There are new treatment handpieces that might afford modest improvement in these areas, but these are definitely difficult to tighten up, perhaps because of the constant stretching of the skin over the joints with flexion.
Pelleve results are subtle and may take multiple sessions. It is non invasive and there is no downtime, but there can be no guarantee that it will work very well.
Pelleve for knees/elbows
Radio frequency devices like Pelleve do promote collagen generation and some tightening of the skin. Results on the face are frequently modest although about 1/3 of people that are treated find it great. Significantly good results on the elbows and knees are very unlikely. In fact using other modalities in addition have also been disappointing for me in these areas. Good luck