Preoperative assessment of the cataract patient and selection of the optimal IOL are data- and time-intensive processes. For our team at Cullen Eye Institute, essential tools for these processes include corneal topography, OCT and two contemporary biometers, combined with the best IOL calculation formulas. In this article, we will walk you through our preop assessment.
CORNEAL TOPOGRAPHY AND TOMOGRAPHY
Our two primary devices are the Galilei (Ziemer) and the Cassini Total Corneal Astigmatism suite (many other excellent devices, in combination, perform most or all of these task as well). The devices have six main benefits:
- Placido ring topography is used to visualize and help to rule out corneal surface pathology, such as dry eye surface changes, epithelial basement membrane dystrophy and Salzmann’s nodular degeneration (Figure 1).
- Tomography aids in ruling out corneal ectatic disease. Together with anterior topography, we can assess corneal regularity to determine if the patient is suitable for a diffractive IOL and if PRK or LASIK would be safe if the eye is ametropic postoperatively. In addition, if we detect keratoconus, we know that hyperopic errors are common after cataract surgery. Therefore, we increase IOL power as the mean central corneal power increases (Figure 2).
- The astigmatic topographic meridians of the topographers are compared with those of the two biometers (Lenstar [Haag-Streit] and IOLMaster 700 [Carl Zeiss Meditec]) to select the optimal alignment of astigmatic treatments — or, if discrepancies exist, to further investigate the ocular surface and to obtain repeat measurements.
- Some devices now provide data on posterior corneal astigmatism, which can aid in selecting toric IOLs.
- Topography is required to assure stabilization after rigid contact lens (CL) wear. We get an image on presentation at three weeks after discontinuing CL wear and at two- to four-week intervals until stable.
- Topographic and tomographic values over the central 2- to 4-mm zones are used to assist in IOL calculations in post-surgical corneas.
OPTICAL COHERENCE TOMOGRAPHY (OCT)
In our clinic, we perform macular OCT scans in all new patients older than age 50 and in all preoperative cataract patients. We are constantly amazed at the OCT findings that had eluded our clinical detection. In fact, in recent studies of routine OCT use in cataract patients, the incidence of clinically undetected macular pathology was as high as 13.2%.1
Accurate assessment of macular health is critical for IOL selection and preoperative patient counseling. A preoperative OCT also provides a valuable baseline to monitor patients who develop visual problems postoperatively.
All preoperative cataract patients in our practice are measured with the Lenstar and IOLMaster 700. The Lenstar uses optical low coherence reflectometry combined with 32 LEDs in two rings to measure anterior corneal curvatures. The IOLMaster 700 is one of the new swept-source OCT biometers (along with the Argos [Movu Inc.] and OA-2000 [Tomey]), and it uses 18 LEDs in three rings to measure corneal power. The IOLMaster 700 provides a full-length OCT image that shows anatomical details of a longitudinal cut through the entire eye. This enables detection of unusual eye geometries, such as tilt or decentration of the crystalline lens. We have also found that the IOLMaster 700 (and by report, the other swept-source biometers) can accurately measure axial length (AL) in many eyes with dense cataracts that previously required immersion ultrasound.2
Measurements with optical biometry are mostly operator independent, and, in the majority of cases, we see nearly identical values obtained from the two biometers. However, careful alignment during the scan and inspection of the measurement quality are still critical for optimal refractive outcomes. Hill et al have championed the concept of having a “preflight checklist” with validation guidelines for optical biometry.3
In our clinic, measurements that warrant additional investigation include: differences between eyes or devices of over 0.2 mm for AL, 0.5 D for corneal power, 0.5 D for corneal astigmatism and > 5 degrees for steep corneal meridian; abnormal LED mires; standard deviation of > 0.3 D for corneal power and 3.5 degrees for steep meridian (per Dr. Hill).3
CASE IN POINT
A recent case demonstrated the need to verify data with multiple measurements. During a preoperative visit, the patient’s Lenstar readings were approximately 1.0 D lower in mean corneal power and had 1.6 D more corneal astigmatism compared to a measurement obtained with the Galilei three months prior. The IOLMaster did not provide a reading for corneal power (Figure 3A-C). Inspection of the reflected LED images obtained from both devices revealed distorted mires inferotemporally (Figure 3D). Dry eye was found and treated. Repeated measurements with both devices one week later showed crisp LED mires as well as average keratometry and corneal astigmatism (less than 0.25 D) values that matched those previously obtained with the Galilei (Figure 3E-G). Following insertion of a non-toric extended depth-of-focus IOL, postoperative uncorrected vision was 20/20, J1 with a refraction of -0.25 D sphere. This case demonstrated the importance of careful inspection and validation of biometric parameters.
IOL POWER SELECTION
The available formulas
The majority of IOL calculation formulas are vergence-based; we use a wide range of them. Based on the number of variables they use to calculate the effective lens position (ELP), these formulas can be categorized into two-variable (Holladay 1, Hoffer Q, and SRK/T), three-variable (Haigis), five-variable (Barrett Universal II) and seven-variable vergence formulas (Holladay 2).4 Two commercially available formulas are based on ray tracing formulas (PhacoOptics [IOL Innovations ApS] and Okulix [Tomey]). The Hill radial basis function (Hill-RBF) is based on artificial intelligence.
While all formulas do quite well in eyes with virgin corneas and ALs between 22 mm and 26 mm, we reported that the Barrett, Hill RBF and Holladay 2 are most accurate.5 For these eyes, we routinely use three IOL formulas with the Lenstar — Holladay 1, Barrett and Hill-RBF — and three IOL formulas with the IOLMaster 700 — Holladay 1, Barrett and Holladay 2. When selecting the IOL power, we examine and compare the predicted refraction from these three formulas on each device. Because the Holladay 1 does not take into account anterior chamber depth in predicting postoperative ELP, we aim for slight hyperopia with this formula in eyes with ACD less than 2.9 mm.
Long and short eyes
Long and short eyes are more challenging. In long eyes, we use the Wang-Koch AL adjustment formula in conjunction with the Holladay 1 formula. The Barrett and Hill RBF are also excellent; we have experienced slightly more consistent results and certainly fewer hyperopic outcomes with the Wang-Koch adjustment.
The formula to adjust AL with the Holladay 1 is as follows: 0.817 x AL + 4.7013, combined with the IOL’s surgeon factor as listed on the ULIB IOL website (except for the Alcon MN/MA60MA IOLs, where we use the manufacturers constant).6 If targeting for distance vision, we reinsert the smaller value for AL into the Holladay 1 and select the first IOL power on the side with minus (-) prediction targets.
In short eyes, due to the high power of the IOL and the relatively short distance from the IOL to the retina, accurate prediction of ELP is critical — but elusive. In eyes with AL of < 22.0 mm, we reported that only around 70% were within 0.5 D of intended postoperative refraction.7 In general, our approach for short eyes is to use the Holladay 1, Holladay 2, Hill-RBF, Olsen and Barrett, favoring the Holladay 2 when there is disagreement.
IOL power calculation in eyes with previous corneal refractive surgery is still challenging. The ASCRS Post-Refractive IOL Calculator is our go-to tool. With the addition of newer IOL power calculation formulas (OCT-based IOL formula and the Barrett True K formula), the accuracy of IOL power calculation in eyes with previous LASIK/PRK has improved, although we still have found only 58.7% to 68.3% within 0.5 D of target refraction.8
IOL power calculation in eyes that have undergone RK is even more difficult — and frustrating. We find unacceptably low accuracy in post-RK eyes, with the percentage of eyes within 0.5 D of target refraction ranging from 30% to 62% for all formulas.9
For post-refractive eyes, we routinely obtain corneal maps using two devices (Atlas 9000 [Zeiss] and Galilei), anterior segment OCT scans (Avanti [Optovue]) and biometry measurements using two biometers (Lenstar and IOLMaster 700). We enter all data available into the ASCRS calculator. The average IOL power displayed in the results section on the calculator tends to perform better, but we pay special attention to the Barrett TrueK, OCT and Haigis-L formulas. With eyes for which the LASIK-induced change in refraction is known, we also like the Masket formula. We warn patients pre-operatively of IOL power calculation difficulties and of the potential for additional postoperative procedures for refractive enhancement, such as PRK, LASIK or, in very rare instances, IOL exchange.
Several approaches are available to guide the selection of toric IOLs. We rely most on the Baylor Toric IOL Nomogram, Abulafia-Koch Formula10 and the Barrett Toric Calculator. We have been encouraged by the progress that the Cassini and IOLMaster 700 have made in measuring posterior corneal astigmatism and look forward to further advances with these technologies. For all patients, we select the IOL that would leave the patient with a small amount of with-the-rule astigmatism, anticipating that there will be against-the-rule drift over time.
AN ABUNDANCE OF ADVANCES
We are so fortunate to work in a time when so many advances have occurred in this area. Further advances on the horizon include automation of data acquisition and analysis, which will reduce errors and improve consistency, and advances in measurement technology and sophistication of our formulas, which will improve accuracy. OM
- McKeague M, Sharma P, Ho AC. Evaluation of the macula prior to cataract surgery. Curr Opin Ophthalmol. 2018;29:4-8.
- Kurian M, Negalur N, Das S, et al. Biometry with a new swept-source optical coherence tomography biometer: Repeatability and agreement with an optical low-coherence reflectometry device. J Cataract Refract Surg. 2016;42:577-581
- Hill WE, Abulafia A, Wang L, Koch DD. Pursuing perfection in IOL calculations. II. Measurement foibles: Measurement errors, validation criteria, IOL constants, and lane length. J Cataract Refract Surg. 2017;43:869-870.
- Koch DD, Hill W, Abulafia A, Wang L. Pursuing perfection in intraocular lens calculations: I. Logical approach for classifying IOL calculation formulas. J Cataract Refract Surg. 2017;43:717-718.
- Gökce SE, Montes De Oca I, Cooke DL, Wang L, Koch DD, Al-Mohtaseb Z. Accuracy of 8 intraocular lens calculation formulas in relation to anterior chamber depth in patients with normal axial lengths. J Cataract Refract Surg. 2018;44:362-368.
- Wang L, Shirayama M, Ma XJ, Kohnen T, Koch DD. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm. J Cataract Refract Surg. 2011;37:2018-2027.
- Gökce SE, Zeiter JH, Weikert MP, Koch DD, Hill W, Wang L. Intraocular lens power calculations in short eyes using 7 formulas. J Cataract Refract Surg. 2017;43:892-897.
- Wang L, Tang M, Huang D, Weikert MP, Koch DD. Comparison of Newer Intraocular Lens Power Calculation Methods for Eyes after Corneal Refractive Surgery. Ophthalmology. 2015;122:2443-2449.
- Ma JX, Tang M, Wang L, Weikert MP, Huang D, Koch DD. Comparison of newer IOL power calculation methods for eyes with previous radial keratotomy. Invest Ophthalmol Vis Sci. 2016;57:OCT162-168.
- Abulafia A, Koch DD, Wang L, Hill WE, Assia EI, Franchina M, Barrett GD. New regression formula for toric intraocular lens calculations. J Cataract Refract Surg. 2016;42:663-671.