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REVIEW ARTICLE
Ahead of print publication  

Toric intraocular lens: A literature review


1 Cataract and Glaucoma Services, Sankara Eye Hospital, Coimbatore, Tamil Nadu, India
2 Sankara Eye Hospital, Coimbatore, Tamil Nadu, India

Date of Submission08-May-2021
Date of Acceptance07-Oct-2021
Date of Web Publication10-Dec-2021

Correspondence Address:
Mithun Thulasidas,
Cataract and Glaucoma Services, Sankara Eye Hospital, Sathy Road, Sivanandapuram, Coimbatore - 641035, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/tjo.tjo_43_21

  Abstract 


Toric intraocular lenses (IOLs) are universally recommended in cataract cases with preoperative corneal astigmatism ≥1.5 D. An optimal surgical outcome depends on careful patient selection, complete preoperative evaluation, accurate IOL power calculation, precise marking of the axis, meticulous intraoperative approach, and methodical postoperative care. Understanding the importance of posterior corneal astigmatism, surgically induced astigmatism, and effective lens position in IOL power calculation and newer techniques to measure them directly have resulted in better postoperative refractive outcomes. We present a brief overview of toric IOLs along with the preoperative evaluation, IOL power calculation, different marking methods, intraoperative approach, and postoperative outcomes. Functional and anatomical outcomes, including uncorrected visual acuity, residual refractive astigmatism, and postoperative IOL misalignment, which have been reported for both toric IOLs and multifocal toric IOLs, are reviewed.

Keywords: Astigmatism, intraocular lens misalignment, multifocal toric intraocular lens, toric intraocular lens



How to cite this URL:
Thulasidas M, Kadam A. Toric intraocular lens: A literature review. Taiwan J Ophthalmol [Epub ahead of print] [cited 2022 Aug 16]. Available from: https://www.e-tjo.org/preprintarticle.asp?id=332205




  Introduction Top


The current generation of cataract surgery has become a refractive procedure, and astigmatism management became an essential part of cataract surgery. Studies have shown that a number of intraocular lenses (IOLs), especially multifocal or aspheric designs, are ineffective if residual astigmatism ≥1.0 diopter (D) is present after surgery.[1] Shimizu et al. first introduced toric IOLs in the early 1990s as three-piece nonfoldable polymethyl methacrylate implants, which can be inserted through a 5.7 mm incision.[2] Since then, toric IOLs are the most effective way of correcting astigmatism during cataract surgery due to their increased predictability and improved safety.[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40] About 33% of the cases undergoing cataract surgery are found to have preoperative corneal astigmatism of ≥1.0 D, with 22% having >1.5 D of astigmatism and 8% having >2.0 D of astigmatism.[9],[41],[42] Although the first models of toric IOL had high incidences of postoperative rotation, advances in the material and design of IOLs have resulted in improved rotational stability and precise visual outcomes.[12],[17],[19],[20],[27],[28],[30],[31],[32],[35]

This review provides a brief overview of toric IOLs along with the preoperative evaluation, IOL power calculation, different marking methods, intraoperative approach, postoperative outcomes, and possible complications. The literature search was performed in MEDLINE and Embase using “toric,” “toric intraocular lenses,” and “cataract surgery,” as keywords. The relevant references cited in those articles were also searched.


  Preoperative Evaluation Top


Getting to know the patient

It is essential to ascertain the visual needs and desires of the astigmatic patients and know their expectations and past ocular and medical history to explore the potential possibilities of their ocular future. The patient should undergo a complete comprehensive ocular examination to rule out any ocular comorbidities that may interfere with the postoperative outcomes. It is difficult to obtain accurate measurements of presenting astigmatism in patients with chronic dry eye disease, narrow palpebral fissures, or deep-set eyes. It is, therefore, vital to treat existing ocular surface disease and normalize the cornea before surgical evaluation. In addition to the general complications of cataract surgery, patients should be warned of a possible refractive surprise. IOL misalignment may occur, which may warrant a secondary procedure

Currently, toric IOLs are available in cylinder powers of 1.0 D to 12.0 D (0.67 D to 8.0 D at the corneal plane) and are intended to correct preexisting regular corneal astigmatism ranging from 0.75 D to 8.0 D.[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[35],[36] Extended series and customized toric IOLs to correct higher astigmatism are also available.[3],[27],[36],[43] Toric IOLs are universally recommended in cases with significant preoperative anterior corneal astigmatism ≥1.5 D.[44] However, in patients undergoing premium IOL implantation (multifocal IOLs) with greater visual demand, a toric multifocal IOL can be preferred for low astigmatism (≥1.0 D).[1] [Table 1] and [Table 2] show the currently available monofocal and multifocal toric IOLs.
Table 1: Currently available monofocal toric intraocular lenses

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Table 2: Currently available multifocal toric intraocular lenses

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Various studies have evaluated the effectiveness of different materials and haptic designs on the stability of IOL.[45],[46],[47],[48],[49] Draschl et al., in their study, found that the hydrophobic IOL was rotationally more stable than the hydrophilic IOL due to the tackiness of the hydrophobic material.[45] However, a recent study by Haripriya et al. observed that the hydrophilic toric IOL had comparable rates of postoperative misalignment and surgical repositioning with the hydrophobic IOL, though the rate of repositioning was nonsignificantly lower with hydrophobic IOL.[46] The advantage of a plate haptic design over open-loop haptic design of toric IOLs is still a question of debate. Patel et al. observed that plate haptic IOLs show greater rotational stability than loop haptics.[47] Nevertheless, studies by Scialdone et al. and Mihaltz et al. could not confirm this finding.[48],[49]

Contraindications for toric intraocular lens implantation

Those with irregular astigmatism caused by forme fruste keratoconus, previous trauma or surgery, dry eye syndrome, epithelial basement membrane dystrophy, Salzmann nodules, pterygium, corneal scars, or other corneal diseases are not ideal candidates. It may be better to treat them initially to regularize the corneal contour prior to astigmatic correction. However, the amount of astigmatism may be reduced with a decreased dependence on spectacles or contact lenses, and such cases may be considered for toric IOLs after proper counseling.[43],[50] Corneal tomography helps determine the effect of these conditions on astigmatism, and if the astigmatism is shown to be stable over the serial examinations, separated by at least 3–4 weeks, a toric IOL can be implanted with the help of accurate keratometry and IOL power calculation. More than one imaging modality should be used to identify inconsistent measurements, which, if present, contraindicate the use of a toric IOL.

In some cases of forme fruste keratoconus, where the manifest axis and magnitude match the corneal axis and magnitude, a toric IOL can be considered. However, those with a decentered apex, radial axis skewing, and significant superior-inferior astigmatic asymmetry are not ideal candidates.

If the irregular corneal tomographic changes are limited to the periphery with a relatively symmetric and stable central cornea (3–5 mm) as in peripheral corneal scars and mild forme fruste keratoconus, a toric IOL can be used. Nevertheless, a toric IOL should be avoided in keratoconus cases in which the patient prefers to wear a rigid contact lens after surgery as the rigid contact lens may effectively unmask the toric IOL power.

Zonular instability and posterior capsular dehiscence is an absolute contraindication as a stable capsular bag-IOL complex is essential for the rotational stability of the IOL.[51]

Insufficient pupillary dilatation is also a relative contraindication, as it may hamper the visualization of the alignment marks, which are located in the periphery of the IOL.[51]

Preoperative investigations

It is essential to evaluate and measure the ocular and optical properties of the patient's eyes, to customize the surgical approach, and to choose a specific IOL. The vital preoperative investigations include corneal tomography for corneal curvature and astigmatism, optical biometry for IOL power calculation, tear film evaluation (meibography) for dry eye, specular microscopy for corneal endothelium status, macular optical coherence tomography (OCT) for underlying retinal conditions, and visual field testing with OCT retinal nerve fiber analysis for manifest glaucoma patients and glaucoma suspects.

The amount, type, and axis of astigmatism are critical points to be considered and can be collected with the topographic evaluation of the cornea. The use of rigid and soft contact lenses should be stopped for at least 3 weeks and 1 week, respectively, before topography.[52] Multiple instruments based on different principles may be used for keratometry estimation, such as manual and automated keratometers, Placido-based corneal topographers, scanning-slit based tomographers, Scheimpflug imaging systems, wavefront aberrometers, color light-emitting diode (LED)-based systems, and anterior segment-OCT (AS-OCT).[53] Taking multiple measurements and employing at least two separate devices based on different principles increases the accuracy of keratometry estimation.[54],[55],[56] Candidates with a similar steep corneal axis on different devices are ideal for toric IOLs. The visual outcomes may not be satisfactory when a considerable difference in both the axis and magnitude of astigmatism is observed on different devices. The measurements should be rechecked and validated if astigmatism is >2.5 D, corneal power is <41.0 D or >47.0 D, or the corneal diameter is <10.75 mm or >13.0 mm.[57]

Posterior corneal astigmatism must be considered while calculating total corneal astigmatism to avoid errors in IOL power calculation. The posterior cornea acts as a minus lens, and the astigmatism is generally against-the-rule (ATR) and constant over time.[44] Furthermore, the anterior corneal astigmatism shifts from with-the-rule (WTR) in the younger age group to ATR in the elderly age group.[58],[59],[60] Depending merely on the anterior corneal curvature measurements could result in residual astigmatism after toric IOL implantation, overcorrecting by a factor of 1.38 in eyes having WTR astigmatism and undercorrecting by a factor of 0.65 in the eyes with ATR astigmatism.[60]

The keratometers and placido-based corneal topography do not measure the posterior corneal curvature, assume a fixed ratio between the anterior and posterior curvature, and are more prone to errors in keratometry estimation.[60] The slit scanning systems, Scheimpflug imaging systems, color-LED topography, and AS-OCT measure both the anterior and posterior corneal curvature and are found to be more accurate. Moreover, they are repeatable.[55],[56],[61],[62]

The image-guided systems such as VERION® and CALLISTO® Eye help in preoperative planning of the location and size of the surgical incisions, capsulorhexis, and positioning of the IOL. The CALLISTO® system also aids in planning the position of limbal relaxing incisions. VERION® unit also helps the surgeons to optimize the incision locations and toric IOL power as per their surgically induced astigmatism (SIA). Furthermore, in femtosecond laser-assisted cataract surgery (FLACS) cases, they assist in determining the position and length of arcuate incisions.[44]

The iTrace® System (Tracey Technologies, Houston, Tx) combines Placido disc corneal topography with ray tracing aberrometry and aids in preoperative planning, decision-making, and postoperative assessment in cases undergoing toric IOL implantation.[63] It gives the corneal curvature and power, measures angle alpha, and calculates the corneal, internal, and total higher-order aberrations. Moreover, the iTrace® system includes an in-built toric IOL planner, which calculates the IOL power and provides the axis of placement after considering SIA. Preoperative reference axis marking accuracy can be evaluated using the Integrated Zaldivar toric caliper and toric calculator. Furthermore, the postoperative toric IOL enhancement software provides the degree of misalignment of the IOL and the direction as well as the magnitude of the required postoperative rotation to achieve optimal results.[44]


  Intraocular lens power calculation Top


Optical biometry is the current gold standard for IOL power calculation. Optical biometers are based on either of the three principles: partial coherence interferometry, optical low-coherence refractometry, or OCT. The main advantage of optical over ultrasound biometry is its improved accuracy by avoiding artefacts due to corneal compression, which may lead to an overestimation of the IOL power.[53],[64] Immersion ultrasound may also yield accurate results, but it is a time-consuming technique and needs an experienced operator.[65] In addition, optical biometry avoids the risk of infection and evaluates other parameters essential for newer generation formulas, such as white-to-white diameter and lens thickness. If surgery is planned in only one eye, the refractive status of the fellow eye should also be taken into account. Various studies have shown that visual acuity is superior when the residual astigmatism is ATR than WTR or oblique.[53],[66],[67]

Multiple formulae and toric calculators are available for IOL power calculation, determining the magnitude of the toric IOL and the axis of placement. An ideal formula should consider the SIA, the posterior corneal curvature, and the effective lens position (ELP). The operating surgeon must determine his SIA using standard astigmatism vector analysis or online tools.[68] ELP is defined as the effective distance between the anterior surface of the cornea and the lens plane, given that the lens was infinitely thin.[69] The ELP is considered to be the major limiting factor for refractive predictability postsurgery, as the accuracy of axial length (AL) and corneal power measurements has been widely demonstrated.[70] Improvements in IOL power calculations over the past three decades are the outcome of improved predictability of the ELP variable.

The Baylor nomogram was published first, which incorporates the posterior corneal curvature in its measurements.[60] Later, Goggin et al. developed coefficients of adjustment to adjust the anterior keratometric power while also taking into account the spherical power of the IOL.[71] The Abulafia–Koch formula introduced by Abulafia et al. incorporates a mathematical regression to estimate the effect of the posterior corneal surface.[72]

The AcrySof® online toric calculator and the iTrace® calculator employ a fixed ratio to convert IOL power to corneal plane power. This results in undercorrection of astigmatism in eyes with long AL and overcorrection in eyes with short AL. The spherical power of the IOL affects the cylindrical power at the corneal plane due to the difference in vergence of the rays. This gives rise to errors in IOLs, especially with high cylindrical powers, and when the patient has high hyperopia.[73],[74] On the other hand, the Tecnis® toric calculator incorporates the anterior chamber depth based on the AL and keratometry values, and the Holladay toric calculator considers the predicted ELP into account.[75] The Barrett toric calculator incorporates the ELP as well as the posterior corneal astigmatism. It was recently updated to introduce keratometry values from different instruments and calculate mean or median keratometry values, which further improve the results. It has better predictability than the other calculators and nomograms.[76],[77],[78] Moreover, the calculator also has the 'Barrett True K Toric Calculator', a customized method for patients who have previously undergone refractive surgery. The revised AcrySof® toric calculator incorporates the Barrett toric algorithm, and the Tecnis® calculator received FDA approval in 2016 to include posterior corneal astigmatism compensation.[44] Other newer toric calculators include the emmetropia verifying optical (EVO) toric formula and the Panacea software, both of which are yet to be tested for accuracy.[53]

Intraoperative wavefront aberrometry estimates the toric IOL power and axis of placement based on the aphakic refraction, especially in eyes with a history of refractive surgery.[44] Yesilirmak et al. in their retrospective study observed that the outcomes of optiwave refractive analysis (ORA) in postrefractive surgery (laser-assisted in situ keratomileusis) eyes were more accurate than those obtained by the standard Sanders-Retzlaff-Kraff-Theoretical (SRK-T) formula and the online American Society of Cataract and Refractive Surgery calculator.[79]


  Marking Methods Top


The axis of the toric IOL must be aligned precisely with the axis of the corneal cylinder for optimal outcomes, which needs exact corneal marking. Different methods have been described to place the axis marks and may be categorized as manual techniques, image-guided systems, intraoperative aberrometry, and iris-detection methods.

Manual techniques

The three-step method is commonly used for toric IOL axis alignment, which involves the preoperative marking of the reference axis, intraoperative alignment of the reference marks using the degree gauge of the fixation ring, and intraoperative marking of the target axis.[80] The reference marks are usually placed in the 3 o', 6 o', and 9 o'clock positions to improve predictability, though some surgeons may prefer to mark only the horizontal 3 o' and 9 o' clock positions. A marking pen or devices such as a rotational thin slit-beam, Nuijts-Solomon bubble marker, weighted thread, or a pendulum marker may be used to perform the marking. This is followed by the intraoperative alignment of these reference marks with the degree gauge of the fixation ring, and the target axis is then marked with a corneal meridian marker intraoperatively. One-step marking may be performed using different devices such as Neuhann one-step toric bubble marker, tonometer marker, electronic toric markers, or Geuder-Gerten Pendulum marker.[81],[82]

The patient should be sitting erect with the back resting against a wall and a straight-ahead gaze while marking the axis to avoid cyclotorsion and inadvertent errors. Topical anesthesia should be administered before marking. However, the cornea should be dry before starting marking. These marking methods are associated with a significant learning curve and may show intersurgeon variability.

In order to eliminate the ink-associated problems with manual marking, Osher ThermoDot® marker (Beaver-Visitec International, BVI, Waltham, Mass.) has been introduced, which uses a bipolar cautery to create an ink-free, precise reference mark during surgery.[44] A 26-gauge needle stained with sterile blue ink can also be used to create anterior stromal puncture and mark the reference axis without smudging.[83]

Image-guided techniques

Osher introduced the concept of iris-fingerprinting in 2010, wherein the iris crypts and brush fields were used as landmarks to place the axis marks, which formed the basis for the development of various image-guided systems, like CALLISTO® Eye and Z align, VERION® system, and the TrueVision® 3D Surgical System.[84],[85],[86],[87]

The image-guided systems capture a preoperative reference image followed by intraoperative registration of the image wherein the limbal landmarks are used to match the two images. A graphic overlay is then superimposed on the surgical field along the target axis, which provides a guide for toric IOL alignment. The image-guided systems also aid in the placement of corneal incisions, the capsulorhexis size, and IOL centration.[44]

Mayer et al. evaluated the efficacy of the CALLISTO® Eye system and compared the results with those achieved using a manual marking technique. They found better IOL alignment and lower deviation from the target-induced astigmatism in the CALLISTO® Eye group than the manual marking group. The mean toric IOL misalignment was significantly lower in the digital group than in the manual group (2.0 ± 1.86° versus 3.4 ± 2.37°). The mean deviation from the target-induced astigmatism was significantly lower in the digital group (0.10 ± 0.08 D versus 0.22 ± 0.14 D). They also found the preoperative procedure and IOL alignment to be faster in the image-guided group.[88]

However, registration may be difficult in highly uncooperative patients and those with deep-set eyes or narrow palpebral apertures. Conjunctival chemosis, ballooning, and bleeding may interfere with intraoperative image registration. Furthermore, the high cost of these devices may limit widespread usage.

Intraoperative aberrometry

Intraoperative aberrometry-based devices like ORA® (WaveTec Vision Systems Inc., CA, USA) and Holos IntraOp® (Clarity Medical Systems, CA, USA) assess the phakic, aphakic, or pseudophakic refraction to give feedback for toric IOL alignment.

ORA® performs a real-time assessment of IOL power and the axis using the aphakic refraction, based on the principle of Talbot-Moire interferometry. It employs a modified refractive vergence formula for accurate IOL power calculation even in complicated postrefractive surgery cases. In addition, it helps achieve minimum residual astigmatism by providing the direction and magnitude of rotation. VerifEye® has been included in ORA with a fast-imaging processor that verifies the stability of the system before taking measurements. Holos IntraOp® also provides continuous real-time refraction throughout the surgery and helps in refining the axis of the toric IOL.[44]

Intraoperative aberrometry-based methods are 2.4 times more likely to have ≤0.50 D residual astigmatism than other standard methods (standard preoperative biometry, conventional IOL power formulas, the Acrysof® toric IOL calculator, and ink marking for positioning).[54] The anterior chamber should be uniformly filled with cohesive ocular viscoelastic devices (OVD) to maintain the intraocular pressure, and a uniform fundal glow is confirmed before taking the measurements. Intraoperative corneal edema, presence of air bubbles, clumps of dispersive OVD in the anterior chamber, or inadequate intraocular pressure and multiple radial keratotomy cuts may affect the accuracy.[89] However, the device needs a significant area or space, as it is mounted directly onto the bottom of the surgical microscope. Furthermore, the high cost involved may limit its use.

Iris-detection method

LENSAR® uses IntelliAxis Refractive Capsulorhexis to produce notches in the anterior capsulorhexis that indicate the steep axis. This iris-fingerprinting technology combined with corneal shape analysis ensures the steep corneal axis by the femtosecond laser in a way that compensates for cyclorotation.[57]

A newer smartphone application, named toriCAM®, has been shown to reduce the reference marking error to 1.28° (67%), compared to the freehand or slit-lamp-assisted marking.[90]


  Intraoperative Approach Top


The target axis is marked at the beginning of the cataract surgery, in case of manual marking, after aligning the preoperatively placed reference marks with the degree gauge of the fixation ring. The clear corneal incisions, size, and centration of capsulorhexis, as well as IOL centration play vital roles in achieving optimal outcomes. Self-sealing clear corneal incisions ≤2.8 mm that are astigmatically neutral or induce minimal astigmatism should be created. Image-guided systems compensate for cyclotorsion and aid in the accurate placement of incisions. A continuous circular capsulorhexis, which is of adequate size (5–5.5 mm) and well-centered, provides an optimum IOL coverage and ensures the stability of the IOL postoperatively. FLACS may further improve the outcomes by providing a precise capsulotomy. Moreover, fewer higher-order aberrations were observed with FLACS than conventional phacoemulsification with toric IOL implantation.[91]

It is essential to center the IOL along the coaxially sighted corneal light reflex, as represented by the first Purkinje image when the patient is fixating on the microscope light. This provides IOL stability and prevents dysphotopsia. The final IOL alignment should be done after removing the OVD and hydrating the wounds. The IOL should be placed about 3°–5° anticlockwise of the final desired IOL position during alignment, as a majority of the open-loop IOLs can be rotated only clockwise, and a rerotation may be required if the IOL rotates clockwise of the target axis during these maneuvers. In eyes with high AL (>24 mm) and those with significant postoperative rotation of IOL, insertion of capsular tension ring may provide better rotational stability by increasing the posterior capsule-IOL contact.[92] It should be kept in mind that posterior capsular rent can lead to IOL tilt and rotation if implanted in the bag and is a relative contraindication.


  Postoperative Outcomes Top


[Table 3] shows the overview of functional and anatomical outcomes reported in the literature after toric IOL implantation.
Table 3: Overview of functional and anatomical outcomes reported in the literature after different types of toric intraocular lens implantation

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Postoperative residual refractive astigmatism

Patients having a toric IOL were more likely to achieve residual refractive astigmatism <0.5 D at six months after surgery. Study outcomes showed a mean postoperative residual astigmatism ranging between -0.71 D and -0.02 D.[10],[11],[25],[28],[37],[38] A lower degree of mean residual astigmatism is observed with toric IOLs as compared to non-toric IOLs with or without limbal relaxing incisions.[9],[14],[16] Toric multifocal IOLs showed residual astigmatism <0.50 D in 80%–98% of cases.[30],[35],[40]

Postoperative uncorrected distance visual acuity

An uncorrected distance visual acuity (UDVA) of 20/40 or better is achieved in 70%–100% of cases receiving toric IOL implantation.[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40] The higher-order aberrations and contrast sensitivity after toric IOL implantation are similar as compared to nontoric monofocal IOLs.[14] Toric multifocal IOLs demonstrate good functional outcomes with UDVA better than 20/40 in 97% to 100% of patients, uncorrected near visual acuity better than 20/40 in 100% of patients, and spectacle independence in 79% to 100% of patients.[30],[35],[40],[93] However, dysphotopic symptoms such as glare and halos associated with multifocal IOLs may limit the overall patient satisfaction.

Mean vision-related quality of life

Satisfactory vision-related quality has been reported by patients receiving toric IOL implantation, and a mean patient satisfaction score of 9.7 ± 0.47 has been reported in a study assessing the outcomes of Tecnis® toric IOL.[21] Nanavaty et al. collected quality of life data and reported that patients with toric IOL were happier than patients who received limbal relaxing incisions.[28]

Postoperative intraocular lens rotation

One degree of misalignment causes a loss of approximately 3% of the effective cylinder power, and the entire toric effect is lost in cases with 30° of misalignment.[94] Studies found a postoperative IOL rotation of >10° in 0%–7% of eyes with AcrySof® toric IOLs, and maximum rotation occurred within the initial 10 days in the postoperative period in patients with high AL.[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16] Studies with Tecnis® toric IOL-implanted cases showed a postoperative rotation of >10° in only 0%–3% eyes.[7],[17],[21],[22],[23],[24],[25],[26]

Rotational stability of the IOL differs with design and material, and better stability has been observed with hydrophobic acrylic lenses. The development of strong adhesions between the IOL and lens capsule in the early postoperative period leads to greater rotational stability. The silicone IOLs have the least rotational stability, though the plate haptic design confers higher stability than conventional three-piece IOLs with polypropylene loop haptics.[95]

Early IOL rotation can be caused by retained OVDs, large capsular bag, small diameter of the haptic, and high AL. The axis of IOL implantation is also associated with postoperative rotation, and a greater incidence of IOL rotation has been observed in with-rule-astigmatism cases with a vertical axis of IOL implantation.[33] Capsulorhexis extension or fibrosis can also cause postoperative rotation.[44]

Realignment of the toric IOL is needed in cases with >10° rotation from the target axis. Rotation <10° changes the manifest refraction by only 0.5 D and usually does not warrant any resurgery.[96] The axis of implanted toric IOL may be assessed at the slit lamp with a rotating slit and rotational gauge. However, the 10° steps on the slit-lamp measuring reticule limit the accuracy of this technique. The online toric outcome analyzer (www.astigmatismfix.com) assists in determining the ideal position of the toric IOL in cases of postoperative malrotation using the patient's postoperative manifest refraction, power, and current axis of the toric IOL.[97] The iTrace® system determines the orientation of the toric IOL based on the internal ocular aberrations. A simple and inexpensive method to measure the toric IOL axis using a camera-enabled cellular phone and (ImageJ) computer software has also been described.[98]

Good results have been observed in cases with early rotation using a long cannula mounted on a balanced salt solution filled syringe to rotate the IOL through the paracentesis incision.[99] Only intraoperative marking is necessary relative to the implanted IOL. This decreases repositioning variability and maximizes outcomes after IOL rerotation. Nevertheless, cases with high residual astigmatism may need an IOL exchange, piggyback IOL implantation, or corneal ablative procedure.[100]


  Conclusion Top


The majority of the patients receiving toric IOLs do extremely good when careful attention is given to patient selection, preoperative evaluation of astigmatism (considering posterior corneal curvature), IOL power calculation (taking into account SIA and ELP), axis marking, and surgical technique. Evaluation of corneal astigmatism by taking multiple measurements and using at least two separate devices based on different principles gives superior outcomes. The Barrett toric calculator, which includes the posterior corneal astigmatism and ELP, has better predictability than the other calculators and nomograms in toric IOL calculation. Image-guided methods for IOL alignment and newer toric IOL designs with better rotational stabilities have helped achieve more predictable outcomes. Future advancements in technology may further refine the results of toric IOL, with greater IOL stability and more accurate visual outcomes.

Financial support and sponsorship

Nil.

Conflicts of interest

The author declares that there are no conflicts of interests of this paper.



 
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