I found a Wall Street Journal article by Kevin Helliker that discussed the epidemic in cruciate ligament injuries in dogs. It's reprinted in the Naples Daily News.
Cruciate-ligament repairs are the most common surgical procedures for injured or diseased dogs. In 2006, the number of dog knees undergoing cruciate-ligament repair each year in America is estimated to now exceed 1.2 million. More dogs knees are repaired than human knees are repaired (about five times the number of human procedures) even though humans outnumber dogs in the U.S. by nearly five to one.
Humans have two legs; Dogs have four legs. But the reason why dogs have more surgeries is not because they have more legs. Not all four legs are susceptible to ACL issues. Front legs are more stable and do not have these types of injuries.
Genes play a role. Researchers identified a gene that appears to predispose Newfoundlands to cruciate tears. In the future, there may be a test that would identify carriers of that gene. If that happens, responsible breeders would not use the dogs with these genes for breeding purposes. But that's in the future.
Anatomy also plays a role. Unlike human knees, dog knees don’t lock—their back legs are always bent. Knee joint ligaments are tense whenever the animal is standing. We ask our four-legged friends stand a lot.
It could be that as humans accept dogs as members of their family, we want to increase our dog's happiness. We are willing to spend more for higher quality of life for them with better food, cushy beds, and even fashionable coats. We are less willing to have them hobble around.
TPLO, what I'm planning for Penny, is also controversial. It's about twice as expensive as normal ACL surgery. Many vets recommend this surgery for for big dogs. Many doctors believe that dogs recover faster and regain more functionality with this type of surgery. Others say that there's no proof. But I haven't talked to a local vet yet who suggests anything else for my big red dog.