In the late 1990s, flying-spot technology was introduced into excimer laser delivery systems. Smoothness and regularity of the ablated surface improved; together with therapy (ie, soft bandage contact lenses and topical steroids), haze, the most frequent late complication of PRK, (Figure 1) was dramatically reduced. A successive improvement in beam profile — from flat-top to Gaussian and truncated Gaussian — and wavefront-guided ablation allowed a significant reduction of tissue removal compared with the first randomized flying ablation pattern (Planoscan; Bausch & Lomb, Rochester, New York).
Reduced transition zone diameter and its conversion into optically efficient transition zone spared a partial amount of corneal tissue (Figure 2). After 2001, the introduction of wavefront-based ablation improved treatment in asymmetric corneas and generally aberrated eyes. From >4 years of experience as well as international LASIK experience, we believe that wavefront-guided treatment is undoubtedly a safe and effective solution, able to supply results that are not inferior to the Planoscan procedure if used for eyes with a low incidence of aberrations, and that are certainly better than Planoscan in aberrated or nonsymmetrical eyes. In more than 90% of cases, we obtained a postoperative UCVA of equal to or less than the preoperative BSCVA; the total effectiveness index was 1.12. Furthermore, in no case have we had the loss of two or more Snellen lines of BSCVA, and we have observed a total safety index of 1.29.
A wavefront-guided procedure is certainly more difficult and takes longer than a preestablished procedure (eg, Planoscan), and evaluating acquired data requires an experienced refractive surgeon. As we have seen in spherical corrections of >4.00 D, there is no significant difference in induced spherical aberrations between this procedure and Planoscan. Thus, the corrective predictability of the spherical component (ie, predictability index of 0.95 for Planoscan versus 0.956 for Zyoptix [Bausch & Lomb]) is not different. Based on these evaluations, we believe that wavefront-guided PRK should be elective for about 10% to 12% of eyes for first treatment and for about 50% to 60% of eyes for retreatments (ie, practically all cases of retreatment with clear cornea) (Figure 3).
In cases of first myopic treatments with cylindrical defects of <1.50 D, it would be preferable — and more predictable — to optimize treatment using the tissue saving with adjustment of K average or tissue saving with aspherical optimization options. These options take advantage of the tissue-saving capacity for enlarging the optical zone or aspherical ablation, both individually or collectively, of the profile necessary for correcting spherical defects.
Therefore, the tissue-saving option, optimizing ablation produced by mean K adjustment in the Munnerlyn formula, allows improvement in ablative precision through better adaptation of the impact angle of the laser in paracentral regions. This leads to more precise ablation produced by a most appropriate tissue removal. Adaptation of the quantity of tissue ablated to the mean radius of curvature1-3 is the main factor leading to greater predictability with respect to Zyoptix WF (Bausch & Lomb) and Planoscan (index of predictability 0.98 vs 0.96 and 0.95, respectively, in a consecutive series of 94 personal cases with follow-up >24 months) and the possibility of 24% greater tissue sparing (range 20% to 30%) versus Planoscan. The Zyoptix TS system sets a new ablative standard for the Bausch & Lomb platform and can implement the US Food and Drug Administration-approved Planoscan results.
PERSONAL EXPERIENCE WITH ZYOPTIX TS
We treated 94 patients with bilateral symmetric myopia or myopia and astigmatism. One eye was treated with Zyoptix TS PRK and the fellow eye with Planoscan PRK. We compared the safety, efficacy and stability of the refractive procedures. The following were evaluated in all eyes at the preoperative exam and at 15 days, 1, 3, 6, 12 and 24 months postoperative: subjective and autorefractometric refraction, UCVA, BSCVA, corneal topography and dynamic pupillometry (CSO EyeTop; Schwind eye-tech-solutions, Kleinostheim, Germany), corneal pachymetry (DGH Pachette [DGH Technology, Exton, Pennsylvania] and Orbscan II [Bausch & Lomb]) and IOP (tono-pen).
The mean preoperative refractive spherical equivalent (SE) error was -3.87 D ±0.41 SD (-1.75 D to -10.00 D) in Zyoptix TS eyes and -3.86 D ±0.39 SD (-1.50 D to -9.00 D) in Planoscan eyes, and the mean BSCVA was 20/21.2 in both groups. Mean pupil diameter (4.93 mm ±0.32) and mean K (43.9 ±1.96) difference were not statistically significant between groups. Scheduled optical zone was >6 mm (mean 6.72 mm, range 6 mm to 7.4 mm) in the Zyoptix TS group and 6.38 mm (range 6 mm to 7 mm) in the Planoscan group. Programmed ablation depth was 59.64 µm (range 26 µm to 102 µm) in the TS group and 73.53 µm (29 µm to 112 µm) in the Planoscan group.
Mean follow-up was 32.3 months. Refractive results were stable in the Zyoptix TS eyes after 1.22 months and the Planoscan eyes after 2.12 months. At the last control, we had an SE refractive error +0.04 D ±0.03 SD in Zyoptix TS eyes and -0.06 D ±0.16 SD in Planoscan eyes. The mean BSCVA was 20/19 in TS eyes and 20/19.6 in Planoscan eyes; no loss was seen in Snellen lines BSCVA in either group. The UCVA was 20/19.3 in TS eyes and 20/20.7 in Planoscan eyes (Figure 4). The difference between preoperative and last control pachymetry, respectively Orbscan II (Bausch & Lomb) and Pachette (DGH Technology), was 60.13 µm and 60.49 µm in the TS group, 76.44 µm and 78.52 µm in the Planoscan group.
The objectives of the Zyoptix aspheric procedure (1) are the same as the TS system and (2) induce limited primary spherical aberration in the central 4.5 mm of the cornea (ie, by using an algorithm with radius of curvature as a variable [aspherical] to remove tissue radius of curvature). The model seems valid in theory. Clinically, the point on which to center the ablation is critical.4-6 If a lens with a constant (spherical) radius of curvature is slightly decentered in the corneal plane (ie, <0.1 mm to 0.2 mm), the optical geometry of the system does not change significantly. If a lens with a variable radius of curvature is similarly decentered, however, more significant optical effects (ie, primary effects7 on lower order aberration and above all secondary effects on paracentral4 third, fifth and other aberrations) will devastate the optical performance of the system. These theoretical aspects suggest that this aspherical ablation profile requires further training of pupillographic eye-tracking systems and better defined indications before it is proposed as a standard and safely used to correct high myopic defects.
Zyoptix TS has a better predictability than Zyoptix WF and Planoscan and a statistically significant difference in refractive outcome (paired t-test P=.00154). Overall, the functional recovery time is shorter than the Planoscan procedure. The ability of Zyoptix TS in tissue saving allows an optical zone enlargement using the same tissue depth to treat eyes with very large pupils (>7 mm) and allows treatment for eyes with high (-8.00 D to -10.00 D) refractive defects.
Aldo Caporossi, MD, FACS, is the head of the department of ophthalmological and neurosurgical sciences at the University of Siena, in Italy. Professor Caporossi states that he has no financial interest in the products or companies mentioned. He may be reached at caporossi@unisi.it.
Stefano Baiocchi, MD, PhD, is from the department of ophthalmological and neurosurgical sciences at the University of Siena, in Italy. Dr. Baiocchi states that he has no financial interest in the products or companies mentioned. He may be reached at stefano.baiocchi2@virgilio.it.
Up Front | Sep 2006
Tissue Saving in Surface Ablation
Optical zone enlargement with the Bausch & Lomb C217 z100 allows surgeons to treat eyes with very large pupils.
Aldo Caporossi, MD, FACS, and Stefano Baiocchi, MD, PhD
