I have both tarlov cysts & osteonecrosis. found out about them at the same time. can the cysts cause the osteonecrosis?

No: the cysts are fluid filled and exert little pressure on the surrounding bone. Changes in the bone if they in fact do occur takes decades. Osteonecrosis of the bone signifies a loss of blood supply to that bone.

Not likely: Tarlov cysts (perineural cysts) are benign sometimes enlarging lesions in the lumbosacral spine that are usually found incidentally by xray or MRI. They can cause back pain, nerve pain and leg weakness. Some require surgical treatment if they are pushing on a nerve. They are not causally related to osteonecrosis.