Corneal biomechanics is a concept encompassing the mechanical properties of the cornea. The cornea is a “viscoelastic body” with both elastic and viscous properties, and its deformation and recovery behavior under external forces characterize its function.
“Elasticity” is the property of a solid deformed by pressure to return to its original shape. “Viscosity” represents the degree of stickiness of a liquid. Most biological tissues are viscoelastic bodies with both properties, and the cornea is one of them.
When pressure is applied to the cornea, it deforms, and upon pressure release, it attempts to return to its original shape. During this process, the loading and unloading curves do not coincide. This phenomenon is called hysteresis. Hysteresis is an indicator of energy dissipated during deformation and recovery, reflecting the mechanical properties of the cornea 1).
The elastic modulus (Young’s modulus) of the cornea has been reported to range widely from 0.1 to 57 MPa depending on in vitro measurement conditions and methods 1). A higher Young’s modulus indicates stiffer tissue that is more resistant to deformation.
Corneal biomechanics is gaining importance in the following clinical scenarios:
Currently, there are two devices that quantitatively evaluate corneal biomechanics in vivo: the Ocular Response Analyzer (ORA) and Corvis ST. The concept of corneal biomechanics is still new, and interpretation of measurement results remains a subject for future research.
Corneal hysteresis (CH) is a parameter measured as the difference in applanation pressures (P1 − P2) between the loading and unloading phases when the cornea is deformed by an air puff. It reflects the viscoelastic properties of the cornea, particularly its energy absorption and dissipation capacity. CH is low in keratoconus; for details, see the section on “Clinical Significance and Applications.”
Corneal biomechanics are influenced by many factors 1). Clinically important factors are summarized below.
Corneal stiffness increases significantly with age 1). This is mainly due to natural increases in collagen cross-linking and fiber modification caused by glycation.
Tahsini et al. (2025) analyzed SSI maps in 72 healthy subjects and showed that the Stress-Strain Index (SSI) increased significantly from 0.938 ± 0.067 in the 20–50 age group to 1.143 ± 0.064 in the 50–80 age group (Pearson r = 0.92, p < 0.001) 3). The rate of stiffening varies by region, progressing faster in areas that are already stiffer.
CCT shows a positive correlation with CH and CRF 1). Thicker corneas yield higher values for these parameters. CCT also affects intraocular pressure (IOP) measurements, with thicker corneas tending to overestimate IOP.
The relationship between intraocular pressure (IOP) and corneal biomechanics is complex 1). Elevated IOP reduces CH measurements, but this is partly due to characteristics of the measurement system rather than true changes in corneal material properties. In glaucoma patients, low CH has been reported as a risk factor for visual field progression.
Estrogen affects collagen cross-linking and reduces corneal stiffness 1). CH fluctuates during the menstrual cycle and pregnancy. Refractive surgery is generally avoided during pregnancy.
In corneal edema, stiffness decreases as water content increases1). Maintaining normal water content (approximately 78%) contributes to mechanical stability.
In diabetes, non-enzymatic glycation of collagen progresses, increasing corneal stiffness1). Some reports indicate that CH and CRF are higher in diabetic patients than in healthy individuals.
CXL is a treatment that uses long-wavelength ultraviolet (UVA) light and riboflavin to strengthen cross-links between collagen fibers, thereby increasing corneal stiffness1). SP-A1 increases after CXL, objectively reflecting improvement in stiffness. The inhibition of keratoconus progression by CXL is detailed in the “Clinical Significance and Applications” section.
The cornea becomes stiffer with age. The main causes are natural increases in collagen cross-linking and glycation. Studies using SSI mapping show that SSI increases by approximately 22% from the 20–50 age group to the 50–80 age group3). However, the rate of stiffening varies by region, progressing faster in the inherently stiffer central and peripheral areas.
Currently, two clinical devices are commercially available for measuring corneal biomechanics in vivo. Experimental techniques such as Brillouin microscopy and optical coherence elastography (OCE) are under development1).
The characteristics of the main measurement devices are shown below.
| Device | Main Parameters | Features |
|---|---|---|
| ORA | CH, CRF | Reflects viscoelasticity |
| Corvis ST | SP-A1, CBI, traumatic brain injury | Deformation analysis with video |
| Brillouin microscope | Elastic modulus | 3D deep mapping |
The ORA is a non-contact air-puff device manufactured by Reichert 2). It monitors the central 3–6 mm of the cornea electro-optically using infrared light while applying and releasing pressure, measuring the deformation and recovery process of the cornea over approximately 30 milliseconds.
The main parameters calculated are as follows:
CH is thought to mainly reflect the viscous properties of the cornea, while CRF mainly reflects the elastic properties2).
The Corvis ST (Oculus) uses a high-speed Scheimpflug camera at 4,330 frames per second to continuously capture an 8.5 mm horizontal cross-section of the cornea, recording corneal deformation induced by an air puff as a video2).
Corneal deformation parameters are calculated at three main time points:
At each time point, the elapsed time, applanation length, deformation velocity, and corneal apex displacement are calculated. The main clinical parameters are as follows:
Corvis ST can also be used as a tonometer like ORA, and calculates biomechanically corrected intraocular pressure (bIOP).
ORA
Bidirectional applanation method using air puff: Tracks corneal deformation and recovery with infrared light.
Parameters: CH, CRF, IOPg, IOPcc. Provides overall viscoelastic indicators.
Released in 2005: The first commercial device enabling in vivo measurement of corneal biomechanics.
Corvis ST
High-speed Scheimpflug camera: Records video of corneal cross-section deformation at 4,330 frames per second.
Parameters: SP-A1, CBI, TBI, SSI, DA ratio, etc. Can calculate integrated indices with tomography.
Expandability: New indices such as SSI map and AI-integrated analysis with Pentacam HR are possible.
Brillouin microscopy analyzes the interaction between light and acoustic phonons to noninvasively map corneal biomechanical properties in three dimensions 1). It estimates the longitudinal elastic modulus of tissue from GHz frequency shifts.
While ORA and Corvis ST measure the average response of the entire cornea, Brillouin microscopy has depth resolution and can visualize local elastic distribution 1). It is expected to be applied for depth-dependent evaluation of crosslinking effects after CXL and detection of localized stiffness reduction in keratoconus. Currently, long measurement time and environmental factors remain challenges, and it has not yet reached clinical application.
Optical Coherence Elastography (OCE) is a technique that measures displacement within the corneal stroma induced by external force 1). It can evaluate strain in the middle and posterior layers of the cornea and is useful for analyzing depth-dependent biomechanical properties.
ORA calculates corneal hysteresis (CH) and corneal resistance factor (CRF) from changes in infrared signals, assessing viscoelasticity globally. Corvis ST records deformation of the corneal cross-section in video using a high-speed Scheimpflug camera and calculates numerous dynamic parameters. Corvis ST is distinctive in that it can utilize AI-based composite indicators such as traumatic brain injury through integration with Pentacam HR.
Measurement of corneal biomechanics plays an important role in early detection of keratoconus, evaluation of refractive surgery, correction of intraocular pressure measurements, and assessment of CXL efficacy.
In corneal ectatic diseases, biomechanical changes occur prior to morphological changes 5). Even at stages where topography or tomography cannot detect abnormalities, biomechanical evaluation may enable early diagnosis.
In early keratoconus, a localized decrease in elastic modulus is linked to collagen fiber disruption, initiating a biomechanical decompensation cycle 7). Stress increases and redistributes, leading to corneal steepening and thinning.
The diagnostic performance of key indicators for early keratoconus detection is as follows 7):
| Indicator | SUCRA Value | Remarks |
|---|---|---|
| Traumatic Brain Injury | 96.2 | Highest accuracy |
| CBI | 83.8 | Second best indicator |
| CRF | 66.4 | ORA-derived |
According to Brar et al., biomechanically suspect eyes are defined as CBI > 0.5 and TBI > 0.29 7). To avoid false negatives, combined use of corneal tomography (e.g., Scheimpflug imaging) and biomechanical assessment is recommended 5)7).
A comprehensive review by Wang et al. (2025) indicates that combined tomographic and biomechanical models outperform individual parameters in detecting FFKC (forme fruste keratoconus). The logistic regression model by Luz et al. achieved an AUROC of 0.953 (sensitivity 85.71%, specificity 98.68%) 2).
In ORA-based studies, FFKC eyes showed significantly lower CH and CRF compared to normal eyes 2). In VAE-NT (very asymmetric ectasia with normal topography) eyes, CH was 8.5 ± 1.5 mmHg and CRF was 8.3 ± 1.5 mmHg, both lower than in normal controls 2).
Corvis ST’s TBI showed an AUROC of 0.985 in VAE-NT eyes, and both CBI sensitivity (99.1%) and TBI sensitivity (99.6%) achieved extremely high values in detecting tomographically abnormal keratoconus 2).
Refractive surgery alters corneal biomechanical properties by resecting or deforming the corneal stroma 4). Postoperative corneal ectasia is rare (0.04–0.6%) but serious, making preoperative biomechanical assessment important 4).
A systematic review by Pniakowska et al. (2023) of 17 prospective studies found that biomechanical reduction was greatest after LASIK (stromal ablation with flap creation), followed by SMILE (lenticule extraction), and surface ablation (PRK/LASEK) 4).
Key findings on postoperative biomechanics:
The general mean values of CH and CRF after surface ablation were CH 8.68 ± 0.94 mmHg and CRF 8.39 ± 1.08 mmHg 4). In SMILE, CH remained significantly higher than in LASEK at 3 months postoperatively, but the difference between the two groups disappeared after 3 years 4).
It has been reported that combining biomechanical indices with topographic parameters improves the prediction accuracy of refractive surgery by more than 25% 7). Patients with lower corneal stiffness have a 2 to 3 times higher risk of postoperative residual refractive error 7).
Intraocular pressure measurement with the Goldmann applanation tonometer (GAT) is influenced by corneal thickness and biomechanical properties 5).
In corneas with ectatic diseases or after refractive surgery, tissue thinning and biomechanical weakening cause artificially low applanation pressure readings 5). The following alternative devices are recommended:
IOPcc calculated by ORA is less affected by CCT and CH and reflects true intraocular pressure more accurately.
CXL is a treatment that strengthens corneal stromal cross-links using riboflavin and UVA, and is effective in halting the progression of keratoconus 6). It increases corneal biomechanical stiffness, but evidence for its mechanism of action at the direct ultrastructural level is insufficient 6).
Larkin et al. (2021) examined the efficacy of CXL in young keratoconus patients in the Keralink multicenter randomized controlled trial. The prevalence of keratoconus reaches 1:375 in the Netherlands and 1:84 in 20-year-olds in Australia 6). CXL has been reported to be effective in halting keratoconus progression in the majority of adults, but confidence intervals are wide and bias risks have been noted 6).
After CXL, SP-A1 increases, and corneal stiffness improvement can be objectively assessed with Corvis ST 1). It should be noted that a certain corneal thickness is required; in cases with severe thinning, modifications such as the sub400 protocol have been reported.
Surface ablation (PRK/LASEK) has the least impact on corneal biomechanics, followed by SMILE and then LASIK 4). In SMILE, because no flap is created, the structural integrity of the anterior cornea is better preserved. Thinner flaps in LASIK and thicker caps in SMILE are advantageous for preservation.
The cornea consists of five layers (epithelium, Bowman’s layer, stroma, Descemet’s membrane, and endothelium), and the stroma, which accounts for about 90% of the thickness, determines corneal biomechanics 1). The stroma is composed of type I and V collagen fibers and proteoglycans. The orientation, density, and cross-linking of collagen fibers are the main factors determining biomechanical properties.
The mechanical behavior of the cornea has the following characteristics 1):
Tahsini et al. (2025) used SSI maps to divide the cornea into 9 zones: central, paracentral (4 zones), and peripheral (4 zones), and analyzed stiffness by region. The mean SSI in the central and peripheral zones was high at 1.153 ± 0.079, while the paracentral zones were low at 0.890 ± 0.057. In particular, the inferior paracentral zone (zones 4 and 5) was the weakest, with an SSI of 0.8333).
The vulnerability of the inferior paracentral zone is consistent with the clinical observation that keratoconus typically develops inferiorly3). This suggests that mechanically weaker areas are more prone to biomechanical decompensation.
A significant difference was also found between the superior paracentral zone (SSI = 0.945) and the inferior paracentral zone (SSI = 0.833), with the nasal side (SSI = 0.903) showing slightly higher stiffness than the temporal side (SSI = 0.879)3).
In the early stages of keratoconus, a local decrease in elastic modulus occurs, initiating collagen fiber disruption and degeneration7). This triggers a biomechanical decompensation cycle:
In the affected area, increased collagen degradation, loss of keratocytes, reduced collagen cross-linking, and marked weakening of the stress-strain response are observed. Genetics, eye rubbing, microtrauma from contact lenses, and atopy are cited as factors contributing to biomechanical degeneration.
According to SSI map analysis, the inferior paracentral zone shows the lowest stiffness value (SSI = 0.833)3). This region coincides with the predilection site for keratoconus, suggesting that inherent mechanical weakness may be involved in disease onset.
SSI II (SSI map) is a new technology that visualizes the stiffness distribution on the corneal surface in two dimensions based on finite element modeling and collagen fiber distribution models2)3).
Tahsini et al. (2025) used SSI maps to analyze age-related regional changes in corneal stiffness. Stiffening progresses faster in inherently stiffer areas (peripheral: 0.0058–0.0067/year) and slower in weaker areas (inferior paracentral: 0.0039/year). A very high correlation (Pearson r = 0.96) was found between SSI of the right and left eyes3).
SSI maps are expected to be useful for understanding the onset and progression mechanisms of keratoconus and for personalizing CXL treatment. Application to personalized treatment according to individual patient age and corneal region is anticipated3).
The introduction of AI and machine learning methods has improved the accuracy of keratoconus detection2).
According to a review by Wang et al. (2025), AI algorithms achieve approximately 98% accuracy in detecting manifest keratoconus, but only about 90% for subclinical cases, leaving a risk of missed detection2).
Using random forest analysis, integration of Pentacam HR and Corvis ST metrics has been reported to achieve 93% specificity and 86% sensitivity in classifying subclinical keratoconus2). A diagnostic model using backpropagation neural networks achieved an AUROC of 0.877, showing better FFKC detection ability than CBI (0.610) and TBI (0.659)2).
Brillouin microscopy is attracting attention as a technology that enables three-dimensional elastic mapping of the cornea1). It has shown utility in depth-dependent evaluation of crosslinking effects after CXL and visualization of localized stiffness reduction in keratoconus. Future directions include improving measurement accuracy through AI integration and clinical application1).
Keratoconus develops bilaterally, but one eye may remain asymptomatic (FFKC/VAE-NT)2). Efforts are underway to predict future risk of onset by biomechanically evaluating the contralateral eye of patients with clinical keratoconus in one eye. TBI shows high detection sensitivity in VAE-NT eyes, and interocular comparison may provide clues for early diagnosis2).
