Your question is an excellent one. In the early days of Thermage (it has been on the market since 2002), higher energies were used and a small number of patients did develop facial fat atrophy. Over the years, advances in Thermage technology and treatment guidelines have essentially eliminated the risk of this complication. At sufficiently high energies or if performed by an inexperienced operator, fat atrophy and other complications can occur with Thermage and other RF treatments.
Ultherapy is not associated with fat loss like thermage, especially its predecessor devices. It seems to be less selective in its energy delivery than focused ultrasound (Ultherapy). The newer Thermage CPT technology has apparently overcome this side effect.
Your question is excellent and related to old technology. The previous Thermage - not the current Thermage with the CPT technology had such reports. The new Thermage is safe and effective and builds up new collagen and does not have any pain or these previously reported reactions. It is very important to consult a board certified dermatologist with experience with Thermage for the best cosmetic results.
I do not have personal experience but have been at a plastic surgical meeting where panel members noted that fat transfer to the face or hands can "melt" with RF devices such as Thermage.
Radiofrequency tightening does not cause any damage the underlying fat. I suggest going only to an expert for this type of treatment. Best, Dr. Emer.