Anterior Segment Techniques
by Richard J. Duffey, MD

There has been a recent trend toward thinner flaps for LASIK to decrease the risk of corneal ectasia following surgery. Other advantages exist as well for patients undergoing thin-flap LASIK.

The IntraLase femtosecond (FS) laser was introduced in the United States by Pulsion (now IntraLase) approximately 4 years ago after receiving FDA approval. The push toward "bladeless" LASIK has been slow but has gained some ground swell over the past year. Professional surveys of refractive surgeons in the United States, done by the American Society of Cataract and Refractive Surgery (ASCRS) and the ISRS/AAO, suggest that IntraLase market share was somewhere from 3% to 8% in late 2003 and growing.^1,2

Most of the reports regarding superiority of FS laser flaps over mechanical microkeratome flaps have compared thin and thick flap cuts with the former with thick flaps cuts with the latter. The purpose of this report is to compare the thin flaps of LASIK cut with the FS laser with thin flaps cut with a mechanical microkeratome, both in the hands of experienced users. Perry Binder, MD, co-medical director of IntraLase, has reported his most recent results of thin flaps with the FS laser³. His data and reporting are the basis of this comparison with thin flaps cut with the 100-μm head of the Moria LSK-One manual microkeratome (MMK) in the hands of the author (RJD).

Four tables have been devised, with Table 1 highlighting flap thickness predictability comparing the 100-μm head of Moria and a 100-μm programmed FS laser cut. The remaining tables compare experience with the FS laser from multiple sources as noted and my experience with more than 7,000 flaps created with the 100-, 130-, and 150-μm heads of the Moria LSK-One.

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Flap characteristics
Table 1 highlights flap characteristics produced with the two different systems in 34 eyes with a 110-μm flap programmed with the FS laser, mean flap thickness was 125 ± 12 μm with a range of 94 to 154 μm³. In 50 consecutive flaps cut with a 100-μ head Moria LSK-One MMK, mean flap thickness was 109 ± 11 μm with a range of 82 to 132 μm.

Standard deviation and range were less with the MMK, suggesting perhaps slightly better predictability of flap thickness with a blade cut versus a laser cut when comparing these two specific systems. However, I suspect that setting the FS laser to a thinner flap setting and cutting a flap at an average thickness closer to 100 μm would have yielded a lesser standard deviation and range comparable with that of the MMK.

The key here, however, is the thin flap. Thin flaps exhibit less standard deviation than thick flaps, whether they are cut with an MMK or a FS laser.

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Flap complication
Although the FS laser has been touted to eliminate buttonholes, free caps, and partial flaps, in my experience these have only rarely occurred in more than 7,000 flaps cut with the LSK-One MMK at multiple flap thicknesses (no buttonholes, one complete free cap, and four partial incomplete flaps).

Free caps, buttonholes, and incomplete flaps did not occur in this small study. Furthermore, they were eliminated altogether in more than 12,000 consecutive flaps using the dual port suction of the disposable ONE USE system by Moria (personal communication, Mark Whitton, MD). Certainly, the dual port suction and the ability to visualize the flap directly as it is developed have contributed greatly to this track record with this disposable MMK system.

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Intraoperative considerations
Epithelial defects and slides did not occur in the 50 eyes treated with MMK in this study; however, slides without defects occurred at a rate of 3% in 7,000 flaps cut with the Moria LSK-One with the 130- and 150-μm heads. Thin flaps cut with an MMK, I suspect, are still more likely to incur epithelial slides due to the shearing force during translation of the blade that is not present with FS laser technology.

Other intraoperative considerations and comparisons are noted in Table 2. Four of the five categories appear to favor MMK over the FS laser. Vaporization of tissue with the FS laser causes a 20-fold expansion of volume from solid to gas with bubbles passing through the corneal stroma (referred to as the OBL, or outer bubble layer) and occasionally into the anterior chamber. This can interfere with the excimer laser ablation (and further FS laser cutting leading to incomplete tissue separation at the flap interface), and 10 to 15 minutes must pass for these gas bubbles to be absorbed before proceeding with excimer ablation.

IOP must be elevated to cut flaps with both technologies; however, it remains elevated for only 10 to 15 seconds with the MMK and for 97 to 210 seconds with the FS laser³. This has now been reduced to an average of 60 seconds with the new fast 15-KHz engine FS laser (personal communication with P. Binder and K. Stonecipher). These longer suction times may increase the risk of retinal vascular abnormalities and optic nerve damage in predisposed patients undergoing FS flap creation.

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Postoperative considerations
Table 3 relates to postoperative considerations when comparing the two technologies. Flap lifting for enhancements can be difficult more than 3 to 6 months after FS laser treatment, but remains relatively easy following routine cutting with an MMK. Re-cutting with either technology, however, risks thin slivers of tissue due to multiple interface planes, which can intersect.

Thin flaps, in general, can be more easily stretched tightly (like a canvas over a drum) into their original position with minimal edema, resulting in improved visual acuity on the first postoperative day. Uncorrected visual acuity on postoperative day 1 in the 50 eyes with MMK flaps ranged from 20/15 to 20/40, with 18% of eyes 20/15 or better, 76% 20/20 or better, 86% 20/25 or better, 90% 20/30 or better, and 100% 20/40 or better.

Almost no microstriae existed in these thin flaps, demonstrating their advantage over thick flaps. When then flaps are cut with an MMK (such as the 100-μm head in this comparative study) there is minimal to no gutter remaining following flap stretching. This allows for more rapid epithelialization along the gutter and perhaps less risk of epithelial ingrowth. A thin flap cut with FS laser has a wider gutter secondary to the often steeper angle of the side cut, higher energy levels for side cuts, and greater physical manipulation of the flap required to lift after cutting.³

Visual recovery is noted to be slower in FS laser-cut flaps because of more flap edema and perhaps a less smooth interface surface.³ Terminal corneal nerve bulbs are cut closer to the epithelial surface (or some not cut at all) in thin flaps, thus requiring less nerve regeneration and perhaps causing less dry eye problems relative to thick flap LASIK.

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Miscellaneous issues
Issues highlighted in Table 4 can have a significant impact on surgeon and patient alike.

Of significant importance is the increase in operative time by 10 to 15 minutes on a bilateral case done with the FS laser. Added surgical time for the patient and decreased surgeon efficiency are negative for the FS laser.

Both technologies require a significant learning curve, more with the MMK. However, a second learning curve must be endured by the MMK surgeon (and patient) switching to the FS laser.

Finally, cost is significantly more for both the laser and disposable items for the FS laser users versus MMK users. Maintenance costs remain to be seen with the FS laser and are certain to be much higher than those of the MMK.

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FS laser as a marketing tool
One of the advantages touted by the FS laser manufacturer over the MMK is that IntraLASIK allows for a totally bladeless, non-cutting procedure. However, the FS laser vaporizes such a miniscule amount of tissue that essentially it acts as a cutting tool⁴ rather than an ablating tool like the excimer laser. Excimer lasers vaporize large amount of tissue and the gas expansion occurs on the surface, allowing it to dissipate into the air aided by the plume vacuum.

However, this same tissue expansion (albeit of a much lesser amount), deep within stromal tissue, that occurs with the FS acts essentially as a tissue plane separator similar to a blade.

If the FS laser could ablate a large-enough volume of tissue intrastromally to effect a refractive change (and simultaneously evacuate the gas expansion without lifting a flap), then the FS laser would be a major technological advancement.

However, at present, the FS laser is analogous to the phaco-laser that has been developed over the past several years for cataract surgery.

Certainly, the phaco-laser is more technologically advanced then traditional ultrasound, but it has added no new advantage over ultrasound phacoemulsificaion (since the laser-produced lens emulsate cannot be removed without intraocular invasion).

Neither the FS nor the phaco-laser has added increased efficiency, decreased risks, or better surgical outcomes in the present state of development.

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Building a better mousetrap
When PRK replaced RK, it was a more expensive technology, but there was a measurable improvement in refractive surgery outcomes and an expansion in the range of correctable refractive error. The same was true with the near-wholesale switch to LASIK a few years later in the United States.

The same cannot be said for the FS laser to date. Any improvement over existing technology must be significant before it will be widely adopted. A decrease in epithelial slides is a definite advantage of the FS laser over MMK; however, in three of the most important thin-flap LASIK issues - predictability of flap thickness, required time of IOP elevation during flap cutting, and the efficiency/cost analysis of the procedure - this MMK equals or outperforms the FS laser.

If efficiency, cost, outcome, and safety can be shown to be improved for patients undergoing FS laser-driven LASIK, then the FS laser (or any other new technology that fits this bill) will be a clear-cut winner and will be widely used by novice and experienced refractive surgeons. Outcomes and safety will lead the way. Build a better mousetrap, and then surgeons and patients alike will beat a path to be given access to it.

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References

1. Duffey R, Learning D. Trends in refractive surgery in the United States: The 2003 ISRS/AAO survey. J Refract Surg 2004 (in press).
2. Duffey R, Learning D. Trends in refractive surgery in the United States: The 2003 ASCRS survey. J Cat Refract Surg 2004 (in press).
3. Binder P. Flap dimensions created with the IntraLase FS laser. J Cat Refract Surg 2004; 30:26-32.
4. Nordan LT. Slade SG. Baker RN, et. al. Femtosecond laser flap creation for laser in situ keratomileusis; six month follow-up of initial US clinical series. J Refract Surgery 2003; 19:8-14.

Author Info
Richard J. Duffey, MD
is affiliated with Premier Medical Eye Group, Mobile AL.

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