Thin-flap LASIK, sometimes specif-referred to as sub-Bowman's keratomileusis, uses thinner, and occasionally smaller-diameter, flaps in order to preserve stromal tissue in an effort to reduce the risk of ectasia. Further, its proponents say it also reduces the incidence of postop dry eye by virtue of severing fewer corneal nerves. Some surgeons argue, however, that, if it's safety you're concerned about, surgeons already have a procedure that severs even fewer nerves, all but eliminates the risk of ectasia by keeping the structure of the cornea intact, and makes enhancements a breeze because you don't have to try to work with a flimsy ultra-thin flap months postop: surface ablation. Here, several surgeons weigh the pros and cons of SBK and PRK.

 

 

Outcomes Compared

Kansas City, Kan., surgeon Dan Durrie, along with his colleague Stephen Slade, coined the term sub-Bowman's keratomileusis to describe a procedure that involves making a flap that's 100 to 110 µm thick and is a bit smaller than the traditional LASIK flap at around 8.2 to 8.5 mm in diameter. He says that, compared to thicker LASIK flaps, the SBK flap cuts fewer nerves, causes fewer dry eyes and, theoretically, as suggested by tests using a Reichert Ocular Response Analyzer, may induce less biomechanical destabilization of the cornea.1 "I don't think SBK is just for doing higher corrections, I think it's better for the cornea," he says. Dr. Durrie thinks a 100 to 110-µm flap is thin enough to promote structural integrity but thick enough to manipulate well.

For surgeons who argue that PRK is just as good, however—they're right. In a study performed by Drs. Durrie, Slade and their colleague John Marshall of London, the researchers randomized 50 patients at two sites to receive SBK in one eye and PRK in the other. "What the study showed was that, from three months on, the eyes were identical in all categories," says Dr. Durrie. "There were no statistically significant differences in safety, acuity, higher-order aberrations or dry eye. That's important, because you can then say that you can do whichever procedure is better for a particular patient sitting in front of you."

 

The Differences Explored

Majid Moshirfar, MD, director of the Moran Vision Center at the University of Utah, uses flaps between 85 and 90 µm for certain patients in whom he wants to conserve stromal tissue, and, though he gets good results, says these SBK flaps present certain challenges. "If someone wanted to bash SBK, he should come at it from the standpoint of that acute surgical phase and the postoperative care that occurs in that first 24 hours," says Dr. Moshirfar. "When I was doing SBK at 110 or 105 µm, I really didn't see a major issue that caused me to change my entire protocol of LASIK postoperative care. But now that I'm doing these 95-, 90- and sometimes 85-µm flaps, I've come to the conclusion that these flaps can develop subepithelial fibrosis in the peripheral aspect of the flap, and, therefore, I usually place a bandage contact lens on these patients after the surgery. I've come to the conclusion that these patients get a little bit more diffuse lamellar keratitis in the peripheral aspect of the flap, where it's most traumatized. Ever since I've begun placing a bandage lens on these thin flaps and putting the patients on Pred Forte q1h and removing the lens the next day, these flaps have had fewer instances of rolling in, less chance of epithelial ingrowth and less risk of fibrosis. Overall, however, I agree with my colleagues that once a flap goes to 90 µm and thinner, there are more issues with flap edge integrity, epithelial fibrosis, shrinkage of the edge of the flap, and perhaps even a higher risk of ingrowth, particularly if you're considering enhancing these patients. I'm very wary about enhancing SBK patients; I'd say I feel more uncomfortable doing an enhancement on an SBK patient than I do on a regular LASIK patient with a 110-µm flap." Clinically, however, Dr. Moshirfar says these flap issues haven't turned into anything significant over time.

San Francisco surgeon Ella Faktorovich agrees that these thinner flaps need extra care in the OR. "It's something the surgeon needs to be aware of, to smooth the flap prior to repositioning it," she says. "And you must take extra care when smoothing it out."

In addition to patients who participate in contact sports and may have the flap displaced if hit in the eye, surgeons say dry-eye risk may also be a consideration when deciding between surface ablation and even a thin-flap procedure such as SBK. "I think there's a little less dry eye with surface ablation than with any flap procedure," says Mountain View, Calif., surgeon Mark Volpicelli. "The stromal nerves come through the anterior third of the tissue, and you're going through more of those with a flap procedure. So, for a post-menopausal female, I'd do surface ablation for fear of epithelial slide and downgrowth. For instance, in a peri- or post-menopausal woman who's +1.5 with marginal dry eye to begin with, I'd avoid a flap procedure. I wouldn't even make a 100-µm or 90-µm flap, because I think it's going to exacerbate the dry eye."

Finally, there's the argument of structural integrity. SBK proponents say the procedure's better for the structural integrity of the cornea because it severs fewer collagen fibrils than the creation of a thicker flap does. However, PRK severs even fewer, so some might wonder if it results in a stronger cornea over the long term.

"My indication for SBK would be a patient who has a somewhat thin cornea but which is otherwise normal on topography and slit-lamp exam," says Dr. Faktorovich. "If a cornea is asymmetric, for example it has inferior steepening greater than 1.5 D, irregularity or there's evidence of epithelial basement membrane dystrophy, I do PRK. An interesting study would be to analyze the long-term outcomes of SBK vs. PRK in patients with mild corneal asymmetry and/or irregularity. Because they're different; with SBK, typically you end up ablating deeper than with PRK. Also, SBK is a different procedure structurally because Bowman's is interrupted peripherally with SBK, whereas PRK involves ablating through Bowman's membrane centrally. That's why I hesitate to do SBK on patients who may potentially have a predisposition toward corneal weakening based on their topographic maps."

Surgeons say, however, that any structural concerns are balanced out by the well-established healing issues with surface ablation. "I understand Dr. Marshall's hypothesis and the importance of the anterior lamellar integrity," says Dr. Moshirfar.
"But let's not forget that we still have no definite knowledge about wound healing's effects. A PRK form -8 D is less predictable than a -8 D SBK because of wound healing.
In PRK, there's an acute phase of seven to 10 days of epithelial healing and keratocyte migration, and there's a large area of stromal exposure because of the 7- or 8-mm epithelial defect. There's a lot more corneal remodeling at the level of Bowman's for a -7 or -8 D PRK. With a 90- or 95-µm SBK flap, however, you're still respecting the biomechanics of the cornea to a degree, but you're not dealing with the haphazard wound healing of surface ablation that we haven't mastered yet."

Also, the length of time it takes for a patient to recover his or her vision is also a strike
against PRK, and may ultimately be the main reason flaps, thick or thin, continue to trump surface ablation. "If someone came up with a way to make PRK have the same speed of visual recovery and comfort as SBK, I'd quit doing SBK and do PRK on everyone, because it's easier, quicker and less expensive," says Dr. Durrie. "However, PRK's disadvantages are why LASIK was invented." 

1. Durrie DS, Slade SG, Marshall J. Wavefront-guided excimer laser ablation using photorefractive keratectomy and sub-Bowman's keratomileusis: A contralateral eye study. J Refract Surg 2008 Jan;24:1:S77-84.