Research Article | Open Access

# Clinical Validation of Adjusted Corneal Power in Patients with Previous Myopic Lasik Surgery

**Academic Editor:**Vasilios F. Diakonis

#### Abstract

*Purpose*. To validate clinically a new method for estimating the corneal power () using a variable keratometric index () in eyes with previous laser refractive surgery. *Setting*. University of Alicante and Medimar International Hospital (Oftalmar), Alicante, (Spain). *Design*. Retrospective case series. *Methods*. This retrospective study comprised 62 eyes of 62 patients that had undergone myopic LASIK surgery. An algorithm for the calculation of was used for the estimation of the adjusted keratometric corneal power (). This value was compared with the classical keratometric corneal power (), the True Net Power (TNP), and the Gaussian corneal power (). Likewise, was compared with other previously described methods. *Results*. Differences between and values obtained with all methods evaluated were statistically significant (). Differences between and were in the limit of clinical significance (, loA [−0.33,0.60] D). Differences between and TNP were not statistically and clinically significant (, loA [−0.50,0.44] D). Differences between and previously described methods were statistically significant (), except with (, loA [−0.37,0.29] D). *Conclusion*. The use of the adjusted keratometric index () is a valid method to estimate the central corneal power in corneas with previous myopic laser refractive surgery, providing results comparable to .

#### 1. Introduction

The precise measurement of corneal power after myopic laser refractive surgery is still currently an issue under debate. Several methods have been proposed during the last years which are classified as methods requiring historical clinical data and methods not requiring historical data. Among those methods requiring previous clinical data, some of them are based on a correction of the corneal power using the refracting change achieved [1–3] and others on performing such correction by adjusting the keratometric index [4–7]. The main disadvantage of all these methods is that they are infeasible if previous clinical patient’s data are not available. For this reason, other methods that do not require patient’s historical data have been developed [3, 5, 8–11]. In this line, our research group has recently proposed a new method for estimating with enough accuracy the corneal power using the keratometric approach that has been found to be valid in both healthy [12] and post-LASIK eyes [13]. This algorithm based on a variable keratometric index was named adjusted keratometric index () and it has been prevalidated clinically in a sample of 32 eyes that had undergone previously myopic LASIK surgery [13]. The aim of the current study is to validate clinically this algorithm for the estimation of the corneal power in eyes with previous myopic LASIK in a larger population including also a larger range of intended refractive corrections.

#### 2. Methods

##### 2.1. Patients and Examination

This retrospective study comprised 62 eyes of 62 patients that had undergone previous correction of a myopic refractive error by means of laser in situ keratomileusis (LASIK) surgery. All LASIK surgeries had been performed using the Pulzar Z1 solid-state laser (CustomVis Laser Pty Ltd., Osborne Park, Australia, currently CV Laser Pty Ltd.) at the Department of Ophthalmology (Oftalmar) of the Vithas Medimar International Hospital (Alicante, Spain). All surgeries had been performed by one experienced surgeon (AA) between October 2012 and December 2013. For this study, only one eye from each subject was chosen according to a random number sequence (dichotomic sequence, 0 and 1). A comprehensive ophthalmologic examination was performed in all cases at least 3 months after surgery, which included refraction, corrected distance visual acuity (CDVA), slit lamp biomicroscopy, Goldman tonometry, fundus evaluation, and the analysis of the corneal structure by means of a Scheimpflug photography-based tomographer, the Pentacam system (Oculus Optikgeräte GmbH, Germany, software version 1.14r01). All patients were informed after surgery about this retrospective study and signed an informed consent document in accordance with the Helsinki Declaration.

##### 2.2. Corneal Power Calculation

Our research group recently proposed the use of a variable keratometric index () for the estimation of the corneal power () using the keratometric approach in patients with previous myopic LASIK surgery [13]. The following expression was defined for considering the ocular conditions of the Gullstrand eye model and the range of anterior and posterior curvature that is commonly found in this kind of patients [13]:where is the postoperative anterior corneal radius.

Furthermore, adjusted keratometric corneal power () was defined as follows [13]:

For comparison purposes, the keratometric corneal power was also calculated using the classical keratometric index .

The Gaussian corneal power was calculated using the following expression:where is the Gaussian total corneal power, is the anterior corneal power, is the posterior corneal power, is the anterior corneal radius, is the posterior corneal radius, is the refractive index of air, is the refractive index of the cornea, is the refractive index of the aqueous humour, and is the central corneal thickness.

Likewise, the True Net Power (TNP) was also recorded, which is the corneal power provided by the Pentacam system (Oculus) based on the anterior () and posterior () corneal radius and calculated by using the Gaussian equation () with the Gullstrand eye model, but neglecting the corneal thickness ():

Besides this, corneal power was also estimated by using other methods described previously for such purpose in eyes with previous myopic laser refractive surgery:(1)Methods requiring previous clinical data:(a)Awwad method [3]: (b)Camellin method [4]: (c)Clinical History method:(d)Jarade method [4]: (e)Savini method [5, 6]: (2)Methods not requiring previous data:(a)Haigis-L method:(b)Shammas method [8]: (c)Seitz method [6]: where , and being the pre- and postsurgery spherical equivalents, is the postsurgery anterior corneal radius, and is the presurgery keratometric corneal power.

For the clinical validation of , it was compared with and TNP. Likewise, the different methods mentioned above were also compared with and in order to demonstrate which was the most accurate approach.

##### 2.3. Statistical Analysis

Statistical analysis was performed using the software SPSS version 19.0 for Windows (SPSS, Chicago, Illinois, USA). Normality of all data distributions was first confirmed by means of the Kolmogorov-Smirnov test. Specifically, the paired Student -test or Wilcoxon test was used for comparing the different methods of calculation depending on whether the normality condition could be assumed or not. The Bland-Altman analysis [14] was used for evaluating the agreement and interchangeability of the different methods for obtaining the corneal power.

#### 3. Results

This study comprised 62 eyes of 62 patients (34 women [54.8%]), with a mean age of years (range 21 to 52 years) and with preoperative myopia between −0.25 and −6.8 D. The sample comprised 31 left eyes (50%). Mean ocular features of the eyes evaluated in the current study can be seen in Table 1. Table 2 shows the values of corneal power estimated with the previously published methods (Awwad, Camellin, Clinical History, Haigis-L, Jarade, Savini, Seitz, and Shammas methods).

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SE_{pre} = preoperative spherical equivalent; SE_{post} = postoperative spherical equivalent; = preoperative radius of curvature of the anterior corneal surface; = postoperative radius of curvature of the anterior corneal surface; = preoperative radius of curvature of the posterior corneal surface; = postoperative radius of curvature of the posterior corneal surface; = corneal thickness. |

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= adjusted keratometric corneal power; = Gaussian corneal power; TNP = True Net Power; = keratometric corneal power using ; = corneal power obtained using Awwad formula; = corneal power obtained using Camellin formula; = corneal power obtained using Clinical History method; = corneal power obtained using Haigis-L formula; = corneal power obtained using Jarade formula; = corneal power obtained using Savini formula; = corneal power obtained using Seitz formula; = corneal power obtained using Shammas formula. |

##### 3.1. Clinical Validation of

As shown in Table 3, there were significant differences (, paired Student’s -test) between and , but not (, paired Student’s -test) between and TNP. The Bland-Altman analysis showed that differences between and were barely clinically significant (mean difference: ; limits of agreement (loA): ) and that differences between and TNP were not clinically significant (mean difference ; loA: ). A very strong and statistically significant correlation was found between and (, ) as well as between and TNP (, ).

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= adjusted keratometric corneal power; = Gaussian corneal power; TNP = True Net Power; = keratometric corneal power using ; = corneal power obtained using Awwad formula; = corneal power obtained using Camellin formula; = corneal power obtained using Clinical History method; = corneal power obtained using Haigis-L formula; = corneal power obtained using Jarade formula; = corneal power obtained using Savini formula; = corneal power obtained using Seitz formula; = corneal power obtained using Shammas formula. |

##### 3.2. Comparison between and Corneal Power Values Estimated with Other Methods

As shown also in Table 3, differences between and the rest of the methods for estimation of were statistically significant (), except for the difference between and (). The Bland-Altman analysis confirmed that all these statistically significant differences were also clinically relevant as the ranges of agreement were quite large (>±0.5 D). Only differences between and (mean: −0.04 ± 0.17 D) did not reach clinical significance (loA: [−0.37 to 0.29 D]).

Table 4 summarizes the results of the comparison between the corneal power calculated considering the curvature of the two corneal surfaces as well as corneal thickness () and those values obtained with the other previously published methods for corneal power estimation in corneas with previous myopic laser refractive surgery. As shown, all differences between and values obtained with such methods were statistically significant (). Considering the Bland-Altman analysis, and provided the lower mean differences with (−0.14 ± 0.24 D and −0.18 ± 0.21 D, resp.) and the smaller ranges of agreement ([−0.6 to 0.33] and [−0.60 to 0.23], resp.). was also close to (mean difference: 0.42 ± 0.14 D), but the range of agreement was larger than that found for and ([0.15 to 0.68 D]).

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= adjusted keratometric corneal power; = Gaussian corneal power; TNP = True Net Power; = keratometric corneal power using ; = corneal power obtained using Awwad formula; = corneal power obtained using Camellin formula; = corneal power obtained using Clinical History method; = corneal power obtained using Haigis-L formula; |