[Comparison of Pentacam and OrbscanII systems for the diagnosis of keratoconus suspects]

To compare the sensitivity of Pentacam and OrbscanII and to analyze the morphology characteristic of topography in normal eyes and different stages of keratoconus, to provide the basis for early screening of keratoconus suspects.

One hundred and ninety six normal eyes, 50 eyes with keratoconus suspect and 73 eyes with clinical keratoconus were enrolled. The changes of corneal anterior/posterior curvature, best fitting sphere (BFS). Inferior-Superior value (I-S) (3 mm and 5 mm), classification of elevation maps and corneal thickness were measured.

There were significant differences including posterior curvature, anterior/posterior BFS, elevation value and corneal thickness in normal eyes between the data measured by these two different instruments, values obtained by Pentacam were less than those by Orbscan. Concerning the elevation maps pattern, the progressively decreasing pattern was dominant in Pentacam with normal eyes and keratoconus suspects, with the progressively increasing pattern accounting for 14.4% (28/196) and 20.0% (10/50) respectively in anterior surface, and 2.0% (41/96) and 26.0% (13/50) in posterior surface. The progressively increasing pattern was in dominant with clinical keratoconus, accounting for 91.7% (67/73) and 94.5% (69/73) respectively in anterior and posterior surface. Progressively increasing pattern in Orbscan was presented in 80.4% (165/196) normal eyes and suspected or clinical stage keratoconus. As to the receiver-operating-characteristic (ROC) curves and cutoff value, Orbscan I-S value (3 mm and 5 mm) of anterior and posterior surface was sensitive for the diagnosis of keratoconus suspects, with the cutoff value of anterior and posterior 5 mm I-S at 1.15 D and 0.65 D, respectively. In addition to the sensitivity of I-S values in Pentacam, the elevation value of anterior and posterior surface also displayed important diagnostic meaning. The cutoff of anterior, posterior elevation values was 4.5 µm and 7.5 µm, respectively.

Numerous measured indicators revealed obviously difference between the two instruments in normal eyes, with slightly smaller values in Pentacam. In addition to I-S values, pattern changes and elevation values in elevation map of Pentacam represent remarkable advantages for the screening of keratoconus suspects. There is an incremental risk of corneal ectasia in the increasing pattern of elevation map.