There are lots of good reasons to take dental X-rays when you're working on a pet's teeth, but I'm only going to talk about one of them today.
Like people, dogs and cats start out with a set of deciduous, or baby, teeth. These are quite small. As the animal grows, these teeth don't. By the time the pet is about three months old, the teeth look too small for the animal, and there is noticeable space between them. That's because the jaws are growing and the teeth aren't. At roughly sixteen weeks, permanent teeth begin to emerge. The first ones will be the upper incisors -- the two teeth right in the middle of the front on the upper jaw (numbers 101 and 201 for your dentists out there). By five to six months, most pets have all of their permanent teeth, with the canine teeth (fangs) coming in last. As the permanent teeth come in, the roots of the baby teeth dissolve, and they just fall out to make room for the new tooth... most of the time, that is.
It's very common to see some baby teeth retained. The permanent tooth has come in beside them, and now we have two teeth in the space meant for one. This is most common with the canine teeth, the fangs, and it's not good. The bottom fang should come up and rest in a groove just in front of the upper fang. If the upper "baby fang" is still present, it pushes the permanent tooth forward, making that groove too small. If the lower "baby fang" doesn't go away, the permanent tooth comes up into the roof of the mouth, instead of that groove. So, if you see permanent teeth coming in while the baby teeth are still there, it's important to have those baby teeth extracted. We want the permanent teeth to come in to the right spot. It's not just a matter of a pretty smile: think how it would feel to have one of your teeth always poking you in the roof of the mouth -- OUCH.
When the baby tooth's root has not dissolved, it can be quite long. In the case of the canine tooth (fang), the root is about twice as long as the tooth you can see. This requires general anesthesia to extract, as you've got to do a bit of digging to get it out in one piece.
These teeth are the reason that I started the post by talking about a dental X-ray unit. This six-months old Yorkie puppy looked like it had a mouth full of permanent teeth, and we just needed to get those two baby fangs that were hanging on. Danged if she didn't have TEN baby teeth hanging around. Some were pretty loose, having no roots, and others were still pretty firm. They all needed to come out and make room for the permanent teeth. Were there more than ten baby teeth hanging on? I don't think so; I do think I got them all, but I don't know for sure. I need that dental X-ray.