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 Table of Contents  
MAJOR REVIEW
Year : 2019  |  Volume : 31  |  Issue : 3  |  Page : 191-201

Intraocular lens power calculation in 2019: The cutting edge


1 Department of Cornea and Refractive Surgery, Giridhar Eye Institute, Kochi, Kerala, India
2 Chakrabarti Eye Care Centre, Cataract and Glaucoma Services, Trivandrum, Kerala, India

Date of Web Publication31-Dec-2019

Correspondence Address:
Dr. Prashob Mohan
Giridhar Eye Institute, Kochi, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/kjo.kjo_71_19

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  Abstract 


In today's world, with the developments in technology to measure ocular biometric parameters more accurately and the availability of sophisticated methods for intraocular lens (IOL) power calculation, cataract surgery has become no less than a refractive surgery. This along with free and easy access to information regarding the latest technology has heightened patient expectations. The demand for a spectacle-free life after cataract surgery is higher than ever. Newer advances in optical biometry, including swept-source optical coherence tomography technology, supplemented by the availability of highly accurate IOL power calculation formulae including ones that use artificial intelligence have the potential to enable surgeons to achieve near-perfect outcomes in majority of their patients. To hit the bull's eye as far as target refraction is concerned, it is necessary to understand the benefits and limitations of currently available cutting edge technology and formulae and apply them to the cataract surgery practice.

Keywords: Biometry, formula, intraocular lens power, optical biometry, swept-source optical coherence tomography


How to cite this article:
Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol 2019;31:191-201

How to cite this URL:
Mohan P, Chakrabarti A. Intraocular lens power calculation in 2019: The cutting edge. Kerala J Ophthalmol [serial online] 2019 [cited 2020 Jan 23];31:191-201. Available from: http://www.kjophthal.com/text.asp?2019/31/3/191/274594




  Introduction Top


Cataract surgery has come a long way since the first intraocular lens (IOL) was implanted by Sir Harold Ridley in 1949.[1] In today's world, it is no longer sufficient to give better vision after cataract surgery. Easy access to information regarding the latest technology has created a demand for near-perfect visual outcomes. One of the most important determinants of the refractive outcome of cataract surgery is IOL power. This necessitates highly accurate calculation of IOL power.

There are two components to accurate IOL power calculation – precise biometry and accurate IOL power calculation formulae.


  Biometry Top


For more than 50 years, ultrasound was the only commercially available tool to measure ocular axial length. This changed in 1998 with the introduction of optical biometry which used infrared light to determine the axial length.[2] Optical biometry has by and largely replaced ultrasound biometry as the technique of choice to measure axial length due to its obvious advantages [Table 1].
Table 1: Advantages of optical biometry

Click here to view


Disadvantages of optical biometry

  1. The cost of optical biometers is significantly more than ultrasound biometers [9]
  2. Dense cataracts and corneal opacities may preclude accurate axial length determination optical biometry.[10] Immersion ultrasound biometry is preferred in such cases, as it is more repeatable due to lack of variable corneal compression.[11],[12] Many of these difficulties have, however, been overcome with the advent of swept-source optical coherence tomography (SS-OCT) technology.[13]


Optical biometry

Optical biometers available today use one of the following technologies:

  1. Partial coherence interferometry (PCI)
  2. Optical low coherence reflectometry (OLCR)
  3. SS-OCT.


Partial coherence interferometry

Fercher and Roth developed this technology in 1986. PCI unlike ultrasound A-scan, uses infrared light. Tissue interfaces with differing refractive indices reflect this light. Interferometric techniques are then used to measure ocular distances.[14]

To minimize the effect of longitudinal eye movements, a dual coaxial beam interferometer is used. Two beams of light are introduced into the eye. These beams have a mutual time delay equal to twice the arm length difference of the interferometer. Tissue interfaces reflect this light, and a PCI signal is produced. The light reflected off the anterior corneal surface and the retinal pigment epithelium are considered for measurement of axial length.[14]

Examples of devices using this technology are the IOL Master 500 from Carl Zeiss, the AL-Scan from Nidek and Pentacam AXL from Oculus.

Optical low-coherence reflectometry

OLCR uses the principle of a Michealson interferometer. Low-coherence infrared light produced by a superluminescent diode is split into two beams by a coupler. One beam is introduced into the eye, and the other is directed toward a scanning reference mirror. Tissue interfaces reflect light. The interference pattern formed by the emitted and reflected light, which travels coaxially, is detected by a detector. The exact location from which the light was reflected from within the eye is determined by scanning the reference beam.[15]

OLCR is being used in the following biometers – Lenstar LS900 (Haag-Streit) and Aladdin (Topcon) and Galilei G6 (Zeimer).

Swept-source optical coherence tomography

SS-OCT based machines do not use a superluminescent diode. Instead, they use a swift sweeping laser as the source. The reference mirror is kept static. The captured interference signal then undergoes Fourier transformation.[16],[17]

The IOL Master 700 (Carl Zeiss) and Argos (Movu) and the Eyestar 900 (Haag-Streit) are the various devices which make use of this principle.

Optical biometers IOL Master 500

The first optical biometer that was commercially available was the IOL master from Carl Zeiss [Figure 1].
Figure 1: IOL Master 500

Click here to view


The IOL Master 500 uses a 780 nm infrared laser as source. It functions on the principle of PCI. It can measure axial length to within 0.02 mm accuracy which is five times more than that of an immersion ultrasound biometer.[18]

IOL A-constants used in optical biometry are different from that used for ultrasound biometry. On the User Group for Laser Interference Biometry website, the optimized A-constants for most commonly used IOLs are available.

Keratometry (K), anterior chamber depth (ACD), and horizontal white-to-white (WTW) distance are also measured by the IOL Master 500 in addition to axial length (AL). A more accurate telecentric method is used to measure keratometry.[19] A projected slit is used to measure ACD.

The formulae available onboard the IOL Master 500 are SRK II, SRK-T, Haigis, Hoffer Q, Holladay-1, Haigis–L, and Holladay 2.

Data can be exported via Universal Serial Bus (USB). There is also a facility to export data via USB toa computer-assisted surgery system called the Callisto Eye, that can display an overlay in the microscope eyepiece during cataract surgery. The IOL Master 500 may be unable to measure axial length accurately in eyes with dense corneal or lenticular opacity. To a certain extent, the accuracy of scans in opaque media was improved by the introduction of a technique where the average of multiple scans was taken.[20]

AL-scan

Nidek's AL-Scan [Figure 2] is a PCI-based biometer. It can measure axial length, ACD, WTW, pupil size, and central corneal thickness (CCT). It uses Scheimpflug imaging to measure ACD and CCT. Rings reflected off anterior corneal surface are used to perform keratometry and corneal topography. The AL-Scan also measures corneal aberrations. The AL-Scan can also capture toric lens assist images for digitally marking the axis of toric IOL placement. The AL-Scan also comes with an ultrasound pachymeter and A-scan.[21]
Figure 2: AL-Scan

Click here to view


All popular IOL calculation formulae are incorporated into this biometer.

Pentacam-AXL

The Oculus Pentacam-AXL [Figure 3] is a combination of an elevation based tomographer and a PCI based optical biometer. A rotating Scheimpflug camera performs corneal tomography and PCI technology is used to determine axial length. It also measures CCT and WTW. The advantage that the Pentacam AXL has is the ability to measure posterior corneal astigmatism that gives it an edge as far as toric IOL planning is concerned. It is also particularly helpful in calculating IOL power in eyes post corneal refractive surgery. Wavefront analysis functions are also incorporated in the device.
Figure 3: Pentacam AXL

Click here to view


The device has the most popular IOL power calculation formulae on board.

Lenstar LS-900

The Lenstar LS-900 [Figure 4] uses OLCR technology to measure ocular distances. It was one of the first devices that could measure lens thickness (LT). A low-coherent beam of light of 820 nm wavelength produced from a superluminescent diode is used to measure axial length, ACD, and LT.
Figure 4: Lenstar LS 900

Click here to view


Thirty-two points placed close to each other are used to perform a dual-zone keratometry. The T-Cone is a reflection-based topographer that comes as an attachment to the Pro Version of the Lenstar LS-900. It can perform anterior corneal topography in a 6 mm zone and is particularly useful in toric IOL planning.

ACD is measured using OLCR as opposed to the slit beam used in IOL Master 500. Horizontal WTW and pupil diameter are the other parameters measured by the Lenstar LS-900.

All modern IOL calculation formulae including Holladay IOL Consultant Professional, the Barrett Suite, Hill-radial basis function (RBF), Masket, Modified Masket, and Shammas No-history are available on board the Lenstar LS-900. Through an additional software interface, Okulix and Olsen formulae are also available.

Aladdin HW3.0

The Alladdin HW 3.0 from Topcon [Figure 5] is a combination of a reflection-based topographer and an optical biometer.
Figure 5: Aladdin HW 3.0

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Keratometry can be performed at 3, 5, and 7 mm zones. 24 placido rings are used to perform anterior corneal topography. The device can also assess mesopicpupillometry and lens centration. Higher order aberrations can be evaluated using Zernicke wavefront analysis.

Axial length is calculated using OLCR technology.

Most popular spherical and toric IOL power calculation formulae are available on board the Alladdin HW 3.0.

AL, ACD, and keratometry measured with the IOL Master 500 and the Aladdin HW 3.0 have good agreement with each other.[22]

Galilei G6

Zeimer's Galilei G6 [Figure 6] is a dual-Scheimpflug camera and placido disk corneal topographer combined with an OLCR biometer. The device provides corneal tomography and three-dimensional anterior chamber analysis along with higher-order aberrations and total corneal astigmatism in addition to AL, CCT, ACD, LT, and WTW measurements. Because it measures posterior corneal astigmatism, it is particularly useful in calculating IOL power in eyes that have undergone corneal refractive surgery. The Galilei G6 also provides access to ray tracing IOL power calculation formulae such as Okulix and PhacoOptics.
Figure 6: Galilei G6

Click here to view


IOL Master 700

The IOL Master 700 from Carl Zeiss [Figure 7] is one of the latest optical biometers to be launched. The device is based on SS-OCT technology.
Figure 7: IOL Master 700

Click here to view


It does rapid scanning at a speed of 2000 scans per second. The device can image the various ocular structures using OCT. This helps in the detection of abnormalities such as lens tilt, posterior polar cataracts, and macular pathologies to some extent. The OCT image of the pit of the fovea allows the operator to ascertain that the fixation is central. The IOL Master 700 measures the curvature anterior corneal surface using telecentric keratometry. Further, it can also measure posterior corneal curvature with the help of SS-OCT technology in order to give a new parameter called total keratometry (TK). Measurement is also possible in eyes with dense cataracts and other media opacities, in which OLCR- or PCI-based optical biometry was not possible.[23],[24]

All the modern IOL power calculation formulae including the Barret suite consisting of the Barret Universal 2, the Barrett True K, and the Barrett Toric are available on board the IOL Master 700. In addition, two formulae, namely the Barrett TK Universal II and the Barrett TK Toric, which utilize TK are available.

The IOL Master 700 also captures a high-resolution image of the eye and can link with the Callisto eye system for markerless toric IOL implantation.

Argos

Argos from Movu [Figure 8] uses SS-OCT technology to perform ocular biometry. Keratometry, AL, CCT, ACD, WTW, LT, CCT, and pupil size can be measured using the Argos. SS-OCT technology enables measurement of AL through dense cataracts. IT is enabled with an analyze mode that enables operators to ascertain the accuracy of measurements.
Figure 8: Argos

Click here to view


Like the IOL Master 700, the argos is capable of making measurements through dense cataracts. The analyze mode allows technicians to check the plausibility of measurements. All popular IOL power calculation formulae are incorporated in the device.

OA-2000

Tomey's OA 2000 [Figure 9] combines a placido disc-based topographer with an SS-OCT based biometer. It measure keratometry, CCT, ACD, AL, LT, WTW, pupillometry, and corneal topography.
Figure 9: OA-2000

Click here to view


All modern IOL power calculation formulae including ones based on ray tracing are available on board.

Eyestar 900

Another SS-OCT based device currently in the final stage of development is the Haag-Streit Eyestar 900 [Figure 10]. It is not commercially available yet. Haag-Streit claims that it can claims to provide biometric analysis of the entire eye from the cornea to the retina, including elevation-based corneal topography.
Figure 10: Eyestar 900

Click here to view


Intraoperative aberrometry

Wavefront aberrometry can be performed during cataract surgery using certain instruments. These intraoperative aberrometers can make aphakic and pseudophakic refractive measurements. These can be attached to the surgical microscope and can provide real-time information on axis of placement of a toric IOL, position of limbal relaxing incisions, etc., via the oculars of the microscope. Intraoperative aberrometers are particularly beneficial in is toric, multifocal, and accommodative IOL implantation and also in cataract surgery in eyes postcorneal refractive surgery.[25],[26] The two commercially available intraoperative aberrometers are: the Optiwave Refractive Analysis (ORA) and the Holos IntraOp.

Intraocular lens power calculation formulae

There are various cutting edge tools available today that can accurately measure ocular biometric parameters. However, all this would amount to nothing in the absence of an IOL calculation formula that can accurately predict postoperative refractive outcome. IOL power calculation formulae have also evolved with the times. A number of formulae which were relevant two decades ago are no more recommended for routine IOL power calculation.

Classification

A schema for classifying the various IOL power calculation formulae is given in [Table 2].[27]
Table 2: Classification of intraocular lens power calculation formulae

Click here to view


Historical/refraction-based formulae

In this method, a fixed power is added to the patient's refraction. It has become obsolete.

Regression formulae

Statistical data acquired previously are solely relied upon to calculate IOL power. These formulae do not take into account the optics of the eye.

Vergence formulae

Vergence formulae arrive at an IOL power using Gaussian optics. Gaussian optics makes an assumption that image vergence = object vergence + lens vergence. Most modern-day IOL power calculation formulae are based on the following equation formulated by Fyodorov.

IOL power = (1336/[AL-ELP]) – (1336/[1336/{1000/([1000/DPostRx] – V) + K} – ELP])

where K is Net corneal power, AL is Axial length, IOL power is IOL power, is Effective lens position (ELP), DPostRx is Desired refraction, and V is Vertex distance.

All the variables other than ELP can be measured directly. However, since ELP cannot be measured directly, some amount of regression is also used in these formulae to estimate ELP. ELP may be estimated using a number of variables. Vergence formulae may be subclassified into 2-variable, 3-variable, 5-variable, and 7-variable formulae depending on the number of variables used to calculate ELP.

Artificial intelligence-based formulae

These formulae do use a form of regression, but they rely on huge databases and employ artificial intelligence-based complex statistical models to arrive identify relationships between variables.

Ray tracing

Formulae that are based on Gaussian optics make certain fallacious assumptions. For example, Gaussian optics assumes the presence of a single optical axis and that all the rays of interest make only very small angles to this axis. Third-generation vergence formulae also assume that all the components of the ocular optical system are thin lenses. They consider the cornea to be a single surface with a uniform refractive index, assuming that the radii of curvature of anterior and posterior cornea bear a fixed ratio to each other in all individuals, which may not be true always. The IOL is also assumed to be a thin lens. Hence, phenomena like spherical aberrations cannot be explained by this model.

Ray tracing is a strategy to calculate the path that a light ray will travel when passed through an optical system. To describe the IOL position in this method, the radii of curvature of both anterior and posterior corneal surfaces, their asphericity, central IOL thickness, and refractive index of refraction are all used. Gaussian optics formulae rely on postoperative refraction to back calculate ELP. This approach is less accurate due to errors in postoperative refraction and incorrect IOL power labeling. Unlike Gaussian optics formulae, ray tracing formulae do not describe a fictitious ELP. The final IOL position is a true geometrical position described by the distance between the apex of the posterior surface of cornea and the apex of the anterior surface of the IOL (postoperative ACD). One advantage that ray tracing formulae have over vergence formulae is that postoperative ACD can be measured directly using PCI and compared with preoperative estimated ACD.

Another advantage that ray tracing formulae have oververgence formulae is their ability to predict IOL power more accurately in abnormally short or long eyes and in eyes that have undergone corneal refractive surgery.[28]

The Olsen formula, Okulix, and PhacoOptics are examples of ray tracing methods to calculate IOL power.[27]

Some of the modern IOL power calculation formulae are discussed in detail below.

Ladas Super Formula

This method of IOL power calculation was jointly developed by Dr. John G. Ladas, Albert Jun, AazimSiddiqui, and Uday Devgan. The points of agreement and disagreement between Hoffer Q, Holladay 1, Holladay 1 with Wang-Koch adjustment, Haigis, and SRK-T formulae for various ranges of axial length and keratometry were identified. The ideal portions from four of the five formulae were used to create a super surface. This is a dynamic formula that is constantly being updated using inputs from surgeons worldwide.[29]

Barrett universal 2

Graham D Barrett developed the Barrett Universal 2. It is based on a theoretical model eye in which ACD is related to axial length and keratometry.

The formula can be used unmodified in eyes of all axial lengths and for all IOL designs. Hence the term universal.

The formula uses five variables – the AL, keratometry, ACD, LT, and horizontal WTW corneal diameter to calculate the ELP. In addition, the lens factor or A constant of the IOL and the desired postoperative refraction should also be entered in the calculator.

Better results are obtained if data from optical biometers are used in the formula.

The formula predicts IOL power accurately in eyes with extremely long axial lengths and even in eyes where negative power IOLs are used. No adjustments are necessary even if the implantation of a meniscus lens is planned.[30],[31],[32]

Holladay 2

Jack T Holladay designed the Holladay 2. It is a 7-variable vergence formula. Axial length, average keratometry, horizontal WTW distance, refraction ACD, LT, and age are the variables used in this formula. It has been determined that the WTW plays a significant role in determining ELP, next only to AL and keratometry. According to Jack T Holladay, eyes can be classified into nine types [Table 3].
Table 3: Holladay schema- 9 types of eyes

Click here to view


In 90% of eyes, there is no correlation between anterior segment size and axial length.[33]

The Holladay IOL Consultant and Surgical Outcomes Assessment Program is available online at www.hicsoap.com. Personalization of IOL A constants leads to a better outcome.

Hill-radial basis function calculator

Warren E Hill along with RBF Calculator Physician Team, Haag-Streit Switzerland, and Mathworks jointly developed the Hill-RBF. The Hill-RBF Calculator is, strictly speaking, not a formula. It is rather a sophisticated algorithm for IOL power calculation that is based on a complex mathematical model that consists of an artificial neural network that uses RBFs as activation functions. It has the property of adaptive learning-a method of pattern recognition and ability to perform tasks which are independent of previously known data. The Hill-RBF calculator can also organize and represent data on its own. It is also a dynamic formula that becomes more accurate in its predictions as more data is fed into it. It is also the only method of IOL power calculation which lets the surgeon about how accurate the predicted IOL power is likely to be by giving an inbounds or out of bounds statement.

The Hill-RBF Calculator has been optimized for the Haag-Streit Lenstar in combination with the Alcon SN60WF biconvex IOL for powers from +30.00 D to +6.00 D and the Alcon MA60MA meniscus design IOL for powers from +5.00 D to −5.00 D.

Kane formula

Developed in September 2017 by Dr. Jack X Kane, with data from close to 30,000 cases, the Kane formula is a theoretical formula, that uses a combination of regression and artificial inetlligence to further refine its results. The power of cloud computing was harnessed in the development of this formula. AL, keratometry, CCT, ACD, LT, and patient gender are the variables used to calculate IOL power. A few clinical studies have shown that the formula is highly accurate.[34],[35],[36]

Olsen formula

Developed by Dr Thomas Olsen, the Olsen formula uses both paraxial and ray tracing optics. A regression method is used to predict the postoperative ACD. ACD and LT are used to determine the ELP. IOL position is described as a fraction of the capsular bag size using a new constant called the C constant. ACD, AL, LT, and keratometry are the variables that need to be entered in this formula.[37]

Okulix

Okulix is a ray tracing formula. Since all optical components of the eye are not considered thin lenses, the radii of curvature of anterior and posterior corneal surfaces need to measured using a corneal topographer. The IOL is also described by the radii of curvature of its anterior and posterior surfaces, thickness, and refractive index.

PhacoOptics

PhacoOptics is also a software that relies on ray tracing to calculate IOL power. It also uses the concept of the Olsen's C constant.

Wang–Koch modification

IOL power calculation using traditional IOL formulae for abnormally long eyes many times leads to postoperative residual hyperopia. A modification to these formulae was suggested by Li Wang and Doug Koch for eyes with AL more than 25.2 mm, to avoid refractive surprise [Table 4].[38]
Table 4: Wang-Koch modification for intraocular lens formulae

Click here to view


FullMonte intraocular lens

FullMonte IOL is a software that uses a method of analysis called the Markov chain Monte Carlo process. A probability instead of a single emmetropic power is displayed at the end of this process. The surgeon can then choose the appropriate IOL power. The software evolves continuously optimizing itself as new data are received. Ultrasound biomicroscopy has enabled us to calculate sulcus-to-sulcus width and sulcus-to-sulcus perpendicular depth. These are variables which can be used by FullMonte IOL to predict IOL power.

UniversIOL calculator

Dr. Samir Sayegh designed the online calculator called the UniversIOL calculator. Instead of proposing a new IOL power calculation formula, the surgeon is provided with a choice of IOL power calculation formulae or combinations of formulae, a choice of computational methods, and a choice of IOL ranking criteria.

Toric IOL calculators

Some of the popular toric IOL calculators available are Assort toric calculator, Barrett toric calculator, Holladay IOL Consultant ToricPreOp Planner, Alcon online toric calculator, AMO easy toric IOL calculator, Care Group toric IOL calculator, and Appasamy Associates toric calculator. All of them are available online, and some of them like the Barrett Toric calculator are also available onboard popular biometers. A few of them have been discussed in detail.

Assort toric calculator

It is an online calculator that can be used to calculate spherical equivalent and toricity using the SRK/T, Holladay, Hoffer Q, or Haigis formulae. It uses a new parameter called Corneal topographic astigmatism (CorT Total), which takes into consideration posterior corneal astigmatism as well.

The calculator also suggests remedies in the form of degrees of IOL rotation/IOL exchange in the event of a postoperative refractive surprise.[39]

Barrett toric calculator

It is a toric calculator that is available both online and on board many popular optical biometers. It uses the Barrett Universal 2 to calculate spherical equivalent. Posterior corneal toricity is determined using a theoretical model. Vector calculations are utilized to arrive at final IOL toricity. It also displays the postoperative spherocylindrical refraction that can be expected after implanting IOL of a specific toricity. The calculator has been optimized for the Haag StreitLenstar optical biometer.[39]

Verion

The Verion [Figure 11] from Alcon laboratories is a tool for seamless IOL power calculation, surgical planning, and intraoperative guidance. It has three components: the Verion Reference Unit, the Verion Planner, and the Verion Digital Marker.
Figure 11: Verion

Click here to view


The Verion Reference Unit measures keratometry and pupil size and captures a high-resolution image of the eye. Anatomical landmarks such as limbus, scleral vessels, iris, and pupil features are detected and used for registration and intraoperative tracking.

Data from the Verion Reference Unit is transferred to the Verion Planner. The Verion Planner is aplatform for surgical planning. It incorporates all popular IOL power calculation formulae including Barrett Universal 2 and Barrett Toric Calculator. Axial length, however, has to be calculated using a biometer and fed in.

Data from the Verion Planner is fed into the Verion Digital Marker, it creates an intraoperative digital overlay that can be viewed by the surgeon in real time by displaying it through the oculars of the operating microscope. It displays site of incisions, axis of IOL alignment, and also a capsulorhexis guide. Using the previously described anatomical data, the Verion Digital Marker automatically corrects for static and dynamic cyclotorsion, thus precluding the need for preoperative marking.

Callisto eye

The Callisto eye [Figure 12] is a tool from Carl Zeiss that, like the Verion assists in markerless toric IOL implantation. It uses data and images captured by the IOL Master 500/700 for registration and displays an overlay through the oculars of the operating microscope that assists the surgeon in making incisions on the right axis, creating the right-sized capsulorrhexis, markerless alignment of the toric IOL, and also in making limbal relaxing incisions.
Figure 12: Callisto eye

Click here to view


Astigmatismfix.com

This is a website developed by John Berdahl and David Hardten. It features a toric results analyzer tool which in cases of a postoperative refractive surprise after toric IOL implantation, predicts whether IOL rotation will reduce postoperative astigmatism and if yes by how many degrees in which direction.

Intraocular lens power calculation in eyes postcorneal refractive surgery

The standard methods of IOL power calculation are inadequate in eyes that have undergone corneal refractive surgery. This is because of many factors. Corneal laser refractive surgery alters the anterior corneal curvature alone. Standard keratometers assume a fixed ratio between anterior and posterior corneal radii of curvature. Since this is altered after corneal refractive surgery, it induces error into measurement of corneal power and hence accurate IOL power calculation. This problem is compounded by the fact that modern IOL power calculation formulae also use corneal power to predict ELP. Radial keratotomy on the other hand flattens both anterior and posterior corneal surfaces. However, the effective optic zone diameter is much smaller than that of standard keratometry. Hence, standard keratometers tend to overestimate corneal power. Furthermore, corneal asphericity is altered after corneal refractive surgery. Corneal asphericity also plays a role in calculation of IOL power.[40]

There are a number of methods that can be used for overcoming this difficulty. A few of them are discussed below.

Haigis-L formula

The standard Haigis formula is modified n the following manner to obtain the Haigis-L formula.

The corneal radius of curvature (r corr) is modified as follows:



where r meas is the measured corneal radius of curvature.[41]

Shammas formula

Shammas described a formula for correction of the keratometric values in post corneal refractive surgery eyes.

Calculated corneal power, K = 1.14 ×TK −6.8

where TK is the post-LASIK corneal topography central K.[42]

Masket method

Samuel Masket developed this regression formula. For eyes with AL >23 mm, the Holladay 1 formula is used to calculate IOL power and for eyes with AL <23 mm, the Hoffer Q formula is used to calculate IOL power. IOL power is adjusted in the following manner.

(LSE × −0.326) +0.101 = Post-LASIK IOL power adjustment

Where LSE is the laser corrected spherical equivalent adjusted for vertex distance.[41]

Modified Masket method

The Masket method was modified by Warren Hill as follows:

(LSE × −0.4385) +0.0295 = Post-LASIK IOL power adjustment.[41]

Aphakic refraction

This method was described by Mackool et al. One hour after cataract extraction, an aphakic refraction is performed. IOL power is calculated in the following manner.[43]

IOL power (D) = Aphakic refraction × 1.75

This method was found to be more accurate when anterior chamber IOLs were implanted.[41]

Ianchulev and Leccisotti described another formula using a similar approach.

IOL power (D) =0.07x 2 + 1.27x + 1.22, where x = aphakic refraction

The Mackool method was found to be more accurate when anterior chamber IOLs were implanted, whereas this approach was found to be more accurate when the IOL was placed posteriorly.[41],[43]

However, both these approaches are cumbersome.

Intraoperative aberrometry

The use of intraoperative aberrometers such as the ORA and Holos can lead to a more accurate IOL power prediction when compared to standard formulae in eyes postcorneal refractive surgery.[41]

Ray tracing

Methods such as Okulix and PhacoOptics which use ray tracing techniques can predict IOL power more accurately when compared to Gaussian optics formulae in eyes that have undergone corneal refractive surgery.[41]

Many web-based tools are available that aim at simplifying IOL power calculation in eyes postcorneal refractive surgery.

ASCRS postrefractive calculator

The ASCRS Postrefractive calculator (www.iolcalc.ascrs.org) enables the surgeon to make use of several available formulae including the Barrett True-K formula in order to calculate IOL power in eyes postcorneal refractive surgery. The IOL power calculated using the ASCRS calculator is reasonably accurate.[41]

Hoffer–Savini LASIK intraocular lens power tool

This is a spreadsheet that can be downloaded from the web address www.iolpowerclub.org/post-surgical-iol-calc. Once biometric data are entered into the spreadsheet, the tool provides modified biometric parameters. These may either be used in any of the popular IOL calculation formulae or the tool itself can give IOL power using various formulae that are available.[41]

Barrett true-K formula

This formula is provided by the Asia Pacific Association of Cataract and Refractive Surgeons (www.apacrs.org).[44]

The McCarthy post refractive intraocular lens calculator

McCarthy et al. studied the refractive outcomes of biometry performed on 173 eyes that previously underwent LASIK or PRK to design the McCarthy Post Refractive IOL Calculator (www.mccarthyeye.com/post-refractive-iol-calculator).[41],[45]

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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