Corneal sublayer thickness measurements with the Nidek ConfoScan 4 (z Ring)

To determine the repeatability of corneal sublayer thickness measurements using the Nidek ConfoScan 4 (CS4) with the z ring on a group of young adult subjects.

Thirty subjects aged 18 to 30 years were invited to have thickness measurements with the CS4 (z ring) on two different days, at similar time of the day to avoid diurnal variation.

Only 22 subjects had valid measurements for analysis. The mean ± SD of central corneal thickness (CCT) was 534 ± 26 μm, epithelial thickness was 42 ± 8 μm, Bowman's layer thickness was 19 ± 7 μm, and stromal thickness (ST) was 472 ± 25 μm. There was no significant difference in the between-visit thickness measurements of each layer with the CS4 (z ring) (paired t-tests, p > 0.05). The limits of agreement of between-visit measurements were -41 (8%) to 37 μm (7%) for CCT, -21 (50%) to 19 μm (45%) for epithelial thickness, -13 (68%) to 17 μm (89%) for Bowman layer thickness, and -46 (10%) to 37 μm (8%) for ST.

Measurements of CCT and ST with the CS4 (z ring) showed reasonably good repeatability (7 to 10%). However, the repeatability of measurements of the thinner corneal layers, such as epithelium and Bowman's layer, was poor.