Approaches to Correcting Astigmatism

Limbal Relaxing Incisions
LRIs can be performed at the time of cataract surgery or afterward to eliminate residual cylinder.

BY ERIC D. DONNENFELD, MD
One of the most important aspects of achieving good surgical outcomes, patient satisfaction, and quality of vision for patients undergoing cataract surgery is the ability to treat astigmatism. Residual astigmatism results in glare, halo, and reduced Snellen visual acuity. There are many techniques for treating astigmatism; I strongly advocate excimer laser photoablation for its accuracy and toric IOLs for their simplicity and safety. However, there are several advantages to limbal relaxing incisions (LRIs) that make this procedure an important tool for refractive cataract surgeons.

ADVANTAGES AND PATIENT SELECTION
LRIs are inexpensive, safe, easy to perform, and do not preclude future excimer laser photoablation. By reducing cylinder, LRIs can make excimer laser photoablation more precise and reduce the amount of tissue removal needed. LRIs can be easily repeated and, most important, they can be performed at the time of cataract surgery. This offers patients the wow effect from having their cataract removed and cylinder reduced during one procedure rather than two separate ones. This aspect of LRIs is particularly important in the United States, as no toric presbyopic IOLs are currently approved for use.

There is a myth in the refractive surgery world that IOL patients will tolerate small residual refractive errors. Nothing could be further from the truth. All cataract surgery patients—particularly presbyopic IOL patients—are incredibly sensitive to even minor cylindrical errors. Refractive IOL surgeons must be willing and able to treat postoperative astigmatism to make patients happy postoperatively. Generally, patients will tolerate 0.50 D of cylinder or less. However, my goal as a refractive cataract surgeon is not merely to give patients vision they will tolerate, but to exceed their expectations. For this reason, I often fine-tune cataract patients who have good results with even 0.50 D of residual cylinder.

In my experience, more than 50% of cataract surgery patients have 0.50 D or more of astigmatism. For these patients with a spherical equivalent of plano, an LRI is an optimal treatment option that can be easily performed without risk of inducing hyperopia or other complications associated with excimer laser photoablation. A toric IOL, in my hands, is the correct treatment for patients expected to have 1.00 D or more of residual cylinder after cataract surgery. Toric IOLs approved in the United States treat up to 2.00 D of cylinder. Patients with more cylinder, or those who do not have complete resolution of their astigmatism, require a concomitant or subsequent LRI.

I use LRIs to treat 1.50 D or less of astigmatism. Treating more than 2.00 D of cylinder with an LRI is less precise, destabilizes the cornea, increases the risk of postoperative dry eye, and can induce higher-order aberrations. In patients who are not candidates for toric IOLs, I will debulk the cylinder with an LRI and then postoperatively treat the residual refractive error with excimer laser photoablation.

SURGICAL PEARLS
It is easy to fix up to 1.50 D of cylinder with one or two LRIs. For preexisting astigmatism, LRIs can easily be performed in the operating room. There are several LRI nomograms for correcting small amounts of cylinder, created by notable surgeons such as Louis D. “Skip” Nichamin, MD,¹ Douglas D. Koch, MD,² and James P. Gills, MD,3 among others. These nomograms are adjusted for age and cylinder axis, making them detailed and complex, giving the impression that the procedure is extremely precise and unforgiving. In my opinion, this simply is not the case. LRIs are as much an art as a science. It is best to practice techniques and develop your own nomogram to achieve consistent results. I have developed a simple nomogram that works extremely well for me (Table 1).

The operating room is the best place to start doing LRIs, and they can be done with routine cataract surgery. It is important to remember that patients' preexisting cylinder is not the only thing to consider when performing LRIs. Surgically induced cylinder is equally important. Vector analysis is required to calculate the new magnitude and axis of cylinder to be treated. I use an online calculator (www.LRIcalculator.com; Figure 1) to input preexisting keratometry (K) values, the axis, and the cylinder induced by my cataract incision. This tool will calculate the resultant cylinder and axis and also create a surgical plan using either the Donnenfeld or Nichamin nomogram that can be printed and brought to the operating room and used as a template for treatment.

For novice LRI surgeons, I suggest peribulbar anesthesia to further facilitate the procedure. I perform LRIs at the beginning of cataract surgery because I prefer a firm eye, one in which the cornea has not been thinned by dehydration under the operating microscope. With a preset diamond knife, I make the arc in the clear cornea 0.5 mm from the limbus. While in the operating room, I use my preoperative corneal topography or LRIcalculator.com printout, which I turn upside down to determine where to place the incisions. I grasp the eye with 0.12-mm Calibri forceps 180° away from the incision. I enter the eye with a diamond knife, hold for 1 second to make certain I have achieved full depth, and then I extend the cut to exactly the length that is required. For most patients, I use a diamond knife with a preset depth of 0.6 mm.

LRIS AT THE SLIT LAMP
There is a myth that surgeons are not comfortable performing LRIs. This is not true. I believe that most surgeons are comfortable performing this procedure but do not use it often because they do not have access to an operating microscope. For these surgeons, there is a very simple solution: perform LRIs at the slit lamp.

I have recently designed a preset diamond knife with Accutome, Inc. (Malvern, Pennsylvania), that has 15° angulation, making it ideal for use at the slit lamp as well as in the operating room. For the slit-lamp procedure, I use lidocaine gel to anesthetize the eye. I use the phoropter to confirm the incision axis, and I operate on this axis exactly as I would under an operating microscope. Making certain that the patient's head is forward, I come from the side, making one incision to achieve 0.50 to 0.75 D of correction. After this 30- second procedure, the patient walks away seeing better that day. Postoperatively, I give the patient prednisolone acetate 1% and gatifloxacin 0.5% four times daily for 5 days.

CONCLUSION
LRIs are underutilized for correcting residual astigmatism after the implantation of presbyopia-correcting IOLs. Ophthalmologists who successfully transition from cataract to refractive IOL surgery do so because they pay attention to the details that improve their patients' visual outcomes. For many of them, this means learning to perform LRIs.

I believe LRIs will become more popular in the future due to increased patient expectations and new technologies designed to improve the accuracy of incisional surgery. Intraoperative aberrometry with WaveTec Vision Systems' ORange (Aliso Viejo, California) device⁴ has significantly increased the accuracy of my LRIs. The exciting technology of femtosecond-assisted cataract surgery will provide the ability to program and perform LRIs at the time of surgery with laser precision.

Eric D. Donnenfeld, MD, is a trustee of Dartmouth Medical School in Hanover, New Hampshire, and a partner in Ophthalmic Consultants of Long Island in Rockville Centre, New York. He is a member of CRST Europe's Global Advisory Board and states that he is a consultant to Abbott Medical Optics Inc. and WaveTec Vision. Dr. Donnenfeld may be reached at tel: +1 516 766 2519; e-mail: eddoph@aol.com.

1. 1.Nichamin LD.Nomogram for limbal relaxing incisions. J Cataract Refract Surg.2006;32(9):1048.
2. Wang L,Misra M,Koch DD. Peripheral corneal relaxing incisions combined with cataract surgery.J Cataract Refract Surg.2003;29(4):712-722.
3. Gills JP. Treating astigmatism at the time of cataract surgery.Curr Opin Ophthalmol.2002;13(1):2-6.
4. Holland EJ,Assil KK,Donnenfeld ED,et al.Advanced astigmatism management using ORange intraoperative wavefront aberrometer:multicenter study.Paper presented at:ASCRS Symposium on Cataract,IOL and Refractive Surgery;April 4,2009;San Francisco.

Toric IOLs
The newest generation of these lenses can reduce or eliminate corneal astigmatism.BY RUDY M.M.A. NUIJTS, MD, PHD; AND NIENKE VISSER, MD
Toric IOLs provide the opportunity to achieve emmetropia in patients with corneal astigmatism. Several surgical strategies may be used to reduce or eliminate preexisting corneal astigmatism during or after cataract surgery, including LRIs, opposite clear corneal incisions, intrastromal corneal ring segments (ICRS), and excimer laser refractive procedures such as PRK and LASIK. However, the refractive changes induced by these procedures are relatively unpredictable, may be unstable over the long term, or may lead to complications. In comparison, toric IOLs provide a safe and predictable alternative for reducing or eliminating refractive astigmatism.

Toric IOLs offer patients with preexisting corneal astigmatism optimal distance UCVA with a cylindrical correction. In my experience, approximately 80% to 90% of patients with toric IOLs do not require spectacles for distance vision. My results are comparable with those of a large study that showed that more than 95% of patients bilaterally implanted with the AcrySof Toric IOL (Alcon Laboratories, Inc., Fort Worth, Texas) achieved spectacle freedom for distance vision.¹

PATIENT SELECTION
Achieving success with toric IOLs depends on selecting suitable patients and performing accurate preoperative corneal astigmatism measurements. Taking into account the minimal available toric IOL power of 1.00 D at the corneal plane and the amount of astigmatism induced by the surgery, patients must have a corneal astigmatism of at least 1.25 D to be candidates for a toric IOL.

Patients with regular bow-tie astigmatism are the best candidates for toric IOL implantation. Corneal topography should be performed to rule out irregular astigmatism and keratoconus. Patients with Fuchs endothelial dystrophy who might need keratoplasty in the future are not good candidates for toric IOL implantation.

Accurate K values and astigmatism axis measurements must be obtained to ensure success with toric IOLs. Our preferred method to measure K values is with the IOLMaster (Carl Zeiss Meditec, Jena, Germany). Its automated keratometry feature minimizes problems related to human error, and it has been shown to be a reliable biometric tool for spherical power calculations.

To determine the astigmatic axis, my colleagues and I compare the values obtained with the IOLMaster and corneal topography. If these values are consistent within 5° of each other, we use the axis measurements obtained with the IOLMaster. If the discrepancy is more than 5°, we use a Javal-Schiotz keratometer (Rodenstock, Dusseldorf, Germany), which we still regard as the gold standard for axis determination.

SURGICAL STEPS
Toric IOLs require only minor modifications to standard cataract surgery and IOL implantation techniques. For the AcrySof Toric IOL—the toric IOL with which our center has the most experience—cylinder power and alignment axis can be calculated using a Web-based toric IOL calculator program (http://www.acrysoftoriccalculator. com). This program takes into account the patient's K values, the axis of astigmatism, and the surgeon's expected surgically induced astigmatism in calculating the optimal IOL power and axis for implantation. It also predicts the postoperative residual astigmatism.

Preoperatively, corneal reference marks are placed at the 0°, 90°, and 180° positions, with the patient sitting upright to account for recumbent cyclotorsion. This can be done using a slit lamp or a Nuijts/Lane Toric Reference Marker (Model No. AE-2791TBL; ASICO, Westmont, Illinois). The advantage of this marker is that the extensions leave slight impressions on the cornea, even if the ink marks may have faded after surgical preparation.

Intraoperatively, the implantation axis is determined using the corneal reference marks and the alignment axis obtained from the toric calculator program. This can be done using a Mendez ring and the Nuijts Toric Axis Marker (Model No. AE-2740; ASICO).

After the surgeon's standard phacoemulsification technique is performed, the foldable toric IOL is inserted through a 2.2- to 2.8-mm limbal incision. The ophthalmic viscosurgical device is removed, and the IOL is rotated to its final position by exact alignment of the reference marks on the toric IOL with the limbal axis marks.

NEW GENERATION OF TORIC IOLS
In the older toric IOL models, such as the STAAR toric IOL (STAAR Surgical Company, Monrovia, California) and the MicroSil toric IOL (HumanOptics AG, Erlangen, Germany), rotation was an issue. This is no longer a problem due to the enhanced capsule-IOL adhesiveness of the materials used in the new generation of acrylic toric IOLs. The STAAR toric and MicroSil are silicone IOLs, whereas the AcrySof Toric IOL is made with a hydrophobic acrylic material. The acrylic material fixates the IOL in the capsular bag within approximately 2 weeks.² We also use the toric Acri.Comfort and the multifocal toric AT.LISA IOLs (both Carl Zeiss Meditec) at our center. We have experienced a low misalignment rate (mean misalignment, 2.5 ±2.4°) with these hydrophilic acrylic lenses.

The AcrySof Toric IOL was initially available in cylinder powers up to 3.00 D at the IOL plane, corresponding to 2.00 D at the corneal plane. This range has now been supplemented with four additional models (SN60T6-T9) with cylinder powers ranging from 3.75 to 6.00 D at the IOL plane, corresponding to 2.50 to 4.00 D at the corneal plane. The Acri.Comfort and AT.LISA toric IOLs have custom cylinder powers up to 12.00 D. The T-flex and multifocal M-flexT IOLs (both by Rayner Intraocular Lenses Ltd., East Sussex, United Kingdom) toric IOLs are available in cylinder powers ranging from 1.00 to 6.00 D, and they can be custom ordered with cylinder powers up to 11.00 D. This provides surgeons the opportunity to correct about 99% of all patients with corneal astigmatism using toric IOLs.³

Cataract surgery with foldable IOLs is usually performed through 2.2-mm incisions. At these sizes, the incisions have been shown to result in only minor surgically induced astigmatism. The multifocal toric AT.LISA and the toric Acri.Comfort IOLs are suitable for sub–2-mm incisions. These incisions are regarded to be astigmatically neutral and will reduce the amount of surgically induced astigmatism even further, resulting in improved visual outcomes.

The number of patients who wish to achieve spectacle independence for distance and near vision continues to rise. In our experience, the multifocal toric AT.LISA IOL provides a mean postoperative distance UCVA of 20/25 and near UCVA of 20/25. Mean axis misalignment with this lens is 2.5°. A recent pilot study showed a UCVA of 20/40 or better in 76% of eyes with moderate to high amounts of corneal astigmatism (greater than 2.00 D). The refractive cylinder was reduced by 90%.⁴

Toric IOLs provide a safe and predictable opportunity for patients with astigmatism to achieve excellent distance UCVA and spectacle independence. As future generations of these lenses are introduced, we will continue to fine-tune our ability to correct astigmatism and thus provide patients with postoperative emmetropia.

Rudy M.M.A. Nuijts, MD, PhD, is an Associate Professor of Ophthalmology in the Department of Ophthalmology at Academic Hospital, Maastricht, Netherlands. He is a member of the CRST Europe Editorial Board. Dr. Nuijts states that he is a consultant to Alcon Laboratories, Inc., Ophtec GmbH, and ASICO, and receives research funding from Alcon and Ophtec. He states that he has no financial interest in the products or companies mentioned. Dr. Nuijts may be reached at e-mail: rudy.nuijts@mumc.nl.

Nienke Visser, MD, is a doctoral (PhD) student in the Department of Ophthalmology at the Academic Hospital, Maastricht, Netherlands. Dr. Visser states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +31 43 3877133; e-mail: nienke.visser@ mumc.nl.

AcrySof Single-Piece Posterior Chamber Intraocular Lenses with Toric Optic,models SA60T3,SA60T4 and SA60T5.U.S.Food and Drug Administration.Document number P930014/S15,2005. 2.Linnola RJ,Sund M,Ylonen R,et al.Adhesion of soluble fibronectin,vitronectin,and collagen type IV to intraocular lens materials. J Cataract Refract Surg.2003;29(1):146-152. 3.Ferrer-Blasco T,Montes-Mico R,Peixoto-de-Matos SC,et al.Prevalence of corneal astigmatism before cataract surgery.J Cataract Refract Surg.2009;35(6):70-75. 4.Alio JL,Agdeppa MC,Pongo VC,et al.Microincision cataract surgery with toric intraocular lens implantation for correcting moderate and high astigmatism:pilot study.J Cataract Refract Surg.2010;36(1):44-52.

ICRS
Implantation of intastromal corneal ring segments is an effective method for the improvement of UCVA and BCVA in keratoconic eyes with astigmatism.
BY EFEKAN COSKUNSEVEN, MD
Implantation of an intrastromal corneal ring segment (ICRS) is a minimally invasive surgical procedure for the treatment of keratoconus. As an additive surgical procedure for keratoconus,^1-20 ICRS implantation effectively improves visual acuity and reduces refractive error and mean K. ICRS implantation decreases irregularity in keratoconic corneas, spherical equivalent, and cylindrical refraction.

Adding material at the corneal midperiphery induces displacement of the local anterior surface forward and flattening of the central portion of the anterior cornea due to the morphologic structure of corneal lamellae (arc-shortening effect).²¹ Short arc-length ICRSs have been shown to be effective for the correction of astigmatism^1,2,7 because they induce less corneal flattening and a significant change in corneal toricity as a result of corneal architecture (as predicted by finite element modeling).²²

BACKGROUND
There are several models of ICRS with varying curvature, width, and optical zones. Initially, only 160° arc rings with optical zones of 5 to 6 mm were available (Keraring [Mediphacos, Belo Horizonte, Brazil], Ferrara Ring [Ferrara Ophthalmics, Belo Horizonte, Brazil], and Intacs [Addition Technology, Inc., Sunnyvale, California]). The Keraring is currently available in 90°, 120°, 150°,160°, and 210° arc lengths and can be used according to special nomograms. Every year, Mediphacos updates the nomogram based on data collected from users.

In my experience, rings with smaller optical zones are good options for treating astigmatism. Generally, when two symmetric 160° arc ICRSs are implanted through incisions on the steep axis, corneal irregularity decreases. Based on the ring's thickness, steep (K1) and flat (K2) K values decrease similarly centrally; spherical equivalent also decreases, but cylinder decreases minimally. Implanting a single ring in a keratoconic cornea with inferior cone may decrease cylindrical error and reduce irregularity more significantly compared with implanting two rings.

Before 90° arc ring segments were available, implantation of two symmetric 160° arc rings in symmetric keratoconic eyes was unsatisfactory because cylindrical error did not decrease as a result of a parallel decrease in both K1 and K2. Today, in patients who necessitate symmetric correction, 90° arc ring implantation is a viable option (Figure 1).

A recently released Keraring nomogram has shown that asymmetric ring implantation can also be effective for the treatment of astigmatism in keratoconus patients. In some cases, steep K values decreased and flat K values did not change or increased. Differences between the K1 abd K2 meridians decreased, resulting in greater astigmatic correction.

A single Keraring with a 210° arc is a good option for corneas in which the ectatic area is on one half of the cornea, such as in the case of inferior cone or in pellucid marginal degeneration (PMD; Figure 2). When this type of ring is implanted, mean K values decrease; however, when we compare the steep and flat K values, there are more changes in the steep K values than in flat K, causing a decrease in cylindrical error.

PRACTICAL APPLICATIONS
The ICRS can be used to correct astigmatism in numerous clinical situations. The following case examples and study results illustrate some of the many applications for the variety of ring designs available.

ICRS can be used to regularize the cornea and to improve astigmatism after penetrating keratoplasty (PKP).²³ A 50-year-old woman was implanted with Kerarings for recurrent keratoconus 15 years after PKP. Two segments (0.15 and 0.25 mm) were inserted using the femtosecond laser to create superior and inferior tunnels. Ten months after the procedure, her UCVA was 20/100, compared with counting fingers preoperatively, and BCVA improved from 20/63 to 20/32 (Figure 3).

An ICRS can be used to regularize the cornea after radial keratotomy (RK).²⁴ A 33-year-old woman with irregular astigmatism 6 years after RK for keratoconus was treated with implantation of a single Keraring segment (0.15 mm thick with a 160° arc) using a femtosecond laser for tunnel creation. The ICRS was inserted in the steepest area (inferior) with no intra- or postoperative complications. Six months postoperative, the patient's UCVA improved from 20/40 to 20/25 and BCVA from 20/32 to 20/20. Mean manifest astigmatic correction decreased from -2.50 to -0.75 D, and corneal topography showed improved inferior steepening and less irregular astigmatism.

Implantation of ICRS can be an effective option for the treatment of second-order and higher-order aberrations in corneas with PMD, as demonstrated in a multicenter study by David P. Piñero, MD, et al.25 Twenty-one consecutive eyes of 15 patients (age range, 21–73 years) with PMD underwent ICRS implantation. Tunnel creation was performed by mechanical means in seven eyes and with femtosecond laser in 14 eyes. Intacs were implanted in three eyes and Kerarings in 18 eyes. At 6 months, UCVA did not improve (P =.11). BCVA improved from a mean preoperative value of 0.54 (Snellen decimal) to a mean postoperative value of 0.75 (P =.06). At 6 months, 44.44% of eyes gained two or more lines of BCVA. Sphere (P =.02), cylinder (P <.01), and spherical equivalent (SE; P <.01) were reduced significantly after surgery (P <.02). Mean K decreased significantly from 44.95 D preoperative to 43.19 D at 6 months postoperatively (P <.01). The root mean square (RMS) values for astigmatism and higher-order residual and coma-like aberrations were significantly reduced with ICRS (P =.03). Additionally, significant negative correlations of preoperative RMS astigmatism and primary spherical aberration with postoperative BCVA were also found. Explantation was performed in four eyes due to poor visual outcome.

ICRS implantation followed by corneal collagen crosslinking (CXL) can be used to improve vision in eyes with keratoconus.²⁶ In a prospective comparative randomized consecutive study by this author and colleagues, CXL was followed by ICRS implantation (group 1) or ICRS implantation was followed by CXL (group 2).

Mean interval between the two steps was 7 months, and mean follow-up was 6 months. Mean astigmatism decreased from 6.59 to 3.14 D. Mean distance UCVA and distance BCVA improved in both groups. Mean SE, cylinder, and mean K values decreased in both groups. Overall, there was more improvement in distance BCVA, SE, and mean K in group 2 than in group 1, suggesting that implantation of ICRS followed by CXL resulted in greater improvement of keratoconus.

ICRS implantation using a femtosecond laser for tunnel creation is a minimally invasive procedure for improving UCVA and BCVA in keratoconus patients.1 Thirty-two patients (50 eyes) with keratoconus underwent Keraring insertion using a femtosecond laser for channel creation. At 1 year, there was a statistically significant reduction in the spherical equivalent refractive error (mean ±standard deviation, -5.62 ±4.15 D [range, -23.62–0.50 D]) compared with that observed preoperatively (-2.49 ±2.68 D [range, -11.12–3.5 D]; P <.001). UCVA before implantation was 20/40 or worse in 47 eyes (range, counting fingers to 20/30), whereas at the last follow-up examination, 14 of 50 eyes had a UCVA of 20/40 or better (range, counting fingers to 20/25). Nine eyes maintained preimplantation BCVA, whereas 39 eyes experienced a BCVA gain of 1 to 4 lines at the last follow-up examination. Only in two eyes (two patients) with advanced keratoconus (stage 3) was there a decrease of up to 2 lines of BCVA. Despite this deterioration, the patients did not want the ICRS removed because they experienced an increase in UCVA.

Complications have been observed with ICRS implantation using a femtosecond laser for channel creation.²⁷ Our center performed a retrospective chart review of 531 patients (850 eyes) who underwent Keraring insertion using a femtosecond laser for channel creation. Intraoperatively, 22 cases of incomplete channel formation occurred.

Other intraoperative complications included laser system malfunction (five eyes), endothelial perforation (five eyes), and incorrect entry of the channel (two eyes). Postoperatively, there were 11 cases of segment migration, two cases of corneal melting, and one case of mild infection. The overall complication rate was 5.7% (49 cases out of 850 eyes).

CONCLUSION
In conclusion, ICRS implantation is an effective option for the treatment of spherocylindrical error and corneal irregularity in keratoconus.^28,29 The inhibiting effect of ICRS on keratoconus progression is still unclear; however, ICRS can improve astigmatism in these eyes, and they remain a viable option for improving vision in our patients.

Efekan Coskunseven, MD, is Director of the Refractive Surgery Department, Dunya Eye Hospital, Istanbul, Turkey. Dr. Coskunseven states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: efekan.coskunseven@dunyagoz. com.

1. 1.Coskunseven E,Kymionis GD,Tsiklis NS,et al.One-year results of intrastromal corneal ring segment implantation (KeraRing) using femtosecond laser in patients with keratoconus.Am J Ophthalmol.2008;145(5):775-779.
2. Shetty R,Kurian M,Anand D,et al.Intacs in advanced keratoconus.Cornea.2008;27(9):1022-1029.
3. Ertan A,Ozkilic E.Effect of age on outcomes in patients with keratoconus treated by Intacs using a femtosecond laser.J Refract Surg.2008;24(7):690-695.
4. Ertan A,Kamburoglu G.Intacs implantation using femtosecond laser for management of keratoconus:comparison of 306 cases in different stages.J Cataract Refract Surg.2008;34(9):1521-1526.
5. Shabayek MH,Alió JL.Intrastromal corneal ring segment implantation by femtosecond laser for keratoconus correction. Ophthalmology.2007;114(9):1643-1652.
6. Zare MA,Hashemi H,Salari MR.Intracorneal ring segment implantation for the management of keratoconus:safety and efficacy.J Cataract Refract Surg.2007;33(11):1886-1891.
7. Kymionis GD,Siganos CS,Tsiklis NS,et al.Long-term follow-up of Intacs in keratoconus.Am J Ophthalmol.2007;143(2):236- 244.
8. Alió JL,Shabayek MH,Artola A.Intracorneal ring segments for keratoconus correction:long-term follow-up.J Cataract Refract Surg.2006;32(6):978-985.
9. Alió JL,Shabayek MH,Belda JI,et al.Analysis of results related to good and bad outcomes of Intacs implantation for keratoconus correction.J Cataract Refract Surg.2006;32(5):756-761.
10. Ertan A,Kamburoglu G,Bahadir M.Intacs insertion with the femtosecond laser for the management of keratoconus:one year results.J Cataract Refract Surg.2006;32(12):2039-2042.
11. Colin J.European clinical evaluation:use of Intacs for the treatment of keratoconus.J Cataract Refract Surg.2006;32(5):747- 755.
12. Kanellopoulos AJ,Pe LH,Perry HD,et al.Modified intracorneal ring segment implantations (INTACS) for the management of moderate to advanced keratoconus:efficacy and complications.Cornea.2006;25(1):29-33.
13. Hellstedt T,Mäkelä J,Uusitalo R,et al.Treating keratoconus with Intacs corneal ring segments.J Refract Surg. 2005;21(3):236-246.
14. Miranda D,Sartori M,Francesconi C,et al.Ferrara intrastromal corneal ring segments for severe keratoconus.J Refract Surg. 2003;19(6):645-653.
15. Siganos CS,Kymionis GD,Kartakis N,et al.Management of keratoconus with Intacs.Am J Ophthalmol.2003;135(1):64-70.
16. Boxer Wachler BS,Christie JP,Chandra NS,et al.Intacs for keratoconus.Ophthalmology.2003;110(5):1031-1040.
17. Ruckhofer J,Stoiber J,Twa MD,et al.Correction of astigmatism with short arc-length intrastromal corneal ring segments: preliminary results.Ophthalmology.2003;110(3):516-524.
18. Siganos D,Ferrara P,Chatzinikolas K,et al.Ferrara intrastromal corneal rings for the correction of keratoconus.J Cataract Refract Surg.2002;28(11):1947-1951.
19. Colin J,Cochener B,Savary G,et al.INTACS inserts for treating keratoconus:one-year results.Ophthalmology. 2001;108(8):1409-1414.
20. Colin J,Cochener B,Savary G,et al.Correcting keratoconus with intracorneal rings.J Cataract Refract Surg.2000;26(8):1117- 1122.
21. Patel S,Marshall J,Fitzke FW III.Model for deriving the optical performance of the myopic eye corrected with an intracorneal ring.J Refract Surg.1995;11(4):248-252.
22. Schanzlin DJ.Studies of intrastromal corneal ring segments for the correction of low to moderate refractive errors.Trans Am Ophthalmol Soc.1999;47:815-890.
23. Coskunseven E,Kymionis GD,Talu H,et al.Intrastromal corneal ring segment implantation with the femtosecond laser in a post-keratoplasty patient with recurrent keratoconus.J Cataract Refract Surg.2007;33(10):1808-1810.
24. Coskunseven E,Kymionis GD,Bouzoukis DI,et al.Single intrastromal corneal ring segment implantation using the femtosecond laser after radial keratotomy in a keratoconic patient.J Cataract Refract Surg.2009;35(1):197-199.
25. Piñero DP,Alio JL,Morbelli H,et al.Refractive and corneal aberrometric changes after intracorneal ring implantation in corneas with pellucid marginal degeneration.Ophthalmology.2009;116(9):1656-1664.
26. Coskunseven E,Jankov MR II,Hafezi F,et al.Effect of treatment sequence in combined intrastromal corneal rings and corneal collagen crosslinking for keratoconus.J Cataract Refract Surg.2009;35(12):2084-2091.
27. Coskunseven E,Kymionis GD,Tsiklis NS,et al.Complications of intrastromal corneal ring segment implantation using a femtosecond laser for channel creation:a survey of 850 eyes with keratoconus.Acta Ophthalmol.2009 Aug 14 [Epub ahead of print].
28. Rabinowitz YS,Li X,Ignacio TS,et al.INTACS inserts using the femtosecond laser compared to the mechanical spreader in the treatment of keratoconus.J Refract Surg.2006;22(8):764-771.
29. Carrasquillo KG,Rand J,Talamo JH.Intacs for keratoconus and post-LASIK ectasia:mechanical versus femtosecond laserassisted channel creation.Cornea.2007;26(8):956-962.