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100Ambrósio Jr R, Ramos I, Luz A, Faria FC, Steinmueller A, Krug M, Belin MW, Roberts CJINTRODUCTIONFrom central keratometry to corneal topography (frontsurface curvature maps), then into 3-D cornealtomography systems, there was a tremendous evolutionon the diagnostic methods for characterizing corneal shape (1,2) .Biological properties, such as wound healing response andbiomechanics, are essential for determining and maintainingcorneal transparency, as well as geometrical and opticalproperties (3) . Corneal biomechanical behavior is well known toinfluence the development of ectatic diseases (4) , and the resultsof surgery (5) . Also, corneal properties influence intraocularpressure measurements (6) , and also may be an independent riskfactor for glaucomatous neuropathy (7-9) .The assessment of corneal biomechanical properties hasbeen limited to laboratory in vitro studies (4,10) , and to virtualmathematical corneal finite element models (6 11,12) . The OcularResponse Analyzer (ORA, Reichert Inc., Depew, NY), amodified non-contact tonometer (NCT) designed to provide amore accurate measurement of IOP through the understandingof compensation for corneal properties, was the first clinical toolfor assessing in vivo biomechanical properties of the cornea (13) .Other nondestructive methodologies have been described,including radial shearing speckle pattern interferometry (14,15) ,Brillouin optical microscopy (16) , OCT (ocular coherencetomography) (17) , and other forms of dynamic cornealimaging (18,19) .We present the Corvis ST (Scheimpflug Technology), anew NCT system integrated with an ultra-high-speed (UHS)Scheimpflug camera that was recently introduced by Oculus(Wetzlar, Germany).Figure 1: Oculus Corvis ST (Wetzlar, Germany)METHODSThe instrument has an ergonomic design (Figure 1), withadjustable head console and chin rest. The patient is comfortablypositioned with proper placement of the chin and forehead. Thepatient is asked to focus at the central red LED (Light EmittingDiodes). A frontal view camera is mounted with a keratometertypeprojection system for focusing and aligning the corneal apex.The exam is programmed for automatic release when alignmentis achieved with the first Purkinje reflex of the cornea. Manualrelease is also possible.The UHS Scheimpflug camera takes over 4,300 frames persecond to monitor corneal response to a metered collimated airpuff with fixed profile that has symmetrical configuration andfixed maximal internal pump pressure of 25 kPa (Figure 2). TheUHS Scheimpflug camera has blue light LED (455 nm, UV free)and covers 8.5mm horizontally of a single slit. Recordingmeasurement time is 30ms, which allows for acquiring 140 digitalframes. Each image has 576 measuring points.This imaging system allows dynamic inspection of the actualdeformation process during NCT. Advanced algorithms for edgedetection of the corneal contours are applied for every frame(Figure 3). The recording starts with the cornea at the naturalconvex shape. The air puff forces the cornea inwards (ingoingphase) through applanation (first or ingoing applanation) into aconcavity phase until it achieves the highest concavity (HC).There is an oscillation period before the outgoing or returningphase. The cornea undergoes a second applanation beforeachieving its natural shape when there is a possible oscillation.The timing and corresponding pressure of the air puff atFigure 2: Graphic presentation of the air pressure (internal) and thecorneal apex signal with detected applanation momentsthe first and second applanations and at the HC moments areidentified. Intraocular pressure (IOP) is calculated based on thetiming of the first applanation event. This initial IOPmeasurement is dependant of corneal resistance. Thedeformation amplitude (DA) is detected as the highestdisplacement of the apex in the HC moment image. The radiusof curvature at highest concavity is recorded. Applanation lengthsRev Bras Oftalmol. 2013; 72 (2): 99-102