Refractive
Keratotomy and the Toric IOL for the Correction of Astigmatism in Clear Cornea
Cataract Surgery
Director of Cataract and Refractive Surgery, Eye Laser Center, Tucson, Arizona USA
Clinical Professor of Ophthalmology, University of Utah School of Medicine, Salt Lake City, Utah
Ik Ho Visiting Professor of Ophthalmology, Chinese University of Hong Kong
Address Correspondence to:
Eye Laser Center
Suite 303
1925 West Orange Grove Road
Tucson, Arizona
USA 85704-1152
Phone: (520) 797-2020 Fax: (520) 797-2235
e-mail: Kershner@EyeLaserCenter.com
web site: http://www.asiteforeyes.com
Introduction:
Since the invention of the intraocular lens by the late Mr. Harold Ridley of England in 1949, lens extraction with the implantation of an intraocular lens (IOL) has become the most commonly performed refractive procedure in the world today. Rather than just for the removal a clouded crystalline lens, cataract surgery today can be utilized to fully correct refractive error. In 1995, I published my results on the technique of clear corneal cataract surgery with the simultaneous correction of myopia, hyperopia and astigmatism. These results demonstrated that we could improve uncorrected visual acuity with the cataract procedure. With the development of new lens designs and materials, the science of vision correction and neutralization of refractive error has attained a new level with the cataract procedure.
The increasing acceptance of refractive procedures to eliminate or reduce the need for spectacle and contact lens correction first gained acceptance with the radial keratotomy technique of the late S. N. Fyodorov of Moscow, Russia. Even today, with the increased technology, incisional keratotomy still provides the simplest, and most reproducible approach to astigmatic correction with cataract surgery. In the early 1990s, the development of the excimer laser enabled surgeons to correct a larger array of refractive errors. Utilized in combination with the automated keratome, laser-in-situ-keratomilleusis, (LASIK), has gained in popularity over the past decade. The difficulty of using a mechanical device to create a consistent corneal flap, and the limitations of removing corneal tissue without compromising the corneal integrity, has lead surgeons to embrace additional approaches to the correction of higher orders of refractive error. Today's cataract procedure with IOL implantation, can fill the need. Much work has been undertaken on the use of refractive implantable lenses for cataract surgery and now for phakic refractive correction, either in the anterior chamber, iris supported, or in the posterior chamber. This approach has held promise also for the correction of presbyopia, the natural loss of accommodative ability that comes with age. Because today's cataract procedure has enabled the use of small (less than 3.0 mm) incisions, the procedure is safer, consistent, and more predictable than ever before. With the development of laser phaco, phakonit, and newer technologies, these incisions are rapidly approaching one-millimeter or less. These very small microincisions are placed through the clear corneal architecture precluding the need for conjunctival dissection, cautery, sutures, injection anesthetics, bandaging and restriction of normal postoperative activities. Microincision cataract surgery has all but eliminated the complications of wound leak, uveal prolapse, and surgically-induced astigmatism. These advances have paved the way for faster, more efficient surgery, with less instrumentation, less intervention and faster visual recovery for the patient.
The application of laser technology and implantable lenses has fallen short of expectations for that segment of the population who have high degrees of refractive error. In addition, individuals who have corneal abnormalities such as, irregularity, keratoconus, corneal dystrophies or scars, may be ineligible for corneal surgery. What do we offer those in whom corneal alteration procedures cannot be considered? For these individuals and for those for whom the visual quality of an intraocular lens implant with the simultaneous correction of refractive error may prove superior, the choice of lens surgery may be a more acceptable alternative.
In 1994, I coined the term keratolenticuloplasty (KLP) to better reflect what we, as cataract surgeons were accomplishing with the simultaneous reshaping of the cornea by tailoring the incision (Figure 1) and replacing the abnormal lenticular architecture with an artificial intraocular lens to correct refractive error. Smaller incision surgery has motivated the IOL industry to develop newer intraocular materials to replace the rigid polymethylmethacrylate (PMMA) lenses of yesterday and replace them with newer silicone, acrylic, thermoplastic and hydrogel materials that can be injected through these microincisions.
Coincident with these advances in microincision cataract surgery has been the increasingly superior visual results that patients have achieved. Myopia and hyperopia are eliminated with the IOL and astigmatism can be corrected with the use of a toric IOL with or without arcuate keratotomy incisions (the so-called limbal or peripheral corneal relaxing incisions). Smaller incision surgery has meant better results for the patient and less complications and worry for the surgeon.
I have developed and adhered to a single incision-single instrument approach to cataract surgery that has benefited my patients over the years that utilizes a clear corneal microincision, simultaneous correction of spherical and astigmatic errors with refractive keratotomy and toric IOLs, in-the-bag phacoemulsification with a mini-phaco flip maneuver, and injection of the IOL through the unenlarged incision. The patient's visual results and immediate recovery without restriction speak for themselves (Figures 2-4). Eighty-nine percent of patients are spectacle-free for most tasks and twenty-nine percent can read without the need for a near correction. The refractive outcomes achieved by following these techniques are the best we have ever achieved, and with incision sizes approaching one millimeter, this technology holds promise for even greater advances in the not too distant future.
Preoperative Evaluation and Surgical Plan:
All patients who present for cataract surgery undergo a comprehensive ophthalmic evaluation which includes careful attention to corneal integrity, signs of pathology, and dilated funduscopy to screen for non-refractive conditions which may impede a successful visual result. In devising the surgical plan, cycloplegic refraction, combined with corneal topography (Figure 5) and ultrasonic biometry, is used to select the best IOL power for complete refractive correction. The strategy is to correct the sphere fully for distance, eliminate less than one diopter (D) of astigmatism with a single incision that doubles as the cataract incision (KLP), and supplement the astigmatic correction for over 1D with the toric IOL. The goal of astigmatic treatment is to fully correct or slightly undercorrect the cylinder, and not overcorrect or shift the cylinder axis. To achieve the proper correction a preoperative surgical plan is developed (See worksheets).
The surgeon has few surgical options at this time with which to correct presbyopia. I am not a proponent of monovision, the process of correcting one eye for near vision and one for distance, unless the patient has been a lifetime success with monovision contact lenses. The multifocal or bifocal IOLs available today are plagued with optical aberrations and loss of contrast sensitivity and therefore are not to be used in any but the simplest and non-demanding of cases. These patients need to be carefully screened and counseled. Those patients who need refractive correction for near tasks following refractive cataract surgery, use a low power reader, at least until such time that a high quality refractive correction for both near and far vision can be achieved. Time will tell which approach will be the best. Presently there is a consciencous effort to evaluate several approaches to the surgical correction of presbyopia. These include new designs in bifocal intraocular lenses, refractive keratotomy (ciliary sclerostomy), implantable scleral segments, implantable accommodative lenses, thermal keratoplasty, and excimer laser resculpting of the cornea. I predict that an implantable, accommodative IOL, will most likely be the ultimate solution to the presbyopic deficiency of conventional IOLs.
The Procedure:
I would encourage any surgeon who is still using scleral tunnel incisions to consider switching to topical anesthesia and clear corneal incisions. Clear corneal incisions allow the surgeon the flexibility to tailor-make the incisional procedure with the optimum refractive outcome in mind. This surgical approach saves time. Patients love the fact that their eyes are fully functional and appear normal almost as soon as they leave the operating room. Patients see better without the dependence on eyeglasses. To achieve this level of perfection, surgeons have to pay attention to corneal curvature, and they must assure the correct selection of intraocular lens power. Clear corneal incisions require that that the surgeon observes a different set of rules. Surgeons need to streamline their surgical technique and master a careful approach to the cornea as a refractive structure not to be ignored. Incisions into the cornea, the most powerful refracting surface in the eye, are all refractive in nature and are unforgiving. They must be properly constructed to assure the best functional and refractive effect. The surgeon will need to pay careful attention to how he creates and manipulates the incision, and how she operates through that incision.
1.
Use Topical Anesthesia
With topical anesthesia, the patient can still see and therefore can be directed by the surgeon to look into the center of the microscope light. Proper self-fixation is critical to stabilizing the eye and accurately determining the optical center of the cornea for correct placement of incisions. Surgeons must recognize however, that patients can feel that what they could not feel with injection anesthetics. It is important therefore, to inform the patient when they they will feel pressure, see the light of the operating microscope, and feel the surgeon touching them, so that they will not be startled and perceive the experience as painful. I always tell the patient when I am about to place the eyelid speculum, start the phaco (infusion causes a proprioceptive sensation of pressure) and when I am about to inject the IOL (as the bag distends, pressure is felt). As long as the patient is informed, they will not startle or move and therefore avoid unnecessary discomfort.
After cycloplegic drops are instilled and a surgical scrub is performed, I instill several drops of 2.5% proparacaine or tetracaine onto the cornea. Do not instill topical anesthesia prior to arrival in the operating room. I also avoid the longer acting anesthetics such as bupivacaine (Marcaine) as they are hyperosmotic, burn and last much longer than required for the surgery. A sterile adhesive drape is applied, and a small, Kershner reversible speculum is positioned under the eyelid margins, away from the incisional site. I then locate the proper steep meridian using an reticle in the ocular of my operating microscope (consulting my worksheets as I do so), mark the cornea with a dry marker, and construct the corneal incisions. It is important to keep the cornea dry until the incisions are properly placed to avoid obscuring the location of the planned incision. Following this, apply one drop of 2.5% hydroxypropylmethylcellulose, (HPMC), rather than balanced salt solution irrigation, to the surface of the cornea. We coat the cornea with HPMC instead of forcing our scrub technicians to direct a stream of balanced salt solution over the ocular surface to keep it clean and moist. In this way, the cornea is protected, coated and left undisturbed. Several ophthalmic products contain HPMC. These include Occucoat, Celluvisc, Refresh artificial tears, and Goniosol. In our practice, we transfer a sterile bottle of Goniosol (maintaining sterility) into 1cc TB syringes on the morning of surgery. Goniosol as the most viscous at 2.5% HPMC, is optically clear, coats the cornea beautifully, and even provides 1.5X magnification. After applying one drop onto the center of the cornea, your scrub tech can do something else. The ocular surface is protected and no irrigation is applied.
2.
Create a Clear Corneal Incision
Unlike scleral tunnel incisions, corneal incisions are not very forgiving. It’s easy to distort, tear or stretch a corneal incision, and when you do, it will leak, induce unwanted astigmatism and heal more slowly. Here are a few basic rules:
• Size. The most common error inexperienced surgeons make when constructing clear corneal incisions is to use a keratome that’s too small for the instruments they plan on passing through the incision. This causes stretching or tearing of the incision, striae that can obscure visualization during the procedure and increase the likelihood of corneal burns during phaco. It can also cause post-operative healing problems. Unlike scleral incisions, corneal incisions do not snap back into place after stretching. If your incision is too small, you will likely wind up distorting the incision when passing instruments through it, causing it to gape like a fish mouth rather than seal shut like a paper cut. The easiest way to avoid this is to use a keratome that is properly sized to accommodate your largest instrument.
Typically, corneal incisions wider than 3.2 mm will induce flattening and unwanted aberration in the refractive power of the central cornea. These incisions usually do not seal on their own, and require suturing. Incisional widths 3 mm or less seal appropriately.
•Blades. Standard disposable steel keratomes used routinely for scleral tunnel incisions won’t work for clear corneal incisions. Only very sharp keratomes can atraumatically penetrate Descemet’s membrane. Many clear cornea surgeons use diamond blades because of their unrivaled sharpness. The cutting edges can be made as thin as 1 µm, enabling these knives to pass through the corneal lamella smoothly and easily, leaving behind an incision as smooth as a paper cut. Unfortunately many surgeons lack the trained surgical team necessary to maintain these expensive instruments properly. For these surgeons, there is an alternative. The clear corneal incision system developed by Becton Dickinson Ophthalmic Surgical in Waltham, Massachusetts USA (Figure 6), features a blade that rivals a diamond’s sharpness, contour and geometry, yet is made from surgical steel. The system is specifically and ideally designed for the clear corneal procedure. The complete disposable kit also includes a hinged, fixation ring to keep the eye stable and an inkless marker that helps to mark the proper size, location and configuration of the ideal clear corneal incision. The marking device creates two marks, a curved one where the entrance to the incision should be palced and a linear mark for the entrance into the eye through Descemet’s membrane. By simply fixing the globe in place with the fixation ring (Figure 7), marking the incision with the inkless marker (Figure 8), the surgeon can properly position the keratome for the ideal incision. For those who prefer and can afford the cost and maintenance of a reusable diamond system, I recommend the Rhein Medical or Diamatrix (The Woodlands, Texas USA) diamond keratome and the Kershner Keratolenticuloplasty Marking Set or Limbal Relaxing Incisions Marker (Rhein Medical, Tampa, Florida USA).
To assure proper geometry and architecture, place the tip of the keratome on the incision entrance line, aim and line up the blade plane parallel to the iris to the second line mark, then pass the blade into the cornea until it reaches the laser mark on the blade. At this point, the tip will enter the eye at the proper angle and the ideal tunnel length will be achieved automatically. The length to width ratio will be maintained at 3:2, which has been proven to be stable (Figure 9).
The clear corneal keratomes are available in a variety of widths to accommodate whatever phacoemulsification tip and lens insertion method you use. The knife has a specially designed double-bevel slit blade in either angled or straight form for proper clear cornea incision construction. An accurate depth blade preset to 550 or 600 microns by the manufacturer is used to construct the two-step arcuate keratotomy incision (Figure 10).
3.
Correct Astigmatism
Where you make the incision is just as important as how you make it. Prior to surgery note in the chart the position of the patient’s steepest meridian on the cornea (see worksheet). Carefully evaluate the patient’s preoperative astigmatism and take the topograph into the operating room with you. Surgeons need to review the concept of flattening the corneal curvature with the incision. The theory behind this process has been well established and has withstood the test of time. Corneal incisions can consistently and predictably alter the corneal power. As eloquently described by Spencer Thornton, MD of Nashville, Tennesse, one of the early American pioneers in refractive keratotomy, all transverse or arcuate corneal incisions will flatten the corneal architecture in the meridian in which they are placed by acting as if tissue has been added to that location. Radial incisions such as those created with radial keratotomy (RK), will flatten the central dome of the cornea and minimize myopia, transverse incisions will flatten only in the meridian in which they are placed. The rule, therefore is, always locate your incision on the steepest meridian. Can't determine the steepest meridian? Simply refract the patient in plus cylinder or analyze a corneal topographic map. Placing the incision anywhere other than the steepest part of the cornea will make the astigmatism worse.
Since most elderly patients have against-the-rule astigmatism, temporal incisions work very well for most, but not all patients. These incisions are also best if the patient has a spherical cornea. The temporal limbus overlies the lateral rectus muscle insertion and is located further away from the optical center of the eye than is the superior limbus. A temporal incision will create less induced corneal astigmatism. Patients with significant pre-existing astigmatism (greater than 0.5 D) will benefit from astigmatic keratotomy (keratolenticuloplasty) at the time of cataract surgery. By simply placing the clear corneal incision on the steepest meridian, all patients will benefit by reducing the corneal astigmatism. Conversely, placing the clear corneal incision on anything but the steep meridian is likely to make the patient worse. Why does this happen? Flattening the corneal dome will induce astigmatism ninety degrees away from the incision placement. This process is known as coupling, and can be used to our advantage to neutralize preexisting astigmatism. If we simply flatten the steep meridian, we can eliminate the corneal astigmatism. But how best can a surgeon flatten the cornea with an incision?
Surgical techniques can be created that incise either a large limbal relaxing incision (LRI) or a small arcuate incision into the cornea (KLP). How do arcuate astigmatic incisions differ from limbal relaxing incisions? Limbal relaxing incisions, because they are placed far peripherally in the corneal scleral limbus, have less flattening effect for a given length. As a result they must be large to have any substantial effect on corneal curvature. When limbal incisions traverse 120 degrees of arc, they effectively denervate the cornea, especially if placed nasally or temporally, where most incisions in an elderly population will end up. In an elderly patient, this can mean an anesthetic cornea, severe dry eye and corneal breakdown. Smaller, arcuate incisions have more effect with less surgery, and as long as they are not placed closer to the optical center of the cornea than a 7mm optical zone, are less problematic, easier to perform and much more predictable, without the downside risk. That is why, in the keratolenticuloplasty technique, I utilize small, arcuate incisions accurately placed into the substance of the cornea.
4.
Use an IOL that can be injected through the clear cornea incision
without enlarging it.
The IOL you use is a matter of personal preference. However, using the clear corneal, microincision approach without enlarging the incision for lens implantation has distinct advantages. The refractive effects of a carefully constructed incision will not be fully realized if the incisional architecture has been breached during the IOL insertion process. That is why I prefer either a plate-haptic single-piece lens, either in silicone or collamer, a collagen copolymer, or a fully injectable three-piece silicone lens that will pass through these incisions without damaging them. These IOLs are fully injectable which makes it possible to use a 2.4 mm incision or smaller. Furthermore, they allow us to use the fully injectable Toric IOL which expands our ability to correct astigmatism and still maintain the smallest of incisions (Figure 11). Toric IOLs are available from Staar Surgical, Monrovia, California USA in cylindrical powers of +2.00 (which corrects 1.4 D in the spectacle plane) or 3.5 D (which corrects 2.3 D in the spectacle plane). If the surgeon prefers a three piece lens, the Pharmacia 911A is a new third generation silicone with truncated edges that has been shown to impede the development of posterior capsular opacification (PCO), and can be injected through a 2.4mm incision with the new Pharmacia injection system. Any incision that is created greater than 3.2 mm in width will induce excessive flattening, seal much less readily and probably should be sutured. That is why it is best to keep the corneal incisions small (less than 3.0mm and preferable less than 2.4mm) and why a lens that can be injected instead of folded is to be preferred. The clear cornea microincision is without question the best way to approach the cataract procedure with today’s technology. More and more surgeons are mastering the finesse of this technique and the challenges of operating through these small incisions. More and more patients are demanding the rapid recovery and clear uncorrected vision that this incisional technique provides.
When implanting the IOL fill the capsular bag with a viscoelastic, being careful not to overfill the chamber, but rather inflate just enough to open the capsulorrhexis. I prefer Healon 5, a very dense viscoadaptive (Pharmacia, Peapack, New Jersey, USA). Placed into the center of the bag after inflation with Healon, the viscoadaptive allows the lens to gently unfold inside the capsular bag. The single piece injectable lens is carefully loaded into the injector, passed into the incision and inserted into the capsular bag in one maneuver. The lens is then injected in one simple maneuver into the capsular bag at the proper meridian. If a toric IOL is used, it is aligned with the steep meridian, and the anterior IOL lens marks are positioned at the proper location.
This surgical procedure is carefully controlled and meticulously planned so that the cornea being reshaped becomes spherical and an optically sound structure (kerato-), and the replacement of the clouded lens with the proper IOL fully corrects the remaining spherical error (-lenticuloplasty). If the ultimate goal of outpatient cataract surgery is less intervention and better visual results then this approach surely offers the greatest opportunity for the optimum refractive outcome and the best surgical result.
The Strategy to Achieve the Best Refractive Outcome from the Clear
Cornea Procedure:
Most surgeons have been slow to accept the techniques of astigmatism management with their cataract procedure because of a resistance to acquire new skills, or the need for new instrumentation. Some of us wrongly assume that there is no real benefit to the patient by creating better uncorrected vision after their cataract procedure. In the United States there is no extra reimbursement for taking the time to create a better visual outcome for the patient, so some feel there is no benefit to the surgeon from a financial standpoint. These arguments against astigmatism management with cataract surgery couldn’t be further from the truth. Astigmatism can and should be managed, because it is better for our patients and because it can be predictably and easily corrected with today’s techniques. By simply adopting a few sound fundamental principals which require the mastery of few if any new techniques, and a minimal if any investment in additional instrumentation, a surgeon can offer a better refractive result for his patients. The surgical correction of astigmatism along with full refractive correction of the spherical error, reduces the need for spectacle correction postoperatively which translates into increased patient satisfaction and more patients. There couldn’t be a better reason to learn these techniques and apply them to every cataract patient.
To assure a precise, reproducible method of achieving better refractive outcomes, we need to adopt a philosophy of refractive correction that works for each and every surgeon and assimilate the discipline needed to follow a set of rules:
Rule #1. To accurately correct astigmatism, we have to accurately measure it.
Rule #2. Never overcorrect the cylinder or shift the cylinder from it’s pre-existing meridian.
Rule #3. Always apply the astigmatic correction on the proper meridian and never operate off axis.
How do we measure astigmatism?
A full and accurate cycloplegic refraction will allow us to determine the magnitude and the orientation of the cylinder axis. Corneal topography and other corneal scanning measurements can allow us to properly analyze the origin of the astigmatism. Naturally, an individual who has a refractive cylinder that does not appear topographically would not require corneal alteration to correct it. Simply removing the cataract will suffice. If the topographic astigmatism, which is usually measured as less than the refractive astigmatism, significantly disagrees with the power or the orientation of the cylinder, then the surgeon must make a judgment of what refractive error to correct. Here is where the art of astigmatic correction with cataract surgery departs from the science. The surgeon must be aware that using a cookbook approach for every patient will not work. It is always better not to attempt a correction rather than perform the incorrect treatment.
Topography is extremely valuable in both determining the qualitative appearance of the astigmatism as well as the location of the cylinder (Figure 5), when choosing whether to utilize symmetrical or asymmetrical incisions. Newer methods of corneal and intraocular analysis utilizing orbscan or wavefront analysis may further provide us with insight into higher order aberrations that could affect the postoperative refractive result. However, our ability to measure astigmatism may exceed our ability to correct it. Not all aberrations will require correction and as recent information is showing us, may in fact, be undesirable. Certain aberrations in the optical system may be necessary for good vision in certain circumstances and probably should be left alone. If the astigmatic correction is regular, clear and present on a single meridian, then it is correctable. Irregular astigmatism, keratoconus, corneal scars and higher order aberrations are probably best left alone, rather than achieve a correction which could result in an undesirable postoperative and irregular cornea.
How do we correct astigmatism?
Early methods of astigmatic control at the time of surgery were limited to elaborate suturing techniques and corneal wedge resection. Although this worked in instances where large corneal incisions were utilized for cataract surgery, and most patients were required to wear a large spectacle correction postoperatively, it has had little role with today’s techniques. We can use the principle that “all incisions placed onto the dome of the cornea will act as if tissue is added where they are placed” to intentionally flatten the steep areas of the cornea and create a more spherical result. Small, arcuate corneal incisions work best when surgeons wish to flatten the cornea at a given location. Arcuate incisions, which closely follow the corneal curvature, placed on the proper latitude of the globe, can flatten in the meridian in which they are placed. This can be utilized to neutralize preexisting corneal astigmatism. The flattening in one meridian will usually result in a steepening of the meridian 90º away, this coupling ratio is approximately 1:1 for arcuate clear corneal incisions. Therefore surgeons need not take into account a change in the spherical power of the eye when they perform astigmatic surgery and IOL power need not be altered as the overall power of the eye will be maintained. Biometry should be carefully preformed preoperatively. The spherical correction desired can be accurately determined and the lens power selected without attention to whether correction of the astigmatism will contribute to an alteration in the lens power.
We can maximize the effect of the clear corneal cataract incision to correct astigmatism by inducing intentional flattening in the meridian in which the incision is placed. Simply by operating on the steepest meridian, we can improve the refractive results for all of our patients. Operate more than 15º off axis, and you will make the postoperative refractive result worse. That is why it is critical to know on what meridian in which to operate. In evaluating the patient for refractive cataract surgery, I always take into account the cycloplegic refraction, review the topography, and look at the A-scan result. The proper IOL is chosen; the location of the steep meridian is marked on the chart. To identify the proper meridian in the operating room, surgeons can make a note in the chart (see the worksheet in this article) of the presence of a small nevus or corkscrew vessel which may help identify the 12:00 o’clock meridian. Using this mark as a guide, the proper meridian for the astigmatic correction can be selected. I prefer to use an ocular reticule in the operating microscope that allows me to align the proper axis with the microscope. Alternatively, the surgeon can use a hand-held degree gauge to determine the proper meridian. Taking into account any rotational movement of the eye when the patient is supine is usually not an issue when topical anesthesia is used. If a peribulbar block is used however, this must be taken into account when determining the proper location for correction. It is often best to mark the proper axis prior to administration of the block if one is used.
We can maximize the effect of the clear corneal incision to flatten the cornea by designing its architecture with the goal of intentionally achieving flattening with the incision (Figure 1).
For astigmatically neutral clear corneal incisions, a one-step plane parallel clear corneal incision is utilized (Figure 1A). A 2.4mm disposable keratome can be used to create the proper architecture for a clear corneal incision that will be self-sealing and maintain a ratio of 3:2 of width to length. If the width of the incision is 3mm, the tunnel length through the cornea should be approximately 2mm. In the KLP technique, we utilize the cataract incision for all the subsequent steps of the surgery to optimize its refractive result. The cataract incision itself can correct up to 1.5 diopters (D) of astigmatism alone when used in a length of approximately 3mm or more (Figure1B). For an astigmatically neutral incision (A) a single-plane incision is created with the keratome. For less than 1D of astigmatism, (B) a two-step clear corneal incision is utilized to flatten in the meridian in which it is placed. By using an accurate depth blade set at a depth of 550 to 600 microns, the incision is made vertically, perpendicular to the cornea. Position the blade handle towards the center of the globe to create a deep groove approximately 85% of corneal depth. The keratome selected for the proper size for the phaco tip and the IOL injector (I usually use a 2.6 mm) is positioned at the base of this keratotomy, and enters the eye in a plane parallel fashion. In this fashion, a two-step clear corneal self-sealing incision is created with the maximum flattening effect for astigmatism correction.
To correct larger degrees of astigmatism, (C) the incision is coupled with an additional arcuate incision on the opposite meridian from the cataract incision, at an optical zone of 9, 10 or 11mm to induce further flattening (Figure1C). Alternatively, the incision can be combined with the implantation of the toric intraocular lens (Staar Surgical, Monrovia, California USA). The toric intraocular lens is presently available in two cylinder powers with the anterior surface of the lens delivering the refractive torus (Figure13). The 2 D lens will deliver approximately 1.4 D of astigmatic correction at the spectacle plane, and the 3.5 D will provide approximately 2.3 D of correction.
The incisions are constructed utilizing the disposable BD limbal relaxing incision system which is comprised of a hinged fixation ring, an inkless marker to mark the proper location for the incision, the accurate depth blade to make the vertical component of the incision and a slit blade to make the proper architecture for corneal entry (Figure 6).
The Operative Procedure:
Patients are administered topical 1% tropicamide in combination with 2.5% phenylephrine drops into the operative eye one drop every five minutes for 3 administrations, prior to surgery. On call to surgery they receive a single drop of 4% topical Betadine suspension. The surgical scrub is performed and a sterile adhesive drape applied to exclude the eyelids and lashes. Several drops of 2.5% tetracaine anesthetic are instilled (if anesthetic drops are used prior to the procedure there may be excessive swelling, drying or sloughing of the corneal epithelium making visualization difficult). The Kershner reversible eyelid speculum (Rhein Medical, Tampa, Florida USA) is positioned under the eyelids and can be rotated out of the way so as to not interfere with the various steps of the procedure. The cornea is kept dry while the proper meridian of the cylinder is identified through the optical reticle in the microscope and the cornea marked with the inkless marker. The globe can be fixated with the disposable fixation ring if necessary, and the incisions created. Astigmatically neutral incisions are created plane parallel to the iris plane. Keeping the handle of the blade flat with respect to the floor and the iris, the blade is passed to create a corneal tunnel with a ratio of incision width to length of 3:2. If a keratotomy incision is being created with the square diamond or the disposable accurate depth blade, it is set to 600 microns and positioned perpendicular to the cornea, which applanates it during the cut. The handle of the keratotomy blade is positioned such that the blade is aimed towards the center of the globe. Next, the cornea is coated with several drops of 2.5% hydroxypropylmethylcellulose (HPMC) which covers the cornea, keeps it moist, protects it, eliminates the need for irrigation during the procedure and provides 1.5X magnification. Hyaluronate viscoelastic (Healon and Healon 5, Pharmacia, Peapack, New Jersey USA) is instilled into the anterior chamber. For shallow chambers or when the lens face is convex towards the anterior chamber, the AC can be filled with Healon 5 (2.3% hyaluronate-Pharmacia) to flatten the capsule and reduce the stress on the zonules.
The Kershner one-step capsulorrhexis forceps (Rhein Medical, Tampa, FL USA) are utilized to create a 5mm round central capsulotomy. Hydrodissection is carried out with a Binkhorst cannula and balanced salt solution irrigation beginning with the subincisional cortex to make sure that this is loosened prior to the phaco procedure. Next an in-the-bag, three-step, single –incision, single-instrument, phacoemulsification technique is performed with a 30º tip, phaco power at 20%, maximum vacuum at 500mmHg, and a aspiration rate of 25cc/min. Central sculpting is performed deeply and widely before the lens is rotated.
Following central sculpting, and removal of cortical rim, the phaco tip is utilized to press on the superior pole of the nucleus until it flips inside the capsular bag. The remainder of the nuclear plate can then be safely removed well separated from the posterior capsule with gentle emulsification and an aspiration. The clear corneal irrigation-aspiration tip (Rhein Medical, Tampa, FL USA) is then introduced to remove residual cortex and irrigate the capsular bag. The capsular bag is then inflated with Healon viscoelastic to open the capsular rim without over inflating the anterior chamber of the eye. A single bolus of Healon 5 is placed into the center of the capsule to facilitate unloading of the lens.
Next the collamer hydrogel IOL (for spherical correction), the Pharmacia 911A three piece silicone or the silicone toric IOL is selected, loaded into the injector cartridge and passed through the incision into the capsular bag at the proper meridian, where it is allowed to position itself without additional manipulation (Figure 14). The injector is then withdrawn from the incision margin. Irrigation and aspiration is used to remove residual viscoelastic. It is important to directly visualize the removal of Healon 5 to avoid postoperative intraocular pressure rises. The lens is then checked for proper centration and orientation on the proper meridian, the eye is reinflated to 20mmHg with balanced salt solution irrigation and a subtenons injection of 0.1cc of betamethasone (Celestone), and 0.1cc of cefazolin (Ancef) is placed under the upper eyelid. The patient is instructed to use artificial teardrops as needed following the procedure. No bandage is provided. The patient is given a pair of sunglasses for use when out-of-doors and is seen postoperatively on the first day, at two weeks, three months, six months and one-year intervals. If a YAG laser capsulotomy is required, it is not performed until the three-month postoperative visit is completed.
The refractive results of clear corneal cataract surgery speak for themselves (Figure 2). Both the sphere (Figure 3) and the cylinder (Figure 4) can be predictably corrected with these techniques. The majority of patients have spectacle-free vision following the procedure and can return to normal activities the same day. Because the incision size is so small, the patient can touch their eyes, resume normal daily activities and need not be concerned that they will undo their procedure. With the microincision technique, the need for long-term postoperative eye drop therapy is unnecessary. I have patients use an antibiotic-steroid combination drop four times a day for one week. This saves the patient both on cost, and inconvenience. The freedom achieved following refractive correction with cataract surgery creates a highly motivated and a most satisfied patient.
Conclusions:
Today’s modern techniques of microincision cataract surgery have enabled surgeons to fully correct refractive error with cataract removal and IOL implantation. Smaller, more flexible injectable intraocular lenses, combined with more efficient methods of phacoemulsification have made it possible to keep incision sizes less than 2.5mm, and as small as 1mm. Careful attention to astigmatic correction, and proper incision construction combined with toric intraocular lenses and judicious selection of the IOL power and can maximize the full refractive correction for each cataract patient. This translates into a more satisfied patient with the less likelihood of postoperative complications, less need for postoperative care, and less need for refractive refinements following the procedure which translates into less dependence on eyeglasses.
Surgeons today have within their grasp, the techniques for optimizing the refractive results of their cataract procedure. Full refractive correction at the time of cataract surgery can and should be accomplished, and must be the goal of every cataract surgeon.
REFERENCES
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