Toric IOLs and Corneal Astigmatic Incisions-Avoiding Problems
©2006. Robert M. Kershner, M.D., M.S., F.A.C.S.
Eye Laser Consulting, Boston, Massachusetts
All Rights Reserved.
Lens extraction with the implantation of an intraocular lens (IOL) has become the most commonly performed refractive procedure in the world today. Rather than use this technology exclusively for removal of a clouded cataract, surgeons are beginning to replace the crystalline lens as a treatment to fully correct refractive error. The techniques of clear corneal cataract surgery and the correction of myopia and hyperopia with the IOL have been available for some time. Some surgeons have also chosen to correct astigmatism at the time of surgery. It has only been recently that correcting presbyopia with an IOL has created a renewed interest in the surgical correction of astigmatism. New lens designs and materials have awakened the science of vision correction and complete neutralization of the total refractive error of the eye is now within our grasp.
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 advanced 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 lens extraction procedure with IOL implantation can fulfill a long overdue need. Much work has been devoted to the use of refractive implantable lenses for cataract surgery and for phakic refractive correction, either in the anterior chamber, iris supported, or in the posterior chamber. The correction of presbyopia, the natural loss of accommodative ability that comes with age, is the latest contribution to our ability to improve vision. Today's cataract procedure uses small (less than 3.0 mm) incisions and as a result 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 cornea precluding the need for conjunctival dissection, cautery, sutures, injection anesthetics, bandaging and the postoperative restriction of normal 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. I have utilized a single incision-single instrument approach to cataract surgery that has benefited my patients over the years. Eighty-nine percent of patients are spectacle-free for most tasks and twenty-nine percent can read without the need for a near correction following implantation with a monofocal optic. With a bifocal or accommodative IOL implanted, the percentage of spectacle free patients rises to 96%. The refractive outcomes achieved with these techniques are the best we have ever achieved, and with incision sizes approaching one millimeter, and new lenses on the horizon, this technology holds promise for even greater advances in visual outcomes in the future.
The application of laser technology and implantable phakic lenses has fallen short of expectations for that segment of the population who have presbyopia or high degrees of refractive error. In addition, individuals who have corneal abnormalities such as, surface irregularity, keratoconus, dystrophies or scars, may be ineligible for corneal surgery. What can we offer those patients who cannot undergo corneal alteration procedures? For these individuals and for those for whom the visual quality of an intraocular lens implant with the simultaneous correction of refractive error is preferable, 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 and replacing the crystalline lens with an intraocular lens to correct refractive error. Arcuate corneal incisions can flatten the steep meridian and correct astigmatism (Figure 1) . Smaller incision surgery has motivated the IOL industry to develop newer intraocular materials, silicone, acrylic, thermoplastic and hydrogels to replace the rigid polymethylmethacrylate (PMMA) lenses of yesterday.
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 one of the two FDA approved Toric IOLs or with arcuate astigmatic 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. Unfortunately, as with all surgical procedures, there are always potential pitfalls. Complications with corneal ablation procedures, toric IOLs and astigmatic keratotomy can and do occur. To correct astigmatism, surgeons must first be able to deliver an astigmatically neutral procedure. If they don't make the pre-existing refractive error worse, they can then set their sights on correcting what is already there. The most common, and often the most preventable problem with surgical intervention, in increasing order of severity, is inadequate correction of the cylinder power, overcorrection, or the worst possible occurrence, shifting the astigmatic axis. If surgeons recognize these potential problems and are prepared to handle them, then successful surgical outcomes will result. The best way to prevent complications of course, is to avoid them in the first place. I will describe several important paradigms that surgeons need to apply when maximizing the success of these technologies.
There are presently only two available FDA approved Toric IOLs, the STAAR TORIC IOL, and Alcon Laboratories Acrysof Toric. These IOLs are single-piece, injectable intraocular lenses (Figure 2) . They incorporate a cylindrical correction on a spherical optic that creates the toric lens. The STAAR Toric lens is available in a range of spherical powers with cylindrical adds of 2.0 D and 3.5 D on the anterior surface of the optic, which deliver, at the corneal plane, 1.4 D and 2.3 D of cylindric correction. The STAAR TORIC IOL TF version measures 10.8 mm and the TL version measures 11.2 mm to accommodate capsular bags of different sizes. Fenestrations in the IOL allow fibrous lens epithelial cells to migrate through and around the lens for increased capsular bag fixation. The lens has been available for almost a decade and over two million STAAR silicone lenses have been implanted worldwide. It is imperative that surgeons measure preoperative astigmatism with accurate keratometry and corneal topography. Refraction should be used to verify the steep corneal meridian. STAAR surgical provides Toric SRK/T calculation software to allow the surgeon to develop a surgical worksheet that includes the patient's measurements and a clearly labeled illustration for the orientation for the lens. At the time of surgery the surgeon needs to align the two markings on the anterior surface of the optic with the steep corneal meridian.
The Alcon Toric IOL is based on the popular single piece Acrysof platform. The toric surface is on the posterior side of the optic and is available in three different dioptric powers, 1.5D, 2.5D, and 3.0D at the IOL plane. The two dots on the lens surface allow the surgeon to orient the IOL with the steepest meridian of astigmatism upon implantation.
Complications of Toric IOLs:
Although the surgical outcome from implantation with a toric IOL can be excellent, surgeons must adhere to several important parameters to assure success. First, one can only correct what can be measured. If the measurement of the pre-existing astigmatism is inaccurate, improvement in post-operative refraction shouldn't be expected. Check and recheck your measurements. You can only operate on what needs to be corrected. With the toric IOL it doesn't matter if the astigmatism is corneal or posterior, as long as it is not due to lenticular changes. 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 an undesirable postoperative correction and an irregular cornea.
Second, always aim to undercorrect rather than overcorrect the cylinder power. It is possible to perform more surgery if necessary, but it can be difficult to take back that which has already been done. Last but certainly not least, even with accurate measurements, if the toric IOL is not implanted on the proper meridian, the patient will be made worse.
One of the potential pitfalls with the implantation of toric IOLs is that even with the best efforts of the surgeon to properly measure the correction and implant the IOL on the proper meridian, the IOL can still move off axis in the postoperative period compromising the optical result. Some have suggested that the one-piece plate haptic STAAR silicone IOL is prone to postoperative rotation. My studies and those of others, have demonstrated that although this may occur in less than 6% of cases, early postoperative manipulation of the IOL optic to the proper meridian can be accomplished rather simply. To reduce the likelihood of off axis migration of the IOL, some surgeons have suggested implanting the STAAR IOL with the anterior torus against the posterior capsule. Use of the longer overall length TF version of the IOL may further reduce this complication. In the case of the Alcon IOL, the torus is on the posterior surface of the optic. The stickier acrylic material may be less prone to movement which could result in less inadvertent rotation. Future studies will be necessary to demonstrate if this is the case.
Patients who present for lens replacement surgery first undergo a comprehensive ophthalmic evaluation which includes careful attention to corneal integrity, signs of ocular 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 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 astigmatism with a single incision that doubles as the cataract incision (KLP), and supplement the astigmatic correction of 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.
Complications of Astigmatic Keratotomy.
I would encourage surgeons to switch to topical anesthesia and clear corneal incisions. Clear corneal incisions allow the surgeon the flexibility to tailor-make the procedure with the optimum refractive outcome in mind. Incisions into the cornea, the most powerful refracting surface in the eye are refractive in nature and therefore 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. Poorly constructed astigmatic incisions can result in irregular astigmatism, scarring, corneal aberrations and compromised visual result.
Use Topical Anesthesia. With topical anesthesia, the patient can still see to look into the center of the microscope light for proper self-fixation. Accurately determining the optical center of the cornea for correct placement of incisions is mandatory. Surgeons must recognize however, that patients can feel that what they could not feel with injection anesthetics. As long as the patient is informed, they will not startle or move and will 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. A sterile adhesive drape is applied, and a Kershner reversible speculum is positioned under the eyelid margins, away from the incisional site. I then locate the proper steep meridian using an ocular reticle in the operating microscope (consulting my worksheets as I do so), marking the cornea with a dry marker, and then 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.
Proper Incision Size. The most common error inexperienced surgeons make when constructing clear corneal arcuate incisions is to use a keratome that doesn't allow proper keratotomy construction length and depth. Corneal incisions longer than 3.2 mm will induce more flattening and unwanted aberration in the refractive power of the central cornea. These incisions usually do not seal well on their own, induce gaping, denervation, dry spots and scarring. Incisional lengths of 3 mm or less seal appropriately and are more predictable in delivering the proper degree of flattening.
Use the Proper Blade. Standard disposable steel keratomes used routinely for scleral tunnel incisions won't work well for clear corneal astigmatic incisions. Only sharp keratomes can atraumatically incise the cornea. Many 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.
Correct Not Overcorrect 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. The rule is, always locate your incision on the steepest meridian. Can't determine the steepest meridian? Simply refract the patient in plus cylinder and refer to the corneal topographic map. Placing the clear corneal incision anywhere but on the steep meridian will make the patient worse. Why? Flattening the cornea in one meridian will induce astigmatism ninety degrees from the incision. Limbal relaxing incisions, because they are placed far peripherally on the corneal-scleral limbus, have less flattening effect for a given length. As a result, they must be long enough 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 the 7mm optical zone, have little downside risk, are less problematic, easier to perform and more predictable.
Achieving the Best Refractive Outcome. Surgeons have been slow to accept the techniques of astigmatism management with their cataract procedure because of resistance to acquire new skills and the need for new instrumentation. To assure a precise, reproducible method to achieve better refractive outcomes, the enlightened refractive lens surgeon needs to adopt a set of rules:
Rule #1. To accurately correct astigmatism, we have to accurately measure it.
Rule #2. Never overcorrect the cylinder power.
Rule #3. Always apply the astigmatic correction on the proper meridian -never operate off axis.
Rule #4. Use the proper tools.
The refractive results of clear corneal lens surgery speak for themselves. Both the sphere and the cylinder 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 corneal incision size is so small, the patient can touch their eyes, resume normal daily activities and need not be concerned that they will interfere with the outcome of their procedure.
Today's modern techniques of microincision lens surgery have enabled surgeons to fully correct refractive error with crystalline lens removal and IOL implantation. Smaller, 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 and presbyopic intraocular lenses can maximize the full refractive correction for each patient.
Surgeons have within their grasp, the techniques for optimizing the refractive results of their lens replacement procedure. Full refractive correction at the time of lens surgery is predictable and easily accomplished. With patients seeking the ultimate vision restoration procedure, it should be the goal of every ophthalmic surgeon.
Legends to Figures.
Figure 1. Arcuate Astigmatic Incisions accurately placed on the steep meridian can predictably flatten the cornea.
Figure 2. Toric IOLs implanted along the steep meridian can neutralize pre-existing astigmatism.
About the Author: Dr. Kershner is Clinical Professor of Ophthalmology at the John A. Moran Eye Center, University of Utah School of Medicine, Salt Lake City, and President of Eye Laser Consulting in Boston, Massachusetts. He has no financial or proprietary interest in the techniques or devices described.