“Astigmatic keratotomy and limbal relaxing incisions: principles, indications, nomograms” Authors: Eric D Rosenberg, Alanna S Nattis, Eric D Donnenfeld

In most surgical subspecialties, the patient views and evaluates two variables: the

incision and the surgeon; however, in an ophthalmic case, we can include one additional

and exceptionally formidable variable: the subjective result. Like it or not, most

ophthalmic surgeries must now be viewed as having three primary end points that include

safety, function, and now refractive outcomes. While safety, or the absence of operative

complications is the outcome by which many of us have analyzed our results for decades,

we have collectively advanced the field to a point where achieving an optimal refractive

outcome holds an almost equivalent weight. As such, astigmatic keratotomies and limbal

relaxing incisions alone or during cataract surgery provide the surgeon with a keystone

procedure that improves patient quality of vision, quality of life, and satisfaction with the

procedure. Limbal relaxing incisions and astigmatic keratotomies require careful

management and execution.

Astigmatism results when the axes of the cornea are unequal and unevenly curved,

and may result in glare, symptomatic blur, ghosting, and halos with as little as 0.50

diopters1 [Figure 1]. Regular and irregular astigmatism can contribute to the meridional

blur that leads to the decreased uncorrected visual acuity experienced by patients. In the

astigmatic cornea, the incoming light rays pass through different meridians causing the

light to focus at more than one location anterior, posterior, or directly on the retina.

Furthermore, regular corneal astigmatism can be subcategorized as with-the-rule (WTR),

where the steep axis of the cylinder is within 15 degrees of the 90 degree vertical

meridian, against-the-rule (ATR), where the steep axis of the cylinder is within 15

degrees of the horizontal meridian, or oblique, where the steep axis is not within 15

degrees of either the vertical or horizontal meridians [Figure 2]. While there are several

different options for treating corneal astigmatism, such as toric intraocular lenses,

excimer laser photoablation, femtosecond laser lenticule removal, conductive

keratoplasty, limbal relaxing incisions (LRIs), and astigmatic keratotomies (AKs), not all

of these options may be available to the cataract surgeon, or correct for the patient. At

times the primary procedure may not correct astigmatism fully and a second procedure

can be additive. Additional considerations in choosing the refractive procedure of choice

include cost effectiveness, convenience, and safety.

LRIs are partial thickness corneal incisions placed adjacent to the limbus along the

steep meridian. The incision for 3 diopters of astigmatism or less reduces the astigmatism

by relaxing the steep axis of regular corneal astigmatism, while simultaneously

steepening the flat axis in a one to one ratio, a phenomenon known as ‘coupling’ [Figure

3]. Arcuate AKs are a similar incisional procedure that is performed more centrally

towards the visual axis. The advantages of LRIs include relative ease of performance, a

reduced tendency to cause axis shift, less irregular astigmatism, less dependent on

pachymetry, less likely to result in over correction, a smoother postoperative topography,

and a quicker post-op stabilization of refraction. Advantages of AKs include a shorter

more powerful incision and a ‘multifocal’ effect.. Relative contraindications to AKs and

LRIs include eyes that had previously undergone radial keratotomies (RKs), keratoconus,

other topographic abnormalities including irregular astigmatism, or known peripheral

corneal disease.2

LRIs tend to work well for low to moderate amounts of astigmatism (<3D), and very

well for 0.5D of cylinder up to 1.5D. When dealing with a patient who has >1.5D of

astigmatism, one should always consider the increased risk for irregular astigmatism and

dry eye disease. Additionally, combination of post-CEIOL excimer laser with an LRI is a

reasonable strategy for “debulking” astigmatism.

Since the conception of the astigmatic incisions to efficiently minimize astigmatism

during surgery, there has been concurrent research into the appropriate incision

placement, length, and depth. Not surprisingly, several LRI and AK nomograms exist for

correcting small amounts of cylinder, and many studies have been conducted in order to

evaluate the efficacy, physics, and outcomes.1,3-17 One study by Bradley et. al. showed

that LRIs produce a 60% average reduction in cylinder, with 79% of patients corrected to

less than one diopter of cylinder, and 59% of patients corrected to less than one half

diopter of cylinder.18 These results compare favorably with the results achieved using

toric IOLs, which resulted in an average 58.4% mean reduction in cylinder.19

Many AK and LRI nomograms are adjusted for age, gender, and cylinder axis,

making them detailed and complex, and may give the impression that these procedures

are extremely precise and unforgiving. However, in our opinion this simply isn’t the case,

and astigmatic incision placement, especially manual ones, still remain as much of an art

as a science. For the novice surgeon a simple nomogram may be favored (table 1), such

as the Donnenfeld nomogram (DONO), which works extremely well for this purpose

(available online at www.LRIcalculator.com) [Figure 4]. The online calculator employs

vector analysis in order to calculate incision parameters based on preoperative patient

keratometry, surgically induced astigmatism, and location of planned primary cataract

incision. If the Donnenfeld or Nichamin nomogram is selected, a visual map of the axis

and lengths of incisions will be provided, and a printout of the LRI calculator can be

brought to the operating room and used as a guide when marking the cornea and

performing LRIs. In general, it is best to practice the techniques of LRIs and develop

your own nomogram to achieve consistent results.

The operating room is the best place to start performing astigmatic incisions, and can

often be combined with routine cataract surgery. When performing cataract surgery it is

important to account for astigmatism that may be preexisting or surgically induced.

Residual astigmatism of 0.50 diopters (D) or even less may result in glare, symptomatic

blur, ghosting, and halos.1 The reduced quality of vision associated with residual

astigmatism following cataract surgery is magnified in patients with multifocal

intraocular lenses. As a result, greater emphasis has been placed on treating corneal

astigmatism at the time of cataract surgery. In the beginning, peribulbar anesthesia and a

conventional monofocal IOL may be preferred, as patients implanted with

presbyopia-correcting IOLs are significantly more sensitive to even minor refractive

errors. Astigmatic incisions should be performed at the start of the case while the eye is

firm, and prior to any manipulation or dehydration of the cornea induced by

instrumentation or operating microscope. The cornea may be marked, especially for

larger cylindrical errors. For the incision, the episclera is grasped at the limbus with a

0.12 forceps approximately 180 degrees away from the intended incision site. While

approaching perpendicular to the cornea, a pre-set diamond knife is advanced into the

cornea 0.5mm central to the limbus, and centered on the axis as determined by vector

analysis of residual cylinder. With multiple companies manufacturing various pre-set

diamond knives, our preference is for the 0.6mm pre-set depth. After advancement, the

knife is held into position for a full second to ensure the full depth of the blade is

achieved (Figure 5). A shallow ineffective incision is one of the most common mistakes

for a novice surgeon. The incision is then extended to its desired length. For control

purposes, it is always preferable to cut towards oneself.

After the surgeon becomes more comfortable with the technique, astigmatic incisions

may be employed on any patient undergoing cataract surgery, especially if they are likely

to end up with half a diopter or more of residual cylinder. Alternatively, the experienced

surgeon may elect to perform this as an in-office stand-alone procedure under the

microscope or at the slit-lamp. Slit-lamp LRIs are a 30 second procedure, and typically

the patients are walking out of the room seeing better than they did entering. To perform

this procedure, the phoropter is used to locate the incision axis and is placed adjacent to

the patient’s eye with the cylinder stripe aligned on the steep axis of astigmatism.

Lidocaine gel is administered into the operative eye, and the patient should be

comfortable with their head placed forward against that slit-lamp’s headband. With an

angled pre-set diamond knife and the surgeon coming from the side, the procedure is

performed as previously described [Figure 6]. Following the procedure, we recommend

topical antibiotics and anti-inflammatory drops four times daily for five days.

As with any surgical procedure, there are potential complications associated with

astigmatic incisions, but most are either temporary or correctable. Possible complications

include overcorrection, undercorrection, perforation of the cornea, infection, decreased

corneal sensation, dry eye, irregular astigmatism, and discomfort. LRIs are generally not

associated with glare or starbursts, but may be experienced by those undergoing AKs. For

both under and overcorrection, we recommend waiting for the refraction to stabilize. A

waiting period of 1-2 months is typically adequate, however this remains highly surgeon

dependent. In patients with significant remaining astigmatism, it may be necessary to

retreat by deepening or enlarging the original incision. For overcorrected patients, the

original incision may be partially sutured closed with a 10-0 nylon or prolene after

cleaning it with the assistance of a Sinskey hook. Placing additional LRIs perpendicular

to the original incision for consecutive cylinder without suturing the original incision is

not recommended, as this may induce irregular astigmatism. In the event of a corneal

perforation, only non-self sealing incisions may need a 10-0 nylon suture.

Not surprisingly, LRIs have been subject to the natural progression of any surgical

procedure with the aim of reducing risk and improving outcomes. The treatment of

astigmatism with femtosecond laser-assisted corneal incisions offers a greater degree of

precision and accuracy than manual methods 20-22, and thus improves the risk profile for

the possible complications mentioned previously. The recent developments in

femtosecond laser technology have shifted the movement from manual LRI and

astigmatic keratotomy procedures to femtosecond laser-guided procedures.

Femtosecond lasers are photodisruptive lasers, in contrast to photoablative and

photocoagulative lasers, with extraordinarily short pulse duration of less than 800

femtoseconds (one femtosecond is 10-15 seconds). This extremely short pulse time allows

the femtosecond laser to cut tissue with considerably less energy than traditional lasers.

Per-pulse energies can be reduced approximately 1,000-fold, from around 1-10 millijoules

for nanosecond lasers (neodymium-yttrium aluminum garnet [Nd:YAG]) to 1-10

microjoules for femtosecond lasers. Both the short 1053nm wavelength, which is not

absorbed by optically clear tissues at low power densities, and the reductions in per-pulse

energy, result in substantially reduced collateral tissue damage, shifting concomitant

damage from a few spherical millimeters with the nanosecond lasers to a few microns

with the femtosecond lasers.

The precision and extremely limited collateral effects of femtosecond pulses on the

cornea have been established in millions of procedures performed using femtosecond

lasers for the creation of LASIK flaps and lamellar and penetrating keratoplasties.23-27

Additionally, the corneal tissue does not absorb the laser wavelength, allowing for a

higher margin of safety. Because a pre-programmed depth is applied to all incisions, a

sizeable distance is kept from Descemet’s membrane, preventing corneal perforation

during femtosecond-guided treatments.

Femtosecond laser-assisted cataract surgery is currently approved by the US Food

and Drug Administration for four clinical indications: primary incisions, arcuate

incisions, lens fragmentation and capsulotomy creation. The use of the femtosecond laser

to create arcuate incisions is a major clinical improvement in that it allows greater

precision due to more accurate arc length, depth, angular position, and optical zone. The

femtosecond laser allows for exact and repeatable incisions, which are necessary for

consistent results not ordinarily achieved through manual methods.20,29

In an early study, using 8-mm arcuate incisions and a 33% reduction of the

Donnenfeld nomogram a 70% reduction in astigmatism was achieved.28 An additional

advantage of the femtosecond laser is that the corneal incisions may be placed

intrastromally (sub-Bowman’s layer), which improves healing by sparing epithelial

damage. This is a major area of interest and nomogram development is currently

underway to optimize this method, which would eliminate the need for corneal wound

manipulation on the surface.

Performing femtosecond laser arcuate incisions requires the parameters of length,

position, depth, and distance from the visual axis where the incisions will be created. For

our practice, we use a 33% reduction of the Donnenfeld nomogram to determine the

length and axis at which the incision should be placed. The depth of our incisions is 85%

of the corneal pachymetry in the area of the incision. We have set our distance from the

visual axis at 9 mm. This programming information is downloaded onto the femtosecond

laser. We then begin the surgical procedure by docking the laser onto the cornea. An

overlay of the incisions is then visible on the surgical screen (Figure 7). Optical

coherence tomography (OCT) imaging of the cornea in the area of the arcuate incision is

then visualized, and the depth is confirmed (Figure 8). The femtosecond astigmatic

incision is then performed (Figure 9). We first perform the capsulotomy, followed by the

lens fragmentation, and then create our corneal incisions. Following the conclusion of the

femtosecond laser treatment, the patient is brought to the operating microscope and the

incisions are opened with a Sinskey hook (Figure 10). OCT confirms the postoperative

depth of the incisions. Some surgeons prefer to open one or both of the incisions

postoperatively under the guidance of the post-surgical refraction. By utilizing the low

energy of the femtosecond laser, the incisions do not have significant effect until they are

opened.

Femtosecond laser-assisted arcuate incisions have brought computerized accuracy

and precision to astigmatism management and cataract surgery. Refractive incisions are

now digitally assisted and do not solely rely on surgeon skill or experience. The use of a

femtosecond laser system will provide faster, safer, easier, customizable, adjustable, and

fully repeatable astigmatic incisions. The ability to perform intrastromal ablations cannot

be achieved with manual incisions and offer major advantages in terms of patient comfort

and safety. Femtosecond laser arcuate astigmatic incisions should enable the majority of

ophthalmologists to enter the field of refractive cataract surgery with confidence.

In conclusion, astigmatic corneal surgery with a diamond knife or a femtosecond

laser dramatically improves cataract surgery refractive results. Astigmatic corneal

surgery can be performed intraoperatively or postoperatively in order to titrate residual

corneal cylinder. The most common rate-limiting factor for refractive results following

cataract surgery is residual astigmatism, and astigmatic incisional corneal surgery is often

the solution to improve patient refractive results and overall satisfaction.

Figure 1: Figure demonstrates normal cornea (left) shaped like a basketball in which both axes are equal and astigmatic cornea (right) shaped like a football in which one axis is steeper than the other. Figure 2: Figure demonstrates WTR (left), ATR (center), and oblique (right) astigmatism. Figure 3: Diagram demonstrating the coupling effect of an LRI, before (left) and after (right).

Figure 4: The Donnenfeld Nomogram found at LRIcalculator.com

Figure 5: LRI performed at the microscope

Figure 6: LRI performed at the slit lamp

Figure 7: Overlay of the planned incision visible on the surgical screen

Figure 8: OCT of cornea with the femtosecond laser planned LRIs visible

Figure 9: The femtosecond astigmatic incision performed

Figure 10: Intraoperative picture showing the opening of an LRI with a Sinskey hook

Table 1: Nomogram table

REFERENCES

1. Nichamin LD. Nomogram for limbal relaxing incisions. J Cataract Refract Surg. 2006;32(9):1048.

2. Mastering Refractive IOLs: The art and science. David F. Chang. Chapter 174. Page 638.

3. Nichamin LD. Astigmatism control. Ophthalmol Clin North Am. 2006;19:485-93.

4. Wang L, Misra M, Koch DD. Peripheral corneal relaxing incisions combined with cataract surgery. J Cataract Refract Surg 2003;29:712-722.

5. Budak K, Friedman NJ, Koch DD: Limbal relaxing incisions with cataract surgery. J Cataract Refract Surg 24:503, 1998

6. Gills JP. Treating astigmatism at the time of cataract surgery. Curr Opinion Ophthalmol 2002;13:2-6.

7. Oshika T, Shimazaki J, Yoshitomi F, et al.: Arcuate keratometry to treat corneal astigmatism after cataract surgery: a prospective evaluation of predictability and effectiveness. Ophthalmol 105:2012, 1998

8. Maloney WF, Grindle L, Sanders D, et al: Astigmatism control for the cataract surgeon: a comprehensive review of surgically tailored astigmatism reduction (STAR). J Cataract Refract Surg 15:45, 1989

9. Price FW, Greene RB, Marks RG, et al.: Astigmatism reduction clinical trial: a multicenter prospective evaluation of the predictability of arcuate keratotomy. Arch Ophthalmol 113:277, 1995

10. Devgan U. Corneal Correction of Astigmatism During Cataract Surgery. Cataract Refractive Surgery Today 2006;7:41-44.

11. Tejedor J, Murube J. Choosing the location of corneal incision based onpreexisting astigmatism in phacoemulsification. Am J Ophthalmol. 2005 May;139(5):767-76.

12. Kaufmann C, Peter J, Ooi K, Phipps S, Cooper P, Goggin M; The Queen Elizabeth Astigmatism Study Group. Limbal relaxing incisions versus on-axis incisions to reduce corneal astigmatism at the time of cataract surgery. J Cataract Refract Surg. 2005 Dec;31(12):2261-5.

13. Muller-Jensen K, Fischer P, Siepe U. Limbal relaxing incisions to correct astigmatism in clear corneal cataract surgery. J Refract Surg. 1999 Sep-Oct;15(5):586-9.

14. Akura J, Matsuura K, Hatta S, Otsuka K, Kaneda S. A new concept for the correction of astigmatism: full-arc, depth-dependent astigmatic keratotomy. Ophthalmology. 2000 Jan;107(1):95-104.

15. Faktorovich EG, Maloney RK, Price FW Jr. Effect of astigmatic keratotomy on spherical equivalent: results of the Astigmatism Reduction Clinical Trial. Am J Ophthalmol. 1999 Mar;127(3):260-9.

16. Price FW, Grene RB, Marks RG, Gonzales JS. Astigmatism reduction clinical trial: a multicenter prospective evaluation of the predictability of arcuate keratotomy. Evaluation of surgical nomogram predictability. ARC-T Study Group. Arch Ophthalmol. 1995 Mar;113(3):277-82. Erratum in: Arch Ophthalmol 1995 May;113(5):577.

17. Oshika T, Shimazaki J, Yoshitomi F, Oki K, Sakabe I, Matsuda S, Shiwa T, Fukuyama M, Hara Y. Arcuate keratotomy to treat corneal astigmatism after cataract surgery: a prospective evaluation of predictability and effectiveness. Ophthalmology. 1998 Nov;105(11):2012-6.

18. Bradley MJ, Coombs J, Olson RJ. Analysis of an approach to astigmatism correction during cataract surgery. Ophthalmologica. 2006;220(5):311-6.

19. Package Insert. Acrysof ToricTM SA60T4IOL, Alcon Laboratories, Inc. 20. Nichamin L. Femtosecond laser technology applied to lens-based surgery.

Medscape Ophthalmology. June 22, 2010. http://www.medscape.com/viewarticle/723864. Accessed July 20, 2011.

21. Nagy Z, Takacs A, Filkom T, Sarayba M. Initial clinical evaluation of an intraocular femtosecond laser in cataract surgery. J Refract Surg. 2009;25(12):1053-1060.

22. Masket S, Sarayba M, Ignacio T, Fram N. Femtosecond laser-assisted cataract incisions: architectural stability and reproducibility. J Cataract Refract Surg. 2010;36(6):1048-1049.

23. Ratkay-Traub I, Ferincz IE, Juhasz T, Kurtz RM, Krueger RR. First clinical results with the femtosecond neodymium-glass laser in refractive surgery. J Refract Surg, 2003;19(2):94-103.

24. Nordan LT, Slade SG, Baker RN, Suarez C, Juhasz T, Kurtz RM. Femtosecond laser flap creation for laser in situ keratomileusis: six-month follow-up of initial U.S. clinical series. J Refract Surg. 2003;19:8-14.

25. Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg. 2004; 30(4):804-811.

26. Montés-Micó R, Rodríquez-Galietero A, Alió JL. Femtosecond laser versus mechanical keratome LASIK for myopia. Ophthalmology. 2007;114:62-68.

27. Steinert RF, Ignacio TS, Sarayba MA. “Top hat”-shaped penetrating keratoplasty using femtosecond laser. Am J Ophthalmol. 2007;143(4):689-691.

28. Slade SG. Femtosecond laser arcuate incision astigmatism correction in cataract surgery. Presented at: the ASCRS Cornea Day; March 25, 2011; San Diego, CA.

29. Abbey A, Ide T, Kymionis GD, Yoo SH. Femtosecond laser-assisted astigmatic keratotomy in naturally occurring high astigmatism. Br J Ophthalmol. 2009;93(12):1566-1569