Orbscan 2

Central corneal thickness (CCT), axial length (AL), aqueous depth (AD, the distance between the posterior corneal surface and anterior lens vertex), anterior chamber depth (ACD, the distance between anterior corneal vertex and anterior lens vertex), and lens thickness (LT) were measured.
Average corneal curvature (Kmed) and corneal astigmatism (difference between maximal and minimal corneal curvature) were assessed using keratometry. Instruments used for biometry were the IOL Master™ (software version V.3.01, Carl Zeiss AG, Oberkochen, Germany), the LENSTAR LS 900® (Haag-Streit Holding AG, Könitz, Switzerland), the ORBSCAN II® (Bausch & Lomb, Rochester, NY, USA) and the GALILEI™ V5.2.1 (Ziemer Ophthalmic Systems AG, Port, Switzerland).

Characterizations of patients, including age, gender, cause, and associated conditions, were collected. The interval from the onset to the start of medical treatment and the interval from onset of therapy to PTK were also recorded. Preoperatively, all patients underwent a slit lamp examination, spherical equivalent measurement, keratometry, corneal topography (Orbscan II, Bausch & Lomb Inc., Rochester, NY, USA), uncorrected visual acuity exam, best spectacle-corrected visual acuity (BCVA) determination when possible, intraocular pressure measurement with Goldmann applanation tonometry, or noncontact tonometry (not possible to perform in all cases). Ultrasound pachymetry (pachymeter SP-3000, Tomey Laboratories, Inc., Phoenix, AZ, USA) was performed on all eyes preoperatively to assess suitability for PTK.
On the day of surgery, digital slit lamp photography was performed to determine the lesion size. Central corneal thickness (CCT), central lesion thickness, and peripheral lesion thickness measurements were taken. Accordingly, the thickness of corresponding areas on the nonaffected eye was also determined. The preoperative BCVA was the last recorded value before the excimer laser procedure was performed. The spherical equivalent and BCVA of the healthy eyes were also measured and recorded.

Orbscan II (Bausch & Lomb, Rochester, NY) provides axial dioptric power maps of both corneal surfaces over the central optical zone diameters of 3 mm and 5 mm and the average pupil diameter under the prevailing conditions. This device is a placido disc scanning slit-lamp system designed to retrieve data from 9000 locations over the cornea [26 (link)]. The scanning slit provides the numerical separation between the two corneal surfaces at each location. These data, in conjunction with the dioptric power maps generated for the front surface, are used to construct an axial dioptric power map of the back surface. The algorithm used is an extension of earlier methods [13 , 25 (link)]. Calculation of corneal back surface powers is based on refractive index values of 1.376 for the cornea and 1.336 for the aqueous humour. The data from the multiple power maps are averaged to provide astigmatic power, and axis, values over two corneal chord diameters (3 mm and 5 mm). Full descriptions and functionality of Orbscan II are provided elsewhere [26 (link), 27 (link)]. This is a long-standing instrument proven to be useful for examining the corneal surfaces in a wide range of studies [11 (link), 28 (link)–33 (link)]. Refractive astigmatism was determined by autorefractometry and by routine subjective refraction.

Anterior corneal topography was obtained and exported from the Orbscan II device (Bausch & Lomb, Bridgewater, NJ, USA). This system projects 40 optical slits, 20 from the right and 20 from the left, onto the cornea at a 45-degree angle. The resulting slit images were captured by a digital video camera and used to reconstruct the topography of corneal surface. The topography maps were repositioned to be centered on the pupil center. The KISA% index was calculated based on the Placido-based axial power maps from the Orbscan II. A Fourier-domain OCT system (RTVue, Optovue, Fremont, CA, USA) was used to acquire corneal pachymetry and epithelial thickness maps. The system works at an 830-nm wavelength and has a scanning speed of 26,000 axial scans per second. The depth resolution of RTVue is 5 μm (full-width-half-maximum) in tissue. The OCT scan pattern for mapping the cornea was “Pachymetry+CPwr,” which consisted of eight evenly spaced radial scans 6 mm in length. The pachymetry and epithelial thickness maps were also centered on the pupil center.