Scanning-slit and Specular Microscopic Pachymetry in Comparison With Ultrasonic Determination of Corneal Thickness

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Abstract
To determine the central corneal thickness values in healthy eyes with the recently developed Orbscan scanning-slit system, contact and noncontact specular microscopic pachymetry and compare the results to conventional ultrasonic pachymetry. In the following sequence, Orbscan, Topcon SP-2000P noncontact specular microscope, AL-1000 ultrasound, and Tomey contact specular microscope were used to record thickness values. Thirty-four healthy right corneas of 34 healthy subjects were investigated. Orbscan pachymetry correlated significantly with ultrasound (r = 0.64, p < 0.001), contact (r = 0.45, p < 0.001), and noncontact specular microscopy (r = 0.72, p < 0.001). Likewise, the Topcon SP-2000P noncontact specular microscopy pachymetry disclosed similar statistical results compared with ultrasound (r = 0.88, p < 0.001), and contact specular microscopy pachymetry (r = 0.76, p < 0.001). The mean central corneal thickness results were significantly higher (p ≤ 0.01) than ultrasonic values (580 ± 43 μm) using the contact specular microscope (640 ± 43 μm) or Orbscan system (602 ± 59 μm) but were significantly lower (p < 0.001) using the noncontact specular microscope (547 ± 49 μm). The results indicate that the devices tested cannot be simply used interchangeably. For long-term patient follow-up, one specific instrument is recommended. Recently developed pachymetry machines are especially helpful when additional corneal data such as thickness profile, elevation maps, anterior chamber depth, and endothelial morphology are required.