III. TREATING PREEXISTING ASTIGMATISM AT THE TIME OF CATARACT SURGERY
Contemporary cataract incisions cause minimal astigmatism, which allows surgeons to focus on correction of preexisting astigmatism in the quest for emmetropia. Corneal biomechanics allow for predictable correction of regular astigmatism with minimal distortion and irregular astigmatism. The layers of the cornea consist of collagen fibrils extending from limbus to limbus with tension evenly distributed along the length of the fiber. In the midcentral corneal stroma, fibrils orient along horizontal and vertical meridians. The fibrils become thicker closer to the limbus and appear to be continuous with the sclera. Hundreds of sheets, or lamellae, of the fibers are regularly packed but are loosely connected to each other, allowing for sliding of the layers and redistribution of tension by bifurcating obliquely oriented fibrils and interspersing proteoglycans.It is possible to manipulate these properties of the cornea to change its shape.
A. Effect of Incisions on Corneal Tissue
One can flatten the cornea in a steep meridian and reduce optical power by relaxing fibrils in that meridian with a perpendicular incision. The degree of flattening is proportional to the number of fibers cut and can be varied by depth of incision and its length. Due to interconnection of corneal fibers and redistribution of tension, incisions also will affect the meridian 90 degrees away. This is called coupling. The effect depends on the type and orientation of the incision.An ideal incision has a coupling ratio of 1:1 (flattening of the incised meridian divided by steepening of the opposite meridian), resulting in a zero spherical shift. Indeed, it was found that 45- to 90-degree incisions in the 5- to 7-mm optical zone have those properties. Most incisions in younger individuals have a coupling ratio of less than 1 leading to small overall steep- ening. The keratometric coupling ratio increases with age and higher preoperative K’s but data are limited and the effect is small.
Due to limited sliding of the fibrils, the effect of an incision is most pronounced near the incision. Consequently, the closer the incision is to the visual axis, the higher the effect of the incision on the visual properties of the eye. Incisions closer to the visual axis are also more likely to induce irregular astigmatism. The redistribution of fibril tension tends to smooth out such irregularities at more peripheral incisions. Because of corneal shape and structure, ATR astigmatism is more easily corrected by smaller incisions than WTR astigmatism.
B. Corneal and Limbal Relaxing Incisions
In the early 1980s, several surgeons investigated the use of corneal inci- sions for correction of astigmatism in cataract patients. Initially, so-called corneal relaxing incisions (CRIs) or astigmatic keratotomy (AK) were used— usually a pair of straight or arcuate clear cornea incisions between 5- and 7-mm optical zone to about 90% to 95% depth of the cornea. To correct greater astigmatism, a second pair of incisions was added peripherally to the first pair, but only 20% to 30% additional effect was achieved with higher irregular astigmatism. Arcuate incisions became more popular because it was believed they better preserved the 1:1 coupling ratio. These incisions were historically used in postkeratoplasty patients with high astigmatism. Their drawbacks included introduction of irregular astigmatism, glare, steepening of the cornea, fluctuations in refraction, and foreign body sensation.
Most cataract patients have moderate astigmatism of 0.5 to 3.0 D. Peripheral corneal relaxing incisions (PCRIs) or limbal relaxing incisions (LRIs) have become more popular than central CRIs for several reasons.
Risk of corneal perforation is reduced because they are performed in the thick peripheral cornea. Risk of irregular astigmatism is reduced because they are located far from the visual axis. Peripheral incisions cause minimal foreign body sensation and glare and allow faster visual recovery. Extensive experimentation resulted in multiple nomograms that take into account age, axis, amount of astigmatism, and specifics of technique. Some nomograms define incisions in millimeter length, but most commonly degrees of arc are used to compensate for variability in corneal diameter.Several nomograms are widely used today—Donnenfeld and Nichamin nomograms for LRIs and Lindstrom nomogram for CRI in 5- to 7-mm optical zone. An online LRI calculation tool utilizing Donnenfeld and NAPA (Nichamin Age and Pachymetry Adjusted) nomograms is also available at http://www.lricalculator.com/. Other nomograms in use include Gills,Wang,and Cristobal.
The incision or multiple incisions are placed on the steep meridian and the length is varied based on the amount of necessary correction. Dr. Osher’s approach keeps the length of the incisions constant at 3 mm and varies the distance from the visual axis.Another technique called paired clear corneal incision in which the cataract incision is placed on the steep meridian and a second identical penetrating incision is placed opposite the first is rarely used because it compromises anterior chamber stability and sterility.
C. Surgical Technique of Corneal Relaxing Incisions or Limbal Relaxing Incisions
Patient selection and education are important in avoiding unrealistic expectations and unhappy patients. The goal is to minimize astigmatism and prevent overcorrections. Choosing the proper axis is very important. Although LRIs are less sensitive to off-axis shift than CRIs/AK, improper placement of the incisions can make vision worse and may be difficult to correct. The most common serious error is being 90 degrees away from the proper axis. Clear written preoperative plans should be reverified before sur- gery. It is helpful to find an iris or scleral landmark close to the proper axis and document it in the office.
Keratometry, corneal topography, and manifest refraction are taken into account and the axis is selected preoperatively if measurements agree. It is wise to delay astigmatism correction until after cataract surgery if the axis and magnitude cannot be reliably determined. The patient is seated (the same position as when taking preoperative measurements) and the steep axis is marked with an indelible pen. Alternatively, intraoperative keratometry may be performed during surgery with a Placido-disk keratoscope and the steep axis marked. The incisions can be placed before or after cataract extraction. Performing LRI at normal IOP with live keratoscopy before cataract extraction helps confirm that all of the astigmatism has been removed. But if one of the incisions is leaking, this may complicate cataract extraction itself. Placing incisions at the end of surgery minimizes that risk and allows for unanticipated complications in the cataract procedure, such as whether the cataract incision was enlarged or sutures were placed, and thus the incisions can be modified or withheld.
After the axis is marked and verified, the length of the incision is marked with the help of an arcuate or radial keratotomy (RK) marker. A fixation ring is often used. A front-cutting calibrated adjustable blade or a preset 600-μm blade is inserted perpendicularly to the corneal surface 0.5 mm central to the anterior border of the surgical limbus and an incision is performed with a fluid motion. An LRI generally should not overlap with a clear corneal incision as it may destabilize it, although the main incision can be placed through an LRI in some cases of ATR astigmatism.
If any small vessels are cut, the bleeding is allowed to stop spontaneously. The incision is reopened and irrigated and the depth is checked with bal- anced salt solution (BSS) on a cannula. Microperforations are rare with a 600-mm setting. A large perforation may occur if the blade is incorrectly set or calibrated. Large perforations associated with anterior chamber collapse should be sutured and sutures removed 1 to 2 weeks postoperatively. Various devices are available for blade support and globe stabilization but are not commonly used.
LRIs can also be performed at the slit lamp on some patients for mild-to- moderate astigmatism to augment previous incisions or for primary incisions. Lidocaine gel allows for adequate anesthesia and speculum is needed for proper exposure. The technique is very similar to OR LRIs but extra care needs to be taken with blade calibration to avoid perforation. Should perforation occur, limiting eye rubbing with a shield and placing a bandage contact lens will allow the incision to be sealed quickly.
The technique is similar for CRIs in the 5- to 7-mm optical zone. The visual axis is marked by corneal reflex while the patient is looking at the microscope light, the optical zone is marked concentric to it, and the arc is marked on the steep meridian. A pachymeter is used to determine corneal thickness at the incision site, and a variable blade is set at 90% to 95% of the thickness. The eye is stabilized with a ring or forceps and the incisions performed similarly to LRI. Precision is more important given the thinner cornea and the proximity to the visual axis, and good wrist and arm support is helpful. The incisions are irrigated and checked for proper depth.
Recently, another option has become available for LRI during cataract surgery—femtosecond arcuate incisions. Initially available on Intralase (Abbott Medical Optics [AMO]) for AK after penetrating keratoplasty, this technology is available now on femtosecond cataract surgery platforms, such as LenSx (Alcon), LensAR (LensAR Inc), Victus (Bausch & Lomb), and OptiMedica. The technique is dependent on the laser system used and nomograms are being fine tuned. Incision position can be adjusted closer to visual axis or further toward the limbus and offer highly precise and clean in- cision. Live optical coherence tomography (OCT) corneal pachymetry measurement allows for proper selected depth (80% to 90%) and minimizes per- forations. Just as in femtosecond LASIK flap creation, the incision needs to be opened with a secondary instrument to break residual collagen bridges and achieve full effect. This can be done at the end of surgery under a Placido ring keratoscope or intraoperative aberrometry, such as ORA System (WaveTec Vision, http://getorasystem.com/). ORA System aberrometer is attached to the microscope and collects a reflected beam wavefront much like eximer wavefront system aberrometer. The eye is usually filled with BSS (or, sometimes, viscoelastic) to normal intraocular pressure, and measurement is made aphakic or pseudophakic to confirm lens power and LRI placement. Femtosecond incisions can be opened in stages under live control to minimize actual measured astigmatism.
D. Toric Intraocular Lenses and Excimer Laser Correction
There are several alternative approaches to astigmatism correction in the setting of cataract surgery. Foldable and nonfoldable toric IOLs are available. STAAR Surgical’s silicone toric IOL has a spherical posterior surface, toric anterior surface, and a plate haptic design (Figure 7-2).
They are available in 2 and 3.5 D toric power for correction of 1.5 and 2.25 D of corneal astigmatism. The lens is placed in the bag through an astigmatically neutral 3-mm corneal wound and rotated into position af- ter viscoelastic removal. Two lenses may be sutured together and placed in the bag to provide up to 4.3 D of correction.18 Use of these lenses is lim- ited by postoperative rotation. Rotation of 10 degrees decreases correction by one third, 20 degrees by two thirds, and >30 degrees increases cylinder. Several studies showed that up to 18% of lenses rotate more than 30 degrees, and 7% >40 degrees, leading to more than 10% reoperation rate.19,20 Plate design IOLs may dislocate posteriorly after yttrium-aluminum-garnet (YAG) capsulotomy.
Another widely used toric lens is based on the popular acrylic Alcon AcrySof SA60 platform. The Food and Drug Administration (FDA) approved smaller power T3-T5 IOLs of up to 3 D in IOL plane on September 14, 2005. In May 2011, FDA also approved higher astigmatism models of AcrySof Toric lens T6-T9 for up to 6 D in the IOL plane, translating into 4.11 D in the corneal plane (Figure 7-3) in regular and aspheric IQ models. Preliminary results from Alcon suggested that this model may demonstrate better stability. The material of this lens has certain adhesiveness for the capsular material, which allows the lens to remain where it was surgically placed.
Typically, the steep axis of astigmatism and/or the horizontal axis are marked preoperatively at the slit lamp. Lens power and model are determined with an online calculator at http://www.acrysoftoriccalculator.com based on preexisting corneal astigmatism. After cataract extraction is complete, the toric lens is inserted into the capsular bag and rotated clockwise to place IOL markings 10 to 15 degrees before the steep axis. Viscoelastic is then removed from under the lens for improved adhesion, the rest of the viscoelastic is removed, and the lens is brought to perfect alignment with the steep axis via final rotation. Intraoperative aberrometry (ORA Systems) can be performed at this point to confirm full correction. The lens can still be adjusted clock- wise at this point but with some difficulty. The final result is very sensitive to proper alignment and requires a meticulous technique.
Toric lenses can also be combined with relaxing incisions to treat high astigmatism that exceeds available IOL power. Using an AcrySof toric IOL in keratoconus or posttransplant patients who are not good candidates for LRI/AK can result in excellent uncorrected vision postoperatively.
The newest FDA approved toric IOL in the United States is the AMO TEC- NIS Toric (Abbott Medical Optics; see Figure 7-3). It is a one piece acrylic IOL with a wave-front designed aspheric optic and a frosted, continuous 360 degree posterior square edge. The IOL has a 6mm optic and 13 mm haptic to haptic diameter. The lens is available from +5 to +34 D, with 0.5-D incre- ments. The TECNIS toric corrects from 1.00 to 4.00 D in the IOL plane.
Future IOLs may include a novel light-adjustable lens (LAL), a silicone IOL under development from Calhoun Vision. This lens can be implanted in a usual fashion and then adjusted in the early postoperative period with low intensity ultraviolet light to provide desired correction. It can also be used with wavefront data to correct total optical error of the eye or for temporary multifocal correction that can be erased if a patient is unsatisfied.
Many surgeons also often use excimer laser after cataract surgery is done and healing is complete. Photorefractive keratectomy (PRK) is often the procedure of choice given relatively small corrections needed, absence of pressure on the eye during the procedure, and age of the patients. Results are good and excellent precision can be obtained.
Cataract surgery has evolved so that it is possible to minimize postoperative refractive error. Patient expectations have increased as a result. Cataract surgeons must be familiar with many refractive tools and techniques. No single approach is best for all patients, and careful planning and attention to detail are essential. Novel technologies will continue to make refractive cataract surgery even more rewarding.
*Dikutip dari Buku Essentials Of Cataract Surgery 2n Ed, halaman 71-77