Asphericity of the anterior human cornea with different corneal diameters

Abstract

There were differences in sagittal corneal height calculations considering constant or variable models of Q. Concern arises when surgical interventions depend on corneal Q-values to predict the outcomes. Surgeons should be aware which procedure is behind Q computing by different corneal topographers and that a constant Q-value cannot reflect the actual shape of the cornea as significant departures from the actual sagittal height can arise depending on which Q-value is assumed.

Characterization of corneal structure in keratoconus

“…It should be considered that the geometry of the anterior corneal surface of the human cornea can be adjusted to a conical section that is characterized by an asphericity (Q) and a specific value for the radius of the apical curvature (r 0 ). 41,42 The Q characterizes the gradual curvature change in the corneal surface from center to periphery. [42][43][44] The most commonly accepted value for Q in young adult patients is À0.23 G 0.08 (prolate ellipsoid).…”

“…[42][43][44] The most commonly accepted value for Q in young adult patients is À0.23 G 0.08 (prolate ellipsoid). 41,45 Positive values of Q would denote the presence of an oblate surface (ie, corneas after myopic ablation with the excimer laser or orthokeratology treatments), and a null value (Q Z 0) would represent a completely spherical surface. In an ectatic cornea (Figure 2), the anterior protrusion generates an increase of the corneal prolatism with an associated negativization of Q (Table 1).…”

Relationship between anterior corneal asphericity and refractive variables

“…In addition, Q-values were calculated for different corneal diameters (3.0 mm, 4.0 mm, 5.00 mm. 6.00 mm, 7.00 mm and 8.00 mm) using the calculation procedures implemented in the Vol-CT 6.89 package (Sarver & Associates Inc.) for a 7 mm pupil size, following the recommendations of González et al [18].…”

“…This idea is consistent with the results of an earlier study by Patel et al [17], in which it was found that the conic section is a poor estimate of the corneal periphery, and the authors suggested the use of different conic sections with different shape factors depending on the region to be represented. Recently, Gonzalez-Méijome et al [18] reported that the rate of change in corneal Q with different corneal diameters increases as corneal astigmatism increases. However, no previous study has been designed to explore the relationship between Q and corneal power for different diameters.…”

A new method to accurately assess post-laser-surgery refractive changes with the intrinsic corneal power changes

“…The explanation for these results is straightforward. Before refractive surgery, the corneal anterior surface is prolate [12], which means the lowest of the radius of curvature is at the apex of the cornea. However, after refractive surgery, the corneal anterior surface is oblate [13,14], meaning that the highest radius of curvature is at the apex of the cornea.…”