If you want a fairly good and safe knee - you might want to try the Geoflex knee from Ohio Willow Wood Company.
I owned one of these knees and walked on it comfortably and with the most secure safety I have felt on any knee I have owned. The only problem with this knee is the early ones uses a rubber return strap, and it tends to shear the pin at the front of the knee where the Flexion functions on the nylon insert to the rear. See the pic.
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The knee is otherwise great - it will safely re-lock (or actually continue support of the wearer) due it's elliptical knee mechanism up to 20 degrees into the swing phase of the knee. The built-in Stance Flexion feature allows for a natural backward bending motion at the knee, creating a smoother stride you can feel.
The problem is - it accomplishes this by compressing a nylon/urethane material round bumper insert at the rear of the knee and it wears out from the repeated compressions - then it clacks on each step until the contact is made to the bumper until you get a new one.
I NEVER fell in this knee - not once. I always provided support on each stumble such that I could either continue the the forward step, or it supported me through the stumble until I could shift my weight to recover.
If you want a safe walking knee and you are not high activity, this might be an answer for you. Having actually walked on this knee, I would recommend this knee to anyone in this activity range looking for a good safe knee. The specs are here => http://www.owwco.com/pdf/catalog/E/OWW%20catalog%20sec%20E_08.pdf
Amputees experience stability for up to 20 degrees of flexion under load, and feel secure and confident with each step. This knee accommodates a hip flexion contracture without giving up knee stability and allows for easier cosmetic finishing. A built-in Stance Flexion feature allows for a natural bending motion at the knee, creating a smoother stride. The roller bearing design allows for increased durability and longevity, while a more refined friction adjustment creates more stability and is much easier to use. The knee has a 4-hole pattern on the proximal end and a 30 mm tube connection at the distal end. The GeoFlex Knee comes with a stance flexion bumper replacement kit, dust cover, and an elastic extension assist.