Comparison of intraoperative subtraction pachymetry and postoperative anterior segment optical coherence tomography of laser in situ keratomileusis flaps

ARTICLE

Comparison of intraoperative subtraction pachymetry and postoperative anterior segment optical coherence tomography of laser in situ keratomileusis flaps Yohko Murakami, MD, Edward E. Manche, MD

PURPOSE: To prospectively compare intraoperative subtraction pachymetry flap thickness measurements and postoperative anterior segment optical coherence tomography (AS-OCT) flap thickness measurements in eyes that had laser in situ keratomileusis (LASIK) flap creation with a femtosecond laser or a mechanical microkeratome. SETTING: Stanford Eye Laser Center, Stanford University School of Medicine, Stanford, California, USA. DESIGN: Comparative case series. METHODS: Each patient received wavefront-guided LASIK using an Intralase femtosecond laser in 1 eye and a Hansatome mechanical microkeratome in the fellow eye. Flap morphology was assessed with an ultrasound pachymeter intraoperatively and an AS-OCT system postoperatively at 1 year. RESULTS: Thirty-six eyes (18 patients) were enrolled. Intraoperative subtraction pachymetry consistently underestimated mechanical microkeratome flap thickness compared with postoperative AS-OCT (P<.001). There was no significant difference between intraoperative subtraction pachymetry and postoperative AS-OCT measurements for femtosecond flaps (PZ.38). The overall mean variation in flap thickness (ie, mean deviation from targeted flap thickness) was 2.6 mm (range 3 to 11 mm) with the femtosecond laser and 14.2 mm (range 17 to 52 mm) with the mechanical microkeratome (P<.001). Postoperative AS-OCT measurements showed femtosecond flaps had a planar configuration and mechanical microkeratome flaps had a meniscus-shaped configuration. CONCLUSIONS: The femtosecond laser created more uniformly planar flaps than the mechanical microkeratome as measured by intraoperative subtraction pachymetry and postoperative AS-OCT. Postoperative AS-OCT measurements varied less than intraoperative subtraction pachymetry measurements for mechanical microkeratome flaps. Financial Disclosure: Neither author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2011; 37:1879–1883 Q 2011 ASCRS and ESCRS

Anterior segment optical coherence tomography (AS-OCT) measurements of flap thickness in eyes after laser in situ keratomileusis (LASIK) surgery show good reproducibility with good interobserver agreement.1–5 These studies report that LASIK flaps created with a mechanical microkeratome typically have a meniscus-type configuration that is thinner centrally and thicker in the periphery.4–7 In contrast, flaps created with a femtosecond laser typically have a more planar flap configuration with uniform thickness throughout Q 2011 ASCRS and ESCRS Published by Elsevier Inc.

the flap.4–6 A few studies8–13 examined the correlation between intraoperative ultrasound (US) subtraction pachymetry and postoperative AS-OCT flap imaging. These studies found that subtraction pachymetry may be subject to greater measurement error with higher standard deviations (SDs) than postoperative AS-OCT.8,10 The present study compared postoperative AS-OCT measurements and the gold standard of intraoperative US subtraction pachymetry measurements of flaps 0886-3350/$ - see front matter doi:10.1016/j.jcrs.2011.05.024

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created with a femtosecond laser and flaps created with a mechanical microkeratome to assess the utility of AS-OCT. PATIENTS AND METHODS This prospective randomized eye-to-eye study compared the thickness and morphology of flaps created using a femtosecond laser in 1 eye and a mechanical microkeratome in the fellow eye of patients having wavefront-guided LASIK for myopia; thus, each patient acted as his or her own control. The flaps were imaged with 2 disparate technologies: US subtraction pachymetry intraoperatively and AS-OCT postoperatively. The study was approved by the Institutional Review Board (IRB), Stanford University School of Medicine, and the study protocol followed the tenets of the Declaration of Helsinki (Clinical Trials Registration Protocol Record #SU-02082009-1758). All patients provided written informed consent. The same surgeon (E.E.M.) performed all LASIK procedures in an academic surgical practice setting. The femtosecond laser (15 kHz Intralase, Abbott Medical Optics, Inc.) was programmed to a flap thickness of 120 mm and a flap diameter of 9.2 mm with a superior hinge in all cases. The mechanical microkeratome (Hansatome Bausch & Lomb) had a 160 mm compression head and a 9.5 mm suction ring. A Sonogage pachymeter (Sonogage, Inc.) was used for intraoperative US subtraction pachymetry, and the Visante system (Carl Zeiss Meditec AG) was used for postoperative AS-OCT. After creation of the LASIK flap and immediately before the flap lift, pachymetry was performed using the pachymeter; 10 US measurements were automatically obtained. The flap was then reflected back and pachymetry of the stromal bed measured. Another 10 US measurements were taken automatically. Flap thickness was calculated by subtracting the mean of the 2 measurements. One year postoperatively, AS-OCT was performed and flap thickness was measured at 5 points in each horizontal OCT image, with a central measurement at the vertex and peripheral measurements at 1.5 mm and 3.0 mm nasally and temporally (Figure 1). Flap configuration was determined by calculating the mean deviation of the 5 AS-OCT values from the target

Submitted: August 21, 2010. Final revision submitted: April 17, 2011. Accepted: May 2, 2011. From the Department of Ophthalmology, Stanford University School of Medicine, Stanford, California, USA. Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, Chicago, Illinois, USA, April 2008; the XXVI Congress of the European Society of Cataract & Refractive Surgeons, Berlin, Germany, September 2008; and the annual meeting of the American Academy of Ophthalmology, Atlanta, Georgia, USA, November 2008. Corresponding author: Edward E. Manche, MD, Department of Ophthalmology, Stanford University, 900 Blake Wilbur Drive, W3002, Palo Alto, California 94304, USA. E-mail: edward.manche@ stanford.edu.

Figure 1. Example of a horizontal cross-sectional AS-OCT image of the cornea created by a femtosecond laser in a uniform planar configuration (top) and a mechanical microkeratome in a meniscus-shaped configuration (bottom). The calipers are located at the 5 measurement points: at the vertex and at 1.5 mm and 3.0 mm nasally and temporally.

thickness divided by the mean achieved flap thickness and expressed as a percentage. A small percentage of deviation from the programmed flap thickness indicated regular planar flap morphology, while a greater percentage of deviation was consistent with a meniscus-shaped morphology with less flap uniformity. Inclusion criteria were age 21 years and older; low to moderate myopia (range 1.50 to 6.25 diopters [D]); and eyes closely matched refractively, with less than 0.50 D difference in spherical equivalent (SE) and less than 0.50 D of refractive astigmatism. Eyes were randomized to receive treatment with the femtosecond laser or the mechanical microkeratome by ocular dominance according to a randomization schedule. Standard refractive surgical exclusion criteria were autoimmune disease, keratoconus, previous ocular surgery, or a history of herpes simplex keratitis. All intraoperative and postoperative complications were assessed at each scheduled and unscheduled study visit. The data were entered into case report forms in the chart, and the IRB was notified of any adverse events within 72 hours. Data were analyzed using SPSS for Windows software (version 18.0, SPSS, Inc.). Mann-Whitney U tests were used to calculate statistical significance, which was set at 0.05 for all tests.

RESULTS The study included 36 eyes of 18 patients (7 men, 11 women) with a mean age of 43.0 years G 8.3 (SD)

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Figure 2. Intraoperative subtraction pachymetry and postoperative AS-OCT measurements. Top: Femtosecond laser flaps. Bottom: Mechanical microkeratome flaps.

(range 30 to 56 years). Preoperatively, the mean SE was 3.83 G 1.42 D. the mean corneal keratometry was 44.0 G 1.37 D, and the mean corneal thickness on US was 547.8 G 23.2 mm. The mean intraoperative central flap thickness measured by subtraction pachymetry was 112.6 G 19.3 mm (range 68 to 151 mm) in the femtosecond laser group and 126.6 G 16.6 mm (range 101 to 163 mm) in the mechanical microkeratome group. The mean flap thickness measured by postoperative AS-OCT was 108.7 G 4.5 mm (range 103 to 124 mm) in the femtosecond laser group and 144.5 G 10.5 mm (range 125 to 165 mm) in the mechanical microkeratome group. The overall mean variation in flap thickness was 2.6 mm (range 3 to 11 mm) in the femtosecond laser group and 14.2 mm (range 17 to 52 mm) in the mechanical group (P!.001). There was no statistically significant difference between intraoperative US subtraction pachymetry and postoperative AS-OCT measurements of femtosecond flaps (PZ.38) (Figure 2, top). Intraoperative subtraction pachymetry significantly underestimated flap thickness compared with postoperative AS-OCT in mechanical microkeratome flaps (P!.001) (Figure 2, bottom).

Postoperative AS-OCT showed the meniscusshaped morphology of the mechanical microkeratome flaps, with the thinnest measurements in the center and thicker measurements toward the flap periphery (Figure 3). The maximum difference in the mean thickness at different measurement points was 52.0 mm, calculated from 178 mm in the periphery and 126 mm centrally in 1 patient. Femtosecond laser flaps had a regular planar morphology, with the maximum difference in the mean values of thickness being 11.0 mm, calculated from 115 mm in the periphery and 104 mm centrally. The mean deviation from targeted flap thickness was 5.8%

Figure 3. Mean OCT measurements at the 5 flap points.

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for femtosecond flaps and 27.8% for mechanical microkeratome flaps (P!.001), confirming the regular planar morphology of the femtosecond laser compared with the morphology of the mechanical microkeratome. The mean deviation from the 120 mm in femtosecond laser flaps was C1.29 mm on AS-OCT. The mechanical microkeratome flaps had good accuracy in achieving the target thickness of 160 mm at the flap periphery (mean difference C2.8 mm) but poor accuracy for central flap measurements (mean difference C34.7 mm). The femtosecond laser was significantly more accurate than the mechanical microkeratome in achieving the desired flap thickness, as shown by measurements with less deviation from the programmed setting (P!.01). DISCUSSION The findings in this study suggest that postoperative AS-OCT imaging is more accurate, with a tighter SD, than intraoperative subtraction pachymetry. The femtosecond laser provided a significantly narrower range of flap measurements regardless of the measurement method used. Postoperative AS-OCT measurements showed that flaps created by the Intralase femtosecond laser were more precise, with a planar flap morphology and uniform flap thickness, than those created by the Hansatome mechanical microkeratome. These findings confirm Stahl et al.’s13 observations that femtosecond lasers create uniform thickness planar configuration flaps while mechanical microkeratomes typically create a meniscus-shaped flap with less uniform thickness. Our findings of greater SDs for flap thickness obtained by intraoperative subtraction pachymetry than by postoperative AS-OCT imaging are consistent with those of von Jagow and Kohnen11 and Li et al.14 One explanation for the observed differences in intraoperative and postoperative measurements is that intraoperative US pachymetry is subject to factors such as edema and compression, which are not present when the AS-OCT measurements are obtained postoperatively. The development of intraoperative AS-OCT capability has the potential to eliminate these inconsistencies. These results were obtained using a single machine of 1 brand of femtosecond laser and a particular mechanical microkeratome. As such, the data cannot be generalized to all femtosecond lasers and microkeratomes. The same surgeon performed all cases. The main limitation of the study was the small number of eyes available for AS-OCT imaging at the 1-year mark.

This study is useful in confirming that the Intralase femtosecond laser is more accurate than the Hansatome mechanical microkeratome in creating LASIK flaps. It found that femtosecond flaps are more uniform in thickness, with a planar flap configuration, than those created by the mechanical microkeratome. It also shows some potential limitations of intraoperative subtraction pachymetry using US. The study suggests that the thickness of flaps created by a femtosecond laser may be more reliable with less scatter of flap measurements than those created with a mechanical microkeratome. It also confirms the usefulness of AS-OCT for imaging LASIK flaps in eyes having LASIK reoperations. The usefulness of OCT measurements 1 year postoperatively points to their utility in late retreatments. Anterior segment OCT can be extremely useful in a refractive surgery clinical practice. REFERENCES 1. Izquierdo L Jr, Henriquez MA, Zakrzewski PA. Detection of an abnormally thick LASIK flap with anterior segment OCT imaging prior to planned LASIK retreatment surgery. J Refract Surg 2008; 24:197–199 2. Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg 2004; 30: 804–811 3. Konstantopoulos A, Hossain P, Anderson DF. Recent advances in ophthalmic anterior segment imaging: a new era for ophthalmic diagnosis? Br J Ophthalmol 2007; 91:551–557. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1994765/pdf/ 551.pdf. Accessed June 3, 2011 4. Medeiros FW, Stapleton WM, Hammel J, Krueger RR, Netto MV, Wilson SE. Wavefront analysis comparison of LASIK outcomes with the femtosecond laser and mechanical microkeratomes. J Refract Surg 2007; 23: 880–887 5. Patel SV, Maguire LJ, McLaren JW, Hodge DO, Bourne WM. Femtosecond laser versus mechanical microkeratome for LASIK; a randomized controlled study. Ophthalmology 2007; 114:1482–1490 6. Kim J-H, Lee D, Rhee K-I. Flap thickness reproducibility in laser in situ keratomileusis with a femtosecond laser: optical coherence tomography measurement. J Cataract Refract Surg 2008; 34:132–136 7. Li Y, Shekhar R, Huang D. Corneal pachymetry mapping with high-speed optical coherence tomography. Ophthalmology 2006; 113:792–799 e792 8. Avila M, Li Y, Song JC, Huang D. High-speed optical coherence tomography for management after laser in situ keratomileusis. J Cataract Refract Surg 2006; 32: 1836–1842 9. Durrie DS, Kezirian GM. Femtosecond laser versus mechanical keratome flaps in wavefront-guided laser in situ keratomileusis; prospective contralateral eye study. J Cataract Refract Surg 2005; 31:120–126 10. Tang M, Li Y, Avila M, Huang D. Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography. J Cataract Refract Surg 2006; 32:1843–1850

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11. von Jagow B, Kohnen T. Corneal architecture of femtosecond laser and microkeratome flaps imaged by anterior segment optical coherence tomography. J Cataract Refract Surg 2009; 35:35–41 12. Wang J, Thomas J, Cox I. Corneal light backscatter measured by optical coherence tomography after LASIK. J Refract Surg 2006; 22:604–610 13. Stahl JE, Durrie DS, Schwendeman FJ, Boghossian AJ. Anterior segment OCT analysis of thin IntraLase femtosecond flaps. J Refract Surg 2007; 23:555–558 14. Li Y, Netto MV, Shekhar R, Krueger RR, Huang D. A longitudinal study of LASIK flap and stromal thickness with high-speed

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First author: Yohko Murakami, MD Department of Ophthalmology, Stanford University School of Medicine, Stanford, California, USA

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