ROBERT MONTÉS-MICÓ, PhD
FLAP CREATION
One recent and interesting technical development in laser refractive surgery is the emergence of ultrashort-pulse lasers.1,2 The femtosecond laser is a focusable infrared laser using ultrafast pulses (ie, in the 100-femtosecond duration range). This laser delivers closely spaced spots that may be focused at a preset depth to photodisrupt tissue within the corneal stroma. It causes minimal inflammation and collateral tissue damage.
During treatment with some femtosecond laser models, a suction applanating lens flattens the cornea to immobilize the eye and allow treatment of a geometrically simpler planar cornea. Recent studies3-6 have demonstrated more predictable flap thickness, an insignificant increase in higher-order aberrations after flap creation, better UCVA and contrast sensitivity, and decreased epithelial injury relative to mechanical microkeratomes.
Flap creation for LASIK represents the first clinical application of the femtosecond laser. My flap creation experience is with the IntraLase femtosecond laser (IntraLase Corp., Irvine, California). The software creates a circular cleavage plane that starts at one side of the cornea and progresses across the cornea using a raster pattern (Figure 1). After the horizontal cleavage plane is created, the pattern changes to vertical and continues through Bowman's layer and the epithelium. It then creates a flap edge using a circumferential pattern of shallower pulses. The software has a programmable angle. An arc along the edge is left uncut, creating the hinge. The software controls all the planned flap diameter and thickness, angle of the side cut, hinge size and location, and all energy settings to create the flap.
In my experience, we program femtosecond laser flaps with the following settings: 120-mm thickness, 9-mm diameter, 45º superior hinge angle (to achieve equivalent corneal stromal surface exposure), 70º side-cut angle, laser raster patterns spot/line separation of 12 mm to 10 mm (respectively), 1.8 microjoules of stromal energy, and 2.4 microjoules of side-cut energy.
Preoperative antibiotic prophylaxis consists of topical ciprofloxacin (Oftacilox; Alcon Cusí, Barcelona, Spain) every 8 hours for 3 days. Antiseptic prophylaxis consists of one drop of povidone-iodine 5% solution to the conjunctiva immediately before surgery. After surgery, I use topical tobramycin and dexamethasone eye drops (Tobradex; Alcon Cusí) every 8 hours for 4 days.
The femtosecond laser requires no moving instruments during flap creation; improved epithelial integrity is a considerable advantage. Flap-related complications (eg, buttonholes, incomplete flaps, diffuse lamellar keratitis), however, have also been reported. We reported5 that, in small and large pupils, myopic LASIK surgery was associated with more postoperative corneal higher-order aberrations versus the femtosecond laser. Spherical aberration increased significantly across both groups, showing a high increase factor for LASIK compared with femtosecond laser LASIK. Geometric differences created on the stromal bed by the femtosecond laser or mechanical microkeratome may play a role in the differences found between the two groups.
The potential advantages of the femtosecond laser over the microkeratome include predictable flap thickness, precise intrastromal cutting not using a razor blade, increased postoperative stability of the flap, and decreased epithelial injury. We have also recently shown6 that a femtosecond laser used for flap creation in laser myopic surgery demonstrated better contrast sensitivity at high spatial frequencies—under photopic and mesopic conditions—than the use of a mechanical microkeratome for LASIK.
A femtosecond laser provides an effective and safe procedure for LASIK treatment of myopia. This technology also improves contrast sensitivity and avoids the negative effect on visual performance found after microkeratome LASIK (eg, increase in higher-order aberrations). Such improvements in visual quality after LASIK with the femtosecond laser are related to the differences in postoperative corneal higher-order aberrations found with femtosecond and microkeratome flap creation.
ROGER F. STEINERT, MD
CORNEAL TRANSPLANT
There are several goals of using laser trephination instead of a conventional blade punch trephine during corneal transplantation. The first is to obtain better quality of vision, with less total astigmatism and fewer aberrations. Second, recovery of useful functional vision should be faster. The third goal is to obtain stronger wound healing, which is safer for the patient and gives the surgeon the option of earlier suture removal.7
Sources of astigmatism and other aberrations derive from the alignment of the donor and host as well as the impact of the suture and uneven wound healing. Ideally, alignment of the donor to host anterior surface is a precise Bowman layer-to-Bowman layer apposition, which must be uniform over 360º. The only way to ensure this is to have some form of lamellar cut in both donor and host that is able to withstand the impact of variables including intraocular pressure. The other source of misalignment is rotational, where more than 50% of the donor is placed in less than 50% of the host rim. Sutures play a role, either through excessive or uneven tension. Uneven postoperative wound healing may lead to distortion in a transplant during the postoperative period, even if the surgery is performed well.
The development of femtosecond laser corneal transplant incisions began with cadaver eye studies at the University of California, Irvine, in 2003. These studies explored the advantages of a shaped edge in improving resistance to wound leakage.8,9 We also began the process of laser alignment incision marks to perfectly match the donor to host tissue. These cadaver eye studies were followed by rapid technical development and clinical implementation. Research and instrumentation development was supported by IntraLase Corp. Clinically, this procedure is most commonly referred to as IntraLase enabled keratoplasty (IEK).
Figure 2 shows six of the more common shapes made with the femtosecond laser. These shapes are derived from the option of performing a midstromal lamellar ring incision, which then may be connected to the anterior and posterior surfaces in either a straight or angled fashion. Initially during patient corneal transplant surgeries, we employed the top hat shape, suggested several years ago by Masimillo Busin, MD, as a manual surgical technique and originally described by José I. Barraquer, MD, in the early 1950s. Performing a manual top hat is a surgical challenge, whereas it is straightforward with the femtosecond laser. My personal experience was that the top hat shape represented an important improvement, but that the inner flange was not necessarily biomechanically stable. The surgeon had to carefully suture through the flange and then place it perfectly within the host rim to obtain the desired effect.
This observation led to the development of the angled incisions, and, most specifically the zigzag. These incisions are more biomechanically favorable, as the tissue layers will naturally align themselves—as long as reasonably accurate placement is made at the time of surgery. The zigzag shape is shown in Figure 3 and as a cross-sectional image from the Visante OCT (Carl Zeiss Meditec AG, Jena, Germany) in Figure 4. In the OCT image, one can see excellent alignment of the anterior and posterior layers of the transplant, and an excellent smooth transition along the anterior surface, achieving the goal of excellent graft-host alignment. (For more information on geometric patterns, see the article on page 70.)
We are currently performing a final analysis of results to date for publication as well as presentations at the upcoming American Society of Cataract and Refractive Surgery (ASCRS) and Association for Research and Vision in Ophthalmology (ARVO) meetings. Most notably, the majority of patients have less than 3.00 D of astigmatism, and a large proportion are under 2.00 D. We also had two patients exhibit rapid loosening of a single 24-bite running 10–0 nylon suture closure, such that full suture removal occurred in 5 months in one patient and in only 2.5 months in the other. In both cases, the donor remained perfectly aligned with no wound slippage or override, and resuturing was unneccessary.
The commercially released software from IntraLase now includes radial alignment marks that are precisely placed on the donor and the host, improving the surgeon's accuracy in precise placement of the corneal graft.
As experience with this technology grows, surgeons will have more options to explore different methods of wound closure, taking advantage of the better alignment and stability from the femtosecond laser incisions. This may mean different suture patterns and materials, and possibly a combination of suturing and glue to further achieve an even incision closure without any tissue tension. Possibly other laser technology will come into play as well, such as using a laser coagulation approach to achieve incision closure.
Surgeons who have the opportunity to employ this exciting technology are consistently enthused about its advantages, and patients readily understand the advantages that they will experience compared with standard corneal transplantation. I believe that femtosecond laser corneal trephination will become the standard for excellent optical results in penetrating keratoplasty.
GüNHAL KAMBURO˘GLU, MD
INTACS IMPLANTATION
Intracorneal ring segment (Intacs; Addition Technology Inc., Des Planes, Illinois) implantation is a refractive procedure that has been used for the correction of mild-to-moderate myopia, surgical management of keratoconus, pellucid corneal marginal degeneration, and iatrogenic postoperative corneal ectasia. There are two methods for creating intrastromal corneal tunnels for Intacs insertion. The first method is a mechanical approach, where a side cut is made by inserting a blade into the desired corneal depth. A specialized blunt dissecting instrument is then aided through the cornea, with the use of a suction centering device guide. The second method utilizes a femtosecond laser that works via photodisruption, creating a channel in the desired depth and location in the cornea (Figure 5).
Ratkay-Traub et al10 reported the first clinical results using the femtosecond laser in Intacs treatment. The investigators found that of 16 eyes receiving intracorneal ring segments, the same refractive results were achieved compared with previous conventional treatments. No intraoperative complications were noted.
Recently, we reported the largest series of Intacs implantation using the femtosecond laser, comprising 118 eyes (69 patients).11 At the end of a 1-year follow-up, UCVA and BCVA significantly increased in 81% and 73.7% of patients, respectively. During a presentation at the American Academy of Ophthalmology (AAO) Annual Meeting, in Las Vegas, Ertan and Colin12 showed that the femtosecond laser, compared with mechanical channel dissection, boasted greater refractive improvement. Change in mean keratometric values was similar between both groups. The study included 205 eyes (150 patients).
The femtosecond laser has several advantages over the mechanical method for Intacs channel creation. Because laser energy is delivered optically to a precise depth, tunnel resections and entry incisions are highly reproducible, with little risk of corneal perforation. Also, the channel size and depth as well as the side-cut position may be changed as desired. Channel depth may be set between 100 µm and 400 µm. We prefer to place the Intacs segments in the 70% depth of the cornea whenever possible.
The inner diameter and the outer diameter of the channel can be set between 4 mm and 9.4 mm and 4.1 mm and 9.5 mm, respectively. We have used two channel sizes in two groups of patients—one narrow and one being wide. In the narrow channel group, the inner diameter and outer diameters were set at 6.6 mm and 7.6 mm, respectively. In the wide channel group, these parameters were set at 6.7 mm and 8.2 mm, respectively. At the end of a 6-month follow-up, the refractive and visual outcomes were similar in both groups, but complications (ie, epithelial plug formation in the side cut, tunnel haze around segments, migration of segments within the tunnel) were seen in the narrow channel group.13
The femtosecond laser also allows easier manipulation in deep-set eyes when compared with the mechanical method. Because no foreign material is introduced into the corneal stroma, it possesses lower risk of infection. Such a channel creation procedure may be completed within seconds and without corneal manipulation.
There are, however, some drawbacks to femtosecond laser-assisted Intacs insertion. Generally, Intacs segments are intended for insertion at a corneal depth of 70%. With the femtosecond laser, however, the maximal depth is achieved at 400 µm. Another disadvantage is that inadequate separation of corneal stroma may be seen in some eyes that require blunt dissection of the stroma. Also, in case of high energy levels, postoperative fibrosis is possible at the incision site.
If the IntraLase FS laser is used for intracorneal tunnel creation in keratoconic eyes, there is a risk of decentered placement of the tunnel in relation to the pupil. During tunnel creation with IntraLase, first the suction ring is introduced, and then the glass applanation lens is pressed against the relatively asymmetric and thin cornea to flatten it. As a result of the pressure, the pupil becomes slightly dilated. The concentric lines—used to guide in the centration process—are placed overlapping the pupil edge in this position, and the tunnel is formed. After removing the glass lens and the suction ring, the cornea and pupil return to their natural positions, and the geometric center of the cornea shifts. In a retrospective study performed on 59 keratoconic eyes (39 patients), we have shown that there was 788.33 ±500.34 µm (range, 30 µm to 2,450 µm) of horizontal deviation in all eyes and 370.83±343.17 µm (range, 0 µm to 1,690 µm) of vertical deviation in 56 eyes.14 The clinical consequence of this finding is not known and has yet to be discussed.
In conclusion, the femtosecond laser for Intacs implantation offers significant safety and other advantages including less corneal trauma, sterilization issues, and reproducibility of the channel over the mechanical technique. There are, however, some limitations that must be improved.
MARK TOMALLA, MD
FEMTOSECOND LASER-ASSISTED
LAMELLAR ENDOTHELIAL KERATOPLASTY
There are many clinical uses for the femtosecond laser.15,16 One new and exciting development is its use in endothelial transplantation. Recently, with the help of my colleagues, I have designed a new technique, called femtosecond laser-assisted lamellar endothelial keratoplasty (FLEK). This past November, the first FLEK operation was performed with the Femtec femtosecond laser (20/10 Perfect Vision, Heidelberg, Germany). This technique is safe, precise, and effective. I will present the FLEK method within, and describe how this technique creates a new standard of care in precision and quality of endothelial transplantations.
Preparing the donor cornea. The donor cornea is stretched and prepared in an artificial anterior chamber. From the endothelial side, a straight laser cut is made and continued into the desired corneal depth. We work with 90º exact cutting angles for the transplant rim. We continue the circular laser cut intrastromally at a depth of 120 µm and with a preselected diameter. After the cut is made, we turn the donor cornea and prepare the reverse side with a flat spatula under optimal conditions.
One advantage to using the Femtec laser is its spherical and curved patient interface. The donor cornea, therefore, does not have to be applanated, and the laser cut occurs very precisely through the naturally curved cornea.
Preparing the host cornea. The advantages of a curved patient interface become even more apparent during preparation of the host cornea, because the naturally curved structures of the eye are cut with exact 90º cutting rims. The first step to preparing the host cornea is connecting the patient's eye with the patient interface. Only minimal suction and pressure are needed, because of the Femtec's curved design. The patient's eye remains a closed system until the endothelial transplant exchange is ready to be made. This provides additional safety to the eye.
The same diameter, cutting depth, and cutting angle that were used on the donor cornea are used on the host. A straight laser cut with an exact cutting angle of 90º is made on the endothelial side of the cornea. The cut is continued until the depth reaches 120 µm. Next, a circular lamellar cut with a preselected transplant diameter is made. Now, the endothelial transplantation is ready to be performed through a 3-mm clear corneal tunnel. Position the endothelial layer of the donor cornea through the paracentesis with instruments and with a ophthalmic viscoelastic device. Once the procedure is completed, create a postoperative tamponade by insufflating air into the anterior chamber.
Presently, we only have 3-month postoperative follow-up with a limited number of patients. One month postoperatively, patients had clear corneas, and their visual acuity was stable (Figure 6).
FLEK not only provides safe, precise, and effective treatments, but it also allows the surgeon to perform the laser treatment intrastromally, with a predefined depth, cutting angle, and diameter. With the Femtec's patented patient interface, the donor and host corneas match together well. Furthermore, because the transplant angles are precise, the risk of postoperative decentration is minimized. The precision of the femtosecond laser is hard to duplicate with a mechanical microkeratome, and therefore, I find FLEK to be a superior technique to mechanically peeling the transplant.
The use of a femtosecond laser requires a careful learning curve, however, skilled corneal and anterior segment surgeons can surely learn to use FLEK to perform more precise procedures. I believe that FLEK will set a new standard in corneal surgery. Further long-term follow-up is planned and will be presented in due course.
Günhal Kamburo˘glu, MD, practices at the Kudret Eye Hospital, in Ankara, Turkey. Dr. Kamburo˘glu states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +90 312 446 6464; fax:+90 312 446 4771; or gunhal@gmail.com.
Montés-Micó, PhD, is Associate Professor at the Human Visual Performance Research Group, University of Valencia, in Spain. Dr. Montés-Micó states that he has no financial interest in the products or companies. He may be reached at robert.montes@uv.es.
Roger F. Steinert, MD, is Professor of Ophthalmology and Professor of Biomedical Engineering, University of California, Irvine. Dr. Steinert states that he has no proprietary or financial interest in the product mentioned, however, IntraLase Corp provides research funding for this work. He may be reached at tel: +1 949 824 0327; fax: +1 949 824 4015; or Steinert@uci.edu.
Mark Tomalla, MD, is Head of the Clinic of Refractive and Ophthalmic Surgery at the Clinic Niederrhein, in Duisburg, Germany. Dr. Tomalla states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +49 203 5081711; fax: +49 203 5081713; or Dr.Mark.Tomalla@web.de.
Up Front | Apr 2007
The Many Functions of the Femtosecond Laser
Cataract & Refractive Surgery Today Europe asked several surgeons to comment on specific uses of this technology.
Montés-Micó, PhD; Roger F. Steinert, MD; Günhal Kamburo˘glu, MD; And Mark Tomalla, MD;
