The evolution of refractive cataract surgery and refractive lens exchange (RLE) has resulted in decreased tolerance for postoperative astigmatism, with patients often expecting spectacle independence after surgery.1 Astigmatism following cataract surgery consists of two components: preoperative astigmatism, which is intrinsic to the patient, and surgically induced astigmatism (SIA), which is a result of the procedure.
In an effort to control astigmatism in the surgical setting, the cataract surgeon must endeavor to know as much as possible about the source of the astigmatism. Intrinsic whole-eye astigmatism is predominantly due to corneal and lenticular astigmatism,2 whereas any procedure that alters the patient’s intrinsic astigmatism causes SIA.
It is systematic for the surgeon to perform keratometry— and in some cases corneal topography—to determine the patient’s preoperative intrinsic corneal astigmatism. However, it is less commonly practiced but equally necessary for the conscientious cataract surgeon to evaluate SIA. The amount of SIA, which is determined by the surgeon’s phacoemulsification technique and principally the type of incison used,3-5 is calculated by comparing pre- and postoperative keratometry values with vector or polar analysis,6,7 both of which are described below.
Standard keratometry should be used cautiously as a sole guide to astigmatism planning, as reliance on the IOLMaster (Carl Zeiss Meditec, Jena, Germany) alone may lead to over-correction when certain types of astigmatism are present.8 Topographic measurement of corneal astigmatism is the current standard of care because, in addition to measuring corneal astigmatism, it identifies irregular astigmatism, which may limit optimum surgical results.8
SURGICALLY INDUCED ASTIGMATISM
Factors affecting SIA include the size,3 site,4 and configuration5 of the incision, as well as preexisting astigmatism, 9 laterality of the eye,5 and the patient’s age.10 Attempts to correct for posture-related ocular cyclotorsion do not influence the calculation of SIA or its variance.11 The surgeon must know the meridian of the incision site when calculating SIA. Therefore, calculating SIA should ideally control for these sources of variability.
SIA CALCULATION METHODS
Refractive data are usually written as sphere, cylinder, and axis. This conventional format may characterize a single refraction but is not suitable for statistical analysis. There are no problems with analysis of the spherical component; the difficulties reside with the astigmatism, which is characterized by a magnitude expressed in diopters and a direction reported in degrees. For statistical analysis, these incommensurable entities must be converted to vectors or similar entities such as polar values.6,12 Below is a description of three SIA calculation methods.
Vector analysis. This method treats the cylinder as a vector (magnitude and direction). The refractive error, which is expressed as sphere, cylinder, and axis, is converted to a vector so that two or more vectors can be compared.7 An example of the difference in SIA calculated using simple arithmetic versus vector analysis is seen in Figure 1.
Polar analysis. Polar values were specifically developed for analysis of the astigmatic component of refractive surgery.12 The meridional polar value denoted by astigmatic polar value (AKP) expresses the surgically induced flattening, and the oblique polar value denoted by AKP(+45) indicates the torque. This pair of polar values characterizes regular astigmatism completely. In bivariate polar value analysis, the average SIA is the combined mean for AKP and AKP(+45), and the spread is a confidence area delineated by an ellipse.12
Online calculators. Two popular online SIA calculators are available at http://doctor-hill.com/physicians/ download.htm (East Valley Ophthalmology, Arizona) and http://www.insighteyeclinic.in/SIA_calculator.php (Insight Eye Clinic, New Dehli, India). Both of these free online calculators use the method of SIA calculation described by Holladay et al.7
Preoperative planning is crucial when attempting to manipulate astigmatic components, particularly SIA. To calculate the SIA associated with the surgeon’s basic phacoemulsification technique, we recommend using a consistent approach for approximately 30 cases and comparing pre- and postoperative data using one of the above techniques. The magnitude of SIA is often between 0.50 and 1.00 D depending on technique and other factors.3-5,9
Any time the surgical technique is modified, such as suturing a clear corneal incision (CCI), we recommend that the case be analyzed separately. Keeping a consistent technique allows the surgeon to have a reasonable estimation of SIA and thus plan to compensate for SIA and other astigmatic components. All corneal sutures should be removed before SIA is calculated.
When planning to correct astigmatism during cataract surgery, aim for a target induced astigmatism and check whether it matches the SIA. Any difference may be reported as the correction index (perfect correction, = 1.0; over-correction, > 1.0; under-correction, < 1.0).13
On-axis incisions. Some surgeons recommend use of a temporal incision to minimize SIA, as the temporal limbus is further form the visual axis than the superior limbus;4 however, this approach precludes the use of an on-axis incision to control SIA. Alternatively, the surgeon may elect to make the incision on the axis of steepest astigmatism. A disadvantage of this technique is that the surgeon may not have calculated the SIA for that given axis, which renders this technique inherently less predictable. Additionally, varying operative position may result in an uncomfortable and difficult surgical experience. Jiang et al14 found that patients with a temporal CCI had better postoperative UCVA than those with with an on-axis CCI. In eyes with preoperative astigmatism less than 2.60 D, on-axis CCI phacoemulsification induced slightly more flattening along the incision meridian than temporal CCI phacoemulsification, although the differences were not significant. The on-axis CCI was also associated with significantly greater absolute torque and angle of error.15
Opposite CCI. In addition to the on-axis incision, the surgeon may make a second CCI of similar dimensions to the original on the opposite corneal meridian. Tadros et al10 reported a mean SIA of 1.57 D (95% confidence interval; 1.42–1.71) using this technique.
Relaxing incisions. The term peripheral corneal relaxing incision (PCRI) has largely superseded the older term of limbal relaxing incision (LRI), as in this technique the peripheral cornea, not the limbus, is incised.2 In the context of a monofocal IOL, PCRIs are useful for treating 1.00 to 1.50 D of regular corneal astigmatism. The optical argument in favor of PCRIs is that a spherical cornea in front of a spherical IOL is better than a toric cornea in front of a toric IOL. PCRIs have the potential to render an eye free of astigmatism, whereas a toric combination induces distortion. However, when more than 1.50 D of correction is necessary, the incision size considerations favor toric IOLs over PCRIs.2
Unwanted under-correction may occur if PCRIs are made after the eye is penetrated.2 A special diamond or metal astigmatic keratotomy blade should be used, oriented perpendicular to the surface of the cornea. Traditionally, the PCRI depth goal is 90% of the minimum corneal pachymetry, but Amesbury et al2 suggest that a consistent PCRI depth of 450 μm is safe and effective when used with their nomogram in the cataract-age population.
Unlike PCRIs, which correct corneal astigmatism at the source, toric IOLs have an astigmatic component that compensates for corneal astigmatism in the IOL plane. The combination of corneal and lenticular astigmatism, however, can lead to unwanted image distortion.
Several toric IOLs are commercially available. The Silicone 1P Toric IOL models AA4203TL and AA4203TF (STAAR Surgical, Monrovia, California) are silicone toric capsular bag IOLs with a plate-haptic design. Chang et al16 reported that the AA4203TL, the longer of the two models, has better rotational stability in myopic astigmatic eyes, and Jampaulo et al17 reported no late rotation of either model.
The AcrySof Toric IOL (Alcon Laboratories, Inc., Fort Worth, Texas) is a one-piece, loop-haptic, acrylic capsular bag IOL. Depending on the model, this lens corrects for 1.50, 2.25, or 3.00 D of astigmatism in the IOL plane, corresponding to approximately 1.00, 1.50, and 2.00 D of toric correction in the corneal plane. Tsinopoulos et al looked at 29 eyes that underwent uncomplicated phacoemulsification and AcrySof Toric IOL implantation. Postoperative UCVA was 20/40 or better in 89.7% of eyes, and the mean toric IOL rotation was 2.2 ±1.5º (range, 0.6–7.8º) at 1 month and 2.7 ±1.5º (range, 0.9–8.4º) at 6 months. In 2009, versions with 3.75, 4.50, 5.25, and 6.00 D of cylinder at the IOL plane became available, enabling correction of corneal cylinder from 3.00 to 5.00 D.
In the Torica line of foldable silicone IOLs (HumanOptics GmbH, Erlangen, Germany), the Torica-s has z-shaped haptics, which are designed to provide rotarotational stability within the capsular bag; the Torica-sS has a larger, 14-mm overall diameter, which makes it suitable for ciliary sulcus placement; and the Torica-sPB is a piggyback toric IOL. All three models are available with selective blue-light filtration.
Rayner Intraocular Lenses Ltd. (East Sussex, United Kingdom) produces the T-Flex, a closed-loop haptic, acrylic toric available in two diameters. Carl Zeiss Meditec (Jena, Germany) manufactures three foldable acrylic toric IOLs (AT.LISA 909M, AT.LISA 909MV, and AT.TORBI), the third of which is a bitoric, aberrationcorrected model.2
For implantation of a toric IOL, a reference mark must be made at the limbus. The Blakewell BubbleLevel (Mastel Inc., Rapid City, South Dakota) has a spirit level that allows the horizontal meridian to be marked while the patient is seated in the upright position.11 The Gimbel/Mendez Fixation Ring and Boris Meridian Marker (both Mastel Inc.) are other options to mark the axis of astigmatism. Both are used under the operating microscope. At the end of the procedure, all viscoelastic material should be removed before the incision is sealed to reduce the chance of postoperative rotation.
Hoffman et al reported using intraoperative retinoscopy at the time of toric IOL implantation to check for skew of the cylinder axis.15 This is a relatively low-tech intraoperative method of screening for large astigmatic components, for example if a toric IOL is inadvertently positioned 90º from the planned orientation.
The intraoperative wavefront aberrometer (ORange; WaveTec Vision Inc, Aliso Viejo, California) checks the power of implanted IOLs and position of toric IOLs and guides corneal astigmatic correction procedures such as PCRIs with immediate objective measurements of effect during surgery.18 The device may also enable surgeons to calculate SIA in real time. However, certain situations limit the use of the ORange system, including small pupils, corneal and foveal disease, wound leakage, nystagmus, local anesthesia block, and use of lidocaine gel.18
Chen et al18 recently evaluated intraoperative refraction using the ORange Gen 1 (pseudophakic settings) and compared it with autorefraction (Humphrey automatic refractor keratometer Model 599; Carl Zeiss Meditec) performed 1 week after cataract surgery and IOL implantation. The mean intraoperative spherical equivalent (SE) was -0.90 ±0.66 D, whereas postoperatively the mean SE was -2.50 ±0.58 D. This represents a mean difference of -1.60 D, which the authors postulate may be due to the difference in pupillary sizes between the intraoperative pharmacologic mydriasis and the regular pupillary size postoperatively. They reported moderate to good correlation between the intra- and postoperative mean SE (Pearson correlation coefficient, r = 0.56; P<.01).
In a retrospective case-control chart review by Packer et al,19 30 eyes that received PCRIs without wavefront measurements (control group) were compared with 37 that had intraoperative wavefront measurements using the ORange system to determine if PCRIs were necessary (aberrometry group). In the aberrometry group, 21.6% of eyes underwent PCRI extension. Postoperatively, the control group had higher mean postoperative manifest cylinder (0.48 vs 0.37 D), but the means and distributions of manifest refractive sphere and SE were similar between groups. Excimer laser enhancement was performed in 3.3% of the aberrometry group and 16.2% of the control group. The authors reported that residual cylindrical refractive error, not sphere, determined the patients’ decisions to have enhancement.
Amesbury et all2 suggest a stepladder treatment of astigmatism at the time of cataract surgery. For less than 1.00 D, they recommend that the phacoemulsification incision be placed on the steep axis; for slightly more astigmatism (1.00–1.50 D), PCRIs may be considered. Further, we advocate that for even more astigmatism (1.50 to 5.00 D), toric IOLs should be the first option, with PCRIs or refractive laser available for enhancement if necessary.
PRK and LASIK can be used to correct residual astigmatism after cataract surgery. Jin et al20 analyzed 57 eyes treated for residual refractive error using LASIK, IOL exchange, or piggyback IOL implantation. UCVA was similar between groups, with 96% achieving 20/40 or better in the LASIK eyes and 93% in the combined group of IOL exchange and piggyback eyes.
UCVA improved significantly in both groups, and no eye lost more than 1 line of BCVA. With a similar length of follow-up, no significant difference in postoperative SE was found between groups (P=.453).
For the cataract surgeon who is not comfortable with corneal procedures, toric IOLs may represent an attractive option for controlling SIA. Toric IOL implantation has many advantages. For starters, it requires only basic equipment, and it is reversible. Additionally, a wide range of lenses is available today, and the issue of initial rotation is much less problematic with the current models than it was in the past.
We recommend that any surgeon embarking on use of toric IOLs consider the following steps: obtain a corneal topographer, calculate SIA using vector or polar analysis for nontoric monofocal lenses, become familiar with toric lens repositioning and PCRIs, and perhaps have a refractive surgeon colleague who can assist with refractive laser when necessary.
SIA is a sample mean, and therefore one must recognize that there are limitations when applying this principle in an individual. Intraoperative retinoscopy is a cheap and effective safety measure, and the advent of intraoperative aberrometry is encouraging. The latter requires further evaluation before it can be recommend for the management of SIA.
Ian John Dooley, MB, BAO, BCh, MSc, MRCSI (Ophth), MRCOphth, practices at Cork University Hospital, Cork, Ireland. Dr. Dooley states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: email@example.com or iandooley@ eustace.net.
Editor’s Note: The author would like to thank Stephen Beatty, MD, FRCOphth, for his assistance with this research.
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