- Email: [email protected]
Corneal Flap Thickness During Laser In Situ Keratomileusis
CORNEAL FLAP THICKNESS DURING LASER IN SITU KERATOMILEUSIS Sheng-Yao Hsu Department of Ophthalmology, Buddhist Tzu Chi General Hospital and Tzu Chi University, Hualien, Taiwan.
To analyze the actual corneal flap thickness (FT) after flap making by MK-2000 microkeratome during laser in situ keratomileusis (LASIK), 42 females and 20 males with myopia or myopic astigmatism were enrolled in this study. FTs were created using a microkeratome with a 130 μm head. Corneal thickness was measured by ultrasonic pachymeter. The correlations between FT and central corneal thickness (CCT), keratometric power and age were analyzed. The mean age at operation for all study subjects was 27.6 ± 4.9 years. The average FT was 133.2 ± 15.4 μm. The average CCT was 540.6 ± 30.3 μm. The average keratometric power was 43.66 ± 1.32 D. There was a positive correlation between FT and CCT and no correlation between FT and keratometric power or between FT and age. We recommend that LASIK surgeons inspect the actual FT when using microkeratome.
Key Words: corneal flap thickness, corneal thickness, keratomileusis (Kaohsiung J Med Sci 2007;23:25–9)
Laser in situ keratomileusis (LASIK) has become a popular refractive surgery for patients with refractive error in recent years. After flap making using a microkeratome, it ablates the partial corneal stromal bed by excimer laser to correct refractive error. There is growing evidence that keratoectasia after LASIK may result from large variation in flap thickness (FT) [1]. Most surgeons do not routinely measure the actual thickness of the corneal flap or the residual stromal bed and treatment decisions are based on the FT announced by the microkeratome manufacturer. In fact, the real FT is variable after flap making [2–8]. To evaluate the variation in FT during the LASIK procedure, we measured the residual stromal bed by ultrasonic pachymeter after flap making with a microkeratome and calculated the actual corneal FT. We also analyzed the correlation
Received: July 6, 2006 Accepted: September 11, 2006 Address correspondence and reprint requests to: Dr Sheng-Yao Hsu, Department of Ophthalmology, Buddhist Tzu Chi General Hospital, 707, Section 3, Chung Yang Road, Hualien, Taiwan. E-mail: [email protected] Kaohsiung J Med Sci January 2007 • Vol 23 • No 1 © 2007 Elsevier. All rights reserved.
between FT and central corneal thickness (CCT), FT and patient age, and FT and keratometric power.
MATERIALS AND METHODS Sixty-two patients, 42 females and 20 males, with myopia or myopic astigmatism between March and August 2004 were enrolled. Baseline ophthalmic evaluation of the patients with myopic or myopic astigmatism included anterior segment and anterior vitreous by slit-lamp biomicroscopy; posterior vitreous, disc, and macula by slit-lamp biomicroscopy with 90-D lens; peripheral retina by indirect ophthalmoscopy; intraocular pressure by non-contact tonometer (CT-80, Topcon, Tokyo, Japan); CCT by ultrasonic pachymeter (MICROPACH 200P+, Sonomed, Lake Success, NY, USA); and keratometric power by autokeratorefractometer (KR-8100, Topcon) and topography (CT 200, Dicon, San Diego, CA, USA). Patients were excluded from the study if they were younger than 20 years, had a history of uveitis, 25
S.Y. Hsu
Flap thickness (μm)
160
y = 0.20x + 17.87 p = 0.001
140
120
100 500
525 550 575 Central corneal thickness (μm)
600
Figure 1. Positive correlation between central corneal thickness and flap thickness.
160
Flap thickness (μm)
glaucoma, ocular trauma, severe dry eye syndrome, collagen disease, and systemic disease, or drug allergy. The preoperative CCT was measured five times using an ultrasonic pachymeter before surgery, not intraoperatively, and the averaged data were noted. After flap making and lift, the stromal bed thickness was measured five times again at the center of the stromal bed using an ultrasonic pachymeter before laser ablation, and the averaged data were also noted. Corneal FT was calculated by subtracting the central corneal stromal thickness from the preoperative CCT. The corneal flaps were created using a MK-2000 (Nidek, Gamagori, Japan) 130 μm microkeratome and a 9 mm size suction ring. The same surgeon performed all of the procedures. The first and second eyes used the same blade for the same subject. Half of the patients were started with flap cutting of the right eye and the other half of the left eye. The FT of the first eye in all subjects was analyzed in this study. The analysis was carried out using a coefficient of variation (CV), which was defined as 100% × standard deviation/mean. Temperature was maintained between 20°C and 22°C, humidity was maintained between 45% and 55%, and barometric pressure was fixed between 65 and 70 mmHg to maintain a constant suction pressure. In this study, statistical analyses were performed by linear regression analysis using SPSS software (SPSS Inc., Chicago, IL, USA). Statistical significance was defined as p < 0.05.
y = – 0.61x + 153.45 p = 0.656
140
120
RESULTS There were 42 females and 20 males with a mean age of 27.6 ± 4.9 years; the mean age for females was 28.8 ± 5.1 and that for males was 27.5 ± 4.9. Mean CCT was 540.6 ± 30.3 μm (range, 495–598 μm). Mean keratometric power was 43.66 ± 1.32 D (range, 47.00–41.25 D). Among a total of 62 eyes, mean FT was 133.2 ± 15.4 μm (range, 114–162 μm). The CV of the FT was 11.6%. There was a positive correlation between CCT and FT (p = 0.001) (Figure 1) and no correlation between keratometric power and FT (p = 0.656) (Figure 2) or between age and FT (p = 0.623) (Figure 3).
DISCUSSION An ideal microkeratome should produce a corneal flap of the desired thickness consistently. Theoretically, 26
100 42
43 44 45 Keratometric power (D)
46
47
Figure 2. There was no significant correlation between keratometric power and flap thickness.
the distance between the fixed microkeratome plate and the edge of the metal blade determines the thickness of the flap during the flap cut [2]. In fact, several other variables are important in determining FT, such as the quality and the entry angle of the blade, translation and oscillation rate, the consistency across the cornea, suction ring pressure setting and suction duration, the mechanism of the cut, room humidity, preoperative CCT, and corneal diameter [2–8]. Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
Corneal flap thickness during LASIK
Flap thickness (μm)
160
y = 0.14x + 122.77 p = 0.623
140
120
100 20
30 40 Age at operation (yr)
Figure 3. There was no significant correlation between age and flap thickness.
In this study, CCT was measured before surgery, not intraoperatively, to reduce the risk of infection during the intraoperative stromal bed thickness measurement. Reviewing the literature where the same microkeratome as in this study was used, a similar FT was found to be achieved: 133.2 ± 15.4 μm. Naripthaphan and Vongthongsri reported a mean FT of 120.52 ± 16.49 μm (range, 84–162 μm) for an 8.5 mm suction ring and 122.06 ± 18.54 μm (range, 84–149 μm) for a 9.5 mm ring [5]. Arbelaez reported a mean FT of 122.46 ± 17.65 μm [7]. Several reports have confirmed a positive correlation between CCT and the cutting thickness, similar to our results. Flanagan and Binder showed, using the automated corneal shaper (ACS) or the Summit Krumeich Barraquer microkeratome (SKBM), that an increase in FT was associated with thicker preoperative pachymetry for both instruments [9]. Similar results have been reported by Jackson et al and Thompson et al for the Amadeus microkeratome, Yi and Joo for the SCMD manual microkeratome, Choi et al for the Innovatome automatic microkeratome, and Gailitis and Lagzdins for the Hansatome microkeratome [6,10–13]. The reason for this may be that a thicker cornea is more compressible in the superficial corneal area than a thinner cornea [8]. Flanagan and Binder reported that a steeper keratometry reading was associated with thinner preoperative corneas. They also reported that a steeper cornea was associated with thinner flaps using the Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
ACS microkeratome but with thicker flaps using the SKBM. Furthermore, they also reported that an increase in patient age was found to be associated with thinner flaps for both microkeratomes [9]. In this study, there was no correlation between keratometric power and FT or between age and FT. Although the first and second eyes used the same blade for the same subject when using the microkeratome, only the FTs of the first eyes in all subjects were analyzed in this study to investigate the actual FT and the variation of the MK-2000 microkeratome for the first cut. Previous reports have shown that a thinner flap results from the second cut using the same blade, and the main reason for this may be due to the fact that the blade becomes duller after the first cut [3,8]. In this study, the FTs still showed variation in a similar way to previous reports and, therefore, it is recommended that LASIK surgeons inspect the actual FT when using a microkeratome. It is also suggested that there be routine measurement of the corneal stromal bed thickness intraoperatively to ensure that enough tissue remains after surgery.
REFERENCES 1.
2.
3. 4.
5.
6.
7. 8.
9.
Genth U, Mrochen M, Walti R, et al. Optical low coherence reflectometry for noncontact measurements of flap thickness during laser in situ keratomileusis. Ophthalmology 2002;109:973–8. Seiler T, Koufala K, Richter G. Iatrogenic keratoectasia after laser in situ keratomileusis. J Refract Surg 1998;14: 312–7. Schultze RL. Microkeratome update. Int Ophthalmol Clin 2002;42:55–65. Miranda D, Smith SD, Krueger RR. Comparison of flap thickness reproducibility using microkeratomes with a second motor for advancement. Ophthalmology 2003; 110:1931–4. Naripthaphan P, Vongthongsri A. Evaluation of the reliability of the Nidek MK-2000 microkeratome for laser in situ keratomileusis. J Refract Surg 2001;17:S255–8. Gailitis RP, Lagzdins M. Factors that affect corneal flap thickness with the Hansatome microkeratome. J Refract Surg 2002;18:439–43. Arbelaez MC. Nidek MK 2000 microkeratome clinical evaluation. J Refract Surg 2002;18:S357–60. Seo KY, Wan XH, Jang JW, et al. Effect of microkeratome suction duration on corneal flap thickness and incision angle. J Refract Surg 2002;18:715–9. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in 4428 eyes. J Refract Surg 2003;19:113–23.
27
S.Y. Hsu 10. Jackson DW, Wang L, Koch DD. Accuracy and precision of the Amadeus microkeratome in producing LASIK flaps. Cornea 2003;22:504–7. 11. Thompson RW, Choi DM, Price MO, et al. Noncontact optical coherence tomography for measurement of corneal flap and residual stromal bed thickness after laser in situ keratomileusis. J Refract Surg 2003;19:507–15.
28
12. Yi WM, Joo CK. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999;25:1087–92. 13. Choi YI, Park SJ, Song BJ. Corneal flap dimensions in laser in situ keratomileusis using the Innovatome automatic microkeratome. Korean J Ophthalmol 2000; 14:7–11.
Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
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Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
29
To analyze the actual corneal flap thickness (FT) after flap making by MK-2000 microkeratome during laser in situ keratomileusis (LASIK), 42 females and 20 males with myopia or myopic astigmatism were enrolled in this study. FTs were created using a microkeratome with a 130 μm head. Corneal thickness was measured by ultrasonic pachymeter. The correlations between FT and central corneal thickness (CCT), keratometric power and age were analyzed. The mean age at operation for all study subjects was 27.6 ± 4.9 years. The average FT was 133.2 ± 15.4 μm. The average CCT was 540.6 ± 30.3 μm. The average keratometric power was 43.66 ± 1.32 D. There was a positive correlation between FT and CCT and no correlation between FT and keratometric power or between FT and age. We recommend that LASIK surgeons inspect the actual FT when using microkeratome.
Key Words: corneal flap thickness, corneal thickness, keratomileusis (Kaohsiung J Med Sci 2007;23:25–9)
Laser in situ keratomileusis (LASIK) has become a popular refractive surgery for patients with refractive error in recent years. After flap making using a microkeratome, it ablates the partial corneal stromal bed by excimer laser to correct refractive error. There is growing evidence that keratoectasia after LASIK may result from large variation in flap thickness (FT) [1]. Most surgeons do not routinely measure the actual thickness of the corneal flap or the residual stromal bed and treatment decisions are based on the FT announced by the microkeratome manufacturer. In fact, the real FT is variable after flap making [2–8]. To evaluate the variation in FT during the LASIK procedure, we measured the residual stromal bed by ultrasonic pachymeter after flap making with a microkeratome and calculated the actual corneal FT. We also analyzed the correlation
Received: July 6, 2006 Accepted: September 11, 2006 Address correspondence and reprint requests to: Dr Sheng-Yao Hsu, Department of Ophthalmology, Buddhist Tzu Chi General Hospital, 707, Section 3, Chung Yang Road, Hualien, Taiwan. E-mail: [email protected] Kaohsiung J Med Sci January 2007 • Vol 23 • No 1 © 2007 Elsevier. All rights reserved.
between FT and central corneal thickness (CCT), FT and patient age, and FT and keratometric power.
MATERIALS AND METHODS Sixty-two patients, 42 females and 20 males, with myopia or myopic astigmatism between March and August 2004 were enrolled. Baseline ophthalmic evaluation of the patients with myopic or myopic astigmatism included anterior segment and anterior vitreous by slit-lamp biomicroscopy; posterior vitreous, disc, and macula by slit-lamp biomicroscopy with 90-D lens; peripheral retina by indirect ophthalmoscopy; intraocular pressure by non-contact tonometer (CT-80, Topcon, Tokyo, Japan); CCT by ultrasonic pachymeter (MICROPACH 200P+, Sonomed, Lake Success, NY, USA); and keratometric power by autokeratorefractometer (KR-8100, Topcon) and topography (CT 200, Dicon, San Diego, CA, USA). Patients were excluded from the study if they were younger than 20 years, had a history of uveitis, 25
S.Y. Hsu
Flap thickness (μm)
160
y = 0.20x + 17.87 p = 0.001
140
120
100 500
525 550 575 Central corneal thickness (μm)
600
Figure 1. Positive correlation between central corneal thickness and flap thickness.
160
Flap thickness (μm)
glaucoma, ocular trauma, severe dry eye syndrome, collagen disease, and systemic disease, or drug allergy. The preoperative CCT was measured five times using an ultrasonic pachymeter before surgery, not intraoperatively, and the averaged data were noted. After flap making and lift, the stromal bed thickness was measured five times again at the center of the stromal bed using an ultrasonic pachymeter before laser ablation, and the averaged data were also noted. Corneal FT was calculated by subtracting the central corneal stromal thickness from the preoperative CCT. The corneal flaps were created using a MK-2000 (Nidek, Gamagori, Japan) 130 μm microkeratome and a 9 mm size suction ring. The same surgeon performed all of the procedures. The first and second eyes used the same blade for the same subject. Half of the patients were started with flap cutting of the right eye and the other half of the left eye. The FT of the first eye in all subjects was analyzed in this study. The analysis was carried out using a coefficient of variation (CV), which was defined as 100% × standard deviation/mean. Temperature was maintained between 20°C and 22°C, humidity was maintained between 45% and 55%, and barometric pressure was fixed between 65 and 70 mmHg to maintain a constant suction pressure. In this study, statistical analyses were performed by linear regression analysis using SPSS software (SPSS Inc., Chicago, IL, USA). Statistical significance was defined as p < 0.05.
y = – 0.61x + 153.45 p = 0.656
140
120
RESULTS There were 42 females and 20 males with a mean age of 27.6 ± 4.9 years; the mean age for females was 28.8 ± 5.1 and that for males was 27.5 ± 4.9. Mean CCT was 540.6 ± 30.3 μm (range, 495–598 μm). Mean keratometric power was 43.66 ± 1.32 D (range, 47.00–41.25 D). Among a total of 62 eyes, mean FT was 133.2 ± 15.4 μm (range, 114–162 μm). The CV of the FT was 11.6%. There was a positive correlation between CCT and FT (p = 0.001) (Figure 1) and no correlation between keratometric power and FT (p = 0.656) (Figure 2) or between age and FT (p = 0.623) (Figure 3).
DISCUSSION An ideal microkeratome should produce a corneal flap of the desired thickness consistently. Theoretically, 26
100 42
43 44 45 Keratometric power (D)
46
47
Figure 2. There was no significant correlation between keratometric power and flap thickness.
the distance between the fixed microkeratome plate and the edge of the metal blade determines the thickness of the flap during the flap cut [2]. In fact, several other variables are important in determining FT, such as the quality and the entry angle of the blade, translation and oscillation rate, the consistency across the cornea, suction ring pressure setting and suction duration, the mechanism of the cut, room humidity, preoperative CCT, and corneal diameter [2–8]. Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
Corneal flap thickness during LASIK
Flap thickness (μm)
160
y = 0.14x + 122.77 p = 0.623
140
120
100 20
30 40 Age at operation (yr)
Figure 3. There was no significant correlation between age and flap thickness.
In this study, CCT was measured before surgery, not intraoperatively, to reduce the risk of infection during the intraoperative stromal bed thickness measurement. Reviewing the literature where the same microkeratome as in this study was used, a similar FT was found to be achieved: 133.2 ± 15.4 μm. Naripthaphan and Vongthongsri reported a mean FT of 120.52 ± 16.49 μm (range, 84–162 μm) for an 8.5 mm suction ring and 122.06 ± 18.54 μm (range, 84–149 μm) for a 9.5 mm ring [5]. Arbelaez reported a mean FT of 122.46 ± 17.65 μm [7]. Several reports have confirmed a positive correlation between CCT and the cutting thickness, similar to our results. Flanagan and Binder showed, using the automated corneal shaper (ACS) or the Summit Krumeich Barraquer microkeratome (SKBM), that an increase in FT was associated with thicker preoperative pachymetry for both instruments [9]. Similar results have been reported by Jackson et al and Thompson et al for the Amadeus microkeratome, Yi and Joo for the SCMD manual microkeratome, Choi et al for the Innovatome automatic microkeratome, and Gailitis and Lagzdins for the Hansatome microkeratome [6,10–13]. The reason for this may be that a thicker cornea is more compressible in the superficial corneal area than a thinner cornea [8]. Flanagan and Binder reported that a steeper keratometry reading was associated with thinner preoperative corneas. They also reported that a steeper cornea was associated with thinner flaps using the Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
ACS microkeratome but with thicker flaps using the SKBM. Furthermore, they also reported that an increase in patient age was found to be associated with thinner flaps for both microkeratomes [9]. In this study, there was no correlation between keratometric power and FT or between age and FT. Although the first and second eyes used the same blade for the same subject when using the microkeratome, only the FTs of the first eyes in all subjects were analyzed in this study to investigate the actual FT and the variation of the MK-2000 microkeratome for the first cut. Previous reports have shown that a thinner flap results from the second cut using the same blade, and the main reason for this may be due to the fact that the blade becomes duller after the first cut [3,8]. In this study, the FTs still showed variation in a similar way to previous reports and, therefore, it is recommended that LASIK surgeons inspect the actual FT when using a microkeratome. It is also suggested that there be routine measurement of the corneal stromal bed thickness intraoperatively to ensure that enough tissue remains after surgery.
REFERENCES 1.
2.
3. 4.
5.
6.
7. 8.
9.
Genth U, Mrochen M, Walti R, et al. Optical low coherence reflectometry for noncontact measurements of flap thickness during laser in situ keratomileusis. Ophthalmology 2002;109:973–8. Seiler T, Koufala K, Richter G. Iatrogenic keratoectasia after laser in situ keratomileusis. J Refract Surg 1998;14: 312–7. Schultze RL. Microkeratome update. Int Ophthalmol Clin 2002;42:55–65. Miranda D, Smith SD, Krueger RR. Comparison of flap thickness reproducibility using microkeratomes with a second motor for advancement. Ophthalmology 2003; 110:1931–4. Naripthaphan P, Vongthongsri A. Evaluation of the reliability of the Nidek MK-2000 microkeratome for laser in situ keratomileusis. J Refract Surg 2001;17:S255–8. Gailitis RP, Lagzdins M. Factors that affect corneal flap thickness with the Hansatome microkeratome. J Refract Surg 2002;18:439–43. Arbelaez MC. Nidek MK 2000 microkeratome clinical evaluation. J Refract Surg 2002;18:S357–60. Seo KY, Wan XH, Jang JW, et al. Effect of microkeratome suction duration on corneal flap thickness and incision angle. J Refract Surg 2002;18:715–9. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in 4428 eyes. J Refract Surg 2003;19:113–23.
27
S.Y. Hsu 10. Jackson DW, Wang L, Koch DD. Accuracy and precision of the Amadeus microkeratome in producing LASIK flaps. Cornea 2003;22:504–7. 11. Thompson RW, Choi DM, Price MO, et al. Noncontact optical coherence tomography for measurement of corneal flap and residual stromal bed thickness after laser in situ keratomileusis. J Refract Surg 2003;19:507–15.
28
12. Yi WM, Joo CK. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999;25:1087–92. 13. Choi YI, Park SJ, Song BJ. Corneal flap dimensions in laser in situ keratomileusis using the Innovatome automatic microkeratome. Korean J Ophthalmol 2000; 14:7–11.
Kaohsiung J Med Sci January 2007 • Vol 23 • No 1
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29