From Magrabi Eye & Ear Center, Dammam, Saudi Arabia.
The author has no financial or 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: Mohamed Hantera, FRCS, ICO, MBA, Magrabi Eye & Ear Center, PO Box 1840, Dammam 31441, Saudi Arabia. Tel: 96 655 255 3322; E-mail: email@example.com
Conventional Munnerlyn-based ablation algorithms have produced excellent outcomes compared with custom ablation for the treatment of myopia.1 Future advances in LASIK and photorefractive keratectomy will likely focus on increasing the quality of vision. The key for enhancing visual quality after LASIK is the reduced induction of higher order aberrations, particularly spherical aberration.2 Spherical aberration can be addressed using aspheric profiles.3,4 Aspheric ablations tend to induce less changes in corneal asphericity compared to conventional ablations.5 A number of excimer laser manufacturers have introduced aspheric algorithms in their platforms.
The NIDEK Advanced Vision Excimer Laser System (NAVEX; NIDEK Co Ltd, Gamagori, Japan) incorporates a number of aspheric ablation profiles for the treatment of primary myopia with or without astigmatism. For example, the optimized aspheric transition zone (OATz) profile is an algorithm that delivers a spherical ablation to the optical zone and an aspheric algorithm to the transition zone. In OATz, the optical zone and transition zone are combined to treat the refractive error unlike conventional ablation, which employs the optical zone diameter for the refractive correction.5 The delivery of an aspheric transition zone reduces the induction of aberrations and increases the effective optical zone postoperatively.2,5 The OATz profile is somewhat similar to “wavefront-optimized” algorithms of other lasers in that the aspheric delivery is based on global corneal curvature and customized to meridional differences in corneal curvature. Using the NAVEX platform, a recent study reported that OATz has better visual quality compared to conventional ablations.5 In the author’s center, OATz has replaced conventional ablation for the treatment of myopic eyes.
An alternative fully aspheric treatment, OPD-guided custom aspheric treatment (OPDCAT), is also available in the NAVEX platform. OPDCAT incorporates fully aspheric optical zones and transition zones based on the preoperative curvature over the entire corneal surface and can optionally treat the ocular (not corneal) higher order aberrations at the surgeon’s discretion. Delivering an aspheric ablation across the treatment zone of the cornea may result in greater gains in visual quality compared to using large aspheric transition zones due to induction of less higher order aberrations. This study presents a prospective comparison of the induction of higher order wavefront aberrations after LASIK with OATz or OPDCAT.
Patients and Methods
In this prospective study, 23 eyes of 13 consecutive patients scheduled to undergo LASIK at Magrabi Eye & Ear Center, Dammam, Saudi Arabia, were evaluated. The induction of higher order aberrations after LASIK was compared in eyes that underwent OATz (OATz group, n=12) or OPDCAT (OPDCAT group, n=11). Patients were randomly selected to undergo either treatment using a randomization schedule.
All surgeries were performed by one surgeon (M.H.). The eyes were prepared for surgery using an iodinepovidone to cleanse the lids and lashes. One or two drops of topical anesthetic were instilled and a sterile drape was used to isolate the surgical field. A lid speculum was inserted to allow maximum exposure of the globe. Additional topical anesthetic was instilled in the operative eye. The Moria M2 microkeratome (Moria, Antony, France) equipped with a 130-μm blade head was used to create lamellar hinged flaps that were 9.5 mm to accommodate the full ablation diameter. The flap was lifted and the excimer laser ablation was delivered to the stroma with the NIDEK CXIII excimer laser. Proper alignment of the eye with the laser was achieved with a 200-Hz infrared eye tracker built into the laser console with the ablation centered on the pupil. Patients fixated on a red fixation light throughout the ablation. All ablations were delivered with profile #5 using a 5.0-mm optical zone and 8.5-mm transition zone. Higher order aberrations were corrected in the OPDCAT eyes at the same optical and transitional zones. All eyes were targeted for emmetropia using a personalized nomogram. The flap was repositioned and the interface was irrigated with balanced salt solution, removing any debris. One drop of topical fluoroquinolone, topical steroid, and artificial tears were instilled and the patient was discharged from the operating suite. Patients were instructed to instill one drop each of topical fluoroquinolone antibiotic, corticosteroid drops four times per day for 1 week, and artificial tears four times per day for 4 weeks.
Main comparisons were for changes in the 5th radial order ocular higher order aberrations for a 6-mm pupil diameter and corneal asphericity that was measured with the OPD-Scan II (NIDEK Co Ltd). Outcomes data were analyzed using Microsoft Excel (Microsoft Corp, Redmond, Wash). Data were compared using the Student t test. A P value <.05 was considered statistically significant. Outcomes at 3 months postoperatively are presented.
The preoperative parameters of both groups are presented in Table 1. No statistically significant differences were noted between groups preoperatively (P>.05). Mean intended spherical correction was −2.83±1.41 diopters (D), and mean intended cylinder correction was −0.73±0.51 D. Postoperatively, all eyes were within ±0.50 D of the intended manifest refraction spherical equivalent. No intra- or postoperative complications occurred. Three months after OPDCAT, no statistically significant changes were noted in mean total ocular higher order aberration and spherical aberration (Table 2). However, 3 months after OATz, statistically significant changes in mean higher order aberration and spherical aberration were noted (Table 3).
Table 1: Preoperative Parameters of Eyes that Underwent LASIK with Optimized Aspheric Transition Zone (OATz) or OPD-Guided Custom Ablation Treatment (OPDCAT)
Table 2: Preoperative and 3-Month Postoperative Higher Order Aberrations and Corneal Asphericity of 11 Eyes that Underwent OPD-Guided Custom Ablation Treatment (OPDCAT)
Table 3: Preoperative and 3-Month Postoperative Higher Order Aberrations and Corneal Asphericity of 12 Eyes that Underwent Optimized Aspheric Transition Zone Ablation (OATz)
Subgroup analysis of the changes between pre- and postoperative higher order aberration are plotted in the Figure. In the OATz group, a continual increase was noted in the postoperative higher order aberrations as the preoperative total ocular higher order aberrations increased (see Fig). In the OPDCAT group, the tendency was the opposite (see Fig). For example, in the OPDCAT group, eyes with >0.30 μm higher order aberrations preoperatively had a lower increase in higher order aberrations postoperatively compared to eyes with ≤0.30 μm higher order aberrations preoperatively (see Fig).
Figure. Change in Higher Order Aberrations (HOA) After LASIK with Optimized Aspheric Transition Zone (OATz) or OPD-Guided Custom Ablation Treatment (OPDCAT) Compared to Preoperative HOA. X Denotes Times.
A number of aspheric ablations have been introduced to address the increase in spherical aberration after LASIK ablation. These profiles range from population averaged (OATz-like algorithms) to fully prolate ablations.6 In this study, minimal changes were found in the corneal asphericity of the OPDCAT-treated eyes compared to the OATz-treated eyes (Tables 2 and 3). A statistically significant difference indicates a greater propensity towards an oblate cornea with the OATz algorithm compared to the OPDCAT algorithm. This observation was likely due to three factors: the delivery of aspheric treatment across the full treatment diameter, the use of patient-specific corneal curvature values over the corneal surface, and the treatment of ocular spherical aberration in the ablation algorithm that also changes the corneal shape.7 On the other hand, the OATz algorithm delivers an aspheric treatment in the transition zone, which is based on the average corneal curvature. This results in flattening of the corneal curvature gradients, as seen after conventional LASIK, which still tend towards creating oblate corneas for eyes with flatter preoperative corneal curvatures.5
Both algorithms address the loss of effective energy in the corneal periphery, which results in an increase in the effective optical zone and reduces the induction of spherical aberration after ablation. This has been verified with the OATz algorithm5; however, no study to date has measured the effective optical zone with the OPDCAT algorithm.
In the OPDCAT group, it was found that the lower the total ocular higher order aberrations preoperatively, the greater the induction of total ocular higher order aberrations postoperatively and vice versa (see Fig). However, in the OATz group, the lower the higher order aberrations preoperatively, the less induced higher order aberrations postoperatively and vice versa. These outcomes verify two previous studies on the OPDCAT, which reported minimal induction of higher order aberrations in eyes with 0.30 to 0.35 μm or more higher order aberrations preoperatively.8,9 A study with the Zyoptix laser (Bausch & Lomb, Rochester, NY) also reported that eyes with higher order aberrations >0.30 μm that underwent wavefront-guided treatment tended to have less induction of higher order aberrations postoperatively compared to eyes with smaller higher order aberrations (<0.30 μm).10 It seems that this observation is specific to the eye rather than the laser platform. Based on these observations, OATz is the recommended treatment for patients with higher order aberrations <0.30 μm and OPDCAT is recommended in patients with higher order aberrations ≥0.30 μm.
Limitations of this study include the small sample size and the fact that different groups of patients were treated as opposed to treating one eye with OATz and the fellow eye with OPDCAT. To reduce variability in outcomes, the same optical zone and transition zone diameters were used for both groups and one surgeon performed all surgeries. A larger sample with longer follow-up is required to verify our preliminary results.
In this prospective evaluation of two aspheric algorithms, the OPDCAT algorithm was shown to reduce the changes in corneal asphericity and induce less statistically significant changes in higher order aberrations and spherical aberration. Patient selection for each aspheric algorithm may be possible based on the level of preoperative higher order aberrations.
- Dougherty PJ, Bains HS. A retrospective comparison of LASIK outcomes for myopia and myopic astigmatism with conventional NIDEK versus wavefront-guided VISX and Alcon platforms. J Refract Surg. 2008;24:891–896.
- Moreno-Barriuso E, Lloves JM, Marcos S, Navarro R, Llorente L, Barbero S. Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci. 2001;42:1396–1403.
- Holladay JT, Janes JA. Topographic changes in corneal asphericity and effective optical zone after laser in situ keratomileusis. J Cataract Refract Surg. 2002;28:942–947. doi:10.1016/S0886-3350(02)01324-X [CrossRef]
- Manns F, Ho A, Parel JM, Culbertson W. Ablation profiles for wavefront-guided correction of myopia and primary spherical aberration. J Cataract Refract Surg. 2002;28:766–774. doi:10.1016/S0886-3350(01)01322-0 [CrossRef]
- Hori-Komai Y, Toda I, Asano-Kato N, Ito M, Yamamoto T, Tsubota K. Comparison of LASIK using the NIDEK EC-5000 optimized aspheric transition zone (OATz) and conventional ablation profile. J Refract Surg. 2006;22:546–555.
- El Danasoury AM. NIDEK optimized prolate ablation for the treatment of myopia with and without astigmatism. J Refract Surg. 2009;25:S136–S141.
- Calossi A. Corneal asphericity and spherical aberration. J Refract Surg. 2007;23:505–514.
- Kermani O, Schmiedt K, Oberheide U, Gerten G. Topographic- and wavefront-guided customized ablations with the NIDEK-EC-5000CXII in LASIK for myopia. J Refract Surg. 2006;22:754–763.
- Venter J. Wavefront-guided custom ablation for myopia using the NIDEK NAVEX laser system. J Refract Surg. 2008;24:487–493.
- Subbaram MV, MacRae SM. Customized LASIK treatment for myopia based on preoperative manifest refraction and higher order aberrometry: the Rochester nomogram. J Refract Surg. 2007;23:435–441.
Preoperative Parameters of Eyes that Underwent LASIK with Optimized Aspheric Transition Zone (OATz) or OPD-Guided Custom Ablation Treatment (OPDCAT)
|Parameter||OATz Group||OPDCAT Group||PValue|
|No. of eyes||12||11|
|Mean age (y)||26.20±4.05||26.00±4.90||>0.1|
|Mesopic pupil diameter (mm)||5.90±1.46||6.17±0.66||>0.1|
|Sim K (D)||45.20±0.65||44.90±0.38||>0.1|
Preoperative and 3-Month Postoperative Higher Order Aberrations and Corneal Asphericity of 11 Eyes that Underwent OPD-Guided Custom Ablation Treatment (OPDCAT)
|Mean±Standard Deviation (Range) (μm)||PValue|
|Higher order aberration||0.70±0.96 (0.13 to 3.40)||0.66±0.49 (0.29 to 1.79)||.819|
|Coma||−0.15±0.28 (−0.42 to 0.35)||−0.14±0.43 (−0.82 to 0.44)||.93|
|Trefoil||−0.15±0.38 (−1.13 to 0.27)||0.26±0.47 (−0.11 to 1.31)||.037*|
|Spherical aberration||−0.034±0.12 (−0.25 to 0.11)||0.14±0.15 (−0.07 to 0.45)||.09|
|Asphericity||−0.17±0.11 (−0.33 to 0.02)||0.19±0.23 (−0.24 to 0.48)||.002*|
Preoperative and 3-Month Postoperative Higher Order Aberrations and Corneal Asphericity of 12 Eyes that Underwent Optimized Aspheric Transition Zone Ablation (OATz)
|Mean±Standard Deviation (Range) (μm)||PValue|
|Higher order aberration||0.29±0.12 (0.16 to 0.60)||0.47±0.19 (0.28 to 0.88)||.003*|
|Coma||−0.02±0.12 (−0.25 to 0.22)||0.05±0.23 (−0.20 to 0.54)||.211|
|Trefoil||−0.16±0.17 (−0.46 to 0.15)||−0.11±0.35 (−0.82 to 0.33)||.818|
|Spherical aberration||−0.042±0.15 (−0.32 to 0.36)||0.13±0.12 (−0.04 to 0.36)||.04*|
|Asphericity||−0.24±0.13 (−0.54 to −0.10)||0.42±0.43 (0.07 to 1.57)||.001*|