Frank A. Bucci Jr, MD
The efficient and accurate correction of residual refractive errors following the implantation of presbyopic-correcting intraocular lenses (IOL) is critical for achieving high levels of both spectacle independence and patient satisfaction. One diopter (D) of astigmatism corresponds to an uncorrected distance vision of 20/30. Almost all patients receiving presbyopic-correcting IOLs, but especially those paying additional fees for these premium IOLs, will experience 20/30 uncorrected distance acuity as unacceptable. They will frequently even perceive a significant difference between 20/25 and 20/20 uncorrected distance visual acuity. Limiting your patient selection to those patients with low amounts of preoperative cornea cylinder will not eliminate your need to acquire skills for correcting residual refractive errors. Wound dynamics and other unpredictable factors inherent in IOL placement will frequently result in residual astigmatism of at least 0.75 D, even though preoperative keratotomy readings predicted that this would not occur.
My general guideline is that 0.50 D or less of spherical error and 0.50 D or less of astigmatism are consistent with acceptable uncorrected visual outcomes by the post–presbyopic-correcting IOL patient. When spherical and cylindrical postoperative refractive errors reach 0.75 D or greater, patients will perceive a meaningful benefit with surgical treatment of their residual refractive error.
The most frequently performed procedures for correcting residual refractive errors following use of posterior chamber IOLs include 1) limbal relaxing incisions (LRIs) at the time of implantation, 2) LASIK at least a few months postoperatively, and 3) postoperative surface laser ablation, such as photorefractive keratectomy (PRK), at least 1 month following implantation. In the majority of cases, I use an alternative technique that I call micro-RK/AK (radial keratotomy/astigmatic keratotomy).
What is micro-RK/AK? This technique is performed no sooner than 3-weeks postoperatively. It includes arcuate incisions in the peripheral cornea that are identical to those used in any other LRI or AK procedure. The surgeon can use his or her nomogram of choice for correcting astigmatism. The second component of the micro-RK/AK technique is the micro-RK. When the implant power is selected preoperatively, the target for the spherical outcome is plano to -0.50. So, in more than 90% of cases, the spherical outcome is either plano or slightly myopic. Micro-RK is used to correct the small degrees of myopia in a relatively noninvasive manner.1
It should be noted that LRIs or AK incisions performed at the time of surgery can now be performed in the traditional manner, with handheld diamond blades or with femtosecond lasers. The femto-AK procedure and incision have distinct advantages and disadvantages when compared with the alternative procedures mentioned above, including micro-RK/AK.
The capacity of femtosecond lasers to correct corneal astigmatism is in its developmental stages. In the future, I believe this emerging technology will compete with or surpass competing techniques for correcting corneal astigmatism at the time of cataract surgery or as an independent procedure. Currently, the character of the femto-AK incision and the procedure performed by the femtosecond laser does not compare favorably with the incision created by an experienced diamond blade incisional AK or LRI surgeon.
The term micro-RK2 is used to distinguish this technique from both mini-RK3 and traditional RK, in which optical zones as small as 3.00 mm are frequently used. In micro-RK, the optical zone is never smaller than 5.00 mm. Since only a very small amount of myopia is being corrected usually, only 1 or 2 micro-radial incisions are necessary. The length of these incisions is always less than 2.50 mm. This is less than the thickness of 2 dimes (2.75 mm). A small portion of the Lindstrom 2-incision mini-RK nomogram (Figure 21-1) is used to select the surgical optical zone. I have been using this nomogram for 25 years, and it is extremely accurate. Fluctuating vision and hyperopic shifts do not occur with radial incisions this small.
ADVANTAGES OF MICRO-RADIAL KERATOTOMY/ASTIGMATIC KERATOTOMY
The advantages of using this technique compared with the alternative techniques are numerous. The advantages of micro-RK/AK vs LRI are mostly related to accuracy, and accuracy is even more critical when dealing with demanding elective presbyopic patients. Since the micro-RK/AK is performed 3 or more weeks postoperatively, the surgeon has the advantage of knowing the exact amount of cylinder, the exact axis of the cylinder, and the exact amount of spherical error. The micro-RK component then provides you with an opportunity to fix small amounts of residual myopia with excellent precision. There is greater ease and precision when this procedure is performed on a nonphaco day, because 1) the pupil is not dilated, 2) it is easier to mark the axis, 3) it is easier to perform pachymetry at each incision site and custom set your diamond blade, and 4) the temporal and paracentesis wounds are more stable.
The advantages of micro-RK/AK over conventional LASIK are also significant. The typical premium IOL patient is usually older than 45 years and at an increased risk for dry eye (especially women). Even lesser degrees of aqueous or evaporative dry eye can compromise the visual outcomes of these already demanding patients. Micro-RK/AK does not create a neurotropic cornea, as does the cutting of a flap with LASIK. The central 5.00 mm of the cornea is untouched. There will be no flap complications, no diffuse lamellar keratitis, and no central epithelial defects.
Figure 21-1. Lindstrom 2-incision mini-RK nomogram. Highlighted area designates the part of the nomogram used for micro-RK. Note that optical zones less than 5.00 mm are never used with micro-RK.
Some surgeons wait up to 3 to 6 months before performing LASIK following an implant, because of concern about wound stability during creation of the flap. Micro-RK/AK can be performed with confidence much sooner. The cost of laser access (purchase or facility fee) and the procedure (procedure card and microkeratome blade) will almost always be significantly greater than this incisional technique. Since the optics of multifocal IOLs are not compatible with custom LASIK, the advantage of a custom ablation cannot be realized when performing LASIK following the implantation of diffractive and refractive IOLs. LASIK enhancements further complicate the patient’s neurotropic dry eye, and cost again becomes a factor. The small incisions of this technique produce no greater postoperative symptoms than LASIK. LASIK becomes my preferred technique when the residual cylinder or sphere increases significantly. As the residual refractive error increases, there is a relative increase in precision with laser vision correction compared with an incisional technique.
The advantages of micro-RK/AK compared with traditional PRK are also substantial. The central 5 mm of the cornea remains untouched. There is less postoperative discomfort. Bilateral surgery is possible, and the visual recovery is quicker for these demanding patients. No bandage contact lens is required, there are fewer postoperative visits, and again this incisional technique is much less costly. There is much less stress on healing of the ocular surface, especially for patients with pre-existing ocular surface disorders, such as aqueous dry eye, meibomian gland dysfunction, anterior blepharitis, or delayed epithelial healing secondary to diabetes. There is frequently a subtle subepithelial haze following PRK that is usually tolerated by the typical PRK patient. In the multifocal implant patient who has already lost contrast sensitivity, the visual consequences of the subepithelial haze may be poorly tolerated.
The advantages of micro RK/AK over femto-AK begin with the architecture of the incisions. The vertical incisions of femto-AK are much less effective and efficient for correcting astigmatism than diamond blade incisions that are constructed perpendicular to the corneal surface. The effectiveness of the incisions is highly dependent on the achieved depth. Limitations in the visualization technology of many femtosecond lasers require a large margin of error when calculating the depth of the AK incision, and effectiveness is sacrificed. Larger arcuate incisions with smaller optical zones are required to correct equal amounts of astigmatism. Many femtosecond incisions require a secondary manual true opening of the incision, because of the “postage stamp” microcorrections of tissue characteristic of femtosecond tissue separation, as observed with femtosecond LASIK flaps.
In general, AK incisions with larger optical zones are less sensitive to small errors in axis alignment. Femto-AK is currently limited to smaller optical zones, because femtosecond incisions cannot be constructed in the peripheral cornea because of the arcus senilis frequently observed in patients undergoing cataract surgery. Axis alignment can also be challenging with the current femtosecond technology. Although companies are developing iris registration to manage torsional rotation, which will eventually improve outcomes, the process is in its early stages of evolution. As with LRI or AK performed at the time of surgery, femto-AK cannot correct small amounts of residual myopia, and the cost is also much greater than micro RK/AK.
I have used micro-RK/AK to correct residual refractive error following implant surgery in more than 2500 cases during the past 25 years. Figures 21-2 and 21-3 summarize the results of this technique in 320 eyes after receiving multifocal IOLs.4 Note that the final uncorrected visual acuity in the Array (Abbott Medical Optics) eyes (n = 111) is slightly better, because this group is exclusively lensectomy patients vs the ReZoom (Abbott Medical Optics) and ReSTOR (Alcon) eyes (n = 209), which are a mix of lensectomy and older cataract patients with less healthy maculas.
Figure 21-2. Postoperative results following micro-RK/AK in 111 eyes status post–refractive lensectomy with a multifocal IOL.
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