if by back tooth you mean the wisdom tooth then it is normal. It is called pericoronitis. In this case the gum covers thre tooth and become infected and is quiet painful . if the tooth is fully erupted then some times the dentist just removes the gums. But if the space is less and if it is there time and again then you can go for the exraction of the wisdom tooth. As such also wisdom teeth does not help in mastication and it is difficult to maintain the oral hygiene in that area. for more info on pericoronitis you can visit the following link on our website..
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