Intraocular lens calculations after laser vision correction

Curr Opin Ophthalmol 2017, 28:16–22

• Corneal measurements and IOL calculations should not  be obtained with standard measurement techniques and formulas in patients who have undergone corneal refractive surgery.
• Adjusted formulas that do not require prerefractive surgery have comparable accuracy to those that do.
• Using a consensus approach for IOL selection yields better refractive results when compared to various single adjusted formulas.
• The spherical aberration profile of the IOL should be taken into account in its selection in order to counterbalance the spherical HOA induced by the refractive corneal ablation pattern.

CHALLENGES IN CORNEAL MEASUREMENTS

1. Corneal power measurement : Standard keratometry  – anterior 3mm radius // the standard corneal index of refraction of 1.3375 is no longer valid, because the ratio between the anterior and posterior curvature is altered, along with the total corneal power and index of refraction . The mean corneal refractive indices have been measured with Dual Scheimpflug Analyzer (Galilei, Ziemer Ophthalmics AG, Port, Switzerland) as 1.3278 in normal eyes, 1.3246 in
   eyes with previous myopic laser in situ keratomileusis (LASIK)/photorefractive keratectomy (PRK), and 1.3302 in eyes with previous hyperopic LASIK/PRK
2. Effective lens position :  In patients with myopic ablation patterns, however, the central cornea has been flattened, and therefore, these formulas predict a falsely shallow ELP recommending insufficient IOL power and resulting in a postoperative hyperopic surprise
3. Higher order aberrations : positive spherical HOA aberration after myopic ablations and increased negative spherical HOA after hyperopic ablations.

Proposed methods for intraocular lens calculations after refractive surgery

• Methods that require prerefractive surgery data
  • historical methods : calculates corneal power by subtracting the refractive change at the corneal plane from corneal powers obtained prior to the refractive intervention
  • the Feiz–Mannis method :  in which IOL power calculations are obtained using the prerefractive keratometry values but current, postrefractive axial length measurements and then adjusting the IOL power by adding the refractive change divided by 0.7
  • the double keratometry method :  in which the prerefractive keratometry is used to calculate the ELP and then the calculated postrefractive  keratometry (clinical history method) is used in the vergence formula to obtain the IOL power.
• Methods that calculate corneal power from postrefractive measurements
  • contact lens over-refraction
  • Regression analysis methods : Wang-Koch-Maloney, Shammas and Haigis-L formulas, and the Galilei total corneal power method.
  • The Barrett True Keratometry formula : Barrett Universal II formula
• Using current corneal measurements with adjustment of intraocular lens power
  • Masket Formula (using biometry readings SRK/T formula and then adjusting the IOL power by 32.6% of refractive
    correction)
  • Modified Masket Formula, which adjusts the IOL power to 43.8% of the refractive change.
• Direct measurements of anterior and posterior cornea after refractive surgery
  • Scheimpflug technology and optical coherence tomography imaging : Gaussian optics thick lens formula (calculates Gaussian equivalent power); the ray tracing method (uses Snell’s law to refract rays passing through the anterior and posterior surfaces) ; or the ocular coherance tomography
    (OCT)-based IOL formula (regression-derived formula to predict the ELP based on an anterior chamber depth-constant, posterior corneal power, and axial length).
• Intraoperative intraocular lens power determination
  • The Optiwave Refractive Analysis (ORA) System (WaveTec Vision, now Alcon, Inc.) obtains an aphakic retinoscopy and refraction intraoperatively
    after removing the crystalline lens but prior to implanting of the IOL, and then through a proprietary algorithm to estimate the ELP, it calculates the
    IOL power based on the aphakic spherical equivalent and the patient’s preoperatively measured axial length and keratometries

ASCRS postrefractive intraocular lens calculator
– a web-based IOL power calculator was designed in 2007 (www.ascrs.org)

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