From Magrabi Eye & Ear Center, Muscat, Oman.
The author has no proprietary interest in the materials presented herein.
Rich Bains, consultant to NIDEK Co Ltd, assisted in the preparation of the manuscript.
Presented at the 10th Middle East African Council of Ophthalmology International Congress; March 26–30, 2009; Manama, Kingdom of Bahrain.
Correspondence: Tamer Gamaly, MD, 106 Rumaila Bldg, Al Nahda St, PO Box 513, Muscat, Oman. Tel: 968 99233137; Fax: 968 24568874; E-mail: firstname.lastname@example.org
Laser in situ keratomileusis (LASIK) is a successful procedure for the treatment of myopia with or without low astigmatism (up to 1.75 diopters [D]).1 However, the treatment of high cylinder is not as effective compared to myopia.2,3 The effectiveness of cylinder correction is reduced largely due to undercorrection and regression.2,3 One strategy to mitigate this lower effectiveness is the use of the cross-cylinder technique.4,5 Using the cross-cylinder technique results in less marked changes in corneal curvature postoperatively.5 The introduction of the cross-cylinder technique has resulted in greater accuracy and predictability for the treatment of high cylinder.5 The introduction of aspheric algorithms has resulted in more physiologic curvature changes in postoperative corneas ranging from mildly oblate to fully prolate.6,7 One example, the optimized aspheric transition zone (OATz) algorithm uses a conventional spherical ablation centrally with a wide diameter transition zone (2.50 mm greater than the optical zone) to reduce the induction of longitudinal spherical aberration.6 The combination of the cross-cylinder technique with an aspheric laser profile may further reduce the induced spherical aberration and enhance the outcomes of cross-cylinder treatment. The smoother changes in curvature by incorporating OATz are more likely to lead to better visual quality postoperatively.6
In this study, the NIDEK Advanced Vision Excimer Laser platform (NAVEX; NIDEK Co Ltd, Gamagori, Japan) with the OATz algorithm and cross-cylinder technique was used for the treatment of patients with 1.00 to 4.25 D of astigmatism.
Patients and Methods
In this prospective study, a total of 74 eyes of 48 consecutive patients were treated with LASIK for myopia with >−1.00 D of cylinder were evaluated preoperatively and at 6 months postoperatively. The cross-cylinder technique with the OATz algorithm was used for all treatments. All eyes underwent an ophthalmic examination including corneal topography, aberrometry, and pupillometry with the OPD-Scan II (NIDEK Co Ltd), uncorrected and corrected distance and near visual acuity, manifest refraction, intraocular pressure measurement, and slit-lamp microscopy at baseline and each examination after surgery. Follow-up examinations were performed at 1 day and 1, 3, and 6 months. Visual acuity and slit-lamp microscopy were performed at 1 day and 1 week postoperatively. Pachymetry, dilated funduscopy, and cycloplegic refraction were performed before and 3 months after surgery. No patients withdrew from the study, and none were lost to follow-up.
LASIK Surgical Technique
All LASIK procedures were performed using the NIDEK CXIII excimer laser, with a treatment optical zone of 6.0 mm and a transition zone of 9.0 mm with the OATz profile.7 The Moria M2 Single Use with a 130-μm microkeratome head (Moria, Antony, France) was used (mean achieved flap thickness was 115.25±25.84 μm; range: 71 to 181 μm) with an intended flap diameter of 9.50 mm.
All procedures were performed by one surgeon (T.G.). One or two drops of tetracaine were placed in each eye and the non-treated eye was patched. Additional drops of the topical anesthetic were placed in the operative eye prior to surgery. Proper alignment of the patient with the laser for cylinder treatments was accomplished by using the torsion error detection and online torsion error correction functions of the laser. Once the patient was supine, just prior to beginning surgery, the eye was aligned with the torsion error detector module of the laser by comparing the iridal landmarks that were obtained during the preoperative evaluation with the OPD-Scan II. A 200-Hz infrared eye tracker along with cooperative patient fixation was used to maintain centration. Suction was applied to the microkeratomepositioning ring and the flap was created with a single, continuous cutting motion after moistening the cornea with buffered sterile saline (BSS). The flap was reflected to expose corneal stroma and the online torsion error function was enabled to continuously monitor the eye for torsional movements during ablation. The eye tracker was enabled immediately after the online torsion error module. After completion of the procedure, the flap was reflected back into position after irrigating the bed and flap with BSS solution, removing any debris. Patients were to administer topical antibiotic/anti-inflammatory drops four to five times a day for 1 week or until the flap was well healed and unpreserved artificial tears for 2 months as required. Patients were seen 1 day after surgery and at the previously indicated surgical intervals. Step-by-step calculations for the cross-cylinder technique have been described previously.8
Refractive outcome and safety and predictability data were analyzed at 6 months postoperatively. At 6 months, patients were asked about the presence or absence of night vision symptoms and whether they had difficulty driving at night; both questions required a “yes” or “no” answer.
The mean preoperative manifest refraction spherical equivalent (MRSE) was −2.10±1.19 D (range: −0.50 to −5.75 D) and mean cylinder was −1.94±0.89 D (range: −1.00 to −4.25 D). Six months after surgery, the mean MRSE was 0.01±0.37 D (range: −0.75 to +1.00 D) and mean cylinder was −0.30±0.35 D (range: 0 to −1.50 D). All eyes had best spectacle-corrected visual acuity (BSCVA) of 20/40 or better 6 months after surgery and 91.9% (68/74) of eyes had BSCVA of 20/20 or better (Fig 1). Six months after surgery, 23% (17/74) of eyes had 20/15 or better uncorrected visual acuity (see Fig 1). Safety at 6 months after surgery is plotted in Figure 2. A primary safety measure is the percentage of eyes with postoperative BSCVA worse than 20/40. Review of the BSCVA data showed that no eyes were worse than 20/40 (see Fig 1). Predictability is plotted in Figure 3. Six months after surgery, 94.6% (70/74) of eyes were within ±0.50 D of the intended MRSE (Figs 3 and 4).
Figure 1. Uncorrected Visual Acuity (UCVA) and Best Spectacle-Corrected Visual Acuity (BSCVA) of 74 Eyes 6 Months After LASIK with the Cross-Cylinder Technique and the Optimized Aspheric Transition Zone Algorithm.
Figure 2. Safety of 74 Eyes 6 Months After LASIK with the Cross-Cylinder Technique and the Optimized Aspheric Transition Zone Algorithm.
Figure 3. Predictability of 74 Eyes 6 Months After LASIK with the Cross-Cylinder Technique and the Optimized Aspheric Transition Zone Algorithm. Green Dotted Lines Represent ±0.50 D of Intended Manifest Refraction Spherical Equivalent (MRSE) and Dashed Green Lines Represent ±1.00 D of Intended MRSE.
Figure 4. Refractive Outcome of 74 Eyes 6 Months After LASIK with the Cross-Cylinder Technique and the Optimized Aspheric Transition Zone Algorithm. SE denotes Manifest Refraction Spherical Equivalent
For all patients, the average MRSE change between 1- and 3-month follow-up and 3- and 6-month follow-up was −0.20 D and −0.10 D, respectively (Fig 5).
Figure 5. Stability of 74 Eyes 6 Months After LASIK with the Cross-Cylinder Technique and the Optimized Aspheric Transition Zone Algorithm. Mean SE Denotes Manifest Refraction Spherical Equivalent.
Six months postoperatively, 6.3% (3/48) of patients responded “yes” to the presence of night vision disturbance and 93.7% (45/48) responded “no.” For the question, “Any problems in driving at night?,” 2.1% (1/48) of patients responded “yes” and 97.9% (47/48) responded “no.”
No intra- or postoperative complications occurred during the study.
The efficacy, predictability, and safety of LASIK treatment for myopia with up to 4.25 D of cylinder was studied with the cross-cylinder technique and OATz algorithm. The outcomes indicated this was an effective treatment strategy, eg, 94.6% (70/74) of eyes were within ±0.50 D of the intended MRSE. Safety was demonstrated with only 5.3% (4/74) of eyes losing two lines of BSCVA.
This study is consistent with previous studies that have shown that cross-cylinder LASIK treatment of high cylinder is a predictable and safe procedure. A study of cross-cylinder treatment for a cohort of patients with a similar preoperative cylinder as the present study (−2.05 D) reported a MRSE of −0.26 D at 6 months postoperatively.9 This is similar to the MRSE of 0.01 D in the present study. A comparison of the cross-cylinder method to treatment by directly programming the cylinder and axis as a single treatment did not find any statistically significant differences.9 However, the authors’ impression was that cross-cylinder treatments were clinically better. For example, eyes that had undergone cross-cylinder treatment had corneal topographies with a more confluent refractive gradient across the treatment diameter.
The addition of an aspheric profile to deliver the cross-cylinder ablation seems to be complementary. For example, OATz-treated eyes have smooth corneal topographies, larger effective optical zones, and better visual quality than eyes with conventional treatment.6 Therefore, the incorporation of aspheric profiles may have similar beneficial effects for patients with moderate cylinder. However, studies incorporating contrast sensitivity, corneal topography analysis, and wavefront aberration data are required to address this issue.
One concern with the use of cross-cylinder ablation is centration of the various steps in the treatment. This was mitigated in the present study by the incorporation of eye and continuous cyclotorsion monitoring. The other is the longer treatment times required due to the cross-cylinder technique and the enhanced peripheral ablation of OATz. However, appropriate nomogram modification can address this concern.
The aspheric cross-cylinder treatment for astigmatism up to 4.25 D was found to be safe and predictable. Meticulous centration and slight nomogram modification are likely required initially.
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- Khan HN, Kaye GB, Febbraro JL. Correction of high astigmatism: case studies using the mixed-cylinder approach. J Refract Surg. 2006;22:S1073–S1078.
- Shin SJ, Lee HY. The efficacy of multi-zone cross-cylinder method for astigmatism correction. Korean J Ophthalmol. 2004;18:29–34.