Toric Intraocular Lenses

This chapter described a short history about the toric intraocular lenses (IOLs) and then discussed some interesting topics such as the measurement of (front and posterior) corneal astigmatism and surgically induced astigmatism; the manual marking techniques and image-guided systems and intraoperative aberrometry-based methods; the new toric lens calculation calculators and toric IOLs formulas; the post operation care of toric IOLs and re-rotation of misaligned toric IOLs; and some relevant issues on multifocal toric intraocular lens. Meanwhile, this chapter also discussed toric IOL in some special cases like keratoconus corneal ectatic disorders, post-refractive surgery and post-keratoplasty, etc.


Introduction
Up to more than one-third of cataract patients have preoperative corneal astigmatism of more than 1.0 diopter (D) [1], while 26.2% have more than 1.5 D [2, 3], 8-14.9% have more than 2.0 [1,3], and 2.6-7.4% have more than 3.0 D [1,3]. Astigmatism is one of the most important factors that affect postoperative vision quality. More than 0.5 D of residual astigmatism can reduce visual performance and patient satisfaction [4][5][6]. Currently, implanting a toric lens is recognized as the most accurate form of astigmatic correction during cataract surgery, especially astigmatism of more than 1 D [7]. Actually, toric IOLs correct preexisting regular corneal astigmatism usually ranging from 0.75 to 4. 75 D [8]. However, the outcomes after toric IOL implantation are still influenced by many factors including accurate preoperative measurement of corneal astigmatism, IOL selection, marking techniques, intraoperative alignment and postoperative care, etc.

A short history and clinical outcomes of toric IOLs
The first article reporting a toric IOL (Nidek NT -98B) was published in 1994 [9], which had a cylinder power of 2.00 or 3.00 D. In the study, Shimizu et al. had relatively positive results, although some negative results still occurred in some eyes of which the lens axis rotated more than 30° [9]. Ever since then, with the predictability increasing and the safety enhancing, toric IOLs have definitely become a considerable option to correct significant astigmatism when undergoing cataract surgery [10,11]. At present, standard toric IOLs are available in cylinder powers of 1.0 to 6.0 D, while higher cylinder powers are also available (see Table 1).
Compared with incisional astigmatic keratotomy, toric IOLs offered better predictability and stability of correction [17], especially in moderate to high astigmatism [30]. In a recent meta-analysis (including 13 RCTs with 707 eyes), toric IOLs provided better distance visual acuity and lower amounts of residual astigmatism, combined with greater spectacle independence, than nontoric IOLs even when relaxing incisions were used [33].
From a social cost-effectiveness perspective, toric IOLs were inferior to monofocal IOLs in a recent prospective study [34], which should be noted in healthcare decision-making.

The measurement of astigmatism
For a toric IOL, the keratometric astigmatism (both axis and magnitude) of the cornea must be accurately measured.

Anterior corneal curvature
Traditionally, keratometry and topography take into account only the anterior corneal curvature [35]. However, nomograms predict total corneal astigmatism based on the power and axis of the anterior corneal astigmatism, assuming a fixed ratio between the anterior and posterior curvature [36]. These methods obviously cannot take outliers and irregularities into account (e.g., post-refractive surgery eyes) [35], thus leading to significant postoperative and/or overcorrection. However, if the agreement of measurement of astigmatism between instruments of different kinds is poor (more than 10°), the selection of toric IOLs requires extra care.

Posterior corneal curvature
The astigmatism of posterior cornea is generally minus lens of against-the-rule. As mentioned above, ignoring effects of actual posterior corneal curvature may lead to inaccuracies in total astigmatism estimation in some eyes. In a recent study [36], for those eyes who received IOLs with 2 diopters of cylinder or less, a coefficient of adjustment of 0.75 for with-the-rule astigmatism and 1.41 for against-the-rule astigmatism can be applied to the corneal astigmatism power value to calculate a more appropriate IOL cylinder power than that be calculated by using unadjusted anterior corneal curvature measurements.
Since minimizing the residual refractive error is especially critical in toric multifocal IOLs [37], imaging systems that measure posterior corneal curvature, as well as the new algorithm that incorporates the effect of posterior corneal astigmatism, are increasingly being invented. For example, the Scheimpflug imaging systems, slit scanning systems, and OCT systems could measure posterior corneal curvature, besides the anterior curvature. In a comparative study [35] including a Scheimpflug tomography (OCULUS Pentacam), a Placido topographer (Tomey TMS-5 in Placido mode), a swept source/Fourier domain OCT (CASIA SS-1000), an autokeratometer (Haag-Streit Lenstar), and a hybrid topographer (Tomey TMS-5), the OCULUS Pentacam has the disadvantage of high measuring noise on posterior corneal curvature. Meanwhile, the highest precision for planning toric IOL power and axis was achieved by combining the keratometry and OCT data. In a recent study, Lu et al. found that a novel multicolored spot reflection topographer system could provide high repeatable measurements in (both anterior and posterior) corneal power and astigmatism [38].

Surgically induced astigmatism
Besides naturally occurring astigmatism, the surgically induced astigmatism (SIA) is also an important factor for the appropriate option of a toric IOL. The SIA could be influenced by position and length of incisions [39]. Meanwhile, to achieve minimum residual refractive astigmatism for specific patients, the incisions could be determined by the magnitude and axis of preoperative keratometric astigmatism [4]. The application of femtosecond laser-assisted cataract surgery (FLACS) could minimize SIA.

IOL power calculation
An accurate biometry is a precondition not only for toric IOLs but also for regular IOL power calculation. The axial length may be measured by either ultrasonic biometry or optical systems, and SRK/T, Holladay 2, Hoffer Q, and Barrett formula are recommended to be used to calculate sphere power. Nguyen et al. adjusted the power of an existing hydrophobic acrylic IOL by a femtosecond laser [40], which is definitely a promising idea.
There are several toric calculators available for surgical planning that have been developed to predict postoperative cylinder power, such as Barrett toric calculator [41], Holladay toric calculator, and Alcon toric calculator(the revised Alcon toric calculator is a derivation of the Barrett calculator). In general, an ideal IOL power calculation formula should take into account the posterior corneal curvature, the effective lens position (ELP), as well as the SIA. And there are a few formulas available such as Abulafia-Koch linear regression formula [42], Baylor nomogram (a method from Koch) [43], Barrett formula, Abulafia-Koch formula, etc.

IOL power calculation considering posterior cornea
A few online toric IOL calculators have been revised to take into account the contribution of the posterior cornea in IOL power calculation, but it proved itself valuable. The Baylor nomogram which incorporates the posterior corneal curvature has been observed to be more precise than traditional Alcon and Holladay toric calculator without posterior corneal astigmatism compensation [44]. However, the revised AcrySof toric calculator incorporates the Barrett toric algorithm, which takes into account both the ELP and the posterior corneal astigmatism, and had better predictability than the Baylor nomogram as well as Holladay and traditional Alcon toric calculator [44]. Other toric IOL calculators such as TECNIS calculator also incorporate posterior corneal astigmatism compensation.

IOL power calculation considering ELP
Failing to consider the anterior chamber depth and cornea thickness may result in inaccurate calculations, especially in eyes with extremes of axial lengths [45]. As mentioned above, the revised AcrySof online toric calculator and iTrace toric planner takes into account the ELP [14,46]. The TECNIS calculator incorporates the anterior chamber depth based on the axial length and keratometry values [46], and the Holladay formula incorporates the ELP in its calculations.

Intraoperative wavefront aberrometry
Intraoperative wavefront aberrometry is increasingly being used to estimate the toric IOL power and axis of placement based on the aphakic refraction, especially in post-refractive surgery cases. A recent study reported only a mean error of 0.43 AE 0.33 D with Optiwave Refractive Analysis (ORA; WaveTec Vision Systems Inc., CA, USA) in post laser-assisted in situ keratomileusis (LASIK) cases undergoing toric IOL implantation, which were more accurate than those obtained by the standard SRK/T formula and the online ASCRS calculator.

Surgery techniques
Many issues, such as accurate marking technique, clear corneal incisions, intraoperative alignment of the toric IOL, capsulorhexis, and IOL centration, play a significant role in achieving optimal outcomes.

Marking techniques
Preoperative reference and axis marking techniques could be broadly categorized as manual methods, image-guided systems, and intraoperative aberrometrybased methods.
The three-step manual technique is at present most commonly used [47], which is fairly accurate [48]. The first step is preoperative marking of the reference axis, which is commonly placed in the horizontal 3'o and 9'o clock positions. The second step is intraoperative alignment of the reference mark. The marking may be performed with a skin marking pen or needle. The patient should be sitting erect in a straight-ahead gaze while marking the reference axis. A change in patient position from sitting to supine may induce significant cyclotorsion; studies reported up to 28°of cyclotorsion in 68% of cases [49]. The manual marking methods have been limited by smudging of the dye, irregular, and broad marks.
Image-guided systems and intraoperative aberrometry have advantages compared with manual marking. The image-guided system based on the concept of landmarks to place the axis marks [50], which could be iris crypts, nevi, brush fields, etc. The systems capture a preoperative reference image and calculated the location of these marks and their distance in degrees from the target IOL axis. Then the system generated a final plan which provides simple angular directions from each reference mark to the planned axis of IOL placement.

Intraoperative toric IOL alignment
Intraoperative IOL positioning is the key procedure to sustain rotation stability. During IOL alignment, the IOL should be left about 3-5°anticlockwise of the final desired lens position, followed by complete OVD removal and hydration of the wounds. Most open-loop IOLs can be rotated only clockwise, and a complete rerotation will be needed if the IOL rotates further clockwise of the target axis.
The image-guided systems and intraoperative aberrometry could be definitely more useful than manual alignment. As mentioned above, the image-guided systems capture a preoperative reference image and an intraoperative image and then match the two images with respect to each other using landmarks. During the operation, 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 and intraoperative aberrometry have improved the precision of toric IOL alignment, with <5°of deviation from the intended axis in the majority of cases.
Compared with manual marking, Elhofi et al. had observed more precise alignment with VERION image-guided system [51], which offers comprehensive astigmatism management, the incision location optimization, toric IOL power calculation, as well as decreasing SIA.
However, Solomon et al. claimed that, compared with the surgeon's standard of care, the use of the VERION combined with intraoperative aberrometry (Optiwave Refractive Analysis system with VerifEye) did not significantly optimize the outcomes [52]. The accuracy of CALLISTO Eye is also very effective [53], and it also assists in planning the position of limbal relaxing incisions.

Complications
Postoperative toric IOL misalignment is the major complication after toric IOL implantation. Toric IOL misalignment could harm visual quality. In a recent experimental study, 5°IOL axis rotation from the intended position determined a decay in the image quality of 7.03%, 10°of IOL rotation caused 11.09% decay, and 30°r otation caused 45.85% decay [54].
Toric IOL misalignment may be attributed to three factors: (1) inaccurate preoperative prediction of the axis of IOL alignment; (2) inaccurate intraoperative alignment; and (3) postoperative IOL rotation. IOL rotation may be observed as early as 1 hour after surgery, and a majority of rotations occur within the initial 10 days [18]. Early IOL rotation likely results from incomplete OVD removal, whereas late postoperative rotation is influenced by the IOL architecture, design, and axial length. In a recently published case report, the toric IOL was rotated more than 115°shortly after a neodymium: YAG (Nd:YAG) laser posterior capsulotomy [55].
Rotational stability of the IOL varies with design and material and strength of IOL capsular bag adhesions. Maximum rotational stability has been observed with hydrophobic acrylic lenses, followed by Hydrophobic acrylic, hydrophilic acrylic, PMMA and silicone. Loop haptic IOLs are better than plate-haptic IOLs on postoperative rotation stability when using silicone IOL, but they are similar when using acrylic IOL. A study of AT TORBI 709 M, which had one-piece hydrophilic acrylic with hydrophobic surface and a supporting four-haptic design, had rotation of more than 5°in 10% cases in 6 months [56]. Another study of AT TORBI 709 M reported 13% eyes had rotation of more than 10° [57], while another study reported 100% rotation of more than 10° [58]. Scialdone et al. found similar results in rotation stability between AT TORBI 709 M and AcrySof toric IOLs [59]. A long-term of 2-year study of AcrySof toric IOLs (hydrophobic acrylic IOL with Flexible loop haptic) reported postoperative rotation of more than 10°in 1.68% eyes, more than 5°in 23.3% eyes [18]. A recent cohort study [60] of 1273 eyes showed that AcrySof toric IOL was less likely to rotate, with 91.9% of eyes rotated 5°in AcrySof toric IOL eyes compared with 81.8% in TECNIS Toric IOL eyes (P < 0.0001); rotation 10°(97.8% Acrysof vs. 93.2% TECNIS, P = 0.0002) and 15°(98.6% Acrysof vs. 96.4% TECNIS, P = 0.02). Furthermore, a hydrophilic IOL with C-flex design (Rayner 600S IOL) was reported to have excellent rotational stability: average 1.83°AE 1.44°at 6 months and no lens rotated more than 5° [61].
In cases with more than 10°of rotation, realignment of the toric IOL is needed [62]. In a study by Oshika et al., 6431 eyes are implanted with toric IOLs, and realignment was performed in 0.653% of cases [63]. An early repositioning performed after 1 week of primary cataract surgery had optical outcomes.
IOL tilt could also induce astigmatism: tilting toric IOLs aligned at 180°would decrease with-the-rule astigmatism, bringing in undercorrection, while aligned at 90°increased against-the-rule astigmatism, bringing in overcorrection [64].
Meanwhile, LASIK, customized surface ablation, or femtosecond laser-assisted intrastromal keratotomies could also be used to correct residual astigmatism [65]. Some toric rotation check, such as https://www.astigmatismfix.com/, could help determine the amount of IOL rotation, and the expected residual refraction. When the large residual cylinder not amenable to correction by rotation alone or refractive surgery, an IOL exchange, piggyback IOLs procedures may be considered.

Multifocal toric IOLs
Toric designs are even more required in multifocal IOLs [66] because patients undergoing multifocal IOLs may not tolerate residual astigmatism of <1 D, and multifocal IOLs without toric design perform best with less than 0.75 D of cylinder [67].
But on the other hand, the selection of multifocal toric IOLs should be more restricted than monofocal toric IOLs, especially for the following candidates: (1) patients who had unrealistic expectations of visual quality when having related ocular comorbidities; (2) patients who may not tolerate dysphoric symptoms such as glare and halos; and (3) patients who had specific contraindications for multifocal IOLs, such as abnormal κ or α angle, etc. Thus, a comprehensive ocular examination should be undertaken to rule out any ocular comorbidities that may interfere with the postoperative outcomes.

Special cases
Normally, cases with irregular astigmatism, corneal ectatic disorders, postrefractive surgery, post-keratoplasty, and high myopia are not ideal candidates for toric IOL implantation, partly because they are unlikely to achieve complete refractive correction with toric IOLs. However, the amount of astigmatism may be partly reduced, decreasing spectacle dependence. And such cases may be considered for surgery after adequate counseling. As a consequence, the applications of toric IOLs are expanding to include special cases such as pellucid marginal degeneration [74,75], mild keratoconus with cataract [76], astigmatism after keratoplasty [77][78][79][80], and high astigmatism [81]; even toric trifocal IOLs were used in high astigmatism cases [82]. In general, the indications of toric IOL are still controversial and expanding.

Conclusions
The outcomes after toric IOL implantation are influenced by a few factors: accurate astigmatism measurement, marking techniques, intraoperative alignment, and postoperative care. The importance of posterior corneal curvature is increasingly being recognized, and advanced toric calculators and fumulars that account for both the anterior and posterior corneal power are becoming the standard of care. The image-guided systems and intraoperative aberrometry could provide a markless IOL alignment and optimize incisions, capsulorhexis size, and optimal IOL centration. New toric IOLs with superior design are still being looked forward although they have already achieved great performance.

Conflict of interest
The authors have declared that no competing interests exist.

Other declarations
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.