Improving toric IOL outcomes, part 1

Reducing a patient’s ocular astigmatism with toric IOLs is on the rise and deservedly so.

Issue: May 25, 2011

ByJack T. Holladay, MD, MSEE, FACS

Jack T. Holladay

The toricity on an IOL is manufactured to within 0.25 D, just as the spheroequivalent power. As with any emerging technology and procedure, there are some pitfalls along the way that need to be addressed. Specifically, in this two-part series, I will discuss 1) measuring corneal astigmatism, 2) toric IOL calculators, 3) toric optimizer, 4) choosing the residual astigmatic target, 5) proper IOL alignment and centration, and 6) managing unfortunate outcomes of residual astigmatism and higher-order aberrations.

Measuring corneal astigmatism

Most surgeons have used a manual or automated keratometer in the past to get the preoperative keratometry readings used for IOL calculations. These instruments usually measure four points on a ring and produce the orthogonal steep and flat meridional power. For a 44 D cornea, the manual keratometer measures points that are 3.2 mm apart; however, the IOLMaster (Carl Zeiss Meditec) measures 2.5 mm apart, and the Lenstar (Haag-Streit) has two rings and measures 2.2 mm and 1.7 mm apart. As long as the astigmatism is perfectly symmetrical (symmetric bowtie shown in Figure 1a) and does not change radially, the result should be the same. Unfortunately, this is rarely the case. The astigmatism often changes as we move radially and is not symmetrical, and sometimes the flat and steep meridian are not even orthogonal (irregular astigmatism). In general, the smaller the sample zone, the smaller the magnitude of the measured astigmatism and axes will change if shaped like a crab claw (Figure 1b).

Figure 1a. Symmetric bowtie astigmatism.

Figure 1b. Irregular crab-claw astigmatism.

Images: Holladay JT

Topography and tomography do a much better job, when irregularity is present. Rather than four points on a ring, thousands of points are measured within a 3 mm to 4.5 mm zone, and a torus or toric ellipsoid is used to perform a least squares fit of the surface to all of the measured points. We first did this on the EyeSys many years ago and named it zonal effective refractive power (Eff RP) over a 3.0 mm zone (Figure 2, values in the first column at the bottom). This value can be compared with the simulated keratometry (Sim K) values in the second column. The Sim K values use the same ring as the manual keratometer and yield essentially the same values. Topography uses Purkinje-Sanson Image 1 (the first reflection of a Placido disk) to determine the curvatures of the surface as a mirror. The reflecting optics are identical to the refracting optics on the front surface, and the analysis can be very accurate, especially to determine the refractive astigmatism. The limitations of Placido-based topography are the central scotoma (unmeasured zone) of 1 mm to 2 mm, depending on the instrument and no measurement of the back surface power.

The back surface power of the cornea is negative and contributes 10% or less to the net corneal power. In the vast majority of patients, the back surface contribution of power never changes the net astigmatism or power of the cornea in a way that cannot be compensated by a constant multiplier. For example, keratometry readings of 44 D × 46 D (2 D of keratometric astigmatism) would convert to a net power of 42.80 × 44.75 (1.95 D of net astigmatism), using 1.3283 as the net corneal index of refraction and 1.3375 for the keratometric standardized index of refraction. Unless the patient has posterior keratoconus, the only clinical condition (other than trauma) that can have a measurable effect, the difference in the astigmatism from not measuring the back surface is negligible.

$Figure 2. Holladay diagnostic summary map illustrating zonal refractive power.$
Figure 2. Holladay diagnostic summary map illustrating zonal refractive power. Notice the average Sim K reading is 43.75 D and the Eff RP is 43 D, 0.75 D steeper due to the limited sample at the 3 mm ring vs. the entire 3 mm zone. Also notice the steep and flat refractive power astigmatism is +0.88 @ 143, whereas the delta Sim K is +1.42 @ 97. In this case, the irregular astigmatism is secondary to over-wear of a rigid contact lens inducing warpage of the cornea.

Figure 3. Holladay report on Pentacam with table of equivalent keratometry reading from 1 mm to 7 mm. Note there is a steady increase in zonal power of 0.8 D (41.8 to 42.6) and 0.4 D increase in astigmatism (0.0 D to 0.4 D). The changes are quite variable from patient to patient.

Tomography measures the elevations of the front and back surface directly with no central scotoma, because the camera is located peripherally. However, similar to measuring the curvatures of a spectacle lens with a Geneva lens clock, a surface must be fit to the elevations and then the power calculated, which is not trivial if the surface is irregular. In normal corneas with only slight irregularities, both tomography and topography determined over a 3 mm to 4.5 mm zone are superior to keratometry. For irregular corneas, measurements with these devices are imperative. In most of these instruments, these area values are referred to as zonal power and zonal astigmatism, like that shown in the Table in Figure 3 on the Pentacam by Oculus.

If one finds a significant difference between the 3.0 mm zonal astigmatism (central zone) and keratometric astigmatism (four points or Sim Ks), then irregular astigmatism is present and one must carefully review the topography/tomography. In summary, the 3.0 mm to 4.5 mm zonal values from topography/tomography will yield better results for both the power and astigmatism of the cornea than the four values from any keratometer.

Toric calculators

Toric calculators have been available since the early ’90s, first used with the toric STAAR Plate IOLs in the U.S. Later the Alcon Toric Calculator became available for its toric IOLs. There are a number of standalone third-party generic calculators, similar to that in the Holladay IOL Consultant, that also perform these calculations.

The commercial calculators have implemented the effect of the incision on the corneal astigmatism determining the crossed cylinder solution of the original astigmatism and the astigmatism induced by the surgical incision. These considerations have continued to improve the results. One source of error, however, is the value that the surgeon enters for the magnitude and axis of the surgically induced astigmatism (SIA). Although the average magnitude of the SIA for a group of surgeons is usually between 0.2 D and 0.7 D for small incisions (2.2 mm to 3.0 mm), in an individual patient the actual magnitude and axis vary widely within this range. Figure 4 shows actual data for 76 cases of the SIA for an excellent surgeon. The average SIA was 0.72 D, but there is a significant variation for each patient. For this surgeon, the variation at 90° and 0° is much greater than in the oblique meridians (45° and 135°). These variations at different axes have been documented in many studies. The meridian of the incision is normally within 7° of the target, but this imprecision also adds to the variability. Surgeons must measure pre- and postoperative astigmatism with the same instrument, to determine SIA as a function the incision meridian (Figure 4) for optimal results.

Figure 4. Double angle plot of surgically induced astigmatism. Note the variation in magnitude and variability as a function of the axis.

All of the toric calculators with which I am familiar, except the Holladay IOL Consultant program and the AMO Express Calculator, use an approximation method rather than the exact solution. This involves using a constant for the ratio of the IOL toricity to corneal astigmatism. The commercial ratios used are from 1.41 to 1.48 and are correct for a 22 D spheroequivalent power IOL and an average effective lens position (ELP) of 5.50 mm (equivalent A-constant of 118.9). The power of the IOL and depth within the eye (effective lens position, ELP) are the two direct factors that influence this ratio. As shown in Table 1, the exact ratio is greater the lower the power of the IOL and the deeper the IOL in the eye (ratio is 1.745 for a 10 D SEQ IOL at 6.5 mm ELP). In contrast, for a 46 D SEQ IOL at 4.00 mm behind the corneal vertex, the ratio is 1.121. Table 2 shows the amount of toricity in diopters necessary for a 10 D, 22 D, 34 D and 46 D IOL (the normal range of IOL powers) to correct 2 D of corneal astigmatism for the range of ELPs from 4.0 mm to 6.5 mm. The maximum difference is 1.25 D (2.24 D to 3.49 D). This is obviously a substantial difference when one is trying to eliminate only 2 D of astigmatism.

The ELP in the two tables is not the lens constant for an IOL, but the actual position of the IOL in the patient’s eye. The lens constant is the average value of these ELPs from all the patients in the sample population for which the IOL was initially tested. Using an IOL with a lens constant of 5.50 mm, the actual ELP of this IOL in a specific patient may range from 4.0 mm in a nanophthalmic eye to 6.5 mm in a high myopic eye.

The only difference among modern IOL formulas today is the prediction of the specific ELP in a patient from the average value reported by the manufacturer that is used in the theoretical vergence formula. The actual vergence formula is more than 140 years old. Richard Binkhorst was the first to adjust the ELP for a specific patient for a given model IOL in 1981. He used the average ELP for a posterior chamber IOL in the sulcus (4.5 mm at that time) and scaled it up or down depending on the patient’s measured axial length compared with normal (23.5 mm). If the axial length were 10% longer than normal, he would use 4.95 (4.5 + 10% × 4.5) (one variable predictor). In 1988, we introduced the Holladay 1 with axial length and keratometry (two variable predictors) for predicting the ELP. Several subsequent two-variable predictors followed, such as the SRK/T and Hoffer Q. Tom Olsen in 1995 developed a four-variable predictor using axial length, keratometry, anatomic anterior chamber depth and lens thickness. In 1996, we introduced the Holladay 2, a seven variable predictor, using axial length, keratometry, anatomic anterior chamber depth, lens thickness, horizontal white-to-white (corneal diameter), age and refraction. Using any IOL formula (Haigis, Hoffer Q, SRK/T, Holladay 1, Olsen or Holladay 2) will yield slightly different spheroequivalent powers, more so the more unusual the eye, but the difference in the toricity needed to correct corneal astigmatism is negligible (< 0.20 D).

The approximation of using a constant ratio between the necessary IOL toricity and the corneal astigmatism, rather than the theoretical vergence formula, will cause errors. It is similar to trying to determine the necessary power of a secondary implant to correct a refractive surprise. The ratio of the refraction to the IOL power is about 1.5 for positive IOLs, if the cornea is 44 D, the IOL is at 5.50 mm, and the primary implant is about 22 D. Nevertheless, as most surgeons are aware, this is only an approximation and the exact calculation must use the refractive vergence formula to be precise. Simply be aware that the more the specific patient’s ocular parameters vary from normal, the more important it is to use the exact rather than the approximation solution. A quick check is to run the calculation for 10 D and 22 D IOL and see if the residual astigmatism changes. If it does not change, then the calculator is using the approximation method.

Part 2 of this article will cover toric optimizer, choosing the residual astigmatic target, proper IOL alignment and centration, and managing unfortunate outcomes of residual astigmatism and higher-order aberrations in the June 10 issue of Ocular Surgery News.

References:

Holladay JT. Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery. J Cat Ref Surg. 2010;36(1):182-183.

Holladay JT. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg. 1997;23(9):1356-1370.

Samuelson SW, Koch DD, Kuglen CC. Determination of maximal incision length for true small-incision surgery. Ophthalmic Surg. 1991;22(4):204-207.

Kohnen T, Koch DD. Methods to control astigmatism in cataract surgery. Curr Opin Ophthalmol. 1996;7(1):75-80.

Kohnen T, Mann PM, Husain SE, Abarca A, Koch DD. Corneal topographic changes and induced astigmatism resulting from superior and temporal scleral pocket incisions. Ophthalmic Surg Lasers. 1996;27(4):263-269.

Binkhorst RD. Intraocular Lens Calculation Manual. A Guide to the Author’s Tl 58/59 IOL Power Module. 2nd ed. New York: Richard D Binkhorst; 1981.

Binkhorst RD. The accuracy of ultrasonic measurement of the axial length of the eye. Ophthalmic Surg. 1981;12(5):363-365.

Holladay JT, Prager TC, Chandler TY, Musgrove KH, Lewis JW, Ruiz RS. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg. 1988;14(1):17-24.

Olsen T, Corydon L, Gimbel H. Intraocular lens power calculation with an improved anterior chamber depth prediction algorithm. J Cataract Refract Surg. 1995; 21(3):313-319.

Holladay JT, Gills JP, Leidlein J, Cherchio M. Achieving emmetropia in extremely short eyes with two piggyback posterior chamber intraocular lenses. Ophthalmology. 1996;103(7):1118-1123.

Holladay JT. Refractive power calculations for intraocular lenses in the phakic eye. Am J Ophthalmol. 1993;116(1):63-6.

Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay Consulting Inc., P.O. Box 717, Bellaire, TX 77402-0717; fax: 713-669-9153; email: holladay@docholladay.com; website: www.hicsoap.com.

Disclosure: Dr. Holladay is a consultant to AcuFocus, Allergan, AMO, Nidek, Oculus, WaveTec and Zeiss.