David Huang

To study the accuracy and repeatability of anterior, posterior, and net corneal power measured by Fourier-domain optical coherence tomography (OCT). Doheny Eye Institute, Los Angeles, California, USA. Cross-sectional study. A Fourier-domain OCT system (RTVue) was used to scan normal eyes, eyes after myopic laser in situ keratomileusis (LASIK), and keratoconic eyes. After the corneal surfaces were delineated, the system calculated anterior and posterior corneal powers by curve fitting over the central 3.0 mm diameter area. Net corneal power was calculated using a thick-lens formula. The repeatability of the calculations was evaluated by the pooled standard deviation of 3 measurements from the same visit. The net corneal power values were compared with standard automated keratometry measurements (IOLMaster). The repeatability of Fourier-domain OCT net corneal power was 0.19 diopters (D), 0.26 D, and 0.30 D in the normal, post-LASIK, and keratoconus groups, respectively. The Fourier-domain OCT net corneal power was significantly lower than keratometry by a mean of -1.21 D, -2.89 D, and -3.07 D, respectively (P<.001). The anterior-posterior curvature ratio was lower in post-LASIK and keratoconic eyes than in normal eyes (P<.001). Corneal power measured by Fourier-domain OCT achieved good repeatability in all 3 groups. The repeatability was better than slower time-domain OCT systems. Because Fourier-domain OCT directly measures both anterior and posterior corneal surfaces, it may produce more consistent results than standard keratometry in post-LASIK and keratoconic eyes in which the anterior-posterior corneal curvature ratios are altered by surgery or disease.

Purpose: To develop an intraocular lens (IOL) power calculation formula based on optical coherence tomography (OCT) that would not be biased by previous laser vision correction. Methods: Twenty-seven eyes of 27 cataract patients without prior laser vision correction who underwent phacoemulsification were included in the study. An optical coherence biometer (IOLMaster, Carl Zeiss Meditec) measured anterior corneal curvature and axial eye length. A high-speed (2000 Hz) anterior segment OCT prototype mapped corneal thickness and measured anterior chamber depth and crystalline lens thickness. Posterior corneal curvature was computed by combining IOLMaster keratometry with OCT corneal thickness mapping. A new IOL formula was developed based on these parameters. One month after phacoemulsification, the manifest refraction spherical equivalent (MRSE) was measured. The prediction error in postoperative MRSE of the OCT-based IOL formula was compared with that of three theoretic formulae: SRK/T, Hoffer Q, and Holladay II. Results: The mean prediction error in postoperative MRSE of the OCT-based formula was 0.04±0.44 diopters (D). The SRK/T was the best of the theoretic formulae, and its prediction error was −0.35±0.42 D. Twenty-one (78%) eyes were within 0.50 D using the OCT formula compared to 18 (67%) eyes using the SRK/T. No statistically significant differences were noted among the formulae. Conclusions: For cataract patients without prior laser vision correction, the OCT-based IOL formula was as accurate as the current theoretic formulae. This new formula is based on direct OCT assessment of the posterior curvature and avoids the calculation errors inherent in conventional IOL formulae.

To evaluate the repeatability of Fourier-domain optical coherence tomography (OCT) pachymetric mapping and compare central corneal thickness (CCT) measurements by OCT, ultrasound pachymetry, and scanning-slit tomography. Doheny Eye Institute, University of Southern California, Los Angeles, California, USA. A Fourier-domain OCT system was used to map the corneal thickness in normal eyes with scans centered on the corneal vertex or the pupil. Repeatability of central and pericentral map sectors was assessed by pooled standard deviation. The CCT measurements were compared between the OCT, ultrasound, and scanning-slit devices. Pupil centration (SD: 1.3 microm central, 1.8 to 3.8 microm pericentral) provided better repeatability than vertex centration (SD: 1.7 microm central, 2.4 to 5.7 microm pericentral) in all sectors (P<.035). The mean CCT was 536.9 microm +/- 27.0 (SD) by OCT, 556.6 +/- 30.5 microm by ultrasound, and 537.2 +/- 32.6 microm by scanning-slit tomography (acoustic factor 0.92). The CCT measured by OCT was significantly thinner than by ultrasound pachymetry (P = .007; mean difference -19.7 microm; 95% limits of agreement [LoA], -40.4 to 0.9 microm) but not than by scanning-slit tomography (P = .2637; mean difference -0.3 microm; 95% LoA, -24.0 to 23.5 microm). The CCT by OCT correlated well with ultrasound and scanning-slit CCTs (r = 0.940 and r = 0.934, respectively). Pachymetric mapping with Fourier-domain OCT was highly repeatable. Repeatability was better with pupil-centered scans than with corneal vertex-centered scans. Ultrasound pachymetry, Fourier-domain OCT, and scanning-slit tomography should not be used interchangeably for CCT assessment.

Uneventful myopic laser in situ keratomileusis (LASIK) was performed in both eyes of a 33-year-old woman. Two weeks after LASIK, examination of the left eye revealed flap striae radiating inferonasally from the superior hinge. The flap was relifted and repositioned to remove the irregular astigmatism and reduce the striae. Before the flap was relifted, Fourier-domain optical coherence tomography (FD-OCT) showed the gap at the temporal flap, which had not been detected by biomicroscopy. After the flap was relifted, FD-OCT documented that the gap was closed. High-resolution FD-OCT was helpful in the visualization and management of flap displacement.

Optical coherence tomography (OCT) is being employed more and more often to image pathologies and surgical anatomy within the anterior segment, specifically in anterior chamber biometry, corneal pachymetric mapping, angle evaluation and high‐resolution cross‐sectional imaging. The cross‐sectional imaging capability of OCT is similar to ultrasound, but its higher resolution allows OCT to measure and visualize very fine anatomic structures. No contact is required. In this review, we describe the utility and limitations of anterior segment OCT.

To study the architecture of clear corneal incisions for phacoemulsification cataract surgery using optical coherence tomography (OCT). Doheny Eye Institute and Department of Ophthalmology, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA. This prospective study comprised 20 eyes of 20 patients 1 month after cataract surgery performed by 1 of 2 experienced surgeons. Temporal clear corneal single-plane incisions were made with 3.0 mm metal keratomes. Each eye was scanned before and 1 month after surgery with a prototype high-speed anterior segment OCT system (Carl Zeiss Meditec, Inc.). The OCT scans were repeated 3 times during the same visit. The length of the corneal incision, thickness of the cornea, and position of the incision (distance from external wound edge to scleral spur) were measured using a computer caliper. The angle of the incision relative to the corneal surface was then calculated. The mean corneal incision length was 1.81 mm +/- 0.27 (SD), the mean corneal thickness at the incision was 747 +/- 67 microm, and the mean distance between the incision and the scleral spur was 1.46 +/- 0.24 mm. The mean angle of the incision was 26.8 +/- 5.5 degrees. The measurements were repeatable to within +/-0.072 mm (pooled standard deviation) for the incision length, +/-11 microm for the corneal thickness, and +/-0.042 mm for the position of the incision. Optical coherence tomography allowed excellent evaluation of corneal incisions in cataract surgery postoperatively. Measurements of wound dimensions using OCT were highly repeatable.