In thermolysis, which factor explains why a shallow insertion is hotter than a deeper insertion?

• A. Current density
• B. Surface area
• C. Resistance
• D. Hair diameter

Answer: (A)

In thermolysis, the heat produced at the follicle comes from the current’s resistance at the electrode-tissue interface. The amount of heat is largely governed by current density—the current per unit area of contact. When the needle is inserted shallowly, the electrode’s contact area with tissue is smaller, so the same current is pressed into a smaller area, raising current density and creating more heat at the surface. Deeper insertions spread the current over a larger area and into more tissue, lowering current density and reducing the localized heat. Hair diameter doesn’t determine the heating at the electrode tip in this mechanism, since the heating is about how concentrated the current is at the contact area and the tissue resistance there.