Advanced Intraocular Lens Power Calculations


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Chapter 4

Advanced Intraocular Lens Power Calculations

4

John P. Fang, Warren Hill, Li Wang, Victor Chang, Douglas D. Koch

Core Messages
■ Accurate IOL power calculations are a crucial element for meeting the ever increasing expectations of patients under-
■ going cataract surgery. Although ultrasound biometry is a wellestablished method for measuring axial length optical coherence biometry has been shown to be significantly more ac-
■ curate and reproducible. The power adjustment necessary between the capsular bag and the ciliary sulcus will depend on the power of the
■ intraocular lens. When the patient has undergone prior corneal refractive surgery, or corneal transplantation, standard keratometric
■ and topographic values cannot be used. Several methods have been proposed to improve the accuracy of IOL power calculation in eyes following corneal refractive surgery; these can be divided into those that require preoperative data and
■ those that do not. Because it is impossible to accurately predict the postoperative central power of the donor graft, there is presently no reliable method for calculating IOL power for eyes undergoing combined corneal transplantation and cataract removal with intraocular lens implanta-
■ tion. The presence of silicone oil in the eye complicates intraocular lens power measurements and calculations.

4.1 Introduction
Accurate intraocular lens (IOL) power calculations are a crucial element for meeting the ever increasing expectations of patients undergoing cataract surgery. As a direct result of technological advances, both our patients and our peers have come to view cataract surgery as not only a rehabilitative procedure, but a refractive procedure as well. The precision of IOL power calculations depends on more than just accurate biometry, or the correct formula, but in reality is a collection of interconnected nuances. If one item is inaccurate, the final outcome will be less than optimal.
4.2 Axial Length Measurement
By A-scan biometry, errors in axial length measurement account for 54% of IOL power error when using two-variable formulas [23]. Because of this, much research has been dedicated to achieving more accurate and reproducible axial lengths. Although ultrasound biometry is a well-established method for measuring ocular distances, optical coherence biometry has been shown to be significantly more accurate and reproducible and is rapidly becoming the prevalent methodology for the measurement of axial length.
4.2.1 Ultrasound
Axial length has traditionally been measured using ultrasound biometry. When sound waves encounter an interface of differing densities, a fraction of the signal echoes back. Greater dif-

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Advanced Intraocular Lens Power Calculations

ferences in density produce a greater echo. By 1. Probe tip/cornea,

measuring the time required for a portion of the 2. Aqueous fluid/anterior lens,

sound beam to return to the ultrasound probe, 3. Posterior lens/vitreous,

the distance can be calculated (d = v × t)/2. Be- 4. Vitreous/retina,

cause the human eye is composed of structures 5. Retina/sclera,

of varying densities (cornea, aqueous, lens, vitre- 6. Sclera/orbital fat.

ous, retina, choroid, scleral, and orbital fat), the

4

axial length of each structure can be indirectly The axial length is the summation of the an-

measured using ultrasound. Clinically, applana- terior chamber depth, the lens thickness, and the

tion and immersion techniques have been most vitreous cavity.

commonly used.

The y-axis shows peaks (known as spikes) rep-

resenting the magnitude of each echo returned to

the ultrasound probe. The magnitude or height

4.2.1.1 Applanation Technique

of each peak depends on two factors. The first is the difference in densities at the acoustic inter-

With the applanation technique, the ultrasound face; greater differences produce higher echoes.

probe is placed in direct contact with the cornea. The second is the angle of incidence at this inter-

After the sound waves exit the transducer, they face. The height of a spike will be at its maximum

encounter each acoustic interface within the eye when the ultrasound beam is perpendicular to

and produce a series of echoes that are received the acoustic interface it strikes. The height of

by the probe. Based on the timing of the echo and each spike is a good way to judge axiality and,

the assumed speed of the sound wave through hence, alignment of the echogram.

the various structures of the eye, the biometer Because the applanation technique requires

software is able to construct a corresponding direct contact with the cornea, compression will

echogram. In the phakic eye, the echogram has typically cause the axial length to be falsely short-

six peaks (Fig. 4.1), each representing the inter- ened. During applanation biometry, the com-

faces of:

pression of the cornea has been shown to range

Fig. 4.1 Phakic axial length measurement using the applanation technique. a Initial spike (probe tip and cornea), b anterior lens capsule, c posterior lens capsule, d retina, e sclera, f orbital fat

4.2 Axial Length Measurement 33

from 0.14 to 0.33 mm [24, 29, 30]. At normal axial lengths, compression by 0.1 mm results in a postoperative refractive error toward myopia of roughly 0.25 D. Additionally, this method of ultrasound biometry is highly operator-dependent. Because of the extent of the error produced by direct corneal contact, applanation biometry has given way to noncontact methods, which have been shown to be more reproducible.
4.2.1.2 Immersion Technique
The currently preferred A-scan method is the immersion technique, which, if properly performed, eliminates compression of the globe. Although the principles of immersion biometry are the same as with applanation biometry, the technique is slightly different. The patient lies supine with a clear plastic scleral shell placed over the cornea and between the eyelids. The shell is filled with coupling fluid through which the probe emits sound waves. Unlike the applanation echogram, the immersion technique produces an additional spike corresponding to the probe tip (Fig. 4.2). This spike is produced from the tip of the probe within the coupling fluid.

Although the immersion technique has been shown to be more reproducible than the applanation technique, both require mindfulness of the properties of ultrasound. Axial length is calculated from the measured time and the assumed average speed that sound waves travel through the eye. Because the speed of ultrasound varies in different media, the operator must account for prior surgical procedures involving the eye such as IOL placement, aphakia, or the presence of silicone oil in the vitreous cavity (Table 4.1). Length correction can be performed simply using the following formula:
True length = [corrected velocity/measured velocity] × measured length
However, using a single velocity for axial length measurements in eyes with prior surgery is much less accurate than correcting each segment of the eye individually and adding together the respective corrected length measurements. For example, in an eye with silicone oil, the anterior chamber depth would be measured at a velocity of 1,532 m/s, the crystalline lens thickness at 1,641 m/s, and the vitreous cavity at either 980 m/s or 1,040 m/s depending on the

Fig. 4.2 Phakic axial length measurements using the immersion technique. a Probe tip—echo from tip of probe, has now moved away from the cornea and becomes visible; b cornea— double-peaked echo will show both the anterior and posterior surfaces; c anterior lens capsule; d posterior lens capsule; e retina; f sclera; g orbital fat

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Advanced Intraocular Lens Power Calculations

Table 4.1 Average velocities under various conditions

By adding the CALF to or subtracting it from

for average eye length [16]. PMMA: polymethyl meth- the measured axial length, the true axial length

acrylate

is obtained.

Another source of axial length error is that

Condition

Velocity (m/s)

the ultrasound beam has a larger diameter than

Phakic eye
4 Aphakic eye PMMA pseudophakic

1,555

the fovea. If most of the beam reflects off a raised

parafoveal area and not the fovea itself, this will

1,532

result in an erroneously short axial length read-

1,556

ing. The parafoveal area may be 0.10–0.16 mm

Silicone pseudophakic Acrylic pseudophakic

1,476

thicker than the fovea.

In addition to compression and beam width,

1,549

an off-axis reading may also result in a falsely

Phakic silicone oil

1,139

shortened axial length. As mentioned before, the

Aphakic silicone oil

1,052

probe should be positioned so that the magni-

Phakic gas

534

tude of the peaks is greatest. If the last two spikes

are not present (sclera and orbital fat), the beam

may be directed to the optic nerve instead of the

fovea.

density of the silicone oil (1,000 centistokes vs. In the setting of high to extreme axial myopia,

5,000 cSt). The three corrected lengths are then the presence of a posterior staphyloma should be

added together to obtain the true axial length. considered, especially if there is difficulty obtain-

Sect. 4.8 describes in greater detail IOL calcula- ing a distinct retinal spike during A-scan ultraso-

tions in eyes with silicone oil.

nography. The incidence of posterior staphyloma

For pseudophakia, using a single instrument increases with increasing axial length, and it is

setting may also lead to significant errors be- likely that nearly all eyes with pathologic myopia

cause IOL implants vary in sound velocity and have some form of posterior staphyloma. Staphy-

thickness (Table 4.2). By using an IOL material- lomata can have a major impact on axial length

specific conversion factor (CF), a corrected axial measurements, as the most posterior portion of

length factor (CALF) can be determined using: the globe (the anatomic axial length) may not

correspond with the center of the macula (the

CF = 1 – (VE/VIOL)

refractive axial length). When the fovea is situ-

CALF = CF × T

ated on the sloping wall of the staphyloma, it may

where VE = sound velocity being used (such as only be possible to display a high-quality retinal

1,532 m/s),

spike when the sound beam is directed eccentric

VIOL = sound velocity of the IOL material being to the fovea, toward the rounded bottom of the

measured,

staphyloma. This will result in an erroneously

T = IOL central thickness.

long axial length reading. Paradoxically, if the

PMMA Acrylic First generation silicone Second generation silicone Another second generation silicone Hydrogel HEMA Collamer

2,713 m/s (Alcon MC60BM) 2,078 m/s (Alcon MA60BM)
990 m/s (AMO SI25NB) 1,090 m/s (AMO SI40NB) 1,049 m/s (Staar AQ2101V) 2,000 m/s (B&L Hydroview) 2,120 m/s (Memory lens) 1,740 m/s (Staar CQ2005V)

Table 4.2 Velocities for individual intraocular lens materials [13]. HEMA: hydroxyethyl methylmethacrylate

4.2 Axial Length Measurement 35

sound beam is correctly aligned with the refractive axis, measuring to the fovea will often result in a poor-quality retinal spike and inconsistent axial length measurements.
Holladay has described an immersion A/Bscan approach to axial length measurement in the setting of a posterior staphyloma [4, 33]. Using a horizontal axial B-scan, an immersion echogram through the posterior fundus is obtained with the cornea and lens echoes centered while simultaneously displaying void of the optic nerve. The Ascan vector is then adjusted to pass through the middle of the cornea as well as the middle of the anterior and posterior lens echoes to assure that the vector will intersect the retina in the region of the fovea. Alternatively, as described by Hoffer, if it is possible to visually identify the center of the macula with a direct ophthalmoscope, the cross hair reticule can be used to measure the distance from the center of the macula to the margin of the optic nerve head. The A-scan is then positioned so that measured distance is through the center of the cornea, the center of the lens, and just temporal to the void of the optic nerve on simultaneous B-scan.
Summary for the Clinician
■ Because the applanation technique requires direct contact with the cornea, compression will typically cause the axial
■ length to be falsely shortened. The speed of ultrasound varies in different media. To account for this, the operator must alter ultrasound speed settings for eyes that are pseudophakic or aphakic or that contain silicone oil in the
■ vitreous cavity. In the setting of high to extreme axial myopia, the presence of a posterior staphyloma should be considered.
4.2.2 Optical Coherence Biometry
Introduced in 2000, optical coherence biometry has proved to be an exceptionally accurate and reliable method of measuring axial length.

Through noncontact means, the IOL Master (Carl Zeiss Meditec, Jena, Germany) emits an infrared laser beam that is reflected back to the instrument from the retinal pigment epithelium. The patient is asked to fixate on an internal light source to ensure axiality with the fovea. When the reflected light is received by the instrument, the axial length is calculated using a modified Michelson interferometer. There are several advantages of optical coherence biometry: 1. Unlike A-scan biometry, the optical coher-
ence biometry can measure pseudophakic, aphakic, and phakic IOL eyes. It can also measure through silicone oil without the need for use of the velocity cenversion equation. 2. Because optical coherence biometry uses a partially coherent light source of a much shorter wavelength than ultrasound, axial length can be more accurately obtained. Optical coherence biometry has been shown to reproducibly measure axial length with an accuracy of 0.01 mm. 3. It permits accurate measurements when posterior staphylomata are present. Since the patient fixates along the direction of the measuring beam, the instrument is more likely to display an accurate axial length to the center of the macula. 4. The IOL Master also provides measurements of corneal power and anterior chamber depth, enabling the device to perform IOL calculations using newer generation formulas, such as Haigis and Holladay 2.
The primary limitation of optical biometry is its inability to measure through dense cataracts and other media opacities that obscure the macula; due to such opacities or fixation difficulties, approximately 10% of eyes cannot be accurately measured using the IOL Master [21].
When both optical and noncontact ultrasound biometry are available, the authors rely on the former unless an adequate measurement cannot be obtained. Both the IOL Master and immersion ultrasound biometry have been shown to produce a postoperative refractive error close to targeted values. However, the IOL Master is faster and more operator and patient-friendly.
Though mostly operator-independent, some degree of interpretation is still necessary for op-

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Advanced Intraocular Lens Power Calculations

timal refractive outcomes. During axial length measurements it is important for the patient to look directly at the small red fixation light. In this way, axial length measurements will be made to

Summary for the Clinician
■ Optical coherence biometry has proved to be an exceptionally accurate and reli-

the center of the macula. For eyes with high to extreme myopia and a posterior staphyloma, being able to measure to the fovea is an enormous

■ able method of measuring axial length. The primary limitation of optical biometry is its inability to measure through

4

advantage over conventional A-scan ultrasonog-

dense cataracts and other media opaci-

raphy. The characteristics of an ideal axial length

ties that obscure the macula.

display by optical coherence biometry are the fol-

lowing (Fig. 4.3):

1. Signal-to-noise ratio (SNR) greater than 2.0.

2. Tall, narrow primary maxima, with a thin, well-centered termination.

4.3 Keratometry

3. At least one set of secondary maxima. How- Errors in corneal power measurement can be an

ever, if the ocular media is poor, secondary equally important source of IOL power calcula-

maxima may be lost within a noisy baseline tion error, as a 0.50 D error in keratometry will

and not displayed.

result in a 0.50 D postoperative error at the spec-

4. At least 4 of the 20 measurements taken tacle plane. A variety of technologies are avail-

should be within 0.02 mm of one another and able, including manual keratometry, automated

show the characteristics of a good axial length keratometry, and corneal topography. These

display.

devices measure the radius of curvature and

5. If given a choice between a high SNR and an provide the corneal power in the form of kera-

ideal axial length display with a lower SNR, tometric diopters using an assumed index of re-

the quality of the axial length display should fraction of 1.3375. The obtained values should be

always be the determining factor for measure- compared with the patient’s manifest refraction,

ment accuracy.

looking for large inconsistencies in the magni-

tude or meridian of the astigmatism that should

prompt further evaluation of the accuracy of the

corneal readings.

Important sources of error are corneal scars

or dystrophies that create an irregular anterior

corneal surface. While these lesions can often be

seen with slit lamp biomicroscopy, their impact

on corneal power measurements can best be as-

sessed by examining keratometric or topographic

mires. The latter in particular give an excellent

qualitative estimate of corneal surface irregular-

ity (Fig. 4.4). In our experience, if the irregularity

is considered to be clinically important, we try

to correct it whenever feasible before proceeding

with cataract surgery. Examples would include

epithelial debridement in corneas with epithelial

basement disease, and superficial keratectomy in

eyes with Salzmann’s nodular degeneration.

When the patient has undergone prior cor-

neal refractive surgery, or corneal transplanta-

tion, standard keratometric and topographic

Fig. 4.3 An ideal axial length display by ocular coherence biometry in clear ocular media [12]

values cannot be used. This topic will be further discussed in Sect. 4.6.

4.5 IOL Calculation Formulas 37

4.4 Anterior Chamber Depth Measurement
A-scan biometers and the IOL Master calculate anterior chamber depth as the distance from the anterior surface of the cornea to the anterior surface of the crystalline lens. In some IOL calculation formulas, the measured anterior chamber depth is used to aid in the prediction of the final postoperative position of the IOL (known as the effective lens position, or the ELP).
4.5 IOL Calculation Formulas
There are two major types of IOL formulas. One is theoretical, derived from a mathematical consideration of the optics of the eye, while the other

is empirically derived from linear regression analysis of a large number of cases.
The first IOL power formula was published by Fyodorov and Kolonko in 1967 and was based on schematic eyes [7]. Subsequent formulas from Colenbrander, Hoffer, and Binkhorst incorporated ultrasound data [3, 5, 14]. In 1978, a regression formula was developed by Gills, followed by Retzlaff, then Sanders and Kraff, based on analysis of their previous IOL cases [8, 26, 28]. This work was amalgamated in 1980 to yield the SRK I formula [27]. All of these formulas depended on a single constant for each IOL that represented the predicted IOL position. In the 1980s, further refinement of IOL formulas occurred with the incorporation of relationships between the position of an IOL and the axial length as well as the central power of the cornea.

Fig. 4.4 Corneal surface irregularity shown on the Humphrey topographic map of an eye with epithelial basement disease

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Advanced Intraocular Lens Power Calculations

4.5.1 The Second and Third Generation of IOL Formulas

have perfectly normal anterior chamber anatomy with normal anterior chamber depth. The error in this assumption accounts for the characteris-

The IOL constants in the second and third gen- tic limited axial length range of accuracy of each

eration of IOL formulas work by simply moving third generation two-variable formula. The Hol-

up or down the position of an IOL power pre- laday 1 formula, for example, works well for eyes

diction curve for the utilized formula. The shape of normal to moderately long axial lengths, while

4

of this power prediction curve is mostly fixed for the Hoffer Q has been reported to be better suited

each formula and, other than the lens constant, to normal and shorter axial lengths [15].

these formulas treat all IOLs the same and make

a number of broad assumptions for all eyes re-

gardless of individual differences. For example, two hyperopic eyes with the
same axial length and the same keratometry may

4.5.2 The Fourth Generation of IOL Formulas

require different IOL powers. This is due to two A recent exception to all of this is the Haigis for-

additional variables: of more importance, the ac- mula [9]. Rather than moving a fixed formula-

tual distance from the cornea that the IOL will specific IOL power prediction curve up or down,

sit in the pseudophakic state (i.e., ELP) and to the Haigis formula instead uses three constants

a lesser degree, the individual geometry of each (a0, a1, and a2) to set both the position and the

lens model. Commonly used lens constants do shape of a power prediction curve:

not take both of these variations into account.

These include:

d = a0 + (a1 * ACD) + (a2 * AL)

SRK/T formula—uses an “A-constant,”

Holladay 1 formula—uses a “Surgeon Factor,” where d is the effective lens position, ACD is the

Hoffer Q formula—uses a “Pseudophakic An- measured anterior chamber depth of the eye (cor-

terior Chamber Depth” (pACD).

neal vertex to the anterior lens capsule), and AL

These standard IOL constants are mostly in- is the axial length of the eye (the distance from

terchangeable—knowing one, it is possible to es- the cornea vertex to the vitreoretinal interface).

timate another. In this way, surgeons can move The a0 constant basically moves the power pre-

from one formula to another for the same intra- diction curve up, or down, in much the same way

ocular lens implant. However, the shape of the that the A-constant, Surgeon Factor, or pACD

power prediction curve generated by each for- does for the SRK/T, Holladay 1, and Hoffer Q

mula remains the same no matter which IOL is formulas. The a1 constant is tied to the measured

being used.

anterior chamber depth, and the a2 constant is

Variations in keratometers, ultrasound ma- tied to the measured axial length. In this way,

chine settings, and surgical techniques (such as the value for d is determined by three constants,

the creation of the capsulorrhexis) can impact rather than a single number.

the refractive outcome as independent variables. The a0, a1, and a2 constants are derived by re-

“Personalizing” the lens constant for a given IOL gression analysis from a sample of at least 200

and formula can be used to make global adjust- cases and generate a surgeon and IOL-specific

ments for a variety of practice-specific variables. outcome for a wide range of axial lengths and

Popular third generation two-variable formu- anterior chamber depths. The resulting constants

las (SRK/T, Hoffer Q and Holladay 1) also as- more closely match actual observed results for

sume that the distance from the principal plane a specific surgeon and the individual geometry of

of the cornea to the thin lens equivalent of the an IOL implant. This means that a portion of the

IOL is, in part, related to the axial length. That is mathematics of the Haigis formula is individu-

to say, short eyes may have a shallower anterior ally adjusted for each surgeon/IOL combination.

chamber and long eyes may have a deeper ante- The Holladay 2 formula uses another inno-

rior chamber. In reality, this assumption may be vative approach, which is to use measurements

invalid. Short eyes and many long eyes typically of corneal power, corneal diameter, ACD, lens

4.6 Determining IOL Power Following Corneal Refractive Surgery 39

thickness, refractive error, and axial length to further refine the ELP calculation. The Holladay 2 formula is based on previous observations from a 35.000 patient data set and has been shown to be advantageous in both long and short eyes.
Summary for the Clinician
■ The shape of the power prediction curve is mostly fixed for each second and third
■ generation formula. Popular third generation two-variable formulas may also assume that the distance from the corneal vertex to the thin lens equivalent of the IOL is, in part, related to the axial length and/or central
■ corneal power. The fourth generation IOL power formulas address these issues.

4.5.3 Capsular Bag to Ciliary Sulcus IOL Power Conversion
Intraocular lens power formulas typically calculate the power of the intraocular lens to be positioned within the capsular bag. Occasionally, this is not possible, as with an unanticipated intraoperative tear in the posterior lens capsule. In order to achieve a similar postoperative refractive result with an IOL placed at the plane of the ciliary sulcus, a reduction in IOL power is typically required.
The power adjustment necessary between the capsular bag and the ciliary sulcus will depend

Table 4.3 Intraocular lens (IOL) power correction for unanticipated sulcus implantation [13]

Capsular bag IOL power
+35.00 D to +27.50 D +27.00 D to +17.50 D +17.00 D to +9.50 D +9.00 D to -5.00 D

Ciliary sulcus power adjustment
–1.50 D –1.00 D –0.50 D No change

on the power of the capsular bag IOL (Table 4.3). The important concept is that for stronger intraocular lenses, the reduction in power must be greater. For very low IOL powers, no reduction in IOL power is required. Table 4.3 will provide good results for most, modern posterior chamber IOLs.
4.6 Determining IOL Power Following Corneal Refractive Surgery
The true corneal power following corneal refractive surgery is difficult to obtain by any form of direct measurement. This is because keratometry and topography measure the anterior corneal radius and convert it to total corneal power by assuming a normal relationship between the anterior and posterior corneal curvatures. However, unlike incisional corneal refractive surgery for myopia, which flattens both the anterior and the posterior corneal radius, ablative corneal refractive surgery for myopia primarily alters anterior corneal curvature. Additionally, standard keratometry measures a paracentral region and assumes that this accurately reflects central corneal power. For these reasons, keratometry and simulated keratometry by topography typically under-estimate central corneal power following ablative corneal surgery for myopia and overestimate it for corneas that have undergone hyperopic ablation.
There is a second and less commonly recognized source of unanticipated postoperative refractive error. As a general rule, IOL power calculations following all forms of corneal refractive surgery should not be run using an uncorrected two-variable, third-generation formula because they assume that the effective lens position is, in part, related to central corneal power. By using axial length and keratometric corneal power to estimate the postoperative location of the IOL, or the ELP, the artifact of very flat Ks following myopic corneal refractive surgery will cause these formulas to assume a falsely shallow postoperative ELP and recommend less IOL power than required. To avoid this potential pitfall, the double K feature of the Holladay 2 formula allows direct entry of two corneal power values by

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Advanced Intraocular Lens Power Calculations

checking the box “Previous RK, PRK…”; if the IOLpre + (ΔD / 0.7) = IOLpost

corneal power value before refractive surgery is where IOLpre = the power of the IOL as if no

unknown, the formula will use 43.86 D as the de- LASIK had been performed,

fault preoperative corneal value. Another option ΔD = the refractive change after LASIK at the

is to apply Aramberri’s “double K method” cor- spectacle plane,

rection to the Holladay 1, Hoffer Q or SRK/T for- IOLpost = the estimated power of the IOL to be

mulas [1] or refer to the IOL power adjustment implanted following LASIK.

4

nomograms published by Koch and Wang [19].

Several methods have been proposed to im-

prove the accuracy of IOL power calculation in eyes following corneal refractive surgery; these can be divided into those that require preopera-

4.6.1.3 Masket IOL Power Adjustment Method

tive data and those that do not.

Masket [22] has developed another method that

adjusts the IOL power based on the amount of

refractive laser correction. Instead of calculat-

4.6.1 Methods Requiring Historical Data

ing IOL power with pre-LASIK data as above, this method modifies the predicted IOL power obtained using the patient’s post-laser correction

4.6.1.1 Clinical History Method

readings by using the following formula:

The clinical history method [18] for corneal power estimation requires accurate historical data and was first described by Holladay as:
Kp + SEp - SEa = Ka
where Kp = the average keratometry power before corneal refractive surgery, SEp = the spherical equivalent before corneal refractive surgery, SEa = the stable spherical equivalent after corneal refractive surgery, Ka = the estimate of the central corneal power after corneal refractive surgery.

IOLpost + (ΔD × 0.326) + 0.101 = IOLadj
where IOLpost = the calculated IOL power following ablative corneal refractive surgery, ΔD = the refractive change after corneal refractive surgery at the spectacle plane, IOLadj = the adjusted power of the IOL to be implanted.
4.6.1.4 Topographic Corneal Power Adjustment Method
There are several approaches to modifying postLASIK corneal power measurements:

4.6.1.2 Feiz-Mannis IOL Power Adjustment Method
Another method that is helpful to use when good historical data are available is the IOL power adjustment method of Feiz and Mannis et al. [6]. Using this technique, the IOL power is first calculated using the pre-LASIK (laser-assisted in situ keratomileusis) corneal power as though the patient had not undergone keratorefractive surgery. This pre-LASIK IOL power is then increased by the amount of refractive change at the spectacle plane divided by 0.7. This approach is outlined as follows:

1. To adjust the effective refractive power (EffRP) of the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System by using the following formulas after myopic or hyperopic surgery respectively [11, 31]:
EffRP – (ΔD × 0.15) – 0.05 = post-myopic LASIK adjusted EffRP EffRP + (ΔD × 0.16) – 0.28 = post-hyperopic LASIK adjusted EffRP
where ΔD = the refractive change after LASIK at the corneal plane.

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Advanced Intraocular Lens Power Calculations