A quantitative scale for the Barrett keratoscope

By allowing the surgeon to measure and modify corneal curvature during wound closure, intraoperative keratometry can reduce postoperative astigmatic errors. A number of keratometric devices have been developed over the last 10 years, each offering a compromise between cost, accuracy and ease of use. The Barrett keratoscope is a simple, inexpensive hand-held device which gives a qualitative indication of the degree of astigmatism. Addition of a transparent overlay, as described in this paper, sets the distance at which the keratoscope is held and allows the magnitude of the astigmatism to be determined, thereby enabling refined adjustment of the suture tension during wound closure. We describe the error in estimation of astigmatism due to the effect of the distance at which the keratoscope is held from the cornea.     

Reliable keratometry with a new hand held surgical keratometer: calibration of the keratoscopic astigmatic ruler

AIM—Some surgeons consider hand held surgical keratometers unreliable. This may be due to incorrect use through not realising that the distance that the keratometer is held from the cornea influences the shape of the image. When a keratometer is held closer to the astigmatic cornea, the elliptical image will appear more circular, particularly for larger degrees of astigmatism. However, the keratoscopic astigmatic ruler (KAR) has design features that correct the hitherto unrecognised problems with the use of a hand held keratometer. This study assesses the reliability and accuracy of measurement of astigmatism using the KAR.
METHODS—The KAR and the Bausch & Lomb keratometer (B&L) were compared using six back surface toric cut contact lens blanks representing 1 to 6 dioptres of astigmatism. Two observers (one experienced in the use of the keratometers, the other a novice) took eight randomly repeated "masked" measurements of each lens blank with the KAR and four measurements with the B&L in a similar fashion.
RESULTS—There was no difference between the measurements with either instrument by each of the observers (p=0.95, ANOVA). The standard error of measurement for the KAR was 0.59 D, for the B&L, 0.31 D. The intraclass correlation coefficient of reliability for the KAR was 0.90 and for the B&L it was 0.97. The coefficient of repeatability for the KAR was plus or minus 0.83 D, and for the B&L plus or minus 0.77 D. The interobserver reliability for the KAR was 0.898, and for the B&L, 0.975.
CONCLUSION—These results suggest that the KAR has good reliability and reproducibility and compares favourably with the B&L keratometer. Inexperience with use does not affect reliability.

     

圆锥角膜患者基于角膜曲率验配硬性隐形眼镜的预测公式

目的:探讨隐形眼镜基线(BC)和角膜曲率结果(Krf)之间简单的数学相关性。方法:本回顾性研究包括350例400只圆锥角膜的眼睛,其先前五年在学院眼科中心验配硬性隐形眼镜。根据角膜曲率结果患者被分为五组,分别为Krf<7,Krf:7-8,Krf>8,Krf-Krs(两个角膜曲率的差异;平坦和陡峭)=0.3-0.6,Krf-Krs>0.6mm1至5组,使用多元线性回归和蒙罗相关系数推测公式。结果:除了第3组患者,可以在所有组中发现线性相关。第1组,BC=0.211×5.904Krf。第2组,BC=0.456×Krf4.160。第4组,BC=0.321×5.219Krf。第5组,BC=0.337×Krf+5.090。结论:RGP验配新公式的发展,可增强眼科医生工作的信心,避免不必要的和频繁的眼镜试验。通常的隐形眼镜验配方法需要更换新的公式,从而帮助节省时间和费用。     

Rigid contact lens fitting based on keratometry readings in keratoconus patients:predicting formula

AIM:To find a simple mathematical correlation between the lens base curve(BC) and keratometry findings(krf).METHODS:This retrospective study included 400 keratoconic eyes(350 patients) previously fit with rigid contact lenses at an academic eye center over a five year period.The patients were classified into five groups based on the keratometry findings(krf<7,krf:7-8,krf>8,krf-krs(difference between two keratometry;flat and steep)= 0.3-0.6,krf-krs >0.6mm as groups 1 to 5,respectively.Multivariate linear regression and Munro’s correlation coefficient were employed to defer the formulas.RESULTS:A linear correlation could be found in all groups except for patients in group 3.For group 1,BC=0.211×krf+ 5.904.For group 2,BC=0.456×krf+4.160.For group 4,BC= 0.321×krf+5.219.For group 5,BC=0.337×krf+ 5.090.CONCLUSION:The development of new formulas for RGP fitting enables ophthalmologists to work with confidence and prevents unnecessary and frequent lens trials.The customary lens fitting methods are needed to be replaced by new formulas,which help to save time and costs.     

Rigid contact lens fitting based on keratometry readings in keratoconus patients: predicting formula

AIM: To find a simple mathematical correlation between the lens base curve (BC) and keratometry findings (krf). METHODS: This retrospective study included 400 keratoconic eyes (350 patients) previously fit with rigid contact lenses at an academic eye center over a five year period. The patients were classified into five groups based on the keratometry findings (krf<7, krf:7-8, krf>8, krf-krs (difference between two keratometry; flat and steep)= 0.3-0.6, krf-krs >0.6mm as groups 1 to 5, respectively. Multivariate linear regression and Munro's correlation coefficient were employed to defer the formulas. RESULTS: A linear correlation could be found in all groups except for patients in group 3. For group 1, BC=0.211×krf+ 5.904. For group 2, BC=0.456×krf+4.160. For group 4,BC= 0.321×krf+5.219. For group 5, BC=0.337×krf+ 5.090. CONCLUSION: The development of new formulas for RGP fitting enables ophthalmologists to work with confidence and prevents unnecessary and frequent lens trials. The customary lens fitting methods are needed to be replaced by new formulas, which help to save time and costs.     

Comparing keratometry readings with manual separation of lids and wire speculum in children under general anesthesia

Purpose:Keratometry (K) readings are crucial for intraocular lens power calculation in cataract surgery. In children who do not cooperate, the keratometry is done under general anesthesia with a handheld autokeratometer. However, there is little consensus regarding the method for the measurement of K readings. The lids can be separated either by fingers or a wire speculum may be placed to separate the lids for measurement.Methods:The children selected for the study were patients cooperative for keratometry reading. Nidek KM-500 handheld keratometer was used first in the awake period. Then under general anesthesia, readings were taken first by separating the lids manually with fingers and then after putting a wire speculum in both the eyes.Results:The average keratometry reading for participants in the OPD, anesthetized with lids manually opened and with lids separated with speculum was 44.7 ± 1.7 D, 44.4 ± 1.9 D, and 44.7 ± 1.7 D, respectively.Conclusion:No significant change was observed in keratometry values in children with manual separation of eyelids or with wire speculum.     

Comparison of total corneal power measurements obtained with different devices after myopic keratorefractive surgery

AIM: To analyze the differences, agreements, and correlation among total corneal power parameters generated by different instruments after myopic keratorefractive surgery. METHODS: The prospective cross-sectional study included patients who underwent myopic keratorefractive surgery and received measurements of corneal power 3mo after surgery. Automated keratometer was used for the measurement of simulated keratometry (SimK), swept-source optical coherence tomography (SS-OCT) based biometer for total keratometry (TK), anterior segment-OCT for real keratometry (RK), and Scheimpflug keratometer for the true net power (TNP), the total corneal refractive power (TCRP) and equivalent K-readings (EKR). The differences among these parameters were analyzed, and the agreements and correlation between SimK and other total corneal power parameters were investigated. RESULTS: A total of 70 eyes of 70 patients after myopic keratorefractive surgery were included. The evaluated corneal power parameters were as follows: SimK 38.32±1.93 D, TK 37.54±2.12 D, RK 36.64±2.09 D, TNP 36.56±1.97 D, TCRP 36.70±2.01 D, and EKR 37.55±2.00 D. Pairwise comparison showed that there were significant differences (P<0.001) among all parameters except for between TK and EKR, RK and TNP, RK and TCRP (P=1.000, 1.000, 1.000, respectively). The limits of agreement between SimK and TK, RK, TNP, TCPR, and EKR were 1.08, 1.08, 1.43, 1.48, and 1.73 D, respectively. All parameters showed good correlation with SimK, and the correlation coefficients were 0.995, 0.994, 0.983, 0.982, and 0.975. CONCLUSION: Among the corneal power parameters after myopic keratorefractive surgery, the value of SimK is the largest, followed by TK and EKR, with TCRP, RK, and TNP being the smallest. The differences among the parameters may be attributable to the different calculation principles. Correct understanding and evaluation of corneal power parameters can provide a theoretical basis for taking advantage of the total corneal power to improve the accuracy of intraocular lens calculation after keratorefractive surgery.     

Determining corneal power using Pentacam after myopic photorefractive keratectomy

Purpose: To assess the accuracy of Pentacam Scheimpflug camera for corneal power measurement in eyes with previous photorefractive keratectomy for myopia. Methods: In this comparative interventional case series, 35 eyes of 35 patients who had myopic photorefractive keratectomy were studied. Corneal power was measured by conventional topography and Pentacam Scheimpflug camera, and equivalent keratometry readings (EKR) in different central corneal rings (0.5 to 4.5 mm), true net power and simulated keratometry (K) measurements as well as those obtained using Shammas no‐history, Koch‐Maloney and Haigis methods were compared with clinical history method. Results: All corneal power measurements except for the topography simulated K and true net power values were statistically similar to the clinical history values. Simulated keratometry and 4.5‐mm EKR values were more closely correlated with clinical history method. Shammas formula, Pentacam simulated K and 3‐, 4‐ and 4.5‐mm EKR provided a 95% confidence interval within ±0.50 D of the mean clinical history method value, among these, the width of the 95% limits of agreement (LoA) was narrower for Shammas and Pentacam simulated K and 4.5‐mm EKR values; however, considerably large 95% LoA were found between each of these values and those obtained with the clinical history method. Estimated preoperative keratometry was statistically similar to the preoperative measurement; however, estimated refractive change was different from actual value. Conclusions: The Pentacam 4.5‐mm EKR and simulated keratometry may be used as an alternative to clinical history method to predict corneal power when pre‐keratorefractive surgery data are unavailable; however, wide LoA should be considered in the calculations.     

Keratometry measurements,corneal astigmatism and irregularity in a normal population: the Tehran Eye Study

Purpose: To determine the mean corneal power, astigmatism, and irregularity in a sample from the population of Tehran. Materials and methods: Four hundred and forty‐two people residing in the first four municipality districts of Tehran were selected through a random stratified cluster sampling approach, 410 of which met the inclusion criteria and were enrolled in the study to have Orbscan II acquisitions between 9:00 am and 7:00 pm. The values obtained for keratometry, corneal astigmatism, and corneal irregularity were studied. Results: The mean keratometry reading was 44.39 diopter (D) [95% confidence interval (CI): 44.22–44.57]. Age, spherical equivalent and corneal diameter were significantly correlated with mean keratometry reading in the multivariate model. Mean corneal astigmatism was 0.98 D (95% CI: 0.89–1.06), also significantly correlated with spherical equivalent in the multivariate model. Mean corneal irregularity in the central 3 mm zone was 1.25 D (95% CI: 1.20–1.29); this was significantly correlated with age. Conclusions: In a sample of the Tehran population, we found higher values of mean corneal power, astigmatism, and irregularity compared to other reports. Considering the high prevalence of cases of suspect keratoconus in the studied population, it is important to pay more attention to corneal indices in keratorefractive surgery.     

角膜曲率的分析

目的：探讨我国人角膜曲率半径的正常值及不同性别、不同年龄的角膜曲率半径差异。方法：对10998只眼的角膜曲率进行检测，并按男、女10岁一组进行统计分析。结果：（1）K1为7.65mm,K2为7.71mm，平均K值为7.67mm。较眼科学正常值K:7.77mm短0.1mm。（2）K的平均值男性较女性的长0.1155mm。且女性各年龄段角膜曲率半径均男性的有不同程度的减短。（3）男女均随年龄的增长，角膜曲率半径大致呈递减趋势，即：角膜曲率半径与年龄成反比关系。（４）男女K1,K2之比，均随年龄增长而增长，即K1逐渐增长而增长，即　K1值逐渐增长，K2逐渐减短。结论：本文测定的角膜曲率较眼科文献中的提供的正常值短0.1mm，并且存在着年龄、性别上的差异。     

Clinical evaluation of keratometry and computerised videokeratography: intraobserver and interobserver variability on normal and astigmatic corneas

AIMS—To evaluate intra- and interobserver variability in measurements on normal and astigmatic corneas with keratometry and computerised videokeratography.
METHODS—Keratometric readings with the 10 SL/O Zeiss keratometer and topographic maps with the TMS-1 were obtained by two independent examiners on 32 normal and 33 postkeratoplasty corneas. Inter- and intraobserver coefficients of variability (COR) for measurements of steep and flat meridian power and location, in addition to the magnitude of astigmatism, were assessed.
RESULTS—Compared with TMS-1, the 10 SL/O keratometer showed a superior repeatability in measuring normal corneas (intraobserver COR for keratometry and TMS-1 respectively: 0.22 and 0.30 D for steep meridian power; 0.18 and 0.44 D for flat meridian power; 0.26 and 0.40 D for astigmatism; 5° and 26° for steep meridian location; 5° and 13° for flat meridian location). Astigmatism intraobserver COR (0.20 D and 0.26 D for the two observers) and interobserver COR (0.28 D) of the keratometer for normal corneas was very good and not affected by observers' experience. Repeatability of the TMS-1 on normal corneas was found to be: (a) observer related, and (b) astigmatism related. A novice observer showed a much greater COR (1.62 D for astigmatism, 30° for flat meridian location) compared with the experienced examiner (0.40 D for astigmatism, 13° for flat meridian location). Higher deviation scores were observed for corneas with higher astigmatism. For the postkeratoplasty corneas, again the keratometer achieved superior reproducibility (astigmatism interobserver COR 1.12 D for keratometry, 4.06 D for TMS-1; steep meridian location interobserver COR 10° for keratometry, 34° for TMS-1).
CONCLUSION—Keratometric readings are more reproducible than topographic data both for normal and postkeratoplasty corneas. The two instruments should not be used interchangeably especially on highly astigmatic corneas. For the TMS-1, users with the same level of experience should be employed in clinical or experimental studies.

Keywords: keratometry; computerised videokeratography; astigmatic corneas     

不同角膜曲率测量方法结果的比较

目的:比较Orbscan眼前节系统与RKT-7700自动式角膜曲率计及手动式角膜曲率计测量角膜曲率的差异。方法:对50例100眼分别进行Orbscan、自动式和手动式角膜曲率计角膜曲率测量。结果:Orbscan、RKT-7700自动式及手动式角膜曲率计角膜曲率测量值分别为:43.42±1.45,43.63±1.50,42.33±1.37D。测量的K1,K2,K,d值均以RKT-7700自动式角膜曲率计为最高,手动式曲率计值最低,手动式角膜曲率计测量值较Orbscan角膜曲率测量值偏低1.0838D(P<0.000);手动式角膜曲率计较RKT-7700自动式角膜曲率计低1.296D(P<0.000);自动式角膜曲率计比Orbscan角膜曲率测量值高0.212D(P=0.294)。Orbscan-II眼前节系统与RKT-7700自动式角膜曲率计间各测量值均无显著性差异。各检测方法间角膜散光值(d)均无显著性差异。手动式角膜曲率计测量的K1,K2,K值与Orbscan-II眼前节系统及RKT-7700自动式角膜曲率计有显著性差异。结论:不同角膜曲率测量仪的测量结果有一定的差异。RKT-7700自动式角膜曲率计测量值最高,手动式角膜曲率计测量值最低,Orbscan眼前节系统与RKT-7700自动式角膜曲率计测量值差异没有显著性。     

Increasing the range of the keratometer

How can the ophthalmologist perform keratometry on a patient whose corneal curvature exceeds the limits of the keratometer? A technique for extending the range of the keratometer to test such patients, e.g., those suffering from keratoconus or corneal plana, is presented.     

Comparing the corneal curvatures obtained from three different keratometers-IOL Master, Bausch & Lomb Manual keratometer and TOPCON KR 8800 autokeratometer

AIM: To compare the corneal curvature and to investigate the agreement between three different keratometers. METHODS: In this prospective study, keratometry was performed using an IOL Master, a Bausch & Lomb manual keratometer and TOPCON KR-8800 autokeratometer on 252 eyes of patients recruited from camps for cataract surgery. The average keratometry values were recorded and compared. The agreements between the instruments were analyzed using the Bland Altman statistical method. The main outcome measure was average keratometry values. RESULTS: The mean corneal power was 44.62±1.52 D with the IOL Master, 44.60±1.52 D with the manual keratometer, and 44.46±1.53 D with the autokeratometer. The paired ttest demonstrated a statistically significant difference in the mean corneal power between the IOL Master and manual keratometer (P=0.001), IOL Master and autokeratometer (P<0.0001), autokeratometer and manual keratometer (P<0.0001). The 95% limits of agreement (LoA) of the IOL Master and manual keratometer were -0.22 to 0.26; IOL Master and autokeratometer were -0.24 to 0.55; autokeratometer and manual keratometer were -0.30 to 0.57 as shown in the Bland-Altman plot. CONCLUSION: Keratometry data obtained with different instruments may not be interchangeable, a fact that has important implications for cataract surgeons with respect to both surgical planning and outcomes auditing.     

Can We Use the Fellow Eye Biometric Data to Predict IOL Power?

Purpose: To study the correlation between right (RE) and left eye (LE) keratometry readings (K) and axial lengths (AL) in a population-based sample of normal subjects. Methods: In a cross-sectional retrospective study conducted at S. Giuseppe Moscati Hospital, Avellino, Italy, 4516 eyes of 2258 patients with a mean age of 67 ± 16.36 years (range 18–96 years) were included. Partial coherence interferometry data obtained in right (RE) and left (LE) eyes were analyzed and correlated. Results: The average K was 44.01 ± 1.50 diopters (D) (range 39.09–49.89 D) in the RE and 44.04 ± 1.53 D (range 39.63–51.89 D) in the LE (p = 0.0075). 4.5% of the patients (101) presented with differences in the corneal power ≥ 1 D, corresponding to a difference of roughly 1 D in the IOL power. The average AL was 23.89 ± 1.77 mm (range 19.09–35.15 mm) in the RE and 23.84 ± 1.68 mm (range 19.23–35.04 mm) in the LE (p = 0.0018). 19.2% of the patients (433) presented with differences in the AL ≥ 0.4 mm, corresponding to a difference of roughly 1 D in the IOL power. Conclusions: In calculating the IOL power, we must be aware of these results when we measure the fellow eye to validate the measurements in the first eye. In the case of postcataract refractive error, the outcome could be used for the second eye only when symmetric biometric findings are present.