Many refractive surgeons are aware of the ways they can detect keratoconus preoperatively, and they also strive to leave the generally accepted 250 µm of residual corneal bed postop in order to avoid ectasia. However, some surgeons say that even these precautionary measures may not be enough in all cases.

Here are some of the experiences surgeons have had with unforeseen ectasia and possible ways to detect abnormal corneas before you operate.

Does ‘Ectatic Surprise’ Exist?

Surgeons are still wrestling with the idea that ectasia can occur in the absence of identifiable risk factors.

In a 2003 retrospective study, researchers from Emory Vision Refractive Surgery Center in Atlanta analyzed four groups: 10 eyes from seven patients identified as having developed corneal ectasia after LASIK; 33 previously reported ectasia cases; 100 consecutive cases of uneventful LASIK; and 100 more consecutive cases of uneventful LASIK that had high preop myopia. The doctors reviewed the preop and postop data for each case and compared them with previously reported cases and those with uneventful outcomes.

The surgeons found that, preoperatively, 88 percent of the ectasia cases met the criteria for forme fruste keratoconus, compared with 2 percent of the first control group and 4 percent of the second control group. Seven of the ectatic eyes (70 percent) had residual stromal bed thicknesses of less than 250 µm, compared to 16 percent of eyes in the first control group and 46 percent in the second group. The mean residual stromal bed thickness for the ectasia cases (223 µm) was significantly less than that of the control groups. Ultimately, the researchers found that all of the ectasia patients had at least one risk factor for the development of ectasia in addition to high myopia, and that no one developed a “surprise” case of ectasia. The risk factors included high myopia, forme fruste keratoconus and a low residual-bed thickness.¹

Empirical data from some practices, however, conflicts with the study’s findings.

Miami surgeon William Trattler performed LASIK on this patient (preop topography, left). He says that, though there were no apparent risk factors for ectasia, the patient went on to develop ectasia three years later (three-year topography map, right).

Chicago refractive surgeon Randy Epstein is taking part in a study in which the researchers, headed up by Shawn Klein, MD, of Roseland, N.J., are collecting data on a series of patients, via an Internet survey, who have “ectasia with no known risk factors.”

“Based on certain criteria, these are the types of cases that any surgeon would do tomorrow, yet they ended up with ectasia,” he says.

In the study, the researchers polled the participants of Keranet (approximately 580 subscribers), the American Society of Cataract and Refractive Surgery (450 subscribers) and isrs.net (525 members), around half of which are duplicate subscribers, since some surgeons belong to multiple groups.

Patients were included in the study if they developed ectasia even though they had a calculated residual stroma bed of at least 250 µm, preop central pachymetry measurements of at least 500 µm, K readings below 47.2 D, a calculated inferior/superior ratio less than 1.4, no more than two retreatments, an initial attempted correction less than 12 D, and an Orbscan II posterior float value less than 50 µm in any cases in which this value was obtained. They excluded any patients who were considered to be complicated cases. Ultimately, the researchers identified a number of patients who developed ectasia but had no evident preop risk factors.

Dr. Epstein says that he can’t share the particulars of the final data of the study because it’s currently being considered for publication, but he does say that identifying these cases that occurred without any warning signs has made him “much more aggressive about recommending surface ablation whenever we have anything at all that we feel might make a patient less than an optimal candidate.”

Dr. Epstein admits that these unexpected-ectasia cases he’s reviewed have made him more gun-shy about performing LASIK on someone whose cornea has something not quite right about it, such as an “asymmetric bowtie” steepening pattern in patients with astigmatism, even of low degrees, and especially in patients with thin corneas and/or high “posterior floats” on Orbscan. He opts instead for surface ablation, sometimes with adjunctive mitomycin-C.

Michael Smolek, PhD Louisiana State University researcher Michael Smolek, PhD, says that the interlamellar strength pattern of the normal human cornea at 50-percent depth (left) seems to correlate with where most cases of keratoconus appear—in the inferior quadrant.

• Ectasia and surface ablation. Simply dropping back and doing a surface ablation procedure instead of LASIK may not eliminate the ectasia risk, though. Jonathan Vukich, MD, surgical director of the Davis Duehr Dean Center for Refractive Surgery in Madison, Wis., has seen a handful of surface ablation cases that developed ectasia unpredictably.

“They caught us by surprise,” he says. “One was a -9 D patient whose corneas were thought to be too thin for LASIK, and in whom there was supposedly 325 µm of residual stroma left after PRK.”

Though the patient had 325 µm of residual stroma, which is well above the 250 µm that most surgeons seem to agree is desired for avoiding postop ectasia, Dr. Vukich thinks there’s more to it than just bed thickness in some cases.

“I think the flap provides structural stability to the cornea, albeit not what it once was,” he says. “In other words, if you took a cornea and performed PRK down to the level of 250 µm, it would not be the same as a 250-µm residual LASIK bed that had another 160 µm of cap on top of it. There is some additional effect there that lends stability to the cornea.”

Dr. Vukich also thinks that, to gauge a particular eye’s risk for ectasia, you have to think three dimensionally, not just in the dimension of depth. “It’s not just the minimal thickness that’s left behind, but the number of square millimeters that’s left at a given thickness,” he explains. “So, in other words, a 6-mm optical zone with no blend zone that has a central thickness down to 250 µm reacts differently than a 6.5-mm optical zone with an 8-mm blend zone that’s down to 250 µm, since the latter is at 250 µm for a greater percentage of the total surface area of the cornea.” He asserts that the latter ablation would leave the cornea structurally weaker.

“I think that many of us are already factoring this in, maybe subconsciously,” Dr. Vukich says. “I think many surgeons are no longer thinking that 250 µm is the absolute safe number, and they’re bumping that up to 275 µm, and, if they can, even higher.

“I believe this is just a reflection of the fact that we want as much left behind as possible. That number, in fact, needs to be higher than what we traditionally thought since we’re now using optical zones that have a minor axis of
6 mm and a greater major axis, and custom treatments that have deeper ablation depths.”

With these thoughts in mind, Dr. Vukich says he’s determined a different corneal thickness to shoot for after surface ablation. “I don’t know specifically what the definite best number is,” he says, “but, personally, I’ve settled on 400 µm as the amount I want to leave behind. That correlates fairly well with a LASIK procedure that leaves behind a cap of 160 µm and a bed of 250 µm, which equals 410 µm.

“My new number isn’t really based on a scientific study, but on the knowledge that the handful of surface-ablation ectasias I’ve seen have all occurred with less than 400 µm of residual cornea.”

Dr. Vukich recently heard about what may be one of the biggest surprises possible: ectasia after hyperopic LASIK, which, he says, is virtually unheard of. An acquaintance of his showed him the patient file, and it represented the first case of its kind he had ever seen. “It is very unusual, but, as we do more of these surgeries, the unusual cases will begin to appear,” he says. The patient did show warning signs preop, however.

Possible Explanations

Michael Smolek, PhD, assistant research professor of ophthalmology at Louisiana State University, has been researching keratoconic risk factors for around a decade, and worked with colleague Steve Klyce, PhD, on some of the original software for keratoconic detection used on corneal topographers. He says he’s come to understand that there may be preop risk factors for ectasia in certain patients that simply aren’t that easy to detect.

“From what I can gather, I think there’s an inherent weakness in the cornea in essentially everyone,” he says. “It’s in the way the cornea’s constructed. As you go down deeper in the stroma, you find yourself in stroma that’s more regularly arranged, and it’s easier to create shearing forces in that stroma, making it easier to disrupt that stroma and stretch it. With LASIK, if you go too deep, you reach stroma that’s inherently weak and biomechanically unstable. The key, then, is simply not to go down too deep.” Dr. Smolek says this may entail leaving 300 µm of residual bed, though no one is exactly sure what the ideal depth may be.

As far as the corneal structure involved with ectasia, Dr. Smolek’s corneal studies have found that, in the anterior half of the stroma, there are fibers that run obliquely from the corneal surface in Bowman’s layer to deep into the cornea. As they run deep toward the posterior half of the stroma, they bifurcate and interlace with the various layers of the anterior half of the stroma. “So, it becomes a structure that’s very tied together and resistant to shearing motions,” Dr. Smolek says. “I have the histological evidence and the biomechanical testing data to show these structures exist.”

He maintains that, once the surgeon makes a cut, and a flap cut in particular, he’s basically isolated all of these structures into the flap, and the remaining cornea is inherently weak. “If you put the flap back and expect it to heal, it’s just not going to heal correctly. It’s never going to go back to a position where you regain 100 percent of the biomechanical strength.”

Dr. Smolek says he’s also found that the cornea is strongest in the periphery, then weakens toward the center. Interestingly, he says he’s also identified an inherent weakness in the inferior aspect of the corneal periphery, “so it ties in very closely with the area where ectasias and keratoconus tend to occur in most cases,” he says.

In addition to depth of ablation, Dr. Smolek says the epithelium’s health may also play a role in cases of ectasia.

“When you do refractive surgery on an eye that’s unhealthy to begin with, particularly one with an unhealthy epithelium, it’s not going to heal correctly,” he says. “I’ve found that, when I review the literature and look at cases of PRK over keratoconus, not only does the keratoconus change more rapidly in these patients, but they also have a lot of epithelial problems, such as haze or epithelial sloughing.” Dr. Smolek theorizes that the surgery is also affecting the health of the epithelium, which, in turn, causes chemical changes in the keratocytes and how they turn over the stroma and repair it.

There may also be something inherently different with naturally thin corneas. “What about patients with 400-µm corneas but no signs of keratoconus?” muses Miami refractive surgeon William Trattler, who has stopped doing LASIK entirely in favor of surface techniques. “What’s their risk for ectasia if they have LASIK? We don’t know if there’s anything that will predispose them toward ectasia, but it’s a concern surgeons have.”


Dr. Vukich agrees. “I think we’re dealing with a functionally different tissue, in terms of how it reacts, when we have a very thin cornea to begin with,” he says. “For instance, an eye with 450 µm or 430 µm of natural corneal thickness, but with no evidence of other corneal problems or progressive disease and which is optically sound, typically doesn’t react the same way as a cornea that was originally 600 µm thick. It’s just a case of different tissue characteristics.”

• Can wavefront help? The jury appears to be out on this. Dr. Smolek says that the prism terms in the wavefront may provide some clues. “It’s interesting, because most people just throw the prism terms away,” says Dr. Smolek. “Prism has nothing to do with focus, it just shifts the image back on the retina, but, as it turns out, that prism component seems to be very sensitive to keratoconus suspects.” He’s currently working on a way to incorporate an ectasia warning system into wavefront map displays that uses the prism and the coma components of the wavefront.

Dr. Vukich, however, is dubious about this. “Wavefront really only analyzes the optical properties of the eye, but doesn’t provide any data about stability,” he says. “It tells us nothing about whether the eye will be able to withstand or tolerate some reshaping. If the wavefront is abnormal because of some corneal irregularity, it will have shown up already in the topography.”

Dr. Vukich urges surgeons to try to tear themselves away from the nuances of wavefront analysis and get back to some of the fundamentals of refractive surgery in order to avoid ectasia. “No matter how accurate our wavefront analysis is and our custom ablations are,” he says, “unless we’re confident that what we’re doing to the cornea will remain a stable part of that eye going forward, the rest of it is just not relevant.” 

1. Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology 2003;110:2:267-75.