3. INTRODUCTION
OVDs also referred to as viscoelastic agents
Have been employed in anterior segment surgery since 1979
Play an important role in maintaining the anterior chamber and protecting the corneal
endothelium
during surgery
Their use has had a profound influence on the evolution of extracapsular and
phacoemulsification surgery
Substances have dual properties :
viscosity of fluid
elasticity of gel or solid
4. IDEAL VISCOLELASTIC SUBSTANCE
Ease of infusion
Retention under positive pressure in eye
Retention during phaco
Easy removal/no removal needed
Doesn’t interfere with instrument insertion
Protect endothelium
Nontoxic
Doesn’t obstruct aqueous flow
6. PHYSICAL PROPERTIES
Viscosity describes resistance to flow and can be thought of as the
“thickness” or “thinness” of a fluid.
It is determined primarily by molecular weight and concentration, so that
substances with high molecular weight and high concentration have the
highest viscosity.
The higher the viscosity, the better the OVD is at displacing tissue and
staying in place.
Measured in centipoise (cPs) or centistokes(cSt)
7. PHYSICAL PROPERTIES
Elasticity refers to the ability of a material to return to its original
shape after being stressed.
It describes the OVD’s ability to re-form after an external force is
applied to the anterior chamber and then removed.
A highly elastic substance is excellent for maintaining space
8. PHYSICAL PROPERTIES
Pseudo plasticity refers to the ease with which a material can change
from being highly viscous at rest to being watery at increasing rates of
shear stress.
This property is found in certain everyday substances such as
toothpaste; when squeezed out of a tube, toothpaste flows easily, but
it retains its shape when it is at rest on the toothbrush.
In clinical terms, at zero shear force an OVD is a lubricant and coats
tissues well, but when forced through a small-gauge cannula it
functions like a liquid.
9. PHYSICAL PROPERTIES
Surface tension describes how the surface of a fluid tends to stick to another
surface. This property is also referred to as coatability, which is inversely
proportional to surface tension.
Thus, an OVD with low surface tension is better at coating tissue but is
harder to remove from the eye.
10. OVDS
Contain one or more of the following substances in varying concentrations:
sodium
hyaluronate
chondroitin
sulfate
hydroxypropyl
methylcellulos
e
Others
11. SODIUM HYALURONATE
Biopolymer, disaccharides
occurs in many connective tissues throughout the body, such as synovial
(joint) fluid and vitreous humor
It was originally isolated from human umbilical cord and rooster combs
Has a half-life of approximately 1 day in aqueous and 3 days in vitreous
12. CHONDROITIN SULFATE
Sulfated glycosaminoglycan, which is an important component of cartilage.
Obtained from shark fin cartilage
Eleminated from AC 24 to 30 hours
13. HYDROXYPROPYL METHYLCELLULOSE (HPMC)
Does not occur naturally in animal tissues, but cellulose is widely distributed in plant fibers such as
cotton and wood.
The commercial product is a cellulose polymer modified by the addition of hydroxypropyl and methyl
groups to increase the hydrophilic property of the material.
Methylcellulose is a non physiologic compound that does not appear to be metabolized intraocularly.
It is eventually eliminated in the aqueous but can easily be irrigated from the eye.
Ease preparation, availability and storage at room temperature
15. COHESIVE OVDS
long-chain, high-molecular-weight, high-viscosity substances
These agents maintain space well at no or low shear rates, whereas at high
shear rates they are easily displaced.
OVDs are easier to remove from the eye because they stick together and are
aspirated as long pieces (similar to spaghetti).
However, they have minimal coating ability and therefore afford less tissue
protection during surgery
16. COHESIVE OVDS
include Healon, Healon GV (Abbott Medical Optics, Santa Ana, CA); Amvisc,
Amvisc Plus (Bausch + Lomb, Rochester, NY); and Provisc (Alcon, Ft Worth, TX)
HYALURONATE PRODUCTS
17. DISPERSIVE OVDS
short-chain, low-molecular-weight, low-viscosity substances with low surface
tension.
These agents provide excellent coating and protection at high shear rates;
however, they are more difficult to remove from the eye because they do not
stick together and are aspirated in short fragments (similar to macaroni).
They are more likely to be retained in the eye after cataract surgery, increasing
the likelihood of angle obstruction with reduced outflow and subsequent IOP
elevation
include OcuCoat (Bausch + Lomb), Viscoat (Alcon), and Healon Endocoat
(Abbott Medical Optics). Discovisc (Alcon) combines qualities of dispersive
and cohesive agents
MOST OF THEM HPMC
20. OTHERS
Some additional OVDs, such as the viscoadaptive agent Healon5 (Abbott
Medical Optics), may need separate classiffcation.
Healon5 is a long, fragile chain with high molecular weight that changes its
behavior at diferent flow rates. The lower the flow rate, the more viscous and
cohesive the OVD is, and the higher the flow rate, the more the chains
fracture.
As a result, this OVD acts as a pseudodispersive agent. However, it must be
carefully removed at the end of surgery because it can cause extremely
elevated IOP if left in the eye
25. USES OF OPHTHALMIC VISCOSURGICAL DEVICES
The space maintenance ability of OVDs keeps the anterior chamber formed despite the presence of
one or more corneal incisions.
With expansion of the chamber, manipulations can be made away from the corneal endothelium and
posterior lens capsule
A cohesive OVD can be used to enlarge a marginally dilated pupil (viscomydriasis)
It can also be used to keep the plane of the anterior capsule flat to assist a controlled continuous
curvilinear capsulorrhexis
Lens implantation is less traumatic to the zonular fibers and the posterior capsule when the capsular
bag is inflated with an OVD
26. CONT..
The coatability of OVDs can be used to protect the corneal endothelium from phaco energy,
particularly in dense cataracts or during long operations. The surgeon must take care to
remove dispersive OVDs completely to reduce the risk of an ocular hypertensive period
caused by angle outflow obstruction.
In the presence of an open posterior lens capsule, a dispersive OVD can be injected over the
tear to provide a vitreous tamponade and prevent prolapse of vitreous anteriorly.
The optical clarity of OVDs has prompted some surgeons to use a layer of OVD on the surface
of the cornea. When slightly moistened with balanced salt solution, the agent coats the
epithelium. This maneuver prevents drying and eliminates the need to irrigate the corneal
surface. It also provides a slightly magnified view of anterior segment structures.
27. CONT…
The choice of OVD varies depending on the clinical scenario. A survey showed
that 97% of surgeons vary their choice of OVD in complicated cases. For
example, in pediatric cataracts or cases with a low endothelial cell count,
shallow anterior chamber, or intraoperative floppy iris syndrome, the choice of
OVD can play a critical role in management.
