Before starting any root canal procedure, dental surgeons reach for a local anesthetic. However, if the patient has a severe toothache, the standard dose may not be enough to keep the pain at bay during the surgery. Figuring out just how much the patient's tooth hurts when he walks through the door may be key to a painless root canal.

A study in the current issue of the journal Anesthesia Progress looked at whether a patient's level of pain before a root canal changed the effectiveness of local anesthesia. Instead of just focusing on whether patients had pain or were pain free before the surgery, the authors tracked the level of pain to determine whether it affected the success of anesthesia.

In a dental emergency, patients often have acute pain and need a root canal right away. The dentist uses local anesthesia to numb nerves in the area that will be targeted during the surgery. However, these same nerves can be overly active if the patient is in extreme pain. The patient's lip may be numb, but the standard dose of anesthesia can't block pain once the dentist's instruments enter the patient's mouth.

The current study focused on the lower teeth of more than 175 adults in India. These patients were rated as having mild, moderate, or severe pain in one lower-jaw molar before their root canal. All were given a standard local anesthetic. The injection was considered a failure if the patient continued to feel pain during the root canal procedure.

The authors found that the amount of pain presurgery affected the successfulness of the anesthesia. Patients with more pain before the root canal were more likely to feel pain during the procedure. In the study, 33% of patients with mild pain before the root canal felt no pain or only mild pain during the procedure, but only 16% of patients with severe pain had a painless surgery.

Age and gender did not appear to change the effectiveness of the anesthesia. Instead, all indications were that the more active pain a patient felt before a root canal, the lower the pain threshold for that patient during the surgery.

Dental clinicians need "to take extra measures in giving local anesthesia to patients with severe pain," said Dr. Vivek Aggarwal, the lead author of the study. "Since such patients are predisposed to a high anesthetic failure rate, a supplementary injection should be given along with a primary injection."