All, 6 y.o., has a slab fracture in a carnasial tooth.  Split in half.   $630 for extraction and cleaning while he's anesthetized.   NOTE: see final photos, showing cleaned teeth.

We would easily have spotted this is we'd been looking.

The only way I can imagine him cracking that tooth is on the beef femur bones I love to buy them.  I've been a negligent, indulgent dog-daddy.  I haven't even looked at their teeth in ages.  This is obvious, and the plaque is terrible.  People here warned about the danger of big heavy bones, and I did not heed. They LOVE to gnaw those bones, for weeks.

I should have taught him to floss more often.  Note here the pink, healthy gum color, while the lower black gum is normal pigment (black gums only in tris?):

Weeks later, the trapezoid-shaped piece with the heavy plaque is a loose flake.  Note the red gum inflammation. 

I wonder if the truncation of the canines is from grinding on gritty tennis balls?  Or traumatic breakage?