Flap thickness accuracy

articles Flap thickness accuracy: Comparison of 6 microkeratome models Kerry D. Solomon, MD, Eric Donnenfeld, MD, Helga P. Sandoval, MD, Oday Al Sarraf, Terrance J. Kasper, MD, Mike P. Holzer, MD, Elizabeth H. Slate, PhD, David T. Vroman, MD, Flap Thickness Study Group Purpose: To determine the flap thickness accuracy of 6 microkeratome models and determine factors that might affect flap thickness. Setting: Magill Research Center for Vision Correction, Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA. Methods: This multicenter prospective study involved 18 surgeons. Six microkeratomes were evaluated: AMO Amadeus, Bausch & Lomb Hansatome威, Moria Carriazo-Barraquer, Moria M2, Nidek MK2000, and Alcon Summit Krumeich-Barraquer. Eyes of 1061 consecutive patients who had laser in situ keratomileusis were included. Age, sex, surgical order (first or second cut), keratometry (flattest, steepest, and mean), white-to-white measurement, laser used, plate thickness, head serial number, blade lot number, and occurrence of epithelial defects were recorded. Intraoperative pachymetry was obtained just before the microkeratome was placed on the eye. Residual bed pachymetry was measured after the microkeratome cut had been created and the flap lifted. The estimated flap thickness was determined by subtraction (ie, mean preoperative pachymetry measurement minus mean residual bed pachymetry). Results: A total of 1634 eyes were reviewed. Sex distribution was 54.3% women and 45.7% men, and the mean age was 39.4 years ⫾ 10.6 (SD). In addition, 54.5% of the procedures were in first eyes and 45.5%, in second eyes. The mean preoperative pachymetry measurement was 547 ⫾ 34 ␮m. The mean keratometry was 43.6 ⫾ 1.6 diopters (D) in the flattest axis and 44.6 ⫾1.5 D in the steepest axis. The mean white-to-white measurement was 11.7 ⫾ 0.4 mm. The mean flap thickness created by the devices varied between head designs, and microkeratome heads had significant differences (P⬍.05). Factors that explained 78.4% of the variability included microkeratome model, plate thickness, mean preoperative pachymetry, Kmin, surgery order, head serial number, blade lot number, and surgeon. Factors such as age, sex, Kmax, Kaverage, white to white, and laser had no significant correlation to flap thickness. Conclusions: The results demonstrated variability between the 6 microkeratome models. Device labeling did not necessarily represent the mean flap thickness obtained, nor was it uniform or consistent. Thinner corneas were associated with thinner flaps and thicker corneas with thicker flaps. In addition, first cuts were generally associated with thicker flaps when compared to second cuts in bilateral procedures. J Cataract Refract Surg 2004; 30:964–977  2004 ASCRS and ESCRS

L

aser in situ keratomileusis (LASIK) has proved to be a remarkably accurate and safe surgical technique to correct ametropia, but there are risks associated with  2004 ASCRS and ESCRS Published by Elsevier Inc.

the surgery. A study by the Magill Research Center for Vision Correction and the American Society of Cataract and Refractive Surgery (ASCRS) (K. Solomon, MD, 0886-3350/04/$–see front matter doi:10.1016/j.jcrs.2004.01.023

FLAP THICKNESS ACCURACY

“2002 Refractive Surgery Survey,” presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, Philadelphia, Pennsylvania, USA, June 2002) indicated that corneal ectasia is the most common reason for patients who have had LASIK to require corneal transplantation. Barraquer’s original recommendation was to leave 300 ␮m of residual corneal tissue,1 but the present consensus is to leave a minimum residual bed of 250 ␮m to minimize the likelihood of ectasia or prevent its occurrence.2–4 Surgeons often modify the optic zone size, blend zone size, and the laser used (if available) to maintain the threshold of 250 ␮m and optimize treatment in particular patients. However, when planning corneal surgery, most surgeons use the microkeratome labeling and the laser ablation depth specific to each procedure to determine the overall depth of surgery and preserve the standard of 250 ␮m of residual corneal tissue. This study compared the flap thickness accuracy of common microkeratome models when used by different surgeons to define isolated variables that could affect flap thickness and to promote standardized labeling of microkeratome devices.

Accepted for publication January 21, 2004. From the Magill Research Center for Vision Correction, Storm Eye Institute (Solomon, Sandoval, Al Sarraf, Kasper, Holzer, Vroman), and the Department of Biometry and Epidemiology (Slate), Medical University of South Carolina, Charleston, South Carolina; the Department of Ophthalmology, Nassau University Medical Center (Donnenfeld), East Meadow, Manhattan Eye, Ear and Throat Hospital (Donnenfeld), New York, and Ophthalmic Consultants of Long Island (Donnenfeld), Rockville Centre, New York, USA. Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, Philadelphia, Pennsylvania, June 2002, and the XX Congress of the European Society of Cataract & Refractive Surgeons, Nice, France, September 2002. Supported in part by the ASCRS Foundation, Alcon Laboratories, Advanced Medical Optics, Bausch & Lomb, and Research to Prevent Blindness. None of the authors has a financial or proprietary interest in any product mentioned. James P. Byrnes provided information technology support and Suzanne Johnston, data entering. Reprint requests to Kerry D. Solomon, MD, Magill Research Center for Vision Correction, MUSC—Storm Eye Institute, 167 Ashley Avenue, Charleston, South Carolina 29425, USA. E-mail: [email protected].

Table 1. Total number of surgeons using each microkeratome. Microkeratome

Number of Surgeons

Amadeus

6

Hansatome

9

CB

1

M2

2

MK 2000

4

SKBM

2

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

Patients and Methods This multicenter prospective study involved 18 surgeons from refractive laser surgery centers in the United States and Mexico. Table 1 shows the number of surgeons using each of 6 microkeratomes. All consecutive patients who had LASIK surgery from March 1, 2002, through April 15, 2002, were included. Intraoperatively, the central pachymetry was measured over the center of the pupil by the surgeon using 1 of 2 types of ultrasound pachymeters (20 MHz Pachette 2, DGH Technology Inc., or 50 MHz Corneal Gage Plus 2, Sonogage), single reading and multi-reading. The accuracy is ⫾0.5 ␮m and ⫾0.4 ␮m with the Pachette 2 and the Corneal Gage, respectively. With a single-reading pachymeter, 3 measurements were taken; if the range of the 3 values was greater than 10 ␮m, 2 more readings were recorded. With a multireading pachymeter, a 15-reading measurement mean was documented. Each surgeon was allowed to use his or her techniques and instruments of choice to create and lift the corneal flap. A pachymetry reading over the center of the pupil of the unmoistened stromal bed was taken (bed measurement) using the same parameters as before the cut. The estimated flap thickness was calculated by subtraction; ie, the mean bed measurement was subtracted from the mean intraoperative pachymetry. After laser ablation, the flap was repositioned and evaluated after surgery according to each surgeon’s standard of care. The microkeratomes evaluated were the AMO Amadeus, 140 and 160 plates; Bausch & Lomb Surgical Hansatome威, 160 and 180 plates; Moria Carriazo-Barraquer, 110, 130, and 150 plates; Moria M2, 160 plate; Nidek MK2000 130, 145, and 160 plates; and Alcon Summit Krumeich-Barraquer (SKBM), 160 plate. Parameters related to the microkeratome included plate thickness, head serial number, ring size, oscillation speed, translation speed, and suction ring. The former 2 parameters were standardized, as illustrated in Table 2. Other factors such as age, sex, surgical order (first or second cut; if a new blade was used to create the flap in the second eye, it was considered a first cut), keratometry (flattest,

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Table 2. Standardization of microkeratome oscillation and translation speeds and ring suction. Oscillation Speed (rpm)

Translation Speed (mm/s)

Amadeus

10 000

3.0

Hansatome

Preset

Preset

CB

15 000

4 s pass

M2

Preset

3.3

9 000

2.0

16 000

2.5

Microkeratome

MK 2000 SKBM

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

steepest, and mean) measured using topography (Orbscan, Bausch & Lomb Surgical, or EyeSys corneal topographer, EyeSys Vision), white-to-white measurement (using Orbscan or calipers), laser (LADARVision威 4000, Alcon Laboratories; Nidek EC-5000, Nidek Inc.; Visx S2/S3), blade lot number, intraoperative pachymetry, and occurrence of epithelial defects were assessed. Epithelial defects were recorded as none, small (less than 10% of the flap epithelial surface area), medium (10% to 25%), or large (greater than 25%). All recorded information was tabulated in Microsoft威 Access 2002 database. All statistical analyses were performed using STATA Version 7 (Stata Corp.) or SAS version 8.02 (SAS Institute, Inc.). Factors that are predictive of flap thickness were determined using a forward-selection multivariable regression analysis.5 Because of the dependence among Kmin, Kmax, and Kav and because Kmin had the greatest marginal correlation with flap thickness, preference was given to the

inclusion of Kmin before the other 2 keratometry variables. Also, because of the known association between ring size and keratometry,6 preference was given to the inclusion of Kmin over ring size. The multivariable regression analyses included all eyes for which the required variables were recorded, resulting in the number of eyes for analyses ranging from all eyes (1634) to 1091 eyes for the model, including the following variables: microkeratome, plate, suction, head serial number, mean preoperative pachymetry, Kmin, blade lot number, surgery order, and surgeon. The suction ring variable contained the largest number of missing observations (430). Statistical tests used for computing P values are indicated in the text. A P value less than 0.05 was considered statistically significant. All P values reported are 2-sided unless indicated.

Results A total of 1634 eyes of 1061 patients were enrolled in the study. The sex distribution of the database was 54.3% women and 45.7% men. The mean patient age was 39.4 years ⫾ 10.6 (SD). The overall mean intraoperative pachymetry was 547 ⫾ 34 ␮m. The surgical order was 54.5% first cut and 45.5% second cut. The mean flap thickness for each microkeratome model per plate thickness is shown in Table 3. The mean keratometry was 43.6 ⫾ 1.6 D in the minimum axis and 44.6 ⫾ 1.5 D in the maximum axis. The mean white-to-white measurement was 11.7 ⫾ 0.4 mm. Table 4 summarizes the mean flap thickness by head serial number and plate thickness.

Table 3. Microkeratome mean flap thickness per plate thickness and sample size. Microkeratome Model Amadeus

Hansatome

CB

Plate Thickness (␮m)*

Number of Eyes†

Mean Flap Thickness ⫾ SD (␮m)

130-2

62

147 ⫾ 24

140

207

134 ⫾ 15

160

169

180 ⫾ 35

160

194

129 ⫾ 21

180

283

136 ⫾ 25

110

13

165 ⫾ 27

130

147

198 ⫾ 26

M2

130

140

146 ⫾ 32

MK2000

130

118

111 ⫾ 19

145

61

103 ⫾ 15

160

81

121 ⫾ 20

160

149

143 ⫾ 23

SKBM

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome *Unknown plates † Plates with n⬍10 eyes were not included.

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Table 4. Microkeratome mean flap thickness per plate thickness and head serial number.

Microkeratome Amadeus

Plate Thickness (␮m)

Head Serial Number*

Number of Eyes

130-2

A13021

36

130130

7

14320018

7

14321418

†

160

Hansatome

†

160

180†

Flap Thickness Minimum (␮m)

Maximum (␮m)

147 ⫾ 26

78

187

149 ⫾ 24

116

180

161 ⫾ 15

149

191

7

142 ⫾ 20

122

169

5

134 ⫾ 18

109

155

14321315

144

130 ⫾ 11

108

181

140140

26

138 ⫾ 22

92

174

A13022 140†

Mean ⫾ SD (␮m)

14321425

19

147 ⫾ 20

96

173

13140

11

143 ⫾ 15

122

176

14321337

5

145 ⫾ 9

133

158

14320425

2

167 ⫾ 13

158

176

320328

65

155 ⫾ 26

60

188

432920

25

197 ⫾ 23

148

239

A1

17

200 ⫾ 34

139

244

A2

11

211 ⫾ 24

178

246

A3

7

167 ⫾ 39

107

220

13160

3

202 ⫾ 12

194

215

4320734

2

175 ⫾ 17

163

187

2723

153

132 ⫾ 21

53

181

4415

13

118 ⫾ 14

102

143

2652

6

128 ⫾ 26

93

154

2803

5

101 ⫾ 20

72

122

3973

4

100 ⫾ 18

82

118

H1

4

135 ⫾ 12

124

152

H2

3

124 ⫾ 41

86

167

2231

2

119 ⫾ 22

104

135

3110

2

118 ⫾ 20

104

132

4551

2

131 ⫾ 25

114

149

4415

43

140 ⫾ 18

102

186

180180

41

148 ⫾ 22

113

200

2803

40

114 ⫾ 12

87

139

H2

40

134 ⫾ 22

81

170

H1

36

152 ⫾ 25

83

199

3973

29

121 ⫾ 21

67

160

4551

26

128 ⫾ 27

78

181

2652

15

147 ⫾ 24

119

194

3110

9

157 ⫾ 32

128

212

4532

2

131 ⫾ 8

125

136

1140

2

159 ⫾ 6

154

163

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Table 4. (cont.)

Microkeratome CB

Plate Thickness (␮m)

MK 2000

SKBM

Number of Eyes

Flap Thickness

Mean ⫾ SD (␮m)

Minimum (␮m)

Maximum (␮m)

110

297

13

165 ⫾ 27

108

209

130†

1505

81

203 ⫾ 26

70

253

32

66

191 ⫾ 24

74

233

422

6

219 ⫾ 20

199

252

150 M2

Head Serial Number*

†

130

130

131

144 ⫾ 30

59

230

1142

9

178 ⫾ 41

110

248

20001

118

111 ⫾ 19

59

217

E0598

145

20001

61

103 ⫾ 15

72

131

160†

20001

58

116 ⫾ 18

69

161

421

17

141 ⫾ 16

116

182

205

4

127 ⫾ 14

114

147

230

2

106 ⫾ 13

96

115

192

31

146 ⫾ 27

91

204

162

29

148 ⫾ 10

123

164

191

27

140 ⫾ 21

101

180

160†

A009H

25

120 ⫾ 16

95

158

174

19

154 ⫾ 15

125

176

A132H

7

178 ⫾ 10

166

191

62

1

166

166

166

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome *Heads without known plate thickness were not included. † Mean flap thicknesses for the head serial number differ significantly (P ⫽ .007) in each marked case.

The flap thickness means and standard deviations reported in Table 3 (by microkeratome and plate only) are computed by combining data across all head serial numbers and ring sizes. Table 4 subdivides the summaries by head serial number. As shown in Table 4, there was significant variation in the mean flap thickness across head serial numbers for some microkeratome–plate combinations (eg, the Amadeus 140); moreover, there are small numbers of eyes with some combinations. There is therefore a potential for the data in Table 3 to be influenced by the measurements from head serial numbers used in only a few eyes. However, this did not appear to be the case. The Amadeus 140, for example, has 3 head serial numbers with 2, 5, and 11 eyes, respectively. Eliminating these 3 head serial numbers from the summary in Table 3 leads to a mean ⫾ SD flap thickness of 133 ⫾ 15 ␮m based on 189 eyes. Removing the head serial number with 19 eyes yields a mean flap thickness of 133 ⫾ 14 ␮m. Another exam968

ple is the Amadeus 160, which has 4 head serial numbers with fewer than 15 eyes. When these 4 heads are removed, the mean flap thickness is 178 ⫾ 34 ␮m in 146 eyes. The summaries of flap thickness reported in Table 3 are largely determined by a small number (sometimes 1) of head serial numbers used for most eyes and are not influenced substantially by head serial numbers used with a few eyes. Furthermore, both the sample mean and the sample standard deviation are such that their precision is approximately proportional to sqrt(n), where n is the number of eyes. For example, the precision of the mean and standard deviation (SD) based on 25 eyes is about half that based on 100 eyes. As an illustration, consider interval estimates for the true mean flap thickness for the Amadeus 140 with head serial number 14321315 (n ⫽ 144) and with head serial number 14321337 (n ⫽ 5). The 90% confidence intervals (CIs) for the mean flap thickness are 129-132 ␮m and 137-154 ␮m, re-

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Table 5. The 95% and 99.7% (2 and 3 SDs, respectively) tolerance intervals per microkeratome, plate thickness, and head serial number. Microkeratome Amadeus

Plate Thickness (␮m)

Head Serial Number*

Mean ⫾ SD (␮m)

95% range (␮m)

99% range (␮m)

130-2

A13021

147 ⫾ 26

95-199

69-225

140

14321315

130 ⫾ 11

108-152

97-163

140140

138 ⫾ 22

94-182

72-204

14321425

147 ⫾ 20

107-187

87-207

160

Hansatome

160

180

CB

13140

143 ⫾ 15

113-173

98-188

320328

155 ⫾ 26

103-207

77-233

432920

197 ⫾ 23

151-243

128-266

A1

200 ⫾ 34

132-268

98-302

A2

211 ⫾ 24

163-259

139-283

2723

132 ⫾ 21

90-174

69-195

4415

118 ⫾ 14

90-146

76-160

4415

140 ⫾ 18

140-176

86-194

180180

148 ⫾ 22

104-192

82-214

2803

114 ⫾ 12

90-138

78-150

H2

134 ⫾ 22

90-178

68-200

H1

152 ⫾ 25

102-202

77-227

3973

121 ⫾ 21

79-163

58-184

4551

127 ⫾ 27

73-181

46-208

2652

147 ⫾ 24

99-195

75-219

110

297

165 ⫾ 27

111-219

84-246

130

1505

203 ⫾ 26

151-255

125-281

32

191 ⫾ 24

143-239

119-263

M2

130

E0598

144 ⫾ 20

104-184

84-204

MK 2000

130

20001

111 ⫾ 19

73-149

54-168

145

20001

103 ⫾ 14

75-131

61-145

160

20001

116 ⫾ 18

80-152

62-170

421

141 ⫾ 16

109-173

93-189

192

146 ⫾ 27

92-200

65-227

162

148 ⫾ 10

128-168

118-178

191

140 ⫾ 21

98-182

77-203

A009H

120 ⫾ 16

88-152

72-168

174

154 ⫾ 15

124-184

109-199

SKBM

160

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome *Head serial numbers with n⬍10 eyes were not included.

spectively, the greater width of the second interval reflecting the greater uncertainty because of the small number of eyes. Similarly, the 90% CIs for the true SDs of the flap thickness for each system based on the sample SDs are 10.1-12.2 ␮m and 5.9-21.5 ␮m,

respectively. The CI for the mean flap thickness discussed here is not to be confused with the tolerance intervals reported in Table 5. A multivariable analysis was used to determine factors that could explain variability in flap thickness. Us-

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Table 6. Mean flap thickness per microkeratome model, plate, and ring size. Microkeratome

Plate

Ring Size (mm)

Number of Eyes*

Mean Flap Thickness (␮m)

Amadeus

130-2

8.5

39

151 ⫾ 20

9.5

23

141 ⫾ 28

8.5

125

133 ⫾ 14

9.5

79

136 ⫾ 17

140

160

Hansatome

160

180

CB

M2

MK 2000

SKBM

8.5

127

181 ⫾ 34

9.5

41

175 ⫾ 37

8.5

138

129 ⫾ 21

9.5

55

131 ⫾ 22

8.5

108

142 ⫾ 24

9.5

172

133 ⫾ 25

110

0

7

167 ⫾ 34

130

0

79

201 ⫾ 25

1

31

193 ⫾ 29

⫺1

32

195 ⫾ 22

2

5

195 ⫾ 31

0

80

146 ⫾ 35

1

35

149 ⫾ 27

2

24

137 ⫾ 27

130

130

8.5

116

111 ⫾ 19

145

9.0

60

103 ⫾ 15

160

8.5

20

130 ⫾ 23

9.0

8

124 ⫾ 20

9.5

53

118 ⫾ 19

20.0

149

143 ⫾ 23

160

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome *Unknown ring size or number of eyes⬍5 were not included.

ing this analysis, it was determined that 78.4% of the flap thickness variability was due to the following factors: microkeratome model, plate thickness and head serial number, mean intraoperative pachymetry, Kmin, surgery order, suction ring, blade lot, and surgeon. Factors such as age, sex, Kmax, Kaverage, white to white, oscillation and translation speeds, ring size, and laser did not correlate with flap thickness. In Table 6, the mean flap thicknesses are reported by microkeratome and plate and ring size and are computed by combining head serial numbers. Subdividing the data further by head serial number would lead to some very small sample sizes, producing highly unreliable summaries. However, the influence of head serial numbers with only a few eyes on the summaries in 970

Table 6 were assessed by eliminating these heads. For example, for the Amadeus 140, using only those ring– head serial number combinations with 10 or more eyes yields a mean flap thickness of 132 ⫾ 14 ␮m for ring size 8.5 mm (based on 118 eyes for 2 head identifications) and 135 ⫾ 17 ␮m for ring size 9.5 mm (based on 69 eyes for 3 head identifications), little change from the values reported in Table 6. As another example, for the Amadeus 160, retaining only those head serial numbers with 10 or more eyes yields mean ⫾ SD flap thicknesses of 174 ⫾ 33 ␮m for ring size 8.5 mm (based on 84 eyes for 3 head IDs) and 174 ⫾ 40 ␮m for ring size 9.5 mm (based on 26 eyes for 2 head IDs). The decrease in mean flap thickness for ring size 8.5 results from the omission of eyes for which the head

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Table 7. Mean flap thickness per microkeratome model, plate, and laser. Microkeratome

Plate

Laser

Number of Eyes

Mean Flap Thickness

Amadeus

130-2

Visx

57

147 ⫾ 24

LADARVision

5

143 ⫾ 20

205

134 ⫾ 15

2

144 ⫾ 20

160

181 ⫾ 34

9

171 ⫾ 37

188

130 ⫾ 21

6

111 ⫾ 24

Visx

224

139 ⫾ 25

LADARVision

59

125 ⫾ 24

140

Visx LADARVision

160

Visx LADARVision

Hansatome

160

Visx LADARVision

180

CB

110

130

M2

130

Visx

7

148 ⫾ 21

LADARVision

6

186 ⫾ 15

Visx

49

193 ⫾ 27

LADARVision

98

200 ⫾ 25

Visx

134

145 ⫾ 32

6

159 ⫾ 33

LADARVision MK2000

SKBM

130

Nidek

118

111 ⫾ 19

145

Nidek

61

103 ⫾ 15

160

Nidek

58

116 ⫾ 18

Visx

23

135 ⫾ 19

Visx

69

144 ⫾ 24

LADARVision

80

143 ⫾ 22

160

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

serial number is missing (elimination of these 36 eyes leads to a mean flap thickness of 175 ⫾ 33). Table 7 summarizes the result by laser, and Table 8 illustrates the correlation between flap thickness and intraoperative pachymetry. Data from patients who had bilateral surgery (n ⫽ 573) using the same blade showed a significant difference in corneal flap thickness between the first and second operated eyes (P⬍ .001, paired t test). The mean flap thicknesses indicated a 6% decrease between first and second eyes from 150 ⫾ 35 ␮m to 141 ⫾ 36 ␮m, respectively (Figure 1). Results by microkeratome model are shown in Table 9. Overall, 8.0% of the eyes developed epithelial defects. Table 10 shows epithelial defects per microkeratome model. The epithelial defect was small in 75.5% of eyes, medium in 20.7% , and large in 3.8%.

The mean age of patients differed significantly between surgeries that did (mean age 45.2 ⫾ 9.9 years) and did not (mean age 38.8 ⫾ 10.5 years) yield epithelial defects; the older the patient, the higher the epithelial defect rate (P⬍.001, 2-sample t test). The mean Kmin and Kave values differed between surgeries with and without epithelial defects; larger values were associated with more epithelial defects (P ⫽ .002 and P ⫽ .014, respectively, 1-way ANOVA test). There was no relation between white to white, Kmax, and the occurrence of epithelial defects. In general, epithelial defects occurred more frequently in the second operated eye than in the first (P ⫽ .029, 1-sided Fisher exact test), as shown in Figure 2. When evaluated by microkeratome model, there was no significant difference between the models, as shown in Table 11. There was no significant difference in the size of the epithelial defect between the first and second cut (P ⫽ .21, Fisher exact test).

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Table 8. Correlation between flap thickness and mean intraoperative pachymetry for each microkeratome model and plate thickness. Number of Eyes†

Mean Intraoperative Pachymetry (␮m)

Mean Flap Thickness (␮m)

Correlation

P Value

547 ⫾ 34

147 ⫾ 24

0.443

.000

533 ⫾ 27

134 ⫾ 15

0.373

.000

540 ⫾ 28

180 ⫾ 34

0.337

.000

194

529 ⫾ 27

129 ⫾ 21

0.196

.006

180

283

558 ⫾ 34

136 ⫾ 25

0.238

.000

110

13

555 ⫾ 37

165 ⫾ 27

0.606

.028

130

147

579 ⫾ 33

198 ⫾ 26

0.493

.000

M2

130

140

535 ⫾ 28

146 ⫾ 32

0.283

.001

MK2000

130

118

545 ⫾ 28

111 ⫾ 19

0.428

.000

145

61

545 ⫾ 29

103 ⫾ 15

0.442

.000

160

81

556 ⫾ 32

121 ⫾ 20

0.229

.040

160

149

553 ⫾ 32

143 ⫾ 23

0.277

.001

Microkeratome

Plate*

Amadeus

130-2

62

140

207

160

169

160

Hansatome

CB

SKBM

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome *Unknown plates were not included. † Plates with n⬍10 eyes were not included.

DISCUSSION Laser in situ keratomileusis continues to be the most common corneal refractive surgery performed worldwide.7 It offers rapid visual recovery, low risk for corneal haze, and reliable results for correction of nearsightedness, farsightedness, and astigmatism. It is performed by first creating an anterior corneal flap with a microkeratome. We evaluated 6 devices and found that several factors affect flap thickness. The most important of these are the microkeratome, plate, suction, head serial number, mean preoperative pachymetry, Kmin, blade lot number, surgery order, and surgeon. Surgeons

Figure 1. (Solomon) Flap thickness by operated eye. 972

must know the mean flap thicknesses of the microkeratome model being used and also understand the thicker and thinner limits. The mean flap thickness was fairly consistent between head designs (P⬎.05), while the variability between microkeratome heads showed statistically significant differences (P ⫽ .007). Thinner corneas were associated with thinner flaps and thicker corneas with thicker flaps, as shown in previous reports.8–10 It is reported that first cuts tend to be thicker than second cuts8,12; we found that first-cut eyes were associated with thicker flaps by about 6%, although this was more pronounced with certain microkeratomes. Overall, 8% of eyes in our study developed epithelial defects. This percentage is low compared to the percentage in studies by Bashour11 and Gimbel et al.,12 who report 14% and 15%, respectively. Lindstrom et al.13 report an incidence of intraoperative epithelial defects of 3.3%. We found that the older the patient, the higher the risk for developing epithelial defects. Bashour11 also reports that patients older than 40 years have a greater risk for developing epithelial defects. Table 3 clearly illustrates that most manufacturers do not label according to mean flap thickness. Additionally, mean flap thickness does not necessarily indicate the range of measurements to the surgeon. Thus, a large percentage of the time, surgeons may be violating the

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Table 9. Statistically significant difference in mean flap thickness by surgical order and microkeratome models per plate thickness. Surgery Order

Microkeratome

Plate Thickness

First Eye Mean Flap Thickness (␮m)

Second Eye Mean Flap Thickness (␮m)

P Value

130-2

160 ⫾ 22

138 ⫾ 22

.00

140

138 ⫾ 17

132 ⫾ 14

.00

160

186 ⫾ 35

170 ⫾ 33

.00

160

132 ⫾ 20

129 ⫾ 22

.15

180

141 ⫾ 25

130 ⫾ 25

.00

Amadeus

Hansatome

110

183 ⫾ 23

162 ⫾ 38

.23

130

201 ⫾ 20

196 ⫾ 26

.06

M2

130

150 ⫾ 32

148 ⫾ 29

.59

MK 2000

130

117 ⫾ 17

109 ⫾ 24

.03

CB

SKBM

145

108 ⫾ 15

94 ⫾ 15

.00

160

125 ⫾ 20

111 ⫾ 16

.01

160

144 ⫾ 21

138 ⫾ 23

.00

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

250 ␮m rule when cutting thicker flaps or perhaps putting patients at risk for very thin flaps and the potential for a buttonhole when cutting thinner flaps. Some devices have less variability than others. For example, the Amadeus 140 and the Nidek MK2000 145 were among the most consistent devices (ie, with the lowest standard deviation). The Amadeus 160, Moria M2 130, Moria CB 110, and Moria CB 130 were among the models with the highest standard deviations. Surgeons must consider not only the labeling of the device (whether the device is labeled by the mean thickness [eg, Amadeus 140] or by how thick the device can cut [eg, Hansatome 180]) but also the variability of each device. For example, the Amadeus 140 had a

mean flap thickness of 134 ⫾15 ␮m, relatively close to the labeling of 140 and comparable to that reported by Jackson and coauthors.14 The Hansatome 180 that cuts ⬍180 ␮m flaps 95% of the time generated a mean flap thickness of 136 ⫾ 25 ␮m, clearly thinner than the labeled upper limit of 180. This finding has been reported by Arbelaez15 and Yildirim et al.8 The Moria units (Carriazo-Barraquer and M2) are intentionally designed to cut flaps 20 to 30 ␮m thicker than labeled.6 However, they cut even thicker flaps, as reported by Arbelaez.15 Knowing how thick a device may cut does not necessarily indicate how likely it is to cut a thin flap and/or create a buttonhole. Additionally, knowing what the mean thickness of a device may be does not

Table 10. Epithelial defect by microkeratome. Total Number of Eyes

Epithelial Defects (%)

Amadeus

438

9.1

Hansatome

479

8.8

CB

166

7.2

M2

140

12.9

MK 2000

262

1.1

SKBM

149

10.1

Microkeratome

CB ⫽ Carriazo-Barraquer; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

Figure 2. (Solomon) Occurrence of epithelial defect per operated eye.

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Table 11. Percentage of epithelial defects per microkeratome model and surgery order. Surgery Order First Eye

Second Eye

Number of Eyes

ED (%)

Number of Eyes

ED (%)

P Value

Amadeus

231

8.6

207

9.7

.742

Hansatome

250

6.8

229

10.9

.145

91

5.5

75

9.3

.380

Microkeratome

CB M2 MK 2000 SKBM

74

8.1

66

18.2

.084

161

1.9

101

0.0

.287

83

10.8

66

9.1

.790

CB ⫽ Carriazo-Barraquer; ED ⫽ epithelial defect; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

indicate how thick or thin surgeons may cut a particular flap in a particular eye. These results clearly indicate that variability is present among all the microkeratome devices. For this reason, surgeons need to measure flaps as frequently as possible, while understanding how their device is likely to cut. Surgeons must not neglect other factors that affect flap thickness such as mean preoperative pachymetry, Kmin, surgery order, and blade lot number. Table 12 summarizes studies that evaluate microkeratome flap thickness accuracy and influential factors. Most found an association between flap thickness and preoperative pachymetry but not mean keratometry. However, our study indicates that Kmin plays a role in flap thickness accuracy. Flanagan and Binder16 compared the Automated Corneal Shaper and the SKBM and report that factors influencing flap thickness included preoperative pachymetry, patient age, mean preoperative keratometry, preoperative refraction, and corneal diameter. Schumer and Bains17 evaluated the Nidek MK 2000 and found a variation between heads. Arbelaez15 found that with the Nidek MK 2000 and the Hansatome, the first cut was thicker than the second one; with the Moria CB, it was the opposite. Using the Hansatome 160 plate with a 9.5 mm suction ring, Spadea and coauthors18 found no correlation between flap thickness and factors such as preoperative keratometry; central corneal pachymetry; and patient refraction, sex, and age. We also did not find a correlation between age and sex. Jackson and coauthors14 found a significant correlation between preoperative pachymetry and flap thickness when using the 140 plate with the Amadeus microkeratome. This correlation was not found when using 974

the 160 or 180 plate; however, there was a trend. No correlation between flap thickness and flat or mean keratometry values was found with the 140 plate, but in first-cut eyes, there was a slight correlation between preoperative flat keratometry and the mean flap thickness with the 160 plate. A new standardized labeling system of microkeratome devices should be created. However, it is difficult to create a perfect method to label the microkeratomes because of patient-related factors. Based on some results produced by this study and others, a good starting point for standard microkeratome labeling would be to use the mean flap thickness ⫾ 2 SDs, as illustrated in Table 13. This reading will give surgeons 95% confidence in their measurements, which will allow better planning. However, since there is variability among the heads, it may be necessary to standardize the labeling of each head device individually. In addition, it is important that microkeratome manufacturers reduce inconsistency between heads. This study had limitations, including that measurements were obtained by 18 different surgeons and many different machines were used. Using ultrasound pachymetry may not be the most accurate way to calculate flap thickness; there could be a difference in the technique when measuring flap thickness, which could increase variability even though it was standardized before the study was initiated. Even when the same surgeon used the same microkeratome with different heads, variability was present, as shown in Table 14. In addition, there were limitations in the number of eyes with certain microkeratomes. Since all eyes were included in the analyses, the P values reported may be biased low be-

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Table 12. Literature review of corneal flap thickness. Study*/Year

Microkeratome

Number of Eyes

Plate Thickness (␮m)

Ring Size (mm)

Mean Flap Thickness (␮m)

Yildirim6/2000

Hansatome

140

180

ND

121 ⫾ 26

SKBM

78

160

9 or 9.5

155 ⫾ 19

SCMD

69

150

8.25

137 ⫾ 34

Amadeus

51

140

8.5 or 9.5

153 ⫾ 18†

50

140

8.5 or 9.5

134 ⫾ 25‡

25

160

8.5 or 9.5

182 ⫾ 26†

25

160

8.5 or 9.5

163 ⫾ 29‡

8

180

8.5 or 9.5

235 ⫾ 24

96

130

8.5 or 9.5

116 ⫾ 19

131

160

8.5 or 9.5

148 ⫾ 22

13

160

8.5 or 9.5

127 ⫾ 33

9

180

ND

133 ⫾ 27

6

130

⫹2§

123 ⫾ 17

160

⫹2

146 ⫾ 27

9

Uc¸akham /2002 10

Yi /1999 14

Jackson /2003

Arbelaez15/2002

MK 2000

Hansatome

CB

25 16

Flanagan /2003

17

Schumer /2001

ACS

1776

160

SKBM

2652

160

Spadea /2002 19

Naripthaphan /2001

20

Jacobs /1999 21

Shemesh /2002

120 ⫾ 23 8.0 to 9.5

161 ⫾ 24




ND

129 ⫾ 22




ND

150 ⫾ 30

46

¶

130

ND

152 ⫾ 25

68

160¶

ND

173 ⫾ 27

Hansatome

50

160

9.5

143 ⫾ 21

MK 2000

148

130

8.5

120 ⫾ 16

32

130

9.5

122 ⫾ 18

20

160

8.5

173 ⫾ 27

MK 2000

158 98

18

§

130 160

LSK-1

93

130

ND

159 ⫾ 28

ACS

46

160

8.5

128 ⫾ 13†

#

123 ⫾ 13‡ Hansatome

38

160

8.5

141 ⫾ 20† 121 ⫾ 27‡

MK 2000

48

130

8.5

127 ⫾ 4† 127 ⫾ 4‡

22

Gailitis /2002

Hansatome

16

160

8.5/9.5

119 ⫾ 20

116

180

8.5/9.5

143 ⫾ 19

ACS ⫽ Automated Corneal Shaper; CB ⫽ Carriazo-Barrequer; SKBM ⫽ Summit Krumeich-Barrequer microkeratome *First author † Right eye ‡ Left eye § Varies from 8.0 to 9.0 according to patient’s K-value
Model 121 ¶ Model 65 # Intended to create a 160 ␮m flap

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Table 13. Proposed standardization of microkeratome labeling. Plate Thickness (␮m)

Proposed Labeling*

Amadeus

140

134/104-164

Hansatome

160

129/87-171

180

136/86-186

CB

130

198/146-250

M2

130

146/82-210

MK2000

130

111/73-149

Microkeratome

CB ⫽ Carriazo-Barraquer *Based on data with n⬎100 eyes. It gives the mean flap thickness/ lower limit-upper limit at 95% CIs (2 SDs).

cause of correlation in flap thickness between the left and right eyes in patients who had bilateral surgery. To protect against this, all analyses were also performed using left eyes only and results were qualitatively the same. The mean flap thickness values in Table 3, for example, change by less than 10 ␮m and the SDs by less than 2 ␮m. In Table 4, the same microkeratome–plate combinations exhibited statistically significant mean flap thicknesses, as found in all eyes (indicated by the asterisks in the table). The mean flap thicknesses reported in Tables 6 and 7 for all eyes are not statistically different from the mean thickness in left eyes only, and the correlations reported in Table 8 change by less than 0.1. Finally, the variables that affect flap thickness retain their statistical significance for predicting the flap thickness in the left eyes only, explaining 79.6% of the variation in the left-eye flap thickness. Flap thickness is a vital component of every LASIK surgical procedure performed. It is common to use the microkeratome manufacturer’s plate thickness as the estimated value for flap thickness when calculating the residual bed. This study confirms that there is variability between microkeratome models. It has also shown that microkeratome labeling does not necessarily represent

the mean flap thickness performed nor is it uniform or consistent. Each manufacturer labels its microkeratome devices on a different basis. Therefore, it is important that surgeons understand the mean flap thickness of each microkeratome model they use not only by plate thickness and head serial number, but also by mean preoperative pachymetry, Kmin, surgery order and blade lot number, since all influence flap thickness accuracy.

Flap Thickness Study Group Flap Thickness Study Group members included Steven Brint, MD, Arturo Chayet, MD, Elizabeth Davis, MD, Eric Donnenfeld, MD, David Hardten, MD, Douglas Koch, MD, Stephen Lane, MD, Richard Lindstrom, MD, Sergio Litwak, MD, Richard Mackool, MD, Miguel Montes, MD, Lee Nordan, MD, James Salz, MD, Thomas Samuelson, MD, Neal Sher, MD, Kerry Solomon, MD, Roger Steinert, MD, and David Vroman, MD.

References 1. Barraquer JI. Keratomileusis for myopia and aphakia. Ophthalmology 1981; 88:701–708 2. Probst LE, Machat JJ. Mathematics of laser in situ keratomileusis for high myopia. J Cataract Refract Surg 1998; 24:190–195 3. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998; 14: 312–317 4. Pallikaris IG, Kymionis GD, Astyrakakis NI. Corneal ectasia induced by laser in situ keratomileusis. J Cataract Refract Surg 2001; 27:1796–1802 5. Myers RH. Classical and Modern Regression with Applications, 2nd ed. Section 4.4. Belmont, CA, PWS-Kent Publishing, 1986 6. Buratto L, Brint S, Salib G. LASIK with a nasal hinge. In: Buratto L, Brint S, eds, Custom LASIK; Surgical Techniques and Complications. Thorofare, NJ, Slack, 2003; 93–117 7. Solomon KD, Fernandez de Castro LE, Sandoval HP, et al. 2003 Refractive Surgery Survey. In press, J Cataract Refract Surg

Table 14. Mean flap thickness obtained by same surgeon using the SKBM 160 with different heads. Head Serial Number

Number of Eyes

Mean Flap Thickness (␮m)

SD

Minimum Flap Thickness (␮m)

Maximum Flap Thickness (␮m)

162

29

148

10.22

123

164

174

19

154

14.79

125

176

A009H

25

120

15.61

95

158

A132H

7

178

9.44

166

191

SD ⫽ standard deviation; SKBM ⫽ Summit Krumeich-Barraquer microkeratome

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8. Yildirim R, Aras C, Ozdamar A, et al. Reproducibility of corneal flap thickness in laser in situ keratomileusis using the Hansatome microkeratome. J Cataract Refract Surg 2000; 26:1729–1732 ¨O ¨ . Corneal flap thickness in laser in situ 9. Uc¸akhan O keratomileusis using the Summit Krumeich-Barraquer microkeratome. J Cataract Refract Surg 2002; 28:798–804 10. 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 11. Bashour M. Risk factors for epithelial erosions in laser in situ keratomileusis. J Cataract Refract Surg 2002; 28:1780–1788 12. Gimbel HV, Anderson Penno EE, van Westenbrugge JA, et al. Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology 1998; 105:1839-1847; discussion by TE Clinch, 1847– 1848 13. Lindstrom RL, Linebarger EJ, Hardten DR, et al. Early results of hyperopic and astigmatic laser in situ keratomileusis in eyes with secondary hyperopia. Ophthalmology 2000; 107:1858–1863 14. Jackson DW, Wang L, Koch DD. Accuracy and precision of the Amadeus microkeratome in producing LASIK flaps. Cornea 2003; 22:504–507

15. Arbelaez MC. Nidek MK 2000 microkeratome clinical evaluation. J Refract Surg 2002; 18:S357–S360 16. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in 4428 eyes. J Refract Surg 2003; 19:113–123 17. Schumer DJ, Bains HS. The Nidek MK-2000 microkeratome system. J Refract Surg 2001; 17:S250–S251 18. Spadea L, Cerrone L, Necozione S, Balestrazzi E. Flap measurements with the Hansatome microkeratome. J Refract Surg 2002; 18:149–154 19. 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– S258 20. Jacobs BJ, Deutsch TA, Rubenstein JB. Reproducibility of corneal flap thickness in LASIK. Ophthalmic Surg Lasers 1999; 30:350–353 21. Shemesh G, Dotan G, Lipshitz I. Predictability of corneal flap thickness in laser in situ keratomileusis using three different microkeratomes. J Refract Surg 2002; 18:S347–S351 22. Gailitis RP, Lagzdins M. Factors that affect corneal flap thickness with the Hansatome microkeratome. J Refract Surg 2002; 18:439–443

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