© Springer International Publishing AG 2018Jean-Luc Febbraro, Hamza N. Khan and Douglas D. Koch (eds.)Surgical Correction of Astigmatismdoi.org/10.1007/978-3-319-56565-1_3
3. Ten Key Points to Optimize Surgical Correction of Astigmatism
Rothschild Foundation, Paris, France
Cullen Eye Institute, Baylor College of Medicine, Houston, TX, USA
University of British Columbia, Victoria, BC, Canada
3.1 Preoperative Evaluation of Astigmatism
Preoperative measurements of corneal astigmatism need to be sufficiently accurate to reduce preexisting astigmatism to within 0.50 (multifocal IOLs)–0.75 D (monofocal IOLs). Our diagnostic tools include manual or autokeratometers, optical biometry, and, importantly, topography or tomography. A prerequisite condition to guarantee the quality of the measurements is a healthy cornea, without any surface irregularities caused by either deficient tear film or corneal pathology:
Corneal astigmatism can be identified with manual and autokeratometers in a repeatable manner. However, these instruments are insufficient because they only measure four points in the central 3 mm of the cornea and are unable to detect astigmatic asymmetries, irregularities, posterior corneal, nor lenticular astigmatism.
Optical biometry provides magnitude and axis measurements at various optical zones (1.65, 2.3, or 3.3 mm depending on the instrument) with variable numbers of points (6, 18, and 32).
Corneal topography has become a mandatory test prior to toric implantation as it allows for the detection of asymmetric and irregular astigmatism. Comparative studies between manual and automated keratometry, Placido-type topography, and simulated keratometry of Scheimpflug systems showed similar results in terms of anterior corneal magnitude, but axis differences were noted [1–3]. Corneal topographers may usually be considered as the final judge in terms of axis, pending verification of the image quality.
Total corneal astigmatism includes anterior and posterior components of the cylinder. Previous methods measured the anterior component only, whereas slit-scanning technology, optical coherence tomography, and Scheimpflug imaging systems allow for the measurement of both anterior and posterior astigmatism. These newer systems use true refractive indices to calculate the anterior and posterior corneal powers (1.376 for the cornea and 1.336 for the aqueous), instead of a standardized corneal refractive index of 1.3375 . Accuracy is still suboptimal, but these devices hold the promise that they can be used to reliably measure posterior astigmatism and optimize the estimation of total corneal astigmatism.
Preoperatively, the different instruments previously described work in symbiosis to give the most accurate measurement of the patient’s astigmatism. The concordance of axis and magnitude of the cylinder, calculated by these instruments, is a key element to optimize the astigmatic correction.
3.2 Patients Selection for Toric IOLs
The surgical correction of astigmatism during cataract surgery, formerly restricted to incisional techniques, has become more widely diffused and accepted, thanks to the availability of toric intraocular lenses. Arcuate incisions, located either in the mid-periphery or adjacent to the corneal limbus, are now generally reserved for treatment of astigmatism up to 1 D. Toric lenses allow for the correction of a more pronounced magnitude corneal astigmatism [5, 6]. Among cataract patients, the prevalence of corneal astigmatism of more than 0.75 D reaches 30%, whereas 22% have more than 1.50 D of cylinder and 8% more than 2D [4, 7, 8]. The surgical management of the varying degrees of astigmatism is crucial in optimizing the postoperative uncorrected visual outcome of our cataract patients:
Patient motivation is the first point to assess during the preoperative cataract consultation. It may vary from zero (patient is happy to wear glasses on a daily basis), to medium (curious to learn more about astigmatic treatments), or high (strong desire to no longer need astigmatic correction). The surgeon needs to provide an adequate response to satisfy the patient’s wishes and expectations.
The patient’s profile is the second most important aspect. Patients with nonrealistic exceptions should be specifically managed to ensure a successful outcome. A realistic setting of expectations is important prior to the surgery, underlining the fact that astigmatism correction provides a reduction in spectacle dependence, not a guarantee of spectacle independence.
Associated corneal or ocular pathologies need to be considered, particularly in cases of dry eyes and corneal dystrophies, such as map-dot-fingerprint dystrophy, as they may increase the chances of artifacts during the preoperative testings or be associated with inaccurate and unstable postoperative refractive outcomes. In these particular cases, toric implantation may be considered but only after a proper treatment of dry eyes and corneal surface pathology. It is to be avoided in patients with certain corneal dystrophies and patients who may require penetrating keratoplasty or DSAEK. It is unclear at this time if DMEK will induce sufficiently small amounts of astigmatism that toric IOLs can be implanted either before or concurrently with this procedure. The stability of the capsular bag should also be examined carefully, as an unstable or subluxated lens is a contraindication for toric IOLs.
The regularity and symmetry of the astigmatism are also important criteria to analyze during the preoperative exams. Toric implantation is ideally indicated for patients with regular and symmetrical corneal astigmatism. Keratoconic patients or post-penetrating keratoplasty corneas may also benefit from toric lenses; however, the astigmatism needs to be stable with low degrees of irregularity in the central portion of the cornea , and the patient needs to be advised of the reduced predictability.
The magnitude of corneal astigmatism is another crucial point to take into consideration. As the final goal is to provide optimal uncorrected postoperative vision at a certain distance, it implies a minimal degree, 0.50 D or less, of postoperative residual astigmatism. Cataract patients with preoperative corneal astigmatism of 0.75 D or more may be therefore good candidates for toric IOLs. In such patients, postoperative uncorrected vision and spectacle independence achieved with toric IOLs are superior compared to monofocal lenses .
3.3 Cataract Incisions and Surgically Induced Astigmatism
An incision on the cornea induces flattening in the incised meridian and steepening in the meridian 90° away. This effect is known as surgically induced astigmatism (SIA). The term SIA is the difference between the amount and the direction of corneal steepening between pre- and postsurgery. The flattening effect corresponds to the flattening at the site of incision, which is calculated by vector analysis, based on pre- and postoperative keratometry. The astigmatic changes are correlated with the incision location, architecture, and size. The larger the incision and the closer to the visual axis, the greater the corneal astigmatic changes [11–15].
Astigmatic analysis can be simple but at the same time extraordinarily complex . The corneal changes inherent to a cataract incision may be measured with a simple but not precise algebraic method or with a more sophisticated and accurate vector analysis. Several methods have been developed to analyze surgically induced astigmatism [17, 18]. They require pre- and postoperative keratometric K readings which include the magnitude and the axis of the cylinder. They allow for the calculation of the surgically induced astigmatism, detail the effects at the site of the incision along with 90° away, and, also, include the spherical corneal change. Most studies published until now used appropriate vector analysis but reported mean/median of the vector magnitudes, without taking into account the angular direction. Instead, SIA should be calculated as a vector, with both magnitude and angle, using aggregate analysis and represented by the centroid of the various vectors on a double angled plot. Aggregate analysis shows significantly lower values for mean SIA. In practical terms, a 2.4 mm or less temporal clear corneal incision induces a mean of 0.1 D, which is almost negligible.
3.4 Posterior Astigmatism and Clinical Implications
Total corneal astigmatism includes not only the anterior portion but also the posterior part of the cornea. The importance of the posterior surface of the cornea was first described by Javal and recently highlighted by Koch [19, 20]. In optical terms, the posterior cornea can be is a minus lens, with a steep vertical meridian in almost 80% of the cases, which creates against-the-rule ocular astigmatism that is relatively stable over time. Posterior corneal astigmatism therefore partially compensates anterior with-the-rule astigmatism, as is common in younger patients, but increases total astigmatism when the anterior cornea has against-the-rule astigmatism, as often occurs in older individuals .
The precise correction of astigmatism requires accurate measurement of net corneal astigmatism with adequate diagnostic instruments. The majority of diagnostic corneal devices, such as manual or automated keratometers, and Placido disc corneal topographers, measure corneal astigmatism based upon the anterior corneal power. New imaging modalities, such as Scheimpflug imaging, OCT, and reflection technology used by a color LCD topographer, can be used to measure posterior corneal astigmatism, but the accuracy of these measurements has not been validated in clinical studies.
Studies have shown that posterior astigmatism contributes significantly to total corneal astigmatism and cannot be predicted correctly with instrumentation measuring the anterior corneal surface. The mean magnitude of posterior astigmatism is approximately 0.30 D but may surpass 0.50 D in almost 10% of the patients. Posterior astigmatism can reach 0.80 D or more in corneas that have anterior with-the-rule astigmatism and up to 0.50 D in similarly detected against-the-rule astigmatism .
In clinical terms, to reduce the chances of refractive errors in patients selected for toric implantation, it is necessary to consider posterior astigmatism. Posterior corneal astigmatism is generally oriented vertically, so its precise assessment would reduce overcorrection in eyes with with-the-rule anterior astigmatism and undercorrection in eyes with against-the-rule anterior astigmatism .
In lieu of the ability to directly measure posterior corneal astigmatism, a number of nomograms have been developed. These estimate the magnitude of posterior corneal astigmatism based on anterior corneal measurements. The first was the Baylor Nomogram, and subsequently other formulas for incorporating estimated posterior corneal power have been developed, including the Barrett toric IOL formula, the Abulafia-Koch formula, and online toric calculators from Abbott Medical Optics and Alcon.
3.5 IOL Calculators
The calculation of the toricity of the intraocular lens in astigmatic patients requires specific toric calculators. The cylindrical power calculation of the lens depends on (1) the total corneal astigmatism, (2) the surgically induced astigmatism of the cataract incision (which as noted above can be entered as 0.1 D for temporal clear corneal incisions at size 2.4 mm or smaller), (3) the effective lens position, and (4) the IOL power. Most toric IOL manufacturers have their own online toric IOL calculators, which provide satisfactory results in the majority of cases. Until recently, none took posterior astigmatism into account. We recommend selecting a calculator that takes into all four factors, and these include:
Holladay Consultant with box checked for posterior corneal astigmatism (which then uses the Baylor nomogram)
Barrett Universal II
Abbott Medical Optics online toric IOL calculator, which was developed from the Baylor nomogram combined with outcome data from AMO’s clinical studies on toric IOLs
Alcon surgical online toric IOL calculator, which uses the Barrett formula
Abulafia and Koch have also developed a formula that uses vector calculations to calculate total corneal astigmatism from anterior corneal measurements. This is being incorporated into the online calculators of some toric IOL companies and will also take into account all four factors .
3.6 Manual and Objective Marking Devices to Compensate for Cyclotorsional Movements
Torsional movements became a subject of interest in the middle of the twentieth century . Numerous studies were performed particularly in the past decades with the onset of new manual techniques and automated technologies developed to measure and compensate for cyclotorsion [24, 25]. Cyclotorsional movements are rotational movements of the eye around the visual axis. They include excyclotorsion when the eye rotates temporally and incyclotorsion when the rotation is nasal.
Preoperative measurements in cataract patients are performed with the patient in a seated position, whereas during the surgery the patient is in a supine position, and the change of position may induce static cyclotorsion. Studies have shown that this position change can cause a mean cyclotorsion of approximately of 3°, with maximum values up to 14° in some patients . Such rotational movements have two adverse optical effects. First they reduce the amount of astigmatic correction along the intended meridian, and, second, they create increasing amounts of astigmatism at an ever-diverging new meridian. For example, 1° of misalignment induces 3.5% of residual astigmatism at a slight difference, and a 30° error leaves the original magnitude of the astigmatism unchanged by shifts to a new meridian. For these reasons, it is crucial to diagnose and compensate cyclotorsion in order to match the marks of the toric IOL with the preoperative calculated axis.
Several techniques and devices are available. They include either manual marking systems or more modern inkless marking technologies.
Manual techniques are based on ink marking devices:
First step is a preoperative marking of the horizontal (0–180°) axis, at the slit lamp or with the patient in the upright position. The patient is asked to look at a distance object with the nonoperated eye in order to avoid cyclotorsion linked to accommodation. One of us (DDK) then takes the additional step of examining the marks and making notation of any deviation from the intended location; this compensates for any imprecise marking that all too often occurs.
Second step takes place at the beginning of surgery, with the patient horizontal on the bed, and under the microscope. The surgeon aligns the preoperative marks with the horizontal meridian of a graduated degree marker.
Third step is the marking of the desired toric IOL axis placement, with a thin ink marked forceps or hook.
Particular attention must be made with regard to the type of ink pencil, as a large ink dot, 20° large, for instance, could already be a source of axis error during the preoperative and intraoperative marking steps. Among the marking instruments, the surgeon may choose between a bubble type, pendular marker, tonometer, slit-lamp, or LED-assisted marker. Studies have shown that pendular marker or slit lamp technique seems more accurate . However, any type of eye marking device is a potential source of error, and particular attention should be paid to the choice of each instrument used during the three steps.
More recently, imaging softwares have replaced ink-based methods. They are able to capture high-resolution preoperative images which allow for anterior segment analysis, in particular, iris details, limbal, and scleral vessels . The preoperative image is taken during the IOL calculation exam and transferred to the operating room via the hospital’s intranet. In the operating room and pre-surgery, the reference image is transferred into the microscope and compared with the intraoperative image (patient is horizontal on operating bed) to detect any eventual cyclorotation. This technology allows for a measurement of the degree of cyclotorsion and, simultaneously, «real-time» tracking of the operated eye to secure alignment of the toric IOL at the intended meridian. These tracking informations are displayed on an external screen throughout the surgery, as well as in the microscope ocular by a digital overlay.
Despite the appeal of automated devices, there are no studies demonstrating superiority to manual marking techniques. Montes de Oca and colleagues reported a mean error of less than 3° for both manual and an automated marking technique, with no errors greater than 10° in either group . Ultimately we believe that automated markers will become widely used due to their convenience and ease of use.
3.7 Manual Arcuate Keratotomy
Corneal incisions used to correct astigmatism were first described in the late 1800s by Lans and were the sole surgical option to correct astigmatism for more than a century . They were challenged by the development of excimer laser platforms in the mid-1990s and more recently by the introduction of toric IOLs. However, relaxing incisions are valuable option today for our cataract and refractive patients.
As a principal, astigmatic incisions are placed perpendicular to the steep axis and induce a flattening at the incised meridian, with an associated steepening 90° away. This induced corneal change is called coupling effect. The coupling ratio is close to one for the majority of arcuate incisions, which means the spherical equivalent remains almost unchanged . Longer astigmatic incisions, close to 90° and more central ones, tend to have a 2:1 or more coupling ratio with induced hyperopic shift. They were originally placed at the mid-periphery of the cornea (7-mm optical zone), initially straight (T-cuts). Arcuate keratotomies (AK) were introduced and deemed to be more effective as the entire incision is equidistant from the visual axis. Factors increasing the magnitude of astigmatic correction include smaller radial distance from the visual axis or pupillary center, greater incision length, greater incision depth, increasing patient age, and unpredictable wound healing responses. Over the years, incision placement has tended to shift more peripherally to zones of 8 mm or greater, so-called peripheral corneal-relaxing incisions (PCRIs). This appears to reduce the likelihood of inducing irregular astigmatism and inherent visual disturbances.