Patients presenting for today's cataract surgery are not only expecting improved vision; they are also expecting spectacle independence. These high patient expectations have increased the demand for surgeons to minimize postoperative refractive error. Correction of spherical error is achieved by choosing the correct power of intraocular lens (IOL) based on the determination of accurate axial length and correct corneal power. Corneal astigmatism is another refractive error that the cataract surgeon must manage and minimize in order to achieve postoperative emmetropia. For true spectacle independence, patients require less than or equal to 0.50 D of astigmatism after surgery. There are various options for reducing corneal astigmatism at the time of cataract surgery and surgeons need to adopt one of multiple approaches based on the amount of preoperative corneal astigmatism. Several recent studies have evaluated the effectiveness of different techniques for the reduction of astigmatism during cataract surgery. Some studies have compared the techniques with each other, whereas others have evaluated the effectiveness of combining different procedures. This review will describe various approaches to the correction of corneal astigmatism during cataract surgery and focus on new technologies aimed at improving refractive outcomes.
The first step to managing astigmatism is a comprehensive preoperative assessment, which must account for the magnitude of astigmatism, location of the axis of cylinder, patient age and the refractive status of the other eye. For spectacle independence, most cataract surgeons aim to treat preoperative corneal astigmatism greater than or equal to 0.75 D.
Several methods are available for evaluating astigmatic refractive error and studies have been done to identify the most reliable and accurate techniques for planning correction. Preoperative assessment starts with the manifest refraction to identify total astigmatism, which includes both lenticular and corneal astigmatism. Because lenticular astigmatism is eliminated with the removal of the cataractous lens, only the correction of preoperative corneal astigmatism is necessary. Therefore, accurate assessment of corneal astigmatism is needed. Manual keratometry, partial coherence interferometry (IOLMaster) and Optical Low-Coherence Reflectometry (Lenstar), corneal topography and corneal elevation mapping may be utilized in planning for the correction of corneal astigmatism. In a recent comparison of four different keratometric methods, a manual keratometer, IOLMaster, Pentacam and autokeratometer were all found to be equally satisfactory in measuring preoperative corneal astigmatism . Others found comparable residual astigmatism in postoperative toric IOL patients who had preoperative planning with either manual keratometry or dual-zone automated keratometry with the Lenstar LS 900 biometer (Haag-Streit International, Koeniz, Switzerland) . New studies have also used a Fourier-domain ocular coherence tomography (OCT) system to measure corneal power . Corneal topography has become a critical tool for identifying the magnitude and axis of astigmatism because it evaluates the entire cornea and can detect irregular astigmatism. More advanced technology utilizing cornea elevation mapping such as the Oculus Pentacam (OCULUS Optikgeräte GmbH, Münchholzhäuser Str. 29, D-35582 Wetzlar, Germany) and Ziemer Galilei (Ziemer Group, Port, Switzerland) evaluate both the anterior and posterior corneal surfaces and may improve the estimation of preoperative astigmatism. There is now evidence that neglecting the posterior corneal surface astigmatism may lead to inaccurate corneal astigmatism estimations in some eyes . Douglas Koch, in his 2012 Charles D. Kelman Innovator's lecture emphasized the importance of posterior corneal effects on total astigmatism and the necessity of accurate preoperative measurement of the posterior corneal shape. In our experience, the best assessment of corneal power comes from careful manual keratometry and the best assessment of astigmatic axis comes from the IOL master interferometry Ks combined with qualitative guidance from corneal topography and corneal elevation mapping.
Once the steep axis of astigmatism is identified preoperatively, the patient's eye must be accurately marked intraoperatively in order to correctly place peripheral corneal relaxing incisions (PCRIs) on axis or align a toric IOL. In surgery, the surgeon manually marks the 6 o’clock and/or the 3 and 9 o’clock positions on the cornea while the patient is sitting upright looking straight ahead with both eyes open to avoid cyclotorsion. Miyata et al. used corneal topography to accurately place reference marks made at the slit lamp for intraoperative identification of the steep axis of astigmatism and found that the topography-based technique increased the accuracy of PCRIs.
Other methods involve digital image capture of iris or conjunctival vessel patterns and mapping the steep axis of astigmatism in relation to these landmarks. SMI Surgery Guidance Technology [SensoMotoric Instruments (SMI) GmbH, Teltow, Germany] links the patient's preoperative diagnostic testing data to the operative procedure without use of manual markers. A reference unit in the office captures images and data such as keratometry and the limbus and pupil location. This information is transferred to the SMI unit in the operating room via a USB memory drive. Real-time eye registration and tracking is generated in the surgeon's microscope view or on a flat screen monitor. This technology helps to provide a consistent coordinate system for axis guidance when placing PCRIs or aligning toric IOLs .
The TrueVision system (TrueVision Systems Inc., Santa Barbara, California, USA) is a stereoscopic high-definition visualization system that displays the surgical field of view in real-time on a three-dimensional flat-panel display in the operating room. The patient's cornea is captured with a three-dimensional image preoperatively in the examination room. Then in the OR, the image is projected onto the view from the surgical microscope. Virtual guidance lines can be superimposed over the limbus to aid LRI incisions and a template can be superimposed over the limbus to aid in toric IOL placement .
SELECTION OF SURGICAL OPTIONS TO CORRECT ASTIGMATISM
For many surgeons the method for astigmatism correction is chosen with a step-ladder approach based on the magnitude of preexisting corneal astigmatism as determined by corneal keratometry and topography [8▪,9]. Less than 1 D of corneal astigmatism can be corrected by placing the clear corneal cataract incision on the steep axis. Incisions as small as 2.4 mm have been shown to flatten the cornea up to 0.5 D in the axis in which they are placed . Longer incisions may be used to correct larger amounts of astigmatism. One to three D of corneal astigmatism can be corrected with PCRIs. Toric IOLs can correct 1–4.5 D of corneal astigmatism. Toric IOL and PCRIs can be combined to correct up to 7 D of corneal astigmatism. Ouchi and Kinoshita  combined toric IOL implantation with PCRIs during bimanual phacoemulsification for eyes with astigmatism more than 2.50 D. They found the technique to be effective with the advantage of using shorter PCRIs with toric IOLs. Eyes with high levels of astigmatism, up to 12 D, can also be corrected with high-power toric IOLs, which are currently only available in Europe, but can be obtained through a compassionate use Food and Drug Administration (FDA) exemption and local institutional review board approval.
Several recent studies have compared the efficacy of various techniques for managing astigmatism at the time of cataract surgery. Mendicute et al. compared toric IOLs with paired, full thickness clear corneal incisions placed 180° apart and found that the two techniques were similar in correcting astigmatism. However, toric IOLs performed slightly better for astigmatism correction of more than 1.2 D. Poll et al. found that PCRIs and toric IOLs performed similarly, but toric IOL was favored for higher degrees of astigmatism (>2.00 D). They concluded that the toric IOL produced better precision and less risk of overcorrection and irregular astigmatism compared with PCRIs. Mingo-Botin et al. also compared PCRIs with toric IOL and found that toric IOL was more effective and predictable with greater spectacle independence and better contrast sensitivity under mesopic conditions with glare. They found no significant difference in visual function scale ratings or patient satisfaction.
The following surgical techniques may be used independently or in combination and are chosen based on the step-ladder approach as described above.
On-axis corneal incision for cataract surgery
A full thickness corneal incision for cataract surgery flattens the cornea in the meridian of the incision and therefore can reduce preexisting astigmatism . The incision is made on the steep axis of astigmatism. The length of the incision can vary based on the magnitude of astigmatism. This is a good approach for correcting small amounts of against-the-rule astigmatism with a temporal incision. On-axis cataract surgery is being used less often because it may require uncomfortable surgical positioning to be on axis and only minimally corrects astigmatism compared with other available options.
Manual peripheral corneal relaxing incisions
PCRIs are a commonly adopted method for managing astigmatism during cataract surgery. These incisions are created in the peripheral clear cornea. The incisions are created based upon the use of an established nomogram that varies based upon the amount of astigmatism, the patients’ age and the location of the steep astigmatic axis. The incisions can be single or paired and the surgical technique can vary based upon the choice of markers, blades and nomograms. The PCRI incisions are usually created at the beginning of cataract surgery when the epithelium is still in good condition and the ocular pressure is predictable. The incisions are created at a 90% depth with either a fixed depth blade or a variable depth single foot plate double cutting diamond blade that is set based on intraoperative pachymetry readings. Two caveats are important to avoid intersecting incisions: if the PCRI incision is co-incident with the temporal clear corneal cataract incision, the PCRI should limited to the length of the cataract incision and only lengthened after the IOL is in place at the end of the case. If the PCRI is near the paracentesis incision, the paracentesis should be made peripheral to the PCRI to avoid intersecting the incisions. PCRIs have resulted in reduced corneal astigmatism and improved vision for patients undergoing standard phacoemulsification through a single astigmatically neutral temporal incision [16▪]. When combined with bimanual microincision cataract surgery (MICS), PCRIs can correct corneal astigmatism with predictable accuracy .
Currently, Toric Multifocal IOLs are not available in the USA, therefore it is important to manage astigmatism in patients receiving multifocal IOLs in order to achieve spectacle independence. Muftuoglu et al. assessed the refractive and visual outcomes of patients who had PCRIs at the time of apodized diffractive multifocal IOL placement and found that PCRIs were safe and effective at reducing astigmatism in these patients.
Femtosecond laser peripheral corneal relaxing incisions
During femtosecond laser-assisted cataract surgery, PCRIs can be strategically placed using the laser . Laser-created PCRIs may prove more effective in correcting astigmatism because the length, location and depth of these incisions are better controlled. However, data to support this are not yet available. Another potential advantage of these incisions is that they can be selectively opened postoperatively to titrate their astigmatic effect.
Toric intraocular lenses
The STAAR toric IOL was the first toric IOL approved by the FDA in 1998, but the lens position was sometimes unstable and it frequently rotated off axis . In 2005, the one-piece acrylic Acrysof Toric IOL (Alcon Laboratories Inc., Fort Worth, Texas, USA) was FDA approved. The lens has excellent rotational stability, with less than 4° of rotation at 1 year .
Toric IOLs are considered for patients with at least 0.75 D of regular corneal astigmatism who desire spectacle-free distance vision after cataract surgery. Acrysof lenses with astigmatism correction as high as 4.11 D at the corneal plane are now available in the USA. A recent study by Visser et al. evaluated the effectiveness of Acrysof Toric SN6AT6 to SN6AT9 model IOLs to correct cylinder powers ranging from 2.50 to 4.50 D. They found that these IOLs were effective and safe at correcting astigmatism in eyes with more than 2.25 D of corneal astigmatism. Toric IOLs are inappropriate for patients with irregular astigmatism, higher order aberrations, or lens capsule instability, from prior trauma [23▪]. These lenses can only be placed in an intact capsular bag, not in the sulcus.
Toric intraocular lens alignment
It is recommend to use the toric IOL that will leave the lowest amount of residual astigmatism, regardless of axis [23▪]. Preoperative evaluation includes accurate keratometry to identify the magnitude and axis of astigmatism. For the Alcon Toric IOL, the astigmatic power and the axis of alignment is identified using an online calculator such as the AcrySof toric calculator from Alcon http://www.acrysoftoriccalculator.com/ (Alcon Fort Worth, Texas, USA). The correct axis must be marked on the eye in order to get appropriate toric IOL alignment. Several methods are available including manual marking and digital capture of ocular reference points as is used in with True Vision and SMI technology. Also, anterior segment OCT can be used for toric IOL alignment and evaluation of postoperative rotational errors . Mingo-Botin et al. compared slit lamp estimation of axis alignment with software-based image analysis of toric IOL alignment and found no significant difference between these two methods used to monitor stability of toric IOLs.
Postoperative IOL rotation significantly decreases the effectiveness of toric IOLS. There is a 3.3% loss of astigmatic correction for every 1° a toric IOL is off axis. Patients with long axial lengths have a higher risk of postoperative toric lens rotation . Fortunately, rotation is less of a problem with newer lens designs. If an IOL is malpositioned because of rotation, tilt or trauma, it can safely be repositioned in the operating room up to 2 weeks after placement . Under- or overcorrection can also be managed with postoperative excimer laser or incisional surgery.
Intraoperative aberrometry devices such as the ORA (formerly ORange) (WaveTec Vision, Aliso Viejo, California, USA) capture wavefront refractive information in real-time during cataract surgery. The data aid in selecting the spherical and cylinder power of the IOL as well aiding in toric IOL alignment. Intraoperative aberrometry is especially useful in patients with prior refractive surgery or in patients that cannot provide adequate preoperative measurements.
The ORA attaches to the operating room microscope and measures wavefront data in the aphakic and pseudophakic state. The intraocular pressure (IOP) is adjusted to within the physiologic range, near 16 mmHg. The patient is instructed to focus on a red fixation light while measurements are taken by the ORA system. In seconds, recommendations for IOL power or toric IOL alignment are displayed. When the measurements are performed at the end of cataract surgery, it is important to confirm physiologic IOP and avoid over hydration of corneal wounds that can artifactually induce astigmatism. Also, the lid speculum and patient eyelid squeezing can induce changes in corneal astigmatism potentially decreasing the accuracy of intraoperative aberrometry .
Overall, intraoperative aberrometry seems to improve refractive outcomes in cataract surgery. Wiley and Bafna  report a mean absolute postoperative spherical equivalent error of 0.36 ± 0.30 D in 215 eyes using the ORange system. Tran  report good refractive outcomes using ORange for post-Lasik patients undergoing cataract surgery. Packer  used intraoperative aberrometry to measure and enhance the effect of PCRIs at the time of cataract surgery and found a trend toward reduction of subsequent excimer laser enhancement in the aberrometry group of eyes. More data are needed to evaluate the efficacy of aberrometry and identify factors that may induce variability in measurements.
In conclusion, the development of new technology is driven by patient expectations for spectacle independence. Careful evaluation of corneal astigmatism preoperatively and intraoperatively with the aid of new technology is likely to improve surgical outcomes for astigmatism correction at the time of cataract surgery.
The authors have no acknowledgments.
Conflicts of interest
The authors have no conflicts of interest to disclose.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 80–81).
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