Buyer’s Guide for Femtosecond / Ultrashort Pulse Lasers

Wendy W. Lee, M.D., M.S.

About Author

Femtosecond laser technology has revolutionized eye surgery. This technology has been widely and increasingly adopted by refractive and cataract surgeons, originally for the creation of the laser in-situ keratomileusis flap secondary to its improved safety and clinical outcomes. Other applications include astigmatic keratotomy, channel creation for intracorneal rings, femtosecond lenticule extraction, corneal inlays and intrastromal presbyopia correction. The latest is the use of femtosecond technology during cataract surgery and corneal transplantation. The success of this technology lies in the fact that it is precise and reproducible. Flap creation with femtosecond laser over that of a microkeratome is due to better contrast sensitivity, increased safety, faster uncorrected visual acuity recovery, less induced aberrations and less IOP variation. There is also a lower incidence of dry eyes post femtosecond LASIK.

Key elements to look at when considering a femtosecond laser are:

1. Laser pulse repetition rate: A low repetition rate needs higher energy to achieve the threshold of photodissection. High repetition rate systems make the procedure shorter and use lower energy with an intention of diminishing the surrounding inflammation.
2. Spot size: The numerical aperture (NA) of a lens influences the laser spot diameter and volume. A higher NA focuses the beam with less dispersion. Higher NA devices use lower energy and increase the depth accuracy and overall precision of the lamellar cut.
3. Pulse energy: If a high-energy photodisrutpion is used, the cavitation bubble that is formed is larger and the pulses do not need to be placed close together. Low-energy systems create a very small bubble, with a greater number of pulses in an overlapping pattern being necessary because the bubbles induce almost no tissue cleaving.
4. Pulse pattern: Every geometrical pattern can be applied with these devices. To perform precise incisions in cataract surgery, an imaging system is required since the position of intraocular structures changes. Two main pulse patterns exist: raster and spiral. Raster involves pulses that are linear and start at the hinge, passing through the center of the cornea and extending to the opposite edge. The spiral pattern is applied when the laser pulses begin centrally and expand centrifugally out to the periphery.
5. Eye Fixation: The amount of induced pressure is higher in devices that applanate the cornea, and lower in devices with a curved applanation docking interface.

The following is a review of available systems used for refractive surgery.

Visumax Femtosecond Laser System
Carl Zeiss Meditec, Inc.

This system has a laser pulse rate of 500 kHz and uses minimal laser energy (<300 nJ)

• The system uses a curved contact interface for minimum IOP increase and corneal distortion during the procedure and has a limbal suction mechanism that is computer controlled for improved patient comfort.
• The VisuMax features a high-quality Zeiss surgical microscope and integrated slit illumination.
• Through the combined use of the VisuMax femtosecond system and the MEL 80 excimer laser, patient positioning between the surgical microscope and the treatment position is performed automatically.
• The CRS-Master can also be integrated as a treatment planning tool for wavefront examination and corneal topography data for the MEL 80.
• There are preset treatment programs that offer a level of standardization. *There is an integrated video camera equipped with a DVD recorder to enable treatment documentation.
• New Keratoplasty option: allows the unit to quickly convert into a workstation for corneal transplantations. Allows precise lamellar corneal grafts for anterior and endothelial transplantations. This unit comes with a high-quality surgical microscope for all surgical phases, a dedicated adapter for donor graft preparation and special treatment pack KP compatible with most artificial anterior chambers.
• Spiral pulse pattern

Other Capabilities:

ReLEx smile: Small Incision Lenticule Extraction
In a single step, the VisuMax creates a refractive lenticule and a small incision of less than 4mm in the intact cornea. The lenticule is removed through the small incision.
ReLEx flex: Femtosecond lenticule extraction 
In a single step, the VisuMax creates the lenticule and the flap access incision. The flap is folded back, the lenticule removed and the flap replaced.
Femto-LASIK:
The femtosecond laser creates the flap. The patient is relocated to the excimer laser, which ablates the corneal tissue. The flap is replaced.
Incision for Intrastromal Corneal Ring Segments:
Corneal incisions created in various positions and depths can be made for ICR implantation. One can choose from 0, 1 or 2 trapezoidal access incisions. The flexible tunnel incision segments can be between 90 and 270 degrees and the angle of the corneal incisions can be adjusted and oriented toward the posterior corneal surface.

VisuMax has no parts that require routine replacement. There is no permanent standby mode.

According to some, VisuMax is the largest, most complex and expensive femtosecond unit.

FEMTO LDV Crystal Line Femtosecond Surgical Laser
Ziemer Group

This unit is the first compact, mobile femtosecond surgical laser capable of an “all-laser LASIK” as well as a wide spectrum of corneal surgeries. The system was CE-marked as a Class 2b device and received 510(k) clearance by the FDA in March 2006. It also has a medical device license for Canada and registration is performed or currently underway in several other countries, such as Korea, China, Australia, Latin American countries and Taiwan.

Applications:

• The main application is preparing lamellar corneal flaps for LASIK procedures. Flap thickness may be selected at typically 110 or 140 microns.
• Sub-Bowman Keratomileusis (SBK): The very high cutting precision of this unit has made it possible to create flaps as thin as 90 microns reproducibly, with diameters of 8.5 to 10mm.
• Lamellar Corneal Surgery (LCS)

Intracorneal Rings: FEMTO LDV flexibility allows dimensions of the corneal tunnels to be set independently for each ring. It can create separate incisions for each ring segment or an all-around tunnel. There are pre-stored tunnel shapes for all current models of rings. Resection without tissue bridges allows for smooth insertion of the rings. With the LCS hand piece (optional accessory), resection depths down to 500 microns are possible.

Intrastromal Pockets (ISP): These incisions can be crafted and positioned for intrastromal inlays. With the MkII and LCS hand pieces, pockets at any required resection depth (from 50 to 500 microns) can be performed.

Lamellar Keratoplasty: The MkII and LCS hand pieces can be used for cutting the bed, even if it requires cutting through opacified or scarred tissue. By means of an Artificial Anterior Chamber, donor buttons are precise.

This laser is fast, has a very small spot size (1mHz) and pulse duration of 200-300 fs with high-aperture optics. It operates at a very low pulse energy (The hand-held delivery system allows for patients to be treated on the bed of the excimer laser so they do not have to be relocated between flap creation and ablation. Fixation is accomplished by an incorporated vacuum suction ring, which is mounted over the application window on the underside of the hand piece. This unit uses flat application.

The FEMTO LDV Crystal Line hand-piece incorporates the TopView Camera, a real-time color video imaging system that delivers a close3-up view of the applanated eye.

Tissue Separation:

This system has a proprietary laser cutting process that generates small bubbles to help visualize the flap edge. The bubbles disintegrate instantly when the flap is lifted. The resection creates a smooth edge with approximately 45 degrees of angulation, ensuring a tight, dynamic fit of the flap into the incision to help avoid epithelial ingrowth.

Procedure Control:

All procedure parameters are controlled via graphical user interface and touchscreen.
Resection Control: Resection dimensions are computer controlled and set through an intuitive set of parameters. The surgeon can select each parameter to customize a flap.
Procedure Status Control: The procedure monitor displays information about the status of principal system modules and about progress of the cutting procedure. Suction and laser can be activated by touching on-screen buttons or by depressing a foot pedal.
Suction Control: Suction is maintained by a computer controlled vacuum system. Should the vacuum fall below a pre-set level, a safety interlock will interrupt the cutting procedure. At the end of the procedure, suction can be automatically released.

There is no direct visualization during the application of pulses and there is a need for a viscoelastic interface. There are limited customizable features.

Consumables:

All components of this system coming into contact with the patient or surgeon are single-use and sterile. The specific contents of a pack are coded into an identification tag attached to the pack. By scanning the identification tag at the Laser’s base station, the system notes suction ring size and resection depth parameters and selects the appropriate cutting trajectory.

This unit is thought by some to be the simplest and cheapest way to replace the microkeratome for a laser.

iFS Advanced Femtosecond Laser
Abbott Medical Optics, Inc.

This system is a 5th generation laser designed as a primary laser for LASIK procedures and advanced corneal surgery procedures. It performs at 150 kHz at low energies (500 – 1300 nJ) for less local injury, and includes comprehensive IntraLase Enabled Keratoplasty (IEK) and ring-channel formation capabilities. Pulse duration is > 500 fs, spot size is 1 – 5 um.

This system is designed with an inverted bevel-in side cut angle that promotes flap stability and positioning and gives the flap tensile strength. The flap can be lifted effortlessly and good adhesions are obtained post-operatively. This system offers advanced options, such as the elliptical flap.

iFS comes with a high-resolution digital video microscope that captures high-resolution digital images throughout the flap creation. Also, tighter spot separation and lower energy are features. There is an advanced user interface, keyboard and touch screen.

This system works on an amplified concept, has been through the greatest number of treated eyes, has manual suction, and a flat laser-cornea coupling.

iFS Advanced Femtosecond Laser just received FDA clearance for use in cataract surgery.

Technolas Femtosecond Workstation
Technolas Perfect Vision

This system operates at 80 kHz, with a flap production of 20 seconds, and works with a similar pulse rate and energy as the IntraLase. It uses a curved applanation docking system, which allows less IOP increase and the suction is permanently computer controlled. This unit features Intracor for the correction of presbyopia and the capability for PKP/LKP, femtosecond laser assisted endothelial keratoplasty (FLEK), tunnels for intrastromal ring segments and arcuate cuts for astigmatic keratotomy. Intracor and Customshape are CE marked but NOT approved for use in the US.

WaveLight UltraFlap FS 200
Alcon

This unit operates at 200 kHZ for flap creation in approximately 6 seconds. There is an automated vacuum control of the patient interface that incorporates a flat applanation docking station that has adjustable flap centration. Not only can it be used for flap creation, but also for intracorneal ring segments and lamellar and penetrating keratoplasty. This unit has a proprietary Beam Control Check that can compensate for Z-Position variances, temperature shift of the laser system’s components, it automatically calibrates for each applanation cone and there is a consistent flap thickness through a reduction of standard deviation. The laser is equipped with a high-quality microscope, and adjustable joystick and a motorized laser arm that moves in 3 axes. WaveLight can be linked through WaveNed, an integrated network with WaveLight EX500 Excimer Laser and WaveLight diagnostic devices for simplified data throughput.

Key elements to look at when considering a femtosecond laser are:

1. Laser pulse repetition rate: A low repetition rate needs higher energy to achieve the threshold of photodissection. High repetition rate systems make the procedure shorter and use lower energy with an intention of diminishing the surrounding inflammation.
2. Spot size: The numerical aperture (NA) of a lens influences the laser spot diameter and volume. A higher NA focuses the beam with less dispersion. Higher NA devices use lower energy and increase the depth accuracy and overall precision of the lamellar cut.
3. Pulse energy: If a high-energy photodisrutpion is used, the cavitation bubble that is formed is larger and the pulses do not need to be placed close together. Low-energy systems create a very small bubble, with a greater number of pulses in an overlapping pattern being necessary because the bubbles induce almost no tissue cleaving.
4. Pulse pattern: Every geometrical pattern can be applied with these devices. To perform precise incisions in cataract surgery, an imaging system is required since the position of intraocular structures changes. Two main pulse patterns exist: raster and spiral. Raster involves pulses that are linear and start at the hinge, passing through the center of the cornea and extending to the opposite edge. The spiral pattern is applied when the laser pulses begin centrally and expand centrifugally out to the periphery.
5. Eye Fixation: The amount of induced pressure is higher in devices that applanate the cornea, and lower in devices with a curved applanation docking interface.

Overall, the ideal device would include a high repetition rate, small spot size and low energy per pulse. The formation of a bubble layer is less common with a softer docking (applanation) pressure, lower energy and faster repetition rate devices.

For an easy reference, Drs. Ronald Krueger and Glauco Reggiani-Mello wrote about femtosecond lasers and created the chart below to map out the differences in the main femtosecond refractive lasers.

* not yet commercially available
ONL: Opaque Bubble Layer