Viscoelastics

Anatomy and Physiology

Ideal OVD Properties

• Optically transparent and clear
• Non-toxic
• Non-inflammable and nonpyrogenic
• Not insight any inflammatory reaction
• Non-immunogenic
• Sterile and free from any reaction inside the human eye
• Should have good viscosity to prevent the collapse of AC at rest
• Should have an ability of retention under pressure, should be highly cohesive with high viscosity
• Should have good endothelium protective properties with minimal surface activity
• Should be easy to remove from the anterior chamber
• Should be cost-effective
• Should not obstruct aqueous outflow
• Should be elastic and have shock-absorbing properties
• Should be liquid enough so that it can be injected through a small-bore cannula[6]

Stabilizing defect - High viscosity at a low shear rate help to maintain space and protect tissues

Mobilizing effect - Moderate viscosity at medium shear rate help in the movement of instruments.

Low viscosity at high shear rates helps in the easy introduction into the eye through a small-bore cannula[7]

Physical and Rheological Properties of Viscoelastics

Viscosity

It refers to the solution's resistance to flow. Higher the viscosity of a viscoelastic, better is its ability to maintain a uniform shape and difficulty in displacement. Viscosity is dependent on concentration, molecular weight, shear rate, and temperature. An ideal viscoelastic should maintain space for accurate instrumentation during the surgery. It is measured in centipoise and millipascals.[8]

Pseudo-plasticity

This is a unique property of viscoelastic. It refers to the ability of viscoelastic to change from gel to a liquid under pressure. It is also defined as the ability to pass from a state of high viscosity at rest to low viscosity at high shear rates. Sodium hyaluronate has greater pseudo-plasticity than hydroxypropyl methylcellulose (HPMC) and chondroitin sulfate. Sodium hyaluronate can be injected through a 30G needle, while HPMC will require a 25G needle.[3]

Viscoelasticity

It is the property of a substance to retain its original form after it has been stretched and deformed. It increases with high molecular weight and increases in the chain length of the molecules. This viscoelastic property helps absorb traumatic maneuvers like shocks during phacoemulsification and the unfolding of IOL during implantation. Sodium hyaluronate has more viscoelasticity than HPMC or chondroitin sulfate.[5]

Rigidity

This is also known as complex viscosity. It means the resistance felt while maneuvering the object through a viscoelastic. It is defined as the square root of the sum of squares of viscosity and elasticity.

Surface Tension

It is a property by which a viscoelastic coat the surface. This property is also governed by the contact angle formed by a drop of viscoelastic on a flat surface. Lower surface tension and low contact angle have better coating ability. The surface tension of sodium hyaluronate is less; hence it has a better coating ability.[9]

Viscoadaptability

This is a viscoelastic property wherein it changes from a supraviscous state at a low shear rate to a fracturable (pseudo-dispersive) at a higher shear rate. Healon 5 has high viscoadaptability and has revolutionized the application of viscoelastics.

Cohesiveness

Cohesiveness is a property by which molecules of viscoelastics adhere to one another and form a clump. Higher the molecular weight, higher the surface tension, and higher the pseudo-plasticity, higher is the cohesiveness of a viscoelastic. Sodium hyaluronate is more cohesive than HPMC and is challenging to aspirate from the anterior chamber during surgery.[10]

Dispersiveness

Dispersive viscoelastics have less tendency to adhere to each other. Viscoelastics with low molecular weight, low surface tension, and low pseudo-plasticity are dispersive viscoelastics.

Coatability

Higher the molecular weight of a viscoelastic, the higher the coatability. More negatively charged sodium hyaluronate and chondroitin sulfate complex has a greater affinity for positively charged instruments, tissues, and implants.[11]

Classification of Viscoelastics

Viscoelastics are classified based on the

• Rheological Properties
• Source[12]

Classification based on Rheological Properties

Arshinoff's classification[13]

Classification According to the Source

Indications

Cataract Surgery

Viscoelastics form an essential component for every step of all types of cataract surgery, whether it is phacoemulsification, manual small incision cataract surgery (MSICS), extracapsular cataract extraction (ECCE), or intracapsular cataract extraction (ICCE). Viscoelastics assist in capsulorhexis, hydrodissection, nucleus prolapse and manipulation, trenching and emulsification of nucleus, IOL implantation, and they also coat the endothelium and prevent corneal damage. The primary function of OVDs is to form the anterior chamber by replacing aqueous humor.

For capsulorhexis, a viscoelastic should have high viscosity, high transparency, good capsular flap control for a spatula effect, and high molecular weight at zero shear rate and must maintain the anterior chamber. The pediatric capsule is highly elastic; hence high density viscoelastic like Healon GV is viscoelastic of choice. In the case of posterior capsular rent during irrigation and aspiration, a dispersive viscoelastic is used to plug the defect.[5]

Viscoelastics Required During Various Steps of Phacoemulsification

Glaucoma Surgeries

In trabeculectomy, hyaluronic acid is the viscoelastic of choice. It helps to prevent endothelial trauma maintain the anterior chamber depth. Some surgeons leave viscoelastic in the anterior chamber to prevent postoperative hypotony and retino-choroidal folds. Viscoelastic can also be injected subconjunctivally for bleb formation.

In viscocanalostomy surgery, viscoelastic is used to open the Schlemm canal. Hyaluronic acid containing viscoelastics is used for viscocanalostomy as they have high pseudo-plasticity and high viscosity. Due to these properties, the viscoelastic can be injected into the Schlemm canal through a small needle, and they help maintain the space.[14]

Corneal Surgeries

In penetrating keratoplasty, the viscoelastic helps maintain the anterior chamber before trephination, provides good support for perfect trephination, helps in the protection of intraocular structures, tamponade the vitreous, and coat the corneal donor button while suturing to prevent endothelial damage. Viscoelastics also play a significant role in deep anterior lamellar keratoplasty (DALK) for layer by layer removal of the host cornea and for exposing the Descemet membrane (DM) during surgery. In Descemet stripping, endothelial keratoplasty (DSEK) and Descemet membrane endothelial keratoplasty (DMEK) viscoelastic help perform DM scoring and any anterior chamber maneuvers. In femtosecond laser application of cornea, viscoelastics help to protect the corneal endothelium.

Viscoanaesthesia

Viscoelastic can be mixed with lidocaine, and the mixture is used for intracameral anesthesia. This treatment modality is labeled as viscoanaethesia. Viscoelastic helps to prolong the effect of lidocaine.[15]

Viscomydriasis

Viscoelastic specially cohesives help to dilate the pupil during the surgery. In small pupil cases, it helps in viscomydriasis.[16]

Intraoperative Floppy Iris Syndrome (IFIS)

Intraoperative Floppy Iris Syndrome (IFIS) is a known complication in patients treated for prostatic hyperplasia with alpha-adrenergic blockers. Viscoadaptive viscoelastics are helpful in such a scenario to dilate and stabilize the pupil. This also helps to prevent iris prolapse.[17]

Retinal Surgery

The membranous adhesions during retinal surgery can be safely separated and excised with the help of viscoelastics. It also helps to release the traction. Sodium hyaluronate is one of the most commonly used viscoelastic during retinal surgeries.[18]

Orbit and Oculoplasty Surgery

Congenital Nasolacrimal Duct Obstruction (CNLDO)- Viscoelastic helps pass the probe through the NLD and keeps the NLD patent.[19]

Ptosis Surgery - It helps to avoid friction with fascia lata.

Dacryocystorhinostomy (DCR) - In DCR surgery, it helps in to separate the adhesions and identify the lacrimal sac

Lacrimal Canaliculus tear - Viscoelastics help to identify the cut ends.

Pediatric Ophthalmology and Strabismus Surgery

In pediatric cataract, viscoelastic help in all the routine steps of cataract surgery. In strabismus, surgery helps in suture adjustment with less force, helps to coat the muscle, and decreases scar formation in the muscular tissues.[20]

Preparation

Sodium Hyaluronate

It is a salt derivative of hyaluronic acid and is present in the connective tissue of bacteria and humans. In homo sapiens, it is found in skin, synovial fluid, and vitreous humor. Sodium hyaluronate was first extracted from the vitreous humor of cows, and Mayer and Palmer named it because it is derived from hyaloids and contains uronic acid. The commercially available formulation is extracted from rooster coombs or streptococcal bacterial culture. Sodium hyaluronate is a long-chain mucopolysaccharide chain with a molecular weight of 2.5 to 4 million Daltons. The viscosity ranges from 100000 to 30000 centipoise with a half-life of 1 day in aqueous and three days in vitreous. The characteristic properties of sodium hyaluronate are sterile, noninflammatory, non-toxic, nonantigenic, and nonpyrogenic.[21]

Chondroitin Sulphate

The physical and chemical characteristics of chondroitin sulfate are similar to sodium hyaluronate. The structure of chondroitin sulfate differs by a sulfated group and double negative charge for every subunit. Commercially available choroid tin sulfate is derived from shark cartilage. It is found in the cornea as one of the three major mucopolysaccharides.[22]

Various Preparations of Chondroitin Sulphate

Isolated Chondroitin Sulphate (20%)

It is a low viscosity preparation and not a space maintainer; instead helps in surface protection. The viscosity can be increased as 50% solution, but this formulation can cause damage to the corneal endothelium.

Chondroitin Sulphate and Sodium Hyaluronate

It is more viscous and has a better coating ability.[23]

Hydroxypropyl Methylcellulose (HPMC)

HPMC is cellulose-derived and extracted from plant fibers like cotton and wood. It is not found in animal tissues. The viscosity of HPMC is 3000 to 4000 centipoise. It is less viscous and lacks the properties of a complete viscoelastic. The molecular weight is 86000; pH is 7.2, and osmolarity of 285 milliosmoles. It is sterilized by autoclaving and has a shelf life of 2 years at room temperature.[24]

Technique or Treatment

Techniques of Viscoelastic Removal

Rock and Roll Technique

In this technique, the irrigation and aspiration (I/A) tips are positioned away from the posterior capsule. The optic is pushed and angled to one side, and viscoelastic is removed from under the optic. A similar maneuver is performed on the contralateral side to remove the remaining viscoelastic.[25]

Compartment Technique Using Bimanual Irrigation and Aspiration

 The irrigation and aspiration tip can be moved back and forth to assist the removal of viscoelastic compartments.[26]

Tapping Technique

The IOL surface is tapped with the I/A tip to dislodge the viscoelastic under the surface of IOL and then can be removed by simple I/A.

Spinning Technique

After IOL implantation, the IOL can be dialed 360 degrees to remove the remaining residual viscoelastic at the equator.

Complications

The various complications associated with viscoelastics are

Secondary Glaucoma

If the viscoelastic is not thoroughly washed intraoperatively from the anterior chamber and capsular bag, it will result in a post-operative IOP spike, usually within 6 to 24 hours. The IOP increases because the trabecular meshwork is clogged, and there is mechanical resistance with large particles of the OVD. In most cases, it resolves spontaneously after 72 hours, but if the IOP rise is persistent, it should be managed with antiglaucoma medications.[27]

Capsular Block Syndrome

Capsular block syndrome (CBS) or capsular block distention syndrome has been reported after viscoelastic use. It refers to the collection of fluid or viscoelastic in the capsular bag behind the nucleus or IOL. The nucleus or IOL is pushed forward, blocking the anterior capsular opening. CBS has been classified as

Intraoperative - Due to collection of balanced salt solution in the capsular bag during hydrodissection.

Early post-operative -  Due to the viscoelastic accumulation in the capsular bag at the end of surgery.

Late post-operative - High-density viscoelastic accumulation in the capsular bag like hyaluronic acid in the capsular bag.[28]

Pseudo-Anterior Uveitis

Due to high viscous property and negative electrostatic charge, the red blood and inflammatory cells may clump and remain suspended in the anterior chamber. This resolves spontaneously within three days. Sometimes the hyphema or the blood clot may get trapped within the vitreous phase and OVD in the anterior chamber mimicking vitreous hemorrhage.[29]

Postoperative Uveitis and Hypopyon

This complication has also been reported after OVD instillation.[30]

Corneal Edema and Decompensation

This is also have been reported after OVD use.[31] 

IOL Surface Crystallization

It occurs due to residues of viscoelastic solution over the IOL surface. The deposits take the pattern of a fern or an amorphous deposit. The IOL calcification mandates an explantation or an exchange.[32]

Calcific Band Keratopathy

This has been reported with chondroitin sulfate-containing OVD.[33]

Hypersensitivity

A small risk of hypersensitivity exists with OVD as sodium hyaluronate is extracted from rooster coombs or streptococcal bacterial culture containing proteins.[34]

Clinical Significance

Techniques Helpful During Surgery

Soft Shell Technique

In the soft shell technique, a dispersive viscoelastic is injected in the anterior chamber forming a small clump on the anterior lens surface. Next, a cohesive viscoelastic is injected below the dispersive on the posterior surface, which pushes the dispersive upwards on the back surface of the cornea, thus protecting the endothelial cells. This technique is helpful during phacoemulsification and irrigation aspiration as it protects the corneal endothelium from trauma due to ultrasound energy and excessive fluid turbulence irrigation. The softshell technique is beneficial in Fuch's endothelial dystrophy, congenital hereditary endothelial dystrophy, extreme crocodile shagreen cases, and denser cataracts (Brown and Intumescent cataract).[35]

Ultimate Soft- Shell Technique

In this technique, a viscoadaptive viscoelastic is first injected in the anterior chamber, and then balance salt solution (BSS) is filled beneath the viscoadaptive viscoelastic. The ultimate softshell technique was introduced to cut down the cost due to the soft shell technique since it employed two different viscoelastics. The viscoelastic in this technique plug the incisional wound, while BSS over the anterior lens surface helps to reduce the resistance while performing surgical maneuvers.[36]

Tri- Soft Shell Technique

This technique first injects a dispersive viscoelastic in the anterior chamber above the lens surface. Then a viscoadaptive viscoelastic is injected below the dispersive to float the dispersive to coat and protect the endothelium.[37]

Enhancing Healthcare Team Outcomes

Viscoelastic are substances with dual properties having a viscosity of a fluid and elasticity of a gel or a solid. They are commonly employed in all kinds of ophthalmic surgical interventions. The nurses, mid-level ophthalmic personnel, pharmacist, allied health, and OT store staff have a crucial role in arranging the viscoelastics during the surgical intervention.

The ophthalmic surgeon should make judicious and timely use of viscoelastics during surgical intervention with no trauma to the ocular structures. It is the responsibility of the operating surgeon to ensure that the viscoelastic is washed thoroughly from the eye at the end of the surgical procedure. In case of any ocular complication of viscoelastic the surgeon and the paramedical staff also has to play an essential role in prompt and meticulous treatment.[6]

Nursing, Allied Health, and Interprofessional Team Interventions

The nurses also play an essential role in ensuring the availability and maintaining the sterility of commonly employed viscoelastics. In addition, the allied health and interprofessional team play a key role in arranging the required viscoelastics and ensuring sterility.

Nursing, Allied Health, and Interprofessional Team Monitoring

The nursing, allied health, and interprofessional team also play a crucial role in monitoring the patient outcome and ensuring the availability of viscoelastics.