New devices, techniques and formulas for selecting IOLs in cataract surgery are continuously being developed. These utilize additional parameters and improved IOL power calculations to reduce refractive prediction errors¹ and help cataract surgeons deliver excellent vision for patients.

This review examines the technology underlying available biometers and topographers and summarizes IOL power calculation formulas.

BIOMETRY DEVICES

Options abound

Ocular biometry is the process of measuring the eye’s dimensions: the axial length (AL), the anterior chamber depth (ACD), white-to-white distance (WTW), pupil size (PS), lens thickness (LT), retinal thickness (RT) and central corneal thickness (CCT). These values help surgical planning and selection of IOL implants.

Keratometers analyze the corneal power of the eye (keratometry [K]) and magnitude of astigmatism, which is also crucial for IOL calculations.

PCI biometry

Partial coherence interferometry (PCI) biometers, exemplified by the IOLMaster 500 (Carl Zeiss Meditec), have a semiconductor laser diode that emits infrared light of two different wavelengths, produces short coherence, and then measures AL, ACD and WTW.² The IOLMaster 500 measures the anterior corneal curvature at six reference points at the 2.30-mm optical zone.² Advantages of this device include rapid acquisition of reliable and reproducible data in silicone-filled and staphylomatous eyes, elimination of direct contact to the patient’s eye (thus reducing the incidence of corneal complications and patient discomfort) and less user dependence.^3,4

Of note, applanation and immersion ultrasound can supplement PCI-based devices for patients with dense cataracts, posterior subcapsular cataracts, corneal opacities, macular disease, vitreous hemorrhages and poor fixation.^2,5

OLCR

Optical low coherence reflectometry (OLCR) utilizes super luminescent diode-emitting light waves with a wavelength of 820 nm to measure AL. Both the Lenstar LS 900 (Haag-Streit) and the OA-2000 (Tomey) feature this technology. These biometers also simultaneously measure CCT, ACD, WTW, K, PS, LT and RT.

The Lenstar uses light with a wavelength of 950 micrometers to measure K at 32 reference points distributed in concentric rings at 2.30-mm and 1.65-mm optical zones. It has an optional T-cone module that provides Placido-ring based topography of the central 6-mm corneal zone.²

The OA-2000 machine evaluates the 5.5-mm optical zone of the anterior cornea with nine Placido discs, each with 256 points of light. Multiple studies confirmed the reliability and reproducibility of the measurements of the OA.² In a study of 1,454 eyes in which 1,079 were analyzed, OLCR technology in conjunction with the Olsen IOL Power formula produced the most accurate IOL calculations compared to eight other formulas.⁶

Swept-source biometry

Swept-source OCT (SS-OCT) features a high-frequency tunable laser with a 1055-nm wavelength that captures cross-sectional B-scans of the whole eye at 30° intervals. Swept-source biometry measures WTW with a light-emitting diode (LED) and the anterior K with a telecentric approach, measuring K at six points in three optical zones with diameters of 1.50-mm, 2.40-mm and 3.20-mm aligned to the visual axis. It calculates total corneal power using the standardized keratometric index of 1.3375.⁵

Compared to other devices discussed, the parameters that the IOLMaster 700 (Zeiss) measures are the corneal diameter, lens geometry, lens tilt and 1.0-mm central retinal cross-sectional image. Thus, it uniquely provides information on fixation patterns, lens decentration and tomography of the eye — all key to ongoing research. For example, ray-tracing models show that lens tilt influences astigmatism.⁷ This device also measures the CCT, ACD, WTW, PD, AL and LT.

Multiple studies demonstrate SS-OCT’s equivalent or slightly improved reliability and reproducibility measuring AL, ACD and keratometry compared to PCI and OLCR and arguably increased precision.^2,5 SS-OCT also provides more accurate biometry under specific conditions, including dense posterior subcapsular lenses and opaque vitreous media, conditions more common in patients with diabetes.^2,5 Its accuracy, precision and additional parameters can improve refractive outcomes and provide avenues for further research.

Intraoperative wavefront aberrometry

Intraoperative wavefront aberrometry measures biometry in real time in the operating room post-cataract removal, theoretically improving the measurements’ accuracy.

The Optiwave Refractive Analysis (ORA, Alcon) uses a super luminescent LED and Talbot-Moire interferometer with a grating system to generate a fringe pattern and measure the refractive error of the central 4-mm optical zone. The device also measures posterior corneal astigmatism and higher-order aberrations — valuable data for premium IOL implantation. These features facilitate execution of corneal relaxing incisions with real-time corneal measurements. Thus, this technology is beneficial for patients with prior corneal refractive surgery and significant astigmatism.

TOPOGRAPHY AND TOMOGRAPHY DEVICES

Placido disc system

The Placido disc system projects concentric rings of light onto the surface of the cornea and measures the anterior corneal curvature in diopters.

The ATLAS 9000 (Carl Zeiss Meditec) utilizes 22 thin rings to generate both a simulated K and K values for three concentric zones: central, mid-peripheral and peripheral corneal zones, with each zone having a radius of 3.0 mm. Poor tear film quality and user-dependent centration can limit the device’s output. The Placido image reveals irregular astigmatism and corneal pathology, such as epithelial basement membrane dystrophy.

Point-source color-LED topography

Point-source color-LED topography generates K values from the central 3.0-mm zone using specular reflection of about 700 red, yellow and green LEDs.

The Cassini (i-Optics) utilizes this technology, the second Purkinje image and a video register to measure posterior corneal as well as total corneal power.

Scheimpflug-based technology

In contrast to data derived from light’s reflection, Scheimpflug-based technology directly images the anterior segment from the anterior corneal surface to the posterior lens capsule, thus including key data on the posterior corneal and total corneal astigmatism.⁸

The Pentacam (Oculus) uses a single 180° rotating Scheimpflug camera with a stationary slit blue LED with 470-nm wavelength to generate ocular biometry with tomography of both the anterior and posterior cornea.⁵

Combined technology

In addition to OLCR, the Galilei G6 (Ziemer Ophthalmic Systems AG) combines the two topographic technologies, utilizing a 20-ring Placido disc system and two rotating Scheimpflug cameras placed 180° apart. In normal eyes, Placido-based technology has tested comparably to Scheimpflug-based devices.^9-11 The Galilei G6 and Lenstar biometer measured values for the AL, LT and WTW with a statistically significant but not clinically significant difference.²

Similarly, OLCR devices generated comparable CCT and ACD but different K values compared to Scheimpflug-based technology.¹²

Anterior segment OCT

Finally, anterior segment OCT uses a 6-mm line scan along eight meridians centered on either the corneal apex or pupillary axis to measure the anterior and posterior corneal K independently, then calculates their summation for the central 3-mm optical zone. The RTVue 100 (Optovue) is a Fourier-Domain OCT; its reference mirror is fixed, the reflections are detected as a spectrum and Fourier transformations produce the A-scans.

Of note, OCT K and subsequent IOL calculations in eyes status post-refractive surgery were equal to or better than other devices.¹³

IOL POWER CALCULATIONS

A review of the formulas

Accurate IOL power calculations — which depend on the quality of the biometric data, the IOL formula and the IOL itself — are paramount to determine the optimal IOL power to achieve the desired refraction following cataract surgery.¹⁴

Historically, IOL formulas have been categorized by generation. Wang et al proposed an alternative methodology-based classification.¹⁴

Pure linear regression formulas, such as the SRK and SRK II formulas, produced calculations based on previous surgical data. More recent formulas, however, leverage the power of artificial intelligence and huge databases for IOL calculations. Examples include the Clarke neural network, which incorporates data from one surgeon with one type of IOL, and the Hill-Radial Basis Function (RBF) calculator, a self-validating method that uses pattern recognition and adaptively learns from large databases.

The Hill-RBF calculator is optimized for the Lenstar LS 900.² Vergence formulas are based on Gaussian optics and estimate the effective lens position. These formulas can be grouped by the number of used variables, ranging from two to seven. The Holladay 1, SRK/T and Hoffer Q formulas use two: AL and K. The Haigis formula adds ACD.

The Barrett Universal II includes the WTW and LT for a total of five variables; this was the most accurate of nine formulas using PCI-based biometry.⁶ The seven-variable Holladay 2 incorporates preoperative refraction and age.¹⁴

Ray-tracing formulas superimpose Snell’s law of refraction onto Gaussian Optics, incorporate new parameters and produce potentially more accurate calculations. Olsen’s Phacoptics formula utilizes ACD, LT, anterior and posterior K and conic coefficients to generate its IOL calculations.¹⁵ Of nine formulas, Olsen’s was the most accurate with OLCR measurements and least accurate using those from PCI, highlighting that specific conditions call for specific formulas.^6,15

The Ladas Super Formula is a situation-specific formula with a component of artificial intelligence. First, the calculator matches the formula for the given situation: The Holladay 1 with the Wang-Koch adjustment when AL is greater than 25.00 mm and the Holladay 1 in other eyes, for example.⁶ The formula then incorporates data from large databases to refine these calculations.

Extreme measures

Most formulas produce minimal refractive predictive error in normal eyes. However, at the extremes (short, long or variable ACD eyes), some formulas work better than others.

A recent study of short eyes (AL less than 22 mm) comparing the Barrett Universal II, Haigis, Hill-RBF, Hoffer Q, Holladay 1, Holladay 2 and Olsen formulas showed no statistically significant difference between them. The Hoffer Q and Holladay 2 formulas produced myopic refractive prediction errors, and the Olsen formula produced slightly hyperopic prediction errors.¹⁶ Another study evaluated the effect of varying ACD in patients with normal axial lengths using these same formulas. In eyes with a normal AL but an ACD 3.0 mm or less, the Barrett Universal II performed better than the Haigis, Hoffer Q and Olsen formulas; otherwise, there was no difference.¹⁷ Eyes with an AL over 26.5 mm have smaller errors using the Holladay 1 and SRK/T formulas with the Wang-Koch adjustments; the Barrett was best with PCI-derived data.^6,18,19

In eyes that have had LASIK or PRK, no formula consistently performed statistically better than others, making the ASCRS calculator, which provides many different formulas, so helpful.^20,21

CONCLUSION

Today, a diverse armamentarium of biometers and topographers utilize a variety of technologies to provide the surgeon with key data on the eye. Advancements in IOL power calculations continue to improve our refractive outcomes. OM

REFERENCES

1. Wang L, Koch DD, Hill W, Abulafia A. Pursuing perfection in intraocular lens calculations: III. Criteria for analyzing outcomes. J Cataract Refract Surg. 2017;43:999-1002.
2. Turczynowska M, Koźlik-Nowakowska K, Gaca-Wysocka M, Grzybowski A. Effective Ocular Biometry and Intraocular Lens Power Calculation. Eur Ophthalmic Rev. 2016;10:94-100.
3. Chia TMT, Nguyen MT, Jung HC. Comparison of optical biometry versus ultrasound biometry in cases with borderline signal-to-noise ratio. Clin Ophthalmol. 2018;12:1757-1762.
4. Canovas C, Alarcon A, Rosén R, et al. New algorithm for toric intraocular lens power calculation considering the posterior corneal astigmatism. J Cataract Refract Surg. 2018;44:168-174.
5. Guimarã Es De Souza R, Montes De Oca I, Esquenazi I, Al-Mohtaseb Z, Weikert MP. Updates in Biometry. Int Ophthalmol Clin. 2017;57:115-124.
6. Cooke DL, Cooke TL. Comparison of 9 intraocular lens power calculation formulas. J Cataract Refract Surg. 2016;42:1157-1164.
7. Weikert MP, Golla A, Wang L. Astigmatism induced by intraocular lens tilt evaluated via ray tracing. J Cataract Refract Surg. 2018;44:745-749.
8. Oliveira CM, Ribeiro C, Franco S. Corneal imaging with slit-scanning and Scheimpflug imaging techniques. Optom CE. January 2011;94:33-42.
9. Shirayama M, Wang L, Koch DD, Weikert MP. Comparison of accuracy of intraocular lens calculations using automated keratometry, a placido-based corneal topographer, and a combined placido-based and dual scheimpflug corneal topographer. Cornea. 2010;29:1136-1138.
10. Shirayama M, Wang L, Weikert MP, Koch DD. Comparison of corneal powers obtained from four different devices. Am J Ophthalmol. 2009;148:528-535.
11. Aramberri, Jaime, Illarramendi I, Oyanarte I, Araiz L, et al. Dual versus single Scheimpflug camera for anterior segment analysis: Precision and agreement. J Cataract Refract Surg. 2012;38:1934-1949.
12. Huang J, Pesudovs K, Wen D, et al. Comparison of anterior segment measurements with rotating Scheimpflug photography and partial coherence reflectometry. J Cataract Refract Surg. 2011;37:341-348.
13. Tang M, Wang L, Koch DD, Li Y, Huang D. Intraocular lens power calculation after previous myopic laser vision correction based on corneal power measured by Fourier-domain optical coherence tomography. J Cataract Refract Surg. 2012;38:589-594.
14. 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.
15. Melles RB, Holladay JT, Chang WJ. Accuracy of intraocular lens calculation formulas. Ophthalmology. 2018;125:169-178.
16. Gökce SE, Zeiter JH, Weikert MP, Koch DD, Hill W, Wang L. Intraocular lens power calculations in short eyes usindoi:10.1016/j.jcrs.2017.07.004
17. 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.
18. Popovic M, Schlenker MB, Campos-Möller X, Pereira A, Ahmed IIK. Wang-Koch formula for optimization of intraocular lens power calculation: Evaluation at a Canadian center. J Cataract Refract Surg. 2018;44:17-22.
19. Wang L, Koch DD. Modified axial length adjustment formulas in long eyes. J Cataract Refract Surg. 2018;44:1396-1397.
20. Hamill EB, Wang L, Chopra HK, Hill W, Koch DD. Intraocular lens power calculations in eyes with previous hyperopic laser in situ keratomileusis or photorefractive keratectomy. J Cataract Refract Surg. 2017;43:189-194.
21. Hill, Warren; Wang, Li; Koch DD. IOL Power Calculator in Post-Myopic LASIK/PRK Eyes. The ASCRS Foundation. http://iolcalc.ascrs.org/ . Accessed July 2, 2019.

About the Authors

Zaina Al-Mohtaseb, MD, is an assistant professor and associate residency director, Department of Ophthalmology, Baylor College of Medicine. She specializes in cornea, external disease and cataract and refractive surgery. Contact her at zaina1225@gmail.com.
Disclosures: Dr. Al-Mohtaseb is a consultant for Alcon, Allergan, Bausch + Lomb, Johnson & Johnson Vision and Carl Zeiss Meditec.

Kanwal Singh Matharu, MD, is a resident in the Department of Ophthalmology, Baylor College of Medicine. He is currently interested in a career in academic medicine. Contact him at matharu@bcm.edu.