Aberrometry in the calculation of intraocular lens power and Toric intraocular lens alignment devices

INTRODUCTION

Aberrometry plays an important role in modern cataract surgery by enabling detailed evaluation of optical aberrations within the eye. It aids in identifying suitable candidates for premium intraocular lenses (IOLs) such as multifocal and toric lenses, assessing corneal aberrations in post-refractive surgery patients, determining the optimal alignment of toric IOLs, and selecting appropriate aspheric IOLs to enhance visual quality. By accurately measuring and analyzing ocular aberrations, aberrometry provides essential data to optimize the visual outcomes of cataract surgery.

This review summarizes current evidence from Google Scholar and PubMed, translating complex concepts into a simplified understanding of aberrometry and its clinical applications. It highlights the principles and practical role of aberrometry in cataract surgery, discusses various devices used for wavefront analysis, and explores both manual and software-assisted marking techniques for precise toric IOL alignment in astigmatism correction. Recent advances in digital technologies and mobile applications for corneal axis marking are also examined.

ABERROMETRY FOR IOL POWER CALCULATIONS

Aberrometry is used to measure the aberrations or imperfections present in the optical system of the eye. The optical path length (OPL) represents the number of times light waves oscillate in reaching from one point to another point. The velocity of light waves changes when they travel through different ocular media. It reduces its velocity when it travels through a medium of high refractive index.^[1] Wavefront aberration refers to the deviation of a wavefront from an ideal wavefront in an optical system. It is a measure of the optical imperfections or distortions introduced by the system. The ideal wavefront represents a perfectly spherical or plane wavefront that would be produced by an ideal light source. It is typically quantified in terms of the root mean square (RMS) value of the deviation. The RMS value provides an overall measure of the average magnitude of the wavefront error across the optical system. The units of wavefront aberration are commonly expressed in microns (μm) since they represent a physical distance measurement.

It is calculated by comparing the OPL of a ray at a specific point in an optical system with the OPL of a reference ray passing through the centre of the pupil. The difference in the OPL between these rays gives the optical path difference (OPD), which represents the phase difference between them. The wavefront aberration is calculated by knowing how much the deviation of the wavefront arises from the defined optical system from the original wavefront from the ideal light source. The factors causing a difference in OPD are summarised in Table 1.^[2]

In 1934, Fritz Zernike introduced a set of polynomials that can be used to expand optical aberrations. These polynomials correspond to specific types of optical aberrations and facilitate the configuration of a wavefront map. The single or double-indexing methods are used to demonstrate the Zernike polynomials; the latter stands for the description of both the order (n) and frequency (m) of the wavefront error.^[3] This double indexing method permits the comparison of wavefront errors by featuring the similarities in aberration shape among those with the same frequency. Zernike polynomials have become a critical tool in the field of optics and have been widely used in various wavefront sensing devices and aberration correction applications.^[4]

The types of aberrations are^[5] lower and higher order. The lower-order aberration (n ≤ 2) can be corrected with the help of spectacles. These include piston, tilt, defocus and astigmatism. The higher-order aberrations (HOAs) (n ≥ 3) cannot be rectified with the use of contact lenses or spectacles. These include trefoil, coma, tetrafoil, secondary astigmatism and spherical aberrations, which are the primary focus of interest as they may impact the visual performance.

The primary goal of cataract surgery is no longer limited to providing good quantitative vision, i.e., defined as 20/20 on the Snellen visual acuity chart, but also good quality of vision. A combination of corneal and internal aberrations, produced by the implanted IOL, contributes to the visual function of the eye after cataract surgery. Pre-existing high aberrations of the cornea may significantly limit desirable visual outcomes of cataract surgery. The bigger the size of the incision, the higher the amount of aberrations induced, so the incision size should be taken into account while assessing post-operative aberrations. Phacoemulsification and particularly microincision cataract surgery have minimal effect on the induction of HOAs of the cornea. Several key determinants, such as biometry, effective lens position, surgery-induced astigmatism, angle kappa, centration of the IOL, pupillary diameter, IOL power and position of the lens, should be considered as they influence the outcome of surgery collectively. A good quality pre-operative aberrometry can be highly supportive in identifying suitable candidates for premium IOLs such as multifocal and toric IOLs, as well as in patients with a previous history of keratorefractive surgery or in patients with corneal pathologies such as keratoconus.^[6] The ocular aberrations can be easily gauged by newer diagnostic tools like an aberrometer, which can assist in identifying potential causes of visual disturbance and guide in the selection of an appropriate IOL.

Aberrometry assists in selecting patients for multifocal intraocular lenses (IOLs) by assessing corneal aberrations, as a highly aberrated cornea manifests reduced contrast sensitivity, often worsening post-multifocal IOL implantation. Severe dysphotopsia – visual disturbances such as glare and halos – has been reported after diffractive multifocal IOL implantation, especially in eyes with anterior corneal coma >0.32 μm. In addition, multifocal IOLs are associated with increased glare and spherical aberrations after refractive surgery, making them less suitable for such patients. A high-angle kappa also contributes to glare and photic phenomena. In post-keratorefractive patients, dissatisfaction despite good uncorrected visual acuity is frequently due to pre-operative HOAs, with symptoms exacerbated by large pupils or small ablation zones. Pre-operative aberrometry is crucial in evaluating corneal aberrations for these cases.^[7]

Related to this, positive dysphotopsia involves unwanted bright visual artefacts such as arcs, streaks, halos and starbursts, often caused by light scattering from the edges or the design of multifocal IOLs, particularly due to sharp-edged optics and concentric rings. These phenomena are especially noticeable in low light and are aggravated by ocular surface and corneal irregularities. In contrast, negative dysphotopsia manifests as a dark shadow or crescent in the peripheral vision, likely due to an illumination gap created by the IOL’s positioning or optic size and is influenced by factors like high-angle kappa. Both types of dysphotopsia contribute significantly to patient dissatisfaction post-multifocal IOL implantation, particularly when combined with pre-existing corneal aberrations and large pupil sizes. Neuroadaptation may reduce symptoms over time, but persistent cases require careful evaluation and sometimes surgical intervention.

This integrated understanding highlights the importance of meticulous pre-operative aberration assessment to minimise dysphotopsia and optimise visual outcomes with multifocal IOLs.^[7]

iTrace (Tracey Technologies, Houston, Tx), a ray tracing technique, precisely determines the alignment and axis of the implanted toric IOL within the capsular bag. It provides information on the magnitude and axis of the toric IOL compared to the ideal position. It also indicates the amount of IOL rotation required to align it correctly and the resulting residual astigmatism after realignment.^[8]

As the eye ages, the lens develops nuclear sclerosis, causing a shift from negative to positive spherical aberration. This deterioration negatively affects vision quality. The primary goal of implanting aspheric IOLs is to reduce or eliminate the positive spherical aberration of the cornea, thus restoring the sharp quality of vision.

Various wavefront analysing devices are available in the market. Shack–Hartmann aberrometry is a widely used technique for evaluating outgoing reflections from the eye. It delivers an infrared laser beam of 785 nm to the retina and assesses the light that is coming back from the retina. This method involves using numerous lenses, which allows for lenslets to focus the reflected wavefront. In the case of the ideal wavefront, a uniform spacing of dot patterns is perceived; however, in eyes with significant lower and HOAs, the pattern appears irregular. This principle is used by several commercially available devices, including the LADARWave (Alcon Surgical, Fort Worth, TX), Zywave Aberrometer/Wavefront Analyzer (Technolas GmbH of Munich, Germany), Bausch and Lomb Orbscan combined aberrometry/topography device (B&L Surgical, Claremont, CA) and WaveScan (VISX, Santa Clara, CA).

Tscherning and Ray tracing aberrometry are commonly used techniques for evaluating the retinal image. The retinal image is perceived using retinal aberrometry and evaluated by charge-coupled device sensors, which determine the deflection of the spots. It projects 256 parallel light rays on the retina, detects where they fall, and forms a retinal spot pattern. This machine assesses the overall aberrations of the eye by determining the retinal spot pattern. The itrace is an example of a machine that uses the ray-tracing principle.

Slit sciascopy is a type of double-pass aberrometry (Nidek OPD-Scan Gamagori, Japan). It uses the light-emitting diode of an infrared wave of 808 nm wavelength. The direction difference and speed of both the incident ray and the reflected ray are noted to construct the optical error. This later helps in constructing the wavefront map. The OPD-scan can also evaluate corneal topography, autorefraction and keratometry.^[9]

Intraoperative aberrometry is a technique that permits real-time assessment of ocular parameters intraoperatively, thus enhancing the accuracy of IOL power calculation and reducing the incidence of residual refractive error. This technique involves measuring the wavefront aberrations intraoperatively and assists in the selection of the appropriate IOL power and axis placement of toric IOL.

There are various intraoperative aberrometers (ORA) available in the market. ORange Intraoperative Wavefront Aberrometer (WaveTec Vision Systems, Inc., Aliso Viejo, California). This device is mounted on the operating microscope and is based on the technique of Talbot-Moiré interferometry, which scans moiré patterns produced by light passing through two gratings. The device uses infrared light and is optimised for the aphakic state (−5–+20 D) to give a complete assessment of the refractive power of the entire eye. This device increases patient satisfaction by improving the predictability of limbal relaxing incisions (LRIs), toric IOLs, and IOL powers to eliminate the need for enhancement.^[10] During cataract surgery, ORange can record a wavefront measurement at any point; the processor then scans the images and measures the refractive state of the operated eye. Preoperatively, the patient’s data is entered and stored in the computer. It can be retrieved during the surgery. The refraction of the entire eye, LRI calculation, toric IOL and IOL power calculation can be selected from the screen. The patient is asked to fix on an internal red light, and the coarse and fine alignment is manually performed so that the ORange is focused on the corneal apex and visual axis. ORange can capture and analyse 40 images in 20 s. The eyeball should be well-pressurised during the capturing of images.

In a study comparing the ORA with Barrett Universal II and Hill-Radial Basis Function (Hill-RBF) 2.0 IOL power calculation formulas, post-operative refraction was within 0.50 D of the target with no significant differences between the groups. The study also concluded that for patients with no previous refractive surgery and good potential visual acuity, the use of the ORA did not help in improving the refractive outcomes.^[11] Gasparian et al. studied the comparison between intraoperative aberrometry (IA) with the Barrett True-K formula done preoperatively for IOL power calculation in eyes with previous refractive surgery.^[12] The study concluded that IA significantly improved the predictability of refractive outcomes compared to the Barrett True-K formula, with a higher percentage of eyes falling within ±1.00 D of target refraction. The study suggests that IA can be an important device in fixing the accuracy of IOL power selection in eyes with post-refractive surgery.^[12]

Gouvea et al. conducted a study about the refractive accuracy of Barrett True K vs IA for the IOL power calculation in post-corneal refractive surgery eyes.^[13] They found that no statistically significant differences were observed between the mean absolute refractive prediction error of pre-operative formulae and IA for any of the four groups (post-hyperopic/post-myopic laser vision correction, post-refractive keratectomy and normal eye). The findings suggest that both the pre-operative formulae and intraoperative aberrometry in eyes having undergone refractive surgery give similar and optimal results.^[13]

Holos intraoperative aberrometry (Clarity Medical Systems) is another intraoperative wavefront aberrometer that gives real-time data display with highly precise wavefront refractive measurements. It uses a proprietary wavefront-analysis method, faster than the interferometry used by ORA, allowing it to evaluate the wavefront refraction more rapidly. Holos takes 90 measurements per second, with a displayed range of −20.00 D–+20.00 D and a dynamic range from −5.00 D to +16.00 D.

TORIC IOL ALIGNMENT DEVICES

Cataract surgery is the most performed procedure in ophthalmology, and its goal is to achieve spectacle independence for the patient. Several modalities are available for correcting astigmatism, including making corneal incisions in the steep meridian, LRIs, arcuate keratotomies and opposite clear corneal incisions,^[14] which can correct up to 1.5 D, but with fewer outcomes and risks of complications. The incidence of corneal astigmatism is <1.25 D in most patients undergoing cataract surgery.^[15]

Toric IOLs have been found to offer greater precision than cornea-based procedures for correcting total astigmatism, with the alignment to the steep axis of the cornea. The correct alignment of the toric IOL is the most critical step, as even malrotation of 1° can result in a loss of astigmatism correction by 3.3%, with zero astigmatism correction following a 30° off-axis from the target axis.^[16] Moreover, improper alignment can induce astigmatism in another meridian and can visually disturb the patient. The misalignment may occur due to intraoperative misalignment or post-operative rotation of the toric IOL. Several techniques are available to achieve precise IOL alignment.

Manual marking is the most common and cost-effective method for achieving precise IOL alignment, with an error margin of <3°. To ensure accuracy, all markings are made after the instillation of topical anaesthetic drops and with the patient in an upright position, as cyclotorsion of the eye from the upright to the supine position can range from approximately 2–4°, up to 15° in individual patients. It involves marking the horizontal meridian first, followed by aligning the marks with the help of a Mendez gauge, and finally using the axis marker to mark the desired axis of IOL implantation.^[17]

Slit lamp marking with a horizontal slit beam can be used to mark using a sterile ink pen after the application of topical anaesthetic drops. The horizontal and vertical marks are made inside the limbus, and proper head and chin alignment is maintained to prevent ocular torsion. A line of light is used to avoid centration errors. Some surgeons prefer to use a 26G needle to mark the reference axis [Figure 1a]. Thin-inked marks are recommended, and local peribulbar anaesthesia should be deferred before marking.

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Figure 1:

Toric alignment devices. (a) Patient undergoing pre-surgery preparation for toric intraocular lens (IOL) placement in the right eye. The slit lamp beam is adjusted horizontally, ensuring it spans from one edge of the limbus to the other. The patient is instructed to focus on a distant object. Following corneal anaesthesia, reference marks are made at the 0° and 180° positions using either a marker pen or a 26G needle. (b) A bubble marker may be used to mark the three and nine o’clock meridians of the cornea in the absence of a slit lamp. (c) The Patwardhan online toric marker app simplifies the process of marking the cornea before toric IOL placement. Surgeons utilise the app by aligning the digital marker with the patient’s eye, ensuring proper positioning of the 0° and 180° axis. By following the on-screen guidance, the surgeon can accurately mark the cornea using either a marker pen or a 26G needle, achieving precise alignment for successful toric IOL placement. (d) Callisto Eye (Carl Zeiss Meditec) projects a live overlay to guide the surgeon in aligning the toric IOL accurately with the intended axis. The three blue lines from the software and three dots on IOL match each other and that denotes the correct alignment of the IOL.

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The bubble marker marks the eye before the start of cataract surgery. The bubble should be in between the two vertical lines, and it ensures that the 0 and 180 marks are parallel to the ground [Figure 1b]. An ink marker pen with gentian violet is used to mark the wedges, and the bubble marker is slowly moved towards the operating eye while the examiner ensures that the bubble is between the vertical lines. Marking the eyes with the non-dominant hand might be difficult.

The pendulum marker (Rumex^® 3-193) is a device with contact wedges at 0°, 90° and 180° on a semicircle that are rotatably attached to a shaft within a handle of a pendulum, and it makes sure that these wedges are aligned horizontally even if the handle is slightly rotated. The patient is asked to look at a distant target, and the dominant hand is used to mark both eyes comfortably. However, it is important that all three wedges contact to avoid rotation and improper marking. Comparative studies have shown that both the bubble marker and pendulum marker are much better ways of marking the cornea than the earlier techniques of slit-lamp and free-hand marking, as they are more accurate and reproducible. The bubble marker and pendulum marker are very convenient since they both can be used to mark in the operating theatre.^[18]

Mobile apps such as the iToric Patwardhan (Android OS, MAQ 4.13) for axis marking have gained popularity in recent years. Using the app, the user first marks the dots on the corneal limbus to indicate the desired axis position. Once the dots are marked, the app allows the user to capture a picture of the cornea using the device camera [Figure 1c]. The app then employs image processing algorithms to assess the correct axis position based on the captured image.^[19]

Laser systems such as the Catalys precision system (Abbott Medical Optics, Inc., Santa Ana, CA) and LENSAR (Orlando, FL, USA) create stromal incisions along the steep keratometry for toric IOL alignment. The recently developed IntelliAxis, which is integrated with the LENSAR laser delivery system, can mark the steep axis at the capsular plane, 180° apart. Cao et al., in their study, found significantly lesser IOL misalignment with Intelliaxis compared to manual markings, though no significant difference was found in post-operative residual astigmatism between the two groups.^[20] An additional advantage of femtosecond laser-assisted capsular marks is the long preservation of capsule marks for later assessment of the toric IOL axis. Iris fingerprinting technique image-guided systems are markerless systems to eliminate potential sources of human error and subjective miscalculations.

The Callisto Eye with Z Align, developed by Carl Zeiss Meditec AG, is an advanced eye tracking technology that gives precise, real-time visualisation intraoperatively. This system provides overlays over the patient’s eye in real time, allowing the surgeon to align the toric lens to the target axis accurately. Once the necessary images are captured, they are transmitted to the Z Align module, and the surgeon can see three parallel lines representing the target meridian for lens alignment [Figure 1d]. This innovative technology provides surgeons with greater accuracy for toric IOL placement, resulting in better outcomes.

The Verion Digital Marker, developed by Alcon laboratories, has a keratometer to take corneal power measurements and capture high-resolution photographs of the scleral blood vessels, limbus and the iris. These images act as reference marks, and any change in position during the surgery can be tracked to find out the amount of cyclotorsion. The Verion system also provides an intraoperative overlay that guides the location of corneal incisions, the making of the capsulorhexis, and the positioning of the IOL. This advanced technology ensures precise and accurate alignment of the toric IOL, reducing the chances of post-operative rotation of the toric IOL and improving the overall visual outcome of the surgery.

The Osher toric alignment system superimposes a 360° protractor over a high-resolution image of the patient’s eye. This permits precise marking of the incision and toric IOL placement using a digital file or as a printout for use in the operating room.

The True Guide software, developed by TrueVision 3D Surgical, Inc., is a digital solution for intraoperative guidance and alignment of toric IOLs. This software utilises the simulated keratometry value of the anterior cornea, which is measured by an LED topographer. The True Guide enables precise alignment of the toric IOL with the intended axis, resulting in better post-operative refractive outcomes compared to manual marking techniques.^[21]

The iTrace system with Zaldivar Toric Caliper from Tracey Technologies provides five features, including autorefraction, keratometry, corneal topography, ray tracing, aberrometry and pupillometry. It provides a simulated E image, as well as a dysfunctional lens index to help quantify the amount of aberrations contributed by the lens and cornea to the total eye aberrations. It utilizes corneal topography data and displays it on the patient photograph, superimposed with a reticule on the cornea and limbus. The Zaldivar Toric Caliper tool is then used to calculate the difference in degrees between the intended toric IOL axis and the iris or limbal markings.^[22]

The iTrace image of the right eye of a patient who underwent phacoemulsification with multifocal toric IOL placement shows that the cylinder axis on the A scan [Figure 2a], Pentacam [Figure 2b], iTrace [Figure 2c] matches with each other and the Toric IOL enhancement display [Figure 2d] shows an aligned IOL with a residual of 0.7 × 5°.

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Figure 2:

iTrace image of the right eye of a patient who underwent phacoemulsification with multifocal toric intraocular lens (IOL) placement. (a) Optical biometry reveals a corneal cylinder of 1.8D × 93°. (b) This finding aligns with the cylinder observed on the Pentacam (1.7D × 93°). (c) The iTrace demonstrates a similar amount of astigmatism of 1.95D. (d) The display of the post-operative toric IOL enhancement check shows that the IOL is well aligned, with a 0.7D × 5° residual cylinder.

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CONCLUSION

Advantages of digital-guided systems in toric IOL surgery include an objective projection of meridians, improved workflow by removing the requirement of a slit lamp or ink marking instruments and more accurate visual outcomes. However, there are also several disadvantages associated with it. Intraoperative registration may fail in cases such as conjunctival chemosis, ballooning or bleeding, and it is difficult in uncooperative patients or abnormal orbital anatomy. Post-operative IOL rotation is not uncommon. Factors such as retained viscoelastic, IOL architecture and design, zonular weakness and large capsular bag in high myopes all potentially cause IOL rotation, affecting the stability. New innovative technological advancements address some of these limitations and improve visual outcomes for patients undergoing toric IOL surgery.