the contact lens exam
Corneal Thickness Measurement
BY AMIR MOEZZI, MSC, BOPTOM & DESMOND FONN, DIPOPTOM, MOPTOM, FAAO
Pachometric corneal thickness measurement has useful clinical and research applications in refractive surgery, glaucoma diagnosis, corneal pathology and contact lens research. We'll briefly describe the different pachometry methods and their precision.
Slit lamp optical pachometry measures the apparent thickness of an optical section of the cornea through a fixed observation angle and converts this value to corneal thickness using a calibration curve.
Jaeger (1952) performed optical pachometry by illuminating the cornea normal to its surface and obliquely viewing (40 degrees) an optic section.
Mandell and Polse (1969) improved the reproducibility of the Donaldson (1966) technique by using two small fixation lights that they could precisely locate with respect to the microscope. They also coupled the pachometer to a potentiometer to electronically record measurements. Using increased illumination in addition to a thinner slit width and wider angle between slit lamp and microscope, Holden and Payor (1979) increased the resolution of slit lamp pachometry and measured epithelial, stromal and total corneal thickness.
Stark et al (1990) reported a precision of 5 µm for optical pachometry. The limitation of optical pachometry is its subjectivity, which can result in inter-observer variability.
Ultrasonic pachometers are contact instruments that necessitate corneal anesthesia. Ultrasonic waves reflect from the adjacent interfaces with different refractive indices, and the probe detects the reflections. The time delay between reflected rays from anterior and posterior corneal surfaces is converted to corneal thickness.
Ultrasonic pachometry is less repeatable because it's difficult to place the hand-held probe on the same corneal location repeatedly.
The Obrascan II/IIz (Bausch & Lomb Surgical, Orbtek) provides simultaneous non-contact topographic surface measurements and pachometry.
Central and peripheral corneal thickness is calculated from the difference in elevation between anterior and posterior corneal surfaces. It provides an advantage over other techniques because it produces fast and repeatable pachometry maps of the central and mid-peripheral cornea with minimum operator skills.
Lattimore et al (1999) and Moezzi (2002) both found an average repeatability of 9 µm for the Orbscan. Marsich et al (2000) compared optical pachometry with both ultrasound and Orbscan and concluded that Orbscan is the most repeatable technique, but it's less precise for peripheral than for central measurements.
Optical Coherence Tomography
Optical Coherence Tomography (OCT) is a noninvasive imaging technique that measures corneal and epithelial thickness based on the same principles as ultrasound pachometry (A-Scan), but uses light. OCT produces a cross-sectional image of the cornea by tracing the back scattered light from anterior and posterior corneal surfaces (B-Scan).
Izatt (1994) found that the axial resolution of OCT is approximately 10 µm to 15 µm, but Drexler et al (2001) have shown that an ultra-high resolution OCT can increase the axial resolution to about 1 µm. Wang et al (2003) found that OCT is an efficient and precise method for measuring topographical corneal and epithelial thickness.
To obtain references visit www.clspectrum.com/references.asp and click on document #108
Fonn is Director of the Centre for Contact Lens Research, Professor, School of Optometry, Waterloo. Moezzi is a research associate at the Centre for Contact Lens Research, University of Waterloo.