Keratoscopy and Placido Disk–based Topography
The keratoscope, which is based on the original prototype invented by Antonio Plácido da Costa in 1880 (Placido disk), projects circular, concentric, illuminated mires onto the corneal surface. Unlike keratometry, which measures quantitative data and is limited to the central 4.0 mm, keratoscopy provides a qualitative evaluation of a larger surface of the cornea, based on the shape and dimension of and the distance between the projected rings. It is helpful in the detection of paracentral and peripheral disorders of corneal contour, such as keratoconus, pellucid marginal degeneration, and astigmatism following penetrating keratoplasty (Fig 2-13). In addition, distorted mires can aid clinicians in identifying surface irregularity.
Figure 2-12 Diagrammatic representation of manual keratometry. A, Misalignment of the horizontal axis (plus sign). B, Correct alignment of the horizontal axis just before the power is corrected (horizontal dial). C, Alignment of axes and vertical and horizontal power.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.