I had no idea! First, I am sorry to hear that Lady developed this condition. This happened to Greta, too, and the vet said it was the angle of her bones...she grew long, straight legs and the angle stretched her ACL. Second, may you have strength during her recovery. They need so much babying and it's sad to see Grace be lonely without her sister to play with. Right now, Betti vocalizes her displeasure several times per day, because I've been spending time nursing Greta and Betti is in a separate room. I keep promising her that soon we will do things together, that she will get so much attention. Third, hang in there. They are so lucky to have you. Some people just put their pups to sleep at the first sign of trouble like this. They would never spend the time and money to correct the tear, but we are not those people. Coming from me, first-hand, when Greta wasn't even two years old her hip fractured and it was expensive to have a pin put in it, but the seven plus years we've spent since then have been remarkable and I could not be more satisfied with the decision to fix her up. Her love is worth more than any amount of money. The past decade spent without Greta is unimaginable.
Much love to Lady during this time of healing.