The tooth looked fine. No cavity, no filling, no crack, no history of injury. From the outside, the upper right first molar of a 31-year-old woman was an ordinary, intact tooth.
It was also loose enough to be graded at the most severe level of mobility; it did not respond to a cold test, and a three-dimensional scan eventually explained why. The tooth had four separate roots instead of the usual three, a fifth canal that belonged to none of them, and an infection that had reached her sinus.
