Highly Accurate IOL Calculations

Accurate IOL calculations begin with identifying the patient's visual goals. No one needs to explain this concept to a LASIK surgeon, but cataract surgeons all too often overlook this basic starting point. With patients' expectations continuing to increase, it is becoming more important to follow the lead of our refractive colleagues and take the time to agree upon a refractive goal with each patient prior to cataract surgery. It may come as a surprise that your patients will often give you an answer other than emmetropia. For example, a patient of mine is a well-known chef who told me that his world exists mainly at arm's length. We agreed before surgery that -1.50D would be our refractive target. Conversely, for a pilot, the refractive goal would undoubtedly be plano. Ascertaining individual preferences, especially if a specific occupational concern is important, can convert those patients who clearly know what they want into a small army of ambassadors for your practice.

THE COMPONENTS OF ACCURATE
CALCULATIONS

For normal eyes, using the best aspects of today's technology makes it possible to consistently achieve highly accurate postoperative results. As always, however, the devil is in the details, meaning that we have to execute all components of the exercise correctly. Patient selection, accurate keratometry, the method of biometry, the IOL power formula selected, and even the surgical technique all play important roles. To concentrate all of our attention on biometry is to miss the point. For example, if the keratometry is off by 0.75D, then the final postoperative refraction will be off by that same amount. Using the SRK/T formula in the setting of high-axial hyperopia will probably produce a hyperopic result. If the capsulorhexis is much larger than the optic of the IOL, a myopic shift may occur following the contraction of the capsular bag. Finally, knowing when to repeat a measurement that does not fall within an established set of validation criteria is as important as knowing how to carry out the measurement correctly in the first place. Highly accurate IOL power calculations are the result of a collection of many nuances, all linked together and each needing optimization.

AN EXCITING TIME

Refractive surprises have occurred ever since Sir Harold Ridley implanted the first IOL in 1949. With steady technological advances, the overall accuracy of our refractive outcomes has generally doubled every 5 to 10 years. With the introduction of the IOLMaster (Carl Zeiss Meditec Inc., Dublin, CA) in North America in 2000, refractive outcomes within 0.25D of the targeted refraction became a reality for the first time. This milestone allows us to set our sights on far more sophisticated endeavors, such as allowing our patients to fully enjoy the correction of spherical aberration. We are clearly entering a very exciting time in the history of IOL power calculations.

There are several patient groups for whom it is not always possible to deliver a highly accurate refractive result, however. At present, consistently accurate refractive outcomes remain elusive for those with prior keratorefractive surgery, keratoconus, extreme axial myopia with posterior staphyloma, nanophthalmic eyes, or eyes with silicone oil.

BASIC KERATOMETRY

When was the last time you calibrated all of the keratometers in your office? If your office has more than one keratometer, or employs several different methods of corneal power measurement (eg, simulated keratometry, automated keratometry, and manual keratometry), multiple instruments will introduce another variable into the process. I strongly recommend assigning a single instrument that was recently calibrated against a set of standard calibration spheres to the task of all pre- and postoperative keratometry.

It is also helpful for each office to establish a set of keratometry validation guidelines. In my office, if the Ks are very flat (less than 40.00D) or very steep (greater than 48.00D), a second person double-checks the measurements and signs the chart. If the total keratometric power between eyes is greater than 1.50D, a second staff member repeats the measurements. If the mires are distorted, or the total astigmatism for either eye is greater than 4.00D, we will typically obtain a topographic axial map to screen for keratoconus. Lastly, if there are any difficulties in obtaining the measurements that cannot be resolved, we ask the patient to return for repeat keratometry on another day.

BIOMETRY

There are currently four methods available for ophthalmic biometry: applanation A-scan; immersion A-scan; immersion A/B-scan; and optical coherence biometry using the IOLMaster.

Surgeons interested in highly accurate outcomes have mostly abandoned applanation biometry, which yields a falsely short axial length due to variable amounts of corneal compression. It is also highly operator dependent and often leads to corneal irritation. Of the ultrasound-based biometric methods, immersion biometry is a much better choice. Although it has the same 10-MHz resolution as the applanation method, it is much more consistent because there is no corneal compression and the measurement displayed is closer to the true axial length. Contrary to popular belief, the immersion technique is actually quite simple to perform, especially when used in conjunction with the Prager shell. Moreover, because immersion biometry is far more consistent than applanation biometry, it often takes less time.
The main limitation to accuracy with 10-MHz A-scan ultrasound is that it uses a relatively broad, low-resolution sound wave to measure the distance from the corneal vertex to the vitreoretinal interface. Moreover, the region surrounding the fovea has a variable retinal thickness, with the foveal center being thinner than the area immediately adjacent to it. Typically, both of these areas are included in an A-scan biometric measurement.

The most sophisticated form of ultrasound-based biometry is a combined immersion vector A/B-scan. By this technique, familiar to our retinal colleagues, a horizontal immersion B-scan is carried out with a simultaneous vector A-scan that can be manually positioned to measure from the center of the corneal vertex to the location of the fovea.1 The disadvantages of A/B biometry are that the equipment is generally somewhat expensive and a higher level of operator skill is required. In my office, if we are not able to use the IOLMaster due to the presence of a dense axial opacity, immersion A/B biometry is our method of choice.

THE IOLMaster

In my opinion, the IOLMaster represents the single most important advance in IOL power calculations since the introduction of ultrasound biometry 3 decades ago. Interestingly, the technological foundation of this instrument is based on principles laid down during the 19th century by the German-American physicist Albert Michelson.2 More than 100 years after its invention, the Michelson interferometer was introduced to ophthalmology via our colleagues in astronomy and physics. It is likely that, in the future, similar technological advancements will come to us from other unrelated disciplines and will have an equally important impact.

One of several reasons why the IOLMaster has a much higher resolution than ultrasound is that the axial-length measurement is based on a very short 780-nm light wave, rather than a much longer 10-MHz sound wave. By optical coherence biometry, the IOLMaster measures the distance from the corneal vertex to the retinal pigment epithelium (not affected by variations in retinal thickness) and then subtracts the foveal thickness. This approximation to an axial length by immersion ultrasound is based on a comparison to the exquisitely accurate Grieshaber Biometric System (Alcon Grieshaber AG, Schaffhausen, Switzerland), an ultra-high–resolution ultrasound biometer that employs four 40-MHz counters and is capable of an astonishing accuracy of 20µm.3 In essence, the IOLMaster is the equivalent of an upright, noncontact, immersion A-scan but with a fivefold increase in resolution.4

There are four situations in which the IOLMaster is best suited for accurate biometry: (1) nanophthalmia or extreme axial hyperopia, because small errors in axial length are important; (2) extreme axial myopia, especially in the presence of a peripapillary posterior staphyloma; (3) prior retinal detachment with silicone oil; and (4) pseudophakia, polypseudophakia, and phakic IOLs. Ophthalmologists are now starting to measure eyes that develop cataracts after phakic IOL implantation, and the IOLMaster can measure straight through the phakic IOL on the phakic setting with excellent results.5

When the IOLMaster debuted, it was presented mostly as a point-and-shoot device with which the axial-length display with the highest signal-to-noise ratio was considered the best choice. Unfortunately, it is not quite that simple. Using the IOLMaster requires the correct interpretation of the axial-length display, with the signal-to-noise ratio being helpful but not the most important determiner of the overall quality of the axial length measurement. Ideally, the axial-length display should have tall and slender primary maxima, with a thin and well-defined termination, much like the familiar silhouette of the Chrysler building in New York City.4 Careful attention to the axial-length display will avoid double peaks and other problems that could lead to potentially inaccurate measurements.

VALIDATION GUIDELINES FOR AXIAL LENGTH

If the preoperative refraction and keratometry are equal between both eyes, but one eye measures 28mm and the other eye measures 26mm, something is obviously wrong. A 27-mm axial length displayed for a patient with a +4.00D refractive error suggests an error. As originally suggested by Holladay,6 it is important to follow a set of axial-length validation guidelines as the basis of a protocol to double-check any measurements that may not correlate with the overall clinical picture. In my office, if the difference between eyes is greater than 0.33mm, a second person independently verifies the results. If the axial length is less than 22mm or greater than 26mm, a second person reviews or repeats the measurements. We do likewise if the axial length correlates poorly with the refractive data or if there is any difficulty in obtaining consistent measurements.

IOL FORMULAe

In North America, the three commonly used, theoretical IOL power calculation formulas (Hoffer Q, Holladay 1, and SRK/T) are derived from the same mathematical backbone. The main difference between these third-generation, two-variable formulas is the way in which they calculate the final position of the IOL, commonly known as the effective thin-lens position.7

Limitations of all third-generation, theoretical, two-variable formulas are that they work best near schematic eye parameters, apply a number of broad assumptions to all eyes, and, apart from the lens constant, predict the final position of the optic of the IOL based solely on central corneal power and axial length. For example, some formulae assume that the anterior and posterior segments of the eye are mostly proportional, or that there is always the same relationship between central corneal power and the effective lens position, which is not always true, especially in axial hyperopia.7,8

By the late 1980s, the Holladay 1 formula was available, which works well for eyes with normal and long axial lengths. This formula was followed in 1990 by the SRK/T formula, which works well for normal-to-moderately long axial lengths. Several years later, the Hoffer Q formula was added, which works well for eyes with short and normal axial lengths. Presently, regression formulae such as Binkhorst II, SRK I, and SRK II are now mostly of historical interest only. Interestingly, SRK II is still widely used by many in spite of its obvious limitations.

In 1991, Wolfgang Haigis, MS, PhD, the head of the Biometry Department of the University of Würzburg Eye Hospital in Germany, published the Haigis formula. Using the same mathematical backbone as other theoretic formulas, the Haigis formula approaches the problem of IOL power accuracy with three constants (a0, a1, and a2) and adds a measured anterior chamber depth for a third required variable. With the a0 constant optimized in a manner similar to SRK/T, and the a1 and a2 constants based on schematic eye parameters, the formula performs as if it were a very good third-generation two-variable formula. When all three constants are optimized by regression analysis based on surgeon-specific IOL data, however, the range of the Haigis formula can be extended greatly to cover both high-axial hyperopia and high-axial myopia. The main limitations to using the Haigis formula for all axial lengths are that only Dr. Haigis and I presently carry out the required regression analysis and a patient database of approximately 200 cases containing a wide range of axial lengths is required.

The Holladay 2 formula, available since 1998, is considered by many to be the most accurate of the theoretic formulas currently offered. The formula is easy to optimize and works well across a wide range of axial lengths. Its main limitations are that it requires the manual input of seven variables and it is relatively expensive to purchase. Surgical practices serious about their refractive outcomes will typically use the Holladay 2 formula.

Because most biometric equipment already comes with several theoretic formulas, a simple rule to follow is to use the Holladay 1 formula for normal-to-long eyes and the Hoffer Q formula for normal-to-short eyes. However, it should eventually be the goal of every surgical practice to use a more modern formula such as Haigis or Holladay 2.

What are useful IOL power calculation validation guidelines? Of course, both eyes should be measured at the same time to serve as a basis for comparison. If the IOL power difference between eyes is greater than 1.00D, or if there is any question about the accuracy of the axial length or keratometry, the results should be double-checked. Also, if the calculated IOL power does not match what you expected to see, such as a +28.00D IOL recommended for an axial myope, repeating the measurements is mandatory.

SURGICAL TECHNIQUE

For highly accurate refractive outcomes, the capsulorhexis should be considered the defining portion of the surgical procedure (Figure 1). Ideally, the capsulorhexis should be round, smaller than the optic, and centered. If carried out correctly, the optic of the lens implant should remain at the plane of the zonules. If the capsulorhexis is much larger than the optic, the forces of capsular bag contraction may shift the optic anteriorly, inducing a myopic shift late in the postoperative course. Also, at the conclusion of the case, the optic of the IOL should be centered directly beneath the capsulorhexis so that the capsular bag can uniformly shrink-wrap around it (Figure 2). This approach is another important step for ensuring consistent refractive outcomes. A failure to pay close attention to the capsulorrhexis can impact on the refractive outcome more than ultrasound-based biometry or keratometry.

SUMMARY

Understand the current limitations of technology. There are some patients to whom you cannot promise a highly accurate outcome. Assign a single instrument for the task of keratometry for added consistency. Use the IOLMaster or immersion biometry rather than an applanation technique. Develop a set of validation criteria for each part of the measurement process and have a second person carefully review and/or repeat any part that falls outside of these guidelines. Consider switching to one of the newer IOL power calculation formulae for improved accuracy. Optimize your surgical technique by making the capsulorhexis round, smaller than the optic, and centered. By embracing current technology and paying careful attention to details, every ophthalmologist can achieve highly accurate refractive outcomes.

Warren E. Hill, MD, FACS, is Medical Director of East Valley Ophthalmology in Mesa, Arizona. He is a consultant for Alcon Laboratories, Inc., and Carl Zeiss Meditec Inc. Dr. Hill may be reached at (480) 981-6111; hill@doctor-hill.com.

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