Study of Clinical Performance of Corneal Topography Measurement Using an Optical Biometer

Corneal topography is a measurement of the shape of the anterior cornea. The shape of a cornea can be fully quantified by providing a map of local power. "Diopter" is the unit of refractive power of a lens. In case of the cornea, the power K [diopter] is related to the radius (curvature) R [mm] of the best fitting sphere by the relation K=337.5/R.

Here, corneal topography measurements are implemented by the Placido method, i.e. by analyzing the reflection image of a ring-shaped illumination.

According to International Standards Organization (ISO) 19980-2012, repeatability of corneal topography is assessed on the central cornea: area with diameter d<=3mm, and middle cornea: 3mm<d<=6mm.

"power difference (2 rep. meas.)": sample mean and 95% C.I. of differences of spatial mean of corneal power between two consecutive measurements.

"power difference 2 std.dev.": sample mean of twice the deviation of local power differences (two consecutive measurements).

For definition of corneal topography, corneal power in "diopter", areas of evaluation and methods, see outcome 1.

This outcome measure aims at testing the equivalence of corneal topography measurement between Lenstar LS 900 Topography (Haag Streit) and Atlas 9000 (Zeiss) by analyzing differences in measurement results for the same eye between both devices.

"power difference (2 devices)" is the sample mean and 95% C.I. of differences of spatial mean of corneal power between two devices. This value quantifies systematic differences between devices (e.g. calibration), ignoring local variations of the power measurement.

"power difference (2 std.dev.)" is the sample mean of twice the standard deviation of local power differences between measurements with both devices. This value quantifies the spatially resolved agreement of corneal shape measurement of the two devices.

For definition of corneal topography, areas of evaluation and methods, see outcome 1 and 2.

Corneal elevation refers to the distance between the measured corneal surface and the best fitting sphere, and is given in µm.

This outcome measure aims at testing the equivalence of corneal topography measurement between Lenstar LS 900 Topography (Haag Streit) and Atlas 9000 (Zeiss) by analyzing differences in measurement results for the same eye between both devices.

"elevation difference (2 std. dev.)" is the sample mean of twice the standard deviation of local corneal elevation differences between measurements with both devices. This value quantifies the spatially resolved agreement of corneal shape measurement of the two devices.

Keratometry radius refers to the corneal curvature R (see primary measure outcome). In this context, the cornea is approximated by a toric surface which can be characterized by a flat meridian (radius R1) and a steep meridian (radius R2) with an angle of 90 degrees between these meridians.

This outcome measure aims at testing the equivalence of keratometry radius measurement between Lenstar LS 900 Topography and Lenstar LS 900 (both Haag Streit) by analyzing differences in measurement results for the same eye between both devices.

Reported are: population mean of difference and 95% confidence interval of mean difference for radius of flat meridian (R1) and radius of steep meridian (R2).

Keratometry axis refers to the axis of the flat meridian of the toric representation of the cornea. For additional information see outcome 4.

This outcome measure aims at testing the equivalence of keratometry axis measurement between Lenstar LS 900 Topography and Lenstar LS 900 (both Haag Streit) by analyzing differences in measurement results for the same eye between both devices.

Reported are: population mean of difference and 95% confidence interval of mean difference for axis of flat meridian.