Trekking shoes have a device to absorbs the shocks, which is usually a gaz or gel layer inside the sole. Unfortunately, this cannot absorb much, since a thick sole would reduce the stability of the foot. By the way, always walking with shock absorbers has bad physiological consequences on the feet and knees.

I think the best way to protect the knees, aside from soft snow and screes, is to lower the body when walking down, and absorb the shock using the thighs' muscles.