Comparison of corneal Q values measured by Topcon KR-1W, iTrace and Topolyzer

Background

The measurement of corneal Q value is essential for corneal refractive surgery and calculation of intraocular lens during cataract surgery.Topolyzer was often used for the measurement of Q value, and recently Topcon KR-1W and iTrace were applied in ophthalmology.However, whether the measured values are interchangeable is unclear.

Objective

This study was to assess the difference and consistency of corneal Q values measured by Topcon KR-1W, iTrace and Topolyzer.

Methods

Corneal Q values were measured on 100 right eyes of 100 healthy subjects under the approval of Ethic Committee of the Sixth Hospital Affiliated to Shanghai Jiaotong University and informed consent of each subject from November to December in 2014 with Topcon KR-1W, iTrace and Topolyzer.Three valid measurements were obtained for each device, and the average values from each device were calculated.The difference of the outcomes among the instruments was compared by repeated measures analysis of variance (ANOVA), and the consistency among the outcomes from different apparatus was analyzed by Bland-Altman plots.

Results

The mean corneal Q values were -0.184±0.112, -0.117±0.167 and -0.269±0.117 from Topcon KR-1W, iTrace and Topolyzer, respectively, with a significant difference among them (P<0.001). The measured Q value by Topcon KR-1W was 0.085±0.010 larger than that by Topolyzer, and the Q values by iTrace was 0.152±0.014 larger than that by Topolyzer, while the Q values obtained by Topcon KR-1W was 0.067±0.016 smaller than that by iTrace (all at P<0.05). The 95% confidence interval of the values between Topcon KR-1W and iTrace, Topcon KR-1W and Topolyzer, iTrace and Topolyzer were -0.106 to -0.028, 0.060 to 0.109 and 0.118 to 0.186, respectively.Bland-Altman plots showed that 6%, 6% and 5% values were outside of 95% agreement of limit (LoA) between Topcon KR-1W and iTrace, iTrace and Topolyzer or KR-1W and Topolyzer, respectively, with the maximal differences of 0.28, 0.43 and 0.38.

Conclusions

Corneal Q values measured by Topcon KR-1W and iTrace are larger than those measured by Topolyzer.Due to the poor agreements among the corneal Q values by the 3 kinds of devices, they are not interchangeable in clinical applications for the measurement of corneal Q value.