It's an awful injury, and a very weird on for anyone to have, but honestly with a bird who has no feathers on it's wings at all, it actually makes sense to me. As someone else mentioned above, Nellie uses her feet/legs for EVERYTHING, and all of her balance, stability, and strength comes from her feet and her ankle, knee, and hip joints, as she has literally no "tools" to use for balance/stability in her upper body. So my assumption is that she took a bad fall, and was probably trying to "right herself" using her leg, and the leg got caught underneath her. Then a horribly strong torsion-force was placed on the knee joint, which tore all of the ligaments of the knee, all the way around the knee: the ACL, PCL, MCL, LCL, and all of the smaller, accessory ligaments...Also, it's probable that the Patellar Tendon was also torn, if not the entire way through, most of the way through, as it's located right at the front of her leg and connects the knee cap to the Tibia about half way down...That's how much Nellie is using her legs for all of her movements and balance, even simply standing is all in her legs and feet...
Paints a pretty clear picture of how devastating Feather-Destructive Behaviors actually are for birds, who all rely on their wings for more than half of their movement and balance.
***The only other thing I can think of that the OP might have meant, or that their friend may have meant, is that rather than having an "Inverted" Knee, Nellie actually has a "Hyperextended Knee", which is not at all the same thing, and would be much less devastating, though still uncommon in birds, because they typically have their wings helping them balance (that's why these types of Ortho injuries are so uncommon in birds, because they usually have their wings assisting their balance, motion, stability, etc.)...It would be nice is we as people had wings, then we wouldn't have these kind of horrible knee injuries as often as we do, lol...
***If it's only a "Hyper-Extended Knee" and not actually an "Inverted Knee", then the prognosis is much better and the surgery isn't nearly as invasive, thought it's still not good or easy by any means...A "Hyper-Extended Knee" means that Nellie's entire knee-joint bent/folded in totally the wrong direction; imagine that your crouching down, but instead of your knee bending in the normal way so that your heels almost touch your butt when you're crouched, instead the back of your knee is almost touching your butt when you're crouched...OUCH! Basically in any Hyper-Extended joint (mainly the knee and the elbow) the joint folds entirely in the wrong direction, which as you can imagine completely stretches all of the ligaments and tendons, often results in bone damage/fractures (especially to the heads of the Tibia, Fibula, and Femur, and often results in stress-fractures/hairline fractures around the "necks" of each of these bones, simply from the torsion forces on them), and sometimes tears the tendons that are stretched the furthest in the wrong direction, but it also typically tears all of the cartilage inside the joint, and usually ruptures the entire bursa sack around the joint...But it's still a much less devastating injury than a completely inverted knee joint...So let's hope that maybe that's what has happened to Nellie, as the surgery is easier, the recovery time is much shorter, the costs will be much less, and the permanent damage is minimal, compared to an Inverted Knee...
Please keep us updated on how Nellie is doing and how your friend is making out...And if she does end-up coming up to Philly to the University of Pennsylvania Vet School with Nellie, if there is anything at all that I can do to help her out, please do not hesitate to PM me. I live a little over 3 hours from Philly, but am totally able and willing to help out in any way I can, I have a good vehicle, and I am very familiar with all of Philadelphia, especially the University City area, as growing-up my mom dated an guy who lived right downtown on the edge of University City and the U of Penn campus, so I spent plenty of time there. If I can give her any help at all I will. My own CAV also graduated from U of Penn vet school, so he may also have some suggestions to help throughout the process. Staying in downtown Philly isn't a good idea, though the University City area is very nice and much safer than the rest of downtown, but you definitely have to be careful where you stay, where you walk, etc. Philly is a dangerous city if you enter the wrong area (pathetically, Temple University, a fantastic medical university, which is not located anywhere near University City but rather in one of the worst neighborhoods in Philly, is literally surrounded/encased in barbwire)...
so we know what happened, plze?
