LASIK consistently reduces corneal hysteresis (CH) — the cornea’s ability to absorb and return from deformation — by approximately 1–2 mmHg on average. The reduction is proportional to tissue ablated, measurable within weeks of surgery, and does not return to preoperative levels. This matters for three reasons: it increases ectasia risk in borderline corneas, causes standard tonometers to underestimate IOP, and complicates long-term glaucoma monitoring. This guide explains the mechanism, the specific values involved, and what post-LASIK patients need to know.
Key Takeaways
- Corneal hysteresis (CH) measures the cornea’s viscoelastic resilience — measured by Ocular Response Analyzer (ORA) in mmHg. Normal range: 9–12 mmHg. Higher = more resilient cornea.
- LASIK reduces CH by approximately 1–2 mmHg — proportional to ablation depth. Higher myopia corrections (more tissue removed) cause greater reduction.
- CH reduction is permanent — unlike curvature, which stabilises, CH does not recover to preoperative levels after LASIK.
- Post-LASIK CH reduction causes standard Goldmann tonometry to underestimate IOP by 2–5 mmHg. Post-LASIK patients need ORA Corneal Compensated IOP (IOPcc) for accurate monitoring.
- Preoperative CH < 9 mmHg is a risk factor for post-LASIK ectasia — particularly combined with borderline corneal thickness or subclinical keratoconus.
- SMILE Pro preserves more CH than flap-based LASIK — SMILE spares Bowman’s layer and anterior superficial lamellae, the biomechanically strongest corneal layers.
What Is Corneal Hysteresis — and What Do CH Values Mean?
Corneal hysteresis (CH) measures the cornea’s viscoelastic properties — how well it absorbs mechanical energy and returns to shape when pressure is applied and released. It reflects the cornea’s dynamic biological response to deformation stress, not just its static structure.
CH is measured by the Ocular Response Analyzer (ORA) — an air-pulse tonometer that records two corneal applanation events; their difference is the CH. A related parameter, Corneal Resistance Factor (CRF), measures overall elastic resistance. Our guide on the Pentacam test for LASIK covers what is assessed in the full pre-operative biomechanical workup.
| CH Value (mmHg) | Clinical Interpretation | LASIK Candidacy Implication |
|---|---|---|
| ≥ 11 mmHg | High — resilient, well-hydrated cornea with strong energy absorption capacity | Favourable — good biomechanical reserve; LASIK-induced reduction less likely to create clinical concern |
| 9–11 mmHg | Normal — adequate viscoelastic properties; typical of healthy adult corneas | Acceptable for LASIK if other criteria met; conservative ablation depth planning |
| 8–9 mmHg | Borderline — requires context; may be normal for some individuals | Borderline — careful Pentacam assessment, residual stromal bed planning, and conservative ablation depth |
| < 8 mmHg | Low — significantly reduced viscoelastic capacity; associated with keratoconus, ectasia, IOP underestimation | Caution / contraindication — elevated ectasia risk; SMILE Pro, Trans-PRK, or ICL more appropriate |
How LASIK Reduces CH — Mechanism and Published Values
LASIK reduces CH through two additive mechanisms: flap creation (severing Bowman’s layer and anterior stromal lamellae — the biomechanically strongest layers) and excimer laser ablation (removing stromal tissue and permanently reducing structural integrity). SMILE Pro involves only ablation-equivalent lenticule removal — no flap.
| Factor | Mechanism of CH Reduction | Published Effect |
|---|---|---|
| Corneal flap creation | Severs Bowman’s layer and anterior stromal lamellae — the biomechanically strongest corneal layers. Disrupting these parallel interwoven fibres permanently reduces tensile strength. | CH reduction occurs independent of ablation. Femtosecond flaps (100–120 µm) are thinner than microkeratome (130–160 µm) — slightly better CH preservation by removing less anterior stroma. |
| Excimer laser ablation | Each dioptre corrected removes ~12–14 µm of stromal tissue — less remaining stroma = less collagen to absorb deformation energy. | CH reduction proportional to depth. Higher corrections (-8D) reduce more than lower (-2D). Published mean post-LASIK CH reduction: 1–2 mmHg from baseline. |
| Preoperative baseline CH | Lower baseline = greater relative impact from same absolute reduction: 1 mmHg matters more at baseline 8 than 12. | CH < 9 mmHg = elevated ectasia screening flag. With thin corneas or subclinical keratoconus → contraindication for standard LASIK. |
| SMILE Pro (comparison) | No flap; lenticule removed through a small incision — Bowman’s layer intact; anterior biomechanically critical layers preserved. | Published studies show less CH reduction with SMILE vs LASIK for equivalent corrections. See our guide on SMILE Pro vs LASIK for the full comparison. |
Clinical Implications of Reduced CH Post-LASIK
For the majority of patients with adequate preoperative biomechanics, reduced CH does not cause immediate problems. The clinical concern is in three areas:
| Clinical Area | How Reduced CH Affects It | What to Do |
|---|---|---|
| IOP accuracy (tonometry) | Post-LASIK corneas with lower CH give falsely low Goldmann IOP readings — typically by 2–5 mmHg. A glaucomatous IOP of 22 mmHg might register as 17–18 mmHg on standard tonometry. | Use ORA IOPcc for all future IOP measurements. Inform every eye specialist of LASIK history — affects all IOP interpretation permanently. |
| Glaucoma monitoring | IOP underestimation means elevated pressure may go undetected — glaucoma can progress further before diagnosis if standard tonometry is used throughout life. | Annual ORA IOPcc or Corvis ST monitoring. Post-LASIK IOP underestimation must be factored at every future eye examination — not only in the immediate post-operative period. |
| Post-LASIK ectasia risk | Low preoperative CH + post-LASIK reduction further reduces the biomechanical barrier against ectasia — particularly when residual stromal bed < 250 microns. | Full Pentacam pre-LASIK; residual stromal bed ≥ 250 µm minimum. Subclinical keratoconus + low CH = LASIK contraindicated. Our guide on post-LASIK ectasia vs keratoconus covers the distinction and management. |
Warning Signs Post-LASIK — When Reduced CH Becomes a Clinical Concern
Reduced CH is a clinical measurement, not a symptom for most patients. But specific warning signs indicate biomechanical changes progressing in a clinically relevant direction — these warrant urgent review:
| What You Notice | Most Likely Cause | Urgency | Action |
|---|---|---|---|
| Vision worsening after initial improvement — 3–12 months post-LASIK | Progressive ectasia — weakened cornea bulging forward, inducing irregular myopia | High — see surgeon within days | Corneal topography + ORA; early diagnosis enables CXL before significant vision loss |
| Increasing halos/starbursts/glare months post-LASIK — not improving | Irregular astigmatism from progressive shape change — early ectasia | High — changing shape, not stable refraction | Urgent corneal topography; do not assume normal if symptoms start after initial recovery completes |
| IOP unusually low on standard tonometry post-LASIK | Artifactual CH-related reading — actual IOP may be significantly higher | Medium — long-term glaucoma monitoring | Request ORA IOPcc; inform every specialist of LASIK history |
| Vision distortion — straight lines curved or wavy | Keratoconus progression or ectasia from biomechanical failure | Urgent | Same-day review; Pentacam + CXL assessment. Early-stage ectasia responds to CXL far better than advanced-stage. |
| Low CH at pre-LASIK screening (< 9 mmHg) — no symptoms yet | Low baseline CH is a risk factor, not a symptom — the concern is residual post-LASIK CH being even lower | Pre-operative decision point | Discuss residual stromal bed plan and ablation depth with surgeon. |
Conclusion
LASIK permanently reduces CH by ~1–2 mmHg — a structural change that does not reverse. Three areas need active management: IOP accuracy (always use ORA IOPcc, not Goldmann), long-term glaucoma monitoring (inform every specialist of LASIK history), and ectasia vigilance (any progressive vision change after initial recovery needs urgent topography). Patients with low preoperative CH (< 9 mmHg) are better served by SMILE Pro or ICL.
Concerned about your corneal biomechanics before or after LASIK? Book a biomechanical assessment at Visual Aids Centre — ORA and Pentacam-based evaluation included in every pre-operative workup.
Frequently Asked Questions
What is corneal hysteresis and why does it matter for LASIK?
CH is the cornea’s viscoelastic property — how well it absorbs and returns from deformation — measured by ORA in mmHg. Normal range: 9–12 mmHg. Low preoperative CH is a risk factor for post-LASIK ectasia; LASIK reduces CH permanently, which also affects IOP accuracy and glaucoma monitoring.
By how much does LASIK reduce corneal hysteresis?
Average reductions of 1–2 mmHg (published series), proportional to ablation — higher myopia corrections cause greater reduction. The reduction is permanent; structural changes to the stroma and Bowman’s layer are irreversible.
Does reduced CH affect glaucoma diagnosis after LASIK?
Yes — reduced post-LASIK CH causes Goldmann applanation tonometry to underestimate IOP by 2–5 mmHg. A genuinely elevated IOP may register as normal. Post-LASIK patients should use ORA IOPcc for all future IOP measurements and inform every eye specialist they have had LASIK.
Does SMILE Pro preserve more corneal hysteresis than LASIK?
Yes — published studies show less CH reduction with SMILE than LASIK for equivalent corrections. SMILE preserves Bowman’s layer and anterior superficial stromal lamellae (the biomechanically strongest layers) — LASIK severs these to create the flap. For borderline CH patients, SMILE Pro is clinically preferable.
What CH level is too low for LASIK?
CH < 9 mmHg is borderline — requires Pentacam assessment, corneal thickness check, and careful ablation depth planning. CH < 8 mmHg combined with other risk factors (thin corneas, subclinical keratoconus, deep ablation needed) is generally a LASIK contraindication — SMILE Pro or ICL is recommended instead.
