Can thread lift for the legs work?

Judging from your photos, the area of concern on your knees is relatively small and liposuction alone should do  well in improving the contour.  Liposuction will not lift the skin but will decrease the fullness in the area.  I have never heard of threadlifts for the legs, have never heard it mentioned in a plastic surgery meeting and never seen an article about it published in any plastic surgery journal.  If it truly was effective, then it would be widely known.  The fact that it is so unknown makes it something to be avoided.

No, I would not recommend a thread lift for the knees.  If you have loose skin you need to have the skin excised.

Thread lifts may have negative results in the knees. Some of the threads may extrude because of the thin skin and the signficiant mobility of the tissues over the bony prominences when you bend the leg. There might be a greater risk of infection in the knee area and if it happens then the threads would have to be removed.