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How far can we push the cornea?
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It never ceases to amaze me how resilient the cornea is. When one considers all it endures on a daily basis, it is remarkable how well the cornea fares. No doubt, every clinician can attest to this delicate structure's ability to ward off infection and to bounce back from life's little traumas.
As if this were not enough, there is the issue of contact lenses. During the past 40 years, the cornea has patiently endured our efforts to develop the ideal contact lens. We have challenged it with PMMA, thimerosal, orthokeratology and extended wear. It has withstood hypoxic, mechanic, toxic and immunogenic insults and — for the most part — bounced back rather well. In spite of it all, the cornea has embraced contact lenses as an extremely safe and efficacious option. Fortunately, contact lenses provide the cornea with the ultimate safe haven — reversibility.
The ultimate test
Arguably, the cornea's ultimate test is refractive surgery. Truly a double-edged sword, refractive surgery offers the cornea tremendous benefits — and significant risks. It is true refractive surgery has a long, rich tradition with many significant advancements. Along the way, the refractive surgery community has learned to temper its surgical approach and to pamper the cornea postoperatively.
Consider the shift from 16-incision radial keratotomy (RK) to mini-RK. The evolution from single-zone photorefractive keratectomy (PRK) to multi-zone, multi-pass PRK. Or the sophistication of laser in situ keratomileusis (LASIK) when compared to automated lamellar keratoplasty. Indeed, refractive surgery is becoming more "cornea friendly." But, will we leave well enough alone?
Surely, it is human nature to push the envelope, and refractive surgery is no exception. Just witness the trend of pushing surgical techniques to correct higher refractive errors or — should I say — "to summit the cornea." For instance, just how far can we drive LASIK for myopia correction?
Serving the cornea
Given corneal flap thickness and high myopia ablation depths, are we ultimately compromising the endothelium, increasing optical aberrations or interfering with tear film dynamics? And what about astigmatic PRK? Considering axis variability in low cylinder powers and the optic zone diameter in high cylinder ablations, is the cornea being well served?
Finally, there's the issue of hyperopic corneal refractive surgery. Whether PRK or LASIK, we're attempting to lessen hyperopia by thinning the midperipheral cornea, thereby inducing a central steepening. Conceptually, it is easy to see how removing corneal tissue lessens myopia. It is a little less clear how removing corneal tissue eliminates hyperopia.
Personally, I find it increasingly difficult to define the boundaries of refractive surgery. I am a strong proponent of this discipline, having witnessed many satisfied patients. However, I am also a card-carrying member of the Corneal Preservation Society and dutifully sworn to the cornea's protection.
To me, it appears that the best surgical solution for certain refractive errors is to just leave the cornea alone. In the final analysis, intraocular lens implantation may prove to be the most appropriate option for certain prescriptions; however, until this debate is resolved we must be vigilant in caring for the cornea. So, will the cornea withstand the challenges of refractive surgery? I suspect it knows, but I am not so sure we do.