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.
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.