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Roughness of Excimer Laser Ablated Corneas With and Without Smoothing Measured With Atomic Force Microscopy

Abstract

PURPOSE:

To analyze the surface roughness of porcine corneas after excimer laser ablation with and without the smoothing procedure by means of atomic force microscopy.

METHODS:

Excimer laser photorefractive keratectomy (PRK) was performed on eight porcine corneas. Immediately following the procedure, smoothing was performed on four corneas using a viscous solution of 0.25% sodium hyaluronate. The corneas were examined in balanced salt solution after fixation in 2.5% glutaraldehyde solution using atomic force microscopy. Quantitative analysis of the ablated stromal surface topography was performed using the section analysis module of the atomic force microscopy software. Repeated measurements were made over small areas (< or =50 microm2) near the center of each ablation, with a vertical resolution of <1 nm.

RESULTS:

Images of the ablated stromal surface showed undulations and granule-like features on the ablated surface of the specimens. The specimens on which the smoothing procedure was performed (root-mean-square [RMS] rough: 0.152 +/- 0.014 microm) were more regular (P < .001) than those on which PRK alone was performed (RMS rough: 0.229 +/- 0.018 microm).

CONCLUSIONS:

Atomic force microscopy analysis requires a simpler preparation of the specimens with respect to that necessary for scanning electron microscopy; for this reason, atomic force microscopy techniques are more reliable for the study of biological surfaces and prove to be a feasible method to establish the differences when comparing different laser techniques. Our investigations highlight that although the laser cut of scanning-spot excimer laser systems is precise in removing even the smallest amounts of tissue, the smoothing technique may still be useful to reduce post-ablation roughness.

Corneal Epithelial Healing After Photorefractive Keratectomy: Analytical Study

Abstract

PURPOSE:

To characterize the velocity of epithelial migration after photorefractive keratectomy (PRK) with 3 different corneal ablation patterns.

SETTING:

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS:

Fifteen patients (30 eyes) with mild to moderate myopia and with simple to compound myopic astigmatism were enrolled for this study. The surgical procedure consisted of standardized PRK with final smoothing performed using the Technolas Keracor 217C excimer laser. The reepithelialization process was evaluated at 0 hours, 20 hours, 40 hours, and 60 hours after surgery using a digital photo camera and custom software for measurement. Digital analysis of the images was performed. Corneal topographies were taken at 1 month, 3 months, 6 months, and 12 months after PRK.

RESULTS:

The mean speed of radial migration in the 10 eyes (33%) in the low spherical ablation group was 0.087 mm/h +/- 0.008 (SD). This was significantly higher than that found in the 10 eyes (33%) in the high spherical ablation group (mean speed 0.078 +/- 0.007 mm/h; P<.001) and in the 10 eyes (33%) in the cross-cylinder ablation group (mean speed 0.055 +/- 0.014 mm/h; P<.001).

CONCLUSION:

Analysis of the data shows that epithelial migration along the photoablated corneal surface depends on the ablation pattern. The epithelial sliding is highly influenced by local variations in the curvature of the stromal surface. The data demonstrate that faster epithelial wound healing after PRK is predictive of optimal visual performance.

Differences in Nasal and Temporal Responses of the Cornea After Photorefractive Keratectomy

Abstract

PURPOSE:

To examine the differences in the biomechanical response of the peripheral regions of the cornea after photorefractive keratectomy (PRK).

SETTING:

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS:

Preoperative and 1-, 3-, 6-, and 12-month postoperative corneal topographies of 70 eyes that had PRK with the Technolas 217C excimer laser (Bausch & Lomb) were obtained. The eyes were divided into 4 groups according to the preoperative spherical equivalent refraction. Preoperative and follow-up topographic data were imported into custom software that computed the average composite corneal map and difference maps in each group to scientifically evaluate the corneal response to the surgery. The software was also used to analyze regional corneal changes after the laser ablation. Corneal peripheries up to 9.0 mm were evaluated.

RESULTS:

The preoperative corneas had a flatter nasal periphery than temporal periphery. The corneal surfaces in the right eyes and left eyes showed a mirror symmetry. Significant differences in the regional response of the cornea were observed (P<.05), with a greater increase in the curvature of the nasal periphery than in the temporal periphery.

CONCLUSIONS:

To refine modeling of the cornea, the different regional anatomic features and biomechanical responses must be considered. Modifying existing ablation algorithms to compensate for the differences between nasal and temporal corneal flattening of the preoperative corneal surface and between the nasal and temporal responses may improve the postoperative corneal shape and quality of peripheral optics.

Smoothing of the Ablated Porcine Anterior Corneal Surface Using the Technolas Keracor 217C and Nidek EC-5000 Excimer Lasers

Lombardo M, Serrao S
J Refract Surg. 2004 Sep-Oct;20(5):450-3.
PMID: 15523956

Abstract

PURPOSE:

To demonstrate efficacy of a smoothing technique to increase regularity of the anterior corneal surface after photorefractive keratectomy (PRK), using two different excimer lasers.

METHODS:

Spherical ablations of -10.00 D were performed on 11 fresh porcine corneas using either the Technolas Keracor 217C scanning-spot or the Nidek EC-5000 scanning-slit beam excimer laser. Following the procedure, we performed a phototherapeutic keratectomy treatment (smoothing technique) on half of the corneal surface. The smoothing technique was performed using a viscous solution of 0.25% sodium hyaluronate, which was spread on the cornea prior to the procedure. The ablation zone was 6 mm in diameter and the transition zone extended to 3 mm. The ablation depth was set at 10 microm. Corneas were then examined with scanning electron microscopy.

RESULTS:

Smoother treatment zones were apparent in porcine corneas in which smoothing was performed following PRK, with both laser systems. Results from the two lasers were not directly compared.

CONCLUSIONS:

The smoothing procedure performed following PRK using a viscous 0.25% sodium hyaluronate masking solution and a scanning laser system rendered the porcine corneal surface more regular.

One-year Results of Photorefractive Keratectomy With and Without Surface Smoothing Using the Technolas 217C Laser

Serrao S, Lombardo M
J Refract Surg. 2004 Sep-Oct;20(5):444-9.
PMID: 15523955

Abstract

PURPOSE:

To assess the efficacy, predictability, stability, and safety of a smoothing technique in patients with myopia immediately after photorefractive keratectomy (PRK) using a scanning-spot excimer laser.

METHODS:

Using the Technolas 217C excimer laser, PRK was performed on 100 eyes of 54 patients. Ablation zone diameter was 6.0 mm and transition zone diameter was 9.0 mm. The eyes were randomized into two groups: in 50 eyes PRK alone was performed and in the other 50 eyes, a smoothing technique was performed after the initial ablation. Preoperative mean spherical equivalent refraction was -4.98 +/- 1.71 D in the PRK only group (range -2.25 to -8.60 D) and -4.82 +/- 1.61 D in the smoothing group (range -2.00 to -8.00 D). Follow-up was 12 months for all patients.

RESULTS:

At 1 year after surgery, mean manifest spherical equivalent refraction was -0.61 +/- 0.50 D (range -2.25 to +0.62 D) in the PRK only group and in the smoothing group, +0.02 +/- 0.32 D (range -0.75 to +0.75 D). Postoperative regularity topographic indices were lower in the smoothing group than in the PRK group (P<.001).

CONCLUSIONS:

Smoothing after PRK for correction of myopia up to -6.50 D increased surface regularity, as expressed by lower topography surface regularity indices, and reduced the incidence and severity of postoperative haze. We observed higher predictability throughout follow-up in the smoothing group, which may be addressed by a nomogram adjustment in the PRK only group.

Photorefractive Keratectomy With and Without Smoothing: A Bilateral Study

Serrao S, Lombardo M, Mondini F.
J Refract Surg. 2003 Jan-Feb;19(1):58-64.
PMID: 12553608

Abstract

PURPOSE:

To assess whether a smoother stromal surface and a faster epithelium regeneration after excimer laser photorefractive keratectomy (PRK) may lead to better visual results.

METHODS:

Ten patients had unilateral PRK and contralateral PRK plus smoothing. The operative outcome was checked by means of digital standardized photographs taken at 0, 20, 40, and 60 hours after surgery. Complete clinical examinations were performed before and 1 and 3 months after surgery.

RESULTS:

Between 20 and 40 hours after surgery, the average velocity of radial epithelial migration in eyes in which smoothing was performed was 0.083 +/- 0.008 mm/h. This was faster than that (0.064 +/- 0.007 mm/h [P<.01]) observed in the eyes without smoothing. The corneal regularity index at 1 and 3 months was worse in the PRK alone group compared to the PRK plus smoothing group (P=.1). At 3 months, better visual outcome and less haze were observed in the PRK plus smoothing group.

CONCLUSION:

When using a flying spot laser, PRK plus smoothing improved the visual results and diminished regression and haze at 3 months, but gave rise to a slight hyperopic shift.

Retinal Sensitivity to Flicker Modulation: Reduced by Early Age-related Maculopathy.

Abstract

PURPOSE:

To evaluate retinal, cone-mediated flicker sensitivity (CFS) in age-related maculopathy (ARM) by quantifying response gain and threshold of the focal electroretinogram (FERG) to flicker modulation.

METHODS:

Nineteen patients with ARM (visual acuity > or =20/30) and 11 age-matched control subjects were examined. Twelve patients had less than 20 soft drusen in the macular region and no hyper-/hypopigmentation (early lesion), whereas seven had more than 20 soft drusen and/or focal hyper-/hypopigmentation (advanced lesion). Macular (18 degree ) FERGs were elicited by a sinusoidally flickering (41 Hz) uniform field (on a light-adapting background) whose modulation depth was varied between 16.5% and 94%. Amplitude and phase of the response's fundamental harmonic were measured.

RESULTS:

In both control subjects and patients with ARM, log FERG amplitude increased with log stimulus modulation depth with a straight line (power law) relation. However, the slope (or gain) of the function was, on average, steeper in control subjects than in patients with either early or advanced lesions. Mean FERG threshold, estimated from the value of the log modulation depth that yielded a criterion response, did not differ between control subjects and patients with early lesions but was increased (0.35 log units) compared with control subjects in those with advanced lesions. In both patient groups, but not in control subjects, mean FERG phase tended to delay with decreasing stimulus modulation depth.

CONCLUSIONS:

Retinal CFS losses can be detected in ARM by evaluating the FERG as a function of flicker modulation depth. Reduced response gain and phase delays, with normal thresholds, are associated with early lesions. Increased response thresholds, in addition to gain and phase abnormalities, may reflect more advanced lesions. Evaluating CFS by FERG may directly document different stages of macular dysfunction in ARM.