The use of the femtosecond laser for the creation of LASIK flaps has gained popularity due to its safety and reproducibility. Several studies have compared the femtosecond laser to the microkeratome and demonstrated more accurate and reproducible flap thickness.1–5 This has led some surgeons to abandon the practice of routinely calculating the flap thickness prior to performing ablation on the stromal bed.
One of the most feared complications of refractive surgery is corneal ectasia. Numerous risk factors have been implicated in the development of post-refractive ectasia, including preoperative topographic changes, corneal thickness, depth of ablation, age, flap thickness, and residual stromal bed.6–8 Ambrósio et al.9 and Santhiago et al.10,11 demonstrated that the percent tissue altered (PTA) is a strong risk factor for development of ectasia. The flap thickness typically accounts for a significant portion of the PTA, usually greater than 50%. A later study by Santhiago et al.12 demonstrated that in patients who developed ectasia due to a high PTA, a larger portion of their PTA was derived from the flap thickness than the ablation. This suggests that flap thickness plays an important role in placing patients at risk for post-LASIK ectasia. Without measuring the flap intraoperatively, the surgeon cannot truly know the PTA or the residual stromal bed.
This study was designed to evaluate the surgeon's ability to visually estimate flap thickness and compare it to ultrasonic intraoperative measurements. Both experienced surgeons and fellows in training were evaluated.
Patients and Methods
The study was evaluated by the Massachusetts Eye and Ear Infirmary institutional review board and was determined to be exempt from review by the Human Studies Committee because of the minimal risk involved. Three experienced surgeons (defined as being fellowship trained and having at least 5 years of post-fellowship experience) and three cornea fellows were selected to perform the study (SAM, KMH, GTF, RRS, CC, PK). All surgeons used a femtosecond laser (Intralase FS60; Abbott Medical Optics, Inc., Santa Ana, CA or Alcon FS200; Alcon Laboratories, Inc., Fort Worth, TX) to create a flap of their desired thickness (range: 90 to 115 μm). The thickness target was chosen based on surgeon preference. Three central measurements were obtained immediately prior to flap creation with the femtosecond laser. The lowest measurement was recorded. After lifting the flap, the surgeon gave the investigator an estimation of the flap thickness based on his or her experience with the appearance and behavior of LASIK flaps. The stromal bed was then measured with ultrasound and a flap thickness was calculated. The surgeon was not told the actual flap thickness or stromal bed unless the measurement was outside agreed-upon parameters (surgeon preference for minimum residual stromal bed). The measuring physician would always alert the surgeon if the estimated residual stromal bed would ultimately be less than 300 μm. Each surgeon provided data on 20 eyes of 10 patients.
A total of 120 procedures were evaluated. The difference between the surgeon estimation and the ultrasonic measured flap was termed the thickness deviation. This value was positive when the actual flap thickness was greater than estimated and negative when the opposite occurred. The average thickness deviation (irrespective of positive or negative) was 15.20 μm (Table 1). The positive thickness deviation may be more clinically relevant because it exposes the patient to greater risk of a higher PTA than planned. Table 2 displays the percentage of positive deviations. The thickness deviation was greater than 10 μm in 44 eyes (36.67%), greater than 20 in 20 eyes (16.67%), and greater than 30 in 12 eyes (10%). There was no statistically significant difference between the number of positive thickness deviations between attending and fellow surgeons (P = .51). Residual stromal bed thickness was not greater than 300 μm for all eyes.
Overview of Data From the Study
Percentage Estimations Resulting in a Positive Thickness Deviation
The difference between the surgeon visual estimation and the flap thickness requested (programmed into the laser) was termed the deviation from planned. The average deviation from planned was 9.55 μm with a standard deviation of 12.54 μm. The deviation from planned was 10 μm or less 65% of the time and within 20 μm 92.5% of the time.
The growing adoption of the femtosecond laser for flap creation has led some surgeons to stop intraoperative measurement of LASIK flap thickness. Despite the improved accuracy and reproducibility of the femtosecond lasers compared to microkeratomes, deviations from intended flap thickness still occur. A retrospective study of more than 200 eyes by Sutton and Hodge4 demonstrated that 13% of femtosecond flaps fell outside their threshold of ±20 μm of intended thickness. The largest deviation was a flap that was 163 μm on an intended depth of 115 μm. This could have exposed the patient to significant risk of ectasia if coupled with an ablation profile that was considered safe for a 115-μm flap but too deep, resulting in an unsafe residual stromal bed or higher than planned PTA, for a 163-um flap.
Ensuring an adequate residual stromal bed is recognized as an important ectasia prevention measure in patients having LASIK.6,7 Although 250 μm has been proposed as a safe limit, many surgeons prefer to preserve a residual stromal bed of 300 μm or thicker. Ambrósio et al.9 and Santhiago et al.10,11 recently demonstrated the importance of PTA as another potential risk factor for ectasia after refractive surgery. They also showed that patients who developed ectasia after LASIK were more likely to have a thicker flap as part of the PTA calculation.12
This study demonstrates that visual estimation of LASIK flap thickness can significantly differ from ultrasonic measurements. The flap was as much as 30 μm thicker than estimated 10% of the time. In one case, the flap was 53 μm thicker than estimated, which significantly altered the PTA calculation. There was no difference in the ability to accurately estimate flap thickness between experienced (> 5 years after fellowship) and inexperienced (current fellows) LASIK surgeons.
Furthermore, the average deviation from planned (9.55 μm) was lower than the average thickness deviation (15.20 μm). This demonstrates that the visual estimation is closer to the surgeon's request than to the true thickness, which may reflect an overconfidence in the accuracy of the femtosecond laser.
Intraoperative pachymetry measurement allows calculation of residual stromal bed and PTA prior to proceeding with excimer laser ablation. This permits the surgeon to modify the amount of tissue ablated by shrinking the optical zone or to abort the surgery if the flap thickness is high enough to result in an unsafe residual stromal bed or PTA. An accurate measurement of LASIK flaps and residual stromal bed is highly recommended as part of ectasia prevention measures. It may also be a prudent measure to implement from a medicolegal standpoint because it allows the surgeon to demonstrate that safety parameters were respected in cases of postoperative ectasia. It has been our experience that the 40% PTA guideline has been a stricter standard when screening for LASIK compared to the residual stromal bed threshold of 300 μm. Thus, simply measuring the stromal bed after lifting the flap may not be sufficient.
There is a concern among some surgeons that contact of the stromal bed by the tip of the ultrasonic probe may increase the risk of infection. We have not encountered an infection in more than 9,000 cases performed with intraoperative pachymetry via an ultrasound probe (unpublished observations). Modern excimer lasers are now fitted with optical pachymeters, allowing intraoperative measurements without physical contact with the cornea and minimizing any delay to the excimer ablation.
A potential limitation to this study was the inclusion of patients who developed opaque bubble layer. Opaque bubble layer may affect pachymetric measurements,13 which could potentially alter the calculation of true flap thickness by subtracting an inaccurate pre-ablation measurement from the total pachymetry. Surgeons were also not masked to the flap thickness requested, which likely altered their visual estimation.
This study demonstrates that visually estimating the thickness of a LASIK flap is not as accurate as ultrasonic measurement and may lead to an unwitting violation of preoperative limits of residual stromal bed and PTA. Obtaining true flap thickness by measuring pachymetry before and after lifting the flap may lead to safer LASIK surgery, especially in the setting of a thinner cornea or deep laser ablation.
- Chen S, Feng Y, Stojanovic A, Jankov MR, Wang Q. IntraLase femtosecond laser vs mechanical microkeratomes in LASIK for myopia: a systematic review and meta-analysis. J Refract Surg. 2012;28:15–24. doi:10.3928/1081597X-20111228-02 [CrossRef]
- Farjo AA, Sugar A, Schallhorn SC, et al. Femtosecond lasers for LASIK flap creation: a report by the American Academy of Ophthalmology. Ophthalmology. 2013;120:e5–e20. doi:10.1016/j.ophtha.2012.08.013 [CrossRef]
- Zheng Y, Zhou Y, Zhang J, Liu Q, Zhai C, Wang Y. Comparison of laser in situ keratomileusis flaps created by 2 femtosecond lasers. Cornea. 2015;34:328–333. doi:10.1097/ICO.0000000000000361 [CrossRef]
- Sutton G, Hodge C. Accuracy and precision of LASIK flap thickness using the IntraLase femtosecond laser in 1000 consecutive cases. J Refract Surg. 2008;24:802–806.
- Santhiago MR, Kara-Junior N, Waring GO 4th, . Microkeratome versus femtosecond flaps: accuracy and complications. Curr Opin Ophthalmol. 2014;25:270–274. doi:10.1097/ICU.0000000000000070 [CrossRef]
- Randleman JB, Russel B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology. 2003;110:267–275. doi:10.1016/S0161-6420(02)01727-X [CrossRef]
- Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115:37–50. doi:10.1016/j.ophtha.2007.03.073 [CrossRef]
- Ambrósio R Jr, Nogueira LP, Caldas DL, et al. Evaluation of corneal shape and biomechanics before LASIK. Int Ophthalmol Clin. 2011;51:11–38. doi:10.1097/IIO.0b013e31820f1d2d [CrossRef]
- Ambrósio R Jr, Dawson DG, Belin MW. Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:1358–1359. doi:10.1016/j.ajo.2014.09.016 [CrossRef]
- Santhiago MR, Smadja D, Gomes BF, et al. Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87–95. doi:10.1016/j.ajo.2014.04.002 [CrossRef]
- Santhiago MR, Smadja D, Wilson SE, Krueger RR, Monteiro ML, Randleman JB. Role of percent tissue altered on ectasia after LASIK in eyes with suspicious topography. J Refract Surg. 2015;31:258–265. doi:10.3928/1081597X-20150319-05 [CrossRef]
- Santhiago MR, Smajda D, Wilson SE, Randleman JB. Relative contribution of flap thickness and ablation depth to the percentage of tissue altered in ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2015;41:2493–2500. doi:10.1016/j.jcrs.2015.05.023 [CrossRef]
- Kaiserman I, Maresky HS, Bahar I, Rootman DS. Incidence, possible risk factors, and potential effects of an opaque bubble layer created by a femtosecond laser. J Cataract Refract Surg. 2008;34:417–423. doi:10.1016/j.jcrs.2007.10.026 [CrossRef]
Overview of Data From the Studya
|Planned flap thickness||90||115||101||9.08|
|Actual flap thickness||55||148||103||19.4|
Percentage Estimations Resulting in a Positive Thickness Deviationa
|> 10 μm||36.67%||36.67%||36.67%|
|> 20 μm||13.33%||20.00%||16.67%|
|> 30 μm||6.67%||13.33%||10.00%|