Comparison of ocular anterior segment parameters measured by three kinds of biometric measuring methods in myopic eyes / 中华实验眼科杂志

Background It is essential to measure and assess the parameters of ocular anterior segment for refractive surgery in myopic eyes.Some different imaging devices can be used for biometric measurement of ocular anterior segment,but which is more accurate and convenient is still under investigation.Objective This study was to compare the anterior segment parameters in myopic eyes measured by anterior segment optical coherence tomography (AS-OCT),Orbscan topography and ultrasonic pachymetry (US).Methods One hundred and forty eyes of 70 myopic subjects with the diopter of-0.75 to-10.25 D,who intended to receive corneal refractive surgery in the Second Affiliated Hospital of Zhejiang University School of Medicine from November 2011 to May 2012,were retrospectively analyzed.Central corneal thickness (CCT) was measured using AS-OCT,Orbscan Ⅱ and US,respectively,and anterior chamber depth (ACD) was measured by AS-OCT and Orbscan Ⅱ,and the angle to angle (ATA) distance and corneal white-to-white corneal distance (WTW) were measured by AS-OCT and Orbscan Ⅱ,respectively.The parameters from different apparatuses were statistically compared.Results The mean CCT were (516.57±30.25) μm in AS-OCT,(523.68±31.87) μm in US and (514.69±38.40) μm in Orbscan Ⅱ,without significant difference among them (F =2.775,P =0.063).Then the patients were divided into three groups based on the US measurement of CCT (＜500 μm group,500-569 μm group,and ≥ 570 μm group).In the ＜500 μm group,there was a significant difference in the CCT among the three methods (F =22.236,P =0.000),significant differences were found between AS-OCT and Orbscan Ⅱ,or Orbscan Ⅱ and US(both at P＜0.05).In the 500-569 μm group,there was no significant difference in the CCT among the three methods (F =3.011,P =0.051).In the ≥ 570 μm group,there was a significant difference in the CCT among the three methods (F =4.133,P =0.021),a significant difference was found between AS-OCT and US(P＜0.05),but there was no significant difference between AS-OCT and Orbscan Ⅱ (P＞0.05).The ACD values measured by AS-OCT was (3.83±0.21) mm,which was higher than (3.75 ± 0.21) mm by Orbscan Ⅱ,with a significant difference between them (t =-8.520,P =0.000).In addition,the ATA value by AS-OCT (12.43 mm±0.74 mm) was higher than the WTW value (11.42 mm±0.33 mm) by OrbscanⅡ,with a significant difference between them(t=-18.088,P=0.000).Conclusions AS-OCT,US and Orbscan Ⅱ can offer accurate CCT value,and they can provide references to one another before refractive surgery.However,the ACD,ATA and WTW values by AS-OCT and Orbscan]Ⅱ have large differences.

The Corneal Thickness Changes in Different Gazes in Orbscan Topography

PURPOSE: To evaluate error of corneal thickness measurement in different gazes in Orbscan topography. METHODS: Sixty eyes of 30 normal subjects were investigated using the Orbscan topography system. The central corneal thickness (Kc) and the thinnest corneal thickness (Kt) on topography map were obtained in 5 different gazes. Gaze directions were classified into center (C), nasal I (NI), nasal II (NII), temporal I (TI), and temporal II (TII), in which I means 13 degree deviation from visual axis and II means 22 degree deviation. RESULTS: Kc in right eye was 540.4+/-37.7 micro meter(mean+/-standard deviation) at central gaze (C) and 550+/-36.9 micro meter at NI ; there was no significant change, but there were in Kc at NII (563.0+/-40.5 micro meter), TI (566.5+/-32.8 micro meter), TII (595.2+/-54.5 micro meter). In addition, the same patterns were seen in left eye in Kc analysis. In contrast, Kt did not change even in 5 gaze changes. In comparison between Kc and Kt at the same gazes, there were significant differences except in central gaze (C). CONCLUSIONS: Though the gaze direction changed, the most reliable measurement for evaluation and follow up of corneal thickness is the thinnest corneal thickness (Kc) in Orbscan topography system. In case that the central corneal refractive power and thickness were needed, it is important that Orbscan topography is performed in exact central fixation.

A Study on the Location of Corneal Thinnest Point and Corneal Thickness in Midperiphery

PURPOSE: To evaluate the association with corneal thickness and corneal thinnest point using Orbscan topography. METHODS: One hundred and four eyes of the 52 normal subjects who were investigated using an Orbscan topography, were devided into following two groups ; one was inferonasal group which has the thinnest point in inferonasal quadrant(23 eyes, 22.1%), and the other was inferotemporal group which has the thinnest point in inferotemporal quadrant. (79 eyes, 76%) Among them, age-matched 30 eyes were randomly selected. The corneal thickness of two groups were measured at eight meridian of each point distant 1.0, 1.5, 2.0, 2.5, 3.0mm from corneal center. The thickest point and the thinnest point of cornea in two groups were compared and analysed. RESULTS: The thickest point of cornea in two groups was located in the superior portion. The thinnest quadrant was located in the inferonasal portion for inferonasal group and in the inferotemporal portion for inferotemporal group. CONCLUSIONS: The quadrant where the thinnest quadrant existed had the thinnest corneal thickness in comparing with any other quadrant.

Comparison of Corneal Thickness Measurements Between Orbscan Topography System and Ultrasonic Pachymetry before and after LASIK

PURPOSE: To report the difference of the values of the corneal thickness measurement measured by Orbscan topography system and ultrasonic pachymetry is different between pre-LASIK and post-LASIK. METHODS: The corneal thickness measured by these two methods was compared in 32 pre-LASIK eyes of 18 subjects and 31 post-LASIK eyes of 20 subjects passed 3 months after laser in situ keratomileusis (LASIK). RESULTS: In the pre-LASIK eyes, the cornea measured thicker by Orbscan topography system than by ultrasonic pachymeter (16.94+/-13.1 microgram, average +/- standard deviation, p=0.001). In the post-LASIK eyes, there was no statistically significant difference in the values of corneal thickness measurement between two methods (-9.58+/-27.88 microgram, p=0.0653). CONCLUSION: We must give attention to the interpretation of values of corneal thickness measurement measured by Orbscan topography system and ultrasonic pachymetry because the correlations between the two methods in pre and post-LASIK eyes are different. We assume that differences of the values of measurement between the two methods may result from either technical errors of the measurers , interference of corneal interface, or change of tear film.

Accuracy of Orbscan Pachymetry Measurements and Ultrasonic Pachymetry before and after LASIK with Orbscan II(R) Topography

PURPOSE: To assess the the accuracy and variability in pachymetry measurements obtained by Orbscan and ultrasonic pachymeter in laser in situ keratomileusis (LASIK). METHODS: Thirty two eyes of 16 patients before LASIK were measured by ultrasonic pachymeter and Orbscan. Thirty eyes of 16 patients that had LASIK were measured by ultrasonic pachymeter and Orbscan postoperatively. The theoretical residual corneal thickness was compared to measurements by both instruments in eyes that had LASIK procedures. RESULTS: The ultrasound pachymeter measurements (543.8+/-24.2 micrometer) were thicker than the Orbscan pachymetry measurements, (541.2+/-24.9 micrometer) by mean of 2.7+/-7.4 micrometer (SD), preoperatively. But this is not statistically significant (p>0.05). Postoperatively, the ultrsound measurements were consistent with theoretical residual corneal thickness, 461.8+/-37.4 micrometer versus 467.9+/-30.71 micrometer (p=0.064), while Orbscan measurements were statistically less than the theoretical residual corneal thickness, 433.2+/-48.75 micrometer versus 464.32+/-33.4 micrometer (p<0.001). CONCLUSIONS: In LASIK eyes, the ultrasonic pachymetry was a more accurate measurement of corneal pachymetry than Orbscan topography system.

Factors Associated with the Posterior Corneal Surface Elevation in Mmyopic Eye

PURPOSE: To know the factors associated with the posterior corneal surface elevation in myopic eye. METHODS: The posterior corneal surface elevation (PCSE), keratometry, central corneal thickness (CCT), peripheral corneal thickness (PCT), thinnest corneal thickness (TCT) were measured in 64 right myopic eyes with the Orbscan topography system. The correlation between PCSE and age, refractive error, intraocular pressure (IOP), keratometry, CCT, TCT, PCT, PCT-CCT, PCT/CCT, PCT-TCT, and PCT/TCT was evaluated. RESULTS: The posterior corneal surface elevation has correlation with CCT (r:.0.3306, p:0.0071), TCT ( r :.0.4094, p:0.0007), PCT-CCT(r:0.3326, p:0.0068), PCT/CCT (r:0.3759, p:0.0020), PCT-TCT (r: 0.4750, p:0.0001), and PCT/TCT (r:0.5145, p:0.0001) but does not have correlation with age, keratometry, refractive error, IOP (r:.0.0627, p:0.6225), and PCT. CONCLUSIONS: In myopic eyes, the posterior corneal surface elevation correlates with central corneal thickness and specifically the ratio PCT to TCT.