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SKILL Corneal shape, structure and thickness
Published in Sam Evans, Patrick Watts, Ophthalmic DOPS and OSATS, 2014
The Pentacam™ (Oculus, Germany) system is based on the Scheimpflug principle, which states that an optical system with a fixed depth of focus will produce crisp images of its subject only if that subject is uniplanar, parallel with the lens and within the area of focus; if the plane of the subject is not coincident with the lens, parts of it will be out of focus (in front and behind the focal point). To overcome this, the lens must be rotated about the focal point, until it and the subject’s plane are coincident. In the case of a curved surface, such as the cornea, this requires constant readjustment of the lens position. By measuring the excursion of the lens as it finds the focused image, the Pentacam can map the curvature of the corneal surface. Scheimpflug cameras obtain very high-resolution images of curved surfaces, and, when used in conjunction with ultrasound biomicroscopy (UBM), render enormously detailed images of anterior-segment structures.
Anterior and posterior corneal higher-order aberrations in early diagnosis and grading of keratoconus
Published in Clinical and Experimental Optometry, 2023
Abbasali Yekta, Hassan Hashemi, Hadi Ostadimoghaddam, Mohsen Hadizadeh, Shokoofeh Rafati, Asgar Doostdar, Payam Nabovati, Mohammad Mehdi Sadoughi, Mehdi Khabazkhoob
Corneal wavefront analysis was performed using Pentacam HR. This instrument is based on the Scheimpflug principle and includes a rotating camera that captures 50 sectional images in 2 seconds, ultimately resulting in the capture of 138,000 distinct elevation points. This elevation data provide a three-dimensional scan of the cornea and anterior chamber. The internal software of the device automatically transforms the data points to corneal wavefront analysis using Zernike polynomials. The root-mean-square (RMS) values were calculated for third-order, fourth-order, fifth-order, sixth-order, and total HOAs for both anterior and posterior corneal surfaces. All Zernike coefficients and RMS values were calculated for a 6 mm corneal diameter in line with most previous studies.17,19–21 Only high quality scans, which displayed ‘OK’ in the scan quality specification (QS) box were considered. To distribute the tear film evenly, patients were asked to blink completely before starting measurement.
Anterior Segment Biometry and Their Correlation with Corneal Biomechanics in Caucasian Children
Published in Current Eye Research, 2019
Inmaculada Bueno-Gimeno, Noelia Martínez-Albert, Andrés Gené-Sampedro, Enrique España-Gregori
Corneal topography and anterior segment biometry were assessed using a rotating Scheimpflug camera (Pentacam; Oculus Inc., Wetzlar, Germany). The Pentacam device is based on the Scheimpflug principle to acquire cross-sectional images of the anterior ocular segment in a non-invasive way. Anterior segment measurements obtained were the following: central corneal thickness in the pupil center (CCT), corneal volume (CV) in a 10.0 mm diameter around the corneal vertex, anterior chamber depth (ACD) measured from the corneal endothelium to the anterior lens surface, and anterior chamber volume (ACV) delimited by a 12 mm diameter around the corneal vertex. Moreover, the measurements obtained of the topographic evaluation was the mean K for anterior and posterior corneal surface from 3 mm central zone.35
Comparison of central corneal thickness measurements with standard ultrasonic pachymetry and optical devices
Published in Clinical and Experimental Optometry, 2019
Optical biometry (AL‐Scan, NIDEK CO., LTD.) employs the partial coherence interferometry principle to measure the axial length and calculates the keratometry using the reflected images over 2.4 and 3.3-mm diameters on the cornea. The White‐to‐White (WTW) is also measured from the obtained image of the anterior segment of the eye. Anterior chamber depth and central corneal thickness are obtained using the Scheimpflug principle.2014