Thermal characteristics of sapphire contact probe delivery systems for laser angioplasty

Contact probes made from synthetic sapphire crystal, designed for general laser surgery, are currently being evaluated for use in laser angioplasty. Their mode of action and safety in the context of arterial recanalisation is unknown, particularly with respect to the degree of probe and catheter heating. Infrared thermal imaging was used to investigate the surface temperature rise of various rounded sapphire probes during emission of continuous wave Nd-YAG (1,064 nm) laser energy. Catheter safety was addressed by analyzing the temperature of the metal interface between the optical fiber and sapphire, as well as the catheter proximal to this junction. Transmission of Nd-YAG energy through each probe was also measured. Five rounded probes of 1.8-3.0 mm diameter (three supplied by Surgical Laser Technologies SLT], two by Living Technology LT]), along with their respective optical catheters, were compared. There was a large temperature gradient between the front and rim of the probes. The maximum surface temperature rise of the sapphire (at 20 W, 5-second exposure) was 314-339 degrees C (SLT) and 90-108 degrees C (LT) P less than 0.001, 3-way ANOVA]. The reason for this difference may be related to "crazing" of the front surface of the SLT sapphires. At all energy levels sapphire temperatures were considerably lower than attained by metal laser thermal angioplasty probes. Forward transmission was slightly higher in the SLT probes (75-85%) than the LT sapphires (54-69%). With fiber perfusion at 2 ml/minute, a minor degree of heating of the metal sapphire holders was recorded (maximum rise 35 degrees C), but heating of the catheter proximal to this was negligible. Therefore, it would appear that the risk of tip detachment or arterial injury due to heating of the connecting metal interface is extremely low. Without perfusion, however, there was a greater degree of interface heating in the LT delivery system suggestive of more laser backscattering by these sapphires compared with the SLT probes P less than 0.001, one-way ANOVA]. The SLT system is, therefore, potentially safer in this respect. These results suggest that some degree of surface heating of contact probes due to energy absorption within the sapphire does occur, but is localised to the front of the probe. This effect may contribute to the process of arterial recanalisation with this device. However, variation in the thermal and optical properties of sapphires from different sources has been demonstrated. The influence of these properties on plaque ablation, and ultimately the clinical performance of different contact probe systems, requires further investigation.