Corneal Tomography: Key Parameters and Take-Home Rules

Quality Specification (QS)

The quality specification (QS) defines the reliability of tomographic capture. If QS indicates poor quality or there is misinformation reproduced by the computer, repeat the capture.

q value

q represents the corneal asphericity of the anterior surface. The ideal measurement is within the central 6 mm. Normal q is approximately between -1 and 0; positive q occurs with oblate corneas. Large negative q can appear after myopic ablation or radial keratotomy (hyperprolate corneas), while positive q can appear after hyperopic ablation or in keratoconus. Oblate and hyperprolate corneas induce spherical aberrations; consult chapter 6 for q-value rules.

Curvature powers and Km

k1: flat meridian curvature in the central 3 mm; normal k1 is greater than 34 D. Each -1 D correction reduces k1 by 0.75 D; final k1 should be > 34 D.
k2: steep meridian curvature in the central 3 mm; normal k2 is less than 47 D. Each +1 D correction adds 1.2 D to k2; final k2 should be < 49 D. Km: mean curvature power of the anterior surface within the central 3 mm; displayed in some machines as average k. Important for flap measurements. Km < 40 D may lead to a free flap; Km > 46 D may lead to a buttonhole.

Kilometers (Km) / mean keratometry

Mean curvature power of the anterior cornea within the central 3 mm, sometimes shown as average k. Used for flap planning. Km < 40 D may result in a free flap; Km > 46 D may result in a buttonhole.

Kmax

Kmax is the maximum curvature power of the entire anterior surface. Normal Kmax < 48 D. Inter-eye difference in Kmax < 2 D. A normal difference is commonly around 1 D. Kmax values > 47 D are considered abnormal; large inter-eye or inter-surface differences can indicate irregularities.

Astigmatism and Axis

Astic is the amount of corneal topographic astigmatism on the anterior surface; normal < 6 D. Axis is the axis of anterior corneal astigmatism within the central 3 mm.

Pachy apex and coordinate system

Pachy apex is the thickness at the apex, treated as the origin where x is horizontal and y is vertical. Positive x is from the patient’s right eye toward the left when seated opposite the physician; positive y is upward. Example coordinates describe the thinning/thickness location relative to the apex.

Pupil center and angle kappa

Pupil center coordinates impact angle kappa and centration. Normal pupil center x coordinate is ≤ 200 μm; x > 200 μm indicates decentered pupil and a significant angle kappa, necessitating centration considerations.

Pupil diameter and illumination

Pupil diameter depends on illumination ( photopic, mesopic, scotopic ). Pupil size should be adjusted for planning: the optical zone should be at least 0.5 mm larger than the scotopic pupil diameter.

Thinnest location and thickness rules

Thickness at the thinnest location: normally > 500 μm; 450–500 μm is suspect; < 450 μm is abnormal. The inter-eye difference in thinnest thickness is typically < 30 μm. The difference between the thinning location and the pachy apex thickness should be ≤ 10 μm. Y coordinate of thinning: categories usually < 0.5 mm normal, 0.5–1.0 mm suspect, > 1.0 mm abnormal.

Anterior chamber metrics

ACV < 100 mm³, ACA < 24°, or ACD < 2.1 mm may indicate risk of angle-closure glaucoma. For phakic IOLs, ACD should be > 3.0 mm and ACA > 30°. PIOLs are contraindicated when ACD < 3.0 mm.

Take-home messages

For myopic treatment, each -1 D correction reduces flat keratometry by 0.75 D; final k1 should be > 34 D. For hyperopic treatment, each +1 D correction increases steep keratometry by 1.2 D; final k2 should be < 49 D. Km thresholds: Km < 40 D may allow a free flap; Km > 46 D may cause a buttonhole. Kmax > 47 D is abnormal; inter-eye differences in Kmax or differences between Kmax and steep Kmax > 1 D (or > 2 D between eyes) are concerning. q value should remain within approximately -1 to 0 within the central 6 mm; corrections beyond -4 D or +3 D require a special profile to avoid q corruption and spherical aberrations. Normal pupil center x ≤ 200 μm; x > 200 μm indicates significant angle kappa and centration is needed. Normal thinnest thickness > 500 μm; 450–500 μm is suspect; < 450 μm is abnormal, with ≤ 30 μm inter-eye difference. Thinning Y coordinate thresholds: < 0.5 mm normal, 0.5–1.0 mm suspect, > 1.0 mm abnormal. Optical zone should be at least 0.5 mm larger than the scotopic pupil; normal anterior chamber angle > 24°, ACV > 100 mm³, ACD > 2.1 mm; PIOLs contraindicated if ACD < 3.0 mm.