Ulthera is usually done to tighten the neck and jowls. I would doubt that you had the silicone injected into these areas. So as long as you do the ulthera in silicone free areas you will be ok. For people who have silicone nodules, we have been using the Ulthera to shrink down these down. So if you do the ulthera over the areas where you have had silicone, there will probably be changes to the contour.
Without having Ultherapy, some patients have had delayed bad reactions to silicone injections. Some immune reactions can occur (granulomas) that develop large bumps under the skin and swelling and redness and pain. This is extremely rare with other fillers such as Voluma, Juvederm, Belotero and Restylane (the hyaluronic acid fillers). There might be an increased risk of developing the immune reaction to Silicone if the Ultherapy energy breaks apart the silicone filler and exposes it more to the body's immune system. This is only a theoretical question and has not been proven. So it's best to avoid if possible, delivering Ultherapy treatment immediately over the Silicone-treated area. I, however, have not been made aware of any cases of such complications.
In most cases, areas of the body treated with silicone injections and areas treated by Ultherapy do not overlap. If you're planning on having Ultherapy performed on regions of the body where it's unlikely that there are silicone injections (such as the face), then I wouldn't be concerned. However, just to be safe, I would contact the doctor who injected you with silicone to learn more information about where you were injected and with how much silicone. That information can prove useful for the specialist who will be performing your Ultherapy treatment.
I woud shy away from areas of the face where there are silicone injectables. Typically the treatment areas that are recommended by Ultherapy are silicone free areas (neck, lateral face) so it would be safe to do the treatments in such areas. I would get detailed knowledge from the previous doctor that injected silicone before an Ultherapy is performed. Read the Ultherapy book on the link below.