Orbscan pachymetry - Implications of a repeated measures and diurnal variation analysis

Introduction: Corneal thickness changes reflect alterations in hydration an d metabolism. Ultrasound pachymetry determinations may be adversely influen ced by fluctuations in tissue hydration, whereas optical systems are appare ntly unaffected by these fluxes, A recently marketed, optical-based, topogr aphic mapping system (Orbscan; Orbtek, Inc.) uses anterior and posterior co rneal surface data to calculate corneal thickness. Objective: This new instrumentation presents as a potentially useful pachym etry tool for evaluation of corneas under hydration flux or challenge (e.g. , postphotorefractive keratectomy [PRK] healing studies) and was therefore evaluated for accuracy and variability. Measurements: Three calibrated standards were measured in repeated fashion. Additionally, 1 test subject was measured 30 times in 1 day (5 measurement s each at 8:00, 9:30, and 11:00 AM and at 1:00, 2:30, and 4:00 PM). Corresp onding measurements were made at 8:00 and 11:00 AM and at 4:00 PM On 3 Sepa rate days to assess repeatability. Grouped data from 18 volunteer subjects were compared to the data of the test subject as well, Results: Pachymetry accuracy on a calibrated standard was determined to be +/-2 mu m (standard deviation, n = 12), Repeated measures on the subject de monstrated a mean standard deviation of 9.08 mu m for 750 thickness data po ints across the central 7 mm of the cornea; peripheral measurement points e xhibited progressively greater variability than at the apex (analysis of va riance; P < 0.0001), A plot of thickness by corneal location and time of da y exhibited a diurnal pattern, with the peripheral cornea exhibiting progre ssively greater thickness changes than the central cornea (two-way analysis of variance; P < 0.00001). The data significantly correlated across days w hen all times of day were considered (r = 0.999). However, thickness values obtained at 8:00 AM were significantly different across days (t test; P < 0.0002), The subject's data correlated very well (r = 0.9996) with the grou ped volunteer data. Conclusions: These data show this system to be useful in corneal research a nd in clinical settings. The data confirm early morning pachymetry to be hi ghly variable. Additionally, the data not only indicate a diurnal variation of corneal hydration over time, but also imply the presence of a diurnal-b ased hydration gradient across the peripheral cornea, both of which can hav e significance for PRK, since excimer tissue ablation effectiveness is infl uenced by tissue hydration.