Laser in situ keratomileusis (LASIK) is now the most commonly performed refractive surgery procedure; however, many questions remain about the key aspect of the procedure—making the flap. Numerous studies have addressed the question of flap thickness,1–12 but there is little information in the literature regarding flap diameter and the variables affecting it. Studies that have measured the corneal flap diameter have found that the actual flap was larger than the predicted suction ring in most cases.4–6,9,10 Small series have suggested a correlation between preoperative keratometry values and the achieved corneal flap diameter.4,12
In this retrospective study, measurements of the actual flap size were compared with the attempted size (suction ring size) and to preoperative keratometry values. This study compared the achieved corneal flap performed by 4 surgeons using 3 Hansatome microkeratomes.
Patients and Methods
This retrospective analysis included 6667 consecutive primary LASIK procedures performed at the Gimbel Eye Centre between January 1998 and October 2002. The study compared the difference in the actual LASIK flap diameter and the microkeratome ring size for 3 Hansatome microkeratomes (Bausch & Lomb) (indicated by serial numbers 2230, 3048, and 3084) in relation to the preoperative keratometry. The surgeries were performed by 4 surgeons from the centre (surgeon 1 = H.V.G., 2 = J.A.V., 3 = J.K., 4 = E.E.P.).
Inclusion criteria were keratometry readings of 39.0 to 49.0 diopters (D), sphere of −0.75 to −12.0 D, cylinder of not more than −6.0 D, corneal thickness that allowed at least a residual bed of 250 μm, no significant corneal asymmetry, age between 18 and 60 years, and stable refraction for at least 1 year. Exclusion criteria were cases performed by other surgeons, cases performed with microkeratomes other than those mentioned, correction that could flatten the cornea to <35.0 D, local corneal diseases and scars, severe corneal dryness, and systemic diseases that could affect the cornea such as collagen vascular disease (rheumatoid arthritis, lupus), Grave's disease, and Steven-Johnson syndrome. Also excluded were flaps with any significant stromal structural irregularities that did not allow laser ablation: buttonhole, thin or irregular flaps, free cap, and incomplete cut.
In the LASIK surgical procedure, the surgeon used a suction ring size of 8.5 or 9.5 mm (8.5 mm = 58%; 9.5 mm = 42%), and after adequate suction was generated, a flap depth of 160 or 180 μm (160 μm = 80%; 180 μm = 20%) was created. The corneal flap was created by 3 Hansatome microkeratomes (#2230, #3048, or #3084), using a superior hinge. After the flap was lifted, laser ablation was performed with a Nidek EC-5000 or EC-5000 CX excimer laser. The flap was repositioned in the stromal bed to match the corneal landmarks followed by interface irrigation with balanced salt solution (BSS®). Excess BSS was gently absorbed with a damp Merocel sponge, and a high magnification inspection of the interface and surface was performed. At the end of the procedure, horizontal diameter of the flap was measured by Castroviejo caliper. Preoperative keratometry measurements were performed by Orbscan (Bausch & Lomb). These keratometry measurements were compared with the difference between the actual flap diameter and the attempted flap size (the ring size). Mean keratometry was 44.09 D (38.75 to 48.99 D). This analysis was done for all surgeons together and for each individual surgeon. The results were analyzed using the Student t test.
This study retrospectively analyzed 6667 eyes of 3485 patients. In 3868 eyes (58%), an 8.5 mm suction ring was used, and in 2792 eyes (42%), a 9.5 mm ring was used. Table 1 summarizes the mean difference between flap diameter and ring size per microkeratome. The mean difference between flap diameter and ring size was not statistically significant (P>.05) between 8.5 and 9.5 mm rings or between keratomes. Overall, the mean difference in flap diameter was 0.66 mm larger than the ring diameter. Microkeratome 3084 resulted in the largest mean difference between flap diameter and ring size. The mean difference between flap and ring diameter was statistically significant per each microkeratome (P<.001). Table 2 summarizes the mean difference between flap and diameter, which was consistently larger in 94.4% of eyes.
The mean flap size per surgeon relative to the microkeratome is summarized in Table 3. The difference in mean flap versus the microkeratome ring size was statistically significant by surgeon (P<.0001). Surgeon 3 consistently created the largest flap diameter, followed by surgeons 1, 2, and 4 (Table 3). Regarding the mean difference between microkeratome ring size and resultant flap size, surgeon 3 had the largest mean, followed by surgeon 2, 4, and 1 (Figure 1). Figure 1 shows that every surgeon had a characteristic pattern for achieving the actual flap diameter, even when using different microkeratomes.
Figure 2 demonstrates the relation between corneal steepness and mean difference between flap diameter and ring size for microkeratome 2230. Higher preoperative keratometry values resulted in larger flap diameters than lower preoperative keratometry values. The correlation between preoperative average Ks and difference in flap diameter was statistically significant by microkeratome (P<.001). The difference between mean flap diameter and ring size increased with steeper corneas with microkeratome 2230, demonstrating the largest positive correlation (P = .45). With microkeratomes 3048 and 3084, R values were 0.28 and 0.23, respectively.
Figure 3 shows the distribution scale of the difference between ring size and actual flap diameter among all eyes and the distribution among the 4 surgeons. A similar pattern was found for surgeons 2, 3, and 4, but a different pattern and more narrow distribution was found for surgeon 1.
A larger flap diameter allows a larger ablation and reduces the risk for ablating the hinge, but it increases the risk for intraoperative bleeding in cases of vascularization of the peripheral cornea, as in chronic contact lens users. In addition, if 2 flap sizes were of equal thickness, the larger flap would create a less stable cornea than a flap of smaller size. Assuming that a surgeon could maintain ideal centration of the flap, the ideal flap would be just large enough to accommodate the laser ablation pattern. Because overcorrection is a possibility, a flap size that is just large enough to accommodate a hyperopic correction is what most surgeons would prefer.
Most experienced LASIK surgeons are aware that the size of the flap created is rarely identical to the size indicated on the suction ring. It is well known from the literature that the actual corneal flap achieved in LASIK is larger than the suction ring size (Table 4).4–6,10,12 In deciding which suction ring to use, it would be helpful to know what flap size would result when choosing a given ring size.
Choi and coauthors12 measured the actual flap size in 268 eyes after LASIK surgery using the Innovatome automatic microkeratome and found that the corneal flap size increased with the preoperative keratometry readings. Spadea and coauthors4 evaluated flap measurements with the Hansatome microkeratome in 50 eyes that had LASIK surgery. The authors found a statistically significant correlation between mean keratometric power and flap diameter as well as correlation between keratometry and flap hinge length and between spherical equivalent refraction and flap diameter.
Our large study shows a strong correlation between preoperative keratometry values and the actual flap size in 3 Hansatome microkeratomes (serial numbers 2230, 3048, and 3084). This relationship may hold true for microkeratomes other than the 3 used in this study as well. Although this correlation was strong, the scatterplot shows that the variability is significant. This limits the usefulness of this information in predicting flap size for any given individual.
There were also consistent differences among surgeons and some differences in how much variation there was for each individual surgeon. One hypothetical explanation for the difference among surgeons is the amount of pressure exerted on the eye with the suction ring while the keratome pass occurs. We believe that higher manual downward pressure on the suction ring may result in a larger flap, but future studies are necessary to prove whether this is the case. The 4 surgeons were interviewed with respect to their technique for measuring the flap. We found that the same endpoints for measurement were used for all surgeons, but we acknowledge that there is still a possibility that some variation may be due to differences in judging these endpoints. We believe these differences are too small to be clinically or statistically significant. Predictably, the 9.5 mm suction ring creates flap sizes larger than 9.5 mm to approximately the same degree as is the case for the 8.5 mm suction ring.
Based on our findings, we have applied the following to our LASIK procedures:
- We frequently use the 8.5 mm suction ring because we know that in most cases, it gives a ring size that is adequate for the primary treatment zone (7.5 mm transition zone).
- Our surgeons modify the decision of ring size based on whether the combination of individual microkeratome and keratometry reading is likely to vary the flap size to larger or smaller than average. For example, if a microkeratome that typically makes a smaller flap is paired with an eye that has flat keratometry values, we may choose to use the larger 9.5 mm ring size rather than the 8.5 mm ring size to reduce the risk of a too-small flap. When the surgeon is deciding which ring size to use to achieve the appropriate flap diameter, he or she must take into account the size of the treatment zone. In planning that treatment zone, the size of the cornea, the pupils, and the refraction are taken in consideration. Some of our surgeons used the 8.5 mm suction ring for hyperopia treatments as well (9.0 mm transition zone) because in almost all cases flap size was larger than 9.0 mm.
- We routinely measure flap size (as well as other parameters) and establish an individual profile for any new microkeratome, knowing that significant variations can exist even for instruments that are theoretically identical.
- We maintain an awareness of surgeon profiles and recommend that new surgeons establish their own LASIK profiles, just as surgeons doing cataract surgery would establish their own A-constant for a particular style of intraocular lens.
In conclusion, estimation of the flap size is an important issue when a refractive surgeon is planning the treatment zone in LASIK surgeries. We found that, in most cases, the actual flap size per surgeon was larger than the attempted size and that corneas with high preoperative keratometry values are more prone to achieve a larger flap size than those having lower keratometry values.
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