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    New approach to corneal power estimation in eyes with previous corneal refractive surgery and keratoconus eyes

     

    In keratoconus, the keratometric approach has been found to lead to significant errors in the estimation of the corneal power.6 These errors may generate relevant errors in IOL power calculation when keratoconus patients require cataract surgery. The keratometric approach in keratoconus has been shown to overestimate the Gaussian corneal power in a range between 0.5 and 2.5 D.6 For this reason, we also developed an adjusted keratometric approach for corneal power estimation, applicable to the keratoconic cornea.7 In this case, eight linear algorithms for different levels of keratoconus severity were developed to obtain the adjusted corneal power by considering only the anterior radius of curvature of the central cornea. With this approach, differences between adjusted and Gaussian corneal power were found to be below ±0.7 D in a sample of 44 keratoconus eyes that was used for the clinical validation of this method of corneal power calculation (Figure 1).7Figure 1: Interchangeability analysis of adjusted corneal power using the Bland–Altman analysis. The Bland–Altman plot shows the differences between adjusted and Gaussian powers against the mean value of both. The upper and lower lines represent the limits of agreement calculated as the mean of differences ±1.96 standard deviations.Figure 1: Interchangeability analysis of adjusted corneal power using the Bland–Altman analysis. The Bland–Altman plot shows the differences between adjusted and Gaussian powers against the mean value of both. The upper and lower lines represent the limits of agreement calculated as the mean of differences ±1.96 standard deviations.

    In conclusion, the use of a single value of keratometric index for the estimation of total corneal power calculation is imprecise, especially in corneas with keratoconus or after myopic laser refractive surgery. The error associated with the keratometric approach can be minimised by using an adjusted keratometric index, consisting of a variable keratometric index that depends on the radius of the anterior corneal surface. Using the adjusted keratometric index may avoid incorrect approaches to keratoconus detection,8 may provide more exact determination of corneal astigmatism and IOL power calculation and may even allow the clinician to perform an improved contact lens fitting. All these potential benefits of adjusted keratometric power are being confirmed in ongoing studies.

     

    References

    1. Camps VJ et al., Optom. Vis. Sci. 2012; 89: 221-228.

    2. Camps VJ et al., Optom. Vis. Sci. 2013; 90: 639-649.

    3. Piñero DP et al., J. Cataract. Refract. Surg. 2012; 38: 1333-1338.

    4. Camps VJ et al., Cornea 2013; 32: 1454-1459.

    5. Camps VJ et al., J. Ophthalmol. 2015; 2015: 824293.

    6. Piñero DP et al., Cornea 2014; 33: 274-279.

    7. Camps VJ et al., Cornea 2014; 33: 960-967.

    8. Piñero DP, Camps V and Caravaca-Arens E. Int. J. Kerat. Ect. Cor. Dis. 2015; 4: 41-46.

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