Improving estimation of corneal refractive power by measuring the posterior curvature of the cornea

Comparison of Mean Corneal Power of Annular Rings and Zones Using Swept-Source Optical Coherence Tomography

This study aims to investigate differences in the mean corneal power of annular zones (corneal power measured over the inner annular zone of difference diameters) and rings (corneal power measured over a ring of different diameters) centered on the corneal apex using the swept-source optical coherence tomography technique. The mean anterior axial curvature (AAC), posterior axial curvature (PAC), and total corneal power (TCP) centered on the corneal apex with the annular rings (0−2 mm, 2−4 mm, 4−6 mm, and 6−8 mm) and zones were assessed using the ANTERION device. The paired-sample t-test was used for data comparison. For the 0−2 mm comparison, the AAC, PAC, and TCP values of rings and zones were interchangeable. For the 2−4 mm comparison, the AAC of the rings was lower than that of the zones (p = 0.004), and the TCP values of the rings were higher than that of the zones (p < 0.001). For the 4−6 mm comparison, the AAC of the rings was lower than that of the zones (p < 0.001), and the PAC and TCP values of the rings were higher than that of the zones (both p < 0.001). For the 6−8 mm comparison, the AAC of the rings was lower than that of the zones (p < 0.001), and the PAC and TCP values of the rings were higher than that of the zones (both p < 0.001). Comparisons between AAC and TCP in each sub-region showed significant differences both in the rings (p < 0.001) and the zones (p < 0.008). Differences in the AAC, PAC, and TCP measured at different diameters (2−4 mm, 4−6 mm, and 6−8 mm) of the rings and zones, centered on the corneal apex, should be noticed in clinical practice. As the diameter increases, the difference between the rings and the zones in terms of AAC, PAC, and TCP increase as well. Clinicians should also pay attention to differences between AAC and TCP for the rings and the zones within the same annular region.

Posterior corneal astigmatism: a review article

Most human eyes show at least a small degree of corneal astigmatism and it can arise from both surfaces of the cornea. The shape of the anterior corneal surface provides no definitive basis for knowing the toricity of the posterior surface. In the previous studies, average astigmatism of the posterior corneal surface was -0.26 to -0.78 diopter. The radius of the posterior corneal surface is less than the radius of the anterior corneal surface. Most studies have found a clear correlation between the anterior and posterior corneal asphericities and the asphericity of the posterior surface is independent of the vertex radius of curvature, refractive error and gender. In contrast to the anterior corneal surface, the asphericity of the posterior corneal surface varies significantly between meridians. The anterior and posterior corneal surface would have approximately parallel principal meridians and both of these surfaces are often flatter in the horizontal meridian than the vertical one. This is especially true in the higher degrees of corneal astigmatism, and then about 10% of any anterior corneal astigmatism is neutralized by an astigmatism arising from the posterior corneal surface. Although the second corneal surface only contributes to about 10% of the total refractive power of the eye, a precise knowledge of its morphology is needed for the correct diagnosis and monitoring the corneal diseases or the surgical interventions and in many eyes neglecting the posterior corneal surface measurement may lead to significant deviations from the corneal astigmatism estimation. In this article, we have reviewed the shape and the toricity of the posterior corneal surface and also the effect of age on it. We investigated the contribution of posterior corneal astigmatism to the total corneal astigmatism and evaluated the accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement.

Agreement between Pentacam and videokeratography in corneal power assessment

PURPOSE

To investigate agreement between a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH) and two corneal topographers (TMS-2 Topography System, Tomey; and Keratron Scout, Optikon 2000 SpA) in measuring the corneal power of normal eyes.

METHODS

The mean corneal powers calculated by simulated keratometry (SimK) with each topographer were compared to those provided by the Pentacam in 71 patients. Specifically, the corneal power values of the Pentacam included in this analysis were the SimK (calculated using the measured anterior corneal radius and standard keratometric index of 1.3375) and the True net power (calculated using the anterior and posterior corneal curvatures and Gaussian optics formula for thick lenses, where the actual refractive index of the air, cornea, and aqueous humor are entered). Bland-Altman plots were used to investigate agreement and analysis of variance (ANOVA) was performed to detect statistical differences.

RESULTS

Although ANOVA did not disclose a statistically significant difference among the mean SimK values (TMS-2: 43.20 +/- 1.51 diopters [D], Keratron Scout: 43.29 +/-1.48 D, Pentacam: 43.25 +/- 1.53 D), the 95% limits of agreement between the TMS-2 and Pentacam and between the Keratron Scout and Pentacam were wide (-1.05 to +0.94 D and -0.95 to +1.02 D, respectively). Agreement was even poorer when considering the mean True net power (42.00 +/- 1.54 D), which was significantly lower than the mean Pentacam SimK (P < .001).

CONCLUSIONS

Although corneal topography and the Pentacam provide similar SimK values, their data should not be used interchangeably as only moderate agreement exists between them. Corneal power values calculated by the True net power are significantly lower than any SimK and cannot be entered into intraocular lens power formulas.

A New Formula for Intraocular Lens Power Calculation After Refractive Corneal Surgery

PURPOSE

When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed.

METHODS

A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial keratotomy [n = 5]) and compared our results to other formulas.

RESULTS

Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosa's method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D.

CONCLUSIONS

In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

Vector summation of anterior and posterior corneal topographical astigmatism

PURPOSE

To determine whether vector addition of posterior corneal astigmatism improves the correlation between topographical and refractive astigmatism.

SETTING

Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, USA.

METHODS

Orbscan topographical maps of 40 eyes (31 patients) showing 1.0 to 6.0 diopters (D) of astigmatism were analyzed. Topographical anterior and posterior corneal surface astigmatism was determined. Refractive astigmatism was compared to topographical astigmatism using 3 methods: anterior topographical astigmatism, overall topographical astigmatism obtained by vector summation of anterior and posterior topographical astigmatism, and simulated keratometry (SimK).

RESULTS

Refractive astigmatism ranged from 0.25 to 5.00 D. The mean error in magnitude between topographical and refractive astigmatism was significantly smaller with the overall topographical method (1.06 D +/- 0.92 [SD]) than the anterior topographical method (1.37 +/- 1.04 D) (P <.0001). The mean error in axis between topographical and refractive astigmatism was smaller with the overall topographical method (10.4 +/- 13 degrees) than with the anterior topographical method (15.5 +/- 30.6 degrees) and with SimK (13.3 +/- 15.1 degrees), but these differences were not statistically significant.

CONCLUSION

Consideration of the Orbscan measurement of posterior corneal surface toricity may improve the prediction of the magnitude of refractive astigmatism.