What is the difference between corneal biomechanics and corneal hysteresis, particularly in relation to intraocular pressure (IOP) measurements in patients with glaucoma or ocular hypertension, and how do devices like Corvis (Corneal Visualization) and ORA (Ocular Response Analyzer) measure these properties?

Corneal Biomechanics vs. Corneal Hysteresis: Understanding the Distinction

Corneal hysteresis is one specific component of overall corneal biomechanics, and yes, Corvis ST and ORA measure different (though related) biomechanical properties using distinct technologies. 1

Defining the Terms

Corneal biomechanics is the broad umbrella term encompassing all mechanical properties of the cornea—including stiffness, elasticity, viscosity, and resistance to deformation. 1

Corneal hysteresis (CH) is one specific biomechanical parameter that represents the cornea's viscoelastic damping capacity—essentially the difference in corneal behavior during inward versus outward deformation, reflecting energy dissipation. 1

How ORA and Corvis ST Differ

Ocular Response Analyzer (ORA)

• Uses a bidirectional air pulse to cause the cornea to move inward and then outward through two applanation events. 1
• Measures corneal hysteresis (CH) as the primary biomechanical output—the difference between inward and outward applanation pressures. 1
• Also measures corneal resistance factor (CRF), another derived parameter reflecting overall corneal resistance. 2, 3
• Calculates "corneal-compensated IOP" (IOPcc) and GAT-equivalent IOP (IOPg) that attempt to account for corneal biomechanical influence. 1, 3
• Does not require topical anesthesia. 1

Corvis ST (Corneal Visualization Scheimpflug Technology)

• Uses high-speed Scheimpflug imaging during air-puff induced corneal deformation to capture dynamic corneal response parameters (DCRs). 4, 5
• Measures multiple biomechanical parameters including stiffness parameter at first applanation (SP-A1), stress-strain index (SSI), integrated radius, deflection amplitude at highest concavity, and inverse concave radius. 4, 5
• Calculates biomechanically corrected IOP (bIOP) that accounts for corneal biomechanical properties. 4, 5
• Provides more comprehensive visualization of the entire deformation process with detailed morphological parameters. 4, 5

Clinical Significance: Why This Matters

Impact on IOP Measurement Accuracy

• Both devices reveal that Goldmann applanation tonometry (GAT) is significantly influenced by corneal biomechanics, not just central corneal thickness (CCT) alone. 1, 6
• In normal-tension glaucoma (NTG), biomechanically corrected IOP was significantly higher than GAT-IOP, suggesting true IOP may be underestimated in these patients with softer corneas. 5, 3
• In primary open-angle glaucoma, bIOP from Corvis ST was significantly lower than GAT-IOP, indicating GAT may overestimate true pressure. 4, 5
• Post-refractive surgery, GAT significantly underestimates true IOP, and biomechanically-adjusted measurements are more accurate. 1, 6

Biomechanics as Independent Risk Factor

• Lower corneal hysteresis (measured by ORA) correlates with worse visual field mean deviation in NTG, independent of IOP or CCT. 2, 5
• NTG corneas are significantly softer and more deformable compared to controls, ocular hypertension, and high-tension glaucoma, as demonstrated by lower stiffness parameters on Corvis ST. 5
• Ocular hypertension eyes have stiffer, less deformable corneas (higher SP-A1 and SSI on Corvis ST), which may explain why they tolerate higher IOP without damage. 4, 5
• CH and CRF are more powerful predictors of NTG progression than CCT alone. 2

Critical Clinical Caveats

Do NOT Use Correction Nomograms

The American Academy of Ophthalmology explicitly advises against using IOP correction formulas based on CCT or biomechanics, as no validated correction nomogram exists. 1, 7, 6

Instead, Use This Approach:

• Measure CCT and corneal biomechanics as independent risk stratification tools, not as correction factors. 7, 6
• Rely on optic nerve examination, OCT imaging of nerve/RNFL, and visual field testing for diagnosis rather than attempting to "correct" IOP values. 1, 6
• Use the same tonometry device consistently from visit to visit, as switching between devices introduces variability that can mask true IOP changes or falsely suggest progression. 6

When to Use Alternative Tonometry

In diseased or post-surgical corneas (keratoconus, post-LASIK, corneal edema, scarring), GAT is highly inaccurate, and alternative methods like ORA, Corvis ST, dynamic contour tonometry, or pneumotonometry should be used. 1, 6

Practical Algorithm for Clinical Use

For glaucoma risk assessment:

• Measure CCT (thinner ≤510 μm = higher risk). 7
• Measure corneal hysteresis via ORA (lower CH = higher risk, particularly in NTG). 7, 2, 3
• Integrate with other risk factors (age, family history, cup-to-disc ratio, perfusion pressure). 7, 8

For IOP measurement in abnormal corneas:

• Use ORA or Corvis ST instead of GAT in post-refractive surgery, keratoconus, or corneal scarring. 1, 6
• Maintain consistency with the same device across visits. 6

For understanding glaucoma susceptibility:

• Softer corneas (lower CH, lower stiffness parameters) suggest higher susceptibility to glaucomatous damage at any given IOP. 4, 2, 5
• Stiffer corneas (higher CH, higher stiffness parameters) may explain tolerance of higher IOP without damage in ocular hypertension. 4, 5