Reviewed by Alessandro Franchini
The use of a keratometric index to estimate total corneal power calculation is imprecise, especially in corneas with keratoconus or after myopic laser refractive surgery. The errors can be reduced by using an adjusted keratometric index, consisting of a variable keratometric index that depends on the radius of the anterior corneal surface.
The calculation of the exact value of corneal power is a key factor in obtaining a precise calculation of the power of IOL to implant in cataract surgery. The anterior and posterior surfaces of the cornea contribute to its total power; therefore, for very accurate calculation of corneal power, the radii of curvature of the anterior and posterior corneal surfaces should be considered as well the central corneal thickness and the true refractive index of the cornea. With all these parameters, the Gaussian corneal power can be calculated assuming paraxial optical conditions.
In clinical practice, total corneal power is usually calculated from the radius of curvature of the anterior corneal surface only. In this context, the concept of keratometric index (nk
) was developed, assuming a corneal model consisting of a single spherical surface with the radius of curvature of the epithelial surface, and a fictitious index of refraction that provides a correction for this simplified model. The simplification assumes that the contribution of the posterior corneal surface to the ocular refractive power is limited, but this may be untrue when anomalous or surgically modified corneal curvatures are present. Our research group demonstrated that the keratometric estimation over- or underestimated the Gaussian corneal power in a range from –1.12 D to +1.80 D in the normal healthy eye.1 Furthermore, the use of keratometric corneal power with the classical keratometric index of 1.3375 was found to induce a maximum underestimation of –3.01 D in IOL power calculation in normal eyes with cataract.2 Therefore, the best approach to obtaining the corneal power is to measure anterior and posterior corneal curvature and to perform a Gaussian calculation.
Adjusted keratometric index
As devices measuring the posterior corneal curvature are not available in all clinical settings, we developed the concept of adjusted keratometric index (nkadj), which is a clinically valid method for estimating an appropriate keratometric index, allowing the corneal power to be calculated using the keratometric approach, but with enough accuracy.3 This method requires postoperative measurement of the anterior corneal radius only, which can be easily obtained in any clinical setting.
In eyes that have had previous keratorefractive surgery for the correction of myopia, several methods have been proposed in recent years for the estimation of corneal power, some of which require historical clinical data that are not always available. Our research group recently proposed a new approach based on the concept previously mentioned, adjusted keratometric index.4 The corneal power calculated with this keratometric index (adjusted corneal power) has been clinically validated in 62 eyes of 62 patients with previous myopic LASIK surgery, comparing it with Gaussian corneal power and other corneal power values calculated with other previously described methods (clinical history, Awwad, Savini, Camellin, Jarade, Shammas, Seitz and Haigis-L).5 The mean (standard deviation) difference between the adjusted corneal power and the Gaussian value in this sample was 0.14 (0.24) D, with a range of agreement from –0.33 to 0.60 D, with only 4 out of 66 cases showing differences of more than ±0.5 D between adjusted and Gaussian power. Compared with the other methods of corneal power estimation, the adjusted corneal power was found to be interchangeable with the estimation obtained using the Haigis-L method, whereas the other methods were observed to provide corneal power values that were significantly different from the adjusted and Gaussian values (overestimation).5 Therefore, the adjusted keratometric index is a valid and easy method to estimate the central corneal power in corneas with previous myopic laser refractive surgery, improving the accuracy of most methods described previously.
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.
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2015; 4: 41-46.