Some prosthetic knees are not worth considering. For instance a "safety knee" is just as dangerous as it is inexpensive.
A "four bar knee" is really reliable, and bends close to the socket being good for a long stump; however, whether hydraulic like the Century 1900, 2000 and 2100, or pneumatic like the Teh-Lin (DAW-USA) they cannot take a lot of walking. They get hot and loose the resistance required for dampening in the swing phase.
The Century 2100 has "cooling fins" this problem is so pervasive - and still they are not enough. The first knee this author walked on was a Century 2000, and it was really nice, except, the hydraulics blew really fast.
The Century 2000 did not get such good press in the "Amazing Race" when in the 10th season of the show Athletic Amputee Sarah Reinertsen had her Century 2000 blow out on her and basically cost her a longer competing position in the race and the boyfriend she was dating at the time because he just wanted her to "keep up" - with a blown Century 2000.
There is a picture of her running in the Century 2000 with a spring foot at http://shareitfitness.wordpress.com/2010/05/10/1056/ She was born with proximal femoral focal deficiency, a bone-growth disorder which eventually lead to her becoming an above-the-knee amputee at the age of seven. After the amputation, she began to run track and broke the 100-meter world record for female above-the-knee amputees at the age of 13. She was formerly the marketing coordinator for Ossur and spokesperson for the Challenged Athletes Foundation.
However it is a good knee - functionally. It clears the ground nicely on standard gait. This author has walked in both the Teh-Lin (DAW-USA) and the Century 2000 knees and you will not stumble on the foot, but the problem is they are completely mechanical and offer no intelligence in the swing phase or stance support for negotiating slopes or stairs or stumble recovery; they just go out from under you.
The Four Bar hydraulic knees are not SNS type (Stance aNd Support) type hydraulic knees.
They do have a feature that you see (in the photo) Sarah using as she is running - when you throw it forward, it locks and you can hit it hard and safely as in a running stride.
In this author's opinion; however the four bar knees are not worth considering for the simple reason they are not rugged nor safe because when they unlock - there is nothing to give you any support from falling, and this can be caused by something as simple as tire rut in a street you are crossing.
While you won't normally trip over the foot in a stride, if you do - you will fall, and that to me outweighs the benefits.