The role of reference body selection in calculating posterior corneal elevation and prediction of keratoconus using rotating Scheimpflug camera

Authors


Illés Kovács
Department of Ophthalmology
Semmelweis University Budapest
Faculty of Medicine
39 Mária str.
1085, Budapest
Hungary
Tel: (36) 20 9617435
Fax: (36) 1 317 90 61
Email: kovacsilles@yahoo.com

Abstract.

Purpose:  To explore the effects of corneal astigmatism and asphericity on posterior elevation values calculated by different reference bodies and to compare their predictive accuracy in the diagnosis of keratoconus.

Methods:  In this prospective observational case series, 44 eyes of 24 patients with keratoconus and 70 eyes of 41 refractive surgery candidates as a control group were measured by the Pentacam Scheimpflug camera. Discriminating ability and predictors of posterior elevation measurements obtained by best fit toric ellipsoid (BFTE) and best fit sphere (BFS) reference surfaces were compared by receiver operator characteristic curves (ROC) and generalized estimating equation (GEE) models. Bland–Altman plots were used to determine the agreement between different reference surfaces.

Results:  Receiver operator characteristic curve analysis showed that posterior elevation measured by a BFTE auto had a significantly higher area under ROC curves (0.99) value than BFTE 8-mm or BFS reference surfaces. ROC analysis identified cut-off values for BFTE auto (9.5-μm), for BFTE 8-mm (10.5-μm), for BFS auto (16.5-μm) and for BFS 8-mm (15.5-μm) reference surfaces. According to GEE models, corneal cylinder and posterior asphericity had the least effect in toric ellipsoid models. Bland–Altman plots showed a systematic bias at higher values of average posterior elevation measured BFS reference surfaces.

Conclusions:  Posterior corneal elevation value measured by the Pentacam camera can effectively discriminate keratoconus from normal corneas although measured values and cut-off points depend on the selection of reference body and corneal asphericity. Toric ellipsoid reference surface seems to be the most sensitive method to differentiate keratoconus.

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