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Up Front | Sep 2008

Surface Ablation Plus Smoothing for High Myopia

We saw a marked improvement in long-term results with smoothing.

Patients with high myopia are usually enthusiastic with their refractive surgery results. Nevertheless, treating patients with at least 6.00 D of myopia is a delicate task. Under correction, haze, regression, glare, and even ectasia may spoil the goal of the final outcome—reducing myopia.

We regularly recur to surface ablation for high myopia excimer laser surgery for several reasons. First, surface ablation provides an optimum tool for correcting high myopia, even in eyes with large pupils that require wider optical zones. Alternatively, the size of the flap used in LASIK procedures limits the size of the optical and transition zones.

Second, surface ablation involves less stroma, reducing its biomechanical effect and the risk of corneal ectasia. Custom ablation with LASIK is another option; however, this technique is based on the patient's preoperative wavefront. The flap cut may change, influencing the patient's final wavefront.

One potential downside to performing surface ablation is the associated corneal irregularities generated by the overlapping ablating shots and intraoperative ocular motion involved with the procedure. In this article, we will discuss the benefits of a technique called smoothing. We have found that performing smoothing during a surface ablation treatment reduces corneal irregularities, therefore affording patients better postoperative outcomes.

If a surface ablation case ends with an irregular corneal surface, haze and regression may result. Recently, Paolo Vinciguerra MD, of Milan, Italy, developed the smoothing technique, to be performed after the refractive ablation. The surgeon applies a masking fluid (hyaluronic acid 4%) to the corneal surface so that the irregularities emerge above the hyaluronic acid. After redirecting the laser beam to the corneal surface, the second ablation hits only the peaks of irregularity and not the valleys, resulting in a perfectly smooth surface—even for large areas of irregularity.

During smoothing, the fluid is carefully and continuously distributed over the corneal surface with a spatula, redistributing it when necessary so that smaller peaks of irregularity progressively emerge and are ablated. Eventually, the surface will be regularized. Ablation is performed with a frequency of 10 Hz to avoid overheating; it is applied over a 10 mm surface diameter and is limited to 10 µm of total ablation.

We performed a long-term, retrospective review of 100 highly myopic eyes (-7.50 D or more) that underwent surface ablation and smoothing between 2000 and 2002. Mean preoperative visual acuity was 0.87 ±0.16 (range, -7.50 to -14.25 D), and cycloplegic spherical equivalent (SE) was -9.55 ±1.79 D. Mean sphere was 8.86 ±1.83 D, and mean cylinder was -1.39 ±0.90 D.

We used the Nidek EC-5000 excimer laser (Nidek Company, Ltd., Gamagori, Japan) and the multiple optical zones technique. The ablation zone ranged from 4.8 to 7 mm, with at least 3 mm of additional transition zone and a cross-cylinder technique for astigmatism. Smoothing, in our opinion, was a key to success in these cases.

Follow-up ranged from 2 to 4 years (mean, 3.1 ±1.6 years). Visual acuity was 0.9 ±0.20 (SE -0.45 ± 0.83 D), with -0.26 ±0.83 D of sphere and -0.37 ±0.46 D of cylinder. Haze was 0.11 ±0.32.

Safety and efficacy are shown in Figure 1. Refractive stability (Figure 2) was remarkable, proving that patients were accurately chosen with the aid of inclusion and exclusion criteria (ie, progressive myopia and a stable postoperative corneal surface). Interestingly, haze continued to decrease over time (Figure 2). Figure 3 shows a case with grade 1 preoperative haze. His refraction was -8.75 -1.50 X 180. Three years later, his refraction was 1.00 -0.75 X 180. Figure 4 shows pre- and postoperative instantaneous map and root mean square aberrations.

Our evaluation of corneal aberrations showed a significant decrease in post- versus preoperative astigmatism. Additionally, the spherical aberration remained unchanged, proving that the transition zone between the treated and untreated corneal areas maintained an acceptably low curvature gradient. Coma increased, indicating that centration in high myopia excimer laser refractive surgery was a delicate issue before eye-tracking systems were commercially available.

Complications included retreatment of two hypercorrected eyes. No relevant corneal haze was observed (scale, 0–3; 0.11 ±0.32 at 3 years). No ectasia or retinal detachment was observed.

This study confirmed that surface ablation provides satisfactory results for high myopia when used in combination with smoothing. Several key points are considered, including (1) an extremely wide, peripheral transition zone induces less spherical aberration, with consequent better visual quality and less halos, and (2) the lower corneal curvature gradient reduces postoperative collagen deposition, preventing regression and unwanted restriction of the optical zone. When we added smoothing, the corneal surface was nicely regularized at the end of a prolonged, deep ablation, and haze and regression decreased.

Patients with successfully treated high myopia are usually among our happiest patients. Surface ablation not only provides a high level of accuracy, but it also reduces the risk of ectasia. Thanks to the adjunct of smoothing, the active corneal healing process that is usually encountered during deep high-myopia ablations can be modulated to prevent regression, haze, and optical zone changes.

Fabrizio I. Camesasca, MD, is an ophthalmologist with the Department of Ophthalmology, Istituto Clinico Humanitas, Milan, Italy. Dr. Camesasca states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +39 02 8224 2311; fax: +39 02 8224 4694; e-mail: fabrizio.camesasca@virgilio.it.

Paolo Vinciguerra, MD, is the Chairman of the Department of Ophthalmology, Istituto Clinico Humanitas, Milan, Italy. Dr. Vinciguerra is a member of the CRST Europe Editorial Board. He states that he has no financial interest in the products or companies mentioned. He may be reached at e:mail: paolo.vinciguerra@humanitas.it.