Viscoelastics in Ophthalmology
Properties, Classification, Clinical Applications & Surgical Decision-Making
Examination question · ~800 words
What are ophthalmic viscosurgical devices? Discuss their chemistry, rheological properties, classification, clinical applications, complications, and surgical decision-making in cataract and anterior segment surgery.
Try to outline your answer mentally before expanding sections below.
Ophthalmic viscosurgical devices (OVDs) are sterile viscoelastic polymers used to maintain intraocular space, protect the corneal endothelium, stabilise ocular tissues, and facilitate surgical manoeuvres during cataract surgery, IOL implantation, corneal transplantation, glaucoma surgery, and complex anterior segment procedures. Their correct use depends on understanding viscosity, elasticity, cohesivity, dispersiveness, and shear-dependent behaviour.
OVDs are high-molecular-weight biological or synthetic polymers that show both viscous and elastic behaviour. They resist flow at rest but may become easier to inject under shear stress.
| OVD / Polymer | Active Component | Typical Molecular Weight | Origin |
|---|---|---|---|
| Sodium hyaluronate | Hyaluronic acid | Approximately 1-5 million Da | Bacterial fermentation / rooster comb |
| Hydroxypropyl methylcellulose | Cellulose derivative | Approximately 80,000-140,000 Da | Synthetic |
| Chondroitin sulfate | Glycosaminoglycan | Variable | Bovine trachea |
| Polyacrylamide | Synthetic polymer | Variable | Synthetic |
Viscosity
- Viscosity is resistance to flow and is usually expressed in mPa.s.
- Zero-shear viscosity is viscosity at rest and determines space-maintaining ability.
- High-shear viscosity is viscosity during injection or cannula flow and determines injectability.
- Pseudoplasticity means viscosity decreases as shear rate increases; this allows a high-viscosity OVD to pass through a small cannula during injection.
Elasticity
- Elasticity is represented by the storage modulus (G').
- High G' indicates a more solid-like, cohesive material that holds shape and is easier to remove.
- Low G' indicates a more fluid-like, dispersive material that spreads and coats tissues.
Surface Tension and Cohesivity
- Higher surface tension favours cohesive behaviour: the OVD remains as a bolus.
- Lower surface tension favours dispersive behaviour: the OVD spreads widely and coats the endothelium.
- Cohesive OVDs are best for space maintenance; dispersive OVDs are best for endothelial protection.
The Arshinoff classification was introduced in 1999 and revised in 2005. It classifies OVDs according to zero-shear viscosity and cohesivity.
| Class | Example | Zero-Shear Viscosity | Cohesivity | Key Feature |
|---|---|---|---|---|
| High-viscosity cohesive | Healon GV | Very high; Healon GV approximately 2 million mPa.s | High | Excellent space maintenance; easy bolus removal |
| Standard cohesive | Healon, Provisc, Amvisc Plus | Approximately 90,000-300,000 mPa.s; Healon about 300,000, Provisc about 90,000, Amvisc Plus about 55,000 mPa.s | Moderate to high | Useful for chamber deepening, capsulorhexis, and IOL implantation |
| Dispersive | Viscoat, Ocucoat | Low; often < 50,000 mPa.s | Low | Excellent endothelial coating; more difficult to remove |
| Viscoadaptive | Healon5 | Ultra-high at rest; low during flow | Adaptive | Behaves cohesively at rest and flows during injection |
| Viscodispersive | DisCoVisc | Moderate | Mixed | Combines endothelial protection with maintainability |
| Combination system | DuoVisc | Dual system: Viscoat + Provisc | Dispersive + cohesive | Designed for soft-shell technique |
Cohesive OVDs: create space, clear quickly, and come out as a bolus.
Dispersive OVDs: distribute widely, defend the endothelium, form droplets, and require more thorough aspiration.
| OVD | Type | Concentration | Molecular Weight | Viscosity | Key Use |
|---|---|---|---|---|---|
| Healon | Standard cohesive | 1% sodium hyaluronate | Approximately 4 million Da | Approximately 300,000 mPa.s | Routine cataract surgery |
| Healon GV | High-viscosity cohesive | 1.4% sodium hyaluronate | Approximately 5 million Da | Approximately 2 million mPa.s | Difficult or complex cataract surgery |
| Healon5 | Viscoadaptive | 2.3% sodium hyaluronate | Approximately 4 million Da | Approximately 7 million mPa.s | Small pupil, IFIS, complex anterior segment cases |
| Provisc | Standard cohesive | 1% sodium hyaluronate | Approximately 2.4-2.5 million Da | Approximately 90,000 mPa.s | Phacoemulsification and IOL implantation |
| Viscoat | Dispersive | 3% sodium hyaluronate + 4% chondroitin sulfate | HA component approximately 500,000 Da | Approximately 40,000 mPa.s | Endothelial protection |
| Ocucoat | Dispersive | 2% HPMC | Approximately 90,000 Da | Approximately 4,000 mPa.s | Cost-effective dispersive OVD |
| DisCoVisc | Viscodispersive | 1.6% sodium hyaluronate + 4% chondroitin sulfate | HA component approximately 1.65-1.7 million Da | High | Combined protection and chamber maintenance |
| DuoVisc | Combination pack | Viscoat + Provisc | Dual formulation | Dual | Soft-shell technique |
| Amvisc Plus | Standard cohesive | 1.6% sodium hyaluronate | Approximately 2 million Da | Approximately 55,000 mPa.s | IOL implantation; not a high-viscosity cohesive OVD |
The corneal endothelium is a non-regenerating monolayer. Normal adult endothelial cell density is approximately 2500-3000 cells/mm²; at birth it is higher, approximately 3500-4000 cells/mm², and declines with age.
- Physical barrier: Dispersive OVDs coat endothelial cells and reduce trauma from nuclear fragments, phaco turbulence, instruments, and IOL contact.
- Hydraulic cushion: Cohesive OVDs maintain anterior chamber depth and reduce endothelial touch.
- Free radical scavenging: Hyaluronic acid has antioxidant properties against phaco-generated free radicals.
- Thermal buffering: OVDs help reduce heat-related endothelial stress.
- Chondroitin sulfate effect: Viscoat provides strong endothelial coating because chondroitin sulfate improves adherence to the endothelial surface.
For endothelial protection in a hard cataract or compromised cornea, choose a dispersive OVD such as Viscoat and use the soft-shell technique. Exact quoted endothelial cell-loss percentages from older teaching sources should not be repeated unless verified from the primary study.
The soft-shell technique described by Arshinoff combines dispersive and cohesive OVDs to maximise endothelial protection and chamber stability.
| Step | Action | OVD Used | Purpose |
|---|---|---|---|
| 1 | Inject dispersive OVD over the corneal endothelium | Viscoat | Coats and protects endothelium |
| 2 | Inject cohesive OVD beneath the dispersive layer | Healon / Provisc | Deepens anterior chamber and pushes iris-lens diaphragm posteriorly |
| 3 | Perform phacoemulsification | Both layers present | Dispersive remains near endothelium; cohesive maintains space |
| 4 | Remove cohesive OVD first | Cohesive bolus | Comes out easily as a single mass |
| 5 | Remove residual dispersive OVD | Dispersive OVD | Requires active aspiration to prevent retained OVD and IOP rise |
The technique exploits differences in surface tension and dispersiveness: the dispersive OVD spreads along the endothelium, while the cohesive OVD remains as a central space-maintaining bolus.
Cataract Surgery
- Capsulorhexis: Cohesive OVD maintains chamber depth and flattens the anterior capsule.
- Phacoemulsification: Soft-shell technique protects the endothelium and maintains chamber stability.
- IOL implantation: Cohesive OVD fills the anterior chamber and capsular bag, preventing bag collapse.
- Posterior capsule rupture: Dispersive OVD helps compartmentalise the anterior segment, protect the endothelium, and stabilise the chamber.
- Small pupil: Healon5 can mechanically expand the pupil and stabilise the iris.
Corneal Transplantation
- PKP: Cohesive OVD maintains chamber depth during open-sky steps and protects donor tissue during manipulation.
- DSAEK / DMEK: Dispersive OVD is useful for endothelial protection during donor manipulation and unfolding.
- DALK: OVD may be used to maintain anterior chamber form during deep dissection.
Glaucoma Surgery
- Trabeculectomy: OVD may protect the lens and cornea during peripheral iridectomy and anterior segment manipulation.
- Tube surgery: OVD maintains anterior chamber depth during tube placement.
- Viscocanalostomy: OVD is injected into Schlemm's canal to dilate it.
- Angle-based procedures: OVD improves chamber stability and visualisation during procedures such as visco-goniotomy or Kahook Dual Blade surgery.
Other Uses
- Anterior segment reconstruction and synechiolysis.
- Selected vitreoretinal procedures as a surgical adjuvant.
- HA-based drug delivery systems for sustained release of anti-VEGF agents or corticosteroids are evolving applications.
| Complication | Mechanism | Common Association | Prevention / Management |
|---|---|---|---|
| Postoperative IOP spike | Retained OVD blocks trabecular outflow | All OVDs; cohesive causes higher shorter spike, dispersive causes lower but longer spike | Thorough I/A removal; antiglaucoma medication in high-risk eyes |
| Corneal oedema | Retained OVD or endothelial stress | Dispersive OVDs, HPMC, hard cataract surgery | Endothelial protection during surgery and complete removal at the end |
| Cystoid macular oedema | Inflammation related to prolonged surgery or retained material | Any OVD if retained | Reduce surgical time and remove OVD thoroughly |
| Pupil block glaucoma | OVD migrates posteriorly and obstructs aqueous flow | Cohesive and viscoadaptive OVDs, especially if retained | Remove OVD behind IOL; ensure patent PI in susceptible eyes |
| Toxic anterior segment syndrome | Contamination or improper handling | Any OVD | Use single-use sterile vials and proper handling |
| Prion transmission risk | Theoretical risk from animal-derived HA | Rooster comb-derived HA | Prefer bacterial fermentation-derived HA where relevant |
IOP rise classically occurs around 4-6 hours postoperatively. Risk is higher in glaucoma, pseudoexfoliation, compromised trabecular outflow, and incomplete OVD removal.
- Bimanual irrigation-aspiration is the standard method for removing OVD.
- Remove OVD from the anterior chamber, behind the IOL optic, and from capsular fornices.
- Use the rock-and-roll manoeuvre by gently tilting the IOL to access trapped OVD behind the optic.
- In the soft-shell technique, remove cohesive OVD first, then actively aspirate dispersive OVD.
- In high-risk eyes, check IOP early postoperatively and treat spikes promptly.
| Property | Cohesive | Dispersive |
|---|---|---|
| Zero-shear viscosity | High | Low |
| Molecular weight | Usually higher | Lower or variable |
| Surface tension | Higher | Lower |
| Elasticity | High | Low |
| Space maintenance | Excellent | Poor |
| Endothelial coating | Poor | Excellent |
| Ease of removal | Easy, bolus removal | Difficult, fragmented removal |
| IOP spike | Higher but shorter duration | Lower but longer duration |
| Best use | AC deepening, CCC, IOL implantation | Endothelial protection, hard cataract, complex phaco |
| Examples | Healon, Provisc, Amvisc Plus, Healon GV | Viscoat, Ocucoat, HPMC |
Healon5 is a 2.3% sodium hyaluronate viscoadaptive OVD with ultra-high zero-shear viscosity of approximately 7 million mPa.s.
- At rest, it behaves like a cohesive gel and maintains space.
- During injection, it becomes easier to flow through a cannula.
- It can mechanically dilate a small pupil when injected beneath the iris.
- It is useful in intraoperative floppy iris syndrome because it stabilises the iris and resists aspiration.
- It may temporarily stabilise zonular weakness, but it does not replace a capsular tension ring when true zonular support is required.
- It must be removed deliberately because retained Healon5 can cause prolonged postoperative IOP elevation.
Intraoperative floppy iris syndrome (IFIS) was described by Chang and Campbell in association with tamsulosin use. OVD choice is central to management.
| IFIS Grade | Features | OVD Strategy |
|---|---|---|
| Mild | Slight iris undulation | Standard cohesive OVD; proceed cautiously |
| Moderate | Iris billowing with progressive miosis | Viscoadaptive OVD such as Healon5 with iris expansion technique |
| Severe | Triad of iris billowing, prolapse, and progressive miosis | Healon5 plus mechanical expansion device such as Malyugin ring; use endothelial protection as needed |
| High-Yield Fact | Answer Point |
|---|---|
| First OVD introduced | Healon, sodium hyaluronate; Balazs, 1970s; first commercial use around 1979 |
| Best endothelial protection | Dispersive OVD, especially Viscoat: 3% HA + 4% chondroitin sulfate |
| Best space maintenance | High-viscosity cohesive OVDs such as Healon GV and viscoadaptive Healon5 |
| Worse peak IOP spike | Cohesive OVDs: higher but shorter spike |
| Longer IOP spike | Dispersive OVDs: lower peak but longer duration due to slower trabecular clearance |
| Healon5 property | Ultra-high zero-shear viscosity; cohesive at rest and easier to inject under shear |
| HPMC mechanism | Synthetic cellulose derivative; non-ionic, cheaper, mainly mechanical space maintenance |
| Soft-shell sequence | Inject dispersive first, cohesive below it; remove cohesive first, then dispersive |
| Best OVD for IFIS | Healon5, because it mechanically stabilises the iris |
| OVD in DSAEK / DMEK | Dispersive OVD protects donor and recipient endothelial surfaces during manipulation |
| Prion risk | Theoretical risk with animal-derived HA; bacterial fermentation-derived HA avoids this concern |
Trap 1 — FALSE
“Healon is the best OVD for endothelial protection during phacoemulsification.”
Viscoat, a dispersive OVD containing 3% sodium hyaluronate and 4% chondroitin sulfate, is better for endothelial coating. Healon is cohesive and mainly maintains space.
Trap 2 — FALSE
“In a tamsulosin patient with IFIS, Viscoat is the primary OVD for iris stabilisation.”
Healon5 is preferred for primary iris stabilisation because its ultra-high zero-shear viscosity mechanically expands and stabilises the iris. Viscoat may still be used for endothelial protection.
Trap 3 — FALSE
“Cohesive OVDs cause a longer postoperative IOP spike than dispersive OVDs.”
Cohesive OVDs usually cause a higher but shorter IOP spike. Dispersive OVDs can cause a lower but longer spike because they penetrate and clear more slowly from the trabecular meshwork.
Trap 4 — FALSE
“In the soft-shell technique, the dispersive OVD is removed first because it was injected first.”
Injection order is dispersive first, then cohesive. Removal order is cohesive first because it exits as a bolus, followed by careful aspiration of the dispersive OVD.
Trap 5 — TRUE
“Healon5 can expand a small pupil because of ultra-high zero-shear viscosity.”
At rest, Healon5 behaves like a solid gel and mechanically displaces the iris; during injection, shear-dependent behaviour allows it to flow through the cannula.
Trap 6 — FALSE
“Amvisc Plus is a high-viscosity cohesive OVD with viscosity above 1,000,000 mPa.s.”
Amvisc Plus has viscosity around 55,000 mPa.s and is better classified as a standard cohesive OVD, not a high-viscosity cohesive OVD.
Trap 7 — TRUE
“The Arshinoff OVD classification was revised in 2005.”
The Arshinoff classification was introduced in 1999 and revised in 2005.
Trap 8 — FALSE
“Normal endothelial cell density of 2500-3000 cells/mm² refers to birth values.”
2500-3000 cells/mm² is a normal adult range. At birth, endothelial cell density is higher, approximately 3500-4000 cells/mm².
Q: Define an OVD.
A: An ophthalmic viscosurgical device is a sterile viscoelastic polymer used intraocularly to maintain space, protect tissues, and facilitate surgical manoeuvres.
Q: What is pseudoplasticity?
A: Pseudoplasticity is the reduction in viscosity with increasing shear rate, allowing easier injection through a cannula.
Q: Which OVD gives the best endothelial protection?
A: A dispersive OVD, especially Viscoat because it contains sodium hyaluronate and chondroitin sulfate and coats the endothelium well.
Q: Which OVD gives the best space maintenance?
A: High-viscosity cohesive or viscoadaptive OVDs such as Healon GV and Healon5.
Q: What is the soft-shell technique?
A: Dispersive OVD is injected first over the endothelium, then cohesive OVD is injected centrally beneath it to maintain chamber depth.
Q: What is the removal order in soft-shell technique?
A: Remove the cohesive OVD first, then aspirate the residual dispersive OVD carefully.
Q: What is the best OVD for IFIS?
A: Healon5, because its viscoadaptive ultra-high zero-shear viscosity mechanically stabilises the iris.
Q: Why are dispersive OVDs harder to remove?
A: They spread into thin layers and droplets and adhere to tissues, so they do not aspirate as a single bolus.
Q: What is the key difference between HPMC and sodium hyaluronate?
A: HPMC is a cheaper synthetic cellulose derivative with mainly mechanical action; sodium hyaluronate is more biocompatible and has additional protective properties.
Q: When does postoperative IOP spike commonly occur after retained OVD?
A: Typically around 4-6 hours postoperatively, especially in glaucoma or pseudoexfoliation eyes.
A 68-year-old patient with advanced pseudoexfoliation syndrome and a 4+ nuclear cataract is scheduled for phacoemulsification. The pupil dilates to 4 mm, pseudoexfoliative material is present on the anterior lens capsule, a 2-clock-hour zonular dehiscence is visible inferiorly, and the patient has taken tamsulosin for 5 years. Describe your complete OVD strategy with scientific justification.
Answer
The scientific basis is to use each OVD for its main strength: Healon5 for mechanical iris and zonular-sector support, Viscoat for endothelial coating, and Provisc for chamber deepening and controlled capsular-bag manipulation.
References
- Arshinoff SA. Dispersive-cohesive viscoelastic soft shell technique. J Cataract Refract Surg. 1999;25(2):167-173.
- Arshinoff SA, Jafari M. New classification of ophthalmic viscosurgical devices. J Cataract Refract Surg. 2005;31(11):2167-2171.
- Chang DF, Campbell JR. Intraoperative floppy iris syndrome associated with tamsulosin. J Cataract Refract Surg. 2005;31(4):664-673.
- Liesegang TJ. Viscoelastic substances in ophthalmology. Surv Ophthalmol. 1990;34(4):268-293.
- Kanski JJ, Bowling B. Clinical Ophthalmology: A Systematic Approach. 8th ed. Elsevier Saunders; 2016.
- Yanoff M, Duker JS. Ophthalmology. 5th ed. Elsevier; 2019.