Louis D. Nichamin, MD
Over the past several years, the concept of refractive cataract surgery has received increased attention from surgeons, and the need for its adoption has recently been made urgent by the approval and availability of new presybopia correcting IOLs. The need to manage preexisting astigmatism is becoming required in the practice of modern phaco surgeons. Experience with keratorefractive surgery has proven that astigmatism as little as 0.75 D may leave a patient symptomatic with visual blur, ghosting, and halos. To fully embrace the possibilities of refractive cataract surgery, surgeons must aspire to a level of accuracy that one would equate with corneal-based refractive surgery. The most popular approach to achieve this goal is through the use of corneal, and specifically, limbal relaxing incisions (LRIs).
Limbal Relaxing Incisions
As Robert Osher, MD, has discussed, the modern era of cataract and astigmatism surgery began in the mid-1980s as he and other surgeons first suggested that astigmatic keratotomy may be combined with cataract and implant surgery.1,2 Over time, many surgeons began to recognize the benefit of moving these incisions to a more peripheral, limbal location.3-5 Experience has shown that LRIs hold several advantages over astigmatic keratotomy incisions placed at a more central optical zone. Advantages include less of a tendency to cause a shift in the resultant cylinder axis and less likelihood of inducing irregular astigmatism. Incisions are easier to create and are, overall, more forgiving. Another important advantage gained by moving to the limbus involves the “coupling ratio,” which describes the amount of flattening that occurs in the incised meridian relative to the amount of steepening that results 90° away; paired LRIs (when kept at or under 90° of arc length) exhibit a consistent 1:1 ratio and therefore elicit little change in spheroequivalent, obviating the need to make any change in implant power.
What to Do?
Perhaps the most challenging aspect of astigmatism surgery involves the determination of the quantity and exact location of the cylinder that is to be corrected. Preoperative measurements – keratometry, refraction, and corneal topography – do not always correlate. Lenticular astigmatism may account for some of this disparity, particularly in patients where there is a wide variance between refraction and corneal measurements; however, some discrepancies are likely due to the inherent shortcomings of traditional measurements of astigmatism. Standard keratometry, for example, measures only two points in each meridian at a single optical zone of approximately 3 mm.
When confounding measurements arise, one may compromise and average the disparate readings; for example, if refraction shows 2 D of astigmatism and keratometry reveals only 1 D, it is reasonable to correct for 1.5 D. Alternatively, if preoperative calculations vary, one may defer placing the relaxing incisions until a stable refraction post-implantation is obtained, and then correct the astigmatism; LRIs may be safely performed in the office in an appropriate treatment-room setting. Corneal topography is helpful when refraction and keratometry do not agree, and it is increasingly becoming the overall guiding measurement upon which the surgical plan is based. Topography is also helpful in detecting subtle corneal pathology such as keratoconus fruste, which would likely negate the use of LRIs, or subtle irregular astigmatism such as that caused by epithelial basement membrane dystrophy.
Nomograms
Once the amount of astigmatism to be corrected is determined, a nomogram must be consulted to determine the appropriate arc length of the incisions. Many popular nomograms are available.6 The author’s nomogram of choice originated from the work of Stephen Hollis, MD, and incorporates concepts taught by Spencer Thornton, MD, particularly his age modifiers.7 As seen in Table 1, astigmatism is considered to be with-the-rule when the steep axis (plus cylinder) is between 45° and 135°. Against-the-rule astigmatism is considered to fall between 0° and 44°, and 136° and 180°. One aligns the patient’s age with the amount of preoperative cylinder to be corrected and finds the suggested arc length that the incisions should subtend.
Table 1
An empiric blade depth setting is commonly used when performing LRIs, typically at 600 microns. This is a reasonable practice when treating patients with cataracts; however, in the setting of refractive lens exchange surgery or when using presbyopia correcting IOLs – where ultimate precision is required – the author prefers to perform pachymetry and use adjusted blade depth settings. Pachymetry may be performed either preoperatively or at the time of surgery. Readings are taken over the entire arc length of the intended incision and an adjustable micrometer diamond blade is then set to approximately 90% of the thinnest reading obtained. Refinements to the blade depth setting, as well as nomogram adjustments, may be necessary depending upon individual surgeon technique, the instruments used, and the style of the blade. In eyes that have previously undergone radial keratotomy, the length of the incisions should be reduced by approximately 50%. In eyes that have undergone significant prior keratotomy, it may be best to avoid additional incisional surgery and use a toric IOL or laser technology instead.
How To Do It: Surgical Pearls
In most patients, the relaxing incisions are placed at the outset of surgery to minimize epithelial disruption. The one exception to this rule occurs when the phaco incision intersects or is encompassed within an LRI of greater than 40° of arc; if it is extended to its full arc length at the start of surgery, then significant gaping and edema may result secondary to intraoperative wound manipulation. In this setting, the phaco incision is first made by creating a shortened LRI with an arc length that corresponds to the width of the phaco and IOL incision. This amounts to a two-plane grooved phaco incision with a depth of either 600 microns or measurement as determined by pachymetry (described above). Following IOL implantation and before viscoelastic removal, while the globe is still firm, the relaxing incision is extended to its full arc length as dictated by the nomogram. When an LRI is superimposed upon the phaco tunnel, the keratome entry is accomplished by pressing the bottom surface of the keratome blade downward upon the outer or posterior edge of the LRI. The keratome is then advanced into the LRI at an iris-parallel plane. This angulation promotes a dissection that takes place at mid-stromal depth, which will help ensure adequate tunnel length and a self-sealing closure.
Proper centration of the incisions over the steep corneal meridian is of utmost importance. According to Euler’s theorem, an axis deviation of 5°, 10°, or 15° results, in effect, in 17%, 33%, and 50% reduction, respectively.8 This reduction holds true for relaxing incisions and toric IOLs. Also, increasing evidence supports the notion that significant cyclotorsion may occur when assuming a supine position.9 For this reason, most surgeons advocate placing an orientation mark at the 12:00 or 6:00 limbus while a patient is in an upright position. This is important when using injection anesthesia wherein unpredictable ocular rotation may occur. An additional measure that may be used to help center the relaxing incisions is to identify the steep meridian (plus cylinder axis) intraoperatively using some form of keratoscopy. The steep meridian over which the incisions are to be placed corresponds to the shorter axis of the reflected corneal mire. Another common way in which the steep meridian is marked uses a Mendez ring or similar degree gauge which is aligned with the previously placed limbal orientation mark, and then locating the cylinder axis on the 360° gauge.
The LRI should be placed at the most peripheral extent of clear corneal tissue, just inside of the true surgical limbus, whether or not pannus is present. If bleeding occurs, it may be ignored and will cease spontaneously. One must avoid placing the incisions farther out at the true surgical limbus because a significant reduction of effect will likely occur due to increased tissue thickness and a variation in tissue composition; these incisions are, therefore, intra-limbal in nature. In creating the incision, it is important to hold the knife perpendicular to the corneal surface to achieve consistent depth and effect, and help avoid gaping of the incision. Good hand and wrist support is important, and the blade should be held as if one were throwing a dart such that the instrument may be rotated between thumb and index finger as it is being advanced, leading to smooth arcuate incisions. Typically, the right hand is used to create incisions on the right side of the globe, and the left hand for incisions on the left side. In most cases, it is more efficient to pull the blade toward oneself, as opposed to pushing it away.
Complications
LRIs have become a safe and effective way of managing astigmatism at the time of cataract surgery. Nonetheless, as with any surgical technique, potential complications exist, and several are listed in Table 2. Of these complications, the most likely is the placement of incisions upon the wrong axis. When this occurs, it typically takes the form of a 90° error with positioning upon the opposite, flat meridian. This results in an increase and likely doubling of the patient’s preexisting cylinder. Diligent attention is required in this regard. The surgeon should consider using safety checks to prevent this complication from occurring such as having a written plan that is brought into the operating room and is kept visible and properly oriented. Incisions are always placed upon the plus (+) cylinder axis and opposite to the minus (-) cylinder axis.
Table 2
Potential Problems
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Although rare, corneal perforation is possible. This may be due to improper setting of the blade depth, or as a result of a defect in the micrometer mechanism. This latter problem may arise after repeated autoclaving and many sterilization runs. Periodic inspection and calibration is warranted, even with preset single-depth knives. When encountered, unlike radial microperforations, these circumferential perforations rarely self-seal and will likely require placement of temporary sutures.
Summary
Refinement of the refractive outcome may be the single-most pressing challenge encountered by cataract surgeons. Along with spherical error, preexisting astigmatism may now be safely and effectively reduced at the time of cataract surgery. Astigmatic relaxing incisions are the most common method used to accomplish this goal. By moving these incisions out to an intralimbal location, the complications and difficulties associated with astigmatic keratotomy have been reduced.
References
- Osher RH. Combining phacoemulsification with corneal relaxing incisions for reduction of preexisting astigmatism. Paper presented at: the annual meeting of the American Intraocular Implant Society; 1984; Los Angeles; Calif.
- Maloney WF. Refractive cataract replacement: A comprehensive approach to maximize refractive benefits of cataract extraction. Paper presented at: the annual meeting of the American Society of Cataract and Refractive Surgery; 1986; Los Angeles, Calif.
- Budak K, Friedman NJ, Koch DD. Limbal relaxing incisions with cataract surgery. J Cat Refract Surg. 1998; 24:503-508.
- Muller-Jensen K, Fischer P, Siepe U. Limbal relaxing incisions to correct astigmatism in clear corneal cataract surgery. J Refract Surg. 1999; 15:586-589.
- Nichamin LD. Changing approach to astigmatism management during phacoemulsification: Peripheral arcuate astigmatic relaxing incisions. Paper presented at: the annual meeting of the American Society of Cataract and Refractive Surgery; 2000; Boston, Mass.
- Gills JP. A Complete Guide to Astigmatism Management. 2003.
- Thornton SP. Radial and Astigmatic Keratotomy: The American System of Precise, Predictable Refractive Surgery. 1994.
- Abrams D. Ophthalmic optics and refraction. In: Duke-Elder SS, ed. System of Ophthalmology. St. Louis, Mo: Mosby; 1970:671-674.
- Swami AU, Steinert RF, Osborne WE, et al. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol. 2002; 133:561-562.