We have treated many female patients with breast implants, both saline and silicone. It is safe and does not injure the implant or silicone. For women with small implants, the implant is far from the treatment area. For women with larger implants, we will hold the implant out of the way during the treatment. I would recommend a plastic surgeon to do the miraDry treatment because they will be more familiar with the depth that they are performing the numbing injections, as well as the location of the implant to avoid any harm.
Provided you see an experienced medical miraDry practitioner there should be no problems. Most implants are mobile and, while they do not extend into the axillary area, can be easily pushed even further away from the treatment zone. Sub-pectoral implants may be less mobile so extra care is required to make sure they implant is eased out of the way.