Figure 1: Normal view of the eye.

It is now claimed that the patient's cornea can be warmed less than four degrees during laser treatment, using Intelligent Thermal Effect Control (ITEC), which is available on the Amaris laser (Schwind).

This is the result of scientific thermodynamic measurements carried out at the Eye Laser Clinic Recklinghausen, Germany (Diego de Ortueta, MD) by the University of Applied Sciences, Aschaffenburg.

Seven myopic eyes were investigated with preoperative spherical equivalent (SEQ) ranging from –2.75 to –9.25 dioptres. The measurements were performed with a high-resolution infrared thermographic camera that recorded thermal images of the eye at a rate of one time per second from a distance of 66 cm. Cornea temperature rise was evaluated within the optical zone as well as in the entire ablation zone.

Figure 2: Eye under the thermographic camera.

The preoperative cornea temperature corresponded to the values cited in scientific publications with measured values ranging from 29.3–31.4 °C.¹ With temperatures higher than 40 °C the proteins of collagen change their structure¹ and there is a possibility that the cornea can be damaged.² This may cause corneal haze³ and in turn may negatively influence the treatment result. In all measured eyes, an intra-operative temperature rise between 2.7 °C and maximum 3.8 °C was observed. The maximum temperature was less than 35 °C. The researchers state that this confirms that the ITEC method efficiently preserves the corneal tissue — despite the 500 Hz pulse frequency of the laser. They also claimed that this showed that with ITEC the amount of the refraction, and consequentially the length of ablation, had no influence on temperature rise.

So what is the method?

Table 1: Result of thermodynamic measurements.

The ITEC method is based on complex mathematical models, simulations and clinical studies. The result is a thermal control system considering all aspects of the dynamics of heat propagation while using a high laser pulse frequency and two energy levels. Among the fundamental principles is that heat deposited locally in the tissue propagates in all spatial directions in the tissue and cools down at the same time. Therefore, the Schwind Amaris sorts the laser pulses spatially and temporally. The result is that a blocked zone becomes smaller during the cooling phase and following laser pulses are able to move closer and more quickly to the position of the last laser pulses. In this manner, the local spot frequency is dynamically limited without significantly affecting the laser pulse frequency. The higher the pulse energy, the lower the maximum allowed local spot frequency and thus greater the blocked zone.

"The measurement results give me even more safety for treatment of higher myopes," commented Dr Diego de Ortueta, one of the researchers at the Eye Laser Clinic Recklinghausen. "The high pulse frequency of the Scwhind Amaris allows a very short treatment time without significantly warming the corneal tissue."

References

1. T. Bende, T. Seiler and J. Wollensak, Graefe's Arch. Clin. Exp. Ophthalmol., 1988;226:277–280.

2. M. Ishihara et al., Lasers Surg. Med., 2002;30(1):54–59.

3. S. Betney et al., Cornea. 1997;16(2):158–161.