Corneal Hysteresis (CH) & Corneal Resistance Factor (CRF): Dynamic Scheimpflug Air-Puff Biomechanics (Corvis ST)

The human cornea is not an elastic spring; it is a viscoelastic tissue exhibiting phase delay between mechanical stress and strain. When an exact collimated air pulse indents the cornea inward and allows it to rebound, two applanation pressure points are recorded ($P_1$ and $P_2$):

$$CH = P_1 - P_2 \quad (\text{Normal Population Average: } \mathbf{10.5 \pm 1.5\ \text{mmHg}})$$

Corneal Hysteresis represents the cornea's capacity to absorb and dissipate mechanical energy. Low $CH < 8.0\ \text{mmHg}$ indicates a soft, compliant cornea with poor damping ability—a critical warning sign for keratoconus progression and glaucoma optic nerve vulnerability.

The Corvis ST records 4,330 frames per second during a $30\ \text{millisecond}$ air pulse, capturing the entire dynamic deformation profile:

1. Deformation Amplitude (DA): Total apex displacement at highest concavity ($DA > 1.2\ \text{mm}$ indicates pathological flaccidity).
2. Radius of Curvature at Highest Concavity: Lower radius ($<6.5\ \text{mm}$) signifies sharp bending and weak stromal cross-linking.
3. Corneal Biomechanical Index (CBI): Logistic regression combining velocity, applanation lengths, and DA, identifying ectasia with $>98\%$ sensitivity.

Following myopic LASIK, Corneal Hysteresis drops by an average of $-2.5\text{ to } -3.0\ \text{mmHg}$ due to the severed lamellae of the flap. PRK and SMILE cause significantly smaller biomechanical reductions ($-1.2\text{ to } -1.8\ \text{mmHg}$), preserving superior post-operative globe resistance.