. I told him I noticed sometimes that I clench my jaw, especially at night. He also suggested a mouth guard because, the dentin is wearing away on some of the molars, the gums are receding and there are little periodontal pockets forming in those teeth.

These cracks in my teeth are vertical, straight lines. They do not look jagged like a crack in the sidewalk. They are dark in color and very thin. It looks like a small piece of hair is stuck in the tooth. I have one on my right side tooth that is on the front facing part of my molar, and may be two on the left molar but facing towards the back of my mouth. When scratching my fingernail over the surface of the vertical line, it is difficult to tell if there is even a difference in the surface texture. I do not know if these lines appear inside my teeth or on the surface. The endodontist did not say anything about these. It is hard to believe he missed them considering I saw them with my naked eye and he was wearing a microscope on his forehead and eyes. Maybe, are they normal and not worth mentioning? The reason I am asking is that the endodontist said that people who grind their teeth or clench their jaw can develop small fractures in their teeth and over time, would cause the need for a root canal.

What could this be?



Welcome to icliniq.com.

  • First of all thank God that you met a good endodontist who diagnosed the right problem. Secondly, through all your case history it is evident that you are having a chronic habit of clenching your jaws that is called 'bruxism' in medical terms.
  • What happens in it is, due to these unintentional excessive forces microfractures occur in the inner or outer layers of teeth. These fracture lines can be termed as cracks in layman term. So, if endodontist talked about fracture lines this automatically means he was referring to these cracked lines in your molars and the other tooth as well.
  • Mostly they are like vertical straight line microcracks (as evident in your case). Initially they can be seen well through the microscope (that your endodontist was using) only and not through the naked eye. At later stage, after months or may be after years due to extrinsic stains (of coffee/tea/sodas) these cracks become stained like brown, black, yellowish and hence become visible to your naked eye.
  • Now, when these cracks got stained they become visible to you. According to you they are present in two of your back teeth and in one next to molar but, this is quite possible that these crack lines are present in all teeth but not stained so they are not visible to you yet!
  • Endodontist would have seen it and addressed cracks in generalized term because clenching of jaws is not a localized problem of one tooth only, it is generalized and all teeth in the jaws are involved in it. Cracks in molars become apparent soon because they bear heavy chewing forces as well.
  • At further later stages of bruxism, if forces are not controlled they may result in cracking down of the upper white layer (enamel) of tooth or inner layers as well. If the problem reaches to this severity then tooth may split or some part of tooth along with upper and inner layers may chip off and then root canal treatment or extraction are the only options you are left with.
  • Well, don't worry, your problem has been diagnosed at the right time, almost in initial phase. You should not be negligent about it and you should wear the mouthguard regularly. Try not to clench you jaws intentionally or unintentionally and then this problem will subside itself soon and these cracks would not propagate further.
  • As you were not able to feel cracks through your finger, it means that they are present in the inner layer of tooth and stains leached into the cracks through micropores, giving them black colour.

Feel free to ask any further questions to an endodontist online --> https://www.icliniq.com/ask-a-doctor-online/endodontist

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