I became intrigued by the spate of articles and presentations given at recent scientific meetings that hail the importance of requiring preoperative pachymetry readings in excess of 500 µm before performing LASIK surgery. Based on my own cases, as well as those of colleagues with whom I have worked closely over the years, I can state that we have collectively performed many thousands of LASIK procedures on patients with preoperative pachymetry readings of less than 500 µm without witnessing a disproportionate degree of post-LASIK ectasia. In fact, the cases of ectasia we have encountered to date (in eight eyes treated by my immediate colleagues and in two eyes that I treated) have all occurred in eyes with preoperative pachymetry readings of more than 500 µm. This finding led me to suspect that there may be other, more important factors that cause or contribute to post-LASIK ectasia. Therefore, I designed a questionnaire for experienced colleagues and friends performing high-volume LASIK surgery. The following questions were included:
1. When did you start performing LASIK?2. How many LASIK procedures have you performed?
3. What microkeratome do you use?
4. What flap thickness do you prefer to aim for?
5. What laser do you prefer to use?
6. Have you had any case(s) of post-LASIK ectasia?
7. If yes, what did you think the most important contributing factor(s) were?
8. What is the minimum pre-op pachymetry that you would require for an ideal -2.00 myopic candidate?
9. Would you do LASIK for a pre-op pachymetry of less than 500 µm?
10. Would you do LASIK for a pre-op pachymetry of less than 480 µm?
11. Would you do LASIK for a pre-op pachymetry of less than 460 µm?
12. What are the steepest pre-op K's that you would do LASIK on for myopia?
13. What are the steepest pre-op K's that you would do LASIK on for hypermetropia?
14. What are the flattest pre-op K's that you would do LASIK on for myopia?
15. What are the flattest pre-op K's that you would do LASIK on for hypermetropia?
16. Any other comments?
REPRESENTATION
The surgeon-respondents represented practices from around the world: two from Australia, eight from South Africa, three from the US and Canada, three from continental Europe, three from the UK, and seven from Ireland.
EXPERIENCE
The number of LASIK procedures performed ranged from 1,000 to more than 30,000 procedures per surgeon. The first surgeons to perform LASIK in this group began in 1993, and the least experienced started in 1998. Therefore, this group represents a broad range of experience, in terms of both number of LASIK procedures performed and years of experience. Interestingly, the average surgeon in this group began performing LASIK in 1995 and has since executed more than 6,600 procedures. This group performed a total of 173,000 procedures.
FLAP-RELATED ISSUES
The respondents most often targeted a flap thickness of 160 µm, but this objective varied between 130 and 180 µm (Table 1). The average flap thickness targeted was 146 µm.
Most surgeons required a residual corneal thickness (RCT; the amount of cornea left after the ablation) of at least 250 µm, although this number also varied. Some were prepared to allow thinner amounts, while others preferred to have at least 300 µm in the corneal bed to accommodate potential re-treatments (Table 2).
Two surgeons further refined their RCTs; they felt that both the patient's age and presenting pachymetry reading influenced the required RCT. If the patient was younger than 30 years, these surgeons preferred a 270-µm RCT, but for patients older than 45 years, the surgeons only required a 240-µm RCT. Another respondent felt that no more than 50% of the cornea should ever be removed, by ablation or flap creation. Therefore, a 500-µm cornea would require a 250-µm RCT, while a 550-µm cornea would require a 275-µm RCT.
LASERS
The questionnaire group favored the Allegretto (WaveLight Laser Technologie AG, Erlangen, Germany) and the Technolas 217z (Bausch & Lomb Surgical, Inc.) excimer lasers. However, other lasers mentioned were the EC5000 (Nidek Incorporated, Freemont, CA), the STAR S4 Excimer Laser System (VISX, Inc., Santa Clara, CA), the LaserScan LSX excimer laser system (LaserSight Technologies, Inc., Winter Park, FL), and the Summit Autonomous (LADARVision4000; Alcon Laboratories, Inc., Fort Worth, TX).
PACHYMETRY
Of the 26 surgeons surveyed, 81% responded that they would perform LASIK on a -2.00 D patient who had a preoperative pachymetry reading of less than 500 µm. Also, 69% of the respondents would perform LASIK on a patient with a preoperative pachymetry reading of less than 480 µm, and 23% would perform the procedure with a preoperative measurement of less than 460 µm. The average preoperative pachymetry measurement required for an ideal -2.00 D myopic patient was 472 µm, although this number varied from 425 to 500 µm (Tables 3 and 4).
POSTOPERATIVE ECTASIA
Fifty-eight percent of the surgeons surveyed had patients who had experienced postoperative ectasia. Specifically, 32 of the total eyes treated by all 26 survey participants (0.018%) developed ectasia after undergoing LASIK surgery. The treating surgeons deemed the following contributing factors most important: (1) forme fruste keratoconus; (2) thicker flaps than anticipated; (3) a residual corneal thickness of less than 250 µm; (4) irregular astigmatism (especially vertical astigmatism); (5) high myopia with or without astigmatism; (6) poor protoplasm; (7) enhancement surgery; and (8) a thin preoperative pachymetry measurement (less than 500 µm). This group of experienced LASIK surgeons therefore rated preoperative pachymetry (as long as it was more than 470 µm) to be the least important contributor for ectasia. By far, the most important factor was undiagnosed forme fruste keratoconus. The respondents also agreed that less sophisticated topography machines made it difficult to diagnose ectasia in the early days of LASIK and that present-day technology was much improved.
One important factor to emerge from the study was the indication of “flaps thicker than anticipated.” In my opinion, this is the one area in which proactive safeguards can enormously improve the outcome and safety of the LASIK procedure. Both participants in this study and various colleagues of mine in personal communications reported variations in flap thickness as large as 80 µm while using the same microkeratome. Such reports imply a 160-µm head can produce flaps both 120 µm thick and 200 µm thick. Presently, the only method by which to gauge a flap's thickness is an intraoperative pachymetry test, and the most important lesson to take away from this survey is the necessity of this testing. Once the surgeon determines the residual corneal thickness, he can decide whether to proceed with the ablation as planned or whether to take action to preserve tissue, perhaps by reducing the size of the optical zone.
It is important to note a number of post-LASIK ectasia cases were recorded in patients who were ideal candidates for the surgery. Although these individuals met all the required criteria (including preoperative pachymetry, corneal topography, and residual corneal thickness), they still developed keratoconus postoperatively. These patients would not register as poor LASIK candidates with any of the routine preoperative diagnostics screenings.
BREAKING DOWN THE RESULTS
I divided the survey respondents into two groups: surgeons who had performed more than 10,000 LASIK surgeries and those who had performed fewer than 10,000. Six of the surgeons had performed more than 10,000 procedures each, for a combined total of 101,000 surgeries. The other 20 surgeons had performed a combined total of 72,718 LASIK procedures. Next, I compared the surgical approaches to flap thickness between more and less experienced surgeons. I found no fundamental differences, except the more experienced surgeons tended to aim for thinner flaps (135 µm on average vs 155 µm) and also generally left the RCT at 260 µm, whereas the less experienced surgeons required 250 µm on average.
I then compared the surgical habits of those surgeons who had experienced a case of corneal ectasia with that of the surgeons who had never encountered the condition. The only significant difference was the group that had experienced ectasia had performed an average of 7,714 procedures, whereas the group that had avoided ectasia had performed 5,772 surgeries. Therefore, the rate of post-LASIK ectasia may directly correlate with the number of surgeries performed.
IN CONCLUSION
The evaluation of a candidate's suitability for LASIK rests on many factors, one of which is his preoperative corneal pachymetry. It appears surgeons can perform LASIK quite safely on patients with preoperative pachymetry measurements of less than 500 µm as long as these individuals meet other preoperative criteria. Finally, the results of the survey emphasize the importance of intraoperative pachymetry—it should become routine for all surgeons performing LASIK and LASIK enhancements.
