Validity of optical pachometry measurements made with contact lenses in situ

Reproducibility of ultrasound pachymetry using the Sonogage Corneo-Gage Plus 2

PURPOSE

The aim of this study was to determine the reproducibility of measurements made using the Sonogage Corneo-Gage Plus 2 (Cleveland, Ohio) ultrasound pachymeter of total corneal and corneal epithelial thickness in 5 different regions of the cornea.

METHODS

Twenty-seven subjects at the New England College of Optometry (NECO) and 20 subjects at the Southern College of Optometry (SCO) were enrolled in this study. Measurements were taken of the central cornea as well as the nasal and temporal regions of the midperipheral and peripheral regions of the right cornea of each subject. Identical measurements were again taken in these subjects 1 week later at approximately the same time of day. Within-subject variations were then assessed using paired t tests.

RESULTS

The only significant measurement differences that were found between visits were for full corneal thickness at both the nasal and temporal midperipheral locations. The mean differences for these locations were 13.5 microm and 13.7 microm, respectively.

CONCLUSIONS

Epithelial corneal thickness and central and peripheral total corneal thickness measurements using the Corneo-Gage Plus 2 pachymeter were reproducible; however, midperipheral total corneal thickness measurements showed poor reproducibility.

Photo-pachometer: an instrument for the objective non-contact corneal thickness measurement

A new instrument to measure corneal thickness is described. The design was based on the well-established optical principle of pachometry and its operation required only basic clinical skills. The instrument measures corneal thickness using a newly developed computer-aided paradigm in order to minimise the inter- and intra-subject variability commonly associated with the original optical technique. Preliminary clinical trials showed that it provides fast, repeatable and objective measurements comparable to ultrasonic pachometry, and no physical contact with the eye was required throughout the procedure. It combines the advantages of the two currently employed techniques and it has potential clinical applications in monitoring the dynamic corneal hydration response in vivo.

Reliability coefficients of three corneal pachymeters

We compared the accuracy and reproducibility of a hand-held portable ultrasound pachymeter, the Pach-Pen (Bio-Rad, Ophthalmic Division, Santa Ana, California); another ultrasound pachymeter, the DGH 1000 (DGH Technology, Inc., Frazer, Pennsylvania); and the Pro-Cem 4 endothelial specular microscope (Alcon-Surgical, Inc., Irvine, California). Each eye of 18 healthy human subjects was examined to determine corneal thickness using the three different instruments. For each instrument, five repeated measurements were obtained at each of five corneal locations (one central, four peripheral), for a total of 25 measurements per eye. The accuracy of the two ultrasound pachymeters was tested by comparing measurements obtained on specially designed test blocks of known thickness. The Pach-Pen was the more accurate of the two ultrasound pachymeters, with measurements within the range of 0.003 to 0.065 mm from the true thickness. The three instruments were most consistent in mean thickness in the center of the cornea. All three instruments showed excellent intraobserver reproducibility, as measured by reliability coefficients over 90%. Overall, the Pach-Pen pachymeter had high reproducibility, and produced more accurate measurements than the DGH 1000 pachymeter.