Erbium:YAG laser surgery of the vitreous and retina

State-switchable and wavelength-tunable gain-switched mid-infrared fiber laser in the wavelength region around 2.94 μm

We demonstrate a gain-switched singly Ho³⁺-doped ZBLAN fiber laser for the first time in the wavelength region around 2.94 μm which circumvents the strong water vapor lines. Four switchable gain-switched temporal states with 1/n (n = 4,3,2,1) pump repetition rates are first observed. The influences of pump power (pulse energy), repetition rate, duty cycle (pulse duration), and laser wavelength on their characteristics are studied, respectively. The results indicate that high pump repetition rate, large pump duty cycle, and short laser wavelength are beneficial for obtaining more gain-switched temporal states. For the case (n = 1), the increased pump repetition rate contributes to the increased pulse duration while decreased pulse energy and peak power. While μs-level pump pulse duration variation has an almost negligible effect on them. By introducing a plane ruled grating, the wavelength tuning was performed yielding a tuning range of 105 nm from 2895.5 nm to 3000.5 nm which just overlays the peak region of liquid water absorption. Finally, further optimizing of laser performances is discussed as well. This demonstration is helpful for preliminarily designing, prior to constructing a mid-infrared gain-switched laser which can find direct applications in laser surgery.

Mid-infrared passively switched pulsed dual wavelength Ho(3+)-doped fluoride fiber laser at 3 μm and 2 μm

Cascade transitions of rare earth ions involved in infrared host fiber provide the potential to generate dual or multiple wavelength lasing at mid-infrared region. In addition, the fast development of saturable absorber (SA) towards the long wavelengths motivates the realization of passively switched mid-infrared pulsed lasers. In this work, by combing the above two techniques, a new phenomenon of passively Q-switched ~3 μm and gain-switched ~2 μm pulses in a shared cavity was demonstrated with a Ho³⁺-doped fluoride fiber and a specifically designed semiconductor saturable absorber (SESAM) as the SA. The repetition rate of ~2 μm pulses can be tuned between half and same as that of ~3 μm pulses by changing the pump power. The proposed method here will add new capabilities and more flexibility for generating mid-infrared multiple wavelength pulses simultaneously that has important potential applications for laser surgery, material processing, laser radar, and free-space communications, and other areas.

Flexible UV light guiding system for intraocular laser microsurgery

BACKGROUND AND OBJECTIVE

Until now, UV lasers could not be applied to vitreoretinal surgery because of the absence of a practical beam guiding system. A flexible, freely mobile hollow core waveguide, capable of delivering UV laser light, was recently developed. We evaluated the feasibility of this UV laser guiding system for vitreoretinal surgery in enucleated porcine and rabbit eyes and in vivo on rabbit retina.

MATERIALS AND METHODS

193-nm argon fluoride (ArF) excimer laser and a 5th harmonic Nd:YAG laser with the wavelength of 213 nm were used as a light source. A hollow core waveguide was connected to a handpiece with a 20-gauge endoprobe as an instrument for vitreoretinal surgery.

RESULTS

Cutting of the retina was possible only with contact technique. With a selected repetition rate of 20 Hz, sharp, full-depth dissection of porcine retinas was achieved only with 1.0 J/cm2. With the repetition rate of 50 Hz, even 0.05 J/cm2 could incise the total thickness of rabbit retina with no histological damage of the underlying retinal pigment epithelium. The ease in using the instrument for vitreoretinal surgery was confirmed by the in vivo experiment.

CONCLUSION

This study revealed the potential use of the new type of the UV laser scalpel, connected to a hollow core waveguide, to perform precise, "cold," and tractionless cutting during vitreoretinal surgery.

Laser-fiber system for ablation of intraocular tissue using the fourth harmonic of a pulsed Nd:YAG laser

We report on a method for delivering high fluence pulsed 266 nm laser radiation to the target tissue via an optical fiber. The fourth harmonic of a Nd:YAG laser was concentrated using a hollow glass taper and launched into an optical fiber. Fluences of up to 2 J/cm(2) were routinely output at the tapered optical fiber tip. The maximum fluence generated before failure of the optical fiber was between 3.5 and 8 J/cm(2). Ablation of ocular tissue was demonstrated using fluences of 1.0 and 0.4 J/cm(2). The delivery system has the potential for use in intraocular surgical procedures.

Mid infrared optical parametric oscillator (OPO) as a viable alternative to tissue ablation with the free electron laser (FEL)

BACKGROUND AND OBJECTIVES

Investigations with a Mark-III free electron laser, tuned to 6.45 microm in wavelength have demonstrated minimal collateral damage and high ablation yield in ocular and neural tissues. While the use of mid-IR light produced by the free electron laser (FEL) has shown much promise for surgical applications, further advances are limited due the high costs of its use. Further investigation and widespread clinical use of six-micron radiation requires the development of an alternative laser source. In this research, we compared a Mark-III FEL and an Er:YAG pumped ZGP-OPO with respect to the effect of pulse duration on ablation efficiency and thermal damage on porcine cornea.

STUDY DESIGN/MATERIALS AND METHODS

A five by seven grid of craters was made about the center of each cornea. Craters were made with a 60-microm spotsize with a 500-microm spacing. Ablation craters were made using 50 pulses per crater at approximately three times the ablation threshold (for water). Histological analysis was used to determine crater depth and thermal damage.

RESULTS

The average zone of thermal damage at 6.1 microm was found to be 4.1 microm for the optical parametric oscillator (OPO) and 5.4 microm for the FEL. At 6.45 microm, the damaged zone was 7.2 microm for the OPO and 7.2 microm for the FEL. At 6.73 microm, the damaged zone was 6.3 microm for the OPO and 7.6 microm+/-0.3 microm for the FEL.

CONCLUSIONS

The OPO caused similar or significantly less thermal damage in porcine cornea when compared with the FEL while generating significantly deeper craters. We determined that the ZGP-OPO has much promise as a bench-top replacement for the FEL for soft tissue ablation.

PULSED ELECTRON AVALANCHE KNIFE IN VITREORETINAL SURGERY

PURPOSE

To evaluate the advantages, disadvantages, safety, and surgical applicability of the pulsed electron avalanche knife (PEAK-fc), a new electrosurgical knife for "cold" and tractionless cutting, in vitreoretinal surgery. PEAK-fc is equipped with an integrated fiberoptic that makes bimanual procedures in intraocular surgery possible.

METHODS

A prospective consecutive trial of 18 eyes in 18 patients who underwent vitreoretinal surgery for proliferative diabetic retinopathy, proliferative vitreoretinopathy, subretinal macular hemorrhage, or macular pucker was performed. The following specific maneuvers were performed with PEAK-fc: transection of epiretinal membranes, retinotomies, retinal vessel coagulation, and posterior membranectomy.

RESULTS

Detached and attached retina could be dissected successfully in eight cases. Intraoperatively, incision edges were sharply demarcated, showing no visible collateral damage. Deeper layers than the neurosensory retina were not affected. With the bimanual approach, epiretinal avascular and vascular membranes could be removed in 10 cases. Hemorrhages occurring during transection of vascularized membranes could be stopped immediately using the coagulation mode of PEAK-fc. Posterior capsule fibrosis was successfully excised in one patient. No complications were observed.

CONCLUSION

PEAK-fc offers precise and tractionless tissue cutting during ocular surgery. Using different waveform parameters, the same device performs cold cutting and/or "hot" coagulation, thus improving the precision, safety, and ergonomics of vitreoretinal surgery.

The effect of free-electron laser pulse structure on mid-infrared soft-tissue ablation: biological effects

Previous studies have shown that changing the pulse structure of the free electron laser (FEL) from 1 to 200 ps and thus reducing the peak irradiance of the micropulse by 200 times had little or no effect on both the ablation threshold radiant exposure and the ablated crater depth for a defined radiant exposure. This study focuses on the ablation mechanism at 6.1 and 6.45 microm with an emphasis on the role of the FEL pulse structure. Three different experiments were performed to gain insight into this mechanism. The first was an analysis of the ablation plume dynamics observed for a 1 ps micropulse compared with a 200 ps micropulse as seen through bright-field analysis. Negligible differences are seen in the size, but not the dynamics of ablation, as a result of this imaging. The second experiment was a histological analysis of corneal and dermal tissue to determine whether there is less thermal damage associated with one micropulse duration versus another. No significant difference was seen in the extent of thermal damage on either canine cornea or mouse dermis for the micropulse durations studied at either wavelength. The final set of experiments involved the use of mass spectrometry to determine whether amide bond breakage could occur in the proteins present in tissue as a result of direct absorptions of mid-infrared light into the amide I and amide II absorption bands. This analysis showed that there was no amide bond breakage due to irradiation at 6.45 microm on protein.

Ablation of vitreous tissue with a high repetition rate erbium:YAG laser

PURPOSE

To quantify erbium (Er):YAG laser ablation of vitreous in relation to different pulse repetition rates < or = 200 Hz, in order to examine the feasibility of laser for removal of vitreous gel (photovitrectomy) in clinically acceptable times.

METHODS

Fresh porcine vitreous samples and saline controls were ablated in air with an Er:YAG laser connected to a sapphire fiber at pulse energies between 1.0 and 21.2 mJ and at pulse repetition rates between 10 and 200 Hz. Net ablation rates were determined by weight measurement.

RESULTS

Reproducible and constant ablation rates were found for given laser parameters. Net ablation rates increased linearly with pulse repetition rate and nonlinearly with pulse energy. Expanded laser parameter domains permitted vitreous ablation rates as low as 1 microg/s to as high as 1031 microg/s. Ablation rates did not differ significantly between vitreous and saline.

CONCLUSIONS

The study documents clinically useful vitreous ablation rates that scale linearly with high repetition rates of Er:YAG laser, and suggests directions for further development of laser technology for enhanced removal of vitreous and other tissues. However, nonlinear effects of pulse energy also exist, indicating need for careful examination of ablation characteristics in various instruments.

High-Power As-S Glass Fiber Delivery Instrument for Pulse YAG:Er Laser Radiation

A 3-mum laser-generation delivery instrument that uses chalcogenide fibers with a unique damage threshold and conical radiation input has been developed for medical applications. A new purification and synthesis scheme has been elaborated that yields fibers with a heterophase inclusion content of less than 10(4) cm(-3). In such fibers the damage threshold is 350 J/cm(2) at an average power density of 0.5 kW/cm(2) in a YAG:Er laser operating in the repetitive pulse free-running regime with a pulse duration of 350 ms. 1-3 x 10(4) laser pulses were transmitted at a repetition rate of 3 Hz and an average output power of 1 W under the condition of a 15% decrease in the output power.

Comparative study of astrocytes in human and rabbit retinae

Immunohistochemical location of Glial Fibrillary Acidic Protein (GFAP) was used to compare the morphology of astrocytes in vascularized and partially vascularized retinae (human and rabbit, respectively). Astrocytes in human and rabbit retinae were found in the same regions as the blood vessels. These cells in partially vascularized retinae differed from those in vascularized retina in several respects. Firstly, there were six morphological types in rabbit retina and only two in human retina. Secondly, in rabbit retinae there were astrocytes only related to blood vessels called "perivascular astrocytes" which were absent in human retinae. Thirdly, the astrocytes were located in the nerve fiber layer and ganglion cell layer in both types of retinae, but in human retinae these cells could also be seen in the internal nuclear layer. These observations demonstrate that there are many differences between astrocytes in human and rabbit retina, suggesting that rabbit retina could be used with caution as an experimental model in comparative studies with human retina.

Intraocular laser surgical probe for membrane disruption by laser-induced breakdown

A fiber probe has been designed as a surgical aid to cut intraocular membranes with laser-induced breakdown as the mechanism. The design of the intraocular laser surgical probe is discussed. A preliminary retinal damage distance has been calculated with breakdown threshold, spot size, and shielding measurements. Collateral mechanical-damage effects caused by shock wave and cavitation are discussed.

Multicenter clinical experience using an erbium:YAG laser for vitreoretinal surgery

PURPOSE

To evaluate the advantages, disadvantages, safety, complications, and surgical applicability of an erbium:YAG laser system for maneuvers in vitreoretinal surgery.

METHODS

A prospective, consecutive trial of 68 eyes in 66 patients undergoing vitreoretinal surgery in which an erbium:YAG laser with graduated output from 0.2 to 5.0 mJ per pulse, repetition rate of 2 to 30 Hz, and equipped with a flexible fiber optic and interchangeable 20-gauge intraocular fiber optic endoprobes was used to perform specific maneuvers, including transection, incision, and ablation of membranes, retinotomy, vessel coagulation, iridectomy, and lens tissue ablation. The patients were treated in five centers in contemporary vitreoretinal surgical settings for surgical indications, including proliferative diabetic retinopathy, proliferative vitreoretinopathy, epiretinal membrane, and retinopathy of prematurity.

RESULTS

One hundred seventy-four maneuvers were performed with an overall surgical efficacy rating of excellent or good in 84% of maneuvers, ranging from a high of 100% for subretinal membrane transection to a low of 25% for coagulation of blood vessels. Complications included retinal break or photocoagulative injury in 5% of epiretinal membrane incisions, minor bleeding from transected retinal vessels during 29% of retinotomies, and intraocular lens damage during two posterior capsulotomies. The most significant limitation was the cautious pace used during maneuvers near the retinal surface.

CONCLUSION

The erbium:YAG laser is capable of versatile new approaches offering precise tissue cutting and ablation in vitreoretinal surgical maneuvers with a high degree of safety. The main limitation encountered was the slow speed of certain critical maneuvers near the retina.

Initial clinical experience with an erbium:YAG laser for vitreoretinal surgery

PURPOSE

We tested the efficacy of an erbium:YAG laser for maneuvers in patients undergoing vitreoretinal surgery.

METHODS

An erbium:YAG laser equipped with a flexible fiberoptic and interchangeable 20-gauge endoprobes of various tip configurations ranging from 100 to 365 microns was used for specific maneuvers in 13 patients referred for vitreoretinal surgery for diabetic traction detachment, proliferative vitreoretinopathy, retinal detachment with posterior break, and epimacular membrane. The following maneuvers were performed: (1) transection of elevated vitreous membranes, (2) incision of epiretinal membranes, (3) drainage and relaxing retinotomy, (4) transection of subretinal membranes, (5) noncontact ablation of epiretinal membranes in air-filled eyes, (6) ablation of lens remnants, (7) posterior capsulotomy, (8) iris surgery, and (9) retinal vascular coagulation.

RESULTS

Forty-eight defined maneuvers were performed with energy levels ranging from 0.2 to 5.0 mJ and repetition rates of 2 to 30 Hz. Transection of elevated membranes, subretinal membranes, and retinotomies were easily performed. Fourteen incisions into vascularized epiretinal membranes in diabetic traction detachment surgery demonstrated a fine margin of coagulation that permitted hemostatic incision. Retinal breaks were created during one of these incisions. Lens remnant ablation, posterior capsulotomy, and iris tissue removal were successful, with a single complication consisting of damage to the posterior surface of an intraocular lens during a pars plana posterior capsulotomy accomplished by means of a side-firing probe. Epiretinal membrane ablations in air-filled eyes were effectively performed in a gradual fashion without hemorrhage.

CONCLUSIONS

The erbium:YAG laser offers precise and effective tissue cutting and removal in vitreoretinal maneuvers. Further study will determine the role of this technology in vitreoretinal surgery.

Measurement and modeling of thermal transients during Er:YAG laser irradiation of vitreous

BACKGROUND AND OBJECTIVE

We investigated the transient thermal behavior of vitreous in order to understand the local thermal effects of laser output, and to predict the potential for unintentional injury during Er:YAG laser vitreoretinal surgery.

STUDY DESIGN/MATERIALS AND METHODS

The output of a free-running Er:YAG laser (2.94 microns, 300 microseconds FWHM) was delivered through a fiberoptic and applied to en bloc samples of bovine vitreous. Temperature was measured with ultrafine thermocouples.

RESULTS

For 6 mJ pulse energy at 10 Hz, a temperature rise of 20 degrees C is measured 500 microns from the laser tip. The temperature rise is localized with a rapid fall-off greater than 1 mm from the energy source. At constant time-averaged laser power, the temperature profile is independent of repetition rate. Our finite-difference model generates results qualitatively consistent with measured data and allows for investigation of the influence of thermophysical parameters on heat transfer.

CONCLUSION

Thermal injury to ocular structures should be limited during intravitreal application of Er:YAG laser energy.

New semiconductor laser for vitreoretinal surgery

BACKGROUND AND OBJECTIVE

We investigated the potential application in vitreoretinal surgery of a CW diode laser with cutting capabilities.

STUDY DESIGN/MATERIALS AND METHODS

A semiconductor CW laser emitting 300 mW optical power at 1.94 microns wavelength was used to perform retinotomies and membrane cutting on rabbit eyes. The device was integrated into a dedicated controller. The laser radiation was delivered through a low attenuation fused silica optical fiber of 200 microns core size, terminating into a 20-gauge endo-ocular handpiece. Histological aspects of threshold lesions obtained on the rabbits' chorioretina were evaluated by light microscopy.

RESULTS

We obtained circular and linear full-thickness retinotomies with contact and noncontact procedures using energy of 120 mJ (240 mW x 0.5 s). Using a non-contact procedure, a larger peripheral coagulation halo around the retinotomies was observed, as compared to the contact method. The adjacent zone of thermal damage ranged from 50 to 200 microns. Lower efficacy was obtained on experimentally induced epiretinal membranes, where only superficial ablation was achieved.

CONCLUSION

The CW 2 microns diode laser will have a promising future in vitreoretinal surgery when a higher output irradiance is available.