Pier Giorgio Gobbi

Photorefractive keratectomy for hyperopia: Long-term nonlinear and vector analysis of refractive outcome

PURPOSE To characterize the refractive changes after excimer laser photorefractive keratectomy for the correction of hyperopia over a follow-up up to 3 years and to assess refractive stability and changes in astigmatism. DESIGN Noncomparative, nonrandomized, retrospective, interventional case series. PARTICIPANTS Thirty-eight hyperopic eyes of 28 patients (age range, 33-62 years) with refraction in the range +1.00 to +8.00 diopters (D). Mean attempted correction was +3.33+/-0.98 D (range, +1.00 to +4.00 D). Data were compared to those from 216 eyes treated for myopia in the range -1.00 to -12.70 D. INTERVENTION The hyperopic correction was made using an erodible mask inserted in the laser optical pathway to produce a circular ablation measuring 6.5 mm in diameter. An axicon was then used to create a blend transition zone from 6.5 mm up to 9.4 mm in diameter. Eyes were evaluated 3 to 11 times (5.5+/-2.4) over a 3- to 34-month follow-up (16.8+/-8.4 months). MAIN OUTCOME MEASURES Vector analysis of refractive error, applying a nonlinear statistical model fitting the spherical equivalent, and the sphere component data. The fit parameters were the long-term error at stabilization (epsilon(infinity)), the amount of regression (epsilon0), being the difference of refractive errors immediately after surgery and at stabilization, and the time constant (T1/2) giving the temporal scale length by which the overcorrection halves (regression half-life). Cylinder was analyzed by a linear regression. RESULTS The initial overcorrection was much larger after hyperopic treatments than myopic ones (epsilon0 = -3.26+/-0.35 D vs. +1.43+/-0.35 D), and it takes typically four times longer to regress (T1/2 = 3.30+/-0.91 months). Refractive stabilization is reached after more than 1 year, with a satisfactory refractive result. The hyperopic treatment induces a mean astigmatism of 1.00 D, which remains constant throughout the follow-up, and tends to be aligned along the with-the-rule meridian. CONCLUSIONS The advantages of a reasonably well-designed algorithm to correct hyperopia (epsilon(infinity) = +0.20+/-0.23 D) are counterbalanced by the long time to refractive stabilization and by the induced astigmatism.

Compound myopic astigmatism correction using a mask in-the-rail excimer laser delivery system. Preliminary results.

Purpose. The latest development in the erodible mask technology is an excimer laser containing the mask in the laser optical pathway. This paper reports the results of the first human series of consecutive treatments performed for the correction of compound myopic astigmatism. Methods. We have treated 83 eyes. Spherical equivalent attempted correction ranged between –1.75 and –11.75 D (mean –7.07 ± 2.45 D), astigmatic attempted correction ranged between –1.00 and –5.00 D (mean –2.42 ± 1.02 D). The sphere correction was made by diaphragm using a multi-zone software with three ablation zones: 100% of the total attempted correction for the central 5.0 mm zone, 70% for the second 6.0 mm zone, 30% for the outer 6.5 mm zone. Cylinder correction was made sequentially after myopic correction using the appropriate mask. Results. One month after treatment, mean refractive error was +1.07 ± 1.24 D (range +4.50/–1.75 D) for spherical equivalent, and –0.49 ± 0.57 D (range +0.75/–2.00 D) for astigmatism. Forty-eight eyes (57.8%) had uncorrected visual acuity of 20/40 or better. At six months, mean refractive error was +0.42 ± 0.97 D (range +3.75/–1.00 D) for spherical equivalent, and –0.44 ± 0.51 D (range +0. 25/–3.00 D) for astigmatism. Vector analysis showed that 57 eyes (68.7%) had 5 or less degrees rotation. Seventy-two (86.7%) and 54 eyes (65.1%) had uncorrected visual acuity equal or better than 20/40 and 20/25 respectively. One eye (1.2%) showed a best corrected visual acuity loss of more than one line, but 8 eyes (9.6%) had a gain of more than one line. One-year results on a smaller series (33 eyes) overlap with the six-month results. Conclusions. The mask in-the-rail excimer laser delivery system appears to be effective and predictable in the correction of compound myopic astigmatism. We observed no significant regression of the astigmatic correction over time. Longer follow-up on larger series is necessary to draw final conclusions.

Laser welding of corneal tissue: preliminary experiences using 810-nm and 1950-nm diode lasers

Laser welding of corneal incisions was performed using two different diode laser wavelengths. Tissue fusion was attempted both with direct absorption of radiation at 1950 nm radiation and with ICG dye-enhanced technique at 810 nm. Thirty deep (not full thickness) corneal incisions of 15 fresh-enucleated porcine eyes were treated. Semiconductor diode lasers emitting in continuous wave at 1950 nm and at 810 nm were used. Both were coupled to an optic fiber handpiece. Wound samples were explanted on day 0 after treatment for histological evaluation. The group of corneal wounds treated with the ICG-enhanced technique revealed tissue welding in 70% of treated wounds. Using 1950 nm tissue fusion was observed in 50% of treated wounds. Macroscopic evaluation revealed a pronounced thermal damage of the epithelium in the samples treated with 1950 nm radiation. Wound closures obtained using 1950 nm were characterized by superficial welding of corneal layers. (Abstract truncated.)

Acoustic transients following excimer laser ablation of the cornea.

Pressure transients occurring inside the eyeball as a consequence of excimer laser ablation of the cornea were measured at various distances along the optical axis, on enucleated porcine eyes. Positive (compressive) pressure spikes up to 90 bars were observed, lasting 100 ns, and developing, as they propagated in the eyeball, satellite rarefaction pulses with negative pressure as high as −40 bar. Such rarefaction wavefronts can trigger the formation of cavitation bubbles, in both the anterior and posterior chamber, depending on the ablation geometry. The potential risks associated with these photoacoustic phenomena, particularly for retina and corneal endothelium, are outlined.

Optical eye model for photorefractive surgery evaluation

Purpose: To improve the applicability of the Gullstrands eye model for retinal image evaluation and prediction of photo-refractive surgery outcome. Methods. The Gullstrands eye model has been modified to allow for variable asphericity of all the refracting surfaces in the model. Formation of the retinal image is studied by means of numerical ray-tracing and by evaluation of modulation transfer function and point/line-spread-function. Results. By comparison of the model line-spread functions with experimental ones taken from the literature, best-fitting values of the asphericity parameters have been derived. The model visual performances are evaluated and compared with other eye models. Application of the model to photo-refractive surgery reveals that proper aspheric curvature in the mid periphery is essential to minimize spherical aberration and guarantee ultimate visual performances in the emmetropic eye, at the limit allowed by diffraction and photoreceptor density in the fovea. Conclusions: The proposed eye model can be useful in vision research as well as in practical applications like the design of ocular lenses and the design of improved algorithms for photorefactive surgery.

Hyperopia correction using an erodible mask excimer laser delivery system coupled to an axicon: preliminary results.

Purpose. This paper presents the results of the first human trial on the correction of hyperopia using an erodible mask excimer laser delivery system coupled to an axicon. Methods. We treated 17 eyes of 17 patients (age range 34–62 years) for the correction of +3.21 ± 1.04 D (range +1.00 to +4.00 D). The hyperopic correction was made using an erodible mask inserted on the laser optical pathway, to produce a circular ablation measuring 6.5 mm in diameter. An axicon was then used to create a blend transition zone from 6.5 mm up to 9.4 mm in diameter. Eyes were evaluated at one, three and six months after surgery. Results. Reepithelization was always observed by the fifth postoperative day, despite the large area of deepithelization (diameter 9.5 mm). Mean refractive error one month after treatment was −2.44 ± 1.59 D (range 0.00 to −6.50 D). Five eyes (29.4%) had a best corrected visual acuity loss more than two to three lines; all eyes showed mild annular haze not involving the central part of the cornea. Six months after treatment, mean refractive error was −0.88 ± 0.99 D (range +0.50 to −3.00 D). Compared to preoperative status, 13 eyes (76.5%) showed an improvement in uncorrected distance visual acuity (1–8 lines), and 14 eyes (82.4%) showed an improvement in uncorrected vision at reading distance (3–7 lines). Two eyes (11.7%) showed a best corrected visual acuity loss of two of three lines. Conclusions. These preliminary results indicate this approach is effective in reducing hyperopia, while its predictability has still to be proved in a larger treatment group with longer follow-up. A cautious approach to this technique is still advisable, especially for higher hyperopic corrections, in view of the large best corrected visual acuity loss seen in two eyes at six months.

Laser-induced stress-wave propagation in the eye

The behavior of stress waves induced by Argon-Fluoride laser ablation of the cornea in the typical operative conditions of clinical laser keratectomy have been studied experimentally and analyzed in porcine eyes and in an artificial eye model. Laser-induced stresses with pressure peaks as high as 100 bar were measured in the anterior chamber of irradiated eyes. Analysis of stress wave propagation the eye evidenced diffraction effects modifying the temporal shape of the stress transient with the formation of a rarefaction phase. Besides, significant pressure enhancements caused by focusing of the stress wave front were observed to occur when the spot diameter exceeded 3 mm. For the maximum laser spot diameter of 6.5 mm, diffraction and focusing effects produced at the acoustic focus in the eye compression peaks of 250 and negative peaks of -90 bar, respectively. Implications to clinical procedures, as possible damages due to tissue stretching and cavitation formation are discussed.

New semiconductor diode laser emitting at 2 um for microsurgery applications

A surgical laser instrument is presented, based on a semiconductor diode emitting in the spectral region around 2 micrometer, in continuous mode. At this wavelength tissue absorption is via histological water, and the interaction is purely thermal. A portable laser station was built, emitting up to 250 mW of optical power at 1.94 micrometers, out of the tip of a low- hydroxil fused-silica optical fiber of 200 micrometer diameter. Experiments have been performed both in vitro with enucleated porcine eyes and in vivo, on rabbits and rats. In vitreo-retinal surgery, retinectomies and incisions of epiretinal fibrotic membranes have been achieved. At reduced output power the laser radiation has been applied to the welding of tissues, in particular cornea, sclera and skin, with interesting results. This new laser source, characterized by ergonomic advantages like compactness, portability, long lifetime, reduced maintenance, is potentially attractive for a number of microsurgical procedures like micro- incisions, shallow coagulations, and welding of hydrated biological membranes.

Experimental study of corneal arcuate excisions performed with the excimer laser on rabbit eyes

Congenital or post-operative high degree astigmatism is currently corrected surgically by corneal arcuate incisions. Due to differences in manual skill in producing corneal incisions with precise depth, width, and length the final refractive results of these surgical techniques may be unpredictable. The aim of the study was to evaluate the feasibility of excimer laser in producing corneal arcuate excisions on rabbit eyes.