Iatrogenic keratectasia: academic anxiety or serious risk?1 (1999)

Rupture of the cornea is an extremely rare condition because of biomechanical safety factors. The force necessary to rupture corneal tissue is 1000 times greater than the physiologic steady-state tension in the cornea. However, this safety factor only applies when sudden forces act on the cornea. In principle, the cornea, like most biologic tissues, demonstrates viscoelastic and elastoplastic behaviors. The safety factor shrinks to approximately 2 times for processes that occur slowly. Andreassen and coauthors1 and Edmund2 have shown that the tensile strength of the normal cornea is only twice that of the keratoconus cornea. One of the painful lessons refractive surgeons have had to learn is that this small safety factor is easily violated. In the Prospective Evaluation of Radial Keratotomy Study and other long-term studies of eyes treated by radial keratotomy, it took more than 5 years to recognize that late keratectasia of the paracentral corneal could lead to progressive hyperopia.3,4 Is it possible that similar processes can happen after laser in situ keratomileusis (LASIK) from the iatrogenic alterations in the biomechanics of the cornea? Clearly, preoperative and postoperative corneal thickness is 1 parameter of concern. A reduced corneal thickness does not primarily influence its infrastructure; however, it does result in an increase in the surface-parallel stress. Reducing the central thickness of a “normal” cornea (520 μm) to 260 μm (as is common in LASIK) increases the surface-parallel stress in the center of the cornea by a factor of 2. After lamellar corneal surgery, such an increase in stress is a long-term condition and individual corneas may react differently. This is the point at which the tensile strength of each cornea and the time factor become relevant. Keratoconus usually starts during puberty. In most cases, it takes more than 5 years (that number again) to manifest clinically. At any stage, the cornea may return to its original tensile strength or at least increase its stiffness, leading to an abortive form of keratoconus, forme fruste keratoconus. We know that the tensile strength of the normal cornea varies by as much as ±25%.5 Unfortunately, we cannot measure the elasticity of the cornea in vivo. Therefore, we have to establish clinical safety guidelines by trial and error. To establish reliable guidelines, a follow-up of more than 5 years seems appropriate. Do we have such long-term data after LASIK? No, we do not. Short-term observations (up to 1 year) have demonstrated iatrogenic keratoconus occurring at a residual stromal thickness of less than 250 μm.6,7 Thirty years ago, Barraquer8 estimated that the minimal thickness of the operated cornea is 300 μm, including a sutured lamella, based on his extensive experience with myopic keratomileusis. Applying this to LASIK with a biomechanically ineffective superficial lamella, maximal myopic corrections (6 mm ablation, 160 μm flap thickness) would be limited to 3 diopters (D) in a thin cornea (500 μm) and 10 D in a thick cornea (600 μm). However, this estimation of maximal correction appears too pessimistic and does not reflect our clinical experience. In my practice, the smallest myopic correction by LASIK that has led to iatrogenic keratoconus is −9.0 D. I was surprised, however, to learn of 5 cases of iatrogenic keratectasia after LASIK for −4.0 to −7.0 D from a colleague in South Africa. Confronted with such confusing information, we as a community of refractive surgeons should stop and reflect. In the year 1999, more than 1 million LASIK procedures will be performed worldwide. Even if iatrogenic keratoconus is a rare condition and occurs in only 1 in 1000 eyes, 1000 eyes will be irreversibly damaged in 1 year. Is the currently published number of fewer than 20 cases only the tip of an iceberg? Would it be safer to use a percentage of the corneal thickness as a minimal residual stromal thickness rather than an absolute number (250 μm) given ultrasound pachymetry measurement errors and biomechanical considerations? I believe the answer is yes, but we need more data to make such a determination. And what are we looking for clinically? Late keratectasia after myopic LASIK would manifest as a central steep island accompanied by regression of the refractive effect and problems with scotopic vision. There are many questions and few answers because of the limited data available in the literature. Therefore, I encourage our professional societies to build a forum on the Internet to collect data on cases of iatrogenic keratectsia. With a larger database, we could determine recommendations for a minimal residual stromal thickness, which would benefit us and our patients.