Overview of Viscoelastics
Viscoelastics, also known as ophthalmic viscosurgical devices, are special fluids used in eye surgeries. These fluids have unique properties – they are viscous, which means they flow slowly, like honey. They also have elastic properties, similar to gels or solids, meaning they can stretch and bounce back to their original shape.
These fluids were developed in 1972 to help in eye surgeries as a replacement for aqueous and vitreous, which are other types of fluids in the eye. The first type of viscoelastic fluid developed was Sodium hyaluronate. It was patented as Healon in 1980. The term “viscosurgery”, which refers to surgery performed using viscoelastic fluids, was introduced by Dr. Endre A Balazs about 30 years ago.
Today, these fluids are often used in cataract surgeries to assist with delicate procedures within the eye. They are also used in corneal surgeries, glaucoma surgeries, eye globe repairs, retinal surgeries, and surgeries for children and misalignment of the eyes. Despite their benefits, viscoelastics can sometimes cause complications such as secondary glaucoma, capsular block syndrome, capsular bag distension syndrome, pseudo anterior uveitis, IOL crystallization, and calcific band keratopathy, which are various types of eye complications.
Anatomy and Physiology of Viscoelastics
In simple terms, OVDs (Ophthalmic Viscosurgical Devices – used by eye surgeons during certain operations) need to have specific qualities. They should be transparent, harmless, and shouldn’t cause any inflammation or immune response in the body. Moreover, they need to be sterile, to prevent infections, and not hinder the natural flow of fluid in your eye. They should also maintain their form under pressure, protect the deeper parts of your eye, be easy to remove, and be affordable. Most importantly, these substances should be able to protect your eye from any damage by acting as a shock-absorber.
Two critical features of OVDs help protect your eye during surgery. First up, their high viscosity (how much it resists flowing) under slow motion (low shear rate) helps keep the operating area stable and protect your eye tissue. Secondly, a moderate degree of viscosity during the operation helps surgical instruments move with ease.
Just like honey is thick and molasses are thin, the viscosity, or ‘thickness’, of OVDs can vary. The more viscous, the better it can support and shape the spaces in your eye. This can be affected by a few factors like the OVD’s concentration and temperature. Another interesting property is that OVDs can alter their consistency – acting gel-like when stationary and flowing like a liquid when under force. This change under pressure is what we call pseudo-plasticity.
Viscoelasticity refers to the OVD’s ability to return to its original shape after being stretched or deformed – a bit like a rubber band. For example, sodium hyaluronate has more viscoelasticity than HPMC or chondroitin sulfate. This property is essential to absorb shocks during eye surgery and help in the placement of artificial lenses (IOL) into your eye.
OVDs also have other remarkable properties like rigidity (resistance to deformation), surface tension (ability to spread across a surface) and cohesiveness (molecular stickiness). Furthermore, these substances can adapt their behaviour based on the pressures applied during surgery. This versatility is what we call viscoadaptability.
OVDs can be sorted based on various factors, such as their rheological properties (how they flow) and source. For instance, we can classify them into ‘Cohesive’ (higher viscosity) and ‘Dispersive’ (lower viscosity) types based on viscosity. Each type has its advantages and disadvantages during surgery, as they behave differently inside the eye.
For example, Hyaluronic acid, a high viscosity OVD, helps to create space and sustain pressure in the eye, but it might escape more quickly and may not protect the deeper parts of your eye efficiently. On the other hand, low viscosity OVDs, like Hydroxypropylmethylcellulose, stay longer in the eye and provide better protection but can obscure the surgeon’s view since they get removed in small fragments.
These are just a few examples, and the choice of OVDs can vary vastly based on their inherent properties and the specific requirements of your eye surgery.
Why do People Need Viscoelastics
Cataract surgery is a procedure that helps improve the vision of people with cloudy lenses, also known as cataracts. During this operation, a special substance called viscoelastics plays a significant role in every step of the process. Viscoelastics assist with opening up the front part of the eye, dealing with the cataract itself and implanting a new, artificial lens.
Viscoelastics possess key qualities, such as high viscosity (thickness), transparency, and the ability to maintain the shape of the anterior chamber (the front part of the eye). It’s also really important that they are able to control the eye’s fine structures, regardless of the type of cataract surgery being performed.
In paediatric cases where a child’s eye capsule (the bag-like structure that holds the lens) is very elastic, high-density viscoelastics like Healon GV are preferred. In some instances, a different type of viscoelastic is used if there’s a tear in the eye capsule. Here are some examples of when and how different types of viscoelastics are used in cataract surgery:
- During ‘capsulorhexis’ (opening the capsule to remove the cataract), a cohesive viscoelastic like Sodium Hyaluronate is used because of its high viscosity and elasticity.
- While ‘trenching’ (making a groove in the cataract to break it up), a dispersive viscoelastic such as Hydroxypropyl Methylcellulose (HPMC) is used for its protective ability.
- Similar to the above, while removing the ‘cortex’ (outer layer of the cataract) and implanting the new lens, dispersive and cohesive viscoelastics are used respectively for their specific qualities.
Viscoelastics are also useful in glaucoma surgeries, corneal surgeries and in managing certain complications such as a ‘floppy’ iris during surgery. They’re even used in surgeries involving the drainage system of the eye, the muscle tissues surrounding eyes and routine procedures in paediatric eye surgery. They also have the ability to reduce friction and can even be mixed with an anaesthesia agent to provide ‘viscoanaesthesia’ for pain-free procedures.
It’s clear that viscoelastics, with their unique properties, are an important cornerstone in various eye surgeries, enabling them to proceed safely and successfully.
Preparing for Viscoelastics
Sodium Hyaluronate
This substance, often described as a salt form of hyaluronic acid, is naturally found in both humans and certain types of bacteria. In humans, it’s usually found in the skin, in fluid around our joints (synovial fluid), and in the jelly-like substance in our eyes (vitreous humor). Sodium hyaluronate was first discovered in the vitreous humor of cows. This commercially available substance is obtained either from the comb of a rooster or from a special type of bacteria known as streptococci. Sodium hyaluronate is a large, sugar-like molecule, which is incredibly viscous (thick and sticky). It is known for being sterile (free from bacteria), does not cause inflammation, is non-toxic (not harmful), and does not trigger an immune response or fever.
Chondroitin Sulphate
This substance has similar physical and chemical characteristics to sodium hyaluronate. The structure is slightly different due to a sulfate group and an extra negative charge included in each of its structural units. It’s commercially produced from shark cartilage. Chondroitin sulfate forms part of the three major sugar-like molecules in the cornea of the eye.
Different Types of Chondroitin Sulphate
Chondroitin Sulphate (20% Isolated)
This formulation is not overly thick and doesn’t fill space like others; rather, it plays an important role in protecting surfaces. If its thickness is increased to a 50% solution, it could potentially harm the cells lining the back of the cornea.
Chondroitin Sulphate and Sodium Hyaluronate Mixture
This combination is much thicker and provides a better protective coating.
Hydroxypropyl Methylcellulose (HPMC)
HPMC is derived from cellulose, a natural material found in plant fibers like cotton and wood. Unlike sodium hyaluronate and chondroitin sulfate, it doesn’t occur in animal tissues. Despite not being as thick as other substances and missing some properties of a viscoelastic (substances that have both liquid and solid characteristics), HPMC has a thickness of around 3000-4000 centipoise (a measure of thickness). It has a neutral pH of 7.2 and contains 285 milliosmoles of particles (a measure of concentration). This substance is sterilized by heat and can be stored for up to 2 years at room temperature.
How is Viscoelastics performed
The medical procedures we’re discussing involve the removal of viscoelastic. This is a substance often used in eye surgeries to protect the eye’s delicate structures. Here are a few techniques doctors use when it’s time to carefully remove this substance:
1. The Rock and Roll Technique: In this method, the doctor uses certain tools to gently push and tilt the lens of your eye to one side. They then remove the viscoelastic from beneath the lens. They repeat this maneuver on the other side to make sure all the viscoelastic is removed.
2. The Compartment Technique Using Bimanual Irrigation and Aspiration: This may sound complex but it boils down to this: the doctor uses a special tip on their tool to move back and forth helping them remove the viscoelastic.
3. Tapping Technique: The eye’s lens is carefully tapped with a tool. This helps remove the viscoelastic under the surface of the lens. Once it’s loosened, the substance can be removed easily.
4. Spinning Technique: After a new synthetic lens has been placed in your eye, your doctor can spin the lens 360 degrees to clear out any remaining viscoelastic.
These procedures are typically carried out during eye surgery after the viscoelastic is no longer needed. It’s important to the health of your eye that all the viscoelastic is completely removed.
Your surgeon will determine which method is the best, most safe, and effective based on your individual situation.
Possible Complications of Viscoelastics
Viscoelastics, often used in eye surgery, can sometimes come with complications. Here are a few of them explained in simple terms:
Secondary Glaucoma
This can occur if the viscoelastic – a substance used to protect your eye during surgery – isn’t fully removed during the operation. This could lead to spikes in your eye pressure within 6 to 24 hours after surgery. The eye pressure increases because the part of your eye that usually drains fluids (trabecular meshwork) gets blocked by particles of the viscoelastic. This usually gets better all by itself after about 72 hours, but if the high eye pressure continues, your doctor might need to prescribe an eye pressure-lowering medication.
Capsular Block Syndrome
Capsular Block Syndrome (CBS) refers to a buildup of fluid or viscoelastic in a part of your eye called the capsular bag. An unpleasant consequence of this buildup can be that the lens of your eye, or an artificial one (intraocular lens or IOL), gets pushed forward, which might block an opening in the front of the capsular bag. CBS can either happen during surgery, soon after surgery when viscoelastic accumulates, or a long time after surgery when a thick viscoelastic substance gets stuck in the capsular bag.
Pseudo-Anterior Uveitis
Because of high thickness and negative electricity of viscoelastic, blood and inflammatory cells could clump together in your eye’s anterior chamber (the front part of the eye). This will typically improve on its own within three days.
Postoperative Uveitis and Hypopyon
Viscoelastics can sometimes cause inflammation of the middle layer of the eye or a pool of inflammatory cells in the front part of your eye.
Corneal Edema and Decompensation
Another complication that could occur due to viscoelastic use is corneal edema, the swelling of the cornea, which is the clear front surface of the eye.
IOL Surface Crystallization
This happens when viscoelastic materials leave residue on the surface of an intraocular lens (a lens implanted in the eye), causing solid deposits that might resemble fern leaves or shapeless deposits. This can lead to the need to remove or replace the lens.
Calcific Band Keratopathy
This condition, related to the use of a type of viscoelastic material containing a substance known as chondroitin sulfate, involves calcium deposits on the cornea.
Hypersensitivity
A small risk of allergic reaction exists with some types of viscoelastics as the sodium hyaluronate in them is derived from a protein found in rooster combs or a specific type of bacteria.
What Else Should I Know About Viscoelastics?
Appendicitis, or inflammation of the appendix, needs to be treated swiftly. This condition can lead to a ruptured appendix, which can lead to severe complications. The standard treatment for this condition is an operation known as an appendectomy, which involves the removal of the appendix.
The operation can be conducted in two ways – through laparoscopic or open surgery. Laparoscopic surgery is less invasive and can lead to less pain, fewer infections, and a quicker recovery time. However, open surgery might be necessary if the appendix has burst or there are other complications.
After your surgery, you would usually be able to go home soon if your appendix hadn’t burst. You would then slowly reintroduce a normal diet. But if the appendix had burst, a longer hospital stay is needed, along with a course of antibiotics.
It’s also worth noting that sometimes, the appendix might look healthy during surgery. But just because it looks normal, doesn’t mean it isn’t diseased. Hence, the decision to remove it would rely on your individual situation.
Laparoscopic surgery is often the better choice for older patients, children, and expecting mothers since it comes with fewer complications. So, if you believe you have appendicitis, it’s important to contact a healthcare provider immediately.
When it comes to eye surgeries, some interesting techniques can be used:
The “Soft Shell Technique” involves injecting a particular substance into the front part of the eye to protect its inner lining from any damage during surgery. This is usually done during a surgery called phacoemulsification (used to treat cataracts) and is particularly useful in cases of certain types of eye disorders and dense cataracts.
The “Ultimate Soft-Shell Technique” was developed to make the soft shell technique more cost-effective. In this approach, a so-called ‘viscoadaptive’ substance is injected into the eye, followed by a special saline solution. This process can reduce friction during surgical maneuvers.
The “Tri-Soft Shell Technique” is yet another method that uses a series of different substances to protect the eye during surgery.