Bottom line: Corneal thickness alone does not predict ectasia risk. Modern biomechanical testing -- using the Ocular Response Analyzer (ORA) and Corvis ST -- measures how your cornea actually behaves under stress, providing the most accurate pre-operative risk assessment available. If your clinic does not offer biomechanical testing, your screening is incomplete.
Why Thickness Is Not Enough
For decades, corneal thickness was the primary safety gate for LASIK. The rule was simple: if the cornea was thick enough to leave adequate residual stromal bed (typically 250-300 microns) after tissue removal, the procedure was considered safe. This rule prevented many cases of ectasia but not all -- because thickness does not capture the full picture.
Two corneas can be the same thickness but have very different biomechanical properties. One might be structurally robust with strong collagen cross-linking; the other might be subtly weakened -- perhaps early in a keratoconus process that has not yet produced topographic changes. Thickness measurements cannot distinguish between them. Biomechanical testing can.
ORA: The Ocular Response Analyzer
The ORA directs a precise air puff at the cornea and measures the pressure at which the cornea flattens inward (applanation 1) and the pressure at which it recovers (applanation 2). The difference between these two pressures is called corneal hysteresis (CH) -- a measure of the cornea's viscoelastic damping capacity.
Higher corneal hysteresis means stronger biomechanics. Lower hysteresis means the cornea absorbs less energy during deformation, indicating weaker structural integrity. Studies have shown that low corneal hysteresis correlates with ectasia risk, glaucoma progression, and keratoconus -- even in corneas with normal thickness and topography.
Corneal Hysteresis Values
Normal corneal hysteresis typically ranges from 9.5 to 12.5 mmHg. Values below 9.0 mmHg raise concern about biomechanical weakness and warrant additional investigation before LASIK is considered. Values below 8.0 mmHg are generally considered a relative contraindication to LASIK in the context of other risk factors.
Corvis ST: Dynamic Scheimpflug Analysis
The Corvis ST takes a different approach. It uses a high-speed camera (4,330 frames per second) to record the cornea's response to an air puff, capturing the entire deformation sequence -- initial inward movement, maximum deformation, and recovery. From this recording, it calculates dozens of parameters describing the cornea's dynamic behaviour.
Key Corvis ST parameters include deformation amplitude (how far the cornea moves inward -- more movement suggests weaker structure), stiffness parameter (resistance to deformation at the applanation point), and the Corvis Biomechanical Index (CBI), which integrates multiple parameters into a single risk score. A CBI above 0.5 suggests possible biomechanical abnormality.
How Biomechanical Screening Changed LASIK Safety
Post-LASIK ectasia was estimated at roughly 1 in 2,000-5,000 cases in the early 2000s. With modern screening that includes biomechanical testing, topographic analysis, and epithelial mapping, ectasia rates have dropped below 1 in 10,000 at centres using comprehensive protocols. The improvement is directly attributable to catching biomechanically at-risk corneas before surgery.
| Screening Level | What It Catches | Estimated Ectasia Risk |
|---|---|---|
| Thickness only | Thin corneas | ~1 in 2,500 |
| Thickness + topography | Thin corneas + early keratoconus patterns | ~1 in 5,000 |
| Thickness + topography + biomechanics | All of the above + subclinical biomechanical weakness | <1 in 10,000 |
💡 What to Ask Your Surgeon
During your consultation, ask: "Do you use ORA or Corvis ST for biomechanical screening?" If the answer is no, ask why. A centre that relies solely on pachymetry (thickness) and topography is using a screening protocol that is adequate but not state-of-the-art. For borderline candidates, biomechanical testing is not optional -- it is essential.
Biomechanical Screening at Colombian Centres
Leading ophthalmology centres in MedellĂn and Bogotá have invested in both ORA and Corvis ST platforms. This is not universal -- not every clinic has biomechanical testing equipment -- which is why asking about screening protocols is part of choosing the right centre. The cost of the equipment is substantial, and its presence is a marker of a centre that takes screening seriously.
For the full screening process, see what happens during a LASIK consultation. For candidates with borderline thickness, thin cornea options covers alternatives including PRK and ICL.
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