Purpose
To evaluate a new Placido topography module "T-cone" for the optical biometer Lenstar LS 900 ("LS 900 TC", Haag-Streit Diagnostics, Koeniz, Switzerland). We report in-vivo repeatability of corneal topography measurements and SimK keratometry. Also we test equivalence of corneal topography measurement of Lenstar LS 900 TC with a corneal topography device Atlas (Atlas 9000, Carl Zeiss Meditec AG, Jena, Germany) as well as equivalence of Lenstar LS 900 TC SimK keratometry with well-established Lenstar LS 900 keratometry.
Methods
Measurements of corneal topography and keratometry were obtained in 50 participants (no or regular astigmatism, N=42) and irregular cornea (irregular astigmatism > 1.5 D, N=8, ). Two consecutive corneal topography measurements were obtained from both Lenstar LS 900 TC (Software: Eyesuite i6.0.1.0, Biometry v2.1.1.) and Atlas. We determined the in-vivo repeatability as the mean absolute difference of mean corneal power. Differences between devices were evaluated by Bland-Altman analysis; equivalence was tested by means of a paired-sample t-test. Keratometry was obtained by Lenstar LS 900 and compared to SimK keratometry of Lenstar LS 900 TC measurements by Bland-Altman analysis; equivalence was tested by means of a paired-sample t-test. In-vivo repeatability of SimK keratometry is reported as mean absolute difference of R1, R2 and AX between consecutive measurements.
Results
For the group of regular corneas corneal topography measurements of Lenstar LS 900 TC are equivalent within ±0.25 D (95% confidence interval) to those of Atlas. In-vivo repeatability was comparable and better than 0.2 D. SimK keratometry measurements (expressed in equivalent refractive error) of Lenstar LS 900 TC are equivalent (95% C.I.) within ±0.25 D to keratometry of LS 900. In-vivo repeatability of LS 900 TC was better than 0.1 D for curvature and better than 0.5 D for AX. Due to small sample size statistically significant equivalence testing is not possible for group of irregular corneas.
Conclusions
Measurements of the LS 900 TC are equivalent to those of the comparison devices and can be used interchangeably in patients with regular cornea. With the current information LS 900 TC permits latest IOL calculation and discovery of corneal anomalies such as keratoconus or post-LASIK prior to cataract surgery. The two combined features (biometry, corneal topography) theoretically enable it to perform ray tracing in near future.
Keywords: 445 cataract •
567 intraocular lens •
733 topography