Dr Luis Real,
ophthalmologist, surgeon from Ecuador.
Specialist in cataract surgery and cornea problems.
https://clinicarealvision.com/
Ophthalmic viscosurgical
devices (OVDs) area 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.
The term
viscosurgery was introduced almost 30 years ago. The
development of the first viscoelastic agent which was made of sodium
hyaluronate (Healon®)
helped to revolutionize the way cataract surgery was performed. Many other
hyaluronates followed, differing from one another in molecular weight,
concentration, and viscosity.Because
of the multitude of important functions that viscoelastic substances serve in
intraocular surgery they have been renamed ophthalmic viscosurgical devices
(OVDs), suggesting that these agents are now to be considered essential
surgical tools, not just corollary products to be used during surgery.
OVDs are unique in
that they own characteristics of both solids and fluids. Surgical applications
of the different OVDs are determined mainly by their physical properties which
are a consequence of their molecular chain length and interactions both within
chains and between chains and eye tissue. They have 4 main physical properties:
-viscosity;
-pseudoplasticity;
-elasticity;
-coatability.
Viscoadaptive OVDs
are substances with high molecular weight and high concentration that contain
fragile long-chain molecules. Their physical properties are different from both
cohesive and dispersive OVDs.
Healon 5 was the
first viscoadaptice OVD to be designed rheologically. This OVD is highly
retentive and maintains the AC shape during surgical manipulation better than
cohesive substances. The term viscoadaptive relates to the different behaviors
they display during surgery according to flow rate. At low flow rate, a
viscoadaptive OVD is highly retentive and maintains the AC shape during
surgical manipulation better than cohesive substances.
At high flow rate,
its chains fracture and coat the endothelium similar to dispersive substances,
being also difficult to fully aspirate.Thus this OVD acts as a pseudodispersive agent.
The appearance of
Discovisc (Alcon) established another category for OVDs.
This substance has
an intermediate to high viscosity but is still highly dispersive. Combining
qualities of dispersive and cohesive agents, it prompted a new expanded
classification that considers cohesion- dispersion independently from
zero-shear viscosity. Discovisc behaves like a cohesive OVD with respect to ease
of removal from the anterior chamber while providing endothelial protection
similar to a dispersive substance.
Cataract surgery
is highly dependent on fluid flow. Understanding the physical properties of
OVDs and their behavior in different surgical environments is therefore key to
comprehending phacoemulsification fluidics and to performing better surgery.
