Method of evaluating ultrasonic pachometers

Although many ultrasonic pachometers are commercially available, little has been reported about their performance. We outline a technique for evaluating the accuracy of ultrasonic pachometers by simulating corneal thickness measurements using hydrogel contact lenses. On-eye contact lens thickness is determined using ultrasonic and optical pachometry. Absolute lens thickness is determined using a thickness gauge. Comparisons are made between measurements from the ultrasonic pachometer and measurements from the thickness gauge and optical pachometer. We used this technique to evaluate the performance of the Vida-55 ultrasonic pachometer. This technique was also used to determine the ultrasonic velocity for cat corneal tissue and two hydrogel lens materials. We determined the velocity of ultrasound in cat corneal tissue to be 1590 m/s at 33 degrees C. The velocity of ultrasound in hydroxyethyl methacrylate (HEMA) was 1740 +/- 24 m/s and 1700 +/- 40 m/s in Snoflex 50. We found the Vida-55 pachometer measured corneal thickness accurately when the appropriate ultrasound velocity was used (SEM = 6 microns).