- Email: [email protected]
Direct measurement of microkeratome gap width by electron microscopy1
Direct measurement of microkeratome gap width by electron microscopy King-yu Liu, FRCOphth, PhD, Dennis S.C. Lam, FRCS, FRCOphth ABSTRACT Purpose: To perform an accurate direct measurement of the microkeratome gap width using scanning electron microscopy (SEM). Setting: Electron Microscope Unit, University of Hong Kong, Hong Kong, China. Methods: The Cambridge Stereoscan S440 scanning electron microscope was used to measure the gap width of 4 SCMD microkeratomes with high accuracy (⫾1.5 m). Results: The manufacturer’s gap specification for the 4 microkeratomes was 150.0 m. The gap width measurements using SEM were 164.7 m, 190.0 m, 200.6 m, and 145.9 m and the respective errors, 9.8%, 26.7%, 33.7%, and 2.7%. Two of the 4 microkeratomes had more than a 25% error in gap width from the specification. Conclusions: The great variation in gap width from the manufacturer’s specification for the 4 SCMD microkeratomes was beyond the standard of tolerance normally accepted in laser in situ keratomileusis (LASIK). Many unexpected LASIK-related keratectasia and corneal perforations may be related to substandard microkeratome manufacturing and calibration. All new microkeratomes and blades should be validated before use to avoid keratectasia and other flap problems in LASIK. J Cataract Refract Surg 2001; 27: 924 –927 © 2001 ASCRS and ESCRS
T
he most important characteristic of the microkeratome is its gap width. The gap width is the shortest distance from the tip of the blade to the footplate of the microkeratome. It is the narrowest part, or the bottleneck. A simple geometry of the gap is assumed here and shown in Figure 1. The gap limits the amount of corneal tissue going through the microkeratome during corneal cutting. It determines the maximum corneal flap thickness and is vital in the calculation of the minimum posterior stromal thickness to prevent corneal ectasia in laser in situ keratomileusis (LASIK). The manufacturer specifies the microkeratome gap width, but sometimes Accepted for publication December 21, 2000. Reprint requests to King-yu Liu, FRCOphth, PhD, Adjunct Assistant Professor, Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, 147K Argyle Street, Kowloon, Hong Kong, China. E-mail: [email protected]. © 2001 ASCRS and ESCRS Published by Elsevier Science Inc.
the specification is inaccurate. Electron microscopy can be used to measure and validate the microkeratome gap width directly and accurately.
Materials and Methods The gap width of 4 SCMD microkeratomes (LASIK Turbokeratome, 150 m fixed head) was measured with the Cambridge Stereoscan S440 scanning electron microscope. The scanning electron microscope has a working principle similar to that of a normal optical microscope except that it uses electrons to form images. It has a high depth of field, and the resolution limit is in the range of 5 nm under high magnification. Because of the high depth of field, the tip of the blade and the footplate of the microkeratome can be focused sharply and the gap width measured directly without distortion. 0886-3350/01/$–see front matter PII S0886-3350(01)00767-2
LABORATORY SCIENCE: MICROKERATOME GAP WIDTH
Figure 1. (Liu) Cross-section of the SCMD microkeratome head.
Figure 2. (Liu) Alignment of the microkeratome in the electron microscope chamber.
Measuring Procedure The electron microscope was calibrated on a standard grid of known dimensions. The microkeratome was mounted onto a clamping stage and inserted into the microscope’s chamber. The stage was tilted so that the top part of the microkeratome head (the reference plane) was almost perpendicular to the electron beam (Figure 2). The stage was then adjusted so that the gap width plane was in focus. The tilting angle of the clamping stage was recorded. The stage was then tilted slightly until the gap width between the tip of the blade and the footplate could be seen clearly. The difference in the tilted angle was calculated (2.8 degrees in the measurement) (Figure 3). At ⫻200 magnification, the gap width was measured by a point-to-point method. The entire procedure was repeated for the remaining 3 SCMD microkeratomes. For consistency, the same person performed all measurements using the same blade. Accuracy Errors can occur in the alignment of the gap with respect to the imaging direction. The maximum misalignment is expected to be in the range of 0.5 degree. The maximum error related to the angle of tilt is about 0.1%, which means that the gap width measurement is relatively unaffected by a small angle of tilt. Furthermore, the gap width was measured based on the images seen on a display monitor of the electron microscope. The maximum error incurred in the measurement was in the range of ⫾1.5 m.
Results The results, corrected for the tilted angle, are shown in Table 1. Two of the 4 microkeratomes had a gap width variation of more than 25% from the manufacturer’s specification.
Discussion
Figure 3. (Liu) Angle of tilt correction in the SEM measurements. Corrected width (c) ⫽ measured width (m) ⫼ cosine ; ⫽ 2.8° during examination; cosine 2.8° ⫽ 0.999.
The minimum posterior stromal thickness to prevent corneal ectasia is not known. However, it is believed that at least 250 to 300 m of residual posterior stromal thickness must be maintained in LASIK.1 Miscalculating the minimum posterior stromal thickness because of an error in the microkeratome gap width specification could be detrimental in LASIK, especially
J CATARACT REFRACT SURG—VOL 27, JUNE 2001
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LABORATORY SCIENCE: MICROKERATOME GAP WIDTH
Table 1. The gap width of 4 SCMD microkeratomes.
Microkeratome
Measured Gap Width (m)
Corrected Gap Width* (m)
Percentage of Error
1
164.92
165.1
9.8
2
190.25
190.4
26.7
3
200.85
201.1
33.7
4
146.07
146.2
2.7
*Corrected gap width ⫽ measured gap width ⫼ cosine 2.8
in compromised eyes with high myopia and a thin cornea. Although the flap thickness produced by a microkeratome can be thicker or thinner than the exact gap width, its exact value is important because when other major factors (eg, intraocular pressure, microkeratome advancing speed, corneal curvature and hydration) are constant, a microkeratome with a larger gap width will produce a thicker corneal flap than a microkeratome with a smaller gap width. If the cornea is dehydrated, the epithelium loose, or the pass very slow, a deeper cut will be made. This is why indirect measurement of the corneal flap thickness,2 by subtracting the residual corneal thickness without the corneal flap from the preoperative corneal thickness, does not truly reflect the microkeratome gap width. Direct measurement of the microkeratome gap width provides all the information about the maximum corneal flap thickness cut by the microkeratome. Nearly all LASIK surgeons assume that the maximum corneal flap
thickness equals the gap width information provided by the manufacturer and use this value in the calculation of the minimum posterior stromal thickness. Using an indirect method, Yi and Joo2 measured a maximum corneal flap of 200 m. If the maximum flap thickness does equal the gap width and if other conditions (eg, corneal hydration) are within normal limits, one could assume that the gap width of the SCMD microkeratome in the Yi and Joo study was 200 m. Direct measurement of the microkeratome gap width would confirm and explain these findings. Although there may be other indirect or direct methods of measuring a microkeratome’s gap width, this is the first report of using a scanning electron microscope to measure and confirm the width directly with an accuracy of ⫾1.5 m. The 4 SCMD microkeratomes tested should have had the same gap width (150 m), with allowance for a ⫾10% error. However, 2 of the microkeratomes had errors greater than 25%. The great variation in gap width from the manufacturer’s
Figure 4. (Liu) The gap width (125 m) of the Chiron ALK micro-
Figure 5. (Liu) The gap width (169 m) of the Chiron ALK micro-
keratome with the original Chiron blade and a 130 m plate measured using the Cambridge S-90 scanning electron microscope.
keratome with the original Chiron blade and a 160 m plate measured using the Cambridge S-90 scanning electron microscope.
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LABORATORY SCIENCE: MICROKERATOME GAP WIDTH
manufacturer (ML Blade, Med-Logics, Inc.) was used with the 160 m plate. This shows that a nonvalidated blade from another manufacturer can significantly change the gap width. Therefore, the gap widths of all new microkeratomes and new blades from other manufacturers should be validated before use to avoid LASIK complications such as keratectasia3 and corneal perforation4 arising from an unexpectedly thick flap causing an exceedingly thin stromal bed.
References Figure 6. (Liu) The gap width (147 m) of the Chiron ALK microkeratome with a blade from a manufacturer other than Chiron and a 160 m plate measured using the Cambridge S-90 scanning electron microscope.
specification in the 4 microkeratomes tested is beyond the standard of tolerance normally accepted in LASIK. One surgeon (K.L.) has used a validated Chiron ALK microkeratome in about 2000 eyes having LASIK over the past 2.5 years with no cases of keratectasia. A Cambridge S-90 scanning electron microscope was used to measure the gap width of the ALK microkeratome with the original Chiron blades. The 130 m and 160 m plates were measured to be 125 m (Figure 4) and 169 m (Figure 5), respectively. The figures are consistent with the manufacturer’s specification. However, the gap width was measured to be 147 m rather than 160 m (Figure 6) when a blade from another
1. American Academy of Ophthalmology. Automated lamellar keratoplasty. Ophthalmology 1996; 103:852– 861 2. Yi W-M, Joo C-K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999; 25:1087–1092 3. Geggel HS, Talley AR. Delayed onset keratectasia following laser in situ keratomileusis. J Cataract Refract Surg 1999; 25:582–586 4. Joo C-K, Kim T-G. Corneal perforation during laser in situ keratomileusis. J Cataract Refract Surg 1999; 25:1165–1167
From the Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, Kowloon, Hong Kong, China. Supported in part by the Mrs. Annie Wong Eye Foundation, Hong Kong. The I-Centre and the 148-Vision Correction Centre, Hong Kong, lent their SCMD microkeratomes for this study, and Mr. Albert Sin and Mr. Eric Lim provided technical expertise and assistance. Neither author has a financial or proprietary interest in any material or method mentioned.
J CATARACT REFRACT SURG—VOL 27, JUNE 2001
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T
he most important characteristic of the microkeratome is its gap width. The gap width is the shortest distance from the tip of the blade to the footplate of the microkeratome. It is the narrowest part, or the bottleneck. A simple geometry of the gap is assumed here and shown in Figure 1. The gap limits the amount of corneal tissue going through the microkeratome during corneal cutting. It determines the maximum corneal flap thickness and is vital in the calculation of the minimum posterior stromal thickness to prevent corneal ectasia in laser in situ keratomileusis (LASIK). The manufacturer specifies the microkeratome gap width, but sometimes Accepted for publication December 21, 2000. Reprint requests to King-yu Liu, FRCOphth, PhD, Adjunct Assistant Professor, Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, 147K Argyle Street, Kowloon, Hong Kong, China. E-mail: [email protected]. © 2001 ASCRS and ESCRS Published by Elsevier Science Inc.
the specification is inaccurate. Electron microscopy can be used to measure and validate the microkeratome gap width directly and accurately.
Materials and Methods The gap width of 4 SCMD microkeratomes (LASIK Turbokeratome, 150 m fixed head) was measured with the Cambridge Stereoscan S440 scanning electron microscope. The scanning electron microscope has a working principle similar to that of a normal optical microscope except that it uses electrons to form images. It has a high depth of field, and the resolution limit is in the range of 5 nm under high magnification. Because of the high depth of field, the tip of the blade and the footplate of the microkeratome can be focused sharply and the gap width measured directly without distortion. 0886-3350/01/$–see front matter PII S0886-3350(01)00767-2
LABORATORY SCIENCE: MICROKERATOME GAP WIDTH
Figure 1. (Liu) Cross-section of the SCMD microkeratome head.
Figure 2. (Liu) Alignment of the microkeratome in the electron microscope chamber.
Measuring Procedure The electron microscope was calibrated on a standard grid of known dimensions. The microkeratome was mounted onto a clamping stage and inserted into the microscope’s chamber. The stage was tilted so that the top part of the microkeratome head (the reference plane) was almost perpendicular to the electron beam (Figure 2). The stage was then adjusted so that the gap width plane was in focus. The tilting angle of the clamping stage was recorded. The stage was then tilted slightly until the gap width between the tip of the blade and the footplate could be seen clearly. The difference in the tilted angle was calculated (2.8 degrees in the measurement) (Figure 3). At ⫻200 magnification, the gap width was measured by a point-to-point method. The entire procedure was repeated for the remaining 3 SCMD microkeratomes. For consistency, the same person performed all measurements using the same blade. Accuracy Errors can occur in the alignment of the gap with respect to the imaging direction. The maximum misalignment is expected to be in the range of 0.5 degree. The maximum error related to the angle of tilt is about 0.1%, which means that the gap width measurement is relatively unaffected by a small angle of tilt. Furthermore, the gap width was measured based on the images seen on a display monitor of the electron microscope. The maximum error incurred in the measurement was in the range of ⫾1.5 m.
Results The results, corrected for the tilted angle, are shown in Table 1. Two of the 4 microkeratomes had a gap width variation of more than 25% from the manufacturer’s specification.
Discussion
Figure 3. (Liu) Angle of tilt correction in the SEM measurements. Corrected width (c) ⫽ measured width (m) ⫼ cosine ; ⫽ 2.8° during examination; cosine 2.8° ⫽ 0.999.
The minimum posterior stromal thickness to prevent corneal ectasia is not known. However, it is believed that at least 250 to 300 m of residual posterior stromal thickness must be maintained in LASIK.1 Miscalculating the minimum posterior stromal thickness because of an error in the microkeratome gap width specification could be detrimental in LASIK, especially
J CATARACT REFRACT SURG—VOL 27, JUNE 2001
925
LABORATORY SCIENCE: MICROKERATOME GAP WIDTH
Table 1. The gap width of 4 SCMD microkeratomes.
Microkeratome
Measured Gap Width (m)
Corrected Gap Width* (m)
Percentage of Error
1
164.92
165.1
9.8
2
190.25
190.4
26.7
3
200.85
201.1
33.7
4
146.07
146.2
2.7
*Corrected gap width ⫽ measured gap width ⫼ cosine 2.8
in compromised eyes with high myopia and a thin cornea. Although the flap thickness produced by a microkeratome can be thicker or thinner than the exact gap width, its exact value is important because when other major factors (eg, intraocular pressure, microkeratome advancing speed, corneal curvature and hydration) are constant, a microkeratome with a larger gap width will produce a thicker corneal flap than a microkeratome with a smaller gap width. If the cornea is dehydrated, the epithelium loose, or the pass very slow, a deeper cut will be made. This is why indirect measurement of the corneal flap thickness,2 by subtracting the residual corneal thickness without the corneal flap from the preoperative corneal thickness, does not truly reflect the microkeratome gap width. Direct measurement of the microkeratome gap width provides all the information about the maximum corneal flap thickness cut by the microkeratome. Nearly all LASIK surgeons assume that the maximum corneal flap
thickness equals the gap width information provided by the manufacturer and use this value in the calculation of the minimum posterior stromal thickness. Using an indirect method, Yi and Joo2 measured a maximum corneal flap of 200 m. If the maximum flap thickness does equal the gap width and if other conditions (eg, corneal hydration) are within normal limits, one could assume that the gap width of the SCMD microkeratome in the Yi and Joo study was 200 m. Direct measurement of the microkeratome gap width would confirm and explain these findings. Although there may be other indirect or direct methods of measuring a microkeratome’s gap width, this is the first report of using a scanning electron microscope to measure and confirm the width directly with an accuracy of ⫾1.5 m. The 4 SCMD microkeratomes tested should have had the same gap width (150 m), with allowance for a ⫾10% error. However, 2 of the microkeratomes had errors greater than 25%. The great variation in gap width from the manufacturer’s
Figure 4. (Liu) The gap width (125 m) of the Chiron ALK micro-
Figure 5. (Liu) The gap width (169 m) of the Chiron ALK micro-
keratome with the original Chiron blade and a 130 m plate measured using the Cambridge S-90 scanning electron microscope.
keratome with the original Chiron blade and a 160 m plate measured using the Cambridge S-90 scanning electron microscope.
926
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manufacturer (ML Blade, Med-Logics, Inc.) was used with the 160 m plate. This shows that a nonvalidated blade from another manufacturer can significantly change the gap width. Therefore, the gap widths of all new microkeratomes and new blades from other manufacturers should be validated before use to avoid LASIK complications such as keratectasia3 and corneal perforation4 arising from an unexpectedly thick flap causing an exceedingly thin stromal bed.
References Figure 6. (Liu) The gap width (147 m) of the Chiron ALK microkeratome with a blade from a manufacturer other than Chiron and a 160 m plate measured using the Cambridge S-90 scanning electron microscope.
specification in the 4 microkeratomes tested is beyond the standard of tolerance normally accepted in LASIK. One surgeon (K.L.) has used a validated Chiron ALK microkeratome in about 2000 eyes having LASIK over the past 2.5 years with no cases of keratectasia. A Cambridge S-90 scanning electron microscope was used to measure the gap width of the ALK microkeratome with the original Chiron blades. The 130 m and 160 m plates were measured to be 125 m (Figure 4) and 169 m (Figure 5), respectively. The figures are consistent with the manufacturer’s specification. However, the gap width was measured to be 147 m rather than 160 m (Figure 6) when a blade from another
1. American Academy of Ophthalmology. Automated lamellar keratoplasty. Ophthalmology 1996; 103:852– 861 2. Yi W-M, Joo C-K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999; 25:1087–1092 3. Geggel HS, Talley AR. Delayed onset keratectasia following laser in situ keratomileusis. J Cataract Refract Surg 1999; 25:582–586 4. Joo C-K, Kim T-G. Corneal perforation during laser in situ keratomileusis. J Cataract Refract Surg 1999; 25:1165–1167
From the Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, Kowloon, Hong Kong, China. Supported in part by the Mrs. Annie Wong Eye Foundation, Hong Kong. The I-Centre and the 148-Vision Correction Centre, Hong Kong, lent their SCMD microkeratomes for this study, and Mr. Albert Sin and Mr. Eric Lim provided technical expertise and assistance. Neither author has a financial or proprietary interest in any material or method mentioned.
J CATARACT REFRACT SURG—VOL 27, JUNE 2001
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