Got this info for you as I suspect this is what has been done.
Pin firing is a therapy that uses a small, red-hot probe to cause cauterization (burning) of tissue in horses with chronic injuries to produce an abundant, serous inflammatory process. As opposed to other inflammation processes such as infections or bruising, serum has little or no fibrin (clotting material) or cellular content and does not coagulate. Firing causes maximal exudation, or oozing, and minimal tissue degeneration. The flooding of serum seems to flush out any chronic irritation, and it does not displace old scar tissue. Firing is done more often in racehorses than in other performance horses, and has been used for more than a century in conditions of recurring injuries such as a splint, curb, or chronic bowed tendon. The process is performed under sedation and local anesthesia, and the pain inflicted is fairly short-lived and usually well-tolerated by the patient.