|Year : 2017 | Volume
| Issue : 3 | Page : 196-210
Update on optical biometry and intraocular lens power calculation
Nazneen Nazm1, Arup Chakrabarti2
1 Department of Ophthalmology, ESI-PGIMSR, ESI Medical College and Hospital, Kolkata, West Bengal, India
2 Chakrabarti Eye Care Centre, Thiruvananthapuram, Kerala, India
|Date of Web Publication||9-Mar-2018|
Cataract and Glaucoma Services, Chakrabarti Eye Care Centre, Kochulloor, Thiruvananthapuram, Kerala
Source of Support: None, Conflict of Interest: None
Intraocular lens (IOL) power calculation is the single most important determinant of functionally improved result of a technically precise cataract surgery. We have discussed recent advances in the field of optical biometry and IOL power calculation formulae as a means to achieve better postoperative visual outcome. The use of automated optical biometry device, the current 'gold standard' of IOL power calculation, dates back to 1999. We have highlighted the evolution of newer optical biometry devices and the technology they are based on, and their advantages and limitations. We have done technical comparison of contemporary biometers and have included contextual current review of literature. We have described newer generation IOL power formulae, IOL power calculation in high to extreme myopia, toric calculators and intraoperative aberrometry, and concluded our discussion with a note on future prospects of IOL power calculation.
Keywords: Barrett universal II formula, Hill-radial basis activation function formula, intraoperative wavefront aberrometry, optical biometry, Wang-Koch's nomogram
|How to cite this article:|
Nazm N, Chakrabarti A. Update on optical biometry and intraocular lens power calculation. TNOA J Ophthalmic Sci Res 2017;55:196-210
| Introduction|| |
Cataract surgery is the most common surgical procedure performed worldwide. The goal of cataract surgery is not just the removal of cataract, but to provide the patient sharp, clear vision without glasses. Despite the growing popularity of laser-assisted in situ keratomileusis (LASIK) and the growing interest in phakic intraocular lenses (IOLs) and other refractive procedures, cataract surgery provides a wider range of refractive error correction than any other surgical procedure, hence emerged the concept of “refractive cataract surgery.” For performing refractive cataract surgery, a cataract surgeon now has, in his armamentarium, a host of technological innovations such as femtosecond laser-assisted cataract surgery and the Zepto capsulotomy device to name a few. To match patient's expectations of crisp and spectacle-free vision, premium IOLs, namely multifocal IOLs, accommodating IOLs and toric IOLs are available. These technological advancements can help achieve better outcomes after cataract surgery. However, the improved outcomes are dependent on precise and accurate biometry. Newer biometry instruments that perform ocular measurements with micron precision and newer IOL calculation formulae to provide precise IOL power required for intraocular implantation, therefore, form the backbone of refractive cataract surgery.
| Optical Biometry|| |
To provide the best possible refractive outcome is the goal of the surgeon, whether the eye of the patient is normal or short or long or postrefractive surgery. Accurate measurement of all ocular parameters to obtain information about the complete geometry of the eye is required to arrive at the correct IOL prediction for each patient.
Optical biometry is a highly accurate noninvasive automated method for measuring the anatomical details of the eye. Accurate anatomical measurements are critical for precise IOL power calculation. For many years, the gold standard of axial length (AL) measurement was ultrasound (US) biometry.
The introduction of optical biometry in the late 1990s revolutionized the precision of IOL power calculation. In 1999, the first automated optical biometry device became available for clinical use – IOLMaster 500 (Carl Zeiss Meditec, Jena, Germany). Because of its ease of use, accuracy, and reproducibility, optical biometry is considered the current gold standard of IOL power calculation in clinical practice and is an indispensable tool for preoperative evaluation of cataract patients. The newer optical biometry devices provide several biometric measurements, namely AL, keratometry (K), anterior chamber depth (ACD), lens thickness (LT), central corneal thickness (CCT), pupil size (PS), and white-to-white distance (WTW).
The crucial advantages of optical biometry are:
- More accurate measurement of AL
- Optical biometry measures to the center of macula, that is, along the visual axis. It thus calculates refractive or optical AL unlike US biometry which measures along anatomic/geometric axis and thus calculates anatomic AL. The visual and geometric axes do not coincide. For IOL power calculation, it is the optical AL which is important and not the anatomic AL
- Optical biometry measures AL from corneal epithelium to the Bruch's membrane, rather than till internal limiting membrane (ILM) measured by US biometer. Since the US biometer measures only till ILM and since average thickness of retina is 200 μ (distance between ILM and photoreceptor layer), the earlier methods of IOL power calculation used to add 200 μ to the measured AL to make up for this difference. However, retinal thickness may vary from 160 to 400 μ. Optical biometer measures the true AL from the anterior corneal vertex to the photoreceptors in the back of retina, and therefore, no such addition/assumption for retinal thickness needs to be made ,
- It uses a partially coherent light source of shorter wavelength than sound wave. Use of shorter wavelength yields more precise AL measurement. The accuracy of AL measurement with US is approximately 0.10–0.12 mm compared to 0.012 mm for optical AL 
- It is a noncontact method. Corneal indentation does not occur. A rigid US biometry tip can cause corneal indentation between 0.1 and 0.3 mm, resulting in error from 0.3 to 1.0 diopters/diopters (D) in IOL power calculation.
- Accurate biometry can be performed in pseudophakic and aphakic eyes and eyes with phakic IOLs. Also AL, measurement is less affected by the type of IOL material ,,,
- More accurate measurements can be achieved in myopic eyes with staphyloma, in children and silicone-oil filled eyes (no need for velocity conversion equation),,,
The currently available optical biometers are based on one of the following technologies: (1) partial coherence interferometry (PCI) (2) optical low-coherence reflectometry (OLCR), or (3) swept-source optical coherence tomography (SS-OCT). [Table 1] lists important optical biometers and the technology they use.
| Iolmaster 500|| |
The IOL Master 500 (Carl Zeiss Meditec AG, Jena, Germany) [Figure 1] is an all-in-one biometer which measures AL, K, and other ocular parameters as well as performs IOL power calculations. It is based on the concept of PCI and operates as a modified Michelson Interferometer.,, PCI biometry was first developed by Austrian physicist Fercher and Roth  who performed the first in vivo AL measurement in 1986. The principle involves a dual beam of infrared (IR) light (780 nm) emitted by a semiconductor laser diode. A signal is produced as a result of interference between the light reflected from the tear film and that reflected by the retinal pigment epithelium. The photodetector receives the interference signal to calculate the optical distance (OD) between the corneal surface and retina. This OD is used to derive the other geometrical intraocular distances.
|Figure 1: IOLMaster 500 (Carl Zeiss Meditec) is the “Gold standard” optical biometer|
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The employment of optical AL instead of anatomic AL has significantly improved the refractive results of cataract surgery. The IOLMaster 500 has been shown to consistently measure AL accurately to within ±0.02 mm. This translates into a 5–10-fold precision in AL measurement. With >100 million power calculations performed worldwide, the IOLMaster 500 is the current gold standard biometer.,
One limitation of IOLMaster was its inability to measure AL reliably in the presence of opaque media such as corneal opacity and dense cataract., This, however, has been addressed to a large extent by a software upgrade (version 5, IOLMaster 500). This software allows averaging of consecutive optical scans, resulting in a composite scan and thus better ability to perform biometry through dense cataracts.
Vogel et al. studied the intraobserver and interobserver reliability and reproducibility of AL, ACD, and corneal radius measurements using the IOLMaster based on PCI. They found reliability of 99.9%, 97.8%, and 99.8%/99.5% for measurement of AL, ACD, and corneal radius (r1/r2), respectively (r1 = flattest radius of corneal curvature; r2 = steep radius; 90° apart from r1).
| Iolmaster 700|| |
IOLMASTER 700 [Figure 2] was the first optical biometer to incorporate SS-OCT technology.,,,,, Its advantages over the earlier devices are as follows:
- It provides full-length OCT image of the eye. The device performs 2000 scans/s. It can identify unusual ocular geometry (e.g., crystalline lens tilt/decentration)
- It is more accurate. Measurements can be verified visually resulting in fewer “refractive surprises”
- The OCT image provides a fixation check. The biometer's fixation check feature alerts the user to a suboptimal scan if the image captured does not show the foveal pit. The fixation check also helps identify macular pathologies such as macular holes and age-related macular degeneration, though the findings need to be verified with a dedicated retina OCT
- Unique telecentric K and distance-independent K: The unique software of IOLMaster 700 allows highly accurate distance-independent corneal surface measurements, independent of PS and even in restless patients
- Better cataract penetration rates: the IOLMaster 700 can perform biometry even through dense cataracts
- Software includes “Haigis Suite” (which includes Haigis, Haigis-T for torics, and Haigis-L for postrefractive surgery eyes) and other IOL power calculation formulae (SRK/T formula: Sanders-Retzlaff-Kraff formula; T for theoretical, Hoffer Q, Holladay 1 and 2, and Barrett Universal II)
- This device is especially suited for Toric IOLs. IOLMaster 700 contains inbuilt toric calculator (Barrett Toric calculator and Haigis-T for toric IOLs), and there is no need to use a separate online toric calculator.
Srivannaboon et al. evaluated the repeatability and reproducibility of IOLMaster 700 with the IOLMaster 500 in 100 eyes of 100 cataract patients. K, AL, ACD, WTW, and IOL power (calculated by the SRK/T and the Haigis formulas) were measured for each device. The repeatability and reproducibility of measurements from the two biometers were high for all parameters. However, the swept-source biometer had better lens penetration than the standard biometer.
Akman et al. also found excellent agreement between the two instruments. However, the IOLMaster 700 was more efficient in acquiring biometric measurements in eyes with posterior subcapsular or nuclear cataracts.
| Lenstar Ls 900|| |
LENSTAR LS 900 [Figure 3] uses the principle of OLCR. Apart from the parameters measured by IOLMaster, the Lenstar also measures LT. Use of LT, in conjunction with the latest state-of-the-art IOL calculation formulas (Barrett, Olsen, Holladay 2), translates into more accurate biometry. The latest version of Lenstar LS900 is equipped with the Hill-radial basis activation function (Hill-RBF), Barrett Universal II, Barrett True-K, and Barrett Toric calculator. Some of its other features are
- Automated positioning system allows for dynamic eye tracking of patient.
- Dual-zone K (at 1.65 and 2.3 mm) and T-cone topography (allows true Placido topography of the central cornea)
- Contains EyeSuite IOL which is a comprehensive set of premium IOL calculation formulae for cataract surgery patients and patients postkeratorefractive surgery. [Table 2] compares the technical specifications of IOLMaster 500 and Lenstar LS 900.
|Table 2: Comparison of technical specifications of IOLMaster 500 and Lenstar LS 900|
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| Eyestar 900|| |
The new device based on SS-OCT was launched in October 2017. The device contains EyeSuite software and provides elevation-based topography maps of both front and back of cornea and provides biometry data of the entire eye from cornea to retina. In addition, it provides two-dimensional (2D) and three-dimensional (3D) images of anterior segment as well as crystalline lens. Data acquisition process is smooth and fast, ensuring patient comfort. The device contains the latest IOL power calculation formulae such as Hill-RBF and Barrett Universal 2.
Various authors have reported excellent agreement between AL measurements by IOLMaster and the Lenstar.,,,, Epitropoulos compared AL acquisition and other parameters by IOLMaster 500 (version 7.1 software) with those from Lenstar LS 900 in 105 cataractous eyes of 63 patients. AL was acquired by the composite mean value of five measurements (composite-5 IM) and 20 measurements (composite-20 IM) of IOL Master 500 version 7.1 software and the standard mean of the first five measurements on standard-5 LS Lenstar LS900. He observed that the IOL Master 500 was more reliable and repeatable with better penetration than the Lenstar in measuring AL in patients with dense cataracts. In comparison, the Lenstar produced higher ACD values and flatter K though these differences were not clinically significant in terms of refractive outcome.
Kołodziejczyk et al. compared the biometric measurements and IOL power calculation obtained by Lenstar LS900 and IOLMaster 500 V.5 on 204 eyes of 106 patients. They concluded that Lenstar allows accurate and repetitive measurement and IOL power calculations comparable with those obtained using IOLMaster 500 V.5. However, the Lenstar, in addition, allowed pachymetry, macular thickness, LT, and pupil measurement [Figure 4].
|Figure 4: A Lenstar LS 900 EyeSuite A Scan printout (here retinal thickness or RT is automatically set at 200 μ).(1, signal strength scale; 2, front surface of cornea; 3, rear surface of cornea; 4, front of lens; 5, rear lens; 6, inner retinal limiting membrane; 7, retinal pigment epithelium)|
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Hoffer et al. compared AL, ACD, and K values obtained by the two instruments in 50 eyes with clear lens and 50 eyes with cataract and reported good correlation between these values in both the groups.
| Oa-2000 (Tomey)|| |
The OA-2000 by Tomey (GmbH, Nurnberg, Germany) was launched in 2014. It combines optical biometry, corneal topography, and K. Fourier-domain OCT provides high-speed tissue penetration and allows measurement of various parameters even through dense cataracts. In case of mature cataract, measurements can be performed with OA-4000 handheld US biometer, which communicates with OA2000 through Bluetooth. IOL power calculations are subsequently performed on OA2000. This biometer is based on Fourier-domain technology and Placido disc-based topography. The machine provides CCD, ACD, AL and K, and corneal topography simultaneously at 2, 2.5, and 3-mm diameter optical zone. The latter helps to create a topography map of cornea to help detect irregular astigmatism and also for comparison of pre- and post-surgery shape of cornea. It is also useful for analysis of eye after LASIK and other refractive procedures and for implantation of toric IOLs (to identify the axis of orientation of toric IOL).
| Argos Advanced Optical Biometer (Movu)|| |
The Argos [Figure 5] uses a 1060-nm and 20-nm bandwidth SS-OCT technology to collect 2D OCT data of the eye. The fast image reconstruction algorithm of the instrument is used to provide real-time 2D imaging of the eye. The 1050 nm light cause less scatter than shorter wavelengths leading to more photons being available to make measurements and hence better penetration through dense cataract. Equipped with Video K with IR light-emitting diode ring illumination, Argos measures AL, CCT, ACD, LT, PS, aqueous depth, WTW, K, and astigmatism. The biggest advantage of Argos is its ability to image through very dense cataracts through an “Enhanced Retinal Visualization” mode [Figure 6] that increases imaging sensitivity of the retinal area by 100 times (without increasing laser power).
|Figure 6: Argos biometric measurement though a dense cataract Axial length was successfully measured by Argos|
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The Argos uses a propriety swept laser source specifically designed for deep imaging (>50 mm) at fast 3000 lines/s A-line rate. The Argos also features an “Analysis mode” which allows the surgeons to verify the results obtained.
Shammas et al. reported good repeatability and reproducibility and comparability of measurements obtained by Argos biometer, IOLMaster 500, and Lenstar LS900. The study was performed on 107 eyes. AL was correctly measured in 96% of cases with the Argos compared with 79% for Lenstar and 77% for IOL Master 500.
| Aladdin|| |
Aladdin [Figure 7] combines OLCR biometry with anterior topography, Zernike corneal wavefront analysis, and pupillometry in one instrument. Following are its important advantages:
- It provides information about corneal asphericity [Figure 8] by mapping 24 Placido rings (on cornea) and analyzing 1024 data points using its real corneal radii technology. It provides extensive information on status of anterior surface of cornea including presence of corneal irregularities, common signs of keratoconus, and information about higher-order aberrations
Dynamic pupillometry allows better assessment of lens centration, constriction, and dilation of pupil in photopic and mesopic conditions to assist in premium IOL selection.
- Zernike wavefront analysis allows evaluation for higher-order aberrations and corneal surface anomalies like keratoconus
- It contains inbuilt toric calculators – Barrett IOL Suite and Abulafia–Koch regression formula
- The 850-nm superluminescent diode allows the Aladdin to penetrate even high-density cataracts
- It generates 3 types of reports
- IOL Power report
- Measurement report which gives an overview of measurements made of both eyes and alerts users to any unusual findings
- Aladdin report gives an overview of important pupillary and topographic features of both eyes which could influence the choice of premium IOL.
A multicenter clinical trial  was conducted in the United States and China to compare the Aladdin with the IOLMaster 500. The US group included a sample of consecutive patients scheduled for cataract surgery. The China group included a sample of healthy individuals with no cataract. In both the groups, AL values by the 2 instruments showed excellent correlation. However, there was a small but statistically significant difference in K and ACD measurements. The Aladdin gave a consistently higher value of ACD in both the groups. Mean K values were found to be flatter with OLCR device in both the US and China groups. Mandal et al. also reported a slightly flatter K value with OLCR device. This study concluded that though there is good correlation between the main biometric parameters of IOL power calculation, small differences in K and ACD exist and should not be overlooked.
|Figure 8: Corneal topography image by Aladdin optical biometer (Topcon. The Aladdin provides accurate corneal topography and is especially useful in selecting patients for toricphakic intraocular lenses|
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| Al-Scan (Nidek)|| |
This easy to use PCI-based biometer [Figure 9] uses an 830 nm IR laser diode for AL measurement. It has following features:
|Figure 9: A Nidek AL Scan (AL-Scan, Nidek Co., Aichi, Japan) biometry and phakic intraocular lens power printout. AL = axial length, ACD = anterior chamber depth, R1 = flattest radius of corneal curvature; R2 = steep radius; 90° apart from r1|
Click here to view
- It contains “3D autotracking” to track patient's eye movements along the X, Y, and Z planes. The “autoshot” feature allows device to capture the scan as soon as it senses correct alignment
- Topography and K with double mire rings help evaluate for aberrations. It measures K at 36 points
- It employs Scheimpflug imaging to measure CCT and ACD (a Scheimpflug system images the anterior segment with a camera perpendicular to a slit beam, thus creating an optical section of cornea and lens). It also provides data about pupil position
- It can perform measurements even through dense cataracts and also contains an inbuilt US biometer.
Kaswin et al. compared the performance of AL-Scan with IOLMaster 500 in 50 eyes and reported excellent correlation in the AL and K obtained by the 2 devices when the AL was in the range of 22–27 mm. Li et al. compared the ocular measurements obtained by AL-Scan with Lenstar in 92 eyes of 92 cataract patients and observed good agreement for AL, CCT, and ACD measurements. Although slight difference in WTW values was noted, they were still in reasonably good agreement. However, PS values were consistently different and showed the worst agreement.
| Galilei G6 Lens Professional|| |
The Galilei G6 [Figure 10] combines OLCR optical biometry, dual-Scheimpflug imaging, and Placido-disc topography. Some of its features are as follows:
|Figure 10: The Galilei G6optical biometer (Galilei G6, Ziemer. Port, Switzerland)|
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- It provides high-definition pachymetry and 3D anterior chamber analysis
- It measures total corneal wavefront, curvature, and K data of anterior as well as posterior cornea, that is, provides complete data to plan cataract or refractive surgery
- Ray-traced posterior corneal surface data to detect bulging or asymmetry in late stages
- The combination of Scheimpflug imaging with optical biometry makes Galilei G6, especially suitable for IOL selection in postkeratorefractive surgery eyes and also (including keratoconus screening) of refractive surgery candidates. It is also helpful in devising corneal implants and in planning and follow-up of keratoplasty patients
- It includes newer IOL power formulas including Shammas No-History, Barrett Universal, and Barrett True-K Toric calculator.
The comparability of biometric measurements and IOL power calculation between IOLMaster 500 and Galilei G6 was studied by Ventura et al. They found similar results for K, AL, ACD, and IOL power values between the two devices.
Several studies have compared the PCI, OCLR, and SS-OCT biometry devices., Most studies have concluded good correlation between PCI and OCLR and SSOCT biometry values. [Table 3] compares the salient features of available optical biometers.
|Table 3: Currently available optical biometers, the parameters they measure and their salient features|
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| Intraocular Lens Power Formulae|| |
The first IOL power formula was published by Fyodorov and Kolonko in 1967 and was based on schematic eyes. Several IOL power formulae are available at present. Important ones are tabulated below [Table 4].
Recently, Koch et al. suggested a new classification of IOL power formula (see below) based on (a) method of calculating IOL power and (b) the data used for these calculations.
- Historical/refraction based
- Regression analysis based: SRK, SRK-II
- Vergence formulae (based on Gaussian optics)
- Two variable
- Holladay 1
- Hoffer Q
- Three variable
- Ladas Super Formula
- Five variabl
- Barrett Universal II
- Seven variable
- Holladay 2
- Artificial Intelligence based
- Clarke neural network
- Ray tracing
| The Newer Intraocular Lens Formulae|| |
As our understanding of the eye's anatomy has increased, there has been a corresponding increase in the complexity of IOL power calculation formulae. For many years, most of the formulae including the Holladay I, SRK/T, and Hoffer Q required only AL and K reading. Thomas Olsen then came up with a formula which required four parameters: AL, K, ACD, and LT. In 1992 Holladay II formula, which required seven variables for IOL power calculation, was released. Later Barrett suggested the Barrett Universal II formula which required AL, K, ACD, LT, and a few optional variables.
The popular newer generation formulae include the Holladay 2, Barrett Universal II, and the Hill-RBF. The common factor in all these formulae (except the Hill-RBF) is the need to predict effective lens position or ELP. ELP is defined as distance from the cornea to the principal plane of IOL. Both anatomical factors (K value, AL, Limbal WTW, Preop ACD, and LT) and IOL-related factors (shape, length, flexibility, anterior angulation if any, material of the haptic, and shape and material of optic) affect ELP. The parameters required for the calculation of important IOL power formulae are depicted in [Table 5].
|Table 5: The parameters required for the calculation of important intraocular lens power formulae|
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| Barrett Universal Ii (Barrett U2)|| |
The formula is called Universal because it is suitable for all types of eyes: short, medium, or long and also for different lens styles. This formula is based on a theoretical model of eye in which ACD is related to AL and K. In this formula, ELP is characterized by ACD and LF (lens factor). The LF is influenced by K, AL, ACD, LT, and WTW in that order. This formula also takes into account the negative value of LF in calculating ELP in the presence of negative-powered type of IOL. The Barrett U2 formula can be openly accessed on www.apacrs.org. Following are the features of Barrett Universal II formula:
- Accurate for all eyes regardless of AL
- Essential variables required for calculation are AL, K, optical ACD, and desired postoperative refraction. Optional variables required are LT and WTW
- Lens factor or “A constant” of the selected IOL is required. If not available, ULIB “A constant” of SRK/T formula is recommended (ULIB is the User Group for Laser Interference Biometry)
- AL and K data from optical biometer (for example, IOLMaster, Lenstar) is required for calculation. However, immersion biometry data may also be used. Since optically measured AL is different from the US measured AL, acoustic A constant will fail to give optimum results when used with optical biometry. Therefore, when AL obtained by US biometry is used in Barrett formula, an appropriate A-constant must be used (This requires pre- and post-operative clinical data and is done on a spreadsheet form in MS Excel format which can be downloaded from the ULIB website ocusoft.de/ulib/)
- Barrett U2 is able to predict for highly myopic eyes and negative powered IOLs without specialized constants or AL modification.
The refractive outcomes using Barrett U2 have been excellent.
| Holladay 2|| |
In 1993, Dr. Holladay led a worldwide study involving 34 cataract surgeons to determine which of the 7 variables were relevant as predictors of ELP. Surprisingly, horizontal WTW measurements emerged as the next most important variable after AL and K. It was also proved that there is almost no correlation between AL and size of anterior segment in 80%–90% of the eyes [Table 6]. This led to the concept of nine types of eyes – not just three (short, medium, or long).
|Table 6: Nine types of eyes. Intraocular lens calculations are more accurate if nine types of eyes are considered (not just short/medium/long eyes), with anterior segment size and axial length as independent variables|
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These results led to the formulation of Holladay 2 formula, an easy-to-use program, in which 7 variables (AL, K, ACD, LT, WTW, age of patient, and previous refraction) are inserted for calculation of ELP and appropriate IOL power.
This newer formula is a great choice for nearly every eye. It is a complete software package that not only allows IOL power calculation in many different types of eyes but also honing of individual results by personalizing the A-constant. This formula is available as part of Holladay IOL Consultant/Surgical Outcomes Assessment Program (HIC-SOAP, available at www.hicsoap.com). It is a paid software.
| Hill-Radial Basis Activation Function (Radial Basis Activation Function Online Calculator)|| |
The new Hill-RBF method [Figure 11] is an advanced, self-validating method for IOL power selection. It was launched in 2016. It is purely “data driven,” independent of ELP and has no data bias. RBF method uses artificial intelligence-driven pattern recognition and sophisticated data interpolation. RBF algorithms are used globally in a variety of technologies such as facial recognition software and thumbprint security scanners. A special feature is that it is the only IOL power calculation formula which provides the user the reliability of result, that is, the software can tell whether it is likely to be correct or whether it is unsure about the calculated IOL power. The older version of Hill RBF online calculator used data from 3400 eyes with a wide range of preoperative ocular parameters. The RBF calculator has been updated in 2017 and includes data from 12400 eyes. The data for normal eyes have been increased by about 7000 eyes. A total of 1000 exceptionally short eyes and axial myopia with IOL power up to -5D have now been included in the latest version. In addition, a target other than plano can be set (e.g., surgeon can aim for slight myopia and calculate the required IOL power accordingly).
|Figure 11: The new Hill-RBF method is an advanced, self-validating method for IOL power selection|
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The Hill-RBF is the product of the efforts of Dr. Warren Hill and his team which included engineers from MathWorks, and 39 investigators from over 17 countries. The Hill-RBF is incorporated in Lenstar Eye Suite and is also available to ophthalmologists globally as an open access web-based calculator (rbfcalculator.com/online). The uniqueness of Hill-RBF lies in the fact that greater the number of surgical outcomes that are fit into the model, greater the accuracy. In other words, the more the number of eyes added to database, the more accurate the calculator becomes.
However, the Hill-RBF has been optimized for biometry data from Lenstar LS900 optical biometer and for a particular IOL (Alcon SN60WF biconvex IOL). It may be used for other biconvex IOLs in the range of -5 to +30D. Biometry data from other sources or other IOL models may lead to suboptimal results.
More information about the calculator can be obtained from Dr. Hill at email@example.com and under “Online Tools” at ascrs.org.
| Intraocular Lens Power Calculation in High to Extreme Myopia|| |
High myopia is one of the most prevalent refractive conditions globally with a high risk of other associated eye conditions.,, Patients of axial myopia (AL >25 mm) are at risk of suboptimal refractive outcome after cataract surgery. The single most important consideration in this setting is to avoid unanticipated postoperative hyperopia. Several authors have reported that AL measured by the optical biometry is more precise than the US in an eye with posterior staphyloma., Second, the use of third-generation formulae may lead to incorrect IOL power calculation resulting in unsatisfied patient postoperatively.
In their landmark article, Wang et al. suggested that modification of AL is required to calculate IOL power with the SRK-T, Holladay 1 and 2, Haigis and Hoffer Q formulas in eyes with AL >25.2 mm. They looked at IOLs (IOL power that was required to be implanted) in 2 groups – power >5D and power 5D or less. In both the groups, it was found that adjusting AL significantly reduced the incidence of postoperative hyperopia. The idea behind the AL modification is that when the original AL is fed into the Wang-Koch's formula [Table 7], a value lower than the original AL value is calculated. When this lower AL value is reinserted into the formulae, an IOL power of higher dioptric value is obtained. This, in turn, eliminates the risk of postoperative hyperopia. However, the Wang-Koch modification may be less accurate in very low power/negative lenses.
|Table 7: Wang-Koch's formula for adjusting axial length in eyes ≥25.2 mm|
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Ghanem and El-Sayed found in their study that though SRK/T, Hoffer Q, Holladay-2, and Haigis work equally well in eyes with high myopia in low-plus powered IOL implantation, Haigis was better in cases requiring minus power IOL implantation. Their study was conducted on 127 eyes of 87 patients with cataract and AL >26 mm.
Newer formulae such as Barrett U2 have also been shown to be accurate in the setting of high myopia.,
| Toric Calculators|| |
The correction of corneal astigmatism with toric IOLs has become a standard of care to improve refractive outcome after cataract surgery. Two components contribute to optimum correction of astigmatism (1) accurate K and (2) reliable method to calculate power of toric IOL. Choosing the correct toric IOL for a patient is more challenging than choosing a spherical IOL. Toric calculators are used to select the desired toric IOL for a given patient. An ideal toric calculator has the following characteristics:
- It is comprehensive, that is, it can be used for preoperative planning, as well as for refractive surprises
- Software should preferably be a part of the biometric device
- It should be applicable to all IOL types
- There should be dynamic display of variables (e.g., dynamic adjustment of astigmatic effects of phaco incision)
- It should provide an alert for “axis-flip”
- It should take into account posterior corneal astigmatism (PCA).
Few generic toric calculators available online are “Assort Toric calculator” (www.assort.com) and “Holladay Toric calculator” (www.hicsoap.com). The Barrett Toric calculator contains Barrett Universal 2 formula to predict the required spherical equivalent IOL power. The formula takes into account lens position as well as PCA without actually measuring it. It derives the posterior corneal curvature based on a theoretical model. The Barrett Toric Calculator is available on the American Society of Cataract and Refractive Surgery (www.ascrs.org/barrett-toric-calculator) and Asia-Pacific Association of Cataract and Refractive Surgeons (www.apacrs.org) websites.
Several toric IOL calculators are available online such as Alcon online toric calculator (https://www.myalcon-toriccalc.com), Acrysof toric calculator (www.acrysoftoriccalculator.com), AMOeasy toric IOL calculator (https://www.amoeasy.com > calc), Care Group toric IOL calculator (www.caregroupindia.com > toric), and Appasamy Associates toric calculator (https://www.appasamy.com > toric). The new Alcon online toric calculator incorporates the Barrett Toric algorithm which takes into account PCA and calculates patient-specific ELP. The Barrett toric nomogram and the Baylor toric IOL nomogram have significantly reduced errors in residual astigmatism predictions in toric IOL calculations.
Abulafia et al. evaluated the accuracy of different methods to measure and predict postoperative astigmatism with toric IOL implantation. Preoperative corneal astigmatism was measured with 3 devices (IOLMaster 500, OLCR and Atlas More Details topographer) and compared with manifest astigmatic refractive outcome postoperatively. The Barrett toric calculator and the OLCR device was observed to achieve the most accurate results, 75% and 97.1% of eyes were within ±0.50D and ±0.75D of the predicted residual astigmatism, respectively.
| Fullmonte Intraocular Lens 2.0|| |
The FullMonte IOL software system is a new adaptive, optimizing process based on Markov Chain Monte Carlo process. It is not a formula, rather a computing process which combines modern theoretical formulas (SRK/T, Holladay I, Haigis etc.) with surgeon's own postoperative refractive record to provide not a single value of IOL power for emmetropia but expected refraction as a graph of probability distributions. The software continuously optimizes itself, adapting to several factors such as short eye/long eye, cases of unique anatomy, or postrefractive patients.
Several studies have recently compared the newer IOL power calculation formulae [Table 8]. To summarize, an analysis of the published literature in the past 50 years reveals that the Haigis, Hoffer Q, Holladay 2, and Barrett Universal 2 formulas are the best options for IOL power prediction in short eyes (<22 mm), whereas the Barrett Universal II, Haigis (with optimized constants), and Olsen formulas provide the most accurate outcomes in long eyes (>26 mm).
|Table 8: Comparison of newer intraocular lens power calculation formulae by different authors|
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| Intraoperative Wavefront Aberrometry|| |
One of the latest developments in the field of cataract surgery is intraoperative wavefront aberrometry. It can perform aphakic and pseudophakic refractive measurements in the operating room on the eye being operated, thus providing real-time intraoperative refractive information. This allows surgeon to confirm or revise the IOL power (calculated through preoperative biometry), optimize the lens location, and tailor arcuate corneal incisions to the eye's astigmatic requirements.,
| Optiwave Refractive Analysis System With Verifeye|| |
ORange wavefront aberrometry system (WaveTec Vision Systems, Inc.) was the first commercially available intraoperative wavefront aberrometer. It has now been replaced by the Optiwave Refractive Analysis (ORA) system [Figure 12]. ORA utilizes IR light and Talbot Moire interferometry, a system in which two gratings are set at a specific angle and distance to produce a fringe pattern as wavefronts are diffracted through the grates. This fringe pattern is then analyzed to provide information on sphere, cylinder, and axis to guide proper IOL selection (including premium IOLs) as well as placement. ORange is attached to a surgical microscope and aphakic, and pseudophakic refractions are performed in the operating room. It takes 40 measurements in less than a minute.
|Figure 12: The Optiwave Refractive Analysis system(ORA, Alcon, Fort Worth, TX)|
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It has a special role in toric IOL implantation. The device shows promise particularly in postrefractive surgery eyes and eyes at the extremes of AL spectra. By providing real-time data to surgeons during cataract surgery, the intraoperative aberrometer allows an unprecedented precision.
ORA has revolutionized premium cataract surgery practice, and some surgeons use it in all their toric, multifocal, and accommodative IOLs in addition to using it as a guide for intraoperative astigmatic keratotomy. Another situation where ORA is useful is in postrefractive surgery patients.
However, there are some drawbacks as well. It has a learning curve. Dr. Mahdavi conducted a survey which revealed that it took 20% of the 101 respondents >100 cases to feel comfortable with ORA. It also prolongs the surgical time by up to 5–6 min.
Woodcock et al. compared intraoperative aberrometry (ORA with VerifEye) with standard preoperative biometry in patients with bilateral cataracts undergoing toric IOL implantation. Intraoperative aberrometry measurement was performed in one eye and standard IOL power calculation with inked axis marking in the contralateral eye. The use of the former technology increased the proportion of eyes with postoperative refractive astigmatism of 0.50 D or less (89.2% vs. 76.6%) and reduced mean postoperative refractive astigmatism at 1 month.
| Holos|| |
Another intraoperative aberrometer is HolosIntraOp [Figure 13]a and [Figure 13]b by Clarity. It utilizes rapidly rotating microelectromechanical mirror and quad detector to measure magnitude of wavefront displacement. Like the ORA, Holos gathers optical wavefront and refraction data intraoperatively to verify the preplanned IOL power and helps choose the size and location of incisions to correct astigmatism. Up to 90 measurements second are taken per se cond. Like the ORA, it is attached to operating microscope for intraoperative refractive measurements.
|Figure 13: (a; see above, B; below): Holos intraoperative aberrometer by Clarity|
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| the Future of Intraocular Lens Power Calculation|| |
The Univers intraocular lens calculator
The UniversIOL was developed by Dr. Samir Sayegh et al. It is a web-based calculator which combines all the high-quality third and fourth generation formulae with a toric IOL calculator. It does not propose a new formula. The surgeon can use one or a combination of formula for IOL power calculation to achieve optimum results. The calculator also tells how much the formula differs from each other so that the surgeon has an idea how close s/he will be to the target. The calculator also contains all IOLs made so that an appropriate power as well as IOL can be chosen.
| Okulix|| |
Okulix is a newer IOL formula calculation program that is based on the principle of “ray-tracing.” It introduces the concept of “true geometrical position” of IOL and uses anterior and posterior central curvature radii, asphericity of IOL surfaces, central IOL thickness, and index of refraction to describe IOL position. It may find particular use in toric and phakic IOLs and IOL power calculation in postkeratorefractive surgery eyes.,
| Conclusion|| |
IOL power calculation in normal and complex eyes has evolved significantly in the last two decades. With ever-evolving lens designs and increasing patient expectations, performing the best possible measurements is the key for successful surgery with good refractive results and a satisfied happy patient. The latest biometry technologies armed with newer IOL power calculation formulae have become necessary tools for the refractive cataract surgeon. With current biometers and newer formulae expecting outcomes in the range of ±0.50D has become a reality in majority of the patients. However, attainment of target postoperative refraction is still not a reality in all cases (such as irregular corneas and postkeratorefractive surgery eyes) and further research is required in this direction.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]