Fig. 3. Immersion B-scan at 10 MHz demonstrating posterior pole staphyloma (arrow).

while displaying the optic nerve image. The B-scan is used to adjust the center of the cornea, lens, and fovea.

Pseudophakic eyes

During the measurement of pseudophakic eyes, the first spike represents the lens implant, followed by multiple signals. IOL implantation causes multiple echoes within the vitreous cavity (Fig. 4). The first spike (IOL echo) should also be aligned along the visual axis and should be of maximum height. Adjustments should be made according to the ultrasonic velocity of the IOL material. Nevertheless, the identification of retinal spikes can be difficult in some cases because of the proximity of the multiple echoes to the retina spike. In these cases, the examiner should decrease the gain for better identification of the retina spike.

Fig. 4. Immersion A-scan of a pseudophakic eye. The spikes correspond to water bath (W), anterior and posterior corneal surface (C), intraocular lens implant (IOL), and retina (R). Multiple spikes (A; artifacts) are a result of IOL implant.

Biometry 197

Holladay and Prager10 described a conversion factor to improve the accuracy of the axial length measurements in pseudophakic eyes. They considered the implant composition, the center thickness, and the amount of vitreous and aqueous crossed by the ultrasonic beam. The conversion factor was obtained by multiplying the center thickness of the IOL by a factor related to the implant’s ultrasonic velocity.

INTRAOCULAR LENS POWER CALCULATIONS

Formulas

First generation

First-generation formulas included regression analysis of previous IOL implantation cases and the predicted IOL position (ACD), which depended upon a specific constant for each IOL. In 1967, Fedorov and colleagues11 published the first formula for IOL calculation based on schematic eyes. Subsequently, Colenbrander12 described his formula, followed by Hoffer13 in 1974. In 1975, Binkhorst14 published a formula that was widely used in United States. Regression analysis was described by Sanders and Kraff15 in 1980, followed by the SRK-I16 comparison to the other formulas. The SRK formula was superior to the other formulas by having a smaller range of error.

Second generation

The predictive relationship between the IOL position within the posterior chamber and the axial length was described to improve the accuracy of first-generation formulas. This direct relationship was calculated by different methods as demonstrated by the SRK-II formula.17

Third generation

The third-generation formulas assumed that the IOL position was related to the axial length. Long eyes would have a deep anterior chamber, whereas short eyes would have a shallow anterior chamber. It has since become well known that this assumption is not valid; hence, at the extremes of axial length, the third-generation formulas produce considerably variable results.

In 1988, Holladay and colleagues18 incorporated the surgeon factor (SF) to the secondgeneration formulas. With this factor, they described the relationship between corneal steepness and the IOL position. The Holladay 1 formula considered the distance from the cornea to the iris plane and from the iris to the posterior chamber IOL position (SF). Retzlaff and colleagues19 in1990 modified the Holladay 1 formula by incorporating the A constants to the SRK/T formula (theoretic).

Hoffer20 modified his own formula in 1993 by replacing the regression formula with a theoretic formula (Hoffer Q). The Hoffer Q formula has been demonstrated to be clinically more accurate than the Holladay 1 and SRK/T formulas in eyes shorter than 22.0 mm.

Fourth generation

The fourth-generation formulas introduced innovative approaches for IOL calculation as follows:

The Haigis’ formula uses three constants for effective lens position settings.21 The formula also includes the ACD (distance of the corneal vertex to the anterior lens capsule) and the AL (distance from the corneal vertex to the macula). The constants are derived by regression analysis and produce an IOL-specific and surgeon-specific factor for different anterior chamber depths and axial lengths.

The Holladay 2 formula uses IOL thickness, corneal power, corneal diameter, ACD measurements, lens thickness, axial length, refractive error, and age to obtain and refine the estimated scaling factor (ESF). A database of 35,000 patients was used to create the Holladay 2 formula.

Selection of the Best Formula

In 1993, Hoffer published an important article regarding the eye’s axial length and formulas. It had been shown that, within the normal range of axial length (22.0 to 24.5 mm), almost all formulas yield the same or similar results; however, at the extremes of axial length, the formulas begin to differ.20 The Holladay 1 formula was the most accurate in eyes from 24.5 to 26.0 mm, whereas the SRK/T worked more adequately in very long eyes (>26.0 mm). The Hoffer Q formula was the most accurate for short eyes (<22.0 mm). More recently, the performance of the Holladay 2 formula was shown to be comparable with that of the Hoffer Q formula in short eyes (<22.0 mm) in a study with 317 eyes.22 Nevertheless, the original Holladay 1 formula was more accurate in eyes with average and medium-long axial lengths.

Special Clinical Situations

Post refractive surgery

Laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and radial keratotomy (RK) change the corneal architecture by flattening the cornea surface. RK for myopia correction flattens the anterior and posterior cornea surfaces while laser ablation changes anterior corneal curvature. Standard keratometry and topography measurements of the cornea power (K) after refractive surgery tend to overestimate the K

readings following laser corrections for myopia and underestimate for hyperopia. The mean of the paracentral cornea measurements (3-mm zone) does not evaluate the real central corneal power (flatter zone). Three methods are described to estimate the post refractive surgery K value.

Clinical history method The estimate of the central corneal power after refractive surgery is obtained by subtracting the difference between preoperative and postoperative spherical equivalent error from the average keratometry power before refractive surgery. Both the preoperative corneal power (keratometry) and the preoperative and postoperative refractive errors are necessary to acquire the final K using this method. This important information should be obtained from the refractive surgeon.

Contact lens method The hard contact lens method requires a known contact lens base curve (BC) and refractive power (PC) in diopters, and the spherical equivalent refraction with (SEcl) and without (SE) the contact lens. By this method, the final estimate corneal power (K) after refractive surgery is calculated as follows:

K 5BC 1PC 1ðSEcl SEÞ

The contact lens method should not be used when a cataract or other media opacities compromise the accuracy of the refraction.23

K value obtained by topography Maloney described a method, further modified by Wang and colleagues24, to obtain the central corneal power after refractive surgery. The final K value is obtained by placing the cursor at the center of the topography axial map, multiplying that value by 1.114, and then subtracting the product by the posterior corneal power (6.1 D).

Double K formulas Another source of postoperative error following refractive surgery is related to the ELP calculation. The ELP is the distance between the surfaces of the cornea (vertex) to the plane of the IOL. Third-generation formulas assume the K power and axial length to estimate the ELP. When using these formulas, very flat keratometric corneal power following refractive surgery will produce a false shallow postoperative ELP. As a result, the calculated IOL power will be underestimated, ensuing in a hyperopic error.

In 2003, Chamon25 described a ‘‘double K’’ method by using the preoperative and postoperative corneal power for IOL calculation after refractive surgery using the Holladay 1 formula. The preoperative K value was determined by topography and the postoperative K value by the