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Flat corneas are the most common cause of free flaps
Modern microkeratomes all but eliminate free flap complications, but experts say it’s important to know how to respond just in case.
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Originally an integral part of automated lamellar keratoplasty and early LASIK surgery, free flaps later became bad news for refractive surgeons. Although an infrequent occurrence, a free, damaged or even lost flap caused great anxiety. But the advent of modern microkeratomes with sophisticated mechanics has all but eliminated this complication.
However, it is a mistake to assume that free flaps are a thing of the past or to become complacent regarding their potential for damage, according to refractive surgeons interviewed for this article.
“Free flaps are a very uncommon occurrence with modern microkeratomes. In fact, I haven’t had one in 3 to 4 years of using the (Bausch & Lomb) Hansatome. However, one should always consider the possibility of a free flap and mark the cornea with an asymmetric mark to aid in realignment of the free flap,” said Mark P. Lesher, MD.
“I’ve done more than 20,000 refractive procedures, and I’ve only had two free flaps in the last 17,000, but I still mark my flaps,” said Karl Stonecipher, MD.
Surgeons such as Louis D. Nichamin, MD, and Dr. Stonecipher suggest that recent LASIK adopters, as well as younger refractive surgeons could benefit from a refresher course in the art of free-flap avoidance and response. Dr. Nichamin noted that young LASIK surgeons need to be aware of the perils of free flaps despite their infrequent occurrence in the same way they need to know how to perform an extracapsular cataract extraction, though modern phacoemulsification’s versatility virtually eliminates ever having to even use their knowledge.
Avoiding the possibility of a free flap has as much to do with careful patient selection as with microkeratome selection. Dr. Nichamin said he has not encountered this complication since packing away his Automatic Corneal Shaper (ACS, Bausch & Lomb) in favor of adding a second Hansatome (Bausch & Lomb) to his armamentarium.
“I used to alternate the ACS with a Hansatome, but I finally said to heck with it, and bought another Hansatome. Since I made that switch, I haven’t had a free flap. But I still mark the cornea and recognize that a free flap is a very real possibility,” he said.
What is it about modern microkeratomes that so drastically reduces the likelihood of a free flap?
“They’re just so much more sophisticated. They’re more highly refined, and they produce good suction more consistently,” Dr. Nichamin said. “One of the more common causes for a free flap, especially on an average eye where the cornea is not too flat, is loss of or at least partial loss of suction. The eye gets softer, and you are more likely to get a small or even a free flap.”
Free-flap mechanism
Free flap reduction is also a result of surgeons having learned the free-flap mechanism — how and why they come about. Avoiding flat corneas is probably the main consideration in avoiding potential free flaps, according to Dr. Nichamin.
“The experienced LASIK surgeon sees fewer free flaps because they are aware of the correlation between flat corneas and free flaps,” he said. “Perhaps the younger LASIK surgeons would do well by recognizing what we’ve learned the hard way over the years. I am very conservative about not cutting a flap on a flat cornea. I think over the past 5 to 6 years surgeons have moved to intentionally creating thicker flaps. I think the majority of surgeons are using a 180 plate instead of a 160 plate for routine cases.”
A flat cornea is the most common cause of a free flap, but anything atypical about the cornea can be a red flag.
“For instance, if the cornea is very large or very small with regard to diameter, that’s something to watch out for,” Dr. Nichamin said.
Free flaps are most likely to occur in patients with K readings of less than 42 D, according to Dr. Lesher.
“In patients with flat Ks, I recommend using a larger diameter suction ring, for example 9.5 mm, to avoid a small diameter flap, with a corresponding short or absent hinge which predisposes to a free flap,” he said.
Dr. Nichamin said that another red flag for a potential free flap is high astigmatism.
“In the case of a high astigmat, the average K may not be too flat, but if the flat meridian happens to be in the same location as the hinge, a free flap may result. So not only do we need to be aware of the average and the central K reading, but also the curvature that corresponds to the hinge location,” he said.
Free-flap management
Dr. Lesher described his routine for free-flap management.
“If a free flap is created, it should be placed epithelial side down in an anti-desiccation chamber with one to two drops of balanced salt solution. Care should be taken in removing the flap from the keratome so as not to tear or damage it. The laser ablation should be performed as usual, and then the flap should be repositioned stromal side down, taking care to align the corneal marks. The flap should be allowed to dry in place for 2 to 3 minutes to guarantee good adherence. The adherence can then be tested with the striae test by pressing on the peripheral cornea and looking for wrinkles that radiate into the flap. The lid can then be taped closed with Steri-Strips, and a shield should be applied to the eye to prevent accidental dislocation of the flap during the first 24 hours.”
Dr. Nichamin said if the cap does not adhere, sutures may be necessary.
“You can put four cardinal sutures in, but probably the safest thing is to put a running anti-torque suture in,” he said.
David R. Hardten, MD, recommends examining the cornea before handing over the microkeratome to the technician. He leaves the ring light on the laser for additional illumination.
“If a free flap has occurred, the corneal light reflex will be duller, similar to what you see when you lift the flap,” he said. Once the flap is located, he said, “If you haven’t handed off the microkeratome, then you can gently grasp the flap with a Hoskins forceps and slide it out [of the microkeratome].”
“When you turn the microkeratome over, the stromal side will be up and the epithelial side down,” Dr. Hardten continued. “I prefer to take it off and then lay it on the conjunctiva with the epithelial side down and the stromal side up, just like the flap would sit if you had just turned it over had it not been free. This way lipid does not get stuck to the stromal side, and the flap doesn’t swell excessively from the tears touching the stroma. Make sure the treatable area is large enough, and then go ahead and do the ablation. Then take the flap and turn it over so it is right side up and replace it.”
Prevention
Cornea specialist Kenneth R. Kenyon, MD, said as with most complication situations, the best strategy is prevention. To this end, he suggested several relevant points.
First, attain adequate vacuum.
“We invariably use a pneumotonometer, rather than a hand-held applanator, to ascertain that intraocular pressure has attained at least 80 mm Hg before initiating the cut,” he said.
Second, don’t lose suction. “During the course of the cut, loss of suction can be equally disastrous, as not only a free flap but also an incomplete or amputated flap can result. Thus it is important to lift up on the suction ring to be certain that scleral suction — rather than just conjunctival suction – has been attained, and to monitor the vacuum level of the suction ring as the cutting stroke proceeds. Both maneuvers assure that adequate scleral suction is attained and maintained.”
If a free flap occurs, it may be of full size — with only the hinge absent, or undersized — usually indicative of a partial loss of suction, according to Dr. Kenyon.
“A full-sized free cap poses no problem at all, as long as it is not inadvertently lost,” he said. “For this purpose, I prefer a microkeratomes with excellent visibility of the flap during the cutting stroke, such as the Moria LSK-One. If the free flap is observed to be of appropriate size in both diameter and thickness, then it is appropriate to complete the ablation according to plan.”
Dr. Kenyon stressed the importance of marking the cornea for proper repositioning. Dr. Hardten suggested identifying a small irregularity in the margin to help ensure that the cap is oriented correctly.
“If not oriented correctly rotationally, it could cause induced astigmatism,” he said. “Make sure the gutter is even, and stroke the flap from center out to make sure the flap sticks in place and the gutters are tight. If you can’t get the flap to stick down, it is upside down.”
Dr. Hardten recommends waiting 1 to 2 days to treat the other eye to prevent the possibility of a free flap in the other eye and the risk of a bilateral flap slip.
Dr. Kenyon agreed.
For Your Information:
- Louis D. Nichamin, MD, can be reached at Laurel Eye Clinic, 50 Waterford Pike, Brookville, PA 15825; (814) 849-8344; fax: (814) 849-7130. Dr. Nichamin has no direct financial interest in any products mentioned in this article. He is a medical monitor for Baush & Lomb.
- David R. Hardten, MD, can be reached at 710 E. 24th St., Suite 106, Minneapolis, MN 55404; (612) 813-3600; fax: (612) 813-3658. Dr. Hardten has no direct financial interest in any products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Karl Stonecipher, MD, can be reached at Southeastern Laser and Refractive Center, 3312 Battleground Ave., Greensboro, NC 27410; (336) 282-5000; fax: (336) 282-5022. Dr. Stonecipher has no direct financial interest in any products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Mark P. Lesher, MD, can be reached at 101 Hospital Loop NE, Suite 203, Albuquerque, NM 87111; (505) 883-6800; fax: (505) 878-9128. Dr. Lesher has no direct financial interest in any products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Kenneth R. Kenyon, MD, can be reached at Cornea Consultants, 100 Charles River Plaza, Boston, MA 02114; (617) 523-2010; fax: (617) 523-4242. Dr. Kenyon has no direct financial interest in any products mentioned in this article, nor is he a paid consultant for any companies mentioned.