Understanding Knee Abduction

Knee abduction is a difficult motion to observe, because the knee is a joint with a bone that confuses thing.  Meaning that the patella that sits on top of the knee joint, and so draws the eye.

The knee joint is the synovial joint that sits between the tibia and femur, in the frontal plane (movements that happen left and right), the knee is capable of 'feeling' abduction and adduction.  

I use the word feeling, but really I mean that the angle between the femur and tibia changes, and that causes a compression and a stretch, on opposite sides of the joint capsule. 

For knee abduction then a stretch would be happening on the inside of the joint and a compression would be happening on the outside of the joint.  Adduction is different at the knee joint, because a straight leg offers a knee adduction, and so it would be better to speaking about adducting.
The difference between talking about abducting and abducted is that if you are -ing then you are travelling, but if you are -ed then you are in a position.  In the diagram above you can see the knee go through abducting, from adducted to abducted.
Returning from abducted to adducted would require abducting.  So really this photo could show both motions happening, depending if you look from left to right or right to left. 

Why is it so hard to see knee abduction?

During motions that are classed as closed chain, then instead of the tibia travelling away from the femur, then the femur would travel in compared to the tibia.  The end result is that when the tibia travels you see the patella travel towards the midline, which people would say is adducting.  They are right, the patella is traveling toward the midline, but the joint is feeling abduction.  This is because the stretch and compression either side of the joint follows that of abduction, a stretch on the inside of the knee and a compression on the outside. 
Here is a one leg squat with opposite side reach to show you what I mean. The movement is small, but it is an example of this motion, and the range of motion you will probably see.