Validity of the keratometric index: evaluation by the Pentacam rotating Scheimpflug camera

PURPOSE: To determine the keratometric index based on actual measurements of the anterior and posterior corneal surfaces using a rotating Scheimpflug camera (Pentacam, Oculus) and evaluate the accuracy of this keratometric index in estimating total and posterior corneal powers. SETTING: Departments of Ophthalmology, Taipei Medical University Hospital and Taipei City Hospital, Taipei, Taiwan. METHODS: The right eye of 221 subjects was measured with the Pentacam system. The radius of the best-fit sphere for the anterior corneal surface (rant) and posterior corneal surface (rpost), mean radius of simulated keratometry (rsimK), and central corneal thickness were obtained. The ratio of rant to rpost (AP ratio) and keratometric index were calculated in each eye. RESULTS: The means for rant, rpost, rsimK, and AP ratio were 7.75 mm +/- 0.28 (SD), 6.34 +/- 0.28 mm, 7.75 +/- 0.27 mm, and 1.223 +/- 0.034 mm, respectively. These parameters were normally distributed. The mean calculated keratometric index (Ncal) was 1.3281 +/- 0.0018. Using the keratometric indices of 1.3281 (Ncal), 1.3315 (Gullstrand schematic eye), and 1.3375 (conventional), the mean arithmetic and absolute estimation errors for the total corneal power were, 0.00 +/- 0.24 diopter (D) and 0.17 +/- 0.17 D, 0.43 +/- 0.23 D and 0.45 +/- 0.21 D, and 1.21 +/- 0.24 D and 1.21 +/- 0.24 D, respectively. The total corneal power was predicted to within +/-0.50 D of the actual value in 95.0%, 60.2%, and 0.9% of eyes, respectively. The mean arithmetic and absolute estimation errors for the posterior corneal power using an AP ratio of 1.223 (this study) or 1.132 (Gullstrand schematic eye) were 0.00 +/- 0.17 D and 0.13 +/- 0.12 D and 0.47 +/- 0.18 D and 0.47 +/- 0.17 D, respectively. The posterior corneal power was estimated to within +/-0.50 D of the actual value in 97.7% and 60.2% of eyes, respectively. CONCLUSION: Using the Pentacam-derived keratometric index improved the prediction accuracies of total and posterior corneal powers.     

Comparison of corneal measurements in keratoconic eyes using rotating Scheimpflug camera and scanning-slit topography

AIM: To compare the anterior segment measurements obtained by rotating Scheimpflug camera (Pentacam) and Scanning-slit topography (Orbscan IIz) in keratoconic eyes. METHODS: A total of 121 patients, 71 males (58.7%) and 50 females (41.3%) (214 eyes) with the diagnosis of keratoconus (KC) were enrolled in this study. Following diagnosis of KC by slit-lamp biomicroscopic examination, central corneal thickness (CCT), thinnest corneal thickness (TCT), anterior chamber depth (ACD), and pupil diameter (PD) were measured by a single examiner using successive instrumentation by Pentacam and Orbscan. RESULTS: There was no significant difference between the two instruments for the measurement of CCT and TCT. In contrast, scanning-slit topography measured ACD (3.46±0.40 mm vs. 3.38±0.33 mm, P=0.019) and PD (4.97±1.26 mm vs 4.08±1.19 mm, P<0.001) significantly larger than rotating Scheimpflug camera. The two devices made similar measurements for CCT (95% CI: -2.94 to 5.06, P=0.602). However, the mean difference for TCT was -6.28 (95% CI: -10.51 to -2.06, P=0.004) showing a thinner measurement by Orbscan than by Pentacam. In terms of the ACD, the mean difference was 0.08 mm (95% CI: 0.04 to 0.12, P<0.001) with Orbscan giving a slightly larger value than Pentacam. Similarly, Orbscan measurement for PD was longer than Pentacam (95% CI: 0.68 to 1.08, P<0.001). CONCLUSION: A good agreement was found between Pentacam and Orbscan concerning CCT measurement while comparing scanning-slit topography and rotating Scheimpflug camera there was an underestimation for TCT and overestimation for ACD and PD.     

Reproducibility and repeatability of central corneal thickness measurement in keratoconus using the rotating Scheimpflug camera and ultrasound pachymetry

PURPOSE: To assess repeatability, reproducibility, and agreement of rotating Scheimpflug camera (Pentacam Oculus, Wetzlar, Germany) and ultrasound pachymetry in measuring central thickness of keratoconic corneas. DESIGN: Method-comparison study. METHODS: In 33 patients with keratoconus (one eye per patient), two examiners each used both pachymetric methods to measure central corneal thickness (CCT); in the same session, measurements then were repeated by examiner 1 (A.M.). The difference between two examiners, and between first and second measurements by examiner 1, with both methods and the difference between the two pachymetric methods in measuring central thickness of keratoconic corneas were noted. RESULTS: With the rotating Scheimpflug camera, interexaminer correlation was higher (intra-class correlation coefficient [ICC], 0.98 vs 0.76) and inter-examiner variability was lower (95% limits of agreement [95% LoA], -14.8 to 13.8 microm vs -18.0 to +49.5 microm) than with ultrasound pachymetry. Both methods showed close first- to second-measurement correlation (ICC, > 90), but the rotating Scheimpflug camera had lower variability (95% LoA, -14.5 to 14.2 microm vs -27.4 to 26.0 microm). Mean CCT was 478.9 +/- 34.6 microm with the rotating Scheimpflug camera and 486.6 +/- 30 microm with ultrasound pachymetry. Although the mean difference was small (-7.8 microm), the 95% LoA (-43.8 to 28.2 microm) showed that the difference between the two methods can be considerable. CONCLUSIONS: In keratoconic corneas, the rotating Scheimpflug camera provides measurements of central thickness that are more reproducible and repeatable than those obtained with ultrasound pachymetry. The rotating Scheimpflug camera seems to be suitable for disease staging and follow-up, when corneal thickness measurements may be repeated over time by different examiners.     

Comparison of anterior segment measurements using rotating Scheimpflug imaging and partial coherence interferometry

METHODS:As part of the first phase of Shahroud Eye Cohort Study with 5 190 subjects of 40 to 64 years of age, CCT and ACD were measured using Scheimpflug imaging with the Pentacam (Oculus, Inc., Lynnwood, WA, USA) and partial coherence interferometry with the Allegro BioGraph (Wavelight, Erlangen, Germany).RESULTS:After applying exclusion criteria, we had data of 4 387 subjects with a mean age of 50.7±6.2 years. Mean CCT with Pentacam and BioGraph were 528.6±33.2μm and 525.6±32μm respectively; the difference was statistically significant (P<0.001), but the correlation was high (R=0.920). Mean ACD measurements using Pentacam and BioGraph were 2.68±0.35mm and 2.62±0.33mm respectively; the inter-device difference was significant (P<0.001) with high correlation (R=0.944). The 95% limits of agreements between devices were -22.65μm to 28.61µm and -0.16mm to 0.29mm for CCT and ACD measurements, respectively.CONCLUSION:For both CCT and ACD, the BioGraph gave significantly lower values than the Pentacam (P<0.05). Despite the high inter-device correlation, the 95% limits of agreements were wide, and this may limit their interchangeability in measuring the CCT and ACD.     

Fourier analysis of corneal Scheimpflug imaging: clinical use in keratoconus

AIM: To evaluate the clinical use of Fourier analysis of videokeratography data in the diagnosis and follow-up of keratoconus (KC). METHODS: We conducted a chart review of consecutive patients presented to our cornea clinic. A team of two experienced cornea specialists divided the patients into three groups: normal cornea, forme fruste KC (FFKC), and clinical KC. The exclusion criteria were a history of previous ocular surgery, any accompanying corneal pathology other than KC, high myopia (>6.00 diopters), amblyopia, pregnancy, breastfeeding, or any current autoimmune disease. The data of Fourier series harmonic analysis were evaluated for their diagnostic capacity using the receiver operating characteristic (ROC) curve. A binary logistic regression analysis was also conducted to construct a diagnostic model. A total of 259 eyes showed progression in the clinical KC group and underwent a combination of accelerated corneal collagen cross-linking and topography-guided customized treatment with an excimer laser. RESULTS: The study included 1262 eyes (618 normal, 530 KC, and 114 FFKC) of 1262 patients. We observed that maximum decentration (MaxDec) was almost as good as maximum keratometry (Kmax) in detecting progressive KC. The area under the curve (AUC) was 0.95 for KC [95% confidence interval (CI): 0.93-0.96] and 0.84 for FFKC (95%CI: 0.79-0.88). Higher predictive accuracy was obtained using a model combining the spherical component, MaxDec, irregularity, and regular astigmatism in the center of the cornea (AUC: 0.97; sensitivity: 89%, and specificity: 96%). CONCLUSION: Decentration, Kmax, and posterior radii of curvatures from a 3.0-mm optical zone centered on the thinnest point of the cornea provide the highest accuracy with low reproducibility of Kmax.     

The role of reference body selection in calculating posterior corneal elevation and prediction of keratoconus using rotating Scheimpflug camera

Purpose: To explore the effects of corneal astigmatism and asphericity on posterior elevation values calculated by different reference bodies and to compare their predictive accuracy in the diagnosis of keratoconus. Methods: In this prospective observational case series, 44 eyes of 24 patients with keratoconus and 70 eyes of 41 refractive surgery candidates as a control group were measured by the Pentacam Scheimpflug camera. Discriminating ability and predictors of posterior elevation measurements obtained by best fit toric ellipsoid (BFTE) and best fit sphere (BFS) reference surfaces were compared by receiver operator characteristic curves (ROC) and generalized estimating equation (GEE) models. Bland–Altman plots were used to determine the agreement between different reference surfaces. Results: Receiver operator characteristic curve analysis showed that posterior elevation measured by a BFTE auto had a significantly higher area under ROC curves (0.99) value than BFTE 8‐mm or BFS reference surfaces. ROC analysis identified cut‐off values for BFTE auto (9.5‐μm), for BFTE 8‐mm (10.5‐μm), for BFS auto (16.5‐μm) and for BFS 8‐mm (15.5‐μm) reference surfaces. According to GEE models, corneal cylinder and posterior asphericity had the least effect in toric ellipsoid models. Bland–Altman plots showed a systematic bias at higher values of average posterior elevation measured BFS reference surfaces. Conclusions: Posterior corneal elevation value measured by the Pentacam camera can effectively discriminate keratoconus from normal corneas although measured values and cut‐off points depend on the selection of reference body and corneal asphericity. Toric ellipsoid reference surface seems to be the most sensitive method to differentiate keratoconus.     

Prospective evaluation of changes in anterior segment morphology after laser iridotomy in Chinese eyes by rotating Scheimpflug camera imaging

Background: To investigate the 3‐D morphology changes of anterior segment in primary angle‐closure suspect and primary angle‐closure after laser peripheral iridotomy (LPI) using rotating Scheimpflug camera. Methods: A total of 37 eyes of 25 patients were enrolled in the study. Just before and after LPI, rotating Scheimpflug camera was performed on each selected eye by one skilled ophthalmologist. Comparison of anterior segment parameters before and after LPI such as central corneal thickness, central anterior chamber depth, peripheral anterior chamber depth, anterior chamber volume, pupil diameter and anterior chamber angle of cross‐section photographs from 0° to 180° (ACA) were analysed. Results: Before and after LPI, central corneal thickness increased from 537.92 ± 27.92 µm to 541.49 ± 27.85 µm (P = 0.074); no statistic changes of central anterior chamber depth were found before LPI (1.72 ± 0.27 mm) and after LPI (1.70 ± 0.24 mm) (P = 0.337); peripheral anterior chamber depth deepen from 0.89 ± 0.26 to 1.14 ± 0.26 mm (P = 0.000); anterior chamber volume increased from 55.54 ± 14.25 to 82.65 ± 17.63 mm³ (P = 0.000); average pupil diameter of all patients are 1.72 ± 0.42 and 1.63 ± 0.46 mm, respectively, showed no statistic difference (P = 0.228). ACA widen from 25.51 ± 5.66° to 28.11 ± 5.67° in 9 o'clock direction(P = 0.005), and from 25.77 ± 5.15° to 27.91 ± 4.87° in 3 o'clock direction after LPI (P = 0.020). Conclusion: LPI induce dramatic changes of 3‐D anterior segment morphology in primary angle‐closure suspect and primary angle‐closure.     

应用旋转Scheimpflug照相系统对角膜参数的研究

目的以旋转Scheimpflug照相技术（Pentacam）研究角膜前表面曲率（Ra）、后表面曲率（Rp）及角膜厚度;根据测量结果,探讨角膜屈光力的计算方法。方法应用Pentacam对302名正常角膜的Ra、Rp、角膜厚度及测量所得K值（Km）进行测量,计算Rp与Ra比值（Rp／Ra）、角膜前表面屈光力（Ka）、后表面屈光力（Kp）,以几何光学公式计算角膜屈光力（Ko）与标定的角膜系数（Nc）,对角膜各参数进行正态分布检验,直线回归分析Rp与Ra的相关性。结果各参数均服从正态分布。Ra为（7．74±0．26）mm,Ka为（48．62±1．61）D,CT为（0．545±0．036）mm,与既往报道的数据相似;角膜后表面参数与既往报道的数据及Gullstrand模型眼不同,Rp为（6．44±0．25）mm,Kp为（-6．22±0．24）D,Rp／Ra为0．832±0．017,低于Gullstrand模型眼的参数;Rp与Ra呈直线相关,回归方程为Rp=0．81×Ra＋0．17（R^2=0．71,P〈0．05）;Nc为1．3288±0．0010,低于标准的角膜系数1．3375。Ko为（42．52±1．42）D,低于Km（43．64±1．44）D,差异有统计学意义（t=151．87,P〈0．01）。K1.3288为（42．52±1．41）D,与Ko比较差异无统计学意义（t=-0．052,P=0．96）。Ko与K1.3288呈直线相关,回归方程为Ko=1．006×K1.3288-0．241（R^2=0．99,P〈0．05）。结论旋转Scheimpflug照相技术与传统角膜地形图仪原理不同,应用该技术发现Rp、Kp、Rp／Ra及Nc与Gullstrand模型眼参数不同,对于标准化角膜光学模型及角膜屈光力的计算具有指导意义。     

两种基于Scheimpflug成像原理的生物测量仪对眼前节参数测量的一致性研究

目的：评估两种基于Scheimpflug成像技术原理的国产Scansys和进口Sirius三维眼前节分析系统，对近视患者眼前节生物参数测量的差异性与一致性。

方法：系列病例研究，选取2022-05/10沈阳爱尔卓越眼科医院诊断为轻中度近视拟行屈光手术(包括全飞秒、半飞秒、TransPRK、ICL植入术)患者103例103眼(所有患者均右眼入组)。收集患者术前角膜曲率(Km)、中央角膜厚度(CCT)、前房深度(ACD_Endo.)、前房角(ACA)、前房容积(ACV)、白到白(WTW)。

结果：Scansys和Sirius测量术前Km为42.88(41.54，44.60)、42.98(41.56，44.52)D，CCT为541.52±29.08、549.55±29.62μm，ACA为42.70°±2.67°、46.63°±5.13°，WTW为12.10±0.60、11.98±0.47mm，差异均有统计学意义(均P<0.01)。Scansys和Sirius测量ACV为194.26±31.06、191.47±25.65mm³，ACD_Endo.为3.40(3.17，3.57)、3.43(3.19，3.56)mm，差异均无统计学意义(均P>0.05)。两种仪器测量Km、CCT、ACA、ACD_Endo.、ACV、WTW值区间范围小，平均差异接近0，95%LoA线外点比例均<5%，一致性较好。

结论：Scansys和Sirius对眼前节参数测量结果差异较小，一致性较好，临床上可以相互参考替代。Scansys理论上可以用来推算ICL型号，或可成为眼前节参数测量的一种新选择。