Pets tend to fracture their teeth much more commonly than do people. And dogs definitely outnumber cats when it comes to traumatic dental problems. Cats don't tend to do the goofy things dogs do, like chew rocks or chase cows, so we see fewer cats for fractured teeth.
The teeth most commonly broken in both dogs and cats are the canine teeth (fangs) and the upper fourth premolars (mostly dogs). The canine teeth break because they are used to grab things that are moving fast (thrown rocks) or running away (collars of other dogs when playing). They also stick up further than any of the teeth around them, making them more prone to being impacted by things carried in the mouth. And being at the front of the mouth, they are what is hit first when the animal is hit in the face (as by a car, or an accidental kick).
Unfortunately, these six teeth are six of only eight teeth in the entire mouth that we really want to keep! We call them strategic teeth because of their importance. Our pets can do without almost any of their other teeth; these are the important guys. The canine teeth keep the lips out of the mouth and the tongue in. The cat on the right is missing an upper canine, and you can see that his upper lip is perpetually becoming caught in his lower tooth. He has a scar on the upper lip from the chronic rubbing and ulceration.
The upper fourth premolars match up with the lower first molars to provide most of the chewing activity when the pet eats. The photo to the left shows a normal set of dog teeth. His nose is to the left. The large tooth to the left is the upper fourth premolar. The big tooth below it is the lower first molar. Notice that they are tall and thin, and fit together much like the blades on a pair of scissors. These are shearing teeth, and the full force of the bite is concentrated in their tips. Breaking these teeth is not a good idea! But try telling that to the dog obsessed with rocks.
Canine teeth tend to break straight across the crown, or at an angle. In order to save these teeth we often need to do a root canal, which removes any painful nerve remnants and pulp, and seals the tooth so that is cannot become infected in the future. In this puppy tooth, you can see that the pulp chamber in the center of the tooth is exposed (where the bit of blood is). This is a direct conduit to the tooth root. If left untreated the root will inevitably become infected (abscessed) at some point in time. In this puppy, we simply extracted the tooth. In an adult, we would do a root canal.
Remember what I said about the chewing teeth having a lot of pressure on the pointy tips? Well, if you get something hard between those two tips and apply a lot of force, there is a ton of outward pressure on the point of the upper tooth. When this tooth breaks, it often forms a "slab fracture", where the tip breaks off and takes the whole side of the tooth with it.
This is an example of a slab fracture. We are looking at the tooth at an odd angle, actually toward the roof of the mouth with the mouth open, so you are seeing the chewing surface of the tooth. The black ridges at the bottom of the picture are the ridges in the roof of the mouth. This dog had some black pigmentation in the mouth, which is normal. The line down the length of the tooth is where it is broken. You can see a brown circle where the very tip of the tooth used to be - it was broken here some time ago, and the brown spot is exposed dentin. The black stuff that looks like it is in the fracture itself actually is - that is bark from the stick the dog was chewing when the tooth broke. This tooth was not repairable even with a root canal, due to the extensive damage.
This tooth, on the other hand, was saved. We don't do a lot of crowns, but this dog is an inveterate chewer. She broke both of her upper fourth premolars at a fairly young age, and had lots of chewing years ahead of her. She broke two of the standard post-root canal restorations used on the teeth, so her owners elected to go with metal crowns and be done with it. She is doing great now, many years later, and no longer has the worry (or expense) of repeatedly breaking these teeth.