Intraoperative technology a plus

Current and future advances improve post-refractive surgery IOL calculations. In this month's Focus on Refractive Surgery, William F. Wiley, MD, describes his experience with an intraoperative wavefront aberrometer (ORange, WaveTec Vision).-George O. Waring IV, MD

Post-refractive power

With keratorefractive surgical procedures continually growing in popularity, an increasing number of cataract surgeries in eyes after keratorefractive surgery is anticipated within a few decades. Although cataract extraction seems to be without major technical obstacles, IOL power calculation turns out to be problematic. Insertion of the measured average keratometry readings after myopic RK, PRK, or LASIK into standard IOL power-predictive formulas commonly results in substantial undercorrection and postoperative hyperopic refraction or anisometropia.

There is much debate surrounding the best method for determining IOL power in post-refractive eyes, and although the "clinical history method" (i.e., subtraction of the spherical equivalent change after refractive surgery from the original K-reading) was initially suggested as the preferred method,¹ Wang and colleagues showed that method as the least reliable in their experience. In analyzing 11 consecutive eyes of eight patients, they found a mean spherical error of –1.02 ±1.13 D (range, –3.25 to 0.97 D) with the clinical history method, a mean spherical error of –0.98 ±0.72 D (range, –2.09 to 0.01 D) with the adjusted effective refractive power method, and a mean spherical error of 0.45 ±0.51 D (range, –0.45 ±1.13 D with the Maloney method.

Toric IOLs

About 20% of patients undergoing routine cataract surgery have visually significant corneal astigmatism,² and this is driving the demand for toric lenses, which can correct both spherical and cylindrical aberrations of the eye.

However, for toric lenses to be effective, the surgeon must choose both the correct sphere and cylinder power, and place the IOL precisely on the axis of the steep meridian of the cornea. About one-third of the effect of the cylindrical correction is lost for every 10° of mal-rotation of the lens. Thus a toric IOL that rotates 30° off axis will have no effect on the patient's corneal astigmatism.³

Additionally, if the wrong cylinder power is chosen, one can under-correct the cylinder or one can inadvertently "flip" the astigmatism axis through overcorrection. Wrong power errors may also be magnified when placed in an off-axis position.