In vivo human corneal deformation analysis with a Scheimpflug camera, a critical review

Abstract

Corneal morphological analysis has greatly improved in recent years, providing physicians with new and reliable parameters to study. Moreover, today corneal functional too is a routine analysis, thanks to biomechanical evaluation allowed by an ocular response analyzer (Reichert Ophthalmic Instrument, Depew, NY, USA). Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany), that relies on the ultrahigh speed Scheimpflug camera, is a new device providing corneal deformation parameters measured ny scanning the cornea response to an air puff; it is an instrument able to measure intraocular pressure too. This device could open up a whole new prospective in screening, detecting and managing corneal diseases, intraocular pressure measurement and in evaluating surgical procedures involving the cornea. This paper provides a comprehensive explanation of Corvis ST measurement principles and parameters and a literature review of scientific studies.

Biomechanical Diagnostics of the Cornea

“…Differences in applied force might confound attempts to directly compare results obtained with these two instruments. 19 …”

“…21,22 Repeatability studies showed that the Corvis can produce reliable measurements of IOP and central corneal thickness (CCT), while the results are more variable when the corneal deformation and biomechanical parameters are analyzed. 19,23–25 These limitations do not diminish the importance of measuring biomechanical behavior in vivo, and both devices have greatly advanced our understanding of corneal biomechanical behavior in many clinically relevant states. Studies that combine tomographic and biomechanical assessment have demonstrated enhanced refractive surgery screening potential 26,27 and suggest that many advances are yet to come in the use of biomechanical measurements for diagnosis and clinical decision making.…”

Applications of Scheimpflug Imaging in Glaucoma Management: Current and Potential Applications

“…The corneal response to the air puff is initially marked by an inward conformational change in the corneal curvature. This initial flattening of the cornea is referred to as the first applanation (Figure 3) [50, 55]. The cornea eventually deforms to a point where it is maximally concave prior to returning to its original shape.…”

“…This event defines the corneal deformation amplitude at the highest concavity (Figure 3). The cornea then naturally returns to its original shape, which is referred to as the second applanation [50, 55]. Various aspects of the corneal deformation response to the air-puff can be quantified, which are reviewed in detail elsewhere [55].…”

“…The ORA functions in a very different way from the Corvis ST; while the air puff of the Corvis ST is applied with a fixed force, the ORA air puff is delivered with a variable force. Although both instruments assess corneal biomechanics, it is difficult to compare the metrics obtained by these two devices [55, 58]. …”