The point of the knee bindings is that, unlike pretty much every other Alpine heelpiece on the market today (except my Dynafit AT bindings), they release laterally as well as vertically, in response to lateral torque, protecting one's ACL. That is a big deal for folks status post knee reconstruction, such as my wife, for whom I bought Kiku's with Knee bindings for Xmas (not being able to find any antique Geze's available). She sets her DINs at 4.0 to protect her knees, and has only come out of the binding once, probably justifiably. The Knee bindings are not hard to find. They are beefy and heavy, and there is no AT version that I have heard of.
Like one's Avalung, beacon, and probe, Knee bindings are something you hope to never actually need, but in fact if they work right you probably won't ever even know that you did need them.