Corneal lenticule harvest using a microkeratome and an artificial anterior chamber system at high intrachamber pressure

PURPOSE: To evaluate the safety and accuracy of a manual microkeratome and an artificial anterior chamber used at high intrachamber pressure to harvest corneal lenticules for lamellar keratoplasty. SETTING: Department of Ophthalmology, University of California, Irvine, California, USA. METHODS: Forty-seven human eye-bank corneoscleral rims were mounted on an artificial anterior chamber. A manual microkeratome was used to perform lamellar keratectomy at a mean intrachamber pressure of 95.8 mm hg +/- 4.8 (sd). Two thicknesses (300 microm and 360 microm microkeratome heads) and diameters (8.0 mm and 9.0 mm) were attempted, and the resultant lenticules were analyzed by pachymetry and digital photography. RESULTS: In the 9.0 mm/360 microm group, corneal perforation occurred in 12 of 17 lenticules (71.2%). Except for this group, 24 of 30 corneas (80.0%) showed a less than 0.5 mm deviation from the expected diameter. Neither the horizontal nor the vertical diameter differences were statistically significant (P >.05). The difference between the horizontal and vertical diameters was within +/-0.2 mm in 23 corneas (76.7%). Three (10.0%) corneal beds contained a slightly uneven keratectomy margin. CONCLUSIONS: This system provided accurate and smooth lenticules for lamellar keratoplasty. The precision and accuracy of the obtained corneal lenticules were better than those in previous reports. However, the 9.0 mm diameter/360 microm thickness head with high intrachamber pressure should not be applied in the clinical setting without further testing. In this laboratory study, a minimal corneal thickness of 588 microm was required to avoid perforation.     

A portable microkeratome for harvesting the human anterior corneal surface

A portable, gas-driven turbine microkeratome device capable of harvesting the entire anterior corneal surface for lamellar transplantation on human donor globes was evaluated. The device consisted of a modified LASIK microkeratome with an enlarged suction ring, head, and blade. Vacuum was achieved by a simple hand pump. Lamellar keratectomy was performed on 5 fresh human donor globes. Lenticule dimensions were measured on days 0, 3, 6, and 9 after storage in preservation media at 4 degrees C. On day 0, the obtained lenticules were 13.9 +/- 0.9 mm and 13.5 +/- 0.4 mm, vertical and horizontal diameters, respectively. The average central lenticule thickness was 152.2 +/- 52 microm. Each lenticule was uniform in thickness over 5 measurement points (P = 0.74). Repeat measurements of corneal thickness over the 9 days showed no statistically significant difference (P = 0.51). On day 9 lenticules were 14.6 +/- 0.3 mm and 14.6 +/- 0.4 mm, vertical and horizontal diameters, respectively. When day 0 was compared to day 9, vertical diameter also showed no statistically significant difference (P = 0.16), whereas horizontal diameter was significantly different (P < 0.001). This device proves to be an economical alternative to electric-powered systems for the harvest of transplantable corneal sections.     

Accuracy and precision of the amadeus microkeratome in producing LASIK flaps

PURPOSE: To evaluate the accuracy and precision of corneal flap thickness following laser in situ keratomileusis (LASIK) performed using the 140-, 160-, and 180-microm heads for the Amadeus microkeratome (AMO, Irvine, CA). SETTING: The study took place at the Cullen Eye Institute, Baylor College of Medicine, Houston. METHODS: In this prospective study, using the Amadeus microkeratome, LASIK flaps were cut in 51 right eyes and 50 left eyes with the 140-microm head, 25 right eyes and 25 left eyes with the 160-microm head, and five right eyes and one left eye with the 180-microm head. The same microkeratome blade was used for bilateral cases with the right eyes always undergoing surgery first. Eyes were grouped by order of blade use for statistical analysis. The effect of preoperative corneal thickness, keratometry values, blade oscillation and translation speeds, and blade reuse on flap thickness was evaluated. RESULTS: Mean flap thicknesses were 153 +/- 18 (range 97-187 microm) OD and 134 +/- 25 microm (range 79-174 microm) OS for the 140-microm head; 182 +/- 26 microm (range 105-220 microm) OD and 163 +/- 29 microm (range 105-216 microm) OS for the 160-microm head; and 235 +/- 24 microm (range 198-258 microm) for the 180-microm head. Flap thickness was significantly thicker for the first eyes cut (right eyes) and was positively correlated with increasing corneal thickness in both eyes. For the first eyes cut, flap thickness was also significantly thicker than the labeled thickness specified by the manufacturer. CONCLUSIONS: With the Amadeus microkeratome, LASIK flap thickness correlated with central corneal thickness for the 140-microm head. Reuse of the microkeratome blades produced significantly thinner LASIK flaps on second eyes cut.     

Predictability of donor lamellar graft diameter and thickness in an artificial anterior chamber system

PURPOSE: To identify factors affecting the predictability and report results of donor lamellar graft diameter and thickness in an artificial anterior chamber obtained with a microkeratome. METHODS: Lamellar lenticules were obtained from 25 human corneoscleral rims mounted in an artificial anterior chamber. Lenticules measuring 9 mm were attempted at two thicknesses (250 microm and 350 microm heads). Intrachamber pressure of 65 mm Hg was confirmed with Barraquer tonometry. Keratometry, pachymetry, limbal white-to-white, and qualitative mires with the diameter applanation lens were evaluated as possible factors predictive of lenticule diameter and thickness. Diameters and thicknesses were measured with calipers and pachymetry, respectively.RESULTS Ninety-two percent (23/25 lenticules; 14/15 in 250 microm, 9/10 in 350 microm) were +/- 0.25 mm of the intended diameter and 76% (19/25 lenticules; 12/15 in 250 microm, 7/10 in 350 microm) were within +/- 100 microm of the intended thickness. Ovoid applanation mires with the diameter applanation lens represented tissue herniation within the artificial anterior chamber and led to ovoid lenticules (2/25). CONCLUSIONS: Lenticules of 9 mm +/-0.25mm in diameter were highly reproducible with proper corneoscleral rim seating. Intrachamber pressure confirmed with Barraquer tonometry is important in obtaining lenticules of consistent diameter and adequate thickness. Ovoid applanation mires may herald improper corneoscleral rim seating and result in a similarly shaped lenticule. A 2 mm or greater corneoscleral rim is recommended to prevent tissue herniation within the artificial anterior chamber used in this study.     

Precision and accuracy of an artificial anterior chamber system in obtaining corneal lenticules for lamellar keratoplasty.

PURPOSE: To determine the precision and accuracy of an artificial anterior chamber and a manual microkeratome in obtaining corneal lenticules for lamellar keratoplasty. SETTING: Department of Ophthalmology, Cornea, External Diseases and Refractive Surgery Service, University of California Irvine, Irvine, California, USA. METHODS: A lamellar keratectomy was performed in 47 human corneoscleral rims. Three lenticule thicknesses (180, 300, and 360 microm heads) and 3 diameters (7.0, 8.0, and 9.0 mm) were attempted. Diameters and thicknesses were measured by planimetry and pachymetry, respectively. RESULTS: Peripheral lenticule thickness was more likely to be within +/-50 microm of the intended depth in thinner cuts (180 microm, 9/15 corneas, 60%; 300 microm, 6/16 corneas, 40%; 360 microm, 3/12 corneas, 33.3%) (P = .045). Eighty percent (32/40 corneas) were within +/-0.5 mm of the expected diameter. Accuracy was best in the 8.0 mm group, with 47.1% (8/17 corneas) within +/-0.2 mm of the expected diameter. A thickness/diameter correlation was not observed (r(s) < or = 0.28). CONCLUSIONS: The precision and accuracy of this system varied according to the attempted thickness and diameter.     

Factors predictive of LASIK flap thickness with the Hansatome zero compression microkeratome

PURPOSE: To determine the explanatory power of preoperative variables and comeal flap thickness in laser in situ keratomileusis (LASIK) using the Hansatome zero compression microkeratome (Bausch & Lomb, Rochester, NY). METHODS: A prospective, nonrandomized, comparative interventional case study was performed on 250 eyes of 129 consecutive patients who underwent LASIK surgery using the Hansatome zero compression microkeratome. A 160-microm or 180-microm microkeratome head and an 8.5- or 9.5-mm suction ring were used in the procedures. Preoperative measurements included refraction, spherical equivalent, keratometry, intraocular pressure, corneal white-to-white, anterior chamber depth, and corneal eccentricity. Corneal thickness was measured intraoperatively using ultrasonic pachymetry before and after flap creation, and the difference was taken as flap thickness. Flap diameter was measured with a corneal gauge. Data were analyzed using simple, multiple, stepwise linear and non-linear regression analyses and two-tailed t tests. RESULTS: The mean flap thickness was 124 +/- 17 microm with the nominal 160-microm head and 142 +/- 20 microm with the nominal 180-microm head. One third (33%) of the total variation in flap thickness could be accounted for by three preoperative variables: average corneal thickness, spherical equivalent refraction, and choice of 160- or 180-microm microkeratome head. A simple correlation of 0.114 was noted between corneal eccentricity and flap thickness, but this variable did not add significant explanatory power on multiple regression analysis. Linear regression analysis allowed determination of a flap thickness nomogram with a standard error of the estimate of 16.9 microm and a 95% confidence interval of +/- 33.1. CONCLUSIONS: Comeal thickness is the most systematic predictor of corneal flap thickness using the Hansatome microkeratome. Because three preoperative variables account for only 33% of the range in flap thickness, future studies should focus on variations in blade extension and corneal biomechanical factors, which may also play an important role in determining flap thickness.     

LASIK flap characteristics using the Moria M2 microkeratome with the 90-microm single use head

PURPOSE: To evaluate the accuracy and consistency of corneal flap thickness, horizontal diameter, and hinge size with the Moria M2 90-microm single use head. METHODS: Fifty-two myopic patients (104 eyes), mean age 32.6 years, underwent bilateral LASIK with a superior hinged flap using the Moria M2 microkeratome (90-microm single use head). Prospective evaluation included flap thickness (subtraction method), diameter, hinge size, interface particles, intraoperative complications, and visual recovery. RESULTS: The mean preoperative spherical equivalent refraction was -5.72 +/- 2.59 diopters (D) (range: -2.88 to -10.75 D) and -5.84 +/- 2.73 D (range: -3.13 to -9.38 D) for right and left eyes, respectively. The mean preoperative central corneal thickness was 548 +/- 24 microm and 547 +/- 25 microm for right and left eyes, respectively. The mean preoperative steepest K was 44.12 +/- 1.28 D and 44.41 +/- 1.27 D for right and left eyes, respectively. Corneal diameter (white-to-white) was 12 +/- 0.4 mm and 11.9 +/- 0.4 mm for right and left eyes, respectively. The mean postoperative flap thickness was 109 +/- 18 microm (range: 67 to 152 microm) and 103 +/- 15 microm (range: 65 to 151 microm) for right and left eyes, respectively. The mean postoperative flap diameter was 9.4 +/- 0.3 mm (expected mean according to the nomogram given by the company was 9.5 mm). The mean postoperative hinge chord was 4.4 +/- 0.4 mm (expected mean 4.2 mm). No interface particles were detected on slit-lamp examination. CONCLUSIONS: The Moria M2 90-microm single use head is safe with reasonable predictability for LASIK flap creation.     

Factors that affect corneal flap thickness with the Hansatome microkeratome

PURPOSE: To evaluate factors that influence corneal flap thickness with the Hansatome microkeratome. METHODS: One hundred thirty-two eyes of 70 patients underwent laser in situ keratomileusis (LASIK). Corneal flap thickness was measured by subtracting the intraoperative corneal bed pachymetry measurement from intraoperative total corneal pachymetry. Variables examined included plate thickness, ring size, blade use, temperature, humidity, barometric pressure, age, average keratometric power, and preoperative corneal thickness. RESULTS: Mean flap thickness using a 180-microm plate was 143 +/- 19 microm (range 61 to 207 microm). Mean flap thickness using a 160-microm plate was 119 +/- 20 microm (range 83 to 159 microm). The difference was statistically significant (P < .05). Mean flap thickness using a 180-microm plate and the same blade on the right and left eye was 151 +/- 21 microm (range 113 to 200 microm) and 137 +/- 21 microm (range 91 to 191 microm), respectively. The 14-microm difference was statistically significant (P < .001). There was a slight negative correlation of flap thickness with humidity. There was a positive correlation with preoperative corneal thickness (pachymetry). CONCLUSION: The Hansatome tended to cut thinner flaps than anticipated based on the plate used. Flaps cut on the first eye were thicker than the second eye using the same blade. Thicker corneas tended to lead to thicker flaps. There was no correlation between flap thickness and microkeratome ring size, temperature, barometric pressure, patient age, or average keratometric power.     

Preparation of donor lamellar tissue for deep lamellar endothelial keratoplasty using a microkeratome and artificial anterior chamber system: endothelial cell loss and predictability of lamellar thickness.

BACKGROUND AND OBJECTIVE: To measure endothelial cell loss and predictability of lamellar thickness after preparing donor tissue for deep keratoplasty with an artificial anterior chamber and microkeratome. MATERIALS AND METHODS: A microkeratome set at a depth of 350 microm and a diameter of 9 mm was used to obtain ten lamellar lenticules from corneoscleral rims mounted in an artificial chamber. A punch trephine then was used to cut the donor tissue 7 mm in diameter. Specular microscopy was performed to evaluate endothelial cell density before the procedure, after cutting with the microkeratome, and after trephination. Pachymetry was performed to determine the predictability of lenticule thickness, before the procedure and after microkeratome incision. RESULTS: Mean post-microkeratome endothelial cell loss was 79 +/- 88 cells/mm2 and post-punch trephination was 85 +/- 94 cells/mm2. This represented a mean percentage loss of 3.2% and 3.5% for the respective steps of this procedure. Nine of the ten lenticules were cut within +/- 75 microm of the intended 350-microm thickness. CONCLUSIONS: Preparing donor lenticules for deep lamellar endothelial keratoplasty with a microkeratome with artificial chamber system caused a relatively small loss of endothelial cells (6.7% of the total) and a reproducible thickness. This may have advantages over manual preparation techniques.     

Evaluation of a microkeratome-based limbal harvester device for limbal stem cell transplantation.

PURPOSE: To assess the cut quality and reproducibility using a novel microkeratome-based limbal harvester. METHODS: An enlarged microkeratome head and stainless steel blades were coupled with a nitrogen gas-driven turbine (15,000 blade oscillations/min) of a microkeratome. A large, 16-mm-diameter suction ring was attached to the globe. A lamellar sclerokeratectomy using head depths of 170 and 200 microm was performed in human donor research eyes. Obtained lenticule thickness was measured by ultrasound pachymetry and the bed size by planimetry. Histologic and scanning electron microscopy (SEM) analyses of the samples were performed. RESULTS: Central lenticule thickness was 294 microm (standard deviation [SD] 37) for the 170 head and 277 microm (SD 91) for the 200 head (p = 0.720). Lenticule diameter was larger in the horizontal meridian using the 170 head (12.8 mm [SD 0.8] vs. 11.9 mm [SD 0.7], p = 0.028), but similar in vertical meridian (12.0 [SD 0.6] versus 11.4 mm [SD 0.6], p = 0.093). Histology showed a multilayer epithelial cell pattern at the lenticule periphery. The SEM showed a smooth cut surface in both the stromal bed and the lenticule. CONCLUSION: Cut reproducibility and quality are similar to those found using standard microkeratomes for corneal lamellar cuts. This system ensures, in a straightforward way, the presence of epithelial cells in the edges of a mechanical sclerokeratectomy for limbal stem cell transplantation.     

Laser in situ keratomileusis flap thickness using the Hansatome microkeratome with zero compression heads

PURPOSE: To evaluate predictability and possible factors affecting flap thickness in laser in situ keratomileusis (LASIK) using the Hansatome microkeratome (Bausch & Lomb Surgical) with zero compression heads. SETTING: Zentrum für Refraktive Chirurgie Münster, Münster, Germany. METHODS: A prospective nonrandomized comparative (self-controlled) trial analyzing flap thickness in 153 consecutive patients having LASIK using the Hansatome microkeratome with zero compression was conducted. Two hundred thirty-eight eyes that had uncomplicated primary LASIK (n = 237) or secondary LASIK (n = 1) by the same surgeon and same technique using 4 different microkeratomes of the same model were evaluated. Each keratome cut was performed with a new Accuglide (Bausch & Lomb) blade in a 160 microm (n = 89), 180 microm (n = 128), or 200 microm (n = 21) Hansatome zero compression head coupled to a 8.5 mm (n = 106) or 9.5 mm (n = 131) suction ring. Only Hansatome elements with the same serial numbers were combined. Ultrasound subtraction pachymetry was routinely used to determine intraoperative flap thickness. Flap thickness was correlated with microkeratome head dimension, suction ring size, preoperative keratometry obtained by Orbscan IIz (Bausch & Lomb), preoperative corneal thickness as obtained by ultrasound pachymetry, refractive error, and age. RESULTS: Measured intraoperative flap thickness was significantly different (P<.01) from predicted flap thickness. The mean flap thickness was 97 microm +/- 18 (SD) (range 65 to 163 microm), 111 +/- 20 microm (range 61 to 177 microm), and 131 +/- 20 microm (range 89 to 162 microm) for the 160 microm, the 180 microm, and 200 microm heads, respectively. There was a good correlation between microkeratome head and corneal flap thickness. However, there was a variability between devices. There was a low correlation between baseline ultrasound pachymetry at the time of surgery and corneal flap thickness (r = .26) and a small effect of ring size. There was no correlation with keratometry, refractive error, or age. CONCLUSIONS: There was a remarkable difference in the flap thickness of microkeratomes of the same make and model. This emphasizes the need to measure intraoperative flap thickness and to evaluate every microkeratome separately. Factors affecting flap thickness seem to be more device dependent than patient related; obtaining flap thickness in the first eye did not enable predictions of the flap thickness in the fellow eye.     

Flap quality in single versus multiple use of the same blade in the Flapmaker microkeratome

PURPOSE: We evaluated experimentally the variability of cut thickness, flap diameter, and cut quality produced by the Flapmaker corneal microkeratome (IOLTech), with single and repeated use of the same cutting blade. METHODS: Keratectomy was performed with twelve cutting heads (8.5-mm diameter, 160-microm cutting depth) on 47 corneas and with six cutting heads (8.0-mm diameter, 180-microm cutting depth) on 18 freshly enucleated swine eyes in a repeated manner. Ultrasonic pachymetry was determined at the initial, central, and final microkeratome pass zones, first before the flap was created and subsequently after the flap was reflected. Flap diameter was measured by planimetry. After the procedure stromal portions were submitted for scanning electron microscopy. RESULTS: On the first blade use, mean central flap thickness was 145 +/- 32 microm and mean vertical flap diameter was 8.4 +/- 0.26 mm with the 8.5-mm blades. With the 8.0-mm blades, mean central flap thickness was 155 +/- 23 microm and mean vertical flap diameter was 8.0 +/- 0.27 mm. Scanning electron microscopy disclosed smooth cut surfaces when new blades were used, but with repeated blade use, increasingly prominent stromal bed folds were observed. CONCLUSIONS: The evaluated blades produced reproducible flap size and thickness and good cut quality with single use, but after the first use, cut quality markedly deteriorated. Repeated use of Flapmaker cutting blades is not recommended.     

Comparison of corneal flap thickness between primary and fellow eyes using three microkeratomes

PURPOSE: To compare corneal flap thickness created in laser in situ keratomileusis (LASIK) in primary (right) and fellow (left) eyes (same blade for both eyes) using three microkeratomes. METHODS: The corneal thickness of 132 eyes (66 patients) was measured preoperatively and intraoperatively after flap creation. Corneal flap thickness was calculated by subtracting stromal bed thickness from total corneal thickness. Three microkeratomes were used: Nidek MK-2000, Bausch and Lomb Surgical Hansatome, and the Chiron Automated Corneal Shaper (ACS). Each patient had both corneas cut by one microkeratome and one blade at the same session. RESULTS: Mean corneal flap thickness created in primary eyes was 128.30 +/- 12.57 microm (range 105 to 147 microm) for the ACS (160-microm plate and 8.5-mm ring) and 122.96 +/- 13.30 microm (range 86 to 140 microm) for fellow eyes; Hansatome (160-microm plate and 8.5-mm ring): 141.16 +/- 20.11 microm (range 101 to 169 microm) in primary eyes and 120.95 +/- 26.95 microm (range 107 to 151 microm) in fellow eyes; Nidek (130-microm plate and 8.5-mm ring): 127.25 +/- 4.12 microm (range 116 to 134 microm) in primary eyes and 127.54 +/- 3.7 microm (range 119 to 134 microm) in fellow eyes. The corneal flap in the ACS and Hansatome microkeratomes was always thicker in the primary than the fellow eye, using the same blade for both eyes. No significant difference was found using the Nidek microkeratome. CONCLUSION: Corneal flap thickness tended to be thinner in fellow eyes than in primary eyes for the ACS and Hanstome microkeratomes. The Nidek microkeratome results were closer to specified corneal flap thickness than the ACS and Hanstome microkeratomes.     

Regularity of human corneal flaps prepared by femtosecond laser technology

PURPOSE: To evaluate the regularity of intracorneal femtosecond laser incisions for preparation of corneal flaps for LASIK. METHODS: Using a corneal contact lens and a femtosecond laser, superficial corneal flaps of two different thicknesses (140 microm and 260 microm) were created in five human donor eyes. The uniform evenness of the corneal flap thickness was assessed histomorphometrically. RESULTS: The coefficient of variation was 0.12 for the thin flaps and 0.77 for the thick flaps. Flap thickness did not vary statistically significantly between the peripheral half of the flap diameter (189.9+/-71.1 microm) and the center of the flap (198.8+/-80.2 microm). Mean thickness was 133.7+/-15.33 microm (median 141.1 microm; range: 105 to 151 microm) for the 140-microm intended flaps, and 268.53+/-20.6 microm (median 269.0 microm; range: 231 to 295 microm) for the 260-microm intended flaps. CONCLUSIONS: Femtosecond laser-prepared corneal LASIK flaps may have acceptable regularity for clinical purposes.     

Bilateral comparison of corneal flap dimensions with the Moria M2 reusable head and single use head microkeratomes

PURPOSE: To compare the Moria (Antony, France) M2 automated microkeratome with the head 130 to a new disposable single use head to evaluate complications, accuracy, and safety of the procedure. METHODS: Ninety-eight eyes of 49 consecutive patients were operated with the Moria M2 microkeratome. One eye was operated with the metallic head 130 and the other with a plastic single use head, both designed to create a 160-microm flap. Intraoperative flap dimensions were correlated to preoperative parameters and evaluated 1 month postoperatively. RESULTS: With the head 130, mean thickness was 153.3 microm (standard deviation [SD] 13.3, range: 102 to 179 microm). When using a single use head, mean thickness was 148.0 microm (SD 9.8, range: 120 to 170 microm). Occasional iron particles were observed in one eye with both head types. No true epithelial ingrowth was detected in any of the eyes, but epithelial dots at the wound edge occurred in one eye, when using the head 130, but not in the eyes operated with a single use head. CONCLUSIONS: On average, both head types created thinner flaps than attempted. Single use heads produced thinner flaps than the head 130. Accuracy in flap thickness in terms of standard deviation was significantly better in single use heads than in the head 130. Single use heads also had fewer microkeratome-related complications. In clinical practice, the single use head was easier to use because no assembly was required. Plastic single use heads also worked more smoothly than the metallic head 130.     

Comparative performance of the Zyoptix XP and Hansatome zero-compression microkeratomes

PURPOSE: To compare flap parameters produced by the Zyoptix XP and Hansatome microkeratomes (both Bausch & Lomb) and to evaluate preoperative variables contributing to flap thickness variation. SETTING: Private practice, St. Louis, Missouri, USA. METHODS: Flap dimensions in 75 eyes that prospectively had laser in situ keratomileusis using the Zyoptix XP microkeratome were compared with a historical control sample of 75 eyes treated with the Hansatome microkeratome. The 2 groups were matched for mean keratometry, central corneal pachymetry, spherical equivalent (SE), age, and microkeratome head size and suction ring diameter. RESULTS: The mean ultrasound-measured flap thickness was 126.54 microm +/- 14.6 (SD) and 143.74 +/- 15.0 microm for the 120 microm and 140 microm Zyoptix XP heads, respectively, and 128.90 +/- 20.4 microm and 143.32 +/- 21.0 microm for the 160 microm and 180 microm Hansatome heads, respectively. The standard deviation in flap thickness was smaller and statistically significant for the Zyoptix XP (+/-14.8 microm) versus the Hansatome (+/-20.7 microm) (P = .0039, F test). Preoperative pachymetry and SE (P<.001) accounted for 20% of the variability in measured flap thickness using the Hansatome. For the Zyoptix XP, preoperative pachymetry was statistically significant in explaining the variation in flap thickness with the 120 microm head (P = .02) but not with the 140 microm head. Variation in flap thickness from either Zyoptix XP head was not statistically related to the preoperative SE. CONCLUSION: Although the 2 microkeratomes produced flaps of similar mean thickness, the Zyoptix XP showed significantly less variation in flap thickness than the Hansatome, was less affected by measurable preoperative variables such as SE, and was closer to nominal labeling.     

Noncontact corneal pachymetry with slit lamp-adapted optical coherence tomography

PURPOSE: The purpose of this study was to determine the accuracy, the reproducibility, and the limits of agreement of noncontact central corneal thickness measurement with slit lamp-adapted optical coherence tomography (OCT). DESIGN: Nonrandomized comparative clinical trial. METHODS: In a prospective comparative observational study, a total of 108 consecutive patients (108 eyes) with normal corneas (92 eyes) and different corneal alterations (16 eyes) participated. Six sequential measurements of the central corneal thickness with slit lamp-adapted OCT and with ultrasound (US) pachymetry at 1640 ms(-1) were performed. The main outcome measures were accuracy, reproducibility assessed with precision and coefficient of variation (CV), and limits of agreement of central corneal thickness measurement. RESULTS: The mean central corneal thickness values were 541 +/- 43 microm (OCT) and 549 +/- 44 microm (US) with a mean precision of +/- 5.8 microm (CV 1.08%) and of +/- 4.0 microm (CV 0.73%), respectively. The method comparison revealed equivalence (+/- 2SD) in the 5% range with a mean difference between both methods of 7.9 microm (1.45%). The relative error was 8.7 microm (1.6%), which corresponded to limits of agreement (+/- 2SD) ranging from -9.5 microm to 25.3 microm. CONCLUSIONS: Central corneal pachymetry with slit lamp-adapted OCT revealed, for clinical purposes, an excellent accuracy and reproducibility with a high degree of agreement compared with US pachymetry. Thus, the presented OCT system seems to be a promising diagnostic modality to objectively measure corneal thickness in a convenient noncontact mode.     

Flap measurements with the Hansatome microkeratome

PURPOSE: To evaluate flap thickness, flap diameter, and hinge length during laser in situ keratomileusis (LASIK) and to correlate these measurements with preoperative keratometric power, central corneal thickness, and patient refraction, gender, and age. METHODS: In this prospective study of 50 eyes of 28 patients (mean age 31 +/- 6.6 yr; range, 24 to 43 yr) results of LASIK for myopia were analyzed (mean spherical equivalent refraction of -7.16 +/- 1.69 D; range, -2.75 to -13.50 D). Corneal flaps were created using the Hansatome microkeratome (Baush & Lomb Surgical) with a 160-microm plate and a 9.5-mm suction ring. Corneal thickness was evaluated using an ultrasonic 50-MHz pachymeter (Sonogage Corneo Gage Plus) and the mean keratometric power was measured with a Corneal Analysis System videokeratographic unit (EyeSys). Data were analyzed using t-test, Pearson product moment correlation coefficient, and Spearman's rho non-parametric correlation coefficients. RESULTS: Mean corneal flap thickness was 142.6 +/- 20.8 microm (range, 107 to 177 microm), mean flap diameter was 9.9 +/- 0.3 mm (range, 9.2 to 10.5 mm), and mean hinge length was 6.2 +/- 0.4 mm (range, 5.2 to 7 mm). Statistically significant correlations (P<.05) were found between mean keratometric power and flap hinge length, mean keratometric power and flap diameter, preoperative spherical equivalent refraction and flap diameter, corneal thickness and flap hinge length, as well as patient age and corneal thickness. CONCLUSIONS: The Hansatome microkeratome was an effective and safe instrument in the creation of corneal flaps for LASIK. Consideration of preoperative keratometric power and corneal thickness may help to reduce or avoid complications.     

Central corneal thickness, tonometry, and ocular dimensions in glaucoma and ocular hypertension.

PURPOSE: To assess possible correlations between central corneal thickness, tonometry, and ocular dimensions. PATIENTS AND METHODS: One hundred seventeen eyes of 117 patients who were not taking any intraocular pressure-lowering medications were studied prospectively. Forty-one patients had ocular hypertension; 13 patients had primary open-angle glaucoma; and 10 patients had normal-pressure glaucoma. Twenty-three healthy eyes were included. Thirty glaucoma suspects (10 patients monitored for possible normal-pressure glaucoma and 20 patients with intermittent ocular hypertension) were included for correlation analysis. Tonometry was performed with Goldmann applanation and pneumotonometry, and central corneal thickness, anterior chamber depth, lens thickness, and axial length were measured ultrasonically. RESULTS: Central corneal thickness was lowest in eyes with normal-pressure glaucoma (538 +/- 51 microm), highest in eyes with ocular hypertension (570 +/- 32 microm), and intermediate and similar in eyes with primary open-angle glaucoma and healthy eyes (547 +/- 34 microm and 554 +/- 32 microm, respectively). These differences were significant (P = 0.028). Goldmann applanation tonometry and central corneal thickness were weakly correlated (r = 0.12, P = 0.205), with a 0.2-mm Hg change per 10-microm variation in central corneal thickness. Pneumotonometry measurements were more strongly correlated with central corneal thickness (r = 0.21, P < 0.05). Lens thickness was strongly correlated with age (r = 0.57, P < 0.001). Anterior chamber depth was negatively correlated with lens thickness and age (r = -0.29, P < 0.005 and r = -0.25, P < 0.01). Axial length was correlated with anterior chamber depth and age (r = 0.5, P < .001 and r = -0.19, P < 0.05). CONCLUSION: Eyes diagnosed as having ocular hypertension have thicker corneas and eyes labeled as having normal-pressure glaucoma have thinner corneas, when compared with healthy eyes or eyes with primary open-angle glaucoma. The effect of central corneal thickness on Goldmann applanation tonometry accuracy appears to be small and usually not clinically relevant. When corneal thickness is markedly different from normal, the clinician may need to factor this into diagnosis and management.     

Residual bed thickness and corneal forward shift after laser in situ keratomileusis

PURPOSE: To prospectively assess the forward shift of the cornea after laser in situ keratomileusis (LASIK) in relation to the residual corneal bed thickness. SETTING: Miyata Eye Hospital, Miyazaki, Japan. METHODS: Laser in situ keratomileusis was performed in 164 eyes of 85 patients with a mean myopic refractive error of -5.6 diopters (D) +/- 2.8 (SD) (range -1.25 to -14.5 D). Corneal topography of the posterior corneal surface was obtained using a scanning-slit topography system before and 1 month after surgery. Similar measurements were performed in 20 eyes of 10 normal subjects at an interval of 1 month. The amount of anteroposterior movement of the posterior corneal surface was determined. Multiple regression analysis was used to assess the factors that affected the forward shift of the corneal back surface. RESULTS: The mean residual corneal bed thickness after laser ablation was 388.0 +/- 35.9 microm (range 308 to 489 microm). After surgery, the posterior corneal surface showed a mean forward shift of 46.4 +/- 27.9 microm, which was significantly larger than the absolute difference of 2 measurements obtained in normal subjects, 2.6 +/- 5.7 microm (P<.0001, Student t test). Variables relevant to the forward shift of the corneal posterior surface were, in order of magnitude of influence, the amount of laser ablation (partial regression coefficient B = 0.736, P<.0001) and the preoperative corneal thickness (B = -0.198, P<.0001). The residual corneal bed thickness was not relevant to the forward shift of the cornea. CONCLUSIONS: Even if a residual corneal bed of 300 microm or thicker is preserved, anterior bulging of the cornea after LASIK can occur. Eyes with thin corneas and high myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.