Corneal biomechanics has two important implications in glaucoma management. First, the physical properties of the cornea (thickness, viscoelasticity) directly affect the accuracy of intraocular pressure measurement3). Second, the biomechanical properties of the cornea may reflect the properties of the connective tissue of the entire eye and serve as an indicator of the susceptibility of the optic nerve head to glaucomatous damage1).
With the increase in surgeries that alter corneal biomechanics, such as LASIK, PRK, and collagen cross-linking, understanding corneal parameters is becoming increasingly important in glaucoma practice3).
| Parameter | Property | Clinical significance |
|---|---|---|
| Central corneal thickness | Static property | Affects IOP measurement accuracy |
| CH | Dynamic property | Predictor of glaucoma progression |
| CRF | Elastic property | Indicator of overall corneal resistance |
Corneal hysteresis (CH) is a biomechanical parameter that reflects the viscous damping capacity of the corneal stroma. Glycosaminoglycans and proteoglycans in the corneal stroma provide viscosity and determine the ability to absorb and dissipate energy under external forces. It is measured with the Ocular Response Analyzer (ORA), with average values of 9.6–10.7 mmHg in normal eyes and lower values of 8–10 mmHg in primary open-angle glaucoma eyes. Eyes with low CH are considered to have a higher risk of glaucoma progression1)5).
Normal central corneal thickness is approximately 540±30 μm3). Central corneal thickness varies by race; reports from glaucoma clinics indicate it tends to be thicker in Caucasians and Hispanics and thinner in African descent.
Corneal refractive surgery: After LASIK, central corneal thickness decreases, leading to significant underestimation of intraocular pressure (IOP) with Goldmann applanation tonometry (GAT)1)3). It is important to maintain records of preoperative central corneal thickness and IOP3).
Corneal edema: Pathologically thick corneas (edema) cause underestimation of IOP with GAT. Physiologically thick corneas cause overestimation1).
Corneal diseases: Corneal diseases such as keratoconus and Fuchs endothelial corneal dystrophy affect measurement accuracy1).
Intraocular pressure: CH and IOP are inversely correlated; as IOP increases, the cornea becomes stiffer and CH decreases.
Central corneal thickness: In healthy individuals, there is a strong positive correlation between CH and central corneal thickness. In glaucomatous eyes, the correlation weakens.
Aging: Viscous substances decrease with age, and CH decreases by 0.24–0.7 mmHg per decade.
Race: African Americans tend to have lower CH than Caucasians.
Central corneal thickness can be measured using contact methods (ultrasound) and non-contact methods (Scheimpflug imaging, anterior segment OCT, specular microscopy)1). The deviation within the same device is 5–15 μm, but differences between devices can reach up to 120 μm; therefore, using the same device is recommended for follow-up.
ORA (Ocular Response Analyzer)
Principle: An air pulse flattens the cornea, and two applanation pressures (P1, P2) are recorded during the inward and outward phases3)4).
Measurements: CH (= P1 − P2), IOPg (Goldmann-correlated IOP), IOPcc (corneal-compensated IOP), and CRF (corneal resistance factor) are calculated.
Reliability: A waveform score of 3.5 or higher indicates good reproducibility. A normal tear film is necessary for accurate measurement.
Corvis ST
Principle: A high-speed Scheimpflug camera (4,330 frames per second) records corneal deformation caused by an air jet as a video.
Measurements: Multiple biomechanical parameters are calculated from images at the first applanation, maximum concavity, and second applanation.
Features: Provides different corneal deformation parameters than ORA, allowing multifaceted evaluation of corneal viscoelastic properties.
All applanation tonometers, including GAT, are affected by corneal biomechanics (thickness, curvature, viscoelasticity) 3). Air-jet and rebound tonometers deform the cornea in a short time, so this effect is greater 3). The same type of tonometer should be used for follow-up of the same patient 3).
Thin central corneal thickness has been shown to be a risk factor for the development of primary open-angle glaucoma in several large studies 1)2). In the OHTS and the European Glaucoma Prevention Study, ocular hypertensive eyes with central corneal thickness less than 555 μm had a higher risk of developing primary open-angle glaucoma compared to eyes with 588 μm or more 1).
However, the association between central corneal thickness and glaucoma progression is inconsistent. Some studies found thin central corneal thickness to be a risk factor for visual field progression, while others found no association 1)2).
| Study | Association with Progression |
|---|---|
| EMGT | Thin CCT is a risk factor for progression |
| Kim & Chen | Thin CCT associated with visual field progression |
| Congdon et al. | CCT not associated, CH associated |
The World Glaucoma Association consensus does not recommend the use of intraocular pressure correction factors based on central corneal thickness for individual patients 1)4). The EGS 5th edition also states that correction algorithms based on central corneal thickness are not validated and should be avoided 4). It has also been pointed out that the association between central corneal thickness and glaucoma may be due to collider bias via measured IOP 5).
CH is an independent risk factor associated with structural and functional progression of glaucoma 1).
Structural changes: Eyes with higher CH have greater optic nerve compliance to withstand IOP spikes. Primary open-angle glaucoma eyes with acquired optic nerve pit (APON) have significantly lower CH.
Functional changes: Low CH is associated with progressive visual field loss over 5 years. For each 1 mmHg decrease in baseline CH, the rate of visual field index (VFI) decline accelerates by 0.25%.
Treatment response: Low CH is associated with greater IOP-lowering response to prostaglandin analogs and SLT.
Corneal hysteresis is classified as having “highly suggestive evidence (class II)” along with IOP and myopia in an umbrella review (meta-analysis of systematic reviews) 5).
There is no universally accepted formula for IOP correction based on central corneal thickness, and the World Glaucoma Association consensus does not recommend the use of correction factors for individual patients 1). The EGS also states that correction algorithms are not validated and should be avoided 4). Central corneal thickness values should be used as a reference for interpreting IOP values and risk stratification, and clinical decisions based on corrected values should be avoided. Using tonometers that account for corneal biomechanics (ORA’s IOPcc, DCT, etc.) is also an option 3).
The corneal stroma constitutes 90% of the total thickness and is composed of collagen fibers and matrix. Collagen fibers provide elasticity, while glycosaminoglycans (GAGs) and proteoglycans (PGs) provide viscosity. The interaction of these two components causes the cornea to behave as a viscoelastic material.
In a stress-strain cycle, the cornea absorbs and dissipates part of the applied energy. This property is measured as hysteresis. A cornea with high CH has greater energy absorption capacity and a stronger buffering function against external forces.
Since the cornea is continuous with the lamina cribrosa as connective tissue, there is a hypothesis that the biomechanical properties of the cornea reflect the properties of the connective tissue of the entire eye. In eyes with low CH, the lamina cribrosa may also be more deformable, potentially increasing the vulnerability of the optic nerve to IOP.
GAT is designed based on the Imbert-Fick law, assuming a corneal thickness of 520 μm. A thicker cornea requires more force to applanate, leading to overestimation of intraocular pressure 3). Conversely, a thinner cornea leads to underestimation. After corneal refractive surgery, measurement errors are particularly large because the stroma has been removed 3).
Evidence is accumulating that corneal hysteresis is a risk factor for glaucoma 5). In a large umbrella review, IOP (OR 2.43), myopia (OR 1.89), and CH (OR 0.18) were classified as “highly suggestive evidence (class II)” 5). Central corneal thickness remained at “suggestive evidence (class III)” 5).
Collagen cross-linking and glaucoma: It has been reported that in glaucomatous eyes, collagen cross-linking is enhanced and CH is reduced, suggesting that inhibition of cross-linking could be a new therapeutic strategy 5).
Evolution of Corvis ST: In addition to ORA, corneal deformation parameters from Corvis ST (such as first applanation time, deformation amplitude, and maximum concavity time) have been shown to be useful for risk assessment 5).
Future challenges:
Multiple prospective studies have shown that low CH is independently associated with structural and functional progression of glaucoma 1). For each 1 mmHg decrease in baseline CH, the rate of VFI decline accelerates. Furthermore, in a large umbrella review, CH was classified as “highly suggestive evidence” along with IOP and myopia 5). CH is considered a useful clinical parameter for risk stratification and setting treatment goals in glaucoma patients.
- American Academy of Ophthalmology. Primary Open-Angle Glaucoma Preferred Practice Pattern®. 2020.
- American Academy of Ophthalmology. Primary Open-Angle Glaucoma Suspect Preferred Practice Pattern®. 2020.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma, 6th Edition. Br J Ophthalmol. 2025.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma, 5th Edition. PubliComm. 2020.
- Khawaja AP, Springelkamp H, Engel SM, et al. Ocular and Systemic Factors and Biomarkers for Primary Glaucoma: An Umbrella Review of Systematic Reviews and Meta-Analyses. Invest Ophthalmol Vis Sci. 2025;66(12):35.
