Characterization of submicrojoule femtosecond laser corneal tissue dissection

Mechanisms of corneal intrastromal laser dissection for refractive surgery: ultra-high-speed photographic investigation at up to 50 million frames per second

Every year, more than a million refractive eye surgeries using femtosecond lasers are performed but the intrastromal cutting process remains an area of development. We investigated the mechanisms of laser dissection in cornea by ultra-high-speed photography. We found that the intrastromal bubble forms multiple lobes along the elongated laser plasma and the overlying lobes expand along the corneal lamellae. Videography demonstrated that the cutting process relies on crack propagation in the stroma along the bubble lobes with the crack originating from the pre-existing bubble layer. These insights are important for further improvement of the cutting mechanisms in refractive surgery.

Reshaping and Customization of SMILE-Derived Biological Lenticules for Intrastromal Implantation

Purpose

To evaluate the feasibility of excimer laser reshaping of biological lenticules available after small incision lenticule extraction (SMILE).

Methods

Fresh and cryopreserved SMILE-derived human lenticules underwent excimer laser ablation for stromal reshaping. The treatment effects in the lasered group were compared with the nonlasered group with respect to changes in surface functional groups (by Fourier transform infrared spectroscopy [FTIR]) and surface morphology (by scanning electron microscopy [SEM] and atomic force microscopy [AFM]). Ten SMILE-derived porcine lenticules, five nonlasered (107-μm thick, -6 diopter [D] spherical power) and five excimer lasered (50% thickness reduction), were implanted into a 120-μm stromal pocket of 10 porcine eyes. Corneal thickness and topography were assessed before and after implantation.

Results

FTIR illustrated prominent changes in the lipid profile. The collagen structure was also affected by the laser treatment but to a lesser extent. SEM exhibited a more regular surface for the lasered lenticules, confirmed by the lower mean Rz value (290.1 ± 96.1 nm vs. 380.9 ± 92.6 nm, P = 0.045) on AFM. The lasered porcine lenticules were thinner than the nonlasered controls during overhydration (132 ± 26 μm vs. 233 ± 23 μm, P < 0.001) and after 5 hours in a moist chamber (46 ± 3 μm vs. 57 ± 3 μm, P < 0.001). After implantation, the nonlasered group showed a tendency toward a greater increase in axial keratometry (6.63 ± 2.17 D vs. 5.60 ± 3.79 D, P = 0.613) and elevation (18.6 ± 15.4 vs. 15.2 ± 5.5, P = 0.656) than the lasered group.

Conclusions

Excimer laser ablation may be feasible for thinning and reshaping of SMILE-derived lenticules before reimplantation or allogenic transplantation. However, controlled lenticule dehydration before ablation is necessary in order to allow stromal thinning.

Comparison of visual performance recovery after thin-flap LASIK with 4 femtosecond lasers

AIM

To compare the speed of visual recovery following myopic thin-flap LASIK with four femtosecond lasers.

METHODS

Eighty-eight eyes of 46 patients who were consecutively scheduled for bilateral LASIK with the IntraLase FS60 (Group 1), Femto LDV Crystal Line (Group 2), Wavelight FS200 (Group 3) and VisuMax (Group 4) femtosecond lasers were enrolled in. Monocular uncorrected distance visual acuity (UDVA), best-corrected distant visual acuity (CDVA), refraction, contrast sensitivity and higher-order aberrations (HOAs) were evaluated at 1, 3d, 1wk and 1mo postoperatively.

RESULTS

Sixteen eyes (72.7%) achieved 20/16 and 8 eyes (36.4%) were 20/12.5 at 1d in Group 2, which was significantly more than other 3 groups. At 1wk, 20 eyes (90.9%) achieved 20/16 in Groups 2 and 4. At 1mo, 20 eyes (90.9%) achieved 20/16 in Group 2 and Group 4, which were significantly more than other two groups. While by 1 mo, the difference of the residual spherical equivalent (SE) was not statistically significant among 4 groups (P=0.121). The induction of spherical aberration (SA) were significantly less for Groups 2, 3, 4 than for Group 1 one day after surgery (P=0.015). The differences among 4 groups were not statistically significant before and after surgery on every time points (all P>0.05).

CONCLUSION

The thin-flap LASIK procedure using the Femto LDV Crystal Line and VisuMax femtosecond laser show faster visual performance recovery.

Changes in ocular biomechanics after femtosecond laser creation of a laser in situ keratomileusis flap

PURPOSE

To evaluate ocular biomechanical parameters with the Corvis ST, a noncontact tonometer combined with an ultra-high-speed Scheimpflug camera, before and after creation of a femtosecond laser-created laser in situ keratomileusis (LASIK) flap.

SETTING

Private practice, Siena, Italy.

DESIGN

Prospective consecutive study.

METHODS

Right eyes of patients having LASIK were assessed with the dynamic Scheimpflug camera before and after femtosecond laser (LDV Z4) flap creation but before mechanical flap lifting.

RESULTS

Twenty-eight eyes of 28 patients were evaluated. Before flap creation, the mean values on the dynamic Scheimpflug camera were intraocular pressure (IOP), 15.04 mm Hg ± 3.99 (SD); central pachymetry, 550.8 ± 101.0 μm; applanation 1 length, 1.721 ± 0.134 mm; applanation 2 length, 1.674 ± 0.287 mm; applanation 1 velocity, 0.126 ± 0.031 m/s; and deflection amplitude, 1.039 ± 0.141 mm. After flap creation, the mean values were IOP, 16.10 ± 3.11 mm Hg (95% confidence interval [CI], 0.44-1.78; P < .05); central pachymetry, 561.8 ± 35.9 μm (95% CI, -28.9 to 50.9; P = .21); applanation 1 length, 1.789 ± 0.1492 mm (95% CI, 0.003-0.134; P < .05); applanation 2 length, 1.759 ± 0.259 mm (95% CI, -0.005 to 0.173; P = .08); applanation 1 velocity, 0.136 ± 0.022 m/s (95% CI, 0.001-0.017; P < .05); and deflection amplitude, 1.029 ± 0.151 mm (95% CI: -0.043 to 0.025; P = .34).

CONCLUSION

The dynamic Scheimpflug camera showed changes in biomechanical properties after femtosecond creation of a LASIK flap as indicated by an increased applanation 1 length and applanation 1 velocity.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Ultrastructural Changes and Corneal Wound Healing After SMILE and PRK Procedures

PURPOSE

To compare keratocyte activation, cellular morphologic changes and wound healing after SMILE and PRK procedures using transmission electron microscope (TEM).

METHODS

In this study, 22 New Zealand white rabbits (10- to 15-week old) were used. The right eyes of all animals underwent SMILE procedure and the left eyes underwent PRK procedure. Cornea samples taken 1 day and 1 week postoperatively were examined using TEM.

RESULTS

Using TEM 1 day after SMILE procedure, the organization of collagen fibers seemed to have been preserved without thermal alterations. Keratocyte activation was observed in the anterior stroma. Disrupted collagen arrangement and debris of cells are visible in the area of damage, and some phagocytic cells and a large number of secondary lysosomes are visible in those cells. At the perimeter zone of the interface, many coenocytes and collagen fragments could be found within the phagocytic cell. One week after SMILE procedure, potential lacuna could be discerned. A large part of the interface of the lenticule extracted had an appearance of clearly being adhered to some mucus secretions. One day after PRK procedure, an irregular epithelial surface was visible using TEM. Keratocytes had been activated and the rough endoplasmic reticulum in those cells had expanded. One week after PRK procedure, the epithelial surface still was irregular and keratinization of the epithelium was still visible in some areas. Corneal endothelium cells were mildly damaged and some vacuoles within the cytoplasm could be discerned. In the anterior stroma, some unhealthy activated keratocytes could still be observed. New collagen fibrils were found present near the activated keratocytes.

CONCLUSION

Using TEM, keratocyte activation could still be observed after SMILE compared to after PRK procedure. Fewer cellular ultrastructural changes were seen after SMILE procedure. Unlike in PRK procedure, no damaged epithelium and endothelium were found after SMILE.

[Importance of wavelength for ultrashort laser pulses in healthy and pathological corneas]

BACKGROUND

A study on the role of laser wavelength in keratoplasty assisted by ultrashort pulse lasers is presented.

MATERIAL AND METHODS

This article gives a summary of the principal physical mechanisms contributing to the transparency of healthy corneas and presents transparency measurements as well as laboratory experiments on tissue with lasers at different wavelengths.

RESULTS

The transparency of a healthy cornea is strongly related to its regular structure at micrometer and nanometer length scales. Many indications for keratoplasty are associated with a perturbation of this structure and therefore with a sometimes strongly reduced tissue transparency. This explains the often unsatisfactory results obtained when using ultrashort pulse lasers for the procedure. Theoretical considerations and laboratory experiments show that the light scattering processes responsible for the loss in laser beam quality depend strongly on wavelength and the use of wavelengths longer than those presently used allows these processes to be almost completely eliminated. The use of a spectral transparency window close to 1.65 µm is suggested.

CONCLUSION

The use of laser wavelengths close to 1.65 µm represents an interesting alternative for the improvement of keratoplasty assisted by ultrashort pulse lasers.

Prospective contralateral eye study to compare 80- and 120-μm flap LASIK using the VisuMax femtosecond laser

PURPOSE

To compare visual outcomes and flap stability of LASIK with ultrathin 80- and 120-μm flaps created with a VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) for moderate to high myopia and to evaluate the effect of corneal flap thickness on outcomes.

METHODS

In a prospective contralateral eye study, 36 consecutive patients (72 eyes) underwent bilateral LASIK for myopia ranging from -2.00 to -10.00 diopters using the VisuMax femtosecond laser and MEL-80 excimer laser (Carl Zeiss Meditec). One eye of each patient was randomized to have the 80-μm flap and the other to the 120-μm flap created with 200-kHz VisuMax femtosecond laser. Preoperative and postoperative tests included visual acuity, manifest refraction, contrast sensitivity, and flap thickness measured by anterior segment optical coherence tomography. Main outcomes and complications were checked at postoperative 1 week and 1, 3, and 6 months.

RESULTS

There were no differences in visual outcome, residual refractive error, or contrast sensitivity between groups during follow-up, except for better uncorrected visual acuity at postoperative 1 day in the 120-μm group. Mean standard deviations of measured flap thickness during follow-up ranged from 3.16 to 3.80 μm in both groups. Opaque bubble layer, a unique complication in femtosecond LASIK, was more frequent in the 80-μm group (7 of 36: 19%) than in the 120-μm flap group (3 of 36: 8%) without a statistically significant difference (P = .301) and was related to thicker central cornea and steeper keratometric value, although it did not influence clinical results. Comparison of the intended versus achieved correction showed no significant differences between groups.

CONCLUSIONS

LASIK using the VisuMax femtosecond laser supplied good clinical results and flap reproducibility in both the 80- and 120-μm flap groups. Patients with relatively thin cornea may benefit from 80-μm flap LASIK.

Unilateral rainbow glare after uncomplicated femto-LASIK using the FS-200 femtosecond laser

PURPOSE

To report a case of rainbow glare following femtosecond laser-assisted LASIK (femto-LASIK) with the Wavelight FS-200 femtosecond laser (Alcon Laboratories, Inc., Fort Worth, TX).

METHODS

A patient was treated bilaterally for myopia with femto-LASIK using the FS-200 femtosecond laser. Postoperatively, he complained of rainbow glare in his right eye.

RESULTS

Three months postoperatively, the induced grating pattern (hyperreflective spot-like zones corresponding to the surgeon-programmed spot and line separation distance of the FS-200 femtosecond laser) was demonstrated with confocal microscopy at the level of the flap interface in the right eye.

CONCLUSIONS

This is the first report of rainbow glare following femto-LASIK with the FS-200 femtosecond laser documented with in vivo confocal microscopy.

Lower energy to make a corneal flap with a 60 kHz femtosecond laser reduces flap inflammation and corneal stromal cell death but weakens flap adhesion

Purpose

To compare corneal flaps created in rabbits with a 60 kHz femtosecond (FS) laser using different levels of raster energy and to measure early inflammation, corneal stromal cell death, and late postoperative adhesion strength.

Methods

Sixty rabbits were divided into three groups of 20 each. A flap 110 µm thick and 9.0 mm in diameter was made in one eye of each rabbit at raster energies of 0.7 µJ, 1.1 µJ, and 2.4 µJ. Histopathological evaluation for inflammation and apoptosis using terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was performed at 4 and 24 hours after flap creation. The adhesion strength of the flaps was measured with a tension meter at 1 and 3 months.

Results

Twenty four hours after flap creation, the 2.4 µJ group had more inflammatory and CD11b-positive cells than the 0.7 and 1.1 µJ groups. The number of TUNEL-positive cells increased with raster energy at 4 and 24 hours. The grams of force (gf) needed to detach the flaps at 3 months was significantly higher in 2.4 µJ group (170 gf) than in 0.7 µJ group (97.5 gf) and 1.1 µJ group (100 gf, p = 0.03).

Conclusions

Using raster energy lower than 1.1 µJ to make a flap with a 60 kHz FS laser decreases inflammatory cell infiltration and corneal stromal cell death in the central cornea but may result in a weaker flap than using higher raster energy (2.4 µJ).

Application of the femtosecond laser LASIK microkeratome in eye banking

PURPOSE OF REVIEW

To review the literature for recent advancements in the femtosecond laser technology with regard to its applications in corneal transplantation and eye banking.

RECENT FINDINGS

Advancements in corneal surgery have encouraged the use of disease-specific corneal subcomponents, utilized in procedures such as anterior-lamellar keratoplasty and endothelial keratoplasty, instead of traditional transplant procedures to minimize adverse effects of penetrating keratoplasty. Femtosecond laser microkeratomes can precisely create flaps for such transplant procedures, achieve better wound stability, and promote healing by shaped wound configurations. Laser microkeratomes have been compared to traditional mechanical microkeratomes for keratoplasty procedures from various aspects and are superior in some aspects and offer unique capabilities.

SUMMARY

Femtosecond laser applications in eye banking include preparation of donor and recipient corneas for use in penetrating keratoplasty, anterior-lamellar keratoplasty, and endothelial keratoplasty. Advantages of femtosecond laser microkeratomes include higher precision of the cut, ability to achieve thinner flaps, and wound configurations that allow greater wound stability, shorter recovery time, and less postoperative pain. However, cost and availability at the eye-bank level may hinder widespread and immediate application.

Finite element model of the temperature increase in excised porcine cadaver iris during direct illumination by femtosecond laser pulses

In order to model the thermal effect of laser exposure of the iris during laser corneal surgery, we simulated the temperature increase in porcine cadaver iris. The simulation data for the 60 kHz FS60 Laser showed that the temperature increased up to 1.23°C and 2.45°C (at laser pulse energy 1 and 2 [micro sign]J, respectively) by the 24 second procedure time. Calculated temperature profiles show good agreement with data obtained from ex vivo experiments using porcine cadaver iris. Simulation results of different types of femtosecond lasers indicate that the Laser in situ keratomileusis procedure does not present a safety hazard to the iris.

Prospect for feedback guided surgery with ultra-short pulsed laser light

The controlled cutting of tissue with laser light is a natural technology to combine with automated stereotaxic surgery. A central challenge is to cut hard tissue, such as bone, without inducing damage to juxtaposed soft tissue, such as nerve and dura. We review past work that demonstrates the feasibility of such control through the use of ultrafast laser light to both cut and generate optical feedback signals via second harmonic generation and laser induced plasma spectra.

Simulation of the temperature increase in human cadaver retina during direct illumination by 150-kHz femtosecond laser pulses

We have developed a two-dimensional computer model to predict the temperature increase of the retina during femtosecond corneal laser flap cutting. Simulating a typical clinical setting for 150-kHz iFS advanced femtosecond laser (0.8- to 1-μJ laser pulse energy and 15-s procedure time at a laser wavelength of 1053 nm), the temperature increase is 0.2°C. Calculated temperature profiles show good agreement with data obtained from ex vivo experiments using human cadaver retina. Simulation results obtained for different commercial femtosecond lasers indicate that during the laser in situ keratomileusis procedure the temperature increase of the retina is insufficient to induce damage.

Temperature increase in porcine cadaver iris during direct illumination by femtosecond laser pulses

PURPOSE

To measure the temperature rise in porcine cadaver iris during direct illumination by the femtosecond laser as a model for laser exposure of the iris during femtosecond laser corneal surgery.

SETTING

Department of Ophthalmology, University of California-Irvine, Irvine, California, USA.

DESIGN

Experimental study.

METHODS

The temperature increase induced by a 60 kHz commercial femtosecond laser in porcine cadaver iris was measured in situ using an infrared thermal imaging camera at pulse energy levels ranging from 1 to 2 μJ (corresponding approximately to surgical energies of 2 to 4 μJ per laser pulse).

RESULTS

Temperature increases up to 2.3 °C (corresponding to 2 μJ and 24-second illumination) were observed in the porcine cadaver iris with little variation in temperature profiles between specimens for the same laser energy illumination.

CONCLUSIONS

The 60 kHz commercial femtosecond laser operating with pulse energies at approximately the lower limit of the range evaluated in this study would be expected to result in a 1.2 °C temperature increase and therefore does not present a safety hazard to the iris.

Human graft cornea and laser incisions imaging with micrometer scale resolution full-field optical coherence tomography

Micrometer scale resolution full-field optical coherence tomography (FF-OCT) is developed for imaging human graft corneas. Three-dimensional (3-D) images with ultrahigh resolution (respectively, 1 and 1.5 μm in the axial and transverse directions), comparable to traditional histological sections, are obtained allowing the visualization of the cells and the precise structure of the different layers that compose the tissue. The sensitivity of our device enables imaging the entire thickness of the cornea, even in edematous corneas more than 800 μm thick. Furthermore, we provide tomographic 3-D images of laser incisions inside the tissue at various depths without slicing the studied corneas. The effects of laser ablations can be observed, along various optical sections, directly in the bulk of the sample with high accuracy, providing information on the interface quality and also imaging tiny changes of the tissue structure. FF-OCT appears to be a powerful tool for subcellular imaging of the corneal structure and pathologies on the entire thickness of the tissue as well as interface quality and changes in the collagen structure due to laser incisions on ex vivo human cornea.

Temperature increase in human cadaver retina during direct illumination by femtosecond laser pulses

PURPOSE

Femtosecond lasers have been approved by the US Food and Drug Administration for ophthalmic surgery, including use in creating corneal flaps in LASIK surgery. During normal operation, approximately 50% to 60% of laser energy may pass beyond the cornea, with potential effects on the retina. As a model for retinal laser exposure during femtosecond corneal surgery, we measured the temperature rise in human cadaver retinas during direct illumination by the laser.

METHODS

The temperature increase induced by a 150-kHz iFS Advanced Femtosecond Laser (Abbott Medical Optics) in human cadaver retinas was measured in situ using an infrared thermal imaging camera. To model the geometry of the eye during the surgery, an approximate 11x11-mm excised section of human cadaver retina was placed 17 mm behind the focus of the laser beam. The temperature field was observed in 10 cadaver retina samples at energy levels ranging from 0.4 to 1.6 microJ (corresponding approximately to surgical energies of 0.8 to 3.2 microJ per pulse).

RESULTS

Maximal temperature increases up to 1.15 degrees C (corresponding to 3.2 microJ and 52-second illumination) were observed in the cadaver retina sections with little variation in temperature profiles between specimens for the same laser energy illumination.

CONCLUSIONS

The commercial iFS Advanced Femtosecond Laser operating with pulse energies at approximately the lower limit of the range evaluated in this study would be expected to result in a 0.2 degrees C temperature increase and do not therefore present a safety hazard to the retina.

Plasma-mediated ablation: an optical tool for submicrometer surgery on neuronal and vascular systems

Plasma-mediated ablation makes use of high energy laser pulses to ionize molecules within the first few femtoseconds of the pulse. This process leads to a submicrometer-sized bubble of plasma that can ablate tissue with negligible heat transfer and collateral damage to neighboring tissue. We review the physics of plasma-mediated ablation and its use as a tool to generate targeted insults at the subcellular level to neurons and blood vessels deep within nervous tissue. Illustrative examples from axon regeneration and microvascular research highlight the utility of this tool. We further discuss the use of ablation as an integral part of automated histology.