Michael Wiltberger

Femtosecond Laser–Assisted Cataract Surgery with Integrated Optical Coherence Tomography

An image-guided, femtosecond laser can create precisely placed, accurate cuts in the eye to improve cataract surgery. The Power of Light As Star Wars fans know, a lightsaber fares better against the Dark Force than does a metal sword. Ophthalmologists, who battle the darkening forces of eye disease, have also learned this lesson, replacing steel scalpels with lasers for creating precise, controlled incisions in the eye. Laser-assisted in situ keratomileusis—commonly known as LASIK surgery—corrects myopia (nearsightedness) and other refractive errors in millions of people each year. Now, Palanker et al. used this approach to devise a more precise, reproducible and automated way to remove cataracts. The authors combine the precise cuts of a laser with the imaging sophistication of optical coherence tomography, a method that uses interference of coherent light scattered by biological tissues to create three-dimensional images of their internal structure. On the basis of the individual patient’s eye anatomy, the laser system calculates the optimal set of cutting patterns for cataract removal and directs the laser to execute these slices, resulting in fast, clean surgery. Two light-based methods made this surgical advance possible. The first, the femtosecond laser, is ideal for use deep inside a fragile eye. Unlike longer pulse lasers, which melt and boil their targets away, producing significant collateral damage to adjacent structures, femtosecond light pulses can turn the material in the focal spot into ionized plasma, allowing dissection of transparent tissues without heat accumulation and minimal disturbance to the surroundings. The resulting cut is smooth and precise. The second method—optical coherence tomography (OCT)—takes advantage of slight variations in the refractive properties of living tissues. Coherent light scattered by structures within the eye allows reconstruction of a 3D image of the live tissue. Palanker et al.’s instrument uses this imaging technique to map the cornea, iris and crystalline lens within the patient’s eye and precisely position the various laser cuts. The laser makes a circular opening in the lens capsule (the membrane that surrounds the lens itself), sections the opaque lens into small pieces that are easily removed, and carves a partial incision in the cornea for later completion of surgery and insertion of the artificial lens under sterile conditions. The laser-created edges in the lens capsule are stronger than those made manually, so they better resist damage when the opaque lens is removed or the new lens is implanted. All the laser cuts are produced without perforating the cornea, so that the procedure can be performed outside the operating room. The laser can also be used to cut the corneal surface for correction of astigmatism and for creating a port for surgical instruments in the operating room. Although the new instrument plans and performs incisions much more accurately than do currently available tools, a surgeon still must remove the lens manually. The benefits of the more precise surgical incisions on visual acuity in patients with various types of intraocular lenses will need to be ascertained in a larger prospective trial, although the preliminary data in the paper are promising and indicate that the laser procedure is safe for ocular tissues. This new instrument will arm surgeons with a precise and automated lightsaber with which to battle the darkening forces of cataracts. About one-third of people in the developed world will undergo cataract surgery in their lifetime. Although marked improvements in surgical technique have occurred since the development of the current approach to lens replacement in the late 1960s and early 1970s, some critical steps of the procedure can still only be executed with limited precision. Current practice requires manual formation of an opening in the anterior lens capsule, fragmentation and evacuation of the lens tissue with an ultrasound probe, and implantation of a plastic intraocular lens into the remaining capsular bag. The size, shape, and position of the anterior capsular opening (one of the most critical steps in the procedure) are controlled by freehand pulling and tearing of the capsular tissue. Here, we report a technique that improves the precision and reproducibility of cataract surgery by performing anterior capsulotomy, lens segmentation, and corneal incisions with a femtosecond laser. The placement of the cuts was determined by imaging the anterior segment of the eye with integrated optical coherence tomography. Femtosecond laser produced continuous anterior capsular incisions, which were twice as strong and more than five times as precise in size and shape than manual capsulorhexis. Lens segmentation and softening simplified its emulsification and removal, decreasing the perceived cataract hardness by two grades. Three-dimensional cutting of the cornea guided by diagnostic imaging creates multiplanar self-sealing incisions and allows exact placement of the limbal relaxing incisions, potentially increasing the safety and performance of cataract surgery.

Semiautomated patterned scanning laser for retinal photocoagulation.

The concept of retinal photocoagulation was introduced by Meyer-Schwickerath for the treatment of diabetic retinopathy in the 1950s and used with some success in the 1960s. The xenon arc photocoagulator utilized for this purpose was large, polychromatic, inefficient, and difficult to operate, prompting a search for a better method of treatment. Further progress was achieved when ruby,1 argon ion,2 and krypton ion3 lasers were coupled to a slit lamp with an articulating arm containing mirrors.4 A contact lens, aiming beam, and movable joystick were used to place the laser beam on the retina. These innovations allowed for creating single laser spots of variable size, power, and duration on the retina with a high degree of precision and ushered in the modern era of retinal laser photocoagulation in the 1970s. The techniques enabled by these devices, termed focal photocoagulation, grid photocoagulation, and panretinal photocoagulation, were refined and shown to be effective in the treatment of proliferative diabetic retinopathy and advanced forms of nonproliferative diabetic retinopathy associated with macular edema in large, prospective, multicenter, randomized trials—the DRS and ETDRS.5,6 These trials validated the efficacy and institutionalized the indications and parameters for treatment that have remained the gold standard since that time. Patients with high risk proliferative diabetic retinopathy who undergo panretinal photocoagulation typically receive between 1,200 and 1,500 laser spots in two to four sessions of 10 minutes to 20 minutes each over the course of 2 weeks to 4 weeks. Because the spots are delivered individually, treatments are time consuming and tedious for the patient and physician alike and can be painful, especially in the retinal periphery. Focal photocoagulation and grid photocoagulation for macular edema are less painful and time consuming, because the spots are applied more posteriorly and are fewer in number, but still are tedious and require a considerable degree of patient cooperation and physician skill to achieve a successful outcome and avoid complications. Until now, little has changed in the general design of the devices used for retinal photocoagulation aside from the substitution of fiber-optic cables for articulating arms and the use of air-cooled solid state lasers rather than water-cooled gas tubes. These innovations have had limited or no impact from the standpoint of the patient or physician on the technique of treatments and clinical outcomes. We reasoned that greater precision and safety in retinal photocoagulation might be achieved by delivering a multiplicity of spots in a pattern created by a scanner rather than as a series of individually placed lesions. We also wondered whether the pattern application time and patient discomfort could be further reduced by using shorter pulses than the conventional 100 milliseconds to 200 milliseconds recommended in the DRS and ETDRS.5,6 Prior efforts toward improvement in retinal photocoagulation systems were principally directed toward (1) fully automated systems with retinal stabilization based on eye tracking7–10 and (2) determination of the optimal dose in each spot using the tissue reflectance– based feedback systems.11 Automated systems required acquisition of an image of the retina before the treatment, planning and aligning all treatment locations with reference to the retinal image, and treating all of these locations automatically. Complex retinal tracking systems were also required in these approaches to ensure alignment between planned treatment locations defined on the acquired image and actual sites on the retina.7–10 The complexity of these fully automated systems hampered the introduction of

Effect of Pulse Duration on Size and Character of the Lesion in Retinal Photocoagulation

OBJECTIVE To systematically evaluate the effects of laser beam size, power, and pulse duration of 1 to 100 milliseconds on the characteristics of ophthalmoscopically visible retinal coagulation lesions. METHODS A 532-nm Nd:YAG laser was used to irradiate 36 retinas in Dutch Belt rabbits with retinal beam sizes of 66, 132, and 330 mum. Lesions were clinically graded 1 minute after placement, their size measured by digital imaging, and their depth assessed histologically at different time points. RESULTS Retinal lesion size increased linearly with laser power and logarithmically with pulse duration. The width of the therapeutic window, defined by the ratio of the threshold power for producing a rupture to that of a mild coagulation, decreased with decreasing pulse durations. For 132- and 330-mum retinal beam sizes, the therapeutic window declined from 3.9 to 3.0 and 5.4 to 3.7, respectively, as pulse duration decreased from 100 to 20 ms. At pulse durations of 1 millisecond, the therapeutic window decreased to unity, at which point rupture and a mild lesion were equally likely to occur. CONCLUSIONS At shorter pulse durations, the width and axial extent of the retinal lesions are smaller and less dependent on variations in laser power than at longer durations. The width of the therapeutic window, a measure of relative safety, increases with the beam size. CLINICAL RELEVANCE Pulse durations of approximately 20 milliseconds represent an optimal compromise between the favorable impact of speed, higher spatial localization, and reduced collateral damage on one hand, and sufficient width of the therapeutic window (> 3) on the other.

System for the automated photothermal treatment of cutaneous vascular lesions

It is well known that the use of tightly focused continuous wave lasers can be an effective treatment of common telangiactasia. In general, the technique requires the skills of a highly dexterous surgeon using the aid of optical magnification. Due to the nature of this approach, it has proven to be largely impractical. To overcome this, we have developed an automated system that alleviates the strain on the user associated with the manual tracing method. The device makes use of high contrast illumination, simple monochromatic imaging, and machine vision to determine the location of blood vessels in the area of interest. The vessel coordinates are then used as input to a two-dimensional laser scanner via a near real-time feedback loop to target, track, and treat. Such mechanization should result in increased overall treatment success, and decreased patient morbidity. Additionally, this approach enables the use of laser systems that are considerably smaller than those currently used, and consequently the potential for significant cost savings. Here we present an overview of a proof-of-principle system, and results using examples involving in vivo imaging of human skin.

Safety of cornea and iris in ocular surgery with 355-nm lasers.

Abstract. A recent study showed that 355-nm nanosecond lasers cut cornea with similar precision to infrared femtosecond lasers. However, use of ultraviolet wavelength requires precise assessment of ocular safety to determine the range of possible ophthalmic applications. In this study, the 355-nm nanosecond laser was evaluated for corneal and iris damage in rabbit, porcine, and human donor eyes as determined by minimum visible lesion (MVL) observation, live/dead staining of the endothelium, and apoptosis assay. Single-pulse damage to the iris was evaluated on porcine eyes using live/dead staining. In live rabbits, the cumulative median effective dose (ED50) for corneal damage was 231  J/cm2, as seen by lesion observation. Appearance of endothelial damage in live/dead staining or apoptosis occurred at higher radiant exposure of 287  J/cm2. On enucleated rabbit and porcine corneas, ED50 was 87 and 52  J/cm2, respectively, by MVL, and 241 and 160  J/cm2 for endothelial damage. In human eyes, ED50 for MVL was 110  J/cm2 and endothelial damage at 453  J/cm2. Single-pulse iris damage occurred at ED50 of 208  mJ/cm2. These values determine the energy permitted for surgical patterns and can guide development of ophthalmic laser systems. Lower damage threshold in corneas of enucleated eyes versus live rabbits is noted for future safety evaluation.