I learned this as I was slowly waking up this morning and the remainder of my top right central incisor (#8 on the tooth numbering chart) broke off.
That tooth had been crumbling for nearly four years and was missing its outer lower quadrant so I wasn't /really/ surprised, but it means I'm going to have to call the dentist on Monday and start the dental reconstruction I'd half-heartedly agreed to since Vogon likes to remind me that fixing my teeth will help my overall health. (I've been studiously trying to ignore them because my mouth has been constantly painful for about five years and the cost of out-of-pocket dental care is stunning when you don't have any money to throw at it.)
The most interesting part is that having approximately a third of a tooth hurts a lot less than having two-thirds of a tooth did. And half of the sore indentation my upper incisors leave on the gum behind my lower incisors when my jaw rests in a semi-neutral position in my sleep might heal up, which is vaguely exciting.
For those of you who haven't seen my mouth in person, I have crumbling teeth and a serious jaw malocclusion. It's been obvious since elementary school but never got corrected because the dental coverage we had back then would have cost my parents about the cost of a brand new car to fix both of the older girls' teeth. Amy's jaw malocclusion is a width difference between the upper and lower jaw that's not noticeable unless you know what visual cues to look for; I have a severe underbite that will probably require surgery to truly fix, but all I really want is for my mouth to not hurt.