Corneal measurements

Total corneal measurements with swept-source OCT and Scheimpflug camera repeatable but not interchangeable

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The total corneal refractive power (TCRP) measurements of the Pentacam Scheimpflug camera and the total corneal power measurements (TK) of the IOLMaster (ZEISS) swept-source ocular coherence tomographer (OCT) both have high repeatability but cannot be used interchangeably, reports Mehdi Shajari MD, Ludwig Maximilians University Hospital, Munich, Germany. Dr Shajari noted that, like the Scheimpflug imaging, the recently introduced optical biometer, the IOLMaster 700, can measure the total corneal power. He and his associates conducted a study to evaluate the repeatability and validity of TK compared to TCRP measurements of the Pentacam. During the 37th Congress of the ESCRS in Paris, France, he described a prospective randomised controlled trial that measured 94 eyes of 94 patients three times with the IOLMaster 700 swept-source optical biometer, and three times with the Pentacam (Oculus) Scheimpflug camera. The investigators took the measurements in a random order. None of the patients had irregular corneas, previous corneal surgery or dry eyes. Also excluded were eyes with corneal astigmatism above 3.0D. They then compared TK values obtained with the IOLMaster 700 to standard K obtained with the same device, and to SimK, total corneal refractive power and the true net power (TNP) values obtained with the Scheimpflug device. They analysed repeatability by calculating the within-subject standard deviations and analysed the validity of the optical biometer measurements by Bland Altman Analysis, Dr Shajari explained. They found that repeatability was 0.4 for TK, 0.42 for standard K, 0.45 for TCRP, 0.43 for TNP and 0.56 for SimK. Bland-Altman analysis revealed no significant difference between standard K and TK (p=0.193), with a mean difference of -0.047D and coefficient of repeatability of 0.326. However, they found a significant difference when comparing TK to TCRP with a mean difference of 0.018D and coefficient of repeatability of 0.612 (p=0.001). Moreover, they found a difference greater than 0.5D between the TK and TCRP astigmatism values in 11 cases. The closest correspondence between the two devices regarding the astigmatism values was between the mean astigmatism vectors of TK and TNP (0.01 @ 3 degrees). “Repeatability of both instruments was high although meridians show great variability, suggesting numerous measurements must be made. In addition, total corneal refractive power measurements between the recently introduced TK and TCRP cannot be used interchangeably. It is always best to stick to one device,” Dr Shajari concluded. Mehdi Shajari: mshajari@med.lmu.de
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