Managing an opaque bubble layer in LASIK cases

August 1, 2014

Surgeons can take steps to manage an opaque bubble layer by reducing femtosecond laser spot line separation during LASIK flap creation.

 

Take-home

Surgeons can take steps to manage an opaque bubble layer by reducing femtosecond laser spot line separation during LASIK flap creation.

 

 

By James Loden, MD, and Emily Graves, MD, Special to Ophthalmology Times

Ever since the use of femtosecond lasers for LASIK flap creation, the presence of an opaque bubble layer has been of some concern for refractive surgeons.

Femtosecond lasers work by a process of photodisruption, in which each laser pulse creates a gas bubble within the corneal stroma. The grid of bubbles creates a lamellar dissection plane that the surgeon then manually separates during lifting of the flap.

An opaque bubble layer may occur when pockets of expanding gas are trapped in the stroma, causing an opaque, whitened appearance to the cornea. The majority of opaque bubble layer encountered in LASIK flap creation is considered “delayed” or “soft” opaque bubble layer, meaning it forms behind the raster line as it passes across the cornea.

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The trapped gas may create difficulty in lifting the LASIK flap, as the gas has not remained in one dissection plane within the cornea. Depending on the location and extent of the opaque bubble layer, it may interfere with pupil tracking during excimer laser ablation if it obscures all or part of the pupil, and it can prevent iris-registered tracking during ablation if it obstructs tracking software from identifying patient-specific iris characteristics. Furthermore, an opaque bubble layer may limit a patient’s ability to fixate properly during ablation.

Karl Stonecipher, MD, recently reported that the presence of an opaque bubble layer was the single greatest influence on the need for enhancement in a large series of more than 4,000 eyes.1 In my own experience, when I attempted a few years ago to proceed with ablation despite the presence of opaque bubble layer, my enhancement rate rose from 1.2% to 10%.

 

NEXT: Study + Video

 

Based on that experience, we quickly returned to the practice of waiting for the opaque bubble layer to resolve before performing the excimer laser ablation. This means that we escort the patient out of the laser suite to rest with eyes closed while the next LASIK patient undergoes surgery, then the patient is brought back to complete flap lifting and excimer ablation.

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However, this approach brings its own challenges. The shuffling of patients impedes surgical flow and ultimately delays the surgical schedule. It can increase patients’ anxiety as they wonder “what went wrong,” and it necessitates an explanation to assuage their concerns.

We recently found that it is possible to reduce the incidence and severity of an opaque bubble layer dramatically during LASIK flap creation by altering laser spot and line separation parameters.

This video illustrates a case in which the patient’s right eye had no opaque bubble layer (OBL) and the left eye had trace OBL. Femtosecond laser flap creation OD (0:40-1:29) and OS (1:50-2:42) are shown first, followed by flap lift, excimer laser ablation, and flap repositioning OD (3:25-5:50) and OS (7:50-11:00). (Video courtesy of James C. Loden, MD).

Opaque bubble layer study

We prospectively evaluated 172 consecutive eyes undergoing bladeless LASIK surgery between January and March 2013. All procedures were performed by a single surgeon using a femtosecond laser (iFS Advanced Femtosecond Laser) and an excimer laser (STAR S4 IR Excimer Laser, both Abbott Medical Optics).

The majority of flaps (n = 153) were created with a temporal-hinged, elliptical flap and a programmed diameter of 8.50 mm; the remaining 19 flaps were 9-mm-diameter elliptical flaps with superior hinges, reflecting preferred flap parameters for myopic versus hyperopic or mixed astigmatism treatments, respectively. All flaps were created using a raster pattern with 5-μm spot separation and 5-μm line separation in the stromal bed. Pocket depth was 230 μm with width of 0.18 mm.

Immediately after completion of flap creation, the presence and severity of an opaque bubble layer was noted by the surgeon and graded on a scale of zero (no opaque bubble layer) to 4 (diffuse, dense opaque bubble layer).

 

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In addition to excluding patients who were not deemed appropriate candidates for LASIK surgery, patients were excluded if they had undergone any prior eye surgery.

Of the 172 eyes undergoing LASIK flap creation using the above parameters, only three (1.7% of eyes) were noted to have an opaque bubble layer to any degree. Of the three eyes with an opaque bubble layer present, one was graded as trace opaque bubble layer and two were graded as 1+.

The opaque bubble layer resolved in all three eyes without any delay in surgery or need to wait for the cornea to clear prior to excimer ablation. Pupil tracking and iris-registered tracking were performed on all three eyes without difficulty during excimer ablation. No subjects experienced gas breakthrough into the anterior chamber.

Visual acuity outcomes were excellent, with all but 4 eyes achieving 20/20 or better, and a very high percentage 20/15 or better. Of the eyes that did not achieve 20/20 with one surgery, all were at least 20/40 or better and none was unexpected. Two had higher hyperopia; one had –8 D of myopia, and the fourth patient had 6.0 D of cylinder that had to be treated with a conventional ablation.

Discussion

In the past, we used a 7 µm × 7 µm spot-line separation, as many other refractive surgeons do by convention. Our incidence of opaque bubble layer, while not formally studied, was probably in the range of 20% to 30% with those parameters. The reduction in opaque bubble in this study to less than 2%, therefore, represents a significant improvement.

There is some precedence for a reduction in the spot-line separation. John Kanellopoulos, MD, recently reported that changes in the flap programming parameters, including tighter line separation, on a different femtosecond laser resulted in less opaque bubble layer.2

Surgeons implanting small-aperture corneal inlays have also reported better-quality pockets and improved results with that technology when they moved from 8 × 8 to 6 × 6 or tighter line-spot separation.

 

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In addition to the reduced rate of opaque bubble layer, a secondary finding was that all LASIK flaps created with the 5 × 5 spot-line separation were much easier to lift compared with those performed in the past using standard 7 × 7 spot-line separation. Using standard spot and line separation parameters, it is not unusual to make multiple passes with a LASIK spatula to separate the flap from the underlying bed. This is due to larger, more numerous “tissue bridges” between femtosecond laser pulses.

Additionally, there is identifiable tension on the flap during lifting when using standard parameters. Adjusting the spot and line separation to 5 × 5 resulted in a faster, easier flap lift.

An initial concern with decreasing spot and line separation parameters was increased flap creation time. Keeping all of the other standard flap parameters the same, the femtosecond laser takes 21 seconds to create a 5 × 5 flap, compared with 14 seconds for a standard, 7 × 7 flap. This nominal increase in laser activation time was more than offset by the ease and speed of the subsequent flap lift, however.

Our study is limited by its design as a prospective case series rather than a case-control study. The surgeon and his operating team were not blinded to the fact that spot and line separation had been adjusted, leaving open the possibility of unconscious observer bias as to the extent of an opaque bubble layer. However, the fact that none of the 172 cases required waiting for opaque bubble layer to clear indicates that the presence and severity of opaque bubble layer was indeed limited.

 

NEXT: Conclusion

 

It is very encouraging to see that opaque bubble layer can be nearly eliminated with a simple decrease in spot and line separation during flap creation using the femtosecond laser. This allows for more efficient surgical flow, improved pupil and iris-registered tracking during excimer ablation, easier flap lifts, and an improved patient experience.

References

1.     Stonecipher KG, Fleischman D. Influences on enhancement rates in laser vision correction. Poster, American Society of Cataract and Refractive Surgery Annual Symposium, April 2014, Boston.

2.     Kanellopoulos AJ, Asimellis G. Essential opaque bubble layer elimination with novel LASIK flap settings in the FS200 Femtosecond Laser. Clin Ophthalmol. 2013;7:765–770.