Ophthalmic viscosurgical device

Last updated March 23, 2023

Ophthalmic viscosurgical devices (OVDs) are a class of clear gel-like material used in eye surgery to maintain the volume and shape of the anterior chamber of the eye, and protect the intraocular tissues during the procedure. They were originally called viscoelastic substances, or just viscoelastics. Their consistency allows the surgical instruments to move through them, but when there is low shear stress they do not flow, and retain their shape, preventing collapse of the anterior chamber. OVDs are available in several formulations which may be combined or used individually as best suits the procedure, and are introduced into the anterior chamber at the start of the procedure, and removed at the end. Their tendency to remain coherent helps with removal,^[1] as the cohesive variants tend to be drawn into the aspiration orifice without breaking up.

Uses

OVDs are used to protect the corneal endothelium from mechanical trauma and to maintain volume and form of the intraocular space during an open incision. The OVD is introduced into the space by syringe through a cannula.^[2] At the end of the procedure they are removed by aspiration and the space filled with a compatible fluid such as a buffered saline solution.^[3]

OVDs are commonly used in cataract, cornea, glaucoma, eye trauma, and vitreoretinal surgery.^[1]

Contraindications

Despite the side effects, the advantages of OVDs have made them indispensable in ophthalmic surgery involving the anterior chamber.

There are no known contraindications to the use of a Sodium Hyaluronate Ophthalmic Viscosurgical Device as a surgical aid in ophthalmic anterior segment procedures.^[4]

Adverse effects

OVDs can cause excessive post-operative intraocular pressure, particularly if any is left remaining in the eye after surgery. The pressure rise is dose-related. It develops in the first day and will usually resolve spontaneously within three days. This effect is assumed to be a consequence of the large molecules of the OVD causing reduced outflow in the trabecular meshwork. Various drugs have been used to limit pressure spikes, and while effective, are not entirely predictable in their effects.^[2]

OVD can be trapped behind the IOL in the capsule during normal surgery, and may cause a forward displacement of the IOL, which in turn shifts the focal plane of the IOL towards near vision.^[2]

Properties

The properties of an ideal OVD would include:^[2]

Ease of placement, low viscosity during placement, and low tendency to entrain bubbles
Retention under positive pressure in the eye
Retention during phacoemulsification
Does not interfere with use of instruments or placement of the IOL.
Protects the endothelium
Nontoxic, sterile, and does not affect intraocular pressure
Does not obstruct aqueous outflow
Optically clear, allowing an unobstructed view.
Easy removal, or no removal required after use

The most relevant physical properties for use in ophthalmic surgery are viscoelasticity, viscosity, pseudoplasticity, and surface tension.^[2]

These physical properties of an OVD are consequences of molecular chain length, and molecular interactions between chains. The rheologic properties of an OVD directly affect its clinical characteristics. An OVD can be chosen that best matches the requirements for a specific procedure, or part of a procedure, and combinations may be useful.^[2]

Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and immediately return to their original state once the stress is removed. This property allows the OVD to retain its shape under low shear stress, and to spring back into shape after a low deforming stress is removed, but also allow instruments to be moved relatively freely when the critical shear stress is exceeded.^[2]

Viscosity is a measure of its the resistance to shear deformation in a fluid at a given rate, or its resistance to flow. It quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. The viscosity of a Newtonian fluid does not vary significantly with the rate of deformation.

Pseudoplasticity is the characteristic of a material to rapidly transform from a gel-like consistency to a liquid and back as the shear stress is varied. A high molecular weight, high viscosity OVD at rest retains its shape well. When under sufficient shear stress it will flow, and alignment of the molecules reduces viscosity to allow rapid flow. Under low shear stress the OVD will quickly revert to an elastic gel, which is a good shock absorber. The highest shear rates occur when the OVD is passed through a cannula, in which state viscosity becomes nearly independent of molecular weight. When the molecules are aligned in the direction of flow, viscosity is determined almost entirely by the concentration.^[2]

Surface tension: The coating ability of an OVD is determined partly by the intermolecular cohesive forces within the OVD, and partly adhesive forces between the OVD and the contacted tissue, instrument or IOL. The contact angle between a drop of the OVD and the other material on a flat surface is an indicator of the ability of the OVD to wet and coat that material. Surface tension is a measure of the cohesion between molecules of the OVD, so a lower surface tension and higher adhesion and contact angle indicate a better ability to wet. A solution of sodium hyaluronate has a significantly higher surface tension and contact angle with the relevant tissues than a solution of chondroitin sulfate, HPMC, or a mixture of sodium hyaluronate and chondroitin sulfate, which indicates better coating by the latter materials.^[2]

Cohesion and dispersion

Cohesive type OVDs adhere to themselves. They have high viscosity and act like a gel. The molecules have long chains, with high molecular weight, and have high surface tension and pseudoplasticity. They are relatively easy to aspirate as they tend to stay in one piece as they are drawn towards the suction orifice.^[1]

Dispersive tyes have lower surface tension and tend to spread and wet contact surfaces. They have lower viscosity, the molecules are not as strongly mutually attracted, they show low pseudoplasticity and have shorter molecular chains and lower molecular weight.^[1]

Mechanism of action

The OVD occupies the volume of the anterior chamber during surgery, maintains its volume and shape without requiring internal pressure, and does not flow out through open incisions when undisturbed. It allows the free passage of instruments, prevents low viscosity fluids from leaking out, and provides the surgeon with a clear view of the internal volume. The cohesive nature of some types facilitates rapid removal by aspiration at the end of the procedure.^[2]

Manufacturing

OVDs are sterile aqueous saline solutions of one or more viscoelatic compounds and buffers to control pH at 7 to 7.5.^[2]

Viscoelastic compounds

Sodium hyaluronate, is a linear polysaccharide molecule of sodium glycuronate and N-acetyl glucosamine.^[1] It is commercially extracted from rooster combs or produced by bacterial fermentation.^[2]
Chondroitin sulfate extracted from shark fin cartilage.^[2]
Hydroxypropyl methylcellulose (Hypromellose) is extracted from wood pulp.^[2]

History

Meyer and Palmer isolated hyaluronic acid from the vitreous cortex in 1934.^[5] Also in 1934, Endre A. Balazs extracted and purified hyaluronic acid from rooster combs and umbilical cord.^[1]

In 1958, Balazs suggested the possibility of using hyaluronic acid as a sustitute for the vitreous substitute during surgery for retinal detachment, and in 1972 made the first injection of hyaluronic acid into the vitreous chamber.^[1]

Ophthalmic viscosurgical devices were introduced in 1972.^[6] Various alternative formulations with varied physical characteristics have been developed since then.^[2]

Balazs developed the procedure of viscosurgery, coined the term, and patented high molecular weight viscoelastic material using purified hyaluronic acid to be used for implantation of IOLs.^[1]

In 1976 an application for the use of the OVD Healon was made with the FDA, and the next year applications were made for its use in surgery for cataracts, IOL implantation, glaucoma, and corneal transplants. In 1979, 510k permission was granted to market Healon, and FDA approval in January 1983. Since then OVDs have become essential tools in ophthalmic surgery.^[1]

Polyacrylamide is a synthetic compound that was used for a while but withdrawn from the market in 1991, after its use was found to be associated with elevated intraocular pressure.^[1]

Related Research Articles

Rheology is the study of the flow of matter, primarily in a fluid state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. Rheology is a branch of physics, and it is the science that deals with the deformation and flow of materials, both solids and liquids.

A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, that is, it has variable viscosity dependent on stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid. Many salt solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as custard, toothpaste, starch suspensions, corn starch, paint, blood, melted butter, and shampoo.

<span class="mw-page-title-main">Vitrectomy</span> Surgery to remove vitreous humor from the eye

Vitrectomy is a surgery to remove some or all of the vitreous humor from the eye.

Eye surgery, also known as ophthalmic or ocular surgery, is surgery performed on the eye or its adnexa, by an ophthalmologist. Eye surgery is part of ophthalmology. The eye is a fragile organ, and requires due care before, during, and after a surgical procedure to minimize or prevent further damage. An eye surgeon is responsible for selecting the appropriate surgical procedure for the patient, and for taking the necessary safety precautions. Mentions of eye surgery can be found in several ancient texts dating back as early as 1800 BC, with cataract treatment starting in the fifth century BC. It continues to be a widely practiced class of surgery, with various techniques having been developed for treating eye problems.

Phacoemulsification is a modern cataract surgery method in which the eye's internal lens is emulsified with an ultrasonic handpiece and aspirated from the eye. Aspirated fluids are replaced with irrigation of balanced salt solution to maintain the anterior chamber.

In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and immediately return to their original state once the stress is removed.

An Intraocular lens (IOL) is a lens implanted in the eye as part of a treatment for cataracts or myopia. If the natural lens is left in the eye, the IOL is known as phakic, otherwise it is a pseudophakic, or false lens. Such a lens is typically implanted during cataract surgery, after the eye's cloudy natural lens (cataract) has been removed. The pseudophakic IOL provides the same light-focusing function as the natural crystalline lens. The phakic type of IOL is placed over the existing natural lens and is used in refractive surgery to change the eye's optical power as a treatment for myopia (nearsightedness). This is an alternative to LASIK.

<span class="mw-page-title-main">Phakic intraocular lens</span> Lens implanted in eye in addition to the natural lens

A phakic intraocular lens (PIOL) is a special kind of intraocular lens that is implanted surgically into the eye to correct myopia (nearsightedness). It is called "phakic" because the eye's natural lens is left untouched. Intraocular lenses that are implanted into eyes after the eye's natural lens has been removed during cataract surgery are known as pseudophakic.

Cataract surgery, also called lens replacement surgery, is the removal of the natural lens of the eye that has developed an opacification, which is referred to as a cataract, and usually its replacement with an artificial intraocular lens.

<span class="mw-page-title-main">Hyaluronic acid</span> Anionic, nonsulfated glycosaminoglycan

Hyaluronic acid, also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans as it is non-sulfated, forms in the plasma membrane instead of the Golgi apparatus, and can be very large: human synovial HA averages about 7 million Da per molecule, or about 20,000 disaccharide monomers, while other sources mention 3–4 million Da.

In rheology, shear thinning is the non-Newtonian behavior of fluids whose viscosity decreases under shear strain. It is sometimes considered synonymous for pseudo-plastic behaviour, and is usually defined as excluding time-dependent effects, such as thixotropy.

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Sodium hyaluronate is the sodium salt of hyaluronic acid, a glycosaminoglycan found in various connective tissue of humans.

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Rheological weldability (RW) of thermoplastics considers the materials flow characteristics in determining the weldability of the given material. The process of welding thermal plastics requires three general steps, first is surface preparation. The second step is the application of heat and pressure to create intimate contact between the components being joined and initiate inter-molecular diffusion across the joint and the third step is cooling. RW can be used to determine the effectiveness of the second step of the process for given materials.

Intraocular lens scaffold or IOL scaffold technique is a surgical procedure in ophthalmology. In cases where the posterior lens capsule is ruptured and the cataract has not yet been removed one can insert the intraocular lens (IOL) inside the eye under the cataract. This way the IOL acts as a scaffold and prevents the cataract pieces from falling inside the back of the eye. The cataract can then be removed safely by emulsifying it with ultrasound and aspiration. This technique is called IOL scaffold and was started by Amar Agarwal from Chennai, India, at Dr. Agarwal's Eye Hospital.

Phacolytic glaucoma (PG) is a form of glaucoma which is caused due to a leaking mature or immature cataract. Inflammatory glaucoma which occurs in phacolysis is a condition which is a result of the leakage of protein within the lens into the capsule of a mature or hyper mature cataract and involves a simple procedure to be cured that is referred to as cataract extraction.

Secondary glaucoma is a collection of progressive optic nerve disorders associated with a rise in intraocular pressure (IOP) which results in the loss of vision. In clinical settings, it is defined as the occurrence of IOP above 21 mmHg requiring the prescription of IOP-managing drugs. It can be broadly divided into two subtypes: secondary open-angle glaucoma and secondary angle-closure glaucoma, depending on the closure of the angle between the cornea and the iris. Principal causes of secondary glaucoma include optic nerve trauma or damage, eye disease, surgery, neovascularization, tumours and use of steroid and sulfa drugs. Risk factors for secondary glaucoma include uveitis, cataract surgery and also intraocular tumours. Common treatments are designed according to the type and the underlying causative condition, in addition to the consequent rise in IOP. These include drug therapy, the use of miotics, surgery or laser therapy.

Uveitis–glaucoma–hyphaema (UGH) syndrome, also known as Ellingson syndrome, is a complication of cataract surgery, caused by intraocular lens subluxation or dislocation. The chafing of mispositioned intraocular lens over iris, ciliary body or iridocorneal angle cause elevated intraocular pressure (IOP) anterior uveitis and hyphema. It is most commonly caused by anterior chamber IOLs and sulcus IOLs but, the condition can be seen with any type of IOL, including posterior chamber lenses and cosmetic iris implants.

References

1 2 3 4 5 6 7 8 9 10 Scholtz, Sibylle (January 2007). "History of Ophthalmic Viscosurgical Devices". crstodayeurope.com. Cataract & Refractive Surgery Today Europe. Retrieved 13 February 2023.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Lane, Stephen S. (2006). "9: Ophthalmic Viscosurgical Devices (OVDs): Physical Characteristics and Clinical Applications". Duane's Opthalmology. Vol. 6. Lippincott, Williams and Wilkins. Retrieved 16 February 2023– via www.oculist.net.
↑ Cionni, Robert J.; Snyder, Michael E.; Osher, Robert H. (2006). "6: Cataract surgery". In Tasman, William (ed.). Duane's Opthalmology. Vol. 6. Lippincott, Williams and Wilkins. Retrieved 16 February 2023– via www.oculist.net.
↑ "Summary of Safety and Effectiveness data (SSED)" (PDF). PMA P200025: FDA Summary of Safety and Effectiveness Data. Retrieved 22 March 2023.
↑ Meyer, K.; Palmer, J.W. (1 December 1934). "The polysaccharide of the vitreous humor". Journal of Biological Chemistry. 107 (3): 629–634. doi: 10.1016/S0021-9258(18)75338-6 .
↑ Davis, G. (January–February 2016). "The Evolution of Cataract Surgery". Mo. Med. 113 (1): 58–62. PMC 6139750 . PMID 27039493.