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.