Kenneth J. Hoffer, MD, FACS
Contents
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Introduction |
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Biometry |
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Formulas |
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Clinical Variables |
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Conclusion |
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CHAPTER HIGHLIGHTS |
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Introduction
Since Sir Harold Ridley experienced a 21 diopter “surprise” in lens power calculation on his first two cases in 1949–50, we have been seeking ways to calculate intraocular lens (IOL) power with greater accuracy (Figure 1-1). The science is rather dry and does not stimulate great interest on the part of the majority of cataract surgeons. To make the subject more understandable, it would be advantageous to break it down into its component parts.
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Figure 4-1 Uncorrected 20/20 vision in an eye with a Ridley posterior chamber IOL implanted by Harold Ridley in 1951. |
The three major components of IOL power calculation are (1) biometry, (2) formulas, and (3) clinical variables. Biometry can be divided into its components needed to calculate IOL power: the axial length, the corneal power, and the IOL position. Formulas can be divided into their generations, their usage and their personalization. I will divide clinical variables into the topics: patient needs and desires, special circumstances, and problems and errors.
When the human lens is replaced with an IOL, the optical status becomes a two-lens system (cornea and IOL) projecting an image onto the fovea. The distance (X) between the two lenses affects the refraction as does the distance (Y) between the two-lens system and the fovea. X is defined as the distance from the anterior surface (vertex) of the cornea to the effective principle plane of the IOL in the visual axis. Y is defined as the distance from the principle plane of the IOL to the photoreceptors of the fovea in the visual axis. It is easy to see that X+Y is equal to the visual axis axial length of the eye (A). Therefore, knowing X and A will allow the calculation of Y (Y = A−X).
Also to calculate the IOL power (P), we must know the vergence of the light rays entering the cornea (refractive error (R)). For emmetropia, R is zero. The relationship of these factors (X, Y (A−X), P, K, R) is such that a formula can be written to describe it. Knowing the values of any four of these variables will allow for the calculation of the fifth.
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Biometry
Axial length
If the crystalline lens (cataract) is to be removed, obtaining an accurate axial length (AL) is mandatory. If the lens has already been removed (aphakia/pseudophakia) or will not be removed (phakic IOL), an AL is not always necessary because the correct implant lens power can be calculated using a refraction formula (see below). Because this formula requires an accurate vertex distance, it is not dependable in cases of aphakia where errors in the vertex distance of a high-powered refraction can have a significant effect.
The important considerations for obtaining accurate ultrasound AL are listed in Table 4-1.
Table 4-1 -- Considerations for obtaining accurate measurements (in order of importance)
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A. Ultrasound Axial Length |
B. Corneal Power |
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A-scan ultrasound instrument |
Instrumentation |
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Real-time oscilloscope screen |
Contact lens wear |
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Immersion technique |
Astigmatism |
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Experienced technician |
Previous refractive surgery |
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Appropriate ultrasound velocities |
Corneal transplant eyes |
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B-scan backup |
Axial Length Instruments
Up to 1999, all axial length measuring instruments have been A-scan ultrasound units. There are many A-scan instruments available and it is important to make sure the unit that you are using has been calibrated and is capable of accurate measurements. It is important to be sure that the instrument has a true analog screen such that true echo spikes are observed in determining axiality. Instruments that merely report a numerical reading of the AL (“black box” or spike simulation) do not allow clinical decision making during the examination and are fraught with potential errors. A major step in improving accuracy would be to replace such an instrument with one that has an oscilloscope screen.
A newer methodology for axial length was introduced in 1999 by Carl Zeiss Meditec (Figure 4-2). It uses laser coherent interferometry to measure AL. The instrument, called the IOLMaster® performs four functions: (1) it measures the AL, (2) it measures the corneal power (K or r), (3) it measures the anterior chamber depth (ACD) (the latter two by optical means), and (4) it performs the formula IOL power calculations using four modern 3rd generation theoretic formulas. The author has performed side-by-side analysis of the accuracy of this instrument compared to our standard immersion A-scan technique and found the instrument to be comparable to immersion ultrasound. A multitude of reports in the literature conclude that the IOLMaster® cannot obtain results in from 10–17% of eyes because of either posterior subcapsular cataracts (PSC) cataract, the density of a cataract, or the patient's inability to fixate. Our results are similar. We have so far noticed considerable difficulty obtaining an AL measurement in eyes with PSC cataracts but have been impressed with the results up to this point and especially its ease of use and repeatability.
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Figure 4-2 The Zeiss IOLMaster® laser tomography axial length measurement instrument. A. Side view, B. Front view. |
WARNING: Be sure the Index of Refraction in the IOLMaster is set to 1.3375 in the Setup screen of the computer for the Hoffer® Q formula to operate accurately.
Immersion Ultrasound Technique
The immersion technique of Ossoinig[1] has been shown to be more accurate than the standard applanation contact technique in several studies[2,][3] over the past 15 years. They report a mean average shortening of the AL of 0.25–0.33mm using applanation compared to immersion. If this shortening error by applanation were a consistent one it could be compensated for by the addition of a correction constant or by IOL power formula personalization. Unfortunately, this is not possible since the error varies from eye to eye.
Arguments against using the immersion technique are that it is time-consuming, more expensive, messy and requires the patient to be totally supine. On the contrary, the examination can be performed in a standard ophthalmic examination chair reclined back at a 45° angle with the headrest set back so that the patient's AL is perpendicular to the floor (Figure 4-3). To maintain a non-leaking fluid bath in the Ossoinig scleral shell (Hansen Ophthalmic Development Labs, 745 Avalon Place, Coralville, IA 52241, 319-338-1285 www.HansenLab.com), we use a 50/50 dilution of 2.5% hydroxypropyl methylcellulose (Goniosol®) in Dacriose® solution. Once the eye is anesthetized topically, the scleral shell is gently placed between the lids and filled 3/4 full with the solution. Any air bubbles should be vacuumed with a short silicone tube attached to a syringe. The latter can also be used to remove the solution at the completion of the procedure. The ultrasound probe is placed into the solution and positioned parallel to the axis of the eye (Figure 4-4). Axiality is judged by watching for the correct spike patterns on the oscilloscope screen as the probe position is adjusted. First the corneal and retinal spikes must be identified and “equally” maximized. An undilated pupil aids the examiner by the fact that eliminating the iris spikes improves the chances of being more axial; a dilated pupil eliminates this advantage.
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Figure 4-3 Immersion ultrasound technique setup for patient in normal ophthalmic examination chair. |
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Figure 4-4 Immersion ultrasound technique showing the probe in the Ossoinig shell filled with 50/50 Goniosol/Dacriose solution. |
Many find the Prager Shell (ESI, Inc., 2515 Everest Lane North, Plymouth, MN 55447, 763-473-2533, tab@eyesurgin.com) easier to use for immersion and studies appear to indicate it being accurate. The author has no experience with it.
WARNING: Measuring the AL of BOTH eyes is prudent and customary.
WARNING: If the AL is very difficult to obtain and the eye appears to have a length greater than 25mm, suspect a STAPHYLOMA. Use the IOLMaster or the Shammas method: By direct ophthalmoscopy (with patient fixating on the cross-hair target), measure the distance from the target (macula) to the edge of the optic nerve (in disc diameters). A B-scan exam is then performed to measure the AL at that distance from the edge of the optic nerve shadow (Figure 4-5).
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Figure 4-5 Staphyloma. B-scan (left) demonstrates staphyloma. A-scans show shorter reading at macula (upper) than at posterior pole (lower). |
WARNING: When measuring an eye containing an IOL, ignore multiple reduplication echoes caused by the IOL seen in the vitreous space.
WARNING: If planning silicone oil injection into the vitreous space, perform an accurate AL measurement before doing so and make this information available to the patient. It is practically impossible to measure a silicone oil eye (try using a velocity of 1000m/s). The Zeiss IOLMaster® is the only way to get an accurate measurement in silicone oil-filled eyes. Alternatively, consider performing a secondary IOL after the aphakic refraction is obtained.
Always measure AL to the nearest hundredth of a millimeter and record it carefully. Errors in AL are the most significant and amount to ~2.5D/mm in IOL power, but it is important to be aware that this error drops to ~1.75D/mm in very long eyes (30mm), but jumps to ~3.75D/mm in very short eyes (20mm). Greater care must be taken in measuring short eyes.
Ultrasound Velocities
The ultrasound velocity[4,][5] for the various parts of the eye, intraocular lens materials and average pseudophakic velocities that I have calculated are shown in Table 4-2.
Table 4-2 -- Ultrasound velocities[4,][5] (at body temperature)
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A. From the following sound velocity values[4,][5] |
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Cornea & lens |
1641 m/sec |
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Aqueous & vitreous |
1532 m/sec |
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PMMA IOL |
2660 m/sec |
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Silicone IOL |
980 m/sec |
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Acrylic IOL |
2026 m/sec |
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Glass IOL |
6040 m/sec |
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Silicone oil |
987 m/sec |
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B. I calculated average sound speeds[4] for various conditions of a 23.5 mm eye |
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Phakic eye |
1555 m/sec |
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Aphakic eye |
1534 m/sec |
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PMMA pseudophakic |
1556 m/sec |
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Silicone pseudophakic |
1476 m/sec |
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Acrylic pseudophakic |
1549 m/sec |
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Glass pseudophakic |
1549 m/sec |
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Phakic silicone oil |
1139 m/sec |
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Aphakic silicone oil |
1052 m/sec |
WARNING: Measuring an eye containing a silicone IOL with standard phakic velocity (1555m/s) can amount to an error of 3–4D.
The nominal average velocity for the normal range AL eye is 1555m/s. Because of the inversely proportional change in the axial ratio of solid to liquid as the eye increases in length, the average phakic velocity of a short 20mm eye is 1560 m/s and that of a long 30mm eye is 1550m/s (Figure 4-6). This factor only amounts to a small (0.25D) error in the extremes of AL, but it can be corrected for. The inversely proportional relationship is greater in pseudophakic eyes but is not a factor at all in aphakic eyes (1534m/s).
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Figure 4-6 Phakic velocity. Graph of decline in average sound velocity of a phakic eye as the axial length increases. |
If an eye has been measured using the wrong velocity, it can be easily corrected without remeasuring the eye by using the following formula:
where V = ultrasound velocity
This is because the instrument does not measure length or distance (d) directly. Instead it measures the time (t) it takes the sound to traverse the eye and converts it to a linear value using the velocity (V) formula where d = V × t.
Optional CALF Method
Holladay[6,][7] has offered an optional method to measure the AL which attempts to decrease the error inherent in changes in average velocity due to the length of the eye. The reasoning behind this method is that, if an “average” eye velocity is incorrect, it affects the entire AL measurement. However, if the estimate of the CALF value is wrong, it only affects a small percentage of the overall AL, i.e. only the lens portion. The method involves measuring all eyes, regardless of status, at a sound velocity of 1532m/s (as if the eye was a bag of water) and to this value is added the Corrected AL Factor (CALF). The CALF value represents the thickness of a lens in the eye whether it is the crystalline lens or IOL(s). The formula for the CALF of any lens (including the cornea or IOL) is:
where TL = the axial thickness of the lens and VL = the sound velocity through that lens.
Holladay computes the thickness of the human cataractous lens using:and the sound velocity through the cataract using:
Substituting the above two formulas into the CALF formula above, the CALF formula for the crystalline lens yields:
The CALF for the cataractous lens is, therefore, calculated using only the age of the patient. Holladay recommends using a CALF value of 0.28 (value for a 70-year-old) for all ages because the value for a 1-year-old is 0.306 and that for a 100-year-old is 0.224. The maximum error in CALF for those younger than 70 years is 0.026 (0.07D) and for those older than 70 years is 0.056 (0.14D).
His formulation, however, ignores the factor of the corneal thickness (0.55mm). To correct this, I recommend using a CALF of 0.32 (0.28+0.037). The correction for the cornea is calculated in Table 4-3B. A similar method can be used for pseudophakic eyes using CALF = TL × (1−1532/VL) and the known VL for each IOL material (Table 4-3A). Knowing the thickness of the implanted IOL, the formulas in Table 4-3C can be used. If the IOL thickness cannot be obtained, Holladay[7] published a table to use. The AL of an eye containing two IOLs of different materials can be obtained using the formula in Table 4-3C.
Table 4-3 -- Formulas for calculating biometric parameters
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A. CALF factors for pseudophakic eyes (using CALF = TL *(1–1532/VL) where VL = the sound velocity for the IOL material in the eye |
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CALFPMMA = TL*(1 − 1532/2660) = +0.424*TL |
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CALFSilicone = TL*(1 − 1532/980) = −0.563*TL |
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CALFAcrylic = TL*(1 − 1532/2026) = +0.243*TL |
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B. The correction for the cornea: |
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CALFCornea = TC*(1 − 1532/1641) = 0.55*(0.066423) = 0.037 |
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C. Knowing the thickness of the implanted IOL*, the following formulas can be used: |
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PMMA eye |
AL = AL1532 + 0.424 * TL + 0.037 |
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Silicone eye |
AL = AL1532 - 0.563 * TL + 0.037 |
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Acrylic eye |
AL = AL1532 + 0.243 * TL + 0.037 |
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Piggyback IOLs |
AL = AL1532 + T1 *(1 − 1532/V1) + T2 *(1 − 1532/V2) + 0.037, where T1 and T2 are the thickness and V1 and V2 are the velocity of each IOL. |
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* |
The IOL thickness can be obtained from the manufacturer. |
Biphakic Eyes (Phakic Eye with a Phakic Intraocular Lens)
The problem here is eliminating the effect of the sound velocity through the phakic lens when measuring the AL using ultrasound. I published a method[8] to correct for this potential error by using the following formula:where AL1555 = the measured AL of the eye at the sound velocity of 1555m/s, T = the central thickness of the phakic IOL and C = the material specific correction factor of +0.42 for PMMA, −0.59 for silicone, +0.11 for collamer and +0.23 for acrylic.
The publication[7] contains tables showing the phakic IOL central thickness of each dioptric power for each phakic IOL on the market today.
Retinal Thickness Factor
Some formula writers add a value to the ultrasonic AL measurement to take into account the additional distance from the surface of the retina to the level of the receptive end of the retinal cones. This value has been estimated to be 0.20–0.25mm and is automatically added to the AL in some formulas (Binkhorst, Holladay) and not used at all in others (Colenbrander, Hoffer® Q).
Corneal power
The first lens in the eye's optical system is the cornea. We usually think of corneal power in terms of diopters of optical power but really we are measuring the radius of curvature of the anterior surface and making assumptions regarding the curvature of the back surface based on the Gullstrand eye. As newer instrumentation evolves, such as the Pentacam (Oculus, Inc USA, Woodenville, WA 888-284-8004 www.oculus.de), we may be able to use Scheimpflug photography to measure the posterior surface of the cornea and, thus, the true total optical effect of the cornea. It has been proposed by many that we should convert to using the radius of curvature (r) rather than diopters (D), but that may take a long time, especially in America.
The important factors to consider in obtaining accurate corneal power are listed in Table 4-1B.
Instrumentation
A manual keratometer measures only the front surface of the cornea and converts the radius (r) of curvature obtained to diopters (K) using an index of refraction (IR) of 1.3375 (some units use a different IR). The formula to change from D to r is (r = 337.5/D)) and from r to D is (D=337.5/r). Many postulate that this index is too high and Holladay[6] recommends using 4/3 instead. To make this correction, one can simply multiply the K reading obtained (in D) by the factor 0.98765431. This will result in ~0.54D decrease in corneal power (range; 0.43D for 35D cornea to 0.62D for 50D cornea). Use the formula 1/3/(IR-1) if your keratometer uses a different index of refraction (IR).
WARNING: Before using this refractive index correction factor clinically, test it on a series of previously operated eyes to see what effect it would have had on your accuracy.
To assure accuracy it is important to calibrate all keratometers (including the IOLMaster®) on a regular schedule.
WARNING: Be sure the Index of Refraction is set to 1.3375 in the Setup screen of the computer on the IOLMaster® for the Hoffer® Q formula to operate properly.
Corneal topography units also supply simulated corneal power values. I performed a prospective comparison study of the manual keratometer (B&L) with one such unit (TechnoMed C-Scan, Tubinger, Germany, www.tmed.com) on 172 cataract eyes. The mean of the central (3mm zone) readings was 0.24D flatter with the topography unit (43.55D vs. 43.79D), which may be explained by the index of refraction discussed above. When personalization was performed on both instrument data sets, however, IOL power calculation accuracy was statistically equal.
WARNING: Hard contact lenses (including gas permeable) should be removed permanently for at least 2 weeks prior to measuring corneal power for IOL power calculation.
Astigmatism
Regular astigmatism is not a factor in IOL power calculation because the goal is to predict the postoperative spherical equivalent refractive error. Therefore, the average of the two K readings is the only value used and should result in mixed astigmatism. If a myopic cylinder were desired, the flattest K reading could be used instead of the average. If astigmatism is surgically corrected at the time of lens implantation, it would be important to know the effect of this surgery on the final average corneal power and adjust the K reading used to calculate the IOL power accordingly. Due to the coupling ratio, this effect is usually zero but an analysis of your previous cases would be useful. Some have reported higher errors in eyes with severe astigmatism.
Keratoconus Eyes
Because a cornea with keratoconus can become very steep, it is important to consider the fact that formulas that use the K reading to estimate the IOL position or effective lens position (ELP) may overestimate this actual postoperative position. One should be aware that the K reading has less of this effect in the Hoffer® Q formula than the other modern theoretic formulas. It is not a factor at all with the Haigis formula since it does not use the K reading at all in estimating the ELP.
Previous Corneal Refractive Surgery
Previous corneal refractive surgery changes the architecture of the cornea such that standard methods of measuring the corneal power cause it to be underestimated (myopia) and overestimated (hyperopia). This was first reported by Koch[9] in 1989. Radial keratotomy (RK) causes a relatively proportional equal flattening of both the front and back surface of the cornea leaving the index of refraction relationship the same. On the other hand, photorefractive keratectomy (PRK), laser-assisted intrastromal keratomileusis (LASIK) and laser-assisted epithelial keratomileusis (LASEK) flatten only the front surface. In myopic eyes, this changes the refractive index calculation creating an underestimation of the corneal power by about one diopter for every seven diopters of refractive surgery correction obtained.
The major cause of error is the fact that most keratometers measure at the 3.2mm zone of the central cornea, which often misses the central flatter zone of effective corneal power; the flatter the cornea, the larger the zone of measurement. There are at least seven methods to more accurately estimate the corneal power in these refractive surgery eyes. There are also seven methods to adjust the target IOL power. To calculate the target IOL power PTARG. Many of these methods require knowledge of some of the following biometric information:
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The planned postoperative refractive error desired RxTARG |
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Refractive surgery preoperative corneal power (K readings) KPRE |
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Refractive surgery preoperative refractive error (spherical equivalent) RPRE |
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Refractive surgery postoperative refractive error (spherical equivalent) RPO. |
Methods to Estimate True Postoperative Corneal Power
Clinical History Method[10–16]
This method is based on the fact that the final change in refractive error the eye obtains from corneal surgery was due only to a change in the effective corneal power. If this refractive change is added to the presurgical corneal power, we will obtain the effective corneal power the eye has now.
WARNING: All patients having corneal refractive surgery should be given the following data to maintain in their personal health records: (1) preoperative corneal power, (2) preoperative refractive error, and (3) postoperative healed refractive error (before lens changes affect it).
They should be told to give these data to anyone planning to perform cataract/IOL surgery on them.
All attempts should be made to obtain the above information from the refractive surgeon's records. Odenthal et al,[17] in 2002, discovered that, although it is optically correct, it is not beneficial to vertex correct the spectacle refraction as was originally recommended. Most recommend not vertexing the refractions because it causes underestimation of the K reading.
For this method, the estimated effective corneal power (K) can be calculated using the following formula:where:
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R = refractive error |
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PREOP = preoperative |
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PO = postoperative. |
Contact Lens Method[10–19]
The contact lens method was first described in 1948 by Englishman Frederick Ridley[18] (the inventor of NaOH IOL sterilization), taught by Joseph Soper[19] in 1974 and popularized by Holladay in the 1990s. This method is based on the principle that if a hard PMMA (not rigid gas permeable) contact lens (CL) of plano power (P) and a base curve (B) equal to the effective power of the cornea, is placed on the eye it will not change the refractive error of the eye. That is, the difference between the manifest refraction with the contact lens (RCL) and without it (RNoCL) is zero. The formula to calculate the estimated corneal power is:where:
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B = base curve |
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CL = contact lens |
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P = power of CL |
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R = refractive error |
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NoCL = bare refraction. |
Again, it is not presently recommended to vertex-correct the refractive errors to the corneal plane. Several computer IOL power calculation programs calculate these two methods automatically when needed (Hoffer® Programs and Holladay® IOL Consultant). There are commercially available hard PMMA CL sets in plano powers with appropriate base curves from Ocusoft (Figure 4-7A) and Eye Scan Consulting (Figure 4-7B) (Decatur, GA, 404-286-9067, eyescan@comcast.net).
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Figure 4-7 Hard polymethyl methacrylate (PMMA) contact lens kits in plano powers for the contact lens method. A, Ocusoft Kit. B, Eye Scan Consulting Kit. |
Maloney Corneal Topography Method[20]
Based on his analysis of corneal topography central Ks (Kt) on LASIK eyes, Robert Maloney[20] developed a formulation in 1998 to predict true corneal power using only the Kt. The formula is:where Kt is postoperative topography central K.
Koch Modification of Maloney Method[21]
In 2003, Douglas Koch[21] analyzed several of these methods and obtained the best results using the Maloney method but only after increasing the constant from 5.5 to 6.1. The formula is:where Kt is postoperative topography central K.
He reported on series of eyes where the best results were obtained using this K estimation and the Aramberri Double-K method with a 3rd-generation formula. He also offered a second method to calculate estimated corneal power if the change in refractive error (RC) of the patient known. The formula is:where:
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Kt = central average K from corneal topography |
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RC = refractive change in refractive error from the surgery. |
Ronje Method[22]
The Ronje[22] Method proposes that the corneal power can be estimated by simply adjusting the flattest postoperative manual keratometry (KPOFLAT) by 25% of the change in the spherical equivalent refractive error (RC) that occurred from the corneal refractive surgery.
For example:where:
RPRE = refractive surgery spherical equivalent
RPO = post refractive surgery spherical equivalent
KPOFLAT = flattest measured postoperative manual keratometry.
Shammas No History Method[23]
Another interesting proposal is by Shammas[23] who studied a series of eyes that have had LASIK. His results led him to propose a formula, in 2003, to predict the effective power of the cornea without needing any of the patient's clinical history, only the postoperative K reading obtained with manual keratometry. The formula is:where:
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K = predicted corneal power |
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KPO = the average corneal power obtained with manual keratometry after corneal refractive surgery. |
Instruments: Topographers and Pentacam
The first instruments to measure corneal power by topography were the Eyesys, Technomed C-Scan and Humphrey units. Holladay developed the “Diagnostic Summary” for the Eyesys unit (Figure 4-8), but still, most of these measurements fall down in post-refractive-surgery eyes.
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Figure 4-8 Eyesys Holladay Diagnostic screen showing corneal power. |
The next instrumentation developed was the Orbscan (Bausch & Lomb), but the difficulty with this unit has been in the post LASIK or PRK cornea. Here the Orbscan has a tendency to overestimate the elevation of the posterior cornea giving an artifactual ectatic appearance[24] (Figure 4-9). Additional error may be induced by a relatively long acquisition time allowing patient movement as the image is being taken.[25] There have also been documented inaccuracies in measurement of the posterior curvature in control standards.[26]
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Figure 4-9 Orbscan screen showing corneal power maps. |
That brings us to the latest and most promising technology, the Oculus Pentacam, which images the anterior segment of the eye using a rotating Scheimpflug camera measurement providing three-dimensional images (Figure 4-10). These images provide a topographic analysis of the corneal thickness, its front surface and most importantly its back surface curvature. In conjunction with software provided by Holladay, the Pentacam is touted to have the ability to generate what they call a “TrueNetPower” map of the cornea and measure the power of the post-refractive-surgery cornea within ±0.55D. This may provide a better estimation of the true corneal power but it has not yet been tested in a large randomized clinical trial.
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Figure 4-10 A, Oculus Pentacam instrument. B, Oculus Pentacam Holladay Power Map screen showing Sim-K and Equivalent K Reading (EKR). C, Scheimpflug photograph of anterior segment by Pentacam. |
Holladay has reported an analysis of the comparison of the postoperative Pentacam reading at 4mm zone to the calculated postoperative power in a series of LASIK eyes. The calculated power was obtained by subtracting the change in refractive error (vertexed to zero) from the pre-refractive-surgery Eyesys Sim-K. The correlation resulted in an R2 of 0.9631 and y of 0.9555 indicating excellent correlation. He then recommended converting the True Net Power of the Pentacam to the Equivalent Sim-K of the Eyesys (e.g. 44.25D = 45.00D). The report provides the “Sim-K” and the “Equivalent K Reading.”
Corneal Power Estimation Summary
In summary, if the results of the above K estimation methods differ, use the lowest estimated corneal power (highest for hyperopic refractive eyes). Rarely are such eyes myopic after IOL surgery. Obviously, some methods cannot be used if the historical data is not available and the CL method is impossible if the cataract precludes performing a refraction. In such cases, it might be wise to delay the IOL implantation and calculate the secondary IOL power using the aphakic refractive error in the refraction formula or use a piggyback lens or phakic refractive lens to correct any deficiency. However, there are other methods available.
Methods to adjust/calculate the target intraocular lens power
Aramberri Double-K Method[27]
Once you have decided on the “best” estimated preoperative K, there is one more consideration. It is the “Double-K” method, one of the most important developments to improve the prediction of corneal power in eyes that have had refractive surgery. It was proposed in 2001 by Aramberri[27] of San Sebastian, Spain. His proposal makes eminent sense. The modern theoretic formulas (except the Haigis) use the corneal power for two purposes; the first is to predict the ultimate position of the IOL (ACD or ELP) and the second (along with AL, target refraction and ELP) is to calculate the power of the IOL. The formulations and algorithms used to predict the ELP are based on the anatomy of the anterior segment which has not been changed by corneal refractive surgery (only the center is flattened and thinned). Therefore, if the postoperative refractive surgery K reading (which is significantly flatter) is used to calculate the ELP it will produce an erroneous ELP value. Since the anatomy has not changed, Aramberri recommends the use of the preoperative K reading to calculate the ELP. The IOL power is then calculated using the postoperative K reading, thus the “Double-K.” His analysis of a small series of eyes proved the benefit of this idea. This method is available for the SRK/T, Holladay and Hoffer® Q formulas on the Hoffer® Programs computer system (see Figure 4-12).
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Figure 4-12 Hoffer/Savini LASIK IOL Power Calculation Spreadsheet organizer showing all data and postoperative results. |
Feiz Method[28,][29]
This method was first described in 2001 by Feiz et al.[28] Their formula was developed by comparing manual keratometry values after LASIK and using the SRK/T formula, the IOL power calculated using the historical method, and what they termed the vertexed IOL power method. In this method two assumptions were made. The first was that to achieve emmetropia, the change in spherical equivalent induced by keratorefractive surgery had to be balanced by the change in IOL power. The second assumption was that for every diopter of change in IOL power, only 0.7D of change will be seen at the spectacle plane. This assumption is not true in all axial lengths.
Performing a linear regression analysis of the vertex IOL power compared to standard keratometry, they developed two linear regression formulas; one for myopic LASIK corneas and the second for hyperopic LASIK corneas:where:
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P= required IOL power |
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PTARG = the IOL power calculated for the desired postoperative Target Rx using the AL and measured (unadjusted) K reading |
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|
RC = the change in refraction (spherical equivalent) caused by the refractive surgery. |
In 2005, they reported a comparison of all three methods for IOL calculation[29] in 19 eyes after myopic LASIK or PRK, which resulted in consistently higher IOL powers using their formula compared to using the post LASIK measurements and the historical method. It still resulted in 16% of eyes being either moderately over- or under-corrected. The hyperopic formula has not undergone any reported study demonstrating its validity.
Below are example calculations (Table 4-4).
Table 4-4 -- Feiz Method example calculations
|
Myopic Eye |
Hyperopic Eye |
|
SRK/T calculates 16.0 D IOL |
Hoffer Q calculates 22.0 D |
|
Change in Rx = −6.0 D |
Change in Rx = +3.0 D |
|
−0.595*(−6) −0.231 = +3.34 |
−0.862*(+3) + 0.751 = −1.84 |
|
P = 16.0 + 3.34 = 19.34 D |
P = 22.0 − 1.84 = 20.16 D |
From their formulae, they produced a nomogram for both types of eyes which might be easier for those without a calculator handy (Table 4-5):
Table 4-5 -- Feiz Method nomogram
|
Feiz[29] Nomogram |
Adjustment to Target IOL Power |
|
|
D Change in Rx from LASIK (X) |
Myopic Eye [−0.595*x−0.231] |
Hyperopic Eye [−0.862*x+0.751] |
|
1.00 |
+0.36 |
0.00 |
|
2.00 |
+0.96 |
−0.97 |
|
3.00 |
+1.55 |
−1.84 |
|
4.00 |
+2.15 |
−2.70 |
|
5.00 |
+2.74 |
−3.56 |
|
6.00 |
+3.34 |
−4.42 |
|
7.00 |
+3.93 |
−5.28 |
|
8.00 |
+4.53 |
−6.15 |
|
9.00 |
+5.12 |
−7.00 |
|
10.00 |
+5.72 |
−7.87 |
Latkany Method[30]
The first proposal by Latkany's group[30] uses the manually measured flattest K of the postoperative cornea in the SRK/T formula. The resulting IOL power is then adjusted by the following formula:where:
P = required IOL power
PFlatK = the IOL power calculated by SRK/T for desired postoperative Rx using the AL and the measured flattest K reading (unadjusted)
PRx = the preoperative refractive surgery spherical equivalent.
The study reported this method to be equal to the historical method for calculation of corneal power. Below is an example calculation (Table 4-6).
Table 4-6 -- Latkany Method example calculations
|
Myopic Eye |
|
SRK/T calculates 22.91D IOL using Flattest K 42.00D |
|
Pre-LASIK Rx = −5.0D |
|
−(0.47*(−5) + 0.85) = +1.50 |
|
P = 22.91 + 1.50 = 24.41D |
|
IOL: intraocular lens; D: diopter. |
Masket Refractive History Method[31]
In 2005, Masket[31] proposed yet another method which adjusts the power of the IOL calculated using the measured IOLM-master data. The formula to adjust the IOL power is:where:
P = required IOL power
PEMM = the IOL power calculated for emmetropia using the AL and measured (unadjusted) K reading
RC = the change in refraction (spherical equivalent) caused by the refractive surgery.
He recommends using the SRK/T formula for myopic ALs and the Hoffer® Q for hyperopic ALs. Here are example calculations (Table 4-7):
Table 4-7 -- Masket Refractive History Method example calculations
|
Myopic Eye |
Hyperopic Eye |
|
SRK/T calculates 16.0 D IOL |
Hoffer Q calculates 22.0 D |
|
Change in Rx = −6.0 D |
Change in Rx = +3.0 D |
|
−0.323*(−6) + 0.138 = +2.076 |
−0.323*(+3) + 0.138 = −0.82 |
|
P = 16.0 + 2.0 = 18.0 D |
P = 22.0 − 1.0 = 21.0 D |
In a series of 28 post-LASIK eyes, he reported 43% of the eyes obtaining a postoperative refractive error of plano, 95% being within ± 0.50D of prediction and a total error range from −0.75D to +0.50D. These early results in a small series by the author of the method are surprisingly impressive.
Wake Forest Method[32]
In 2005, Michael Gagnon[32] presented an alternative calculation method by the group at Wake Forest University, which has been discussed by others over the years. This method simply uses the patient's preoperative refraction before LASIK as the target or “desired” postoperative refraction in the calculation, and the measured AL and K readings without modification.
Ianchulev Intraoperative Aphakic Refraction Method[33]
In 2003, Sean Ianchulev[33] proposed calculating IOL power by performing an aphakic refraction on the operating table immediately after the cataract has been removed using a handheld automated refractor. The resultant refraction is modified by the following formula:
where:
P = emmetropic IOL power
AR = automated refraction
A = IOL A constant.
His early results are quite promising. This method would completely eliminate the need for axial length and corneal power measurements, in addition to the problems with LASIK and silicone oil-filled eyes. However, it would require a large inventory of IOL powers available in the OR.
Following on this idea, in 2006, Mackool[34] published a small series of patients who had cataract extraction without IOL implantation under topical anesthesia. Thirty minutes after the surgery, an aphakic manifest refraction was performed at a vertex distance of 12mm. The following formula was then used to calculate IOL power:
where:
P = emmetropic IOL power
AR = aphakic refraction
A = IOL A constant.
After the IOL calculation was carried out, the patient was immediately returned to the operating room for IOL implantation using the calculated power. Using the above formula in 12 eyes, he reported a mean absolute refractive error of 0.30D, and an average refractive error of −0.18D. However, having two separate surgeries would seem to be inconvenient.
Hoffer/Savini Excel Spreadsheet Tool
In 2006, it became obvious that there were so many methods to perform these calculations that it was becoming very confusing. This author in collaboration with Giacomi Savini of Italy decided to place all the various calculations on one Microsoft Excel spreadsheet so it would be easy to see what data needed to be collected (Figure 4-11). Once all or most of the data are entered into the appropriate cells, the calculations are performed automatically. Then the results of all the methods are displayed side by side allowing the surgeon to select the most appropriate calculation. Anecdotally, the first use of this spreadsheet (Figure 4-12) led to a 2-month postoperative refractive error of −0.25D (SE) when the target Rx was −0.50D (UCVA 20/25; BCVA 20/20). The tool is free and downloadable at www.EyeLab.com.
|
Figure 4-11 Hoffer/Savini LASIK IOL Power Calculation Spreadsheet organizer before data entry. |
Retinal Detachment Eyes: Hoffer Double-Axial Length Method
Just as the Double-K method uses two K readings because the formulas use the K reading to predict the ELP, this method, proposed by the author in 2000, instead uses two ALs. The postoperative retinal detachment (RD) AL of the eye is used to calculate the IOL power. Since most post-encircling band RD eyes have a 1.0 mm increase in AL, and the ACD is not affected by the encircling band, it would be best to use the AL-1 in the part of the formula that calculates the predicted ELP. This method results in making the IOL a little weaker than would be predicted using all the modern formulas. Alternatively, one would just lower the recommended IOL power in such RD eyes.
Corneal Transplant Eyes
A problem also arises when attempting to predict what the corneal power will be after corneal transplantation. Some have suggested using the corneal power of the other eye (if it isavailable) or using an average of one's post-transplant corneal powers, but published reports show a very large range of prediction and refractive errors using these attempts. Performing the IOL implantation after the corneal transplant has settled down was suggested by this author[35] in 1986 and in 1990, Geggel[36] reported excellent refractive results (Figure 4-13) using this two-stepped approach (66% 20/40 or better acuity without correction). A secondary piggyback toric IOL or toric phakic IOL is another alternative to correct residual ametropia.
|
Figure 4-13 Corneal transplants. Dramatic decrease in range of IOL prediction error when IOL is implanted secondarily after transplant heals (5.62D) vs. a triple procedure (9.82D). |
Corneal Scar Eyes
The problem of getting an accurate corneal power measurement in eyes with corneal scarring and irregular astigmatism has not received much attention. Cua et al[37] studied this in two eyes needing IOL exchange due to large “IOL surprises” of +5 and −7.5D. They compared six methods to ascertain the corneal power and found the hard contact lens over refraction method to be the most accurate; decreasing the error they would have obtained with the manual keratometer of +4–5D to −0.4–1.6D. This may be a useful clinical tool in such cases.
Intraocular lens axial position
This factor was historically referred to as the anterior chamber depth (ACD) because the optic of all IOLs in the early era was positioned in front of the iris, in the anterior chamber. Because most IOLs today are positioned behind the iris, new terminology has been offered, such as effective lens position (ELP) by Holladay[6] and actual lens position (ALP) by the FDA.
ACD is defined as the axial distance between the two lenses (cornea and lens or IOL) or, more exactly, the distance from the central front surface (anterior vertex) of the cornea to the effective principle plane of the IOL (or front surface of the crystalline lens). This value is required for all formulas and it is incorporated into the A constant specific to each IOL style for regression formulas or as an ACD, both supplied by the manufacturer. Some have proposed that it would be useful to measure the preoperative anatomic ACD (corneal epithelium to anterior capsule) either with an A-scan unit or by optical pachymetry. The author performed such a comparison study on 44 eyes and showed that the optical method resulted in a mean 0.20 (±0.35)mm deeper ACD than obtained by ultrasound using 1548m/s (3.14 vs.2.93mm).
The IOL position has been considered the least important of the three variables as a cause of IOL power error, but in 1998, the author saw an early postoperative IOL patient with a shallowed ACD and myopia of −2.50. After 3 days, the chamber deepened by 2.0 mm and the refractive error changed to plano. IOL position has received the most attention from formula writers over the past 10 years. The major effort has been toward better prediction of where the IOL will ultimately rest. A recent study by the author on a series of 270 eyes receiving a silicone plate haptic lens showed that the IOL shifted a mean of 0.06 mm posteriorly (ACD deepened) at 3 months, compared to its position on the first day after surgery. This was commensurate with a mean 0.21 D shift toward hyperopia.
WARNING: An IOL intended for capsular bag placement should be decreased by 0.75–1.25 D (depending upon the IOL power) when placed in the ciliary sulcus.
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Formulas
Generations
First Generation
The first IOL power formula was published by Fyodorov[38] in 1967. Colenbrander[39] wrote his in 1972, followed by the Hoffer[40] formula in 1974. Binkhorst[41] published his formula in 1975, which became widely used in America. In 1978, first Lloyd and Gills[42,][43], followed by Retzlaff[44] and, later, Sanders and Kraff[45] each developed a regression formula based on analysis of their previous IOL cases. This work was amalgamated in 1980 to yield the SRK I formula.[46] All these formulas depended on a single constant for each lens that represented the predicted IOL position (ACD).
Second Generation
In 1982, at the Welsh Cataract Congress in Houston, the author[47,][48] demonstrated a direct relationship between the position of a PMMA posterior chamber IOL and the axial length, and presented a formula to better predict ACD. Others (Binkhorst[49], SRK II[50] (1988)) developed different mechanisms to apply this predictive relationship which Holladay defined as the second generation.
Third Generation
In 1988, Holladay[51] proposed a direct relationship between the steepness of the cornea and the position of the IOL. He modified the Binkhorst formula to incorporate this as well as the axial length relationship. Instead of ACD input, the formula would calculate the predicted distance from the cornea to the iris plane (using a corneal height formula by Fyodorov) and add to it the distance from the iris plane to the IOL. The latter he called the surgeon factor (SF) and it is specific to each lens. Retzlaff[52] followed suit and modified the Holladay I formula to allow use of A constants calling it the SRK/T theoretic formula in 1990. It was intended to replace the previous SRK regression formulas that 50% of American surgeons still use despite this. In 1992, Hoffer developed the Q formula[53] using a tangent function to accomplish the same effect.
Fourth Generation
In 1990, Olsen et al.[54] proposed using the preoperative ACD and other factors to better estimate the postoperative IOL position and published algorithms for this approach. After several studies showed that the Holladay I formula was not as accurate as the Hoffer® Q in eyes shorter than 22 mm, Holladay used the preoperative ACD measurement, as well as corneal diameter, lens thickness, refractive error and age, to calculate an estimated scaling factor (ESF) that multiplies the IOL-specific ACD. This Holladay II formula has been promulgated since 1996 but has yet to be published.
In 1999, Wolfgang Haigis[55] proposed using three constants based on the characteristics of the eye and the IOL to predict the position of the IOL. The formula calculates the predicted postoperative ELP by:where:
ELP = predicted IOL position
a0 = a lens-specific constant
a1 = a constant to be effected by the measured preoperative ACD
a2 = a lens-specific constant to be effected by the measured preoperative axial length
ACD = the measured axial distance from the corneal apex to the front surface of the lens
AL = axial length.
As in the Holladay formula, the constants must be optimized (personalized) to each IOL style and surgeon. Single optimization only optimizes the a0 and creates accuracy equal to the Hoffer® Q and Holladay, but triple optimization of all three constants creates additional accuracy. The problem is that triple optimization requires a series of 500–1000 cases of one lens style and the eyes in the series must statistically cover all axial lengths from very short to very long. This may be quite difficult to achieve for the average surgeon.
Refraction Formula
Holladay[56] published a formula in 1993 to calculate the power of an IOL for an aphakic eye or ametropic pseudophakic eye (piggyback IOL) or a refractive lens (PRL) for a phakic eye. It does not need the AL, but does require the corneal power, preoperative refractive error and desired postoperative refractive error, as well as the vertex distance of both. The author does not recommend its use in aphakic eyes because the vertex distance is difficult to measure accurately and, owing to the high power of their refractive error, greater errors can result. It is, however, a good check against the AL formula calculation.
Usage
Based on Axial Length
My study[53] of 450 eyes (by one surgeon using one IOL style) (Figure 4-14) showed that in the normal range (72%) of axial length (22.0–24.5mm) almost all formulas function adequately, but that the SRK I formula is the leading cause of poor refractive results in eyes outside this range. It also showed that the Holladay I formula was the most accurate in medium-long eyes (24.5–26.0 mm) (15%) and the SRK/T was more accurate in very-long eyes (>26.0mm) (5%). In short eyes (<22.0mm) (8%) the Hoffer® Q formula was most accurate and this was confirmed (P >0.0001) in an additional large study of 830 short eyes as well as in a multiple-surgeon study by Holladay. Holladay has postulated that the other formulas overestimate the shallowing of the effective lens position (ELP) in these very short eyes.
|
Figure 4-14 Error range. Range of intraocular lens power error in 450 eye study using regression formulas compared to modern theoretic formulas. |
A more recent study[57] performed by the author on 317 eyes, showed that the Holladay II formula equaled the Hoffer® Q in short eyes but was not as accurate as the Holladay I or Hoffer® Q in average and medium-long eyes (Table 4-8). Eyes shorter than 19 mm are extremely rare (0.1%) and may well be benefited by using the Holladay II formula. It appears that in attempting to improve the accuracy of the Holladay formula, the addition of more biometric data input has improved the Holladay II formula in the extremes of axial length but deteriorated its excellent performance in the normal and medium-long range of eyes (22.0–26.0mm), which accounts for 82% of the population.
Table 4-8 -- Results of accuracy of four theoretical formulas on 317 eyes using the Holladay IOL Consultant for analysis [Shaded = recommended formulas][46]
|
Mean Absolute Error |
All 317 Eyes |
|||||||
|
Formula |
Short <22.0 |
Normal 22.0–24.5 |
M-long 24.5–26.0 |
V-long >26.0 |
Long <24.5 |
All eyes |
Max Error |
>± 2 D error |
|
Holladay 2 |
0.72 |
0.56 |
0.51 |
0.49 |
0.50 |
0.55 |
−1.60 |
0% |
|
Holladay 1 |
0.85 |
0.42 |
0.37 |
0.56 |
0.43 |
0.43 |
−1.44 |
0% |
|
Hoffer Q |
0.72 |
0.43 |
0.47 |
0.58 |
0.50 |
0.45 |
−1.61 |
0% |
|
SRK/T |
0.83 |
0.46 |
0.35 |
0.44 |
0.36 |
0.44 |
−1.45 |
0% |
|
AVERAGE |
0.78 |
0.47 |
0.42 |
0.52 |
0.45 |
0.47 |
||
|
BEST |
H–Q H–2 |
H–Q H–1 |
S/T H–1 |
S/T |
S/T |
|
Where M-long = medium long, V-long = very long, Long = all long eyes, Max = maximum. |
Methodology
There are several means by which to use these newer formulas including A-scan instruments, handheld calculators, and computer programs that run on DOS, Windows and Macintosh systems, as well as for the palm PDA operating system (Figure 4-15). You can also adapt the published ones yourself to a spreadsheet program. It is important to check the errata in references 41 and 42. The most popular commercial programs are the Hoffer® Programs System* (the first computer program for IOL power in 1994) and the Holladay® IOL Consultant* (1997), which include several formulas that can be personalized and provide routines to deal with odd clinical situations.
|
Figure 4-15 Hoffer® IOL power program on a palm personal digital assistant. A, Main calculation screen using Hoffer Q®, Holladay and SRK/T formulas. B, Next screen showing refractive results of different intraocular lens (IOL) powers. C, Clinical history method screen. D, Contact lens method screen. E, Personalization screen for adding new postoperative eyes. F, Personalization screen for various IOLs. |
Personalization
The concept of personalizing a formula based on a surgeon's past experience and data was introduced by Retzlaff [52,][58] using the A constant to refine the formula. Holladay incorporated this concept into backsolving for the Surgeon Factor and Hoffer backsolved for his personalized ACD. Several studies have proved that formula personalization definitely improves formula accuracy significantly.
The following parameters are required from postoperative eyes:
|
1. |
Axial length (preoperative) |
|
|
2. |
Corneal power (preoperative) |
|
|
3. |
IOL power |
|
|
4. |
Postoperative refractive error (stable). |
The eyes should all contain the same lens style by one manufacturer that has been implanted by one surgeon. The same biometry instruments and technician should also have been used. Eyes with postoperative surprises or acuity worse than 20/40 should not be included in the analysis; this is due to poor accuracy in obtaining refractive error. Personalization involves backsolving for the exact IOL position that would produce the resultant refractive error with that IOL power, AL and K. Then all the “ideal” IOL positions are averaged to arrive at the personalized value to use in the future. Personalization can be easily performed using the Hoffer® Programs or Holladay® IOL Consultant computer programs.
* Available from EyeLab, Inc. 1605 San Vicente Blvd, Santa Monica, CA 90402, 310-451-2020, KHofferMD@AOL.com.
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Clinical variables
Patient needs and desires
Most surgeons have developed their own plan for deciding on the clinical needs of their patients. It has often been recommended to aim patients for mild postoperative myopia (−0.5 to −1.5D), so if the error is on the plus side, they will be emmetropic and if on the minus side, they will have reading vision. This is necessary because of the larger range of IOL power errors generally experienced. When the bell-shaped curve of prediction error is squeezed down to 67% within ±0.50D, it is then possible to aim most patients for emmetropia. This is even more important when implanting a multifocal IOL. Senior citizens are much more active today then in the past and in emergency situations it would be a lot safer if they were emmetropic.
There are several exceptions, however. Patients who have been life-long myopes are never happy being hyperopes postoperatively. Patients that would wind up with a large anisometropia should be stimulated to be fitted with a contact lens in the other eye prior to deciding on an emmetropic IOL. Monocular contact lens wearers are more successful than binocular. It is wise to document all discussions in unusual situations.
Special circumstances
Monocular Cataract in Bilateral High Ametropia
The dilemma is whether to make the ametropic eye emmetropic or to match the large ametropia of an eye that may never need surgery. Up until now, the author has convinced most patients to accept a monocular contact lens or ignore the unaffected eye and go for the “brass ring” of emmetropia. In the future, those that can't tolerate contact lenses could have a phakic IOL either placed in the unaffected eye or placed over the IOL to eliminate aniseikonia and have it removed should the affected eye ultimately need surgery.
Pediatric Eyes
Children have always posed a dilemma[59] in IOL power selection in that the eye will grow in length and become more myopic if a fixed emmetropic power is implanted. The study of pediatric eyes by Gordon and Donzis[60] shows a steep axial length growth rate from premature babies to age 2 years, increasing by 6 mm (~20 D), while corneal power drops from 54D to 44D offsetting 10D. If IOLs are used in this age group it might be best to place piggyback lenses with the more posterior IOL having the average adult emmetropic power and the anterior IOL being the added power needed to reach emmetropia now. As the child grows, they can be corrected with myopic glasses until they are old enough to have the anterior IOL removed.
Between the ages of 2 and 5 years growth slows to about 0.4mm per year and between the ages of 5 and 10 years the total growth is only another 1mm, while corneal power remains stable. From age 2 to 10, it might be wise to aim for 1.5–2D of hyperopia postoperatively, which allows for reasonable uncorrected vision and light spectacle correction in amblyopia treatment. When they mature, they will wind up emmetropic or mildly myopic, depending on the age at implantation. Growth slows after age 10–15 and emmetropia can be the aim. Future use of implantable phakic refractive lenses over the top of IOLs may be very helpful in these children since they can easily be exchanged as the eye grows, keeping them emmetropic throughout life.
Plager et al.[61] reported on 38 eyes of 27 subjects who had received an IOL in childhood. Based on their results, they recommend the following scheme for the refractive age-dependent goal for children (Table 4-9).
Table 4-9
|
Age |
3 |
4 |
5 |
6 |
7 |
8 |
10 |
13 |
|
Goal |
+5.00 |
+4.00 |
+3.00 |
+2.25 |
+1.50 |
+1.00 |
+0.50 |
Plano |
Multifocal Intraocular Lens
In 1991 the author[62] reported that to obtain −2.75D myopia (reading at 14–16 inches) the IOL power in the near vision region must be about 3.75–4.00D stronger than the emmetropic power. It was also shown that the amount of this additional power in a bifocal IOL is not affected at all by the axial length and very little by the corneal power. It is affected, however, by the IOL position and an AC lens needs less add power than a PC lens. Obviously, to negate the need for any glasses, it is important to aim for emmetropia, but mild postoperative hyperopia is far better than even the mildest myopia. The distance vision will be reasonable in the former (the patient can easily obtain readers if necessary); while in the latter it will not. Bifocal IOL patients with myopia are not happy and everything should be done to avoid this situation, since minus power “readers” are not readily available. A phakic IOL could be implanted over the top of the bifocal to make the eye emmetropic.
Silicone Oil Refractive Effect
The second problem that arises when the vitreous is replaced with silicone oil is that the refractive index of the oil is much less than that of the vitreous and it acts as a negative lens in the eye, which must then be offset with more power in the IOL. This effect is dependent upon the shape factor of the back surface of the IOL, such that a biconvex IOL creates the worst problem and a concave posterior lens (no longer commercially available) has practically no effect. In between the two is the plano-posterior lens, which is recommended in these cases. With a plano-convex lens, 2–3D must be added to the IOL power to compensate for this silicone effect, but much more is needed for biconvex lenses.
Piggyback Lenses
Either piggyback lenses can be placed primarily or the second lens placed secondarily over a previously healed IOL. In the former, the anterior IOL forces the posterior IOL more posteriorly; a distance equal to the central thickness of the anterior lens. This causes the posterior lens (whose focal point is moved more posteriorly) to require more power to maintain the same focus. This effect diminishes the thinner (lower power) the anterior lens is and a thinner lens is easier to remove if that should be necessary. Primary piggyback lenses need special calculations to adjust for the posterior lens shift. One can simply add one-half the central thickness of the anterior IOL to the ACD being used by the formula.
Secondary lenses can be calculated using the refraction formula or by a more simple formulation based on the fact that the healed primary IOL is more stable. Because of the differences in the effects on vertex power changes between plus and minus lenses, the following formulation works well:
where Rx = PO spherical equivalent refractive error.
Problems and errors
The major problem is an unacceptable postoperative refractive error. The sooner it is discovered, the sooner it can be corrected. Therefore, it is wise to perform K readings and a manifest refraction on the first postoperative day. The author has long recommended immediate surgical correction[63] (24–48h); this allows easy access to the incision and the capsular bag, one postoperative period, and excellent uncorrected vision. The majority of medico-legal cases today are due to a delay in the diagnosis and the treatment of this iatrogenic problem. Up until now, we could only correct this problem by lens exchange, which creates the dilemma of determining which factor created the IOL power error: axial length, corneal power or mislabeled IOL or a combination of all three. Today, with the advent of low-powered IOLs, the best remedy may be a piggyback IOL. When using a piggyback IOL, it is not necessary to determine what caused the error or to remeasure the axial length of the freshly operated pseudophakic eye. It is possible to confirm the power of an explanted IOL by using the McReynolds lens analyzer (Figure 4-16) (Vision & Hearing Center, PO Box 488, 1111 Main St. Quincy, IL 62301, 217-222-6656).
|
Figure 4-16 McReynolds intraocular lens power analyzer used in standard clinical lensometer. |
It is important to remember that a shallow AC can lead to as much as 3D of myopia (depending on the power of the IOL), which will disappear when the AC reforms. An RK eye has a propensity for the cornea to flatten postoperatively causing large hyperopic surprises. It may take up to 3 or 4 months for the cornea to re-steepen, therefore, surgical correction should not be attempted until then.
Handling the Intraocular Lens Power Surprise
An inappropriate postoperative refractive result is disappointing to both the patient and the surgeon. It is often difficult to determine what caused this prediction error.
|
1. |
According to most studies, the most common cause is an error in measuring axial length:
|
||||||||||||||||||||||
|
2. |
Incorrect measurement of the corneal power is the second most common reason for IOL power error:
|
||||||||||||||||||||||
|
3. |
The third and least effective factor in prediction error is the healed effective position of the IOL in the eye. This is referred to as the A constant, the surgeon factor (SF) or the Hoffer anterior chamber depth (ACD). Holladay instituted the replacement term, effective lens position (ELP), since most IOLs today are not in the anterior chamber:
|
||||||||||||||||||||||
|
4. |
The use of formulas is a cause of IOL power error, especially regression formulas, for example the SRK I regression formula, when used in eyes outside the normal AL range of 22–24.5mm. This has been shown in so many studies[14] over the past 12 years, it would be impossible to reference them all here. |
||||||||||||||||||||||
|
5. |
There are other miscellaneous causes for IOL power errors that can be just as serious as those mentioned above. A rare manufacturer labeling error can be very serious and very difficult to pick up before the patient is discharged from the facility. If the operating room nurse hands the surgeon the wrong IOL power during the surgery this may not be easily recognized in time to correct the error. Lastly, transcription mistakes can cause some of the largest errors seen. |
Prevention of Common Errors
|
• |
Use the IOLMaster® or immersion A-scan to measure the AL. |
|||||||||||||
|
• |
Suspect a staphyloma in eyes >25 mm: use IOLMaster and/or Shammas A/B-scan technique. |
|||||||||||||
|
• |
Use CALF method: measure eye using 1532m/s and add +0.32mm to the result to correct for any error in sound velocity. |
|||||||||||||
|
• |
Employ a well-trained, experienced technician. |
|||||||||||||
|
• |
Regularly calibrate manual keratometers. |
|||||||||||||
|
• |
Carefully evaluate the IOLMaster® scan for reliability. |
|||||||||||||
|
• |
Keep contact lenses out for 2 weeks prior to keratometry (at least in one eye.) |
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Silicone-oil eyes need IOLMaster® if possible or ultrasound AL times 0.71. |
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Use the Hoffer® Q formula in eyes <22mm and in post-refractive surgery eyes. |
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Use the Holladay® I formula in eyes 24.5–26mm in length. |
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Use the SRK/T formula in eyes longer than 26mm. |
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Never use the SRK Regression formulas (SRK I or II). |
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Personalize your ELP factors in the formulas. |
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Surgeon should personally select the IOL power for the individual patient. |
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Prepare a sheet with all IOL powers that may be needed and place it on the wall and also on the microscope in the OR for the surgeon and OR nurse to verify the correct IOL power. Use red paper for right eyes and yellow paper for left eyes. |
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Be sure to set the IR to 1.3375 in the setup screen of the IOLMaster®. |
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Use the ‘clinical history and contact lens methods’ (have PMMA CLs in the clinic) for postrefractive surgery corneas and use the lowest calculated K (highest for hyperopes):
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Consider delaying the IOL implantation until the cornea has healed after a penetrating keratoplasty rather than performing a “triple procedure.” |
Suggestions for Diagnosing and Treating Intraocular Lens Power Surprises
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• |
Make it a routine to perform a manifest refraction on postoperative day 1 so as to discover the problem early enough to take the patient back to the operating room and correct the problem in the first 48 h. The patient is immediately pleased and medico-legal actions are completely eliminated. |
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Consider the use of a piggyback IOL or phakic IOL if the eye has healed beautifully and removal of the errant IOL would be more traumatic to the eye. For myopic error use 1 times the error and for hyperopic errors, use 1.5 times the error (or the Shammas Formula). |
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Consider a minimal four-incision RK if repeat intraocular surgery is not possible. |
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• |
Measure the power of a removed IOL using the McReynolds Analyzer (William McReynolds 217-222-6656) or ask a manufacturer to be present in the operating room to do it. |
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Conclusion
Simple steps and attention to detail can be very useful in preventing IOL power errors and recent advances in IOL power range availability has made this problem more easily corrected. Since performing the first American ultrasound IOL power calculation[64] in 1974, the past 32 years have seen great improvement in the accuracy of postoperative refractive prediction. Future improvements may some day eliminate the problems we have left.
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