Measurement definitions

Zones and center

Depending on the Center line choice on the results screen, one of the two definitions below applies.

Center line: Vertex

Center definition: the anterior (outer) surface is modeled as a smooth arc by a conic fit over the full valid extent; the APEX of this arc (the steepest point) is the zone center. This is a disease-independent ANATOMICAL landmark, so the zone frame keeps the same meaning in non-keratoconus assessments (the thinnest point is a disease-oriented landmark and is shown separately). Zones are absolute-distance rings around this center: z1 |d| < 1 mm (center), z2 1–2, z3 2–3, z4 3–4, z5 4–5 mm (periphery); each ring is the union of both sides and reading from center to periphery follows the disease gradient. The thinnest point (TP) is displayed separately; the Apex box states which zone it falls in (TP–center separation is informative in keratoconus). TP-window measurements (TP ± 0.5 mm) remain anchored to TP. Honesty note: the steepest point is an ESTIMATE from a single meridian and may be affected by scan tilt; the true Kmax location requires multi-meridian topography. Thicknesses are measured perpendicular to the anterior surface; zone membership is defined by horizontal distance.

Center line: Thinnest point

Center definition: the zone center is the THINNEST POINT (TP) — the column with the minimum median-smoothed total perpendicular thickness among quality-passing columns. Being derived from the thickness profile, TP is unaffected by scan tilt and is the clinical reference point of keratoconus assessment (the Pentacam pachymetry map and the Belin-Ambrósio protocol are TP-centered); this choice serves DISEASE-ORIENTED follow-up. Zones are absolute-distance rings: z1 |d| < 1 mm (center), z2 1–2, z3 2–3, z4 3–4, z5 4–5 mm (periphery). The steepest point (conic apex) is shown separately as an ANATOMICAL landmark; its distance and zone appear in the Apex box. TP-window measurements (TP ± 0.5 mm) stay anchored to TP under either choice. Thicknesses are perpendicular to the anterior surface; zone membership is horizontal distance.

Perpendicular thickness

Thicknesses are measured PERPENDICULAR to the anterior surface, not as vertical pixel distance: perpendicular thickness = vertical extent × cos θ, where θ is the local surface slope. Vertical measurement systematically overestimates thickness in the sloped periphery; perpendicular measurement corrects this.