Modified Fiber Tips for Laser Angioplasty: Mechanisms of Action

Laser angioplasty with modified fiber tips has become a common procedure for the recanalization of totally occluded peripheral arteries. We evaluated the contribution of optical, thermal, and mechanical effects to the mechanism of recanalization of various probes, theoretically and experimentally. Temperature behavior and tissue penetration were measured in relation to axial force exerted by metal laser probes, and optical contact probes coupled to continuous‐wave and pulsed Nd:YAG lasers. Modified fiber tips only penetrated tissue when the contact surface of the probe exceeded a temperature threshold of about 225°C in the fatty tissue model used. Metal laser probes had to be insulated from a liquid environment to attain this temperature. Optical probes needed to have an absorbing layer of carbonized tissue particles to attain this temperature. Tissue penetration by modified fiber tips was force dependent, especially with optical probes. Since the diameter of the probe was larger than the ablative laser beam, the atraumatic probe had to distend soft tissues mechanically. Because the metal laser probes delivered their energy in all directions, undesired heating in the radial direction has to be reduced by motion. The recanalization mechanism of modified fiber tips will depend on the properties of the obstruction. Sometimes the probes will recanalize the obstruction without the use of laser energy (Dotter effect). Moderate heating of the tissue by either direct absorption of light, or by heat conduction from the heated surface of the probe, may help to remodel the obstruction. Tissue temperatures around 100°C will vaporize the water compound, and temperatures higher than 225°C will ablate the solid compounds of the tissue creating a channel. The present unsteerable probes will not penetrate heavily calcified obstructions. Calcific deposits may deflect the probe into the wall with the risk of perforation.