|Year : 2020 | Volume
| Issue : 3 | Page : 244-251
Astigmatism correction with toric intraocular lens during cataract surgery
N Sandhya, Keerthana Zacharia, SJ Saikumar
Department of Cataract Services, Giridhar Eye Institute, Kochi, Kerala, India
|Date of Submission||02-Sep-2020|
|Date of Acceptance||03-Sep-2020|
|Date of Web Publication||23-Dec-2020|
Dr. N Sandhya
Senior Consultant, Giridhar Eye Institute, Kochi, Cochin - 682 020, Kerala
Source of Support: None, Conflict of Interest: None
Currently toric intraocular lens (TIOL) implantation is the most reliable method to correct pre-existing corneal astigmatism of 1D or more at the time of cataract surgery. A good surgical outcome depends on proper patient selection, accurate preoperative measurements, precise intraocular lens (IOL) power calculation and execution of proper surgical plan. Accurate keratometry measurement on a pristine cornea is crucial in TIOL power calculation. The recognition of the importance of posterior corneal astigmatism and its inclusion into various formulas and recent methods to measure it directly have resulted in better postoperative refractive outcomes. Fine-tuning of surgery with digital guidance in TIOL alignment and new lens designs with better rotational stabilities have helped achieve more predictable outcomes. Postoperative IOL rotation is the major cause of patient dissatisfaction after TIOL surgery. However, various correction algorithms and formulas are available which can help optimal corrective re rotation to address this problem satisfactorily.
Keywords: Astigmatism, cataract surgery, toric intraocular lenses
|How to cite this article:|
Sandhya N, Zacharia K, Saikumar S J. Astigmatism correction with toric intraocular lens during cataract surgery. Kerala J Ophthalmol 2020;32:244-51
|How to cite this URL:|
Sandhya N, Zacharia K, Saikumar S J. Astigmatism correction with toric intraocular lens during cataract surgery. Kerala J Ophthalmol [serial online] 2020 [cited 2022 Jun 30];32:244-51. Available from: http://www.kjophthal.com/text.asp?2020/32/3/244/304544
| Introduction|| |
The aim of modern cataract surgery has shifted from just removal of cataract to the provision of spectacle free vision. Currently, around 40% of people undergoing cataract surgery have astigmatism of more than 1D and 8% more than 2D.,, Residual postoperative astigmatism is an important cause of patient dissatisfaction after cataract surgery.
Astigmatism can be corrected by limbal relaxing incisions, astigmatic keratotomies, opposite clear corneal incisions, or by currently toric intraocular lenses (TIOLs). Corneal incision techniques are less predictable and results are more likely to regress over time., In comparison, TIOLs are more predictable and stable.
At present, standard TIOLs are available in cylinder powers of 1.5D to 6.0D and are intended to correct preexisting regular corneal astigmatism from 0.75D to 4.75D. Extended series and custom made TIOLs can correct cylinders up to 11-12D. Postoperative residual refractive corneal astigmatism of up to 0.5D may not need correction, but astigmatism of 0.75D or more should be addressed in patients seeking spectacle independence, especially in those choosing multifocal intraocular lenses (IOLs).
The 1st TIOL was a three piece non-foldable lens introduced by Shimizu et al. in 1992. The first foldable TIOL became available in 1994. However, these lenses had a relatively high postoperative rotation rate. Since then, various modifications have come in TIOL models.
TIOL is different from the spherical IOL in its toricity on the anterior, posterior, or both surfaces. There are marks near the haptic optic junction, which represent the plus axis cylinder of the lens, which needs to be aligned with the steep axis of the cornea. Successful TIOL outcome depends on proper patient selection, acquisition of accurate preoperative data, precise IOL power calculation and execution of the proper surgical plan.
| Patient Selection|| |
Patients with regular bowtie astigmatism are most suitable for TIOL implantation. Patients with irregular astigmatism are not good candidates though spectacle dependence can be reduced with TIOLs.
Patients must have a healthy and pristine ocular surface to obtain reliable and reproducible keratometry readings. Dry eye and other ocular surface diseases should be addressed before taking corneal measurements. Remember to measure the cornea before refraction, tonometry and dilatation.
A stable IOL-bag complex is a prerequisite for the rotational stability of the TIOL. Zonular instability and posterior capsule dehiscence are relative contraindications for TIOL implantation. Patients who have undergone prior intraocular surgeries may not achieve the intended results with TIOLs due to the surgically induced anatomical changes and the increased chance of IOL rotation.
| Intraocular Lens Power Calculation|| |
Manifest refraction should never be taken into account for IOL power calculations because it involves both corneal and lenticular astigmatism. Only corneal astigmatism is considered when planning TIOL.
In with-the-rule (WTR) astigmatism, the vertical corneal meridian is steeper. In against the rule (ATR) astigmatism, the horizontal meridian is steeper. Normally anterior cornea has WTR astigmatism and this gets modified by the posterior cornea, which has ATR astigmatism and hence this needs to be taken into account in TIOL power calculations to avoid overcorrection of WTR (by a factor 0.5D) and under correction of ATR astigmatism (by a factor 0.3D). Because of this, the threshold for putting a TIOL in the case of WTR astigmatism is around 1.25D and for ATR astigmatism around 0.4–0.5D. Studies have also shown that ignoring posterior corneal astigmatism (PCA) may lead to axis errors of 7.4°±10.3° due to the lack of accurate prediction of total corneal astigmatism (TCA.)
Precise keratometry (K) measurements are important in identifying the correct power and axis of corneal astigmatism. The keratometers and Placido based topography systems measure only the anterior corneal curvature. They assume a fixed ratio between the anterior and posterior curvature and estimate the PCA. Point-source color LED topographer, OCT and Scheimpflug image-based tomographers measure anterior and posterior corneal curvatures. Devices using these technologies, including Galilei, Pentacam HR, and Cassini, provide TCA and power. Although the use of direct measurements of PCA is theoretically useful in decreasing postoperative astigmatism after TIOL implantation, studies have shown that outcomes in normal eyes using these measurements are not better than those achieved by using mathematical models that estimate PCA such as Barrett's toric calculator.
Be cautious if the average corneal power difference between the two eyes is >0.9D, and the patient cannot adequately fixate during keratometry measurements. It is better to further validate the measurements if astigmatism is >2.5D and if corneal power is <41.0D or >47.0 D or if the corneal diameter is <10.75 mm or >13.0mm.
For better outcomes you can take keratometry measurements from 2 devices that use different principles., Cases with similar steep corneal meridian on different devices are good candidates for TIOL implantation. However, if significant variability in both the axis and magnitude of TIOL is observed on different devices, the patient should be evaluated to rule out coexistent ocular comorbidities. The visual outcomes may not be satisfactory in such cases. The highest precision for planning TIOL power and axis is achieved by combining the keratometry and OCT data.
An ideal IOL power calculation formula should take into account the surgically induced astigmatism (SIA), PCA as well as the effective lens position (ELP). An incision into the cornea will alter its astigmatism. However, the magnitude and direction of change depend on the age of the patient, size and shape of the incision, location on the cornea and corneal biomechanical properties. Prediction of SIA is often difficult, and therefore, many surgeons use a value of approximately 0.5D. However, recent reports have shown that the range of SIA is less predictable and a “centroid” value defined as a two dimensional mean of SIA of 0.1D can be used for temporal MICS.
In general, there are two approaches for TIOL calculations. The first is to use mathematical models (e. g. Barrett toric calculator (BTC), Abulafia-Koch) that estimates the predicted postoperative residual refractive astigmatism based on anterior corneal measurements. The second is the use of TCA measurements (Scheimpflug imaging with Panacea Toric Calculator, ray tracing software such as PhacoOptics). Comparative studies found that in normal eyes, the direct measurement approach was not superior to the estimated mathematical approaches.
Various IOL power calculating formulas are available, and currently the BTC is the most widely used. It takes into account both PCA and ELP and is robust throughout the axial lengths. It is available online through the ASCRS and APACRS websites. BTC also has the option of input of 3 different K readings from three different devices through integrated K and can also use the measured K values.
In addition, most of the IOL manufacturers have their own toric calculators, most of which take into account PCA like AcrySof toric calculator and the Tecnis toric calculator, which are available online.
Studies have been performed comparing BTC and new Alcon AcrySof toric calculator and found that these two methods are highly accurate to calculate toric lens power compared to older methods. A study comparing AcrySof and Tecnis calculators found that the Tecnis calculator tends to predict a higher astigmatic power, however, no postoperative results were compared. All these calculators incorporate PCA and ELP, still tend to overcorrect WTR astigmatism and under correct ATR astigmatism.
| Special Situations|| |
Eyes with a history of previous corneal surgery or irregular contours due to diseases such as Keratoconus poses a challenge in IOL power calculations. Traditional keratometry measures only the anterior cornea and estimates the diopteric power of cornea based on a fixed ratio of the radii of the two surfaces. Corneal refractive surgery alters the anterior surface without altering the posterior surface of the cornea, affecting the ratio unpredictably. Barrett True-K TK Toric Calculator can be of help in these eyes because it incorporates the direct measurements of the posterior cornea avoiding estimation errors.
In eyes with keratoconus there is an irregular surface, and the apex is offset. TIOL implantation can be deferred in such situations. However, if you are planning a TIOL, elevation-based topography devices (Pentacam or Galilei) have been reported to be advantageous to measure both the anterior and posterior corneal curvatures and to calculate the TCA.
| Execution of Proper Surgical Plan|| |
The most important aspect in TIOL implantation is marking the steep axis of the cornea and aligning the mark on the TIOL with it. This is because one degree of misalignment of TIOL from the intended axis can cause a loss of approximately 3% of the effective cylinder power, and 30° misalignment can cause complete loss of its toric effect. The axis marking can be done by iris fingerprinting technique, image-guided systems, intraoperative aberrometry based systems or manually.
| Manual Marking|| |
Here first, the reference axis and then the target axis are marked. Reference marking is the marking of 3 and 9 o'clock meridia. This should be done in the sitting position to take care of cyclorotation happening on lying down. This is approximately 2°–4° on average but can be up to 15° in individual patients.
Adequate topical anesthesia should be administered, and the patient should be looking at the distance to avoid convergence. The cornea should be dry and the marking should be done on the cornea and not on the conjunctiva (conjunctiva is mobile). The marking may be performed with a skin marking pen in a freehand manner or with the help of various devices such as a thin slit beam [Figure 1], weighted thread, pendulum marker, or Nuijts Solomon bubble marker [Figure 2]. This is followed by the intraoperative alignment of these reference marks to the degree gauge on a fixation ring, and the target axis is then marked with a corneal meridian marker [Figure 3]a, [Figure 3]b, [Figure 3]c. Anterior stromal puncture using an ink-stained 26G needle produce marks lasting longer.
|Figure 1: Reference axis marking with slit beam. Mark the point where the bright reflex meets the limbus|
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|Figure 2: Reference marking with bubble marker. Mark the cornea at the limbus when the bubble is in the center|
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|Figure 3: (a) Patient under microscope after reference axis marking. (b) Target axis marking under microscope. (c) Final alignment of the TIOL with the target axis. The 3 dots near the haptic optic junction aligned with the target axis marks|
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One step axis marking may be done with various devices such as tonometer markers, Neuhann one step toric bubble marker, and Geuder Gerten Pendulum marker. A mean error of 2.4° ± 0.8° has been observed during axis marking with a bubble marker. The least rotational deviation was observed with a pendular marker (mean 1.8°) in a study comparing four methods of manual marking.
Osher ThermoDot Marker employs a bipolar cautery to create an ink-free, precise reference mark. Other marking systems include the RoboMarker system and electronic TIOL markers.
The manual marking methods have some inherent sources of errors such as smudging and fading of the dye, irregular and broad marks, which can be minimized by a meticulous approach. Digital technologies can be used to further refine this.
| Digital Technologies|| |
In 2010, Osher introduced the concept of iris-fingerprinting, which uses the landmarks of the iris to place the axis marks. The Osher Toric Alignment System constituted the basis for the modern image-guided systems.
These technologies can exclude human errors, transcription errors and provide a higher degree of accuracy in intraoperative TIOL alignment. Interconnected devices can measure IOL power, process data and share information to the surgical microscope. These technologies include real-time eye-tracking that compensates for cyclorotation and eye movements to ensure that the superimposed visual overlay always remains in the same position in relation to the eye.
There are three main principles used by these technologies:
- Limbal registration (image-guided) [Table 1]: An image of the vascularization of the conjunctiva and sclera is captured during biometry and matched with the image obtained by the surgical microscope. An overlay then indicates the steepest axis. Alcon VERION Image-Guided System [Figure 4], Zeiss Callisto Eye with Z align [Figure 5], and TrueVision 3-D Surgical System work on this principle.
Factors altering conjunctival vasculatures like pharmacologic mydriasis, application of topical anesthesia and conjunctival chemosis, ballooning or bleeding may interfere with intraoperative registration in these devices.
- Intraoperative wavefront aberrometry (IA): IA devices such as ORA system and Holos IntraOp perform a real-time assessment of the aphakic or pseudophakic refraction to provide feedback for TIOL power calculation and alignment.
Solomon et al. found that the current generation formulas produce clinical outcomes with TIOLs that are as good or better than those achieved with IA. The accuracy of aberrometry readings may be affected by intraoperative corneal edema, pressure by eyelid speculum, presence of air bubbles or clumps of the dispersive ocular viscoelastic device (OVD) in the anterior chamber, centrally located cortex or by dry corneal surface. Before obtaining the readings, ensure a uniform fundal glow and fill the anterior chamber uniformly with OVD to maintain an intraocular pressure of around 20-30.
- Iris fingerprinting (iris detection): LENSAR uses IntelliAxis Refractive Capsulorhexis to create notches in the anterior capsulorhexis to indicate the steep axis [Figure 6]. Here combining this technology with corneal shape analysis ensures marking of the steep corneal axis by the femtosecond laser in a way that compensates for cyclorotation.
The markings can be virtual that are visible only as projections during surgery (image-guided systems, IA) or real semi-permanent or permanent (intrastromal corneal or capsular markings). Permanent marking is helpful to monitor postoperative misalignment of TIOL.
| Surgical Procedure|| |
Phacoemulsification is not different from the standard one. The capsulorhexis should overlap the optic of the IOL all around uniformly for better centration of the IOL. The ideal capsulorhexis diameter should be adjusted to the IOL optic diameter and ranges from approximately 4.5-5.5 mm. The mark on the IOL needs to be aligned with the target axis. First, the lens is grossly aligned by rotating it clockwise as it unfolds till it is left about 3°–5° anticlockwise to the final desired axis. Rotate it to the final axis after removing OVD completely and tap the lens gently onto the posterior capsule. While hydrating the corneal incision, care should be taken not to propagate the fluid wave behind the IOL and not to leave the eye hypotonous as both can lead to untoward rotation of the implant. Hydrating the second paracentesis before removing the irrigation handpiece from the eye helps to reduce chamber collapse and rotation of the IOL after the infusion is removed.
In high myopes and in whom the previously operated eye had undergone significant postoperative rotation of IOL, insertion of capsular tension ring (CTR) is an option to increase the rotational stability by increasing the posterior capsule-IOL contact. In a study by Vokrojova et al. presence or absence of CTR did not make a significant difference in the rotational stability of the TIOL in normal eyes, but in eyes with an axial length of 24 mm or greater, better IOL alignment was observed in the group with CTR. Long axial length is associated with larger capsular bags and lower power IOLs, which are thinner.
| Current Intraocular Lens Models|| |
Currently, available IOL designs are shown in [Table 2] and [Table 3]. The choice of IOL depends on the surgeon's comfort and patient expectations.
|Table 2: Material, design and power of currently available mono focal toric intraocular lens|
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|Table 3: Material, design and range of power of commercially available multifocal toric intraocular lens|
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| Postoperative Management|| |
Patients undergoing TIOL implantation do well with excellent visual outcomes; however, these lenses can rotate after surgery, especially in the early postoperative period, with a majority occurring within 10 days after surgery.
Accuracy of IOL alignment depends on the precision of marking of the target axis, uniformity of IOL overlap with the capsulorhexis, completeness of removal of OVD, water tightness of incisions, and on a stable IOP. Stability also depends on the material, design, and overall diameter of the IOL, axial length of the eye, and capsule contraction. Hydrophobic acrylic IOLs showed the highest capsular adhesive properties, followed by hydrophilic acrylic IOLs and finally silicone IOLs. Limiting physical activities in the first 24 h after surgery has also shown to reduce the chance of IOL rotation.
In a study conducted by Miyake et al. on AcrySof toric IOLs, postoperative rotation of >10° was observed in only 1.68% of eyes after 2 years Lee and Chang et al. found that nearly all of the TIOL rotations occurred within the first postoperative hour. The mean rotation was 3.79° for tecnis TIOLs and WTR axis IOL orientation was more associated with >10° rotation, which could reflect the effect of gravity on haptic position for vertically oriented IOLs. In their study, the Tecnis TIOLs predominantly rotated counter-clockwise irrespective of the IOL's axis. Postoperative toric IOL misalignment was the major factor for unsatisfactory visual outcomes after surgery, with realignment needed in up to 0.65%–3.3% of cases.
The IOL position should be confirmed postoperatively if a significant difference is found between expected and measured postoperative refraction. The axis of implanted TIOL may be assessed in the postoperative period at the slit lamp with a rotating slit and rotational gauge. The 10° steps on the slit lamp measuring reticle limit the accuracy of this method. Another easy way to measure the axis of TIOL is using various camera-enabled cellular phones and computer software.
In cases of postoperative misalignment, www.astigmatismfix.com can be used to determine the axis of TIOL realignment. This calculator uses the patient's postoperative refraction, IOL power, and axis to predict the new axis of alignment. Barrett Rx Formula can also be used to determine possible piggyback IOL and ideal IOL axis.
Early rerotation tends to lead to improved outcomes. In cases where the lens is felt to be at high risk of repeat rotations, the IOL optic may be captured in the capsular bag to improve rotational stability. In cases with larger amounts of postoperative refractive error which cannot be completely negated by IOL rotation alone, options include IOL exchange, piggyback IOLs, or corneal ablative procedures.
Newer modular IOL systems in clinical trials, such as the ClarVista Harmoni, which have an outer base with haptics and an inner removable optic, would theoretically allow for easier IOL explantations. Another technology to manage postoperative refractive outcomes is the Light Adjustable Lens (LAL, RxSight). When it is selectively exposed to ultraviolet light, it undergoes spherical and cylindrical power changes and allows the surgeon to adjust the final refraction after the lens has stabilized in the eye.
| Conclusion|| |
The outcome of TIOL implantation has improved dramatically in recent times. Improved corneal measurement techniques, predictive IOL formulas, newer IOL designs and intraoperative alignment techniques have helped surgeons to provide excellent uncorrected visual acuity.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3]