Korean J Ophthalmol.  2018 Jun;32(3):163-171. 10.3341/kjo.2017.0075.

Comparison of Anterior, Posterior, and Total Corneal Astigmatism Measured Using a Single Scheimpflug Camera in Healthy and Keratoconus Eyes

• Choi Y
• Eom Y
• Song JS
• Kim HM

• KMID: 2412794
• DOI: http://doi.org/10.3341/kjo.2017.0075

Abstract

PURPOSE
To compare the effect of posterior corneal astigmatism on the estimation of total corneal astigmatism using anterior corneal measurements (simulated keratometry [K]) between eyes with keratoconus and healthy eyes.
METHODS
Thirty-three eyes of 33 patients with keratoconus of grade I or II and 33 eyes of 33 age- and sex-matched healthy control subjects were enrolled. Anterior, posterior, and total corneal cylinder powers and flat meridians measured by a single Scheimpflug camera were analyzed. The difference in corneal astigmatism between the simulated K and total cornea was evaluated.
RESULTS
The mean anterior, posterior, and total corneal cylinder powers of the keratoconus group (4.37 ± 1.73, 0.95 ± 0.39, and 4.36 ± 1.74 cylinder diopters [CD], respectively) were significantly greater than those of the control group (1.10 ± 0.68, 0.39 ± 0.18, and 0.97 ± 0.63 CD, respectively). The cylinder power difference between the simulated K and total cornea was positively correlated with the posterior corneal cylinder power and negatively correlated with the absolute flat meridian difference between the simulated K and total cornea in both groups. The mean magnitude of the vector difference between the astigmatism of the simulated K and total cornea of the keratoconus group (0.67 ± 0.67 CD) was significantly larger than that of the control group (0.28 ± 0.12 CD).
CONCLUSIONS
Eyes with keratoconus had greater estimation errors of total corneal astigmatism based on anterior corneal measurement than did healthy eyes. Posterior corneal surface measurement should be more emphasized to determine the total corneal astigmatism in eyes with keratoconus.

Keyword

Astigmatism; Cornea; Keratoconus; Posterior corneal astigmatism; Total corneal astigmatism

MeSH Terms

Astigmatism*
Cornea
Humans
Keratoconus*
Meridians

Figure

• 

  Fig. 1 Linear regression analysis of the relationship between anterior and posterior corneal cylinder power. The solid line represents a linear regression line (Y = 0.156X + 0.270, R2 = 0.470, p < 0.001) for the keratoconus group (filled triangles), and the dashed line represents a linear regression line (Y = 0.174X + 0.194, R2 = 0.422, p < 0.001) for the normal controls (open circles). CD = cylinder diopters.

• 

  Fig. 2 Linear regression analysis of the relationship between anterior and total corneal cylinder power. The solid line represents a linear regression line (Y = 0.914X + 0.361, R2 = 0.824, p < 0.001) for the keratoconus group (filled triangles), and the dashed line represents a linear regression line (Y = 0.894X − 0.011, R2 = 0.928, p < 0.001) for the normal controls (open circles). CD = cylinder diopters.

• 

  Fig. 3 Linear regression analysis of the relationship between posterior corneal cylinder power and cylinder power difference between the simulated keratometry (K) and total cornea. The solid line represents a linear regression line (Y = 0.928X − 0.868, R2 = 0.240, p = 0.004) for the keratoconus group (filled triangles), and the dashed line represents a linear regression line (Y = 0.786X − 0.172, R2 = 0.592, p < 0.001) for the normal controls (open circles). CD = cylinder diopters.

• 

  Fig. 4 Linear regression analysis of the relationship between absolute flat meridian difference and cylinder power difference between the simulated keratometry (K) and total cornea. The solid line represents a linear regression line (Y = −0.027X + 0.249, R2 = 0.370, p < 0.001) for the keratoconus group (filled triangles), and the dashed line represents a linear regression line (Y = −0.010X + 0.300, R2 = 0.592, p < 0.001) for the normal controls (open circles). CD = cylinder diopters.

• 

  Fig. 5 Double-angle plots of the vector difference between the astigmatism of the anterior corneal surface and total cornea. The red ellipse indicates one standard deviation. (A) Keratoconus group. (B) Normal controls. D = diopters.

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