Continuing Education Activity

Corneal astigmatism occurs due to the inequality of the corneal curvatures and needs to be addressed to achieve spectacle independence and optimal vision. Corneal astigmatism can be calculated by corneal topography, corneal tomography, keratometry, and intraoperative aberrometry. Toric intraocular lenses (IOLs) are considered the most predictive way of correcting corneal astigmatism in a patient undergoing cataract surgery, as they may correct astigmatism as low as 0.75 diopters. Monofocal and multifocal toric IOLs are available and are chosen based on the patient's expectations. Toric IOLs are unsuitable for patients with pre-existing posterior segment pathology, zonular instability, and irregular astigmatism. The most common concern is the postoperative misalignment of the IOL, which can be prevented by washing the viscoelastic from the anterior chamber and the capsular bag to avoid postoperative rotation. This activity reviews the indications of toric IOLs and highlights the role of ophthalmologists, optometrists, and paramedical staff in evaluating and treating corneal astigmatism.

Objectives:

• Assessing patients who are candidates for toric intraocular lenses.
• Implement the preoperative workup for toric intraocular lens implantation.
• Apply best practices when performing toric intraocular lens implantation.
• Identify the common postoperative complications post toric intraocular lens implantation and their management.

Introduction

Ocular astigmatism accounts for 13% of the total refractive errors encountered in patients.[1] The first treatment of astigmatism dates back to 1825, when George Airy used cylindrical lenses to correct his own refractive error.[2] Uncorrected ocular astigmatism results in a blurred image and significant deterioration of visual acuity.[3]

The cause of astigmatism is still not known; several etiologies, including genetics, the pressure of eyelids over the globe, tension due to extraocular muscles, and visual feedback mechanisms, have been attributed.[4] Total ocular astigmatism is considered to be a sum of corneal astigmatism which is contributed by the corneal surface changes, and internal or residual astigmatism, which is contributed by the irregularities of the crystalline lens. The anterior and posterior corneal curvatures contribute to the total corneal astigmatism.[5]

Initially, the estimation of corneal astigmatism was based on the measurement of anterior corneal curvature by keratometry and videokeratography. The calculation of the refractive power was based on empirical estimation of the posterior corneal surface.[6] With advanced imaging technologies like Scheimpflug imaging, Purkinje images-based technologies, and optical coherence tomography, the measurement of the posterior corneal surface has helped us aim for better refractive outcomes in cataract surgery. Posterior corneal astigmatism was evaluated to range from -0.26 to -0. 78 D.[7][8]

Modern-day cataract surgery has emerged as a refractive procedure that aims to eliminate spherical and cylindrical power and achieve spectacle independence.[9] The prevalence of corneal astigmatism >1.00D is 40%, greater than 1.50D was 20% in patients, and >2. 00 D was found in 8% of patients with corneal astigmatism undergoing cataract surgery.[10][11] 

The various methods of correcting corneal astigmatism during cataract surgery include toric intraocular lenses (IOL), the placement of the clear corneal phacoemulsification incision on the steeper corneal axis, paired opposite clear corneal incisions over the steeper meridian, and limbal relaxing incisions over the steeper meridians.[12]

Toric IOLs are considered the most predictable way of correcting corneal astigmatism.[13] Shimizu et al introduced toric IOLs 1992 as a rigid 3-piece polymethylmethacrylate lens.[14] The toric IOLs have undergone multiple improvements since then to address stability and postoperative alignment issues.

Indications

Toric IOLs are ideal for candidates with preexisting corneal astigmatism greater than 1 D. 

1. Age-related cataracts with corneal astigmatism: Toric IOLs are indicated in patients with preexisting corneal astigmatism of >1.00 D and a significant grade of cataract planned for cataract surgery.[15] Patients who desire spectacle-free near vision can be counseled for multifocal toric IOLs.
2. Corneal ectatic disorders: Patients with mild-to-moderate grades of keratoconus and other ectasias with regular astigmatism, which has been stable over time, are also suitable candidates. Off-label use of customized IOLs may also benefit patients with irregular astigmatism but should be prescribed only after assessing patient expectations.[16][17]
3. In patients who have undergone penetrating keratoplasty and who have high levels of irregular astigmatism and stable keratometry post-suture removal, customized or conventional toric IOLs can be implanted.[18]
4. They may also be used in patients with stable astigmatism due to corneal scars following healed corneal ulcers, pterygium excision, and corneal tear repairs.[19]

Contraindications

For patients with preexisting zonulopathy due to various congenital and acquired causes and zonular strength is a prerequisite for IOL stability.

Corneal scars and irregular astigmatism are relative contraindications for the placement of toric IOLs. They may not be relieved of astigmatism completely, but the magnitude of astigmatism may be reduced and may be taken up after adequate counseling depending on patient expectations.[20]

Poorly dilating pupils may hamper visualization for adequate alignment.

Patients with preexisting retinal pathologies and who have undergone vitreoretinal surgical procedures may not be suitable candidates as their visual outcomes may be compromised due to the retinal pathologies.[21] Patients with uveitic pathologies, glaucoma, and patients who have undergone glaucoma surgeries may not achieve good visual outcomes by their primary pathologies.

Intraoperative posterior capsular rent is a relative contraindication as it may lead to IOL decentration or tilt in the postoperative period.

Patients with unrealistic visual expectations.

Equipment

 This table summarises the types of equipment required to perform various steps to implant a toric IOL. 

 Toric IOL design

They are single-piece hydrophobic acrylic IOLs with dot or line axis markings on their posterior surface. These markings indicate the flatter axis and must be aligned with the pre-operative markings. They are more stable when compared with silicone IOLs.[35] 

With greater than 5-degree misalignment seen in only 6.9% of the cases. Silicone IOLs with C-shaped haptics had a rotation in an anticlockwise direction within two weeks of the postoperative period.[36][37] They can correct astigmatism from 1.00 D to 6.00 D, and IOLs are customized to correct higher astigmatism. Silicone plat haptic IOLs are also available. Toric IOLs can be monofocal, multifocal, extended depth of focus, and phakic toric IOLs.[38][39][40][41]

Personnel

Trained optometrists are required to perform pre-operative workup of the patient, including measurement of uncorrected and best-corrected visual acuity, refraction, and ocular biometry, including axial length and keratometry values. An anesthetic evaluation is required to assess the patient's fitness for the planned procedure.

The surgery is generally performed under local anesthesia, but anxious patients may require sedation to complete the procedure. Patients with systemic medical conditions like cardiac and neurological pathologies may require monitoring throughout the surgery.

An ophthalmologist usually performs standard phacoemulsification or a femtosecond laser-assisted cataract surgery (FLACS) to implant the intraocular lens. A skilled operating theatre assistant usually assists while performing the surgery. The ward nurses or the ophthalmologist can do postoperative care and counseling.

Preparation

Patient Selection

The patient should be thoroughly evaluated clinically to grade the cataract assess pupillary dilatation and tear film status, and rule out significant anterior and posterior segment disorders. Any dry eye or ocular surface disorders should be addressed adequately before cataract surgery. The patient is clearly explained about the surgical procedure and what to expect on the day of surgery. Informed written consent is obtained from the patient. 

Steps in Preparing For Toric IOL Implantation

1. Computing total corneal astigmatism
2. Spherical power calculation
3. Pre-planning of incision and IOL alignment
4. Phacoemulsification/ FLACS for implanting IOL
5. Post-surgical care

Computing Total Corneal Astigmatism

Preoperative estimation of anterior and posterior corneal astigmatism is essential to calculate the IOL power. It can be calculated using one of the abovementioned devices or the Baylor toric nomogram to estimate corneal astigmatism.[42]

Spherical Power Calculation

Intraocular lens power calculation should consider the posterior corneal curvature, effective lens position, and surgically induced astigmatism.[43] Axial length measurement is undertaken using ultrasonic or optical measurement systems. Keratometry to assess the corneal curvatures and astigmatism can be done using manual and automated keratometry. They may include slit scanning systems, optical coherence tomography-based systems, Scheimpflug imaging systems, Placido-based topographers, and an aberrometer. The various measurements are done on two different devices with different principles, and results cross verified for accuracy. Intraoperative wavefront aberrometry rapidly replaces all these calculators to determine the power and axis.

The Barrett toric calculator has better predictability since it considers both estimated lens position and posterior corneal power.[44][45]

Pre-Planning of Incision and IOL Alignment

Axis marking is essential to implant the toric IOLs in the desired axis and achieve optimal visual outcomes. It helps in planning the incision and in the intraoperative alignment of the IOL. The marking can be done manually or using advanced technologies like intra-operative marking systems and intraoperative aberrometry-based methods. Various online calculators and formulae are available for calculating the IOL power and axis of implantation. They include AcrySof online toric calculator, iTRACE calculator, TECNIS calculator, and the Hollday formula. 

Various commercial brands of toric IOLs are available, and a choice of a particular IOL depends on availability, financial considerations, and surgeon preference refraction. The IOL with the least amount of residual astigmatism is chosen. Overcorrection of corneal astigmatism leads to postoperative residual astigmatism acting in an axis perpendicular to the preoperatively measured axis, termed as flipping of the axis.[46] Preoperative reference marking of 0 and 180 degrees is done before the patient is shifted to the operating table using manual marking, slit lamp marking, bubble, or a pendulum marker.[31][47][48]

Technique or Treatment

Steps of IOL Implantation

1. Anaesthesia

The patient can be operated on under topical, sub-tenon or peribulbar anesthesia, depending on the patient's and the surgeon's comfort.  

2. Intra-operative axis alignment and toric axis marking 

Manual Marking

This 3-step technique consists of preoperative reference marking by free hand marking, slit lamp, or a Nuijt-Solomon bubble marker. This is done preoperatively under topical anesthesia. The ocular surface should be dry and excess drops or tear fluid should be removed. Primary gaze in a sitting position is preferred as a change in position from lying down to sitting can cause significant cyclotorsion. The second step is the intra-operative alignment of the horizontal axis using a Mendez gauge. The third step is the intraoperative marking of the desired axis, along which the IOL markings should be aligned. 

FLACS Assisted Marking

It aids in making two markings 180 degrees apart in the steep axis. The advantages included avoidance of parallax error found with manual markings and aiding in making astigmatically neutral arcuate incisions.[49]

Image-guided Marking

Various image-guided systems are available to overcome the disadvantages associated with the manual marking of the axis. These images capture high-resolution images of the iris architecture, iris, and limbal vessels, which serve as landmarks to plan the incisions intraoperatively. They help plan the location of clear corneal incisions, limbal relaxing incisions, and capsulorhexis. They may also help optimize the results based on the individual surgeon’s surgically induced astigmatism (SIA).

Placido disc-based topography with ray tracing aberrometer may provide power maps, corneal curvatures, and internal, corneal, and total higher-order aberrations. Considering the SIA, they have an inbuilt toric IOL power calculator that guides the incision placement.[50]

3. Wound Construction

Clear corneal incisions that do not leak and are self-sealing are placed in astigmatically neutral sites, and their placement may be guided by the systems mentioned above.

4. Capsulorhexis

Adequate-sized continuous curvilinear capsulorhexis covering 0.5 mm of the edges is essential to prevent postoperative rotation. A centered round rhexis is required for ideal IOL positioning. A large rhexis may lead to instability of IOL.

5. Removal of Lens Material

The lens material is removed by phacoemulsification, and cortex wash is done. Thorough polishing of the capsule is done to prevent posterior capsular opacification, which may cause IOL tilt and visual discomfort to the patient.

6. IOL Implantation

A cohesive viscoelastic is preferred over a dispersive viscoelastic to inflate the bag for IOL implantation, considering the ease of removal. The IOL is implanted into the bag with the axis markings left around 3 to 5 degrees anticlockwise to the final lens position. 

If the patient is operated on under topical anesthesia, they are asked to fix at the microscope light, and the IOL is perfectly centered based on the first Purkinje image. The viscoelastic is removed completely from the anterior chamber and behind the IOL in the bag, and the wound is hydrated before the final rotation of the IOL is made to align with the axis. Intraoperative aberrometry is advantageous in estimating the residual astigmatism of the eye and adjusting the IOL position intraoperatively.

7. Postoperative Care

The patient is asked to maintain a supine position an hour after surgery to avoid changes in the IOL position. The postoperative IOL alignment can be confirmed by refraction, dilated examination to look for the IOL axis, keratometry, and ray tracing aberrometer. Postoperative steroids are prescribed in tapering doses.

Complications

The complication unique to toric IOL implantation is IOL misalignment. IOL alignment is influenced by various factors, including marking technique, capsulorhexis size, coverage of IOL edge by rhexis margins, corneal incision sealing at the end of the surgery, rotational stability of IOL, and surgeon experience.  

IOL misalignment may occur secondary to either incorrect calculation of the IOL axis, incorrect placement of the IOL intraoperatively, or due to postoperative rotation. When the misalignment exceeds 30 degrees, it may induce a cylindrical power in a new meridian. One degree of misalignment leads to a 3% loss of effective power, and 30 degrees of misalignment results in the loss of the entire toric effect with significantly worse visual acuity.[51][52]

Postoperative IOL rotation may occur as early as one hour to up to ten days post-surgery. Incomplete removal of the viscoelastic from the bag results in early postoperative rotation, and late postoperative rotations may occur due to IOL design, rhexis extension, large bag size, axial length, and inadequate coverage of IOL by the rhexis margins. With-the-rule astigmatisms were associated with a higher rate of postoperative rotations.[53]

Realignment of the IOL has to be considered in cases with more than 10 degrees of malalignment. An intraoperative marking indicates the new target axis in relation to the current misaligned axis. A side port incision is made, and a long cannula is mounted on a syringe filled with the balanced salt solution is used to rotate the IOL to the desired position.[54] Corneal ablative procedures, piggyback IOLs, and IOL exchange procedures are required for higher degrees of astigmatism not amenable to rotation alone.

The complications inherent to routine phacoemulsification may be encountered while or after the surgery for implanting toric IOLs.

Intraoperative Complications

• Corneal wounds, burns, leaky corneal wounds
• Iris prolapse, iris chaffing by the phacoemulsification probe
• Smaller rhexis, large rhexis, rhexis margin run out
• Posterior capsular rent
• The nucleus or cortical matter drops into the vitreous
• Intraoperative zonular dialysis
• Suprachoroidal hemorrhage

Postoperative Complications

Early

• Corneal edema, striate keratopathy
• Raised intraocular pressure
• Ocular hypotony due to wound leak
• Toxic anterior segment syndrome
• Hyphaema
• Residual lens material
• Refractive surprise

Late

• Posterior capsular opacification
• Cystoid macular edema
• Endophthalmitis
• Retinal detachment
• Subluxation/ dislocation of intraocular lens
• Corneal decompensation/ bullous keratopathy
• Recurrent uveitis

Clinical Significance

Toric IOLs result in better patient satisfaction, and uncorrected visual acuity of 20/40 or better is achieved in most patients.[55][56][57] 

Appropriate pre-operative workups for patient selection, intraoperative IOL alignment, and incidence of intraoperative and postoperative complications determine the outcomes of post-toric IOL implantation. The facilitation of posterior corneal power measurement by newer investigations has made estimations of both anterior and posterior corneal curvature possible. Femtosecond laser-assisted cataract surgery, when combined with implantation of toric IOLs, may significantly reduce higher-order aberrations and superior visual outcomes.[58] 

Newer advanced toric IOLs have better stability limiting postoperative rotation and predictable visual outcomes. They may have an extended spectrum of indications like irregular astigmatism, higher corneal astigmatisms, post keratoplasty, and corneal ectatic cases.

Enhancing Healthcare Team Outcomes

A coordinated interprofessional team approach is essential for ensuring optimum outcomes with toric IOL implantation. The optometrist evaluates the best-corrected spectacle refraction and performs corneal topography and optical biometry required for IOL power and axis calculation.

They upload the patient's values in online toric IOL calculators and generate patient reports pre-operatively which helps in deciding the axis of implantation and the planning of incisions. The operating theatre personnel ensures the appropriate IOL and other instruments required for the pre-operative marking and performing phacoemulsification. An ophthalmologist/ optometrist or trained personnel can perform the pre-operative axis marking on the patient.

A trained ophthalmologist performs the surgery under peribulbar/sub-tenon/ topical anesthesia. An anesthetist may be required to monitor the patient intraoperatively in patients with systemic ailments and in anxious or uncooperative patients to provide intravenous sedation.

The postoperative care of the patient is taken care of by the ward nurses who counsel the patient regarding the dosage and the procedure of administration of topical medications and other postoperative instructions. The optometrist and ophthalmologist can do the postoperative follow-up of patients at routine intervals.