The occurrence of iatrogenic keratectasia following laser in situ keratomileusis (LASIK) is a concern in modern lamellar refractive surgery and has been reported since 1998. Although the incidence appears rare, LASIK surgeons are paying close attention to this potential problem, particularly in preoperative diagnostic examinations. In this issue, Comaish and Lawless (pages 2205–2212) provide an overview of current knowledge about corneal biomechanics in progressive post-LASIK keratectasia and the parallels with keratoconus. Their article points out that we do not know the real cause. Is it purely a biomechanical result of LASIK, in which case weakening the cornea by LASIK may put everyone at risk if overtreatment occurs or the residual stromal bed is thin? Or is it a chronic disease process affecting only predisposed individuals? A third option is a combination of the 2.
The general thinking is that residual post-LASIK stromal thickness should not be less than 250 μm. For some researchers, determining the residual stromal thickness by the initial corneal thickness (eg, 55% of the initial corneal thickness) seems more logical as it better reflects the individual nature of each cornea. Obviously, precise measurement of the overall corneal thickness and the thinnest corneal point is mandatory; preoperatively, to include or exclude patients; intraoperatively, to abort the ablation; and postoperatively, to qualify patients for further treatment (overcorrection, undercorrection, or irregular astigmatism). In current practice, ultrasound measurements are supplemented by slit-scanning technology, confocal microscopy, and optical coherence tomography. But can we provide a precise measurement of corneal thickness?
Rainer et al. (pages 2141–2144) report an interesting result of corneal thickness measurements. The mean corneal thickness measured with 3 ultrasound pachymetry devices was similar but statistically different. The measurement with partial coherence interferometry (PCI) was about 20 μm to 26 μm thinner than the ultrasonic measurements; PCI was the more precise method, with better intraobserver variability. This shows that the measurement of corneal thickness for LASIK requires careful attention to determine the “real” value. Perhaps better devices than those currently in use are necessary.
For the clinician, the important steps in preventing keratectasia have been summarized in recent editorials1–3: (1) Evaluate topography and pachymetry before each case. (2) Avoid LASIK in eyes with abnormal or suspicious topography. (3) Calculate the estimated residual posterior corneal thickness based on corneal thickness, ablation depth, and anticipated flap thickness. (4) Measure flap thickness and posterior corneal thickness intraoperatively. (5) Collect, evaluate, and report keratectasia cases.
I would like to propose an additional guideline that I now follow in my practice: Do not perform LASIK in patients with corneas less than 500 μm at the thinnest point. I base this on 1 published case (both eyes)4 and verbal reports of ectasia occurring after LASIK in these corneas. Admittedly, posterior bed thickness is unknown in these cases. However, excellent options such as surface ablation or phakic intraocular lenses are available for these patients.
The effort to identify corneas that could develop keratectasia after ablation surgery, particularly lamellar procedures, should continue. The upper limits of LASIK in each individual case will primarily depend on this identification and on quality-of-vision issues. Fortunately, the number of keratectasia cases appears to be very low. With current technology, we are achieving excellent results in most of our patients. However, the overall success of refractive surgery will depend on stable and safe long-term results, and we should ensure this outcome.
1. Koch DD. The riddle of iatrogenic keratectasia [editorial]. J Cataract Refract Surg 1999; 25:453-454
2. Seiler T. Iatrogenic keratectasia: academic anxiety or serious risk? [editorial]. J Cataract Refract Surg 1999; 25:1307-1308
3. Kohnen T. Need for intraoperative measurement of corneal thickness during LASIK [editorial]. J Cataract Refract Surg 2000; 26:1695-1696
4. Amoils SP, Deist MB, Gous P, Amoils PM. Iatrogenic keratectasia after laser in situ keratolmileusis for less than −4.0 to −7.0 diopters of myopia. J Cataract Refract Surg 2000; 26:967-977