Noncontact measurements of central corneal epithelial and flap thickness after laser in situ keratomileusis

PURPOSE: To investigate the changes in the epithelium and flap after laser in situ keratomileusis (LASIK), when measured with optical coherence tomography (OCT). METHODS: Twenty-eight eyes of 14 patients (age: 39.9 +/- 8.6 years) underwent LASIK. The central thickness of corneal epithelium and flap were measured with a real-time 1310 nm OCT 1 day, 1 week, and 1 month after surgery. A custom software program was used to process multiple images of each eye on each visit. RESULTS: After surgery, the corneal epithelium changed significantly (ANOVA: F((3, 81)) = 12.3, P = 0.000) with not statistically significant thinning at one day (mean +/- SD: 57.8 +/- 5.9 micro m, P = 0.26, compared with baseline: 59.9 +/- 5.9 micro m) and statistically significant thickening at 1 week (60.8 +/- 5.8 micro m, P = 0.04, compared with 1 day) and 1 month (64.6 +/- 6.1 micro m, P = 0.008 compared with all others). There were statistically significant changes in the corneal flap thickness (ANOVA: F((2, 54)) = 4.59, P = 0.01) with thickening in the intervals between 1 day (143.3 +/- 20.6 micro m) and 1 week (149.7 +/- 24.6 micro m, P = 0.12), and between 1 week and 1 month (152.7 +/- 19.3 micro m, P = 0.01). There was a strong correlation (r = 0.898) between the difference of corneal thickness before and after surgery and predicted laser ablation depth. CONCLUSIONS: OCT is a useful noncontact tool for thickness measurements of the epithelium, flap, and total cornea. After LASIK, the epithelium and flap showed thickening during the study period.     

Imaging interface fluid after laser in situ keratomileusis with corneal optical coherence tomography

A 41-year-old myopic patient who had laser in situ keratomileusis 6 months earlier was treated for a complete retinal detachment (RD) with proliferative vitreoretinopathy. Surgical treatment consisted of an encircling band, pars plana vitrectomy, and silicone oil filling. Postoperatively, the patient developed marked corneal edema with no increase in intraocular pressure (IOP) as measured by applanation tonometry. Interface fluid was confirmed by corneal optical coherence tomography. Quantification of the corneal structures revealed that corneal edema was in the residual posterior stroma predominantly. The epithelial and flap thickness did not change significantly. The case demonstrated that after vitreoretinal surgery for RD repair, transient corneal endothelial cell dysfunction developed, causing marked edema of the posterior corneal stroma and interface fluid accumulation. However, an increase in IOP cannot be excluded.     

Estimating residual stromal thickness before and after laser in situ keratomileusis

PURPOSE: To determine the factor(s) that influences measurement of residual stromal thickness (RST) after laser in situ keratomileusis (LASIK) surgery. SETTING: Clinical office-based excimer laser refractive surgery center. METHODS: In this retrospective comparative interventional case study of 6235 eyes, ultrasonic corneal pachymetry was performed immediately before and after flap creation and immediately after laser ablation in the primary procedure and after 647 enhancements. Differences in the methods for calculating RST were compared statistically. RESULTS: Using the RST measured at enhancement as the actual RST, measurements of RST immediately after laser ablation underestimated residual thickness due to laser-induced stromal dehydration and microkeratome effects (P<.001). Estimates of RST using a "standard" or estimated flap thickness were less accurate predictors of residual thickness (P<.001) than use of the theoretical laser resection with a measured flap thickness (RST-4) (P =.78) or a modified flap thickness subtracted from the postoperative corneal thickness (RST-8) (P =.98), which provided the best RST estimates. CONCLUSIONS: Before LASIK, the best means of estimating RST is to subtract the theoretical laser resection obtained from the laser computer and the expected flap thickness normally obtained with a given microkeratome system from the preoperative central corneal thickness. After LASIK, the most accurate means of calculating RST is to subtract the original flap thickness from the postoperative central corneal thickness.     

High-speed optical coherence tomography for management after laser in situ keratomileusis

PURPOSE: To report applications of optical coherence tomography (OCT) in the management of laser in situ keratomileusis (LASIK) related problems. SETTING: Doheny Eye Institute and Department of Ophthalmology, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA. METHODS: Five patients referred for LASIK-related problems were enrolled in a prospective observational study. Clinical examination, ultrasound (US) pachymetry, Placido ring slit-scanning corneal topography (Orbscan II, Bausch & Lomb), and high-speed corneal OCT were performed. RESULTS: In cases of regression and keratectasia, OCT provided thickness measurements of the cornea, flap, and posterior stromal bed. Locations of tissue loss and flap interface planes were identified in a case with a recut enhancement complication. The information was used to determine whether further laser ablation was safe, confirm keratectasia, and manage complications. Optical coherence tomography measurements of central corneal thickness agreed well with US pachymetry measurements (difference 6.4 microm +/- 11.7 [SD]) (P = .026), while Orbscan significantly underestimated corneal thickness (-67.5 +/- 72.5 microm) (P = .17). CONCLUSIONS: High-speed OCT provided noncontact imaging and measurement of LASIK anatomy. It was useful in monitoring LASIK results and evaluating complications.     

Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness

ObjectiveTo determine whether ectasia after laser in situ keratomileusis (LASIK) is related to residual corneal bed thickness.DesignRetrospective noncomparative case series.ParticipantsThirty-two eyes of 16 patients with refractive errors of −4.00 to −18.00 diopters were examined.InterventionLASIK was performed. The topography of the posterior corneal surface was examined with the Orbscan slit scanning corneal topography/pachymetry system.Main outcome measuresThe difference in the elevation of posterior corneal surface regarding the best-fit sphere was measured.ResultsAfter surgery, mean bulge of 17.2 ± 7.2 μm was found in eyes with residual corneal bed of 250 μm or greater, whereas 41.0 ± 22.1 μm was seen in eyes with bed thickness less than 250 μm (t = 4.29; P = 0.000).ConclusionPosterior corneal bulge is correlated with the residual corneal bed thickness. The risk of ectasia may be increased if the residual corneal bed is thinner than 250 μm.     

Flap thickness reproducibility in laser in situ keratomileusis with a femtosecond laser: optical coherence tomography measurement

PURPOSE: To compare ultrasound (US) pachymetry, Orbscan, and optical coherence tomography (OCT) measurements of the central corneal thickness (CCT) before laser in situ keratomileusis (LASIK) and evaluate the reproducibility of flaps created with the IntraLase femtosecond laser (IntraLase, Inc.) using OCT. SETTING: Department of Ophthalmology, IIsan Paik Hospital, Goyang, Korea. METHODS: Central corneal thickness was measured using OCT, US pachymetry, and Orbscan in 59 eyes of 30 patients before femtosecond laser LASIK. The postoperative corneal flap thickness, measured using OCT, was compared with the preoperative intended thickness. RESULTS: Optical coherence tomography, US pachymetry, and Orbscan measurements provided similar CCT values (P>.05). Corneal thickness values obtained using US or Orbscan correlated well with those obtained by OCT, with the correlation coefficient ranging from 0.804 to 0.889 (P<.05). The OCT measurements showed no significant difference between the postoperative flap thickness and the intended flap thickness (P>.05). CONCLUSIONS: Optical coherence tomography was comparable to US pachymetry and Orbscan in cornea thickness measurement. Optical coherence tomography was easy and relatively accurate to use preoperatively and in the early postoperative period. The femtosecond laser created highly reproducible flaps that corresponded with the preoperative intended thickness.     

Comparison of residual stromal bed and flap thickness in primary and repeat laser in situ keratomileusis in myopic patients

PURPOSE: To compare the change in residual stromal thickness and flap thickness between primary laser in situ keratomileusis (LASIK) and repeat LASIK in myopic patients. SETTING: Melbourne Excimer Laser Group, East Melbourne, Australia. METHODS: This retrospective nonrandomized comparative trial comprised 46 eyes of 34 patients who had repeat LASIK. The thickness of the residual stromal bed was calculated by subtracting the calculated stromal ablation from pachymetry of the stromal bed after cutting the flap in primary treatment and directly measuring during retreatment. The thickness of the LASIK flap in primary and repeat LASIK was calculated by subtracting the central pachymetry of the stromal bed after creating the flap from pachymetry before cutting and lifting the flap, respectively. The main outcome measures were comparison of the residual stromal bed and flap thickness between the primary treatment and the retreatment. RESULTS: The mean thickness of the calculated residual stromal bed after primary treatment was 329.8 microm +/- 40.8 (SD), and the mean measured residual stromal bed at retreatment was 317.3 +/- 42.8 microm. The mean difference in residual stromal bed thickness was 12.5 +/- 13.0 microm (P<.001). Sixteen eyes (34.7%) had a decrease in bed thickness between 11 microm and 20 microm. The mean flap thickness during primary LASIK and repeat LASIK was 145.2 +/- 17.1 microm and 169 +/- 18.3 microm, respectively. The mean interval between primary treatment and retreatment was 7.4 +/- 4.1 months. The mean change in flap thickness was 23.8 +/- 15.2 microm (P<.001). Fifteen eyes (32%) had an increase in flap thickness between 11 microm and 20 microm. There was a negative correlation between refractive error before primary treatment and the difference in flap thickness. No correlation was found between the difference in flap thickness and the interval between the primary treatment and the repeat treatment. CONCLUSIONS: Intraoperative pachymetry of the stromal bed during retreatment is strongly recommended as the residual stromal bed and flap thickness changes between primary and retreatment. There is a tendency for the measured stromal bed at retreatment to be thinner than the calculated stromal bed and for the flap to be thicker than previously measured.     

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.     

LASIK术中微型角膜板层刀对角膜厚度的影响

目的　比较 L ASIK手术前后角膜厚度的变化 ,分析微型角膜板层刀对角膜厚度的影响。方法　 48例(96只眼 )的角膜厚度在 L ASIK术前及术后不同时期进行测量 ,比较术后理论值与实测值的差异 ,分析理论值与实测值的差异所在 ,从而判断微型角膜板层刀是否对角膜厚度产生一定的影响。结果　术后 3天、7天的角膜厚度实测值与理论值有显著性差异 (P <0 .0 5 ) ,术后 1月实测值与理论值差异无显著性 (P >0 .0 5 )。结论　 L ASIK术中微型角膜板层刀的切割作用对角膜厚度会产生一定的影响。     

Monitoring corneal structures with slitlamp-adapted optical coherence tomography in laser in situ keratomileusis

PURPOSE: To monitor corneal structures with slitlamp-adapted optical coherence tomography (OCT) in laser in situ keratomileusis (LASIK). SETTING: Department of Ophthalmology, Vivantes Klinikum Neuk?lln, Berlin, Germany. METHODS: In this prospective, nonrandomized, comparative clinical case series of consecutive patients who had LASIK for myopia and myopic astigmatism, the corneal structures were studied with slitlamp-adapted OCT at a wavelength of 1,310 nm. The central corneal thickness (CCT) and epithelial, flap, and residual stromal thicknesses were assessed preoperatively, immediately after surgery, on postoperative day 1, and then, on average, after 8, 35, and 160 days. RESULTS: Twenty-five eyes of 13 patients were included. The attempted mean spherical equivalent correction was -6.11 diopters (D) +/- 2.16 (SD) with a mean calculated stromal ablation depth of 92 +/- 24 microm. The CCT was 516 +/- 26 microm preoperatively and 453 +/- 40 microm postoperatively (P<.001). The epithelial thickness increased from 57.0 +/- 7.7 microm preoperatively to 61.0 +/- 7.5 microm postoperatively (P =.04). Imaging of the hyperreflective interface was possible in all patients for up to 15 months. The flap and residual stromal thickness was 211 +/- 28 microm and 344 +/- 48 microm, respectively, immediately after LASIK and 164 +/- 21 microm (P<.001) and 284 +/- 32 microm (P<.001), respectively, on postoperative day 1. There were no further significant changes during the follow-up. The overall mean reproducibility was +/-4.50 microm (coefficient of variation [CV] 0.94%) for CCT, +/-4.99 microm (CV 8.57%) for epithelial thickness, +/-6.25 microm (CV 3.55%) for flap thickness, and +/-7.09 microm (CV 2.42%) for residual stromal thickness. CONCLUSION: Slitlamp-adapted OCT can be used to longitudinally monitor the variable structures of the cornea, epithelium, flap, and residual stroma in LASIK.     

Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography

PURPOSE: To measure total corneal power using optical coherence tomography (OCT). SETTING: Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA. METHODS: Thirty-two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high-speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography. RESULTS: The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK-induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (-18.8%) (P<.001). CONCLUSIONS: Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK-induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post-LASIK eyes.     

应用谱域OCT观察LASIK对黄斑区视网膜厚度的影响

目的使用谱域OCT（SD-OCT）观察LASIK对手术患者黄斑区视网膜厚度的影响。方法系列病例研究。使用随机数字表选择2012年10月至2013年5月接受LASIK的30例（60眼）屈光不正患者,采用SD-OCT观察患者术前、术后10 d以及术后30 d的黄斑总容积（TMV）和中央视网膜厚度（CRT）的变化。数据分析采用重复测量设计和Pearson相关分析法。结果30例（60眼）患者术前BCVA（logMAR）为-0.063±0.041,TMV为（7.31±0.31）mm3,CRT为（220.3±21.1）μm,术后10 d时相应的指标分别为-0.043±0.081、（7.32±0.26）mm3、（221.6±20.8）μm,术后30 d时相应的指标分别为-0.074±0.061、（7.35±0.29）mm3、（219.8±22.9）μm。患者术后视力满意,术后10 d的CRT较之术前、术后30 d的差异均有统计学意义（P＜0.05）,而术后10 d的TMV仅与术后30 d的差异有统计学意义（P＜0.05）,同时术前术后的TMV、CRT的改变程度与术前等效球镜度不相关。结论LASIK术后短时间内黄斑区存在一定程度的视网膜水肿,程度与等效球镜度无关,并且不影响预后视力。     

Correlation between intraocular pressure and corneal stromal thickness after laser in situ keratomileusis

PURPOSE: To assess the relationship between intraocular pressure (IOP) and corneal stromal thickness following laser in situ keratomileusis (LASIK) for myopia or myopic astigmatism. SETTING: Private clinic in Turkey. METHODS: The database of patients who had LASIK was retrospectively reviewed. Preoperative and postoperative IOP and corneal thickness were measured in both eyes of 353 patients by noncontact tonometry and ultrasonic pachymetry, respectively. The correlation between changes in IOP and corneal thickness was evaluated by regression analysis. RESULTS: Laser in situ keratomileusis was associated with a mean decrease in pachymetry of 46.7 microm +/- 28.9 (SD) and a mean decrease in IOP of 2.8 +/- 2.1 mm Hg. There was a significant correlation between the decreases in IOP and pachymetry (P <.001). CONCLUSION: The decrease in IOP after LASIK was related to the decrease in corneal stromal thickness, although there may be other contributing factors.     

Hansatome零压力角膜板层刀切瓣厚度的预测性研究

目的评价Hansatome零压力角膜板层刀切出角膜瓣厚度的预测性,探讨不同因素对其的影响。方法对行LASIK者326例652眼应用超声角膜测厚仪分别于手术前后测量角膜瓣厚度,计算实际角膜瓣厚度,并对相关影响因素进行分析。结果Z160刀头第1刀角膜瓣厚度为(113.50±15.62)μm,第2刀为(104.75±13.76)μm,第2刀切瓣厚度较第1刀薄(P<0.05)。Z180刀头第1刀角膜瓣厚度为(123.24±14.35)μm,第2刀为(116.58±16.34)μm,较标示厚度薄(P<0.05)。Z160刀头配合使用8.5mm和9.5mm负压环切瓣厚度分别为(108.32±12.64)μm和(110.20±14.39)μm,Z180刀头配合使用两种负压环切瓣厚度分别为(119.50±14.52)μm和(120.34±16.38)μm,统计学处理均无显著差异。结论Hansatome零压力板层角膜刀切出角膜瓣厚度较标示厚度薄,负压环的大小与角膜瓣厚度无关。     

高度近视LASIK治疗中角膜瓣厚度的临床分析

目的分析高度近视LASIK治疗中,影响角膜瓣厚度的因素。方法40例(80只眼)屈光度为-7.00D~-9.00D的高度近视患者,运用MoriaM2板层刀制作角膜瓣,对角膜曲率不同、板层刀负压吸引力不同、M2刀片新旧不同进行分组,对术前、术后屈光度、视力及剩余角膜基质床厚度进行分析比较。结果Ⅰ组板层刀负压吸引力相同时角膜曲率不同,制作出的角膜厚度不同,两者有明显差异(P<0.05);Ⅱ组角膜曲率相同,板层刀负压吸引力不同,制作出的角膜瓣厚度不同,两者有明显差异(P<0.05);Ⅲ组板层刀片新旧不同,制作出角膜瓣的厚度不同,两者有明显差异(P<0.05);结论LASIK治疗高度近视时,对一个有经验的手术医生制作一个80~110μm的薄角膜瓣是合理的,能有效防止术后屈光回退和医源性圆锥角膜的发生。     

Flap and stromal bed thickness in laser in situ keratomileusis enhancement

PURPOSE: To evaluate whether flap thickness changes after the primary laser in situ keratomileusis (LASIK) procedure and to assess the accuracy of intraoperative pachymetry and ablation depth measurements in predicting stromal bed thickness before enhancement in eyes that have had primary myopic LASIK. SETTING: Bascom Palmer Eye Institute, Miami, Florida, USA. METHODS: This retrospective noncomparative interventional case series comprised 57 eyes of 42 patients who had LASIK enhancement between June 2001 and September 2002. Exclusion criteria included previous ocular surgery or complications during the first LASIK procedure. Only patients who had had LASIK and enhancement by the same surgeon at our institution and had intraoperative pachymetry readings for both procedures were included. The original flap was relifted in all enhancement procedures. Corneal thickness was routinely measured intraoperatively by ultrasound pachymetry. The age, eye, refraction, date of primary LASIK, central corneal thickness (CCT) and central stromal bed thickness at primary LASIK, depth of ablation, flap thickness (subtraction pachymetry), date of enhancement, CCT and central stromal bed thickness at enhancement, and flap thickness at enhancement were recorded. RESULTS: Thirty-one eyes of 26 patients were myopic and 26 eyes of 16 patients were hyperopic before primary LASIK. The mean time between LASIK and enhancement was 218 days +/- 115 (SD) (193 +/- 88 days in myopic eyes and 248 +/- 136 days in hyperopic eyes [P = .068]). The flap tended to be thicker at enhancement than in the primary LASIK procedure by 9.3 +/- 25.7 microm in myopic eyes (P = .054) and 10.5 +/- 16.6 microm in hyperopic eyes (P = .004). A strong correlation was found between flap thickness in the first and second procedures in myopic and hyperopic eyes (r = 0.6). In myopic eyes, the mean difference between the estimated stromal bed thickness after the first procedure (central bed thickness- ablation depth) and the stromal bed thickness measured directly at enhancement was not statistically significant (3 +/- 29 microm; P = .54, paired t test). A strong correlation was found between the 2 measurements (r = 0.8, P<.001). Another strong correlation was found in myopic eyes between the estimated corneal thickness after the primary LASIK and the corneal thickness measured at enhancement (r = 0.81, P<.001). No correlation was found between the difference in flap thickness and the time to enhancement (r = 0.09 in myopic eyes and r = 0.01 in hyperopic eyes). CONCLUSIONS: Flap thickness tended to be thicker at enhancement than at primary LASIK. Intraoperative pachymetry and ablation depth measurements proved to be precise tools to predict stromal bed thickness before enhancement in eyes that had had primary myopic LASIK. This information may help in planning LASIK enhancements.