Tutorial
Introduction
Patient Selection
Change in BCVA
Intacs Channels
Conclusion
References

Slides

Femtosecond Laser and Intacs

Lav Panchal, MD · Tracy Swartz, OD, MS, FAAO · Ming Wang, MD, PhD

Introduction

Keratoconus and pellucid marginal degeneration (PMD) are progressive corneal diseases with bilateral corneal thinning that result in corneal distortion and reduced vision. A typical clinical presentation is shown in Slide 1, and a typical topographical map is shown in Slide 2 and Slide 3. Historically, management of keratoconus and PMD was limited to gas-permeable contact lenses followed by penetrating keratoplasty at the advanced stage. However, penetrating keratoplasty is associated with significant risks, including intraocular surgery risk, graft rejection, prolonged use of steroids and glaucoma risk, endothelial cell loss, anisometropia, long period of rehabilitation and irregular astigmatism.¹

SLIDE 1

SLIDE 2

SLIDE 3

Various surgical options have been tried to treat keratoconus and PMD with limited success in patients who have become contact lens intolerant. These options include corneal weakening procedures such as PRK (to remove corneal haze and improve the surface of the cornea) and PRK and LASIK to correct astigmatism and nearsightedness. LASIK may, in fact, worsen the keratoconus/PMD disease due to tissue removal. To address the root cause of ecstatic conditions such as keratoconus and PMD, namely corneal wall weakening, procedures have been developed to treat keratoconus by reinforcing the strength of the cornea, which include epikeratoplasty and lamellar keratoplasty. Intracorneal ring segments represent a new treatment for keratoconus/PMD by tissue additive effects and tissue tenting effects. Intacs ring implantation (Intacs, Addition Technology, Inc., Des Plaines, Ill.) in eyes with keratoconus was first performed in 1997. The U.S. Food and Drug Administration granted limited approval with a Humanitarian Device Exemption for the placement of Intacs in patients with keratoconus in July 2004. The segments are shown in Slide 4, and an illustration of implantation is seen in Slide 5. Slit lamp photographs of a double-segment and a single-segment implantation are seen in Slide 6 and Slide 7. A typical topographic map of an eye prior to surgery and following a double-segment implantation is shown in Slide 8 and Slide 9.

SLIDE 4

SLIDE 5

SLIDE 6

SLIDE 7

SLIDE 8

SLIDE 9

Patient Selection

In the majority of studies, the Intacs ring placement was performed in patients with keratoconus who had clear central corneas and experienced contact lens intolerance.^2-4 Boxer Wachler and colleagues⁵ reported that the severity of corneal damage in patients who received Intacs rings ranged from a mild form of the disease (forme fruste) to advanced disease cones with scarring. The FDA approved the Intacs procedure for patients >20 years old who have experienced progressive deterioration in their vision, have clear central corneas with a minimal thickness of 450 mm at the proposed incision site and for whom corneal transplantation is the only remaining option to improve visual function.

In the United States, 0.25-mm, 0.30-mm, and 0.35-mm Intacs segments are currently available. A variation in size and site of implantation was noted among various studies. Boxer Wachler and colleagues⁵ used a thick-ring segment inferiorly and a thin-ring segment superiorly. The size of ring segment depends upon the amount of myopia present (e.g., the larger the refractive error, the larger the segment size required) and the thickness of the cornea.

Colin and colleagues³ used 0.45-mm segments superiorly and 0.25-mm segments inferiorly, although Siganos and colleagues² reported using two 0.45-mm segments in all patients. Kymionis and colleagues⁵ used two 0.45-mm segments and noted that a smaller segment size for the inferior cornea may decrease the likelihood of perforation in thin corneas, a common occurrence in patients with keratoconus and PMD.

SLIDE 10

Complications that may be associated with Intacs implantation include infection, inflammation, segment migration, expulsion and perforation. Significant edema may result following implantation, as shown in Slide 10, and is addressed using topical steroids. Segments migration can be seen in Slide 11. Boxer Wachler and colleagues⁵ described one eye that experienced a superficial channel dissection with anterior Bowman's perforation. Although using a deeper channel, the segment was implanted on the same day. Mild inflammatory reactions were reported for two eyes. Segment migration and externalization were found 1 day after surgery in one eye with severe keratoconus. This segment was explanted, followed by the second segment in the same eye and the pair in the fellow eye (the fellow eye also had severe keratoconus). Foreign body sensation was noted in three eyes, and all required explantation of the segment.

SLIDE 11

Some studies reported mild-to-moderate intralamellar channel deposits.^2,4 One eye showed neovascularization at the wound site 2 months postoperatively, but the condition did not progress.² Segment migration may occur when segments are positioned too close to the wound site.⁷ Other reported findings include corneal staining, epithelial cysts, induced astigmatism, a temporary reduction in corneal sensation, elevated IOP, epithelial plug formation, neovascularization (pannus), conjunctival discharge, incision gape, aqueous flare, corneal infiltrate, anterior uveitis/iritis and stromal haze. These complications are observed less frequently in patients who had their channels made with a femtosecond laser.

Generally, Intacs implantation aims to decrease myopia and flatten keratometric values, but its effect on astigmatism is not well understood. However, some patients have achieved a decrease of 2.43 D in refractive myopia with a significant improvement in refractive astigmatism following the implantation of Intacs.⁵ Siganos and colleagues¹ found a mean reduction of 1.82 D in spherical equivalent, with no significant change in refractive astigmatism. Colin and colleagues⁴ reported a mean increase of two lines of best-corrected visual acuity (BCVA) over baseline and the largest reduction in manifest refractive cylinder (2.70 D).

SLIDE 12

SLIDE 13

In reference to keratometry, all authors reported a flattening of central corneal curvature. Colin and colleagues⁴ found an average flattening of 4 D using keratometry, as well as a qualitative reduction of corneal ectasia as seen with topographic mapping. Siganos and colleagues² reported a mean reduction in mean keratometry value of 1.94 D.

The authors have also used the segments to stabilize corneas post-surgery. Slide 12 illustrates segment implantation in a corneal graft in a patient whose keratoconus recurred, resulting in severe topographic changes, irregular astigmatism and decreased vision. Slide 13 illustrates segments implanted in a cornea of a patient who had undergone radial keratotomy and whose refraction was shifting >5 D due to structural weakness and instability. The segments stabilized refraction with a two-line increase in BCVA.

Change in BCVA

Siganos and colleagues² reported that four of 33 eyes lost one to two lines of BCVA, and the remaining eyes experienced a one- to six-line improvement. In a separate study, patients achieved a mean improvement of two lines at 12 months.² Colin and colleagues⁴ reported a mean increase of two lines of BCVA over baseline.

Intacs Channels

Manual channels
Intacs channels may be created manually using either a diamond knife or a femtosecond laser. When using a manual channel maker, a 1.2-mm incision is made in the cornea at the edge of 7-mm optical zone and at 70% depth. Intrastromal channels are dissected using the vacuum-centering guide and dissection tools. The Intacs are then delicately threaded into the channels, and the incision is closed using a 10-0 suture. This procedure requires more surgical intervention (using a diamond knife to create a channel and suture to close it) than the femtosecond laser procedure. The manual channel requires more frequent follow-up to ensure proper healing, and suture removal, if needed, occurs 1 to 4 weeks after surgery. Patients typically report significant discomfort. Studies comparing results of femtosecond-created flaps vs. microkeratome-created flaps report faster healing using the laser technique.⁸ Based on the authors' experience with laser Intacs, advantages of Intacs were similar to use of the Intralase (IntraLase Corp., Irvine, CA) laser.

Femtosecond laser channels
The use of a femtosecond laser for lamellar dissection added a new dimension to corneal surgery. At low-power densities, visible and near-infrared laser light freely passes into the eye without effect. However, at high-power densities, the nonlinear optical properties of these structures lead to absorption, generating plasma. The infrared neodymium-glass femtosecond laser emits a wavelength similar to Nd:YAG laser, which is used widely in ophthalmic surgery. Each femtosecond laser pulse is approximately 10,000 times shorter in duration, compared to a Q-switched Nd:YAG laser. Unlike photothermal lasers, the high peak intensities of the femtosecond laser create plasma inside transparent tissues, such as the cornea, without interfering with surface tissue.

Femtosecond laser pulses require significantly less energy to produce photodisruption than longer pulsewidth lasers, such as picosecond and nanosecond lasers,⁹ because laser power is inversely proportional to pulse duration. This lower energy threshold is advantageous not only to avoid thermal burn and collateral damage with a high-energy light beam, but also because the smaller cavitation bubble size allows for nearly contiguous placement of laser pulses. A femtosecond laser has been used for making flaps and creating intrastromal tunnels for intrastromal corneal ring segments, as well as for femtosecond laser keratomileusis with lenticule removal and intrastromal PRK, in which corneal tissue is removed without disturbing the epithelium. The authors have developed a protocol in using the femtosecond laser to create a corneal pocket for the implantation of artificial cornea Alphacor (CooperVision Surgical Inc., Perth, Australia), which is now the authors' standard method of implanting the Alphacor in end-stage corneal blind eyes.¹⁰

Several advantages of creating laser channels for placement of Intacs exist, when compared with manual blade cut. When using the femtosecond laser to create the channels, the depth of insertion, which ranged from 66% to 77% of the thinnest corneal measurement,⁵ can be reasonably controlled. The femtosecond laser was used to fashion a channel of external diameter of 7.8 mm and internal diameter of 6.8 mm at the authors' center. Visual acuity improves faster after surgery than when a blade is used to create a channel after surgery,¹¹ thus, suturing is not required. The patient comfort level is higher than when using surgical instruments to make the channel. The laser is much less operator-dependent than manual dissection because it reliably produces a channel of consistent size and depth through laser focusing, rather than manual tissue deformation and cutting, maximizing visual outcomes and safety.

The advantages offered by femtosecond laser for creating the intrastromal corneal ring (ICR) segment channels when compared with manual dissection with a diamond knife are numerous. The laser beam can be focused at any level in the stroma within 10 µm of accuracy and creates a circular channel of uniform depth along its entire circumference. As a result, surgeons can avoid the risks of anterior chamber perforation, shallow ring placement and nonuniform placement of the two ring segments that can accompany manual dissection.

The laser also provides surgeons the advantage of precision control of the channel width. A channel that is too wide can take longer to close down over the ring and, during that healing period, more fluctuation in visual acuity may occur. If the channel is too tight, then the surgeon must push harder to insert the ring, possibly increasing corneal edema, which may prolong visual rehabilitation.

In addition, using a femtosecond laser, one can precisely control the location of the ring placement. This is particularly advantageous when treating recurrent keratoconus on a corneal graft, in which one may want to choose the precise location of the ring and avoid graft-host junction dehiscence. The authors first used a femtosecond laser during Intacs ring implantation on a keratoconus graft¹² and have since used the technique to reduce ectasia on graft in a series of patients.

Another positive feature distinguishing laser channel creation from a manual approach is the architecture of the vertical exit cut. The laser consistently creates a 1-mm side cut that is completely perpendicular to the channel. Thus, a surgeon can easily maneuver the ring segment into the channel. In addition, the site of the exit incision created by the femtosecond laser is more reliably self-sealing than that created with a diamond blade. Consequently, sutures have not been needed to address wound gape in the eyes undergoing the laser procedure. The authors have not encountered an infection resulting from the entrance cut. The exit cut can be positioned anywhere in the cornea using the laser and can be astigmatically neutral. Finally, because the laser introduces a light beam only into the cornea, the risk of contamination with foreign material or microorganisms is minimized.

The authors have thus far performed nearly 50 femtosecond laser Intacs ring procedures, with a high success rate of improvement of corneal shape and increased the ease of contact lens fitting, and with a small to moderate amount of reduction of myopia and improvement of BCVA. They are encouraged by these results, but data from larger studies and with longer follow-up are needed to confirm these findings.

Conclusion

• The use of Intacs in patients with keratoconus appears to be a safe, cost-effective alternative to penetrating keratoplasty.
  
  
• The appropriate size and placement site of Intacs in patients with keratoconus require further evaluation. These parameters vary among studies, and no studies to date compare different segment sizes or placement.
  
  
• It must be established whether Intacs segments inhibit the progression of keratoconus as well as delay the need for a penetrating keratoplasty procedure or eliminate the need for the procedure.
  
  
• Using a femtosecond laser helps earlier visual recovery and minimal surgical trauma compared to using a blade to make the channel.
  
  
• Results with Intacs placement using a traditional nonlaser approach were good.
  
  

References

1. Smolin G, Thoft RA, eds. Keratoplasty. In The Cornea. Little, Brown & Co.; 1997: 659-663.
   
   
2. Siganos CS, Kymionis GD, Kartakis N, et al. Management of keratoconus with Intacs. Am J Ophthalmol. 2003;135:64-70.
   
   
3. Colin J, Velou S. Implantation of Intacs and a refractive intraocular lens to correct keratoconus. J Cataract Refract Surg. 2003;29:832-834.
   
   
4. Colin J, Cochener B, Savary G, et al. INTACS inserts for treating keratoconus: One-year results. Ophthalmology. 2001;108:1409-1414.
   
   
5. Boxer Wachler BS, Christie JP, Chandra NS, et al. Intacs for keratoconus. Ophthalmology. 2003;110:1031-1040.
   
   
6. Kymionis GD, Aslanides IM, Siganos CS, Pallikaris IG. Intacs for early pellucid marginal degeneration. J Cataract Refract Surg. 2004;30:230-233.
   
   
7. Rodriguez-Prats J, Galal A, Garcia-Lledo M, et al. Intracorneal rings for the correction of pellucid marginal degeneration. J Cataract Refract Surg. 2003;29:1421-1424.
   
   
8. Durrie DS, Kezirian GM. Femtosecond laser versus mechanical keratome flaps in wavefront-guided laser in situ keratomileusis: Prospective contralateral eye study. J Cataract Refract Surg. 2005;31:120-126.
   
   
9. Heisterkamp A, Mamom T, Kermani O, et al. Intrastromal refractive surgery with ultrashort laser pulses: In vivo study on the rabbit eye. Graefes Arch Clin Exp Ophthalmol. 2003;241:511-517.
   
   
10. Wang MX, Haddad W, Abdelmalak H, Swartz T. Femtosecond laser - Alphacor artificial cornea implantation. Paper presented at: The American Society of Cataract and Refractive Surgery Annual Meeting; May 1-5, 2004; San Diego, Calif.
    
    
11. Ratkay-Traub I, Ferincz IE, Juhasz T, et al. First clinical results with the femtosecond neodymium-glass laser in refractive surgery. J Refract Surg. 2003;19:94-103.
    
    
12. Wang MX, Coward D, Abdelmalak H, Swartz T. Femtosecond laser - Intacs for recurrent keratoconus on a corneal graft. Paper presented at: The American Society of Cataract and Refractive Surgery Annual Meeting; April 15-20, 2005; Washington, DC.