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.

Imaging pulsatile retinal blood flow in human eye.

A functional Fourier domain optical coherence tomography instrument offering spectral Doppler imaging of in vivo pulsatile human retinal blood flow was constructed. An improved phase-resolved algorithm was developed to correct bulk motion artifacts. Spectral Doppler imaging provides complementary temporal flow information to the spatially distributed flow information of the color Doppler image by providing direct visualization of the Doppler spectrum of the flow whose pattern can be further quantified with various velocity envelope curves and their corresponding flow indices. The coefficient of repeatability on resistance index measurement was assessed by analyzing 14 measurements on two vessels within two normal subjects.

Top hat wound configuration for penetrating keratoplasty using the femtosecond laser: a laboratory model.

PURPOSE: To evaluate the mechanical stability and induced astigmatism of a modified multiplanar "top hat" wound configuration for full-thickness penetrating keratoplasty (PK) using the femtosecond laser as compared with PK in a laboratory model. METHODS: Eight human corneoscleral rims were mounted on an artificial anterior chamber. Four samples were assigned to the traditional PK group. Four samples underwent full-thickness keratoplasty with the femtosecond laser: a 9.0-mm cylindrical cut was made from the anterior chamber into the stroma, followed by a ring-shaped (outer diameter 9.0 mm, inner diameter 7.0 mm) horizontal lamellar resection at two-thirds corneal depth and a 7.0-mm cylindrical cut from the lamellae to the corneal surface. Mechanical stability was evaluated after placement of the cardinal sutures and the running sutures. RESULTS: In the "top hat" PK group, wound leakage occurred at 19 +/- 3.36 mm Hg after placement of the cardinal sutures and at 86.25 +/- 9.74 mm Hg after placement of the running sutures. In the traditional PK group, leakage occurred at 0 +/- 0 mm Hg and 76.25 +/- 20.98 mm Hg after placement of the cardinal sutures and running sutures, respectively. Both techniques induced steepening of the corneal curvature postop. The modified wound group showed a mean change in average K of 3.43 +/- 3.62 D, whereas the traditional PK group showed a mean change in average K of 3.21 +/- 6.67 D. CONCLUSION: The femtosecond laser-produced "top hat" wound configuration for PK was found to be more mechanically stable than that produced by the traditional method.

Femtosecond laser-assisted intracorneal keratoprosthesis implantation: a laboratory model.

PURPOSE: To demonstrate femtosecond laser-assisted intracorneal keratoprosthesis implantation and determine the mechanical stability as a function of intraocular pressure. METHODS: Eight human corneoscleral rims were mounted on an artificial anterior chamber. The femtosecond laser microkeratome was used to create a 2.5-mm diameter posterior corneal cap. A 7.2-mm-diameter lamellar stromal pocket was then created at mid-corneal depth. Finally, a 6-mm arc opening to the corneal surface was created at the periphery of the lamellar cut. The posterior lenticule was removed using corneal forceps and a 7.0-mm biopolymer keratoprosthesis was inserted into the stromal pocket. The surface wound was sealed using two 10-0 nylon sutures. A 3.0-mm anterior corneal opening was trephined to expose the keratoprosthesis. Intrachamber pressure was raised until wound leak was observed. RESULTS: Seven of the 8 implants withstood pressures of at least 135 mm Hg without implant extrusion. CONCLUSION: Femtosecond laser corneal dissection provides an alternative to more challenging manual dissection methods for keratoprosthesis implantation. Use of the femtosecond laser microkeratome will further refine keratoprosthesis surgical technique and may allow rapid and easy execution of the surgery.

Femtosecond laser posterior lamellar keratoplasty: a laboratory model.

PURPOSE: To evaluate feasibility of femtosecond laser application in posterior lamellar keratoplasty. METHODS: To evaluate the laser's effectiveness through opaque corneas, anterior corneal caps were resected from opaque corneas induced with 80% acetone solution. To evaluate the femtosecond laser posterior lamellar keratoplasty surgical procedure, human corneoscleral rims were mounted on an artificial anterior chamber. After corneal pachymetry, the femtosecond laser was used to create a 6-mm-diameter, 200-microm-thick endostromal lenticule. Access to the lenticule was provided by a small perilimbal surface opening, also created by the laser. The lenticule was removed using a pair of corneal forceps. A donor lenticule of similar dimensions was created, its endothelial surface coated with viscoelastic, inserted, and positioned on the recipient bed. Two sutures were placed to seal the small surface opening. RESULTS: The femtosecond laser produced an effective and smooth dissection through opaque corneas even at deeper settings. Graft transplantation was fairly simple and effective. CONCLUSION: Femtosecond laser posterior lamellar keratoplasty is a procedure that may provide an alternative to penetrating keratoplasty or the technically challenging manual posterior lamellar keratoplasty.

Subsurface photodisruption in human sclera: wavelength dependence.

BACKGROUND AND OBJECTIVE: Approximately 105 million people worldwide have glaucoma, and approximately 5 million are blind from its complications. Current surgical techniques often fail because of scarring of the conjunctival tissue, Tenon's tissue, or both. Femtosecond lasers can create highly precise incisions beneath the surface of a tissue, as previously demonstrated in the transparent cornea. Because the sclera is a highly scattering subsurface, photodisruption has not been previously possible. MATERIALS AND METHODS: To overcome scattering, a laser operating at 1,700 nm was used to make subsurface cuts in human sclera in vitro via photodisruption. RESULTS: Sub-10-microm width incisions were created beneath the surface without collateral tissue effects, something not possible with shorter wavelengths used to date in corneal applications with the femtosecond laser. CONCLUSION: Completely subsurface photodisruptions can be accomplished in human sclera in vitro. In vivo studies are required to evaluate the potential use of this technology for scleral applications.

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.

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.

Corneal response to femtosecond laser photodisruption in the rabbit.

In this report we evaluated the effect of femtosecond laser energy on the development of corneal haze and keratocyte activation in rabbits following intra-stromal photodisruption to create LASIK flaps using a modified commercial femtosecond surgical laser. Three groups of flap parameters were studied: 1.5 microJ/pulse with 10 microm spot separation and complete side cut (Group 1); 3.5 microJ/pulse with 14 microm spot separation and complete side cut (Group 2); 3.5 microJ/pulse with 14 microm spot separation and partial (50 microm) side cut (Group 3). All flaps were left attached without lifting to avoid epithelial contamination. Rabbits were then evaluated pre- and post-operatively by quantitative in vivo and ex vivo confocal microscopy. The achieved flap thickness 1 week after surgery averaged 88.9+/-12.8, 90.8+/-6.9 and 86.5+/-6.8 microm for Groups 1-3 respectively (p=NS). Interface thickness was significantly greater (p<0.05) in the higher energy groups averaging 40.0+/-11.2 and 37.7+/-5.7 microm for Groups 2-3 compared to 28.6+/-4.5 microm for Group 1. Corneal haze was barely detectible and not significantly different between groups, although haze was detected in the region of the side-cuts in Groups 1 and 2. No clinically significant changes in stromal or epithelial thickness were noted. Laser confocal microscopy showed the presence of small diameter cells within the flap interface that resided within disrupted regions of the corneal collagen lamellae. Keratocyte activation was only detected in regions of the 100% side cut and not over the flap interface. In conclusion, the results of this study indicate that photodisruption of the corneal stroma alone without flap elevation regardless of laser energy does not induce significant corneal haze in the rabbit. However, a thicker stromal interface was seen with the higher energy suggesting greater stromal damage.

Finite-element modeling of posterior lamellar keratoplasty: construction of theoretical nomograms for induced refractive errors.

PURPOSE: To estimate the theoretical corneal refractive error induced by mechanical weakening effects from posterior lamellar keratoplasty (PLKP) in the human cornea. METHODS: The refractive effects of PLKP are simulated by finite-element modeling (FEM) as a mathematical function of the thickness of the excised posterior lamellar corneal button, with a nonlinear formulation of stress-strain relation for the corneal material. A theoretical nomogram was developed to correlate the refractive changes to button thickness. RESULTS: The predicted refractive change after PLKP is less than 1 dpt for a 170-microm thickness posterior corneal button over a broad range of Young's modulus. Thicker buttons result in greater surgically induced refractive errors. CONCLUSIONS: According to FEM analysis, the excision of a posterior lamellar button of less than 170 microm thickness produces a minimal predicted refractive change (< 1 dpt) in the cornea after PLKP.