Does Corvis ST Measure Corneal Hysteresis?
No, Corvis ST does not directly measure corneal hysteresis—that parameter is specifically measured by the Ocular Response Analyzer (ORA), which is a different device. Corvis ST is a dynamic Scheimpflug analyzer that captures corneal deformation and generates different biomechanical parameters, while corneal hysteresis is the proprietary measurement of the ORA system 1.
Understanding the Two Different Technologies
Ocular Response Analyzer (ORA)
- The ORA uses a collimated air pulse to cause bidirectional corneal applanation (inward then outward movement) and directly calculates corneal hysteresis (CH) and corneal resistance factor (CRF) as its primary biomechanical outputs 1.
- The device measures the difference in pressure between the inward and outward applanation events, which defines corneal hysteresis 2.
- CH varies over a dynamic range of 1.8 to 14.6 mm Hg and is only weakly correlated with corneal thickness 2.
- The ORA also calculates "corneal-compensated" IOP and GAT-equivalent IOP 1.
Corvis ST (Corneal Visualization Scheimpflug Technology)
- Corvis ST is a dynamic Scheimpflug analyzer that uses high-speed Scheimpflug imaging to capture the corneal deformation process during a standardized air puff, generating up to 12 different biomechanical parameters—but not corneal hysteresis 1.
- The device generates parameters including: time of highest concavity, applanation times (A1 and A2), applanation lengths, velocities, deformation amplitude, peak distance, and radius measurements 3, 4.
- These parameters provide detailed structural observation of corneal deformation but are fundamentally different measurements than ORA's corneal hysteresis 5.
The Relationship Between Corvis ST Parameters and Corneal Hysteresis
While Corvis ST doesn't measure corneal hysteresis directly, research has examined correlations between the two devices:
- Corvis ST parameters show only weak to moderate correlations with ORA-measured corneal hysteresis 5.
- The strongest correlations found were: deformation amplitude ratio (corneal softness, R=-0.51), SP A1 (corneal stiffness, R=0.41), and Inverse Radius (R=-0.44) 5.
- A multivariate model using multiple Corvis ST parameters (A1 time, A2 time, highest concavity deformation amplitude, and CCT) achieved R=0.67 correlation with CH, but this still represents only moderate association 5.
Clinical Implications
For Keratoconus and Ectasia Detection
- Both devices are being evaluated for early keratoconus detection, but they measure different aspects of corneal biomechanics 1.
- The American Academy of Ophthalmology notes that corneal hysteresis (from ORA) appears to provide independent risk information for glaucoma progression, though this is supplementary rather than essential 6.
- Corvis ST has demonstrated utility in characterizing pellucid marginal degeneration and monitoring post-crosslinking changes, but serves as an additional diagnostic tool rather than first-line 7, 3.
For Post-Refractive Surgery Assessment
- Both devices show altered biomechanical properties after LASIK, ReLEx flex, and ReLEx smile procedures 4.
- ORA-measured CH and CRF are significantly lower after all keratorefractive procedures compared to controls 4.
- Corvis ST parameters (such as A1 deflection length and HC time) also change after refractive surgery, but these are distinct measurements from corneal hysteresis 4.
Common Pitfall to Avoid
Do not assume that Corvis ST and ORA are interchangeable or that Corvis ST parameters can substitute for corneal hysteresis measurements. While both devices assess corneal biomechanics using air-puff technology, they employ fundamentally different measurement principles: ORA derives parameters from pressure information during bidirectional applanation, while Corvis ST provides detailed structural observation through high-speed Scheimpflug imaging 5. The weak to moderate correlations between devices mean they provide complementary but not equivalent information 5.