Intraocular Lens Calculations After Myopic Refractive Surgery

With the popularity of refractive surgery rising since the 1990s, more patients who have previously undergone refractive surgery are developing visually significant cataracts. The number of patients in this group will only continue to rise as more refractive surgery patients reach a cataract forming age. Calculating intraocular lens (IOL) power after refractive surgery will, therefore, become increasingly important. Unfortunately, traditional methods of IOL calculation often lead to a “hyperopic surprise” in patients who have previously undergone myopic refractive surgery. As a result, refractive surgeons have been slow to meet patients’ expectations, which may be high due to previously successful refractive surgery. The purpose of this article is to describe the sources of error in calculating IOL power after refractive surgery and to discuss current and future methods, which may improve accuracy of these IOL calculations.

Sources of Error

Errors in IOL calculation after refractive surgery are largely due to 3 reasons. First, manual keratometers and topography units are unable to accurately measure anterior corneal curvature after refractive surgery. Traditional keratometry assumes a spherical corneal surface to determine the measurement of central corneal power. After myopic refractive surgery, where central corneal tissue is ablated, this assumption is no longer valid. Topography is likely more accurate, because it measures over 1000 points in the central 3.0 mm zone, while traditional keratometry only measures 2 points located 3.2 mm and 2.6 mm from the corneal center. With topography, the central corneal power should be used, as this gives a more reliable corneal power measurement in post-refractive surgery patients as opposed to simulated keratometry (SimK).

Second, predicting corneal power using anterior corneal curvature measurements and the standard index of refraction (1.3375) incorrectly assumes that there is a constant relationship between the anterior and posterior curvature of the cornea, which is untrue after laser refractive surgery. Ablation by the excimer laser during myopic treatment causes a change in the anterior corneal curvature. The change in the posterior curvature, if any, is unknown. Therefore, it would not be advisable to use an effective refraction index of 1.3375 after myopic refractive surgery. This error would lead to an underestimation of IOL power.

Finally, reliance upon post-refractive surgery keratometry readings will likely lead to an inaccurate estimation of effective lens position (ELP). This is true for most IOL calculation formulas, except the Haigis formula. Aramberri proposed a “Double K” formula, where the pre-refractive surgery keratometry is used to calculate the ELP, while the post-refractive surgery keratometry is used to calculate the corneal power (J Cataract Refract Surg. 2003;29:2063-2068). For previously myopic patients, the Double K formula was found to be more accurate than traditional methods.

Current Methods

The most widely used method for keratometric calculation post refractive surgery is the clinical history method introduced by Holladay (Refract Corneal Surg. 1989;5:203). In this method the amount of refractive surgery correction is subtracted from the preoperative keratometry. Accurately calculating this method can pose some challenges. Errors can be encountered when nuclear sclerosis-induced myopia is incorporated into this formula. Also, pre-refractive surgery information, which is necessary, may be unavailable.

Alternative methods for corneal power determination include the contact lens method. In this method the difference in manifest refraction before and after insertion of a plano hard contact lens with a known base curve is determined. The change in refraction is subtracted from the known base curve, and this is assumed to be the curvature of the central cornea. Zeh and Koch evaluated this method for cataract patients with normal corneas and those without cataracts, and the researchers found it to be accurate in patients with vision as low as 20/80 due to cataracts (J Cataract Refract Surg. 1999;25:898-903). Other methods which bypass post-refractive surgery corneal power include the vertex method and the back-calculated method. Reports by Feiz et al (Cornea. 2001;20:792-797) and Walter and associates (J Cataract Refract Surg. 2006;32:425-429) have shown these methods to be accurate as well.

To further improve accuracy of IOL calculation regression, Latkany and colleagues (J Cataract Refract Surg. 2005;31:562-570) and Masket and Masket (J Cataract Refract Surg. 2006:32: 430-434) analyzed traditional methods, which calculated IOL power using keratometry readings in the SRK/T formula. They found that regression formulas that called for an adjustment based on the amount of refractive surgery correction produced more accurate IOL power measurements.

Future of IOL Calculations after Refractive Surgery

Using the Gaussian optics formula to determine central corneal power would be theoretically more accurate than current methods. In this formula the anterior and posterior curvature must be accurately measured. Currently, measurement of the posterior corneal curvature remains difficult. Options include pan corneal scanning, slit-beam topography with Orbscan or Pentacam or stereoscopic topography. The reliability of the posterior curvature measurements and how they may aid in IOL calculations after refractive surgery are currently being evaluated. Also, newer modalities to adjust the power of the IOL after implantation may correct any “surprises” that are encountered.

Refractive patients must be well informed about the inexactitude of postoperative IOL power calculations, for any refractive surprises may need to be corrected with more refractive surgery. All refractive surgeons should, therefore, provide a card to each of their patients that notes their preoperative keratometric and refractive measurements as well as their stable postoperative refractive measurements.

References

Author Disclosure

The author states that he has no financial relationship with the manufacturer or provider of any product or service discussed in this article or with the manufacturer or provider of any competing product or service.