Journal of Refractive Surgery

Original Article 

Accuracy and Reproducibility of Cap Thickness in Small Incision Lenticule Extraction

Dan Z. Reinstein, MD, MA(Cantab), FRCOphth; Timothy J. Archer, MA(Oxon), DipCompSci(Cantab); Marine Gobbe, MST(Optom), PhD

Abstract

PURPOSE:

To evaluate the accuracy and reproducibility of cap thickness for small incision lenticule extraction (SMILE) with the VisuMax femtosecond laser (Carl Zeiss Meditec, Inc., Jena, Germany).

METHODS:

Artemis very high-frequency digital ultrasound (ArcScan Inc., Morrison, CO) measurements were obtained before and 3 months after SMILE in 70 eyes of 37 patients with intended cap thickness between 80 and 140 μm. True cap thickness at the time of creation was calculated as the addition of the preoperative epithelial thickness and the postoperative stromal component of the flap and mapped for the central 5-mm diameter zone. Cap thickness accuracy was calculated as the difference between the mean and intended cap thickness. Reproducibility was evaluated as the cap thickness standard deviation between eyes. Accuracy and reproducibility of cap thickness were calculated for all eyes and grouped by intended cap thickness. Within-eye variation in cap thickness was calculated as the standard deviation of all data within the central 5-mm diameter zone.

RESULTS:

Mean cap thickness accuracy was between −2.3 and 6.5 μm and was −0.7 μm centrally (range: −11 to +14 μm), including all eyes. Cap thickness reproducibility was 6 μm or less for the majority of the central 5-mm diameter zone and was 4.4 μm centrally. Cap thickness accuracy and reproducibility were similar for different intended cap thicknesses. Average within-eye variation in cap thickness was 4.3 μm.

CONCLUSIONS:

SMILE cap thickness using the VisuMax femtosecond laser was found to be accurate and reproducible across the central 5-mm diameter zone for intended cap thicknesses over the range of 80 to 140 μm.

[J Refract Surg. 2013;29(12):810–815.]

From London Vision Clinic, London, United Kingdom (DZR, TJA, MG); the Department of Ophthalmology, Columbia University Medical Center, New York, New York (DZR); and Centre Hospitalier National d’Ophtalmologie, Paris, France (DZR).

Dr. Reinstein is a consultant for Carl Zeiss Meditec (Jena, Germany) and has a proprietary interest in the Artemis technology (ArcScan Inc, Morrison, Colorado) and is an author of patents related to VHF digital ultrasound administered by the Cornell Center for Technology Enterprise and Commercialization, Ithaca, New York. The remaining authors have no financial or proprietary interest in the materials presented herein.

Prepared in part fulfillment of the requirements for the doctoral thesis of Dr. Reinstein for University of Cambridge.

AUTHOR CONTRIBUTIONS

Study concept and design (DZR, TJA, MG); data collection (DZR, TJA); analysis and interpretation of data (DZR, TJA, MG); drafting of the manuscript (DZR, TJA); critical revision of the manuscript (DZR, MG); statistical expertise (DZR, TJA)

Correspondence: Dan Z. Reinstein, MD, MA(Cantab), FRCOphth, London Vision Clinic, 138 Harley Street, London W1G 7LA, United Kingdom. E-mail: dzr@londonvisionclinic.com

Received: May 17, 2013
Accepted: July 15, 2013
Posted Online: October 30, 2013

Abstract

PURPOSE:

To evaluate the accuracy and reproducibility of cap thickness for small incision lenticule extraction (SMILE) with the VisuMax femtosecond laser (Carl Zeiss Meditec, Inc., Jena, Germany).

METHODS:

Artemis very high-frequency digital ultrasound (ArcScan Inc., Morrison, CO) measurements were obtained before and 3 months after SMILE in 70 eyes of 37 patients with intended cap thickness between 80 and 140 μm. True cap thickness at the time of creation was calculated as the addition of the preoperative epithelial thickness and the postoperative stromal component of the flap and mapped for the central 5-mm diameter zone. Cap thickness accuracy was calculated as the difference between the mean and intended cap thickness. Reproducibility was evaluated as the cap thickness standard deviation between eyes. Accuracy and reproducibility of cap thickness were calculated for all eyes and grouped by intended cap thickness. Within-eye variation in cap thickness was calculated as the standard deviation of all data within the central 5-mm diameter zone.

RESULTS:

Mean cap thickness accuracy was between −2.3 and 6.5 μm and was −0.7 μm centrally (range: −11 to +14 μm), including all eyes. Cap thickness reproducibility was 6 μm or less for the majority of the central 5-mm diameter zone and was 4.4 μm centrally. Cap thickness accuracy and reproducibility were similar for different intended cap thicknesses. Average within-eye variation in cap thickness was 4.3 μm.

CONCLUSIONS:

SMILE cap thickness using the VisuMax femtosecond laser was found to be accurate and reproducible across the central 5-mm diameter zone for intended cap thicknesses over the range of 80 to 140 μm.

[J Refract Surg. 2013;29(12):810–815.]

From London Vision Clinic, London, United Kingdom (DZR, TJA, MG); the Department of Ophthalmology, Columbia University Medical Center, New York, New York (DZR); and Centre Hospitalier National d’Ophtalmologie, Paris, France (DZR).

Dr. Reinstein is a consultant for Carl Zeiss Meditec (Jena, Germany) and has a proprietary interest in the Artemis technology (ArcScan Inc, Morrison, Colorado) and is an author of patents related to VHF digital ultrasound administered by the Cornell Center for Technology Enterprise and Commercialization, Ithaca, New York. The remaining authors have no financial or proprietary interest in the materials presented herein.

Prepared in part fulfillment of the requirements for the doctoral thesis of Dr. Reinstein for University of Cambridge.

AUTHOR CONTRIBUTIONS

Study concept and design (DZR, TJA, MG); data collection (DZR, TJA); analysis and interpretation of data (DZR, TJA, MG); drafting of the manuscript (DZR, TJA); critical revision of the manuscript (DZR, MG); statistical expertise (DZR, TJA)

Correspondence: Dan Z. Reinstein, MD, MA(Cantab), FRCOphth, London Vision Clinic, 138 Harley Street, London W1G 7LA, United Kingdom. E-mail: dzr@londonvisionclinic.com

Received: May 17, 2013
Accepted: July 15, 2013
Posted Online: October 30, 2013

Small incision lenticule extraction (SMILE) is a new laser refractive surgery procedure in which two interfaces are created using a femtosecond laser to isolate a lenticule of stromal tissue that can be manually removed from the cornea through a small incision.1–4 For the SMILE procedure to be refractively accurate, the accuracy of the femtosecond laser interface creation must be sufficiently high. Femtosecond lasers have been used for creating LASIK flaps since 20025 and studies have demonstrated how the accuracy and reproducibility have improved over time.6–14 Currently, SMILE has only been performed using the VisuMax femtosecond laser (Carl Zeiss Meditec, Inc., Jena, Germany), for which the flap thickness reproducibility has been reported to be between 3.8 and 14.4 μm.10–14

The aim of the current study was to evaluate the accuracy and reproducibility of cap thickness for myopic SMILE treatments with the VisuMax femtosecond laser using Artemis very high-frequency (VHF) digital ultrasound (ArcScan Inc., Morrison, CO).

Patients and Methods

Patients

This study was a retrospective, noncomparative case series of patients who underwent a SMILE procedure at the London Vision Clinic, London, United Kingdom, between May 2011 and December 2012. A complete ocular examination was performed to screen for corneal abnormalities and determine patient candidacy for refractive surgery. Eyes with ocular pathology such as keratoconus, corneal scar, corneal dystrophy, and previous ocular surgery were excluded. Our routine preoperative assessment was performed and has been described previously.15 All patients also had an Artemis VHF digital ultrasound scan as described below. Informed consent and permission to anonymously use their data for analysis and publication were obtained from all patients.

Surgical Procedure

All SMILE treatments were performed using the VisuMax 500-kHz femtosecond laser by a single surgeon (DZR). The VisuMax femtosecond laser has been described in detail previously.1–4,10–14 Details of the geometry and software set-up of the SMILE lenticule, cap, and small incision have been described previously.1–4 In the current study, two incisions were created in all eyes: a 2-mm incision superonasally and a 3-mm incision superotemporally. The optical zone diameter used was between 5.75 and 7.00 mm.

During the SMILE procedure, at the moment of contact between the disposable curved contact glass and the cornea, a meniscus tear film appears, at which point the patient is able to see the fixation target clearly because the vergence of the system is focused according to the patient’s refraction. In this way, the patient essentially auto-centrates the system, and hence the center of the lenticule on the corneal vertex, which closely approximates the visual axis. The femtosecond laser ablation was performed as described previously.1–4 Total suction time was approximately 35 seconds, independent of refractive error treated.

The 3-mm superotemporal incision was opened and the upper and lower edges of the lenticule were delineated, so that the tissue planes were well defined. If it was not possible to delineate the edge of both interfaces, the second 2-mm superonasal incision was opened to find the edge(s) that had yet to be delineated. The upper interface was separated first using a standard lamellar corneal surgical technique of waving the instrument back and forth. The lower layer was then dissected in a similar fashion. Once both layers had been separated, the lenticule was removed from the cornea using a pair of retinal microforceps.

The patient was then taken to the slit lamp and fluorescein was instilled and the full distention of the cap centrally was achieved by a dry microspear sponge to ensure that any redundant cap (due to mismatch of cap vs bed length) was not left in the central cornea, but rather redistributed to the periphery.

Postoperative Evaluation

Patients were instructed to wear plastic shields for 7 nights. Tobramycin/dexamethasone (Tobradex; Alcon Laboratories, Inc., Fort Worth, TX) and ofloxacin (Exocin; Allergan Ltd., Marlow, UK) drops were prescribed four times daily for the first week. Patients were observed at 1 day, 1 month, and 3 months. Artemis VHF digital ultrasound was performed at the 3-month postoperative visit.

Artemis Cap Thickness Calculation

The Artemis VHF digital ultrasound technology is capable of measuring individual layers within the cornea in three dimensions; the details of the Artemis system and patient set-up have been described previously.16–21 Cap thickness was measured by the Reinstein Flap Thickness Method of adding the thickness of the stromal component of the flap measured 3 months after surgery to the preoperative epithelial thickness, as we have described previously for measuring true LASIK flap thickness at the time of creation.16,17 Measuring the cap thickness at least 3 months after surgery ensures that postoperative edema has resolved, and using the preoperative epithelial thickness accounts for any postoperative epithelial changes known to occur after LASIK.19,20 Because the SMILE procedures were all centered on the corneal vertex and the Artemis scans were also centered on the corneal vertex, the central cap thickness was defined as the thickness at the Artemis coordinate (0,0). The repeatability of Reinstein Flap Thickness measurements has been shown to be 2.79 μm within the central 4-mm diameter, with a central repeatability of 1.68 μm.18

Statistical Analysis

Accuracy of cap thickness was calculated as the difference between the intended and measured flap thickness. Reproducibility of cap thickness was evaluated as the cap thickness standard deviation between eyes. The accuracy and reproducibility were calculated for all eyes and grouped by intended cap thickness and plotted for the central 5-mm diameter zone using DeltaGraph v5.0 (SPSS, Inc., Chicago, IL). Epithelial thickness values for left eyes were reflected in the vertical axis and superimposed onto the right eye values so that nasal/temporal characteristics could be combined. A Bland–Altman plot was performed to visualize the accuracy and reproducibility of central cap thickness; a scattergram of the difference between intended and measured cap thickness were plotted against the intended cap thickness.22 A horizontal line delineating the mean difference and two further lines delineating a range of 1.96 standard deviations were also plotted, which corresponded to the range in which 95% of the error values lie.

The standard deviation of flap thickness within the central 5 mm diameter zone was determined to represent the within-eye variation of flap thickness for each eye. The average within-eye variation in the population was calculated as the square root of the mean variance.

Microsoft Excel 2010 (Microsoft Corporation, Redmond, WA) was used for data entry and statistical analysis.

Results

The study included 70 eyes of 37 patients. The contralateral eye in 3 patients underwent LASIK, and one patient had one eye treated. The mean age of the population was 34.8 ± 11.1 years (range: 21 to 61 years). The mean maximum myopic meridian treated was −7.81 ± 2.33 diopters (D) (range: −2.25 to −12.50 D). The mean cylinder treated was 0.57 ± 0.42 D (range: 0.00 to 1.50 D). The mean thinnest corneal thickness by handheld ultrasound was 532 ± 32 μm (range: 468 to 591 μm). The optical zone diameter was 5.75 mm in 6 eyes (9%), 6.00 mm in 22 eyes (31%), 6.25 mm in 12 eyes (17%), 6.50 mm in 16 eyes (23%), 6.75 mm in 3 eyes (4%), and 7.00 mm in 11 eyes (16%). The mean minimum thickness programmed at the edge of the lenticule was 9 ± 4 μm (range: 3 to 25 μm).

The intended cap thickness was 80 μm in 1 eye (1.4%), 100 μm in 1 eye (1.4%), 106 μm in 1 eye (1.4%), 110 μm in 15 eyes (21%), 120 μm in 13 eyes (19%), 125 μm in 5 eyes (7%), 128 μm in 1 eye (1.4%), 130 μm in 25 eyes (36%), 135 μm in 6 eyes (9%), and 140 μm in 2 eyes (3%). The mean intended cap thickness was 123 ± 11 μm (range: 80 to 140 μm).

Figure 1 shows a horizontal Artemis B-scan image of the cornea of one eye 3 months after SMILE. Figure 2 shows the maps of the accuracy and reproducibility of cap thickness for all eyes and grouped by cap thickness. On average the cap was thinnest (2.3-μm thinner than intended) 0.8-mm superior to the corneal vertex and thickest (6.5-μm thicker than intended) 2.5-mm inferior to the corneal vertex.

Artemis very-high frequency digital ultrasound (ArcScan Inc., Morrison, CO) horizontal B-scan of a cornea 3 months after a small incision lenticule extraction procedure using the VisuMax femtosecond laser (Carl Zeiss Meditec, Inc., Jena, Germany). Digital signal processing is performed on the B-scan signal and layer thickness measurements are obtained by a computer algorithm on the I-scan, resulting in the red line image of the interfaces.

Figure 1:

Artemis very-high frequency digital ultrasound (ArcScan Inc., Morrison, CO) horizontal B-scan of a cornea 3 months after a small incision lenticule extraction procedure using the VisuMax femtosecond laser (Carl Zeiss Meditec, Inc., Jena, Germany). Digital signal processing is performed on the B-scan signal and layer thickness measurements are obtained by a computer algorithm on the I-scan, resulting in the red line image of the interfaces.

Topographical maps of the accuracy and reproducibility of cap thickness centered on the corneal vertex for the central 5-mm diameter zone. All left eyes were mirrored so that positive x-values represented the nasal cornea and negative x-values represented the temporal cornea. A Cartesian 1-mm grid was superimposed with the origin at the corneal vertex. All eyes were included after normalizing the cap thickness data for the intended cap thickness. For the accuracy map, the break point was set to 0 so that positive values (yellow/red) represented areas where the cap was thicker than intended and negative values (green/blue) represented areas where the cap was thinner than intended. The mean cap thickness was displayed to represent the accuracy grouped for 110, 120, and 130 μm intended cap thicknesses. The color scale represents the cap thickness in microns and the break point was set to the intended cap thickness so that yellow and red represented areas where the cap was thicker than intended and green and blue represented areas where the cap was thinner than intended. The color scale for all reproducibility maps represented the standard deviation in microns. The break point was set to 6 μm so that all areas where the reproducibility was better than 6 μm were displayed in green or blue.

Figure 2:

Topographical maps of the accuracy and reproducibility of cap thickness centered on the corneal vertex for the central 5-mm diameter zone. All left eyes were mirrored so that positive x-values represented the nasal cornea and negative x-values represented the temporal cornea. A Cartesian 1-mm grid was superimposed with the origin at the corneal vertex. All eyes were included after normalizing the cap thickness data for the intended cap thickness. For the accuracy map, the break point was set to 0 so that positive values (yellow/red) represented areas where the cap was thicker than intended and negative values (green/blue) represented areas where the cap was thinner than intended. The mean cap thickness was displayed to represent the accuracy grouped for 110, 120, and 130 μm intended cap thicknesses. The color scale represents the cap thickness in microns and the break point was set to the intended cap thickness so that yellow and red represented areas where the cap was thicker than intended and green and blue represented areas where the cap was thinner than intended. The color scale for all reproducibility maps represented the standard deviation in microns. The break point was set to 6 μm so that all areas where the reproducibility was better than 6 μm were displayed in green or blue.

Table 1 shows the accuracy and reproducibility of central cap thickness for all eyes grouped by intended cap thickness. The central cap thickness was within 2 μm of the intended thickness in 50% of eyes (35 of 70), within 5 μm in 83% of eyes (58 of 70), and within 10 μm in 96% of eyes (67 of 70). Figure 3 shows the Bland–Altman plot comparing the intended cap thickness with the difference between the intended and measured central cap thickness. The average within-eye variation in flap thickness was 4.3 μm (range: 2.1 to 8.2 μm).

Accuracy and Reproducibility of SMILE Cap Thickness for All Eyes and Grouped by Intended Cap Thickness

Table 1:

Accuracy and Reproducibility of SMILE Cap Thickness for All Eyes and Grouped by Intended Cap Thickness

Bland–Altman plot of the difference between the intended cap thickness and VisuMax-Artemis measured central cap thickness plotted against the intended cap thickness. The mean accuracy (intended – measured central cap thickness) of −0.7 μm was plotted as the bold green line, whereas the dotted green lines delineate the 95% limits of agreement (−9.3 to +7.9 μm, calculated as the mean difference ±1.96 × the standard deviation of 4.4 μm). The VisuMax femtosecond laser is manufactured by Carl Zeiss Meditec, Inc., Jena, Germany, and the Artemis is manufactured by ArcScan Inc., Morrison, CO.

Figure 3:

Bland–Altman plot of the difference between the intended cap thickness and VisuMax-Artemis measured central cap thickness plotted against the intended cap thickness. The mean accuracy (intended – measured central cap thickness) of −0.7 μm was plotted as the bold green line, whereas the dotted green lines delineate the 95% limits of agreement (−9.3 to +7.9 μm, calculated as the mean difference ±1.96 × the standard deviation of 4.4 μm). The VisuMax femtosecond laser is manufactured by Carl Zeiss Meditec, Inc., Jena, Germany, and the Artemis is manufactured by ArcScan Inc., Morrison, CO.

Discussion

In this study, cap thickness in SMILE treatments with the VisuMax femtosecond laser system was both accurate and reproducible with a mean central accuracy of −0.7 μm and reproducibility of 4.4 μm. The cap thickness was found to be uniform with only a small asymmetry of 8.8 μm in the vertical meridian. The reproducibility was found to be consistent with a reproducibility of less than 6 μm in the majority of locations. The accuracy and reproducibility were also found to be consistent and independent of intended cap thicknesses between 80 and 140 μm.

In a previous study using one of the original VisuMax 200-kHz femtosecond laser systems, we reported that the accuracy of 110-μm LASIK flaps was +2.3 μm with a reproducibility of 7.9 μm measured using the Artemis VHF digital ultrasound scanner and the same method as in the current study.17 The flap thickness reproducibility with the VisuMax 200-kHz system has also been reported using other techniques to be 3.8 μm,11 5.1 μm,12 7.9 μm,13 13.9 μm,14 and in the range 7.5 to 14.4 μm (for different flap thicknesses).10 This is the first study to report the reproducibility for the 500-kHz version of the VisuMax system. Table A (available in the online version of this article) provides a summary of the accuracy and reproducibility of central flap thickness for all published studies using femtosecond lasers.

However, these studies were performed with several different measurement methods, each of which has associated potential errors. We have previously discussed in detail the relative unreliability of other methods compared to the method used in the current study.17,23 To summarize, there are three main issues:

  1. The repeatability of the measurement device. The Artemis has a flap thickness repeatability of 1.68 μm,18 compared with 4.2 to 7.4 μm with the RT-Vue OCT (Optovue Inc., Fremont, CA)24,25 and 4.8 to 8.7 μm with the Visante OCT (Carl Zeiss Meditec),6,26,27 whereas handheld ultrasound has a repeatability of residual stromal bed thickness measurements of 4.9 μm.28

  2. Postoperative epithelial thickness changes. The changes in the epithelial thickness profile after a LASIK procedure can be up to approximately 20 μm for myopia20 and 24 μm for hyperopia,21 with significant epithelial changes occurring overnight and during the first month after surgery.19 Therefore, any study based on postoperative measurement of flap thickness will include errors due to epithelial thickness changes. On the other hand, the method used in the current study avoids these errors by combining temporally displaced measurements of the epithelial thickness preoperatively and the stromal component of the flap postoperatively (once all edema has subsided).

  3. Intraoperative handheld ultrasound can also be affected by stromal hydration,29 lack of coincidence of the probe location for corneal and residual bed measurements, and compression of tissue by the application of the probe.

The cap thickness was within 2 μm of the intended thickness across the horizontal meridian, demonstrating the uniform nature of the cap. There was a slight vertical asymmetry with the cap 2.3 μm thinner than intended superiorly and 6.5 μm thicker than intended inferiorly. This small tilt has been determined to not be optically or clinically significant (personal communication, Carl Zeiss Meditec, December 19, 2007).

The cap thickness was found to be highly uniform compared to microkeratome flaps, such as we have previously reported for the standard and zero compression Hansatome microkeratomes (Bausch & Lomb, Salt Lake City, UT).30 The within-eye variation of 4.3 μm for the VisuMax femtosecond laser was 60% better than the 10.7 μm for the standard Hansatome microkeratome and 10.4 μm for the zero compression Hansatome microkeratome.30 This result was similar to other studies in which femtosecond laser flaps were found to be significantly more uniform than microkeratome flaps.6–9,13,14

It is difficult to compare flap thickness uniformity among studies due to the difference in which locations were measured and the method of measurement. Almost all studies that investigated flap thickness uniformity were done using postoperative myopic optical coherence tomography data. Therefore, it could be that a finding of uniform flap thickness might actually mean that the true flap thickness profile was thinner centrally than paracentrally due to lenticular epithelial thickening with thickest epithelium centrally.

SMILE cap thickness using the VisuMax femtosecond laser was found to be accurate and reproducible and was independent of the intended cap thickness. These elements are critical to the refractive surgical success rates of the intralamellar femtosecond lenticule extraction technique.

References

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  30. Reinstein DZ, Archer TJ, Gobbe M. LASIK flap thickness profile and reproducibility of the standard vs zero compression Hansatome microkeratomes: three-dimensional display with Artemis VHF digital ultrasound. J Refract Surg. 2011;27:417–426. doi:10.3928/1081597X-20101110-01 [CrossRef]
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Accuracy and Reproducibility of SMILE Cap Thickness for All Eyes and Grouped by Intended Cap Thickness

Cap Thickness All 80 to 110μm 120μm 125 to 130μm 135 to 140μm
No. of eyes 70 18 13 31 8
Accuracy (mean) −0.7 +0.4 −0.6 −1.5 −0.5
Reproducibility (SD) 4.4 4.7 5.5 3.9 3.3
Range −11 to +14 −9 to +11 −11 to +14 −9 to +9 −6 to +5

10.3928/1081597X-20131023-02

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