You might call various pet insurance companies to see what their exact coverages are. Pre-existing conditions and/or genetic disorders (such as hip dysplasia, hemophilia, etc.) are not covered, but I don't know about previous injuries and you would want to be very specific about that if you spoke with a rep about it. You can choose different types of coverage, too, including plans that cover only accidents, so you might be able to go that route.
Personally, I have had very good luck with my pet insurance company. I have been with them for six years now. The insurance is not cheap, but I have two large dogs (chocolate labs, each weighing about 100 pounds) and vet exams/diagnostic tests/surgery/medicine for them is costly. My policy doesn't cover everything, but it takes care of a lot and it provides me with a certain degree of security just knowing that it at least makes most treatments/surgeries "doable" --especially for the big things. (They have paid out a lot more to me than I have paid to them.)
I have insurance through the AKC Pet Healthcare Plan, but there are many others out there. I have heard good things about VPI (Veterinary Pet Insurance). I think they are a little less expensive than mine, and they offer very specific information about their coverage on their website. My veterinarian has many clients who use VPI, and he has gotten a lot of good feedback about them.
I know there are other companies, too, but I have heard nothing specific about them so cannot speak to that. Maybe if there is a veterinarian on the this forum, she/he could offer you more info on the various companies.
Good luck.
MissL