Nd:YAG fourth harmonic (266-nm) generation for corneal reshaping procedure: An ex-vivo experimental study

Corneal reshaping is a common medical procedure utilized for the correction of different vision disorders relying on the ablation effect of the UV pulsed lasers, especially excimer lasers (ArF) at 193 nm. This wavelength is preferred in such medical procedures since laser radiation at 193 nm exhibits an optimum absorption by corneal tissue. However, it is also significantly absorbed by the water content of the cornea resulting in an unpredictability in the clinical results, as well as the high service and operation cost of the commercial ArF excimer laser device. Consequently, other types of solid-state UV pulsed lasers have been introduced. The present work investigates the ablation effect of solid-state laser at 266 nm in order to be utilized in corneal reshaping procedures. Different number of pulses has been applied to Polymethyl Methacrylate (PMMA) and ex-vivo rabbit cornea to evaluate the ablation effect of the produced laser radiation. PMMA target experienced ellipse-like ablated areas with a conical shape in the depth. The results revealed an almost constant ablation area regardless the number of laser pulses, which indicates the stability of the produced laser beam, whereas the ablation depth increases only with increasing the number of laser pulses. Examination of the ex-vivo cornea showed a significant tissue undulation, minimal thermal damage, and relatively smooth ablation surfaces. Accordingly, the obtained 266-nm laser specifications provide promising alternative to the traditional 193-nm excimer laser in corneal reshaping procedure.

Lasers in corneal surgery

New laser methods have introduced new possibilities and partly replaced traditional methods in corneal surgery. Femtosecond lasers have traditionally been used to replace the surgeons' knife in corneal refractive surgery and corneal transplantation. Excimer laser has been used from the beginning of 1990 for the reshaping of cornea. With the development of excimer laser, traditional photorefractive corneal surgery has improved and new methods have been found such as PRK, PTK, epi-LASIK, and LASEK. Today LASIK, the most commonly used refractive surgery, uses both femtosecond and excimer lasers. SMILE, the most recent method in corneal refractive surgery, is solely based on the use of femtosecond laser.

New instruments for lenticule extraction in small incision lenticule extraction (SMILE)

Small incision lenticule extraction (SMILE) is an alternative to Laser-Assisted in situ Keratomileusis (LASIK) for correction of myopia. In cases where surgeons inadvertently dissect the posterior surface first, identification of the anterior surface and subsequent removal become difficult since the anterior surface of the lenticule is compacted against the anterior stromal surface. This may result in incomplete lenticule removal, and a remnant of intrastromal lenticule in SMILE may lead to visual sequelae. In order to aid surgeons in lenticule removal, we have designed and developed 5 novel SMILE lenticule strippers to locate and extract the lenticules more easily. The aim of this study was to investigate and compare the efficacy and quality of these lenticule strippers in assisting SMILE. Thirty porcine eyes were used. The ease of extraction and removal of the lenticule with different strippers was graded by an experienced SMILE surgeon, the extracted lenticule circularity was evaluated by calculating the lenticule circularity, and the intactness of the extracted lenticule edge was assessed using scanning electron microscopy. We found these novel strippers can be of great help to improve the safety and quality of SMILE surgery, particularly in those cases of difficult lenticule extraction.

Visual and refractive outcomes of femtosecond lenticule extraction and small-incision lenticule extraction for myopia

PURPOSE

To compare the visual and refractive outcomes of femtosecond lenticule extraction (FLEx) and small-incision lenticule extraction (SMILE) in eyes with myopia.

DESIGN

Prospective, randomized, masked clinical trial with intraindividual comparison.

METHODS

This study evaluated 52 eyes of 26 consecutive patients with spherical equivalents of -4.19 ± 1.65 diopters (D) (mean ± standard deviation) who underwent FLEx in 1 eye and SMILE in the other eye by randomized assignment. Before surgery, and 1 week and 1, 3, and 6 months after surgery, we assessed safety, efficacy, predictability, stability, and adverse events of the 2 surgical techniques.

RESULTS

LogMAR uncorrected and corrected distance visual acuity was, respectively, -0.17 ± 0.10, -0.20 ± 0.07 in the FLEx group and -0.15 ± 0.10, -0.19 ± 0.07 in the SMILE group 6 months postoperatively. In the FLEx and SMILE groups 6 months postoperatively, 96% and 100% of eyes, respectively, were within 0.5 D of the targeted spherical equivalent correction. Changes of -0.02 ± 0.39 D and 0.00 ± 0.30 D occurred in manifest refraction from 1 week to 6 months in the FLEx and SMILE groups, respectively. No clinically significant complications occurred in the FLEx or the SMILE group.

CONCLUSIONS

Both FLEx and SMILE performed well in the correction of myopia throughout the 6-month observation period. FLEx may be essentially equivalent to SMILE in terms of safety, efficacy, predictability, and stability, suggesting that the presence or absence of lifting the flap does not significantly affect these visual and refractive outcomes.

[Intrastromal corneal segments implantation in keratoconus]

Up-to-date models of intrastromal corneal segments are reviewed. Different techniques of up-to-date models of intrastromal corneal segments implantation in keratoconus are described. Mechanical and laser techniques of forming of corneal tunnels are analyzed. Information about potential complications of intrastromal corneal segment implantation is given.

First results of Intracor procedure in Croatia

This study reports early outcomes of a cohort of presbyopic patients treated with Intracor. The study took place from December 2010 to May 2011 and was conducted in University Eye Hospital "Svjetlost", Zagreb, Croatia. 95 eyes were enrolled in this prospective clinical trial (49 patients with non dominant eye and 23 with bilateral treatment). All patients gave informed consent prior to enrollment. Follow up consisted of uncorrected and corrected distant and near visual acuity, record of topographic changes, visual disturbances and patient satisfaction at 1 week, 1 and 3 months after the surgery. In this study Intracor procedure presented as both safe and effective with all eyes gaining several lines of uncorrected near visual acuity (UNVA), and achieving good uncorrected distant visual acuity(UDVA) as well. UDVA was affected by a mild myopic shift, which was effective in reducing mild preexisting hyperopia in some patients but led to a mild myopic outcome in previously emmetropic patients. Statistically significant improvement in UDVA and UNVA was observed in all time points. At 3 months of postoperative follow up all patients gained several lines of UNVA with monocular UNVA Jaeger system 1.67 +/- 0.28. UDVA showed slight improvement over time and initial myopic shift showed tendency of slight decrease with all patients achieving 1.0. Overall patients satisfaction was very high (98%) with only a few (3 patients, 5 eyes) reporting mild halo and glare at 3 months postop.Intracor procedure has proven its short-term safety and efficacy in treating presbyopia. However, longer follow up period is needed.

Effect of air-flow on the evaluation of refractive surgery ablation patterns

An Allegretto Eye-Q laser platform (Wavelight GmbH, Erlangen, Germany) was used to study the effect of air-flow speed on the ablation of artificial polymer corneas used for testing refractive surgery patterns. Flat samples of two materials (PMMA and Filofocon A) were ablated at four different air flow conditions. The shape and profile of the ablated surfaces were measured with a precise non-contact optical surface profilometer. Significant asymmetries in the measured profiles were found when the ablation was performed with the clinical air aspiration system, and also without air flow. Increasing air-flow produced deeper ablations, improved symmetry, and increased the repeatability of the ablation pattern. Shielding of the laser pulse by the plume of smoke during the ablation of plastic samples reduced the central ablation depth by more than 40% with no-air flow, 30% with clinical air aspiration, and 5% with 1.15 m/s air flow. A simple model based on non-inertial dragging of the particles by air flow predicts no central shielding with 2.3 m/s air flow, and accurately predicts (within 2 μm) the decrease of central ablation depth by shielding. The shielding effects for PMMA and Filofocon A were similar despite the differences in the ablation properties of the materials and the different full-shielding transmission coefficient, which is related to the number of particles ejected and their associated optical behavior. Air flow is a key factor in the evaluation of ablation patterns in refractive surgery using plastic models, as significant shielding effects are found with typical air-flow levels used under clinical conditions. Shielding effects can be avoided by tuning the air flow to the laser repetition rate.

Surface ablation of corneal stroma with few-cycle laser pulses at 800 nm

We report measurements of crater diameter and surface ablation threshold as a function of laser fluence in porcine corneal stroma and fused silica with pulse durations of 7 fs (2.7 optical cycles), 30 fs and 100 fs at 800 nm. For laser pulses with Gaussian radial intensity profile, we show experimentally that the square of the crater diameter is a linear function of the logarithm of the fluence in fused silica, while it is closer to a linear function of the fluence in corneal stroma. Extrapolating these relations to zero diameter indicates that for both media the minimum fluence required for surface ablation is reduced with shorter pulse duration. A simple theoretical model suggests that this effect is due to a more significant contribution of photoionization as the laser pulse duration shortens.

Corneal shaping and ablation of transparent media by femtosecond pulses in deep ultraviolet range

PURPOSE

To assess the performance of a newly developed solid-state femtosecond ultraviolet (UV) laser system in corneal ablation.

SETTING

Vilnius University, Laser Research Centre, Vilnius, Lithuania.

METHODS

Femtosecond pulses in the deep UV range (205 nm) were obtained by the generation of the fifth-harmonic of an amplified Yb:KGW laser system (fundamental output at 1027 nm). Coupled with galvanometric beam-scanning mirrors, this system allowed ablation shaping of transparent media, including poly(methyl methacrylate) (PMMA), collagen, and ex vivo porcine corneas. The surfaces of ablated structures were characterized using a noncontact confocal optical profiler.

RESULTS

Spherical structures were successfully formed in all 3 materials tested. A 10.0 diopter refraction change in the cornea was produced in 180 seconds. The resulting surface quality was significantly higher (roughness length >100 microm versus approximately 6 microm) in gelatin and ex vivo corneas than in PMMA.

CONCLUSION

The solid-state femtosecond UV laser system seems an attractive alternative to the currently used ophthalmic argon-fluoride excimer laser system because of its small footprint, silent operation, and ability to generate femtosecond light pulses at both 1027 nm (suitable for flap creation) and 205 nm (corneal ablation).

Suitability of Filofocon A and PMMA for experimental models in excimer laser ablation refractive surgery

Experimental corneal models in plastic (in PMMA, and more recently in Filofocon A, a contact lens material) have been proposed recently to overcome some of the limitations of the theoretical approaches aiming at improving the predictability of corneal reshaping by laser ablation. These models have also been proposed for accurate assessment of corneal laser ablation patterns. In this study Filofocon A and PMMA optical and ablation properties were studied using an experimental excimer laser set-up. The effective absorption coefficient and the ablation thresholds of these materials were obtained as a function of the number of pulses. Both materials follow a Beer-Lambert law in the range of fluences used in refractive surgery, and the number of incubation pulses is less than 4 (PMMA) and 2 (Filofocon A) above 140 mJ/cm2. We found that above 40 pulses for Filofocon A and 70 pulses for PMMA, ablation threshold and effective absorption coefficients can be considered constant (F th = 90 mJ/cm2 and alpha eff = 36000 cm(-1), for Filofocon A, and F th = 67 mJ/cm2 and alpha eff = 52000 cm(-1) for PMMA, respectively). The absence of ablation artifacts (central islands), a lower number of incubation pulses, a lower pulse-number dependence of the ablation threshold, and a good correspondence between alpha eff and the absorption coefficient alpha estimated from spectroscopic measurements make Filofocon A a more appropriate material than PMMA for experimental models in refractive surgery and for calibration of clinical lasers.

[First Brazilian refractive surgery survey]

PURPOSE

To perform the first Brazilian refractive surgery survey.

METHODS

Between August 2001 and February 2002, a questionnaire was mailed to 7890 Brazilian ophthalmologists. The questionnaire presented questions about demographic aspects, technology, instruments, market, trends, practice patterns and cost of refractive surgery.

RESULTS

Nine hundred and twenty questionnaires (11.67%) were answered. LASIK is the preferred technique for correction between +5.00 and -7.00 D. The mostly used excimer laser is Nidek EC 5000, and Hansatome is the mostly used microkeratome. Others aspects like: demographic, techniques, practice patterns, trends, market, and pre-, intra- and postoperative care, were analyzed.

CONCLUSION

With this survey, the Brazilian ophthalmologists can observe and monitor these aspects of refractive surgery in Brazil, and compare them with others surveys.

In-vivo intratissue ablation by nanojoule near-infrared femtosecond laser pulses

Non-invasive intratissue ablation was performed in the cornea of living rabbits by using 80 MHz near-infrared intense nanojoule femtosecond laser pulses. The intratissue surgical effect was induced by multiphoton absorption at a wavelength of 800 nm and was ascertained by histological examination. Highly precise intratissue ablation was obtained with no detrimental effects to the overlying or underlying layers. Activated keratocytes in the laser-treated corneas were detected with two-photon imaging postoperatively. Intratissue femtosecond laser ablation thus has potential as a effective technique in refractive surgery for the treatment of visual disorders.

The Ideal Excimer Beam for Refractive Surgery

PURPOSE

To identify essential considerations for the optimal excimer laser beam for refractive laser surgery and further to determine specific beam parameters fulfilling the essential requirements.

METHODS

Starting with the fundamental laser tissue interaction, the various parameters defining the laser beam are analyzed and optimal values for each are evaluated.

RESULTS

Gaussian beams generally produce smoother ablated surfaces than "flat top" beams. Optimal ablation efficiency for a Gaussian profile occurs at a peak laser fluence of approximately 450 mJ/cm2. To treat realistic optical aberrations the Gaussian spot size should be < 0.9 mm. The laser repetition rate for such a beam can be multiple hundred hertz without causing thermal damage if a "flying spot" strategy is used. Accurate treatment under such conditions requires both high-fidelity laser spot placement and high-performance eye tracking.

CONCLUSIONS

Surgical systems using the characteristics defined in this study should yield excellent treatment outcomes.

Ablationsprofile in der kornealen refraktiven Laserchirurgie

The predictability and quality of results in corneal refractive laser surgery are determined by a number of factors. Here, the calculation and choice of the ablation profile represent central elements. Our growing knowledge about the physical and optical properties of the eye in recent years has led to the development of different strategies in the generation of ablation profiles. This review describes the currently used ablation profiles with their advantages and disadvantages and provides an outlook on future methods for the calculation of ablation profiles.

[Supernormal sight and high-resolution retinal imaging: a review]

BACKGROUND

An analysis of the wavefront in human eyes has yielded new findings with respect to the cones and other elements of the retina. But the correction of higher-order wavefront aberrations in photorefractive processes is still associated with basic difficulties.

MATERIALS AND METHODS

The world literature relating to wavefront corrections in experimental research and the correction of photorefractive aberrations in the human eye is surveyed.

RESULTS AND CONCLUSIONS

The correction of higher-order aberrations has permitted a vast improvement in the resolution of the cone-pattern picture existing in vivo. As a consequence, new findings with regard to colour physiology have been forthcoming. On the other hand, the application of wavefront corrections in photorefractive procedures has been only partially successful.