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Pravin K. Vaddavali, Deabrun Dutta, Parthasarathy Kalaiselvan, Swathi D; Validation Of Criteria For Diagnosis Of Keratoconus On The Orbscan And Agreement Of Orbscan Pachymetry With Ultrasound Pachymetry. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4177.
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To validate the criteria for diagnosing keratoconus using the Orbscan IIz and to test the agreement between pachymetry on the Orbscan, ultrasound pachymeter, Visante optical coherence tomography (OCT) and Optovue optical coherence tomography
100 eyes of 100 patients with clinically diagnosed keratoconus with no corneal scarring were imaged with Orbscan with an acoustic factor of 0.92 on two occasions. They also underwent pachymetry measurements with 9 point ultrasound pachymetry, Visante OCT and Optovue OCT. Maximum K values, 3 and 5 mm irregularity, minimum pachymetry, posterior float and posterior elevation were analyzed. 100 eyes of 100 normal subjects also underwent Orbscan topography and the same criteria for diagnosis of keratoconus was applied
ROC curves revealed maximum K value of 46.95, 3mm zone irregularity of 1.45, 5mm zone irregularity of 2.5, posterior float of 54.5D, posterior elevation of 44 microns and a thinnest pachymetry value of 465 microns had 99% sensitivity and 65% specificity in diagnosing keratoconus. Intersession repeatability scores were (>0 .9) for all parameters of Orbscan except for 3mm, 5mm zone irregularity and Sim ‘K’ astigmatism axis(0.633,0.777 and 0.837) respectively. Mean central ultrasound pachymetry was 450 microns and was 436 microns on the Orbscan, 437 m on the Visante OCT and 445 on the Optovue OCT
The current proposed criteria to diagnose keratoconus on the Orbscan are highly sensitive but not specific. Mean pachymetry is overestimated on ultrasound pachymetry contrary to popular belief and seem to be equal on Orbscan, Visante OCT and Optovue OCT estimation
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