New algorithm accurately estimates corneal power after LASIK

Clinicians can accurately estimate the central corneal power in corneas that have undergone myopic laser refractive surgery using an adjusted keratometric index, researchers said.

Results with the index were comparable to the Haigis-L method, reported Vicente J. Camps, University of Alicante, San Vicente del Raspeig, Spain, and colleagues.

The researchers published their findings in the Journal of Ophthalmology.

Researchers have long debated the best way of measuring corneal power after myopic laser refractive surgery.

The methods generally fall into two categories: those that require previous clinical data, and those that don’t.

Those requiring historical data are useless if the clinician cannot obtain the necessary historical data.

Dr. Camps and his colleagues have developed a new keratometric approach and previously validated it in healthy eyes, as well as in a sample of 32 eyes that had undergone LASIK.

They wanted to validate the algorithm in a larger population of patients who had undergone LASIK and also in a larger range of intended refractive corrections.

So they enrolled 62 eyes in 62 patients that had undergone LASIK. All LASIK surgeries were performed using the Pulzar Z1 solid-state laser (CustomVis Laser Pty, Osborne Park, Australia) at the Department of Ophthalmology of the Vithas Medimar International Hospital in Alicante, Spain by one surgeon between October 2012 and December 2013.

Of the patients, 34 (54.8%) were women. They had a mean age of 33.42 years. Preoperative myopia ranged from -0.25 D to 6.8 D. Half the sample were left eyes.

Formulas

The researchers estimated corneal power using a variable keratometric index (n_kadj). Considering the Gullstrand eye model and the range of anterior and posterior curvature that is commonly found in this kind of patient, they defined the following expression for n_kadj: n_kadj = -0.0064286r_1c + 1.37688, where r_1c is the postoperative anterior corneal radius.

They also calculated the keratometric corneal power using the classical keratometric index, the Gaussian corneal power, and the True Net Power.

The researchers estimated corneal power with historical data using the Awwad, Camellin, Clinical History, Jarade and Savini methods. They made similar estimates using methods that do not require historical data: the Haigis-L, Shammas, and Seitz methods.

The researchers found significant differences (P < 0.01) between P_kadj and Gaussian corneal power, but this was barely clinically significant, with the mean difference only 0.14 D.

They did not find a statistically significant difference (P = 0.319) between P_kadj  and the True Net Power, and this was also not clinically significant, with the mean difference 0.03 D.

They calculated a strong correlation between P_kadj and Gaussian corneal power (r = 0.994) and the True Net Power (r = 0.994).

There were only four cases out of 66 that showed differences between P_kadj and Gaussian corneal power of more than ±0.5 D. 

The differences between P_kadj and the rest of the methods for estimating corneal power were statistically significant (P > 0.01), except for the difference between P_kadj and the Haigis-L method (P = 0.09).

Difference between Gaussian corneal power and other methods

In addition, the researchers found that the difference between the Gaussian corneal power and all the other methods for estimating corneal power were also statistically significant (P <0.01).

 “Therefore P_kadj is an acceptable method for estimating the corneal power of corneas with previous myopic laser refractive surgery as 100% of estimation were within ± 0.7 D,” the authors wrote.

Gaussian corneal power is the most exact method of calculating central corneal power in paraxial optics, and it, along with P_kadj and the Haigis-L method, may be considered interchangeable, the study suggested.

Therefore, the researchers wrote, both P_kadj and the Haigis-L Method “can be considered appropriate methods for estimating the corneal power in corneas with previous myopic laser refractive surgery when posterior corneal surface data are unknown in clinical practice.”

They found that the other methods tended to overestimate Gaussian corneal power. For example, they found that the classical keratometric approach for corneal power estimation, based only the anterior corneal radius, induces significant overestimations, ranging from 1.07 D to 1.97 D.

However, the Clinical History Method was the most variable in this study; its results differed from Gaussian corneal power in a range from -1.55 D to 3.76 D.

Dr. Camps and colleagues wrote that the results confirm an inappropriate method for estimating corneal power in eyes with previous laser refractive surgery can lead to an overestimation of corneal power and thus to an underestimation of the IOL power required when cataract surgery is planned for these eyes.

They concluded that their adjusted keratometric index, P_kadj, is “a valid and easy method to estimate the central corneal power in corneas with previous myopic laser refractive surgery, improving the accuracy of methods described previously for such purpose.”