When my husband broke his pelvis he was told to wear them until he was weight-bearing, which took three months. He wore them faithfully every day. I think it's the "weight bearing" and general activity level that is key. My understanding is that the body's musculature helps to support the circulatory system and its function and if the musculature isn't doing that because you're not moving your body as you normally would, like after surgery, your risk of developing a blood clot is increased. My husband was remarkably active while on crutches but still wasn't considered weight-bearing; he washed those socks out in sinks all over China and Europe!