Inter‐observer variability of central corneal thickness measurements using non‐contact specular microscopy after laser in situ keratomileusis

Background: The aim of this study was to investigate the inter‐observer variability of central corneal thickness measurements carried out with a non‐contact specular microscope in patients who had undergone myopic laser in situ keratomileusis. Methods: Twenty‐six eyes of 26 subjects who had undergone myopic laser in situ keratomileusis were studied with the Topcon SP‐2OOOP non‐contact specular microscope (Topcon Corp, Tokyo, Japan). The mean of three consecutive measurements of the central corneal thickness was recorded by two investigators prior to and one month after myopic laser in situ keratomileusis. Results obtained by each of the two physicians were compared. Results: Prior to surgery the mean central corneal thickness was 536.2± 22.2 μm for physician 1 and 536.3 ± 22.2 μm for physician 2 (p = 0.980). The pre‐operative 95 per cent confidence interval was 522.7 to 544.2 μm (physician 1) and 523.1 to 544.1 μm (physician 2). The mean central corneal thickness was 485.8 ± 29.3 μm for physician 1 and 485.6 ± 28.0 μm for physician 2 one month after laser in situ keratomileusis (p = 0.977). The post‐operative 95 per cent confidence intervals were 470.1 to 496.2 μm and 470.3 to 495.4 μm for physicians 1 and 2, respectively. Conclusion: Central corneal thickness measurements can be carried out by different clinicians after laser in situ keratomileusis with non‐contact specular pachymetry, without there being any significant differences among them.     

To compare central corneal thickness measurements obtained by Pentacam with those obtained by IOLMaster 700, Cirrus anterior segment optical coherence tomography and Tomey specular microscopy in normal healthy eyes

Purpose:To compare central corneal thickness measurements obtained by Pentacam with those obtained by IOL Master 700, Cirrus Anterior segment optical coherence tomography and Tomey Specular microscopy in normal healthy eyes.Methods:Two hundred and six eyes of healthy subjects were included in the study. Each subject was assessed by four different methods of measuring central corneal thickness using Pentacam, IOL Master 700, Cirrus AS-OCT and Tomey Specular microscopy by a single examiner.Results:The mean CCT [± standard deviation (SD)] for Pentacam, IOL Master 700, Cirrus AS-OCT and Tomey Specular microscopy were Pentacam (Oculus), AS-OCT (Cirrus), IOL Master 700 and Specular microscopy (Tomey) were 523.75 (±27.75), 525.29 (±28.81),517.13 (±28.43) and 512.82 (±27.60) μm, respectively. All the means were significantly different from one another (P < 0.000). The differences between pairs of mean central corneal thickness (CCT) for Pentacam and IOL Master, Pentacam and anterior segment- optical coherence tomography (AS-OCT), and Pentacam and Specular microscopy are statistically significant. Bland–Altman plots showed that pentacam and IOL Master 700 have the closest agreement, followed by AS-OCT. Specular microscopy was found to have the poorest agreement with Pentacam.Conclusion:We found that CCT measurements of Pentacam did not correlate with measurements of IOL Master, or AS-OCT or Specular microscopy. In clinical practice, the devices analyzed should not be used interchangeably due to low agreement regarding CCT values.     

Reliability and repeatability of the Pentacam on corneal curvatures

Background: The purpose of this study was to assess the intra‐observer reliability and inter‐session repeatability of the Pentacam system (Oculus, Inc.) on the simulated keratometry readings (Sim K) and peripheral corneal curvatures (PCC) at both the anterior and posterior corneal surfaces. Methods: One eye was randomly selected from each of the 39 normal subjects from the School of Optometry at The Hong Kong Polytechnic University. In the first session, three readings were taken for reliability analysis. Measurements were repeated in a second session scheduled one to two weeks later for repeatability analysis. Two approaches were used, comparing the first reading between the two sessions and comparing the average from three readings between the two sessions. Both the anterior and posterior Sim K and PCC were calculated from the tangential and axial maps. The inter‐session repeatability was reported as the coefficient of repeatability (COR = ± 1.96 × standard deviation of differences). Relative repeatability (RR) was a percentage of the ratio of COR to the mean. Results: All parameters showed good intra‐observer reliability (Cronbach's alpha ≥ 0.925; intra‐class correlation coefficient, ICC ≥ 0.804), except for the posterior power vector J45 (= 0.896; ICC = 0.742) and tangential PCC at the superior region (= 0.891; ICC = 0.732). The inter‐session repeatability on anterior Sim K (COR ≤ ±0.21 D; RR ≤ 0.48 per cent) and posterior Sim K (COR ≤ ±0.09 D; RR ≤ 1.39 per cent) was good. Axial PCC showed better repeatability than the tangential PCC. The COR from using the first approach for all the posterior PCC was within 0.23 D, while the COR reduced to within 0.16 D from using the second approach. Conclusions: The Pentacam showed good intra‐observer reliability on Sim K and PCC measurements. Averaging three readings from one visit could improve repeatability between visits.     

Comparison of central corneal thickness as measured by non-contact specular microscopy and ultrasound pachymetry before and post LASIK

Purpose: To compare central corneal thickness (CCT) measurements made by ultrasound pachymetry and non‐contact specular microscopy in eyes before and post laser in situ keratomileusis (LASIK). Methods: Forty eyes of 20 consecutive refractive surgery candidates were included in this study. The CCT in both eyes was measured before and 3 months after LASIK with a non‐contact specular microscope and a ultrasound pachymeter. Result: Both before and after LASIK, the difference of the mean CCT between non‐contact specular microscopy and ultrasound pachymetry was statistically significant (P < 0.001 before surgery, and P = 0.02 after surgery). The difference between them was −14.41 μm and −3.69 μm, respectively. In both situations, non‐contact specular microscopy and ultrasound pachymetry measurements were highly correlated (r = 0.959, before surgery, and r = 0.979 after LASIK). Bland–Altman analysis showed that either before or post LASIK, the 95% limits of agreement have a span of approximately 30 μm. Preoperatively, the mean standard deviation of repeated measurements was 4.26 μm by ultrasound pachymetry, and 4.02 μm by non‐contact specular microscopy, whereas it was 3.83 μm by ultrasound pachymetry, and 3.89 μm by non‐contact specular microscopy, postoperatively. No statistically significant differences were found between the mean standard deviations by these two devices in both situations (P = 0.561 before surgery, and P = 0.849 after LASIK). Conclusions: Both before and after LASIK, non‐contact specular microscopy is not in complete agreement with ultrasound pachymetry in measuring CCT. Both devices provided comparable repeatability of measurements.     

三种仪器测量圆锥角膜中央角膜厚度的对比研究

目的:对比A型超声角膜测厚仪、角膜内皮镜和Pentacam眼前节分析仪测量圆锥角膜中央角膜厚度的差异。方法:选择2012-07/10圆锥角膜患者31例55眼,分别使用A型超声角膜测厚仪、角膜内皮镜和Pentacam眼前节分析仪测量中央角膜厚度,并对测量结果进行F检验和Pearson相关性分析。结果:A型超声(55眼)、角膜内皮镜(45眼)和Pentacam(52眼)测量值分别为469.87±57.56,479.00±42.39,487.02±44.64μm,三者测量值结果相比差异没有统计学意义(P>0.05)。对三种仪器的测量结果进行直线相关分析,A型超声与角膜内皮镜,r1=0.758,P<0.01;A型超声与Pentacam眼前节分析仪,r2=0.949,P<0.01;Pentacam眼前节分析仪与角膜内皮镜,r3=0.685,P<0.01。结论:三种仪器的中央角膜厚度测量值存在正相关,Pentacam眼前节分析仪测量角膜厚度不仅与A超结果更接近,而且方法安全简便,全面反应角膜各点厚度,更适用于圆锥角膜患者的角膜厚度检查及科研研究。     

Pentacam系统与A超角膜测厚仪测量中央角膜厚度的一致性和重复性

目的 探讨Pentacam三维眼前节分析诊断系统测量角膜厚度的可重复性,比较其结果与A型超声角膜测厚仪测量结果的一致性。方法 112例拟行准分子激光角膜屈光手术的受检者,只选取右眼作为研究对象,分别由两位检查者肓法使用Pentacam系统和A超角膜测厚仪测量中央角膜厚度3次,将结果进行对比分析。角膜厚度值的比较采用配对t检验进行统计学分析;一致性检验采用Bland-Altman法。结果 角膜厚度均值分别为Pentacam系统(538.63±31.55) μm,超声测厚仪(541.02±30.45) μm,两者差异具有统计学意义（t=- 3.414,P=0.001）,两者的差值平均为（-2.39±7.42）μm。两法具有较好的一致性,95％的一致性界限为（- 16.93 μm,12.15 μm）。Pentacam系统与超声测厚仪测量值的组内相关系数分别为0.86和0.88,两种仪器的测量结果都比较稳定、重复性好。结论 Pentacam系统与超声测厚法具有较好的一致性,测量结果的重复性也较好,但要注意增加测量次数。     

Pentacam系统测量正常人眼表面麻醉前后中央角膜厚度的对比研究

目的 比较Pentacam三维眼前节分析系统与A超角膜测厚仪测量正常人中央角膜厚度的差异,并研究Pentacam系统与A超测量正常人眼表面麻醉前后中央角膜厚度的变化,为临床角膜屈光手术方式的选择提供参考.方法 对25例(50眼)正常人分别应用Pentacam系统及A超角膜测厚仪测量中央角膜厚度,Pentacam系统的测...     

Pentacam和Sirius地形图系统与超声测厚仪角膜厚度的对比研究

目的:探讨Sirius与Pentacam及超声测厚仪测量中央角膜厚度的一致性.方法:对51例102眼正常人采用Pentacam和Sirius及超声角膜测厚仪三种仪器测量角膜厚度,记录并比较三种方法测量所得中央角膜厚度结果,同时运用Bland-Altman法绘制图表分析两两间一致性.结果:A超中央角膜厚度平均值低于 Pentacam的平均值,Sirius中央角膜厚度的平均值最接近A超平均值.Sirius与Pentacam一致性的比较:其Bland-Altman图中3.92%的点在95%一致性界限以外.在一致性界限以内,Sirius与Pentacam中央角膜厚度差值的绝对值最大为19.5μm.Sirius与A超一致性的比较:其Bland-Altman图中2.94%的点在95%一致性界限以外.在一致性界限以内,Sirius与A超中央角膜厚度差值的绝对值最大为18.667μm.Pentacam与A超一致性的比较:其Bland-Altman图中5 82%点在95%一致性界限以外.在一致性界限以内,Pentacam与A超中央角膜厚度差值的绝对值最大为25 33μm.结论:Sirius系统与Pentacam系统具有良好的一致性,且Sirius系统比Pentacam系统更接近A超角膜测厚的结果.     

不同角膜厚度下Pentacam系统对压平眼压计测量值的校正

目的 比较Goldmann压平眼压计(Goldmann applanation tonometry,GAT)和Pentacam眼压校正系统在中央角膜厚度(central corneal thickness,CCT)不同的正常眼测量值的差异,评价其校正准确性.方法 对69例(69只眼)按CCT不同,分为正常组(520～<580μm)42只眼、较薄组(450～<520μm)16眼、增厚组(580～640μm)11只眼.应用Pentacam前房分析仪检查和Goldmann压平眼压计测量眼压,并用Pentacam系统所提供的4种校正方法对眼压测量值进行校正.结果 角膜正常组和角膜增厚组中,平均GAT、各组眼压校正值之间差异均无统计学意义(P>0.05),GAT、各组眼压校正值与CCT之间无线性相关(P>0.05);角膜较薄组中,平均GAT、各组眼压校正值之间差异均有统计学意义(P<0.05),GAT值与CCT之间线性相关(IOP=0.067×CCT-16.582,r2=0.730,P<0.05),Shah组校正值与CCT之间亦存在线性相关(IOP=0.076×CCT-25.349,r2=0.328,P<0.05);其余各组与CCT之间无明显相关(P>0.05).结论 Pentacam三维眼前段分析仪根据角膜厚度校正眼压测量值,尤其对角膜较薄组使用Dresden校正公式,有助于对其诊断和治疗提供更加准确的依据.     

Effect of central corneal thickness on intraocular pressure and comparison of Topcon CT‐80 non‐contact tonometry with Goldmann applanation tonometry

Background

To compare intraocular pressure (IOP) measurements obtained with the Topcon CT‐80 non‐contact tonometer (NCT) and Goldmann applanation tonometer (GAT), in different ranges of IOP in normal and glaucoma subjects, and to assess the influence of central corneal thickness (CCT) on the IOP measurements in Asian Indian eyes.

Methods

Four hundred and two eyes of 402 subjects (193 newly diagnosed primary open angle glaucoma [POAG] and 209 normal) were enrolled for this prospective study. For each eye, IOP was measured with GAT by a glaucoma specialist and NCT by a trained optometrist. The IOP values were compared among the tonometers in the three different IOP ranges (≤ 12 mmHg, 13–20 mmHg and ≥ 21 mmHg) using Bland–Altman graphs. Correlation between GAT and NCT was assessed by Pearson correlation co‐efficient. CCT was measured with ultrasound pachymetry and its correlation with GAT and NCT was analysed using linear regression analysis.

Results

The mean paired difference of IOP between NCT and GAT was 1.556 ± 2.69 mmHg (r = 0.26, p = 0.006) at IOP range of ≤ 12 mmHg, ?1.665 ± 2.6 mmHg (r = 0.51, p < 0.0001) in IOP range of 13–20 mmHg and ?2.202 ± 3.44 mmHg (r = 0.82, p < 0.0001) in the IOP range of ≥ 21 mmHg. Linear regression analysis showed a mean IOP variation of 0.27 mmHg per 10 μm change in CCT for NCT (p < 0.0001) and IOP change of 0.19 mmHg per 10 μm change in CCT for GAT (p = 0.01).

Conclusion

In this study of normotensive and POAG subjects, the Topcon CT‐80 NCT showed an overestimation of IOP at the lower range and underestimation of IOP in normal and higher ranges of IOP. Clinicians should keep in mind that CCT influences IOP measurement with both types of tonometer and that the IOP readings obtained with these tonometers are not interchangeable.

     

Tear-side re̊gulation of desquamation in the rabbit corneal epithelium: A specular microscopy study

Specular microscopy of the corneal epithelial surface reveals cells varying greatly in reflectance. Older cells appear brighter than new cells. With in vitro preparations bright cells desquamate one by one. Initially they bulge out at their centres, and finally they break lateral attachments. Cell fenestrations are not a normal feature of old or desquamating cells. The surface life of a cell is normally about nine hours, but this depends on the solution bathing the epithelium. When glutathione-bicarbonate Ringer is replaced with saline, cells desquamate in sheets at a rate of about 10×normal. The surface appearance of cells is also altered. me desquamatory behaviour of cells, but not their surface appearance, is normal when calcium is included in the tear-side bathing solution. Both surface appearance and desquamation rate return to normal when glutathione-bicarbonate Ringer's is substituted for saline. It is concluded that tears have a nutritional role in maintaining the integrity of the corneal surface     

四种非接触式光学仪器测量中央角膜厚度一致性的研究

目的 比较Pentacam HR三维眼前节分析系统、角膜光学显微镜SP-3000P、扫频源光学生物测量仪OA-2000及相干光眼前段成像仪(Optovue Angiovue OCT)测量中央角膜厚度(CCT)的一致性。设计诊断技术评价。研究对象 2021年9-11月北京同仁医院眼科角膜正常的白内障患者118例(1 18眼)。方法 分别采用上述4种仪器测量角膜厚度,比较角膜顶点CCT值的差异性和一致性,并比较Pentacam HR测量角膜顶点及瞳孔中央CCT值的差异,以及Optovue OCT测量角膜顶点及中央2 mm直径平均CCT值的差异。主要指标CCT值、组内相关系数(ICC)、Bland-Altman图中平均值差值及95%一致性界限(LoA)。结果 Pentacam HR、SP-3000P、OA-2000及Optovue OCT测量角膜顶点CCT值分别为(532.29±30.35)μm、(512.20±30.88)μm、(516.89±31.25)μm、(515.58±29.78)μm;4种仪器比较差异具有统计学意义(F=559.469,P<0.001);其中OA2000测得...     

Comparison of central corneal thickness measurements by Orbscan II and Pentacam after corneal refractive surgery

PURPOSE: To compare the accuracy of postoperative pachymetry between Orbscan II (Bausch & Lomb) scanning-slit corneal topography/pachymetry and the Pentacam (Oculus) rotating Scheimpflug camera. METHODS: Central corneal thickness (CCT) was determined in 24 patients (48 eyes) before and after laser in situ keratomileusis (LASIK) or Epipolis LASIK (Epi-LASIK) procedures. All eyes were examined by Orbscan II and Pentacam prior to refractive surgery and at the first, fourth, and twelfth week postoperatively. The residual CCT (RCCT) measured by each instrument was compared to the theoretical RCCT. RESULTS: On the first, fourth, and twelfth week after the refractive surgery, the mean RCCT measurements by Orbscan II were 413 +/- 72, 435 +/- 65, and 440 +/- 69 microm, respectively. Those of Pentacam were 434 +/- 51, 436 +/- 53, and 438 +/- 50 microm, respectively. Orbscan II measurements at the postoperative first and fourth week were significantly smaller than the theoretical RCCT (P < 0.01, P < 0.01, paired t test). There was no statistical difference between the theoretical RCCT and the Pentacam measurements at any stage. CONCLUSION: The Orbscan II measurement values of postoperative corneas tended to be thinner than the theoretical values, but not those of the Pentacam.     

干眼症对角膜地形图角膜厚度检测结果的影响

目的:探讨干眼症对Pentacam三维眼前节分析仪角膜厚度检测结果的影响。方法:利用Pentacam三维眼前节分析仪及A超对有干眼症的30例患者进行检测,记录治疗前后的角膜厚度,采用SPSS11.5统计分析软件处理检测数据,用配对t检验分析治疗差异,以P<0.05为有统计学意义。结果:干眼症患者治疗前Pentacam三维眼前节分析仪测得的角膜厚度范围428～557μm,均值为504.5±37.3μm,A超角膜测厚仪测得的角膜厚度范围477～596μm,均值为532.7±32.4μm。两种测量方法所得数值差异有统计学意义(P<0.05)。治疗后Pentacam三维眼前节分析仪测得的角膜厚度范围469～607μm,均值为541.2±41.1μm,A超角膜测厚仪测得的角膜厚度范围472～593μm,均值为538.4±30.7μm。两种测量方法所得数值差异无统计学意义(P>0.05)。结论:干眼症会对Pentacam眼前节分析仪的角膜厚度测量结果产生影响,对有干眼症的患者,应在给予适当治疗后进行厚度评估,以便取得更精确的厚度信息。     

Sirius和Pentacam测量前房深度及中央角膜厚度值的比较研究

背景 眼前节解剖结构的生物学测量参数对于青光眼、眼外伤的诊治及人工晶状体(IOL)度数的测量具有重要意义.目的 比较Sirius与Pentacam测量前房深度及中央角膜厚度(CCT)的差异,并对两种仪器的测量结果进行一致性评价.方法 收集温州医学院眼视光学院在校学生38名,其中男20人,女18人;年龄23 ～ 32岁.每位受检者右眼分别用Pentacam和Sirius对前房深度和CCT进行测量,每种仪器对每眼测量3次,取平均值进行各种仪器的重复性和一致性评价,并对两种测量结果进行差异比较和一致性评价.结果 Pentacam测量的前房深度为(3.18±0.21) mm,组内相关系数(ICC)为0.995,变异系数(CV)为0.066;Sirius测量的前房深度为(3.22±0.21 )mm,ICC为0.996,CV为0.065.两种仪器测量前房深度的差值为0.04 mm,差异有统计学意义(t=-6.225,P＜0.05);两种仪器测量前房深度的相关系数r=0.977;95％一致性区间为(-0.04 ～0.13)mm,位于均值的1个标准差区间±0.21 mm内,此结果临床上可以接受.Pentacam测量的CCT为(535±33) μm,ICC为0.994,CV为0.062;Sirius测量的CCT为(537±36) μm,ICC为0.999,CV为0.067.两种仪器测量的CCT相差2μm,差异无统计学意义(t=1.771,P＞0.05);两种仪器测量CCT的相关系数r=0.985;95％一致性区间为(-11.64 ～15.65)μm,位于均值的1个标准差区间±34.27 μm内,此结果临床上可以接受.结论 Sirius与Pentacam测量前房深度及CCT的一致性较好,两种方法在对眼前节参数的测量时可相互替代,为临床眼前节参数的测量提供了一个新的选择.     

Corneal thickness and endothelial cell density measured by non-contact specular microscopy and pachymetry in Rhesus macaques (Macaca mulatta) with laser-induced ocular hypertension

PURPOSE: Sustained increase in intraocular pressure (IOP) in humans results in a loss of corneal endothelial cells and an increase of corneal thickness. The effects of chronically elevated IOP on the corneal endothelium of monkeys with laser-induced ocular hypertension, a commonly used animal model of human glaucoma have not been documented. This study examined the central corneal thickness (CCT), the corneal endothelial cell density (ECD), and the corneal endothelial cell size (ACS) in Rhesus monkeys with experimental ocular hypertension.Materials and methods. Ten male monkeys with argon laser-induced ocular hypertension in one eye for an average duration of 2.4+/-0.7 years, were sedated with ketamine hydrochloride, and the CCT, ECD, and ACS measured at the center of the cornea of both eyes with a Topcon SP-2000P non-contact specular microscope (Topcon America Corporation((R)), Paramus, NJ, USA). CCT was also measured using a DHG-500 Pachette ultrasonic pachymeter (DHG Technology Inc., Exton, PA, USA). Mean and standard deviation (S.D.) of CCT, ECD and ACS for each eye was calculated and statistically compared.Results. Mean CCT in the hypertensive and normal eyes measured by specular microscopy was 0.477+/-0.023mm and 0.468+/-0.020 mm, respectively. Mean ECD in the hypertensive and normal eyes was 2601.7+/-631.8 and 3990.2+/-402.9 cells mm(-2), respectively. The mean size of the endothelial cells was 252.4+/-23.9 micro m(2) in the normal eye and 408.7+/-115.0 microm m(2) in the hypertensive eye. No significant difference in the measurement of CCT was observed between the specular microscope and the pachymeter (p=0.46).No significant difference in the mean CCT was observed between the two eyes (p=0.4820), whereas the mean ECD was significantly lower in the hypertensive eye than in the normal eye (p<0.001). The ECD was inversely related to the length of IOP elevation (p<0.001). CONCLUSIONS: No difference in the corneal thickness measurement was observed between the specular microscopy and the pachymetry techniques. Chronic ocular hypertension did not significantly affect the CCT, but caused a significant loss of endothelial cells in the center of the cornea of the laser treated eyes compared to the normotensive eyes. The duration of elevated IOP was the most important factor affecting the ECD.     

Pentacam HR及Optovue iVue OCT测量圆锥角膜和近视患者角膜厚度的研究

目的 分析Pentacam HR、Optovue iVue OCT在圆锥角膜和近视患者中角膜厚度测量值的一致性,以期对角膜交联手术和角膜屈光手术患者角膜厚度测量提供指导.设计诊断技术评价.研究对象2018年10月-2020年10月北京同仁医院圆锥角膜和拟行角膜屈光手术的近视患者181例(359眼),其中近视166眼,圆...     

Pentacam三维眼前节分析仪测量近视眼角膜厚度的可靠性分析

目的:比较Pentacam三维眼前节分析系统、OrbscanⅡ角膜地形图系统、A型超声测厚仪在近视眼患者角膜厚度中的测量,为临床应用提供理论依据。方法:分别利用上述3种测量方法对在我院行LASIK手术的患者进行角膜最薄点厚度的测量并进行统计学分析。结果:分别采用Pentacam,OrbscanⅡ,A超角膜测厚仪对56例56只近视右眼进行角膜厚度测量,测得角膜厚度的平均值分别为(547.04±3.65)μm、(535.41±32.93)μm和(539.46±29.15)μm。将3种不同仪器测得的角膜厚度平均值进行方差分析,F=1.992,P=0.140,差异无统计学意义;三种仪器测量角膜厚度有很好的线性相关;Penta-cam仪测量角膜厚度具有较好的重复性。结论:用Pentacam三维眼前节分析系统进行LASIK术前角膜厚度的测量,其结果准确、全面、方便,具有较好的临床应用价值。     

On the use of Orbscan II to assess the peripheral corneal thickness in humans: a comparison with ultrasound pachometry measures.

PURPOSE: To compare the measures of corneal thickness measurements obtained by an optical scanning slit method with those obtained by an ultrasound (US) pachometer, with special interest in the mid-peripheral (2.5 mm from centre) and peripheral (4.5 mm from centre) region of the cornea. METHODS: Three measures of corneal thickness were taken using Orbscan II and then by US pachometry (under topical anaesthesia with benoxinate 0.4%) on 24 adults, aged 20-58 years and with up to 8.5 D of myopia. The full Orbscan topography maps were used to extract single point data along the horizontal corneal meridian for the geometric centre, 2.5 mm from centre (nasal and temporal) and 4.5 mm (nasal and temporal) from centre. No correction factor was used for the Orbscan data. The same set of measures were made with the US pachometer. In all cases, the averages of three (centre) or six (mid-periphery and periphery) readings were taken as the measurements from each cornea. RESULTS: Orbscan readings on the right eyes averaged 0.576, 0.632 and 0.712 mm for central, mid-peripheral and peripheral sites with average values for emmetropic subjects (<1 DS, n = 12) being marginally higher than for myopic subjects (average - 4.00 DS, n = 12). For US pachometry, the average values were however 0.522, 0.554 and 0.606 mm. Similar results were obtained on left eyes. Combining data from both eyes also showed that the mean difference between Orbscan II and US measures was not constant across the cornea, being 0.055 +/- 0.014 mm at the centre, 0.080 +/- 0.019 mm at mid-peripheral locations and 0.107 +/- 0.046 mm at the peripheral sites. These differences persisted after application of the generally recommended acoustic factor (x0.92) to all of the Orbscan readings. CONCLUSIONS: A single acoustic factor correction cannot be applied to all corneal thickness measures made with an Orbscan II to equate the measures to those made with an US pachometer.