Vitreous Substitutes

Dr. Shylesh B DabkeResident, Dept. of Ophthalomolgy

Kasturba Medical College, Mangalore

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These are gases or liquids which are used as intra operative instruments to reestablish intraocular volume, to assist in separating membranes adherent to retina, to manipulate retinal detachment and to mechanically flatten retina.

The vitreous humor occupies 2/3rd of the volume of the eye.

Is a gelatinous substance composed of water (98–99%) & Matrix of collagen fibers and hyaluronic acid gel.

Its functions include holding the retina in place and circulating metabolites throughout the eye.

This gel is itself is damaged in various vitreoretinal disorders and needs to be replaced.

VITREOUS HUMOR

Good tamponade effect

High surface tension

Sufficient buoyancy

Should be inert, slowly biodegradable and transparent

Maintain stable refractive index and shock absorbing properties

Avoid serving as a scaffold for cellular proliferation

Expected to have :

Conventional• Air – SF6, C3F8• Liquid – BSS, Perfluorocarbons,

Silicon oilNewer

• Semifluorinated Alkanes• Silicon oil-semifluorinated alkane

combinationExperimental• Polymers – Hydrogel, Smart

Hydrogel, Thermosetting gels

• Implants• Cell culture

Used as irrigation fluid during vitrectomy and also to maintain normal intraocular volume and pressure after drainage of SR fluid during RD surgery & choroidal detachment.

BSS enriched with bicarbonate, dextrose & glutathione better tolerated by ocular tissue, especially endothelium

BASAL SALT SOLUTION

It offers no tamponade effect

It may pass through retinal breaks & may lead to RD

BSS not preferred because

Ohm(1911)-1stintravitreal air injection for RD

Rosengren(1938) concept of internal gas tamponade

Norton(1973) reported favorable results with SF6

Air & other gases

Buoyancy Surface tension

Interruption of intraocular

current

Intra ocular air bubble is based on 3 features

Non expandable• Air• Nitrogen• Helium etc.

Expandable

• SF6• C3F8

Three phases of gas transfer in expanding gases

Bubble expansion

Nitrogen equilibrium

Bubble diffusion

First gas to be injected intravitreally

Effect lasts for 3 days

Can be used in RD to maintain IOP after SRF drainage

Now not used commonly as internal tamponade

Air

Expandable gas

Colourless, odourless, nontoxic gas

Max expansion after 36 Hrs (doubles in volume)

Disappears in 10 days

It was shown that this inert gas provided longer, more

effective tamponade than air

Relatively free of harmful side effects

Sulfur hexafluoride

Inert, colorless & inflammable

6 times heavier than air

Max expansion occurs after 2 days – 4 times the initial volume

Perfluorocarbon gases

Indications

• Presence of superior retinal detachments that do not have too much subretinal fluid

• Retinal breaks occurring from the 10 to 2 o’clock meridians

• Breaks in the periphery between the ora serrata and the equator

• Macular holes & other posterior retinal breaks

• Re-detachment after scleral buckling

• Contra-indications to GA

Contraindications

• Inferior breaks

• PVR – pre op traction on retinal tear

• Inability to maintain appropriate position

• Severe glaucoma

• Cloudy media- precludes full assessment of retina

• Aphakia / pseudophakia with PCO

• If patient has to travel by air within 1 week

Postoperative Care

Clinical Estimation of Gas Volume

• Patient must maintain prone or particular head position after gas injection• IOP should be measured 6-8 hrs post operatively• Prophylactic timolol/acetazolamide is indicated

• Most clinicians describe “gas fills” as a percentage of the vitreous cavity based upon ophthalmoscopy

Complications

• Pupillary block glaucoma• Corneal endothelial decompensation• Shallow AC• CRVO • Expansion during air travel • lens opacity• Subretinal gas migration• Dislocation of IOLs

Gas Pulling on the Vitreous and Retina Leading to Hemorrhage and Tears and Shifting of Subretinal

Fluid to Macular Area

Air CF4,SF6 XE Perfluorocarbons

Choice of gases

• When the volume of gas is adequate to tamponade the break

• when its desirable to achieve larger bubble size to tamponade multiple retinal breaks or folds• when large volume of SRF is drained

• when greater expansion properties are needed –GRT, macular holes

• Sodium hyaluronate (1%; Healon) has the most favorable viscoelastic properties

• Used to unfold the retina during repair of giant retinal tears and to manage hemorrhage

• Also used in separation of epi-retinal membranes*

VISCOELASTIC SUBSTANCES

Cibis et al(1962)-silicone oil in retinal Surgery

“Silicone oil”- “lighter than

water”

“Fluorosilicone”, “heavier than

water”

SILICONE OIL

• Clear, transparent, inert, non carcinogenic, heat resistant

• High surface tension

• Immiscible in water

• RI – 1.404

• Lighter than water

PROPERTIES OF SILICONE OIL

Tamponade Space filler Mechanical inhibition of membraneous contraction Haemostasis

MODE OF ACTION

• Acts as internal permanent tamponade in attaching traction free RD*

• immiscibility of waterlimits free movts of proliferative cells & biochemical mediators withinvitreous*

• Inhibits diffusion of angiogenic mediatorshelps in rubiosis iridis

• redirects tractional forces from radial to tangential traction so re-detachments are usually flat & confined to periphery sparing macula

• Tamponades bleeding vessels in PDR and from the edge of retinotomy & retinectomies

Optics

In Phakics & Pseudophakics

In Aphakics

• Concave anterior surface of the globule it causes hypermetropia of about +5D

• Convex anterior surface of the globule power of the eye is increased such that the aphakic refractive error (pronounced hyperopia) is reduced

• Long-term tamponade of the retina

• PVR

• Giant retinal tear

• PDR

• Traumatic RD

• RD complicated by iris neovascularization

• Patient not compliance with positioning & post op positioning• After failure of previous scleral buckling, vitrectomy, membrane dissection &

intraocular gas injection for Rx of RD

INDICATIONS

Emulsification

COMPLICATIONS

Cataract

Band shaped keratopathy

Stromal opacification

Glaucoma

• Immediate post op rise due to inflammation• Acute pupillary block in aphakics- prevented by inferior (Ando’s/Japanese) iridectomy• Emulsified oil trapped in TM chronic glaucoma

Silicon Syndrome

Hypotony

• Retinopathy (toxic) -not proved• Redetachment after removal of

silicone oil

• PFC were first evaluated as artificial blood substitutes

• Haidt et al.(1983) first used PFC experimentally as a vitreous substitute

• Colorless, odorless, non-flammable

• Chemically and biologically inert when pure.

• They are stable to temperatures as high as 400 to 500º Celsius.

• High specific gravity*

• Significant tamponading effect*

PERFLUOROCARBON LIQUIDS

• Optically clear

• Immiscible with water or blood or other common organic compounds

• Low viscosity*

• High vapor pressure rate

• Refractive index significantly different from aqueous

Optical Clarity• All PFC liquids are optically clear and relatively free of sources of reflection/optical aberration

• optical clarity allows the application of laser energy to the attached retina during the surgical procedure

• Since they do not absorb visible light and have a higher boiling point than the thermal burn, PFC liquids are considered a safe medium for the delivery of laser energy

Immiscible with Intraocular Fluids• The ability of the material to resist incursion by blood or intraocular fluids makes it a valuable aide in improving visibility in cases involving heavy or uncontrolled bleeding.

• PFCL can also be used as a “unit”, making them helpful in retrieving intraocular or crystalline

lenses by floating them off the retina and up into the pupillary space.

Refractive Index

• The ability to accurately control intraoperative PFC liquid is dependent on the surgeon’s ability to visualize the material in the eye. PFCL have an index significantly different than aqueous. Hence, a distinct interface between the perfluorocarbon liquid and aqueous can be clearly visualized.

• Also helps to remove the PFC at the conclusion of surgery.

Injected slowly keeping the tip of the cannula just within the meniscus of the expanding bubble, and centered over the optic disc if it is visible.

Administration of PFCL

• Giant Retinal Tears*

• Proliferative Vitreoretinopathy(PVR)

• Trauma

• Dislocated Lenses

INDICATIONS

Dislocated Lenses

• The specific gravity of perfluorocarbon liquids is greater than that of crystalline lenses, PMMA (IOL’s) or silicone intraocular lenses, and allows the surgeon to gently float the dislocated lens or fragment off the retina following a “complete” vitrectomy,

• Retinal detachments secondary to macular holes can be managed by injecting a small amount of PFCL to flatten the hole so that endophoto-coagulation can be carried out

• Surgical Management of retinal detachments in advanced ROP

• Endophthalmitis: PFCL can be injected to cover posterior pole during vitrectomy for endophthalmitis, which prevents contact of the antibiotic with macula, avoiding possible macular toxicity

• Control of Bleeding During Pars Plana Vitrectomy

• Removal of Intraocular Foreign Bodies

• Retinal break from forceful injection of PFCL into the vitreous cavity*

• Dispersion of PFCL into multiple bubbles can occur if the level of PFCL goes above the infusion cannula or if injection is not done into the PFCL bubble

• Large amount of PFCL can damage the corneal endothelium in aphakic and pseudophakic eyes and should be removed completely

Complications

• Useful in the treatment of complex retinal detachments. Such a combination supports both superior and inferior areas of the pathology

• Silicone oil greater viscosity is less likely to enter the anterior chamber in aphakic eyes than are PFCLs & resist the movement of PFCL into the anterior chamber

• Silicone oil tends to delay the emulsification of PFCLs when both are used together

Combination of PFCL and Silicone oil

Experimental• Polymers – Hydrogel, Smart Hydrogel, Thermosetting gels

• Implants

• Cell culture