OPHTHALMIC VISCOSURGICAL

DEVICESDr A DEBATA

Dr S R PATI

INTRODUCTION Viscoelastic substances are solutions

with dual properties – act as viscous liquid as well as elastic solids/gels

Viscosurgery was a term coined by Balazs

Sodium Hyaluronate was 1st used in ophthalmic surgery as viscoelastic in 1972 as a replacement for vitreous & aqueous humor

PROPERTIES The rheological properties of OVDs are

most relevant for its usefulness. Viscoelasticity – Elasticity refers to the

ability of the solution to return to its original shape after being stressed.

It allows the ant. Chamber to reform after deformation by depression on the cornea when the external forces are released

Amount of elasticity of an elastic compound increases with increasing mol.wt. & greater chain length of the molecule

Viscosity – Reflects a solution’s resistance to flow

- Measured in Centipoise (Cp) or centistokes (Cst)

Liquids have viscosities < 10000 Cst at rest Solutions having viscosities > 10000 Cst

are gel like - Viscosity depends on degree of

movement of solution (shear stress) & varies inversely with temperature

- Viscosity of a solution can be increased by increasing the mol.wt/conc.of the solution

Pseudoplasticity - a solution’s ability to transform when under pressure, from a gel like substance to a more liquid form.

- A change in molecular structure occurs - Clinically a high mol.wt, high viscosity

OVD at rest(zero shear force) acts as an excellent lubricant, coats tissues & maintains space very well.

- Under influence of stress(high shear rate),OVD become elastic molecular system & behave as an excellent shock absorbing gel.

When a soln. is passed through a cannula viscosity becomes independent of MW & solely determined by concentration

Pseudoplastic soln have a low viscosity at high shear rates & can be extruded easily through a small cannula

Cohesion-dispersion index (CDI) - Defined as the percentage

viscoelastic agent aspirated/100 mm Hg

Surface tension - The coating ability of an OVD is

determined not only by the surface tension of material itself but also by the surface tension of the contact tissue, surgical instrument or IOL.

- By measuring the angle formed by a drop of OVD on a flat surface (contact angle),the coating ability is estimated.

- At lower surface tension & lower contact angle, better ability to coat.

RHEOMETRIC PATTERNS OF BEHAVIOR OF FLUID VISCOSITY IN RESPONSE TO INCREASING RATES OF SHEAR

CLASSIFICATION OF OVDS1.High viscosity-cohesive OVDs a.superviscous-

cohesive(>1,000,000mPs) b.viscous cohesive(1,00,000-

1,000,000mPs)

2.Lower viscosity-dispersive OVDs

3.Viscoadaptive OVDs

HIGH VISCOUS COHESIVE Superviscous cohesive- eg. Healon GV,I Visc plus Viscous cohesive- eg.I Visc, Provisc, Healon, Amvisc,

Amvisc plus etc. All products contain Na.hyaluronate Indications of highviscous cohesive OVD- -To deepen the AC -To enlarge small pupils -to dissect adhesions -during IOL implantation

Healon GV-A sterile,nonpyrogenic agent

Conc. Of Na.hyaluronate is 1.4% MW-5 million daltons Use-Cataract Sx, IOL implantation,

corneal transplant Sx, glaucoma Sx It has 3times more resistence to

pressure than healon I Visc-a healon GV clone Superior viscous & cohesive properties

at lower shear viscosity.

Super viscous cohesive are preferred in topical & intracameral anaesthesia technique

Easy removal as a single mass at end of Sx & thus minimizes risk of raised IOP post-op.

LOWER VISCOSITY DISPERSIVE When injected, fracture into smaller bits

& disperse throughout AC. Eg. Viscoat, Cellugel, Vitrax, Ocucoat Most of them are Hydroxypropyl methyl

cellulose Use- To hold vitreous out of surgical field

as in zonular disinsertion It divides AC into - OVD occupied space - Surgical zone

Disadvantages Do not maintain or stabilise spaces Tend to get aspirated during I/A & leads

to irregular viscoelastic-aqueous interface causing obscuration of surgical view of post. capsule

Can form micro-bubbles & further obscure the view

Difficult to remove fully at the end of surgery

VISCOADAPTIVE OVD Behaviour changes at different flow

rates Acts as a viscous cohesive agent at

lower flow rate & as a pseudo-dispersive agent at higher flow rates

Adapts its behaviour to surgeon’s needs during surgery

Highly purified non inflammatory high mol.wt. Na Hyaluronate at a 2.3% conc. dissolved in a physiological buffer

ADVANTAGES OF VISCOADAPTIVE OVDS Different functions at diff. flow rates Crystal clear & has higher refractive index

than aq.humor, so increases clarity within surgical field.

Ability to bind to & to protect delicate corneal endoth. cells from debris & turbulence during phaco

Helpful in small pupil as it causes viscomydriasis

Neutralises the +ve vitreous pressure & prevents the capsulorrhexis extension.

Cleaned by Rock and roll technique/by two compartment technique(TCT)

SOFT SHELL TECHNIQUE Developed by Arshinoff Use of both lower viscosity dispersive &

high viscosity cohesive OVDs together to minimise their drawbacks & to get best properties of both

Use - Corneal endothelial protection during lens removal ,During IOL implantation

E.g. Duovisc – Provisc + Viscoat

THE SOFT SHELL OR ULTIMATE SOFT SHELL TECHNIQUE A mound of dispersive OVD is placed on the

surface of the crystalline lens and it is pressurized up against the corneal endothelium by injecting a cohesive (soft shell technique) or viscoadaptive (ultimate soft shell technique) OVD below it

In the ultimate soft shell technique, another layer of balanced salt solution is injected below the viscoadaptive agent

During the ensuing phacoemulsification, the ultimate soft shell technique preserves the viscoadaptive layer, whereas the cohesive OVD is likely aspirated out of the anterior chamber with the soft shell technique.

THE SNOWMAN TECHNIQUE

David Leaming developed an implantation technique

This involves using the 4% chondroitin sulfate/1.65% sodium hyaluronate OVD in the chamber and balanced salt solution in the bag to improve toric procedures.

DESIRED PROPERTIES OF IDEAL OVD Ease of infusion Retention under +ve pressure in eye Retention during phaco Easy removal/no removal needed Doesnt interfere with instruments/IOL

placement Protects endothelium Nontoxic Does not obstruct aq.outflow

USES OF OVDS Cataract Sx Corneal Sx & penetrating keratoplasty Glaucoma Sx Ant.segment reconstruction as a result

of trauma Vitreoretinal Sx Strabismus Sx Oculoplastic Sx

Different jobs demand different OVDs 1. Maintain space: e.g. AC during rhexis

bag during IOL insertion cohesive best

2. Create space: e.g. Creating sulcus shift lens material cohesivebest

3. Sealing off: e.g. Sealing capsular tear keeping iris away dispersive best

4. Coating: e.g. Protect corneal endothelium lubricate cornea dispersive best

CATARACT & REFRACTIVE SX Capsulorrhexis-4 basic features help in

doing rhexis- 1.High mol.weight & high viscosity at

zero shear rate which maintains AC. 2.Excellent visibility provided by its high

transparency 3.High elasticity & pseudoplasticity 4.It should give a good capsular flap

control,providing the soft & permanent spatula effect.

Cleavage of lens structure by OVD- -OVD keeps AC shape unchanged

during BSS injection & avoids increase in pressure

Nuclear emulsification- -OVDs preserve the space & also

because of their low cohesiveness ,they remain in the AC despite high irrigation flow.

-OVDs adhere to corneal endothelium,thus protecting the endoth.cells

Irrigation & aspiration- -protects corneal endothelium due to high

adhesiveness. -it remains where it is placed ,without

mixing with cortex because of its low cohesiveness thus helping in easy removal of cortex.

Capsular bag filling & IOL implantation- -OVDs expand the capsular bag for easy

IOL implantation. -helps in correct positioning ,centering ,&

allowing for possible IOL rotation maneuvers

OVDs also used for implantation of other IOL designs

Eg.anterior chamber,iris fixated,artisan lenses etc.

Cataract Sx in pediatric cases- -use of Healon GV causes effective

push in the opposite direction & capsulorrhexis becomes easier.

-high density viscoelastic agents stabilize the post.chamber & push back the vitreous face during post capsulorrhexis.

During IOL implantation ,capsular bag is kept open & the AC is well formed.

OVDs also help to dilate the pupil & maintain a good intraoperative mydriasis.

GLAUCOMA SX Visco-canalostomy Means opening of schlemm’”s canal by

OVD. A Nonpenetrating

procedure ,independent of external filtration

Advantages-decrease risk of infn, -decrease incidence of cataract -hypotony -flat AC -Excludes risk of late infn &

conjunctival & episcleral scarring

OVDs should have high pseudoplasticity to allow injection through a small needle

Should have high viscosity at zero shear rate to maintain the spaces as long as possible

Eg.Healon GV & Healon-5 are usually used

VISCOSTAINING OVDs used as a vehicle to deliver capsular dyes for

use during cat.Sx. Dyes used are-Fluorescein Na,ICG,Tryptan blue Techniques-staining from above under an air

bubble & intracameral subcapsular inj.of Fl.Na ( staining from below)with blue-light enhancement.

Any instrument entering eye will cause some air to escape with rise of lens-iris plane

A small amount of high density viscoelastic placed near incision prevents air escape & minimizes risk of sudden collapse.

Alternatively-dye mixed with OVD called as viscostaining of ant.lens capsule covers ant capsule without coming in contact with corneal endoth.

VISCOANAESTHESIA Mixture of OVD with an anesthetic soln

(known as VISTHESIA) had advantages of viscosurgery,maintainence of ACD,capsular bag expansion,protection of corneal endoth.

Prolongs anesthesia No extra surgical step for intracameral

inj. Of lidocaine Contains topical component -0.3%

hyaluronic acid with 2% lidocaine in a single dose unit

Intracameral component-1.5%hyaluronic acid with 1% lidocaine

Can be used externally to provide corneal & conjuctival epithelial protection

OVDs have soft instrument effect during iris & other Sx.

Used to form a mechanical barrier to control hemorrhage both intraocularly & extraocularly.

Decreases the risk of post op CME. Removal techniques- -Rock & Roll method -Two compartment technique -Bimanual irrigation & aspiration technique

COMPLICATIONS OF OVD USE

Post-op. increase in IOP - Occurs in 1st 6-24 hrs & resolves

spontaneously within 72 hrs - Due to mechanical resistance at TM Crystallization of IOL surfaces - Due to precipitation or deposition of

viscoelastic soln. - Fern like or amorphous appearance - IOL should be explanted & exchanged

Capsular block syndrome or Capsular bag distension syndrome (CBS)

Characterised by accumulation of liquefied substance within a closed chamber inside the capsular bag, formed because the lens nucleus or the PCIOL optic occludes the ant. capsule opening created by capsulorhexis

Classified as : 1.Intra-op – time of nucleus luxation

following hydro-dissection 2.Early post-op 3.Late post op. – with liquefied after

cataract-eg.Use of high density viscoelastic agent

like Healon GV causes late CBS Calcific band keratopathy - Occurs with chondroitin sulphate

containing OVDs

Pseudo Anterior uveitis - Due to OVDs viscous nature & the

electrostatic charge of it - RBCs & inflammatory cells remain in

AC giving it appearance of uveitis - Spontaneously resolves within 3 days - Intra ocular hemorrhage may be

trapped between vitreous space & OVD in AC mimicking VH

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