Comparison of total keratometry with corneal power measured by optical low-coherence reflectometry and placido-dual Scheimpflug system

Purpose: To compare the total keratometry (TK) and astigmatism measurements in eyes with cataract using automated keratometry of swept-source optical coherence tomography (ss-OCT), optical low-coherence reflectometry (OLCR), simulated keratometry (SimK), and total corneal power (TCP) of combined placido-dual Scheimpflug imaging system. Setting: The study was conducted at LV Prasad Eye Institute, Hyderabad, India. Design: Retrospective evaluation of electronic medical records of patients who were evaluated for cataract surgery Methods: Twenty-eight eyes of 28 patients were included in the study. All patients evaluated for cataract surgery underwent corneal power measurements using three devices: ssOCT, OLCR, and combined placido-dual Scheimpflug imaging were included in the study. Vector analysis was performed to evaluate corneal astigmatism and Bland-Altman analysis was conducted to evaluate the limits of agreement of similar parameters among devices. Results: The mean TK was statistically significantly different from the keratometry obtained from optical biometers and values measured by the Scheimpflug imaging system. The magnitude of mean difference was greater between TK and TCP (0.75 ± 0.25) compared to other variables. The mean difference in astigmatism between TK, ss-OCT-K (0.09 ± 0.12, p = 0.48), OCLR-K (0.10 ± 0.48, p = 0.91), and TCP (0.09 ± 0.47, p = 0.31) was not statistically significant but was statistically significant between TK and SimK values (0.23D ± 0.49D). The axis of orientation (<20°) of astigmatism was comparable (100%, 28 eyes) between two keratometry variables measured by ss-OCT. Conclusion: There appears to be a greater correlation of automated keratometry, and TK values obtained from ss-OCT compared to other variables studied. The measurements from TK, Simk, and TCP cannot be used interchangeably.

Pupil barycenter configuration in patients with myopia and hyperopia

Purpose To compare the pupil barycenter configurations that can be used to calculate the kappa angle in terms of distance in patients with myopia and hyperopia. Method : This prospective study evaluated 394 eyes of 197 patients. The patients were divided into two groups according to their spherical equivalent values: the myopic group (mean spherical equivalent refraction < − 0.50 D) and the hyperopia group (mean spherical equivalent refraction > + 0.50 D). The two groups were further subdivided according to severity (myopic group: mild, <−0.50 and ≤ − 3.00 D; moderate, <−3.00 and ≤ − 6.00 D; severe, <−6.00 D; hyperopic group: mild, ≥+0.50 and ≤ + 2.00 D; moderate, >+2.00 and ≤ + 4.00 D; severe, >+4.00 D). The pupil and iris barycenter distance measurements and other parameters were obtained through optical low-coherence reflectometry. Results Of the 197 patients, 109 (55.3%) were female and 88 (44.7%) were male, and their age ranged from 7 to 60 years (mean, 35.16 ± 14.75 years). The average pupil barycenter measurements originating from the kappa angle distances were 0.38 ± 0.15 and 0.21 ± 0.11 mm in patients with hyperopia and myopia, respectively. Corneal and lens thickness measurements were higher in patients with hyperopia, whereas anterior chamber depth and pupil diameter measurements were higher in patients with myopia. No significant difference in astigmatism or white-to-white measurements was observed between patients with hyperopia and those with myopia. Conclusion The kappa angle distances are higher in hyperopia than in myopia. The pupil barycenter measurement from the kappa angle distance can be an alternative for the evaluation of the kappa angle.

Comparison of Preoperative Angle Kappa Measurements in the Eyes of Cataract Patients Obtained from Pentacam Scheimpflug System, Optical Low-Coherence Reflectometry, and Ray-Tracing Aberrometry

Background Angle kappa plays a vital role in the implantation of multifocal intraocular lens (MIOLs). Large angle kappa is related to higher risk of postoperative photic phenomena. This study aims to compare preoperative angle kappa in the eyes of cataract patients obtained from Pentacam Scheimpflug system (Pentacam), optical low-coherence reflectometry (Lenstar), and ray-tracing aberrometry (iTrace). Methods One hundred thirteen eyes of 113 patients with cataracts were included. Each eye was examined by the devices to obtain angle kappa (X-Y Cartesian coordinates) and pupil diameter. When considering dependent eyes for one individual, angle kappa in right eyes and left eyes should be analyzed separately. The difference, correlation, and agreement between values for horizontal and vertical components of angle kappa were evaluated by paired t-tests, Pearson or Spearman tests, and Bland‑Altman analysis, respectively. Results No significant differences in the pupil diameter were found between Pentacam and Lenstar or Lenstar and iTrace in both eyes (P > 0.05). Positive angle kappa (nasal light reflex) was found in most cataract patients (79.25–84.91%) through 3 different devices in both eyes. The significant differences of angle kappa were not found between Pentacam and Lenstar compared to Pentacam and iTrace or Lenstar and iTrace in both eyes (P > 0.05). Pentacam and Lenstar showed strong correlation in the horizontal and vertical components of angle kappa compared to Pentacam and iTrace or Lenstar and iTrace in both eyes (r = 0.906 to 0.939). The 95% limits of agreement (LoA) in the horizontal and vertical components of angle kappa with Pentacam and Lenstar were narrower respect to Pentacam and iTrace or Lenstar and iTrace in both eyes. Conclusions There were no differences, strong correlation, and good agreement in angle kappa with Pentacam and Lenstar for cataract patients. Based on these findings, the measurement of preoperative angle kappa in the eyes of patients with cataracts by Pentacam and Lenstar has good agreement, and can be used interchangeably, but caution is warranted for outcomes-based iTrace that report angle kappa.

Signal-to-Noise Ratio of Brillouin Grating Measurement with Micrometer-Resolution Optical Low Coherence Reflectometry

Signal-dependent speckle-like noise was the dominant noise in a Brillouin grating measurement with micrometer-resolution optical low coherence reflectometry (OLCR). The noise was produced by the interaction of a Stokes signal with beat noise caused by a leaked pump light via square-law detection. The resultant signal-to-noise ratio (SNR) was calculated and found to be proportional to the square root of the dynamic range (DR) defined by the ratio of the Stokes signal magnitude to the variance of the beat noise. The calculation showed that even when we achieved a DR of 20 dB on a logarithmic scale, the SNR value was only 7 on a linear scale and the detected signal tended to fluctuate over ±14% with respect to the mean level. We achieved an SNR of 24 by attenuating the pump light power entering the balanced mixer by 55 dB, and this success enabled us to measure the Brillouin spectrum distributions of mated fiber connectors and a 3-dB fused fiber coupler with a micrometer resolution as examples of OLCR diagnosis.