Year : 1996 | Volume
: 44 | Issue : 4 | Page : 191--206
Fluid vitreous substitutes in vitreo retinal surgery
Sandeep Saxena1, Lingam Gopal2,
1 Department of Ophthalmology, K.G. Medical College, Lucknow, U.P, India
2 Vision Research Foundation, Madras, India
Dept. of Ophthalmology, K.G. Medical College, Lucknow, U.P
Advances in the surgical instrumentation and vitreoretinal techniques have allowed intraoperative reapproximation of retina to a more normal position. The use of intravitreally injected liquid materials (viscoelastic liquids, liquid perfluorocarbons and silicone oil), as adjunctive agents to vitreo-retinal surgery play an important role in facilitating retinal reattachment. These materials are used as intraoperative instruments to re-establish intraocular volume, assist in separating membranes adherent to the retina, manipulate retinal detachments and mechanically flatten detached retina. Over the longer term, silicone oil maintains intraocular tamponade. One should be cognizant of the potential uses, benefits and risks of each of these vitreous substitutes.
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Saxena S, Gopal L. Fluid vitreous substitutes in vitreo retinal surgery.Indian J Ophthalmol 1996;44:191-206
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Saxena S, Gopal L. Fluid vitreous substitutes in vitreo retinal surgery. Indian J Ophthalmol [serial online] 1996 [cited 2019 Sep 22 ];44:191-206
Available from: http://www.ijo.in/text.asp?1996/44/4/191/24561
The success of surgery for retinal detachment is dependent on attaining closure of retinal break(s), relieving traction on the retina and minimizing the recurrence of traction. In complicated retinal detachments, there has been a greater recognition of the pathoanatomic changes in vitreoretinal relationships and advances in the surgical instrumentation and techniques allow intraoperative reapproximation of the retina. The use of intravitreally injected liquid materials play a vital role in facilitating retinal reattachment. Viscoelastic fluids and perfluorocarbon liquid (PFCLs) are used as intraoperative instruments. Postoperatively, silicone oil maintains the neural retina and retinal pigment epithelium in apposition. This article discusses the properties and uses of various liquid vitreous substitutes.
Viscous fluids used in vitreoretinal surgery include sodium hyaluronate, chondroitin sulphate and hydroxypropyl methylcellulose. These are polymers with molecular weights ranging from approximately 30,000 to 4 million daltons. It is the viscoelastic properties of these materials that make them most useful for intraocular surgery. Shearing occurs when fluid is made to flow. At zero shear rate (steady state) they exhibit high viscocity whereas at high shear rates their viscocity decreases. This behaviour allows the material to be injected through small gauge cannula and yet ensures that the material will regain its shape. Sodium hyaluronate (1%; Healon) has the most favorable viscoelastic properties. Its molecular weight is nearly 4 million daltons and has a viscocity greater than 400,000 centistokes at near zero shear. At high shear, the viscocity decreases to 110 centistokes. The viscoelastic properties of sodium hyaluronate can be helpful in the separation of epiretinal membranes. The term viscodissection describes the hydraulic elevation of epiretinal membranes as the fluid is injected into the plane between the retina and proliferative tissue. The membranes are separated from the retinal surface with less trauma and the remaining attachments can be dissected more easily. In proliferative diabetic retinopathy blood can be displaced so that the planes between membranes and retina can be seen more easily. This approach should be used cautiously when the retina is atrophic because iatrogenic retinal breaks may develop if too much injection force is used. Sodium hyaluronate has also been used to unfold the retina during repair of giant retinal tears and to manage haemorrhage. Postoperative elevation of intraocular pressure can result if large volumes are left in the vitreous cavity after surgery. Sodium hyaluronate has been used in surgery for Stage V retinopathy of prematurity, as a volume maintainer. Being a viscous substance, it does not leak out rapidly and enables bimanual dissection through 2 ports although infusion is absent. During fluid air exchange, in aphakic and pseudophakic eyes, problem in visualisation can occur sometimes. In aphakic eyes, especially wherein the epithelium has been removed, or wherein the cornea has been cut and sutured (as in combined cataract surgery and pars plana procedure), descemet's folds form or get exaggerated during fluid air exchange. This substantially interferes with visualisation. The situation can be countered by coating the posterior corneal surface with viscoelastic substances. In pseudophakic eyes with a posterior capsulotomy, the exposed portion of the IOL tends to become dry during fluid air exchange leading to break down of fluid film into fine droplets. This again can interfere with visualisation and can be countered by coating this surface with sodium hyaluronate or methyl cellulose.
PERFLUOROCARBON LIQUIDS (PFCL)
(PFCL) Low viscocity liquid perfluorocarbons are optically clear compounds, having a specific gravity higher than saline, which make them a useful surgical tool for flattening a detached retina, intraoperatively. Since their initial use in vitreous surgery as a surgical tamponade in 1987, they have been found to be useful in the management of complex vitreoretinal conditions.
Several low viscocity PFCLs have been studied for their potential intraoperative use. These include, perfluoro-tributylamine (C12F27N),[10-13] perfluoro-decalin (C10F18),[12-15] perfluoro-phenanthrene (C14F24),[16-17]perfluoro-ethylcyclohexamine (C8F16), foralkyl AC-6,perfluoro-octylbromide and perfluoro-n-octane (C8F18). Perfluoro-n-octane, perfluoro-tributylamine, perfluoro-decalin and perfluorophenanthrene are the liquid perfluorocarbons currently in use. Their properties are listed in [Table:1]. They are optically clear with indices of refraction similar to that of water. Hence, no optical aberrations occur when working through PFCLs. An interface can be visualized between the PFCL and saline, since they are immiscible. Visualizing the interface is important as it facilitates the complete removal of PFCL and reduces the incidence of residual droplets in the vitreous cavity. The specific gravity of PFCL is nearly twice that of water. Thus, the tamponade force exerted by PFCL against retina is considerably greater than that exerted by equivalent volume of fluoro-silicone oil. This characteristic makes it possible for the PFCL to flatten the retina intraoperatively. The interfascial tension of PFCL with water is roughly equivalent to that of silicone oil and the material tends to be cohesive, so that liquid remains in one large bubble. The surface tension though not as pronounced as long acting gas still provides some deterrance to passage of PFCL through a break. The low viscocity of PFCL (0.8 to 8 Centistokes at 25° C vs 1000 to 5000 Centistokes at 25° C of silicone oil) facilitates easy introduction and removal with small gauge instruments and allows them to demonstrate areas of residual traction. PFCLs have variable vapour pressures. A higher vapour pressure is desirable because a residual layer of PFCL, on the surface of retina, will evaporate during fluid-air exchange, thus reducing the incidence of residual droplets. The boiling point of PFCL exceeds that of saline and this allows safe endo-photocoagulation without causing vaporization.
Perfluoro-n-octane is the preferred PFCL. It is obtained as a highly purified compound. It does not contain protanated impurities (NMR spectroscopy analysis). The major impurities that could be present in PFCLs are hydrogen containing and would result from the incomplete fluorination of the hydrocarbon precursor; hydrogen-containing impurities are suspected to cause tissue reactivity. In addition to being free of detectable impurities, it can also be manufactured with a uniformity of 99.9%. It has a relatively lower boiling point and higher vapour pressure than do other PFCLs.
The high stability of the carbon-fluorine bond in a PFCL can render the liquid virtually inert. Experience with PFCLs as artificial blood replacements also suggests that they are biologically inert. Various studies, evaluating short-term tolerance of different PFCLs have not shown electro-physiologic or morphologic evidences of retinal toxicity for as long as 48 hours. Electron microscopic studies of intravitreal PFCLs have not shown any toxicity in pig eyes for a period upto 3 hours.,, Perfluorophenanthrene has shown reduced incidence of dispersion into small droplets during long-term tamponade and appears to be well tolerated intravitreally upto 1 week and perhaps longer. Experimental extended vitreous replacement with perfluoro-n-octane, in rabbit eyes, elicited a macrophage response with epiretinal membrane formation on the surface of retina. Narrowing of the outer plexiform layer and thinning of the outer nuclear layer, in the inferior retina, was observed as the PFCL remained longer in the eye.These changes suggest the mechanical effect of PFCL. Similar changes, due to pressure-induced mechanism, have been observed in the superior retina of rabbit eyes filled with silicone oil. Small droplets of perfluoro-n-octane injected into the vitreous cavity of rabbit elicited mild macrophage response but no retinal alterations at 6 months. Clinically, no observable inflammatory effects have been seen after 6 months, in patients with residual PFCL droplets., Nevertheless, whether these small droplets can induce intraocular proliferation is unknown and the long-term effects of residual PFCLs are uncertain. In general PFCLs are used as intraoperative tools and not as post operative tamponading agents, in view of the propensity for emulsification, although perfluoro phenanthrene has been used as a short term tamponading agent.
Applications of PFCL
1. Proliferative Vitreoretinopathy (PVR): [Figure:1]
The use of PFCLs in the management of retinal detachment complicated by severe degrees of PVRwas first described by Chang. Subsequent studies found a reattachment rate of 78% and 96.5% with the use of PFCL. These results compare favourably with the silicone study group. The surgical technique in most of these cases includes removal of the crystalline lens to allow anterior dissection. Intraocular lenses are left in place. A scleral buckle or encircling band is used to support the vitreous base. In eyes with anterior PVR, PFCLs open peripheral folds by pulling the peripheral retina posteriorly. This action allows better visualization of the membranes and therefore their complete removal. The anterior proliferations in the vitreous base are better defined when the posterior retina is immobilized by PFCL. The use of PFCLs permit initial dissection of posterior PVR., Posterior membrane dissection is begun at the optic nerve and 0.5 - 1.0 ml PFCL is injected over the optic nerve. This opens the funnel and flatten the posterior retina, exposing residual membranes and creating counter-traction to allow anterior dissection. PFCL interface is kept posterior to areas of epiretinal membranes and their removal continues in a postero-anterior direction. As the retina becomes more mobile further PFCL is added. The level of PFCL is kept posterior to the retinal breaks and to the scleral buckle area. Once the retina is completely flattened, PFCL-air or a PFCL-silicone oil exchange is done.
There are numerous advantages of PFCLs. A posterior retinotomy is not necessary for drainage of subretinal fluid because it is displaced and aspirated through peripheral breaks. This reduces injury to the retina and the risk of reproliferation from the retinotomy site. Areas of retinal traction can be readily visualized and it can be determined if all traction has been relieved or when subretinal membrane removal or retinotomies are needed.. The size of relaxing retinotomies can be precisely monitored because PFCLs flatten the retina as it is cut. Even 360° retinotomies can be easily handled by PFCLs without need for agents such as retinal tacks. Besides flattening and immobilizing the posterior retina, it acts as a barrier, protecting the posterior retina from cells and biochemically active substances produced during surgery which may contribute to reproliferation. Finally, PFCLs incontrast to air do not cause pupillary miosis when injected into the eye to flatten the retina.
2. Proliferative Diabetic Retinopathy
PFCLs can be usedful in the management of patients with severe proliferative diabetic retinopathy. PFCLs make it possible to flatten the retina in cases of combined traction-rhegmatogenous detachments to facilitate the removal of fibrovascular proliferation and to allow endophotocoagulation to be applied intraoperatively. Care should be taken in cases where there is significant bleeding, since droplets may be hidden under blood clots, making their removal difficult. Anatomic reattachment, using PFCLs, has been reported in eyes with diabetic traction detachments complicated by a rhegmatogenous component.
3. Giant Retinal Tears : [Figure:2]
Liquid perfluorocarbons offer the greatest advantage in the management of giant retinal tears. Previously described techniques involved use of silicone oil, fluorosilicone, prone fluid-gas exchange, sodium hyaluronate, and the expanding bubble method. Chang et al initially described the use of PFCLs for giant retinal tears. Subsequent reports showed a reattachment rate of 100%, and 90% in giant retinal tear cases complicated with PVR. Vitrectomy is necessary when the flap of the tear is inverted and immobile. Lensectomy is done if better visualization is required. A central and anterior vitrectomy is done and epiretinal membranes, if present, are removed. The flap of the tear is bimanually unfolded and a small amount of PFCL is introduced over the disc. More PFCL is added to flatten the retina upto the edge of the tear. Membranes on the edge of the tear are removed and the vitreous base is trimmed. After all the traction has been removed, the level of PFCL is raised over the edge of the tear. A fluid-air exchange or a fluid-silicone exchange is done, removing all the fluid anterior to the PFCL. The PFCL is then aspirated antero-posteriorly. If slippage occurs in the air-filled eye, a small amount of saline is injected into the vitreous and the patient is rotated postoperatively to steamroll the retina. The choice of tamponade depends on the extent of tear and presence of PVR. If giant tear is around 90 to 180° with no PVR, gas tamponade with perfluoro propane gas may be preferred. Silicone oil is preferred in most other situations. When silicone oil is used, it is preferable to do direct PFCL silicone oil exchange. The retinal edge is manipulated hydraulically, thus avoiding trauma by mechanical manipulation. In cases without PVR, a scleral buckle can be avoided.
Traumatic retinal detachments are often complicated by vitreous haemorrhage, subretinal or choroidal haemorrhage, lens injury, foreign bodies and severe proliferation. PFCLs have been used in the management of penetrating ocular trauma.,,In cases with extensive vitreous haemorrhage and retinal detachmnet, PFCLs can stabilize the retina, drain subretinal fluid anteriorly and permit safe removal of blood from the vitreous base with a posteriorly flattened retina. Injected into subhyaloid space PFCL can induce posterior vitreous separation and make vitrectomy simpler.If subretinal haemorrhage is present, the PFCL displaces it anteriorly so that it can be aspirated through a peripheral break. The higher specific gravity of PFCL allows stabilisation of eye, preventing collapse and reducing risk of intraocular haemorrhage. Traumatic retinal detachments
complicated by epiretinal or subretinal membrane or large breaks and dialyses are managed in the same manner as PVR. PFCLs may assist in the management of traumatic retinal incarceration to stabilize the retina during manipulations and retinectomies. Intra-ocular wood or plastic foreign bodies can be floated on the surface of PFCL and removed. Metallic foreign bodies can be held in place by covering them with PFCL and removing them through PFCL thus minimizing the potential for retinal damage.
5. Retinal detachment in Acquired Immunodeficiency syndrome
Cytomegaloretinitis occurs in approximately 25% cases of AIDS. Intravenous gancyclovir has proven to be successful in treating this condition. Subsequently, 15% to 29% of treated eyes develop rhegmatogenous retinal detachments with up to 50% incidence of bilateral retinal detachments., The acute retinal necrosis syndrome also leads to rhegmatogenous retinal detachments. Preservation of at least ambulatory vision in such patients increases their quality of life. PFCL has been used to flatten the retina displacing subretinal fluid through pre-existing breaks in the retina. This technique obviates the need for a posterior retinotomy and permits direct fluid-silicone exchange.
6. Posteriorly dislocated crystalline lens and intraocular lens : [Figure:3]
PFCLs facilitate the safe removal of a posteriorly dislocated crystalline lens, nucleus,, and nuclear fragments. The PFCL allows the crystalline lens to float off the retinal surface into the anterior vitreous cavity. In the presence of retinal tear, PFCL prevents the retina from detaching. Prior to aspiration, lens fragments may be produced intentionally. The crushed material may remain on the surface of the bubble, avoiding damage to the retina from posteriorly falling particles. Retrieval, removal, repositioning or exchange of a posteriorly dislocated IOL by using PFCL, eliminates the risk of injuring the posterior retina.,, While elevating the IOL, the high density vitreous substitute can flatten an associated retinal detachment without the need for a posterior drainage retinotomy and an air-fluid exchange. It also supports the eye from collapsing, thus allowing a safer IOL exchange or fixation. PFCLs have been used in the management of retinal detachments associated with dislocated crystalline or intraocular lenses.,,, PFCLs, while elevating the lens, displace subretinal fluid through an anterior break, successfully flattening the retina.
7. Surgical excision of subretinal membrane
PFCLs have been used during the surgical excision of subfoveal membranes. Following removal of The subretinal membrane, PFCL is injected to tamponade any bleeding from subretinal space
8. Retinal incarceration
Retinal incarcerations may be repaired by injecting PFCLs. PFCL applies countertraction which helps to pull the incarcerated retina back into the vitreous cavity. Unrelieved incarcerations may be managed by relaxing retinotomy.
PFCL can be injected to cover posterior pole during vitrectomy for endophthalmitis, which prevents contact of the antibiotic with macula, avoiding possible macular toxicity. However, this application of PFCL is not very popular with most surgeons.
10. Retinal detachments secondary to macular holes
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. In cases where there is doubt whether there is a peripheral break in addition to macular hole, PFCL can be injected to flatten the retina. If a peripheral break is present, the fluid will drain through it flattening the retina peripherally.
11. Massive subretinal haemorrhage
Cases of massive subretinal haemorrhage are managed by creating retinotomy through which a fibrinolytic agent is injected to dissolve the clotted blood. PFCL can be injected into the eye to express the liquified blood through the retinotomy.
12. Retinopathy of prematurity
PFCLs can also be used in the management of open-funnel or closed-funnel retinal detachments of advanced retinopathy of prematurity.
13. Retinal detachments associated with choroidal coloboma
Perfluoro perhydrophenanthrene, in the area of retinal detachment associated with an inferior coloboma provides prolonged inferior tamponade.
14. Retinal detachments associated with posterior retinal breaks
It was thought initially that PFCL could not be used in situations where posterior posterior breaks were present since the liquid would migrate subretinally. PFCL has been injected into eyes with complicated retinal detachments with posterior breaks without complications to help anterior dissection and endophotocoagulation.
15. Supra Choroidal Haemorrhage
Desai et al have described the use of perfluoro phenanthrene in the management of 3 cases of non-expulsive supra choroidal haemorrhage. The PFCL was inserted into the vitreous cavity with simultaneous drainage of blood from anterior sclerotomies.
Complications of Perfluorocarbon liquids
1. Retinal break
A retinal break can occur from forceful injection of PFCL into the vitreous cavity. This complication can be avoided by slow injection of the fluid and by directing stream at the optic disc.
2. Subretinal migration of PFCL
PFCL can migrate subretinally during surgery. It can either be aspirated by an extrusion needle or displaced anteriorly and evacuated through a peripheral break by filling the vitreous cavity with more PFCL.
3. Dispersion of PFCL
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.
4. Residual PFCL
Large amount of PFCL can damage the corneal endothelium in aphakic and pseudophakic eyes and should be removed. Patients with residual droplets of PFCL in the subretinal space and vitreous cavity have been followed without any evident inflammatory problems., These small droplets tend to decrease in size over time and eventually disappear.
Paul Cibis, of St. Louis, was responsible for the early popularity of intraocular silicone oil. John Scott used silicone oil as an instrument to separate membranes and push the retina back, without vitrectomy. It was Haut, who first combined use of silicone oil as internal tamponade after vitrectomy. Zivojnovic popularised this technique and contributed immensely to it's successful usuage by defining its role clearly. The observations of Reidel et al defined the complications of intravitreal silicone oil and aided in the purification of the material and production of a silicone oil with more homogenous chain length, thereby reducing emulsification significantly.
Silicone oils are polymers of a number of molecular weights of polydimethylsiloxane. Differences among silicone oils are determined by the length of the polymer, which affects the viscocity and the hydrocarbon radicals, which constitute the side groups of the polymer. It is not miscible with water.
Silicone oil is transparent and has a refractive index of 1.40, which is higher than that of vitreous. Hence the optics of silicone oil filled eye changes. In the phakic eye, if the silicone globule is in contact with the lens and the anterior surface of the globule is thus concave, a hyperopic change to about +5D is induced. Since the front of the silicone globule is convex in an aphakic eye, the refractive power of the eye is increased such that the hyperopia is reduced., The convex anterior surface of the silicone oil can be altered by the position of the head, thus making it difficult to correct an aphakic silicone oil filled eye by utilizing a spectacle lens or contact lens.
The viscocity of silicone oil is designated in centistokes. The viscocity of a specific silicone oil is determined most importantly by the average molecular weight and the entanglement of individual strands. High-viscocity silicone oils have been speculated, less likely, to undergo emulsification., A 1000 centistokes silicone oil with an average molecular weight of 30,000 daltons has 403 such repeating units. Silicone oil of 5000 centistokes has a molecular weight of 50,000 daltons and 673 repeating units. Most clinical studies have been performed with 1000 and 5000 centistoke silicone oils. Although higher centistoke silicone oil may reduce the risk of emulsification, they are more difficult to insert and remove from the eye.
The main purpose of silicone oil is to permit long-term tamponade of the retina. Its surface tension is significantly less than a gas-water interface. Because it has a specific gravity of approximately 0.971 (less than that of water) it produces a buoyant force within the eye. This buoyant force is only one-third of that of the pressure produced by a gas bubble, which may provide a helpful force in maintaining reattachment of the superior retina if it is free of traction but is insufficient to resist radially directed tractional forces on the retina., There are other mechanisms by which intravitreal silicone oil can deter redetachment. The presence of silicone globule in close proximity to the surface of the retina may impose a redirection of tractional forces, so that they are realigned parallel to the retina (tangential) and are correspondingly less effective than radially transmitted traction. When a large silicone globule occupies the vitreous cavity, the posterior retinal surface is covered by only a thin layer of aqueous,, Perhaps under these conditions, intraocular fluid currents are not sufficient to redetach the retina unless the traction is significant.,
Long-term tamponade force is produced primarily by interfascial surface tension. It is important to avoid blood and surgical debris within the eye, in order to maintain the highest possible surface tension. The silicone oil bubble has the ability to produce a tamponade effect as well as to compartmentalize the eye. The interfacial tension of the silicone oil with water forms a bubble that does not easily migrate through open holes. The blockage of open retinal holes prevents passage of fluid into the subretinal space and may allow the retinal pigment epithelium to pump out any residual fluid. Because it is impossible to completely fill the eye with silicone oil, there is always an inferior space filled with vitreous fluid, having restricted movement. Occasionally, inferior open holes close spontaneously because the pigment epithelium can remove fluid with enough rapidity to overcome the diminished fluid currents, thereby resulting in spontaneous retinal reattachment. This restricted fluid space can also harbour the potential for perisilicone proliferation. The small remaining fluid cavity gradually concentrates proteins and other factors that may stimulate tissue proliferation, thereby explaining the propensity for reproliferation in oil-filled eyes treated for retinal detachments.
A standard three port vitrectomy is done with the goal of removing all formed vitreous. It is of extreme importance that all retinal traction be released and the retina be completely reattached before the use of silicone oil. The PFCLs can be useful in producing traction in the posterior pole and stabilizing the anterior retina, to facilitate dissection of membranes. PFCLs may be removed before the gas exchange or during fluid-air exchange. An inferior iridectomy is performed in aphakic eyes, which may also be beneficial in eyes with a posterior chamber intraocular lens if zonular rupture is suspected. Extremely high buckles prevent the silicone oil from forming a proper tamponade in areas posterior to the buckle and therefore should be avoided. Pre-existing very high buckles may need to be removed. Following reattachment of the retina by the fluid-air exchange and endolaser photocoagulation, the silicone oil is infused into the eye through a 19 or 20 gauge thinwalled cannula attached to the silicone pump. Fluid silicone oil exchange also can be done. In eyes where PFCL is used, direct PFCL silicone oil exchange is done. The interface between the two liquids is well made out, and this avoids the unnecessary step of PFCL air exchange. This also eliminates the miosis, corneal descemet's folds and other visualisation problems that go with an air filled eye. The eye should be left normotensive or slightly soft at the end of the procedure. The anterior chamber is formed with B.S.S. at conclusion. The eye must be relatively soft before this maneuver to avoid migration of B.S.S. posteriorly. [Figure:4] shows fundus photograph of an eye with silicone oil.
The technique of oil removal is relatively simple. A formal three port vitrectomy is favourable as any residual or recurrent preretinal membranes or other media opacities can be removed. Balanced salt solution is infused through the pars plana using the infusion cannula. In aphakic patients, where additional surgery is not required, a small corneal incision made by the Micro Vitreoretinal (MVR) blade is sufficient to allow the egress of the oil. An 18 or 19 gauge thin- walled needle inserted through the pars plana and attached by a small piece of tubing to a 60 ml syringe allows generation of sufficient suction force to evacuate oil in phakic or pseudophakic eyes. Mechanised viscous fluid extractors can also be used. Silicone oil tends to be trapped in the vitreous base and under the iris and in membranes. Flute needle wash out of small bubbles of silicone oil may need to be done for a long time. However, total removal of all oil traces is never possible [Figure:5]. The minimal time after which silicone oil can be removed is considered to be 3 weeks. In general, it is preferred that the silicone oil be removed by 3 to 6 months postoperatively.[93-95] and even earlier. There are, however, several cases in which silicone liquid has remained in eyes for several years.
Applications of Silicone Oil
1. Proliferative Vitreoretinopathy (PVR)
Rhegmatogenous retinal detachments complicated by PVR constitute the major group of eyes in which silicone oil could be of some benefit. These eyes exhibit a propensity for reproliferation that has resulted in the anatomical success rate of only 40% to 70%. In general, perfluoropropane is indicated in most patients as a primary procedure for severe PVR. The use of C3F8 helps avoid a second surgery for oil removal and results in similar or slightly better results in eyes untreated with previous vitrectomy. Silicone oil should be considered in patients if previous vitrectomy has not achieved in patients if previous vitrectomy has not achieved retinal reattachment. One eyed patients can regain mobility faster with silicone oil. It may be preferable in patients who are not able to position themselves. In eyes with an intraocular pressure less than 5 mm Hg and recurrent severe retinal detachment, silicone oil may be preferred. Despite the potential benefits, because of its buoyancy it rises to the superior pole of the vitreous cavity and is thus most effective when a retinal teardetachment is located superiorly and is less effective for inferior lesions. Consequently, the inferior quadrants in which PVR also tends to be most severe,, are usually the locus of persistent or recurrent detachments following silicone oil injection, Redetachment has obvious consequences for visual function and may also secondarily precipitate lens and corneal complications by forcing the intravitreally placed silicone globule anteriorly. Buckling of an inferior tear, in conjunction with intravitreally placed silicone oil, is advocated so that the silicone globule, deformed by the buckle, will contact the retinal tear.
The 20% incidence of retinal detachment with oil removal is similar to the ultimate retinal detachment rate in eyes with oil retention. Silicone oil removal may, however, help in avoiding longterm corneal and retinal complications and often results in immediate improvement in visual acuity. The oil removal operation can be combined with further surgery to peel off any residual or reproliferated membranes. Silicone oil removal should not be considered a simple procedure. A thorough evaluation of the retina pre and intraoperatively is needed to look for (1) recurrent preretinal fibrosis (2) areas of shallow detachments especially in the periphery that can be easily missed, (3) open retinal breaks that may not cause retinal detachment in the presence of silicone oil but may produce the same following it's removal. Careful removal of reformed membranes, and treatment of identified retinal breaks helps to reduce incidence of recurrent retinal detachment following silicone oil removal. The thoroughness of initial vitrectomy (especially peripheral traction relief), very often dictates the type of recurrence that occur under silicone oil. In the presence of significant recurrence, further surgery is done under silicone oil to remove the traction and reattach the retina. In this case a silicone oil filled syringe is connected to the infusion cannula to top up the eye whenever needed.
Comparison of silicone oil and intraocular gas for intraocular tamponade is shown in [Table:2]. The silicone oil study was a multicenter, randomized clinical trial supported by the National Eye Institute, Bethesda, Maryland. The outcome of the study has been published as several reports. Silicone oil tamponade has been found to be superior to sulfur hexafluoride, but when compared to perfluoro propane gas (C3F8), silicone oil had similar anatomic and functional results. Abnormalities in intraocular pressure postoperatively were noted both in silicone oil and C3F8 groups. In eyes needing relaxing retinotomy also, C3F8 was found to be as effective as silicone oil. Removal of silicone oil was found to lead to increased likelihood of improved vision as well as recurrent retinal detachment. Oil removed eyes in general showed a trend towards reduced incidence of complications. Corneal abnormalities were noted in both gas and oil filled eyes.
2. Proliferative Diabetic Retinopathy
Vitrectomy used with aggressive sectioning and removal of fibrovascular membranes, endophotocoagulation and improved case selection result in 70 to 80% anatomical success rate in eyes with proliferative diabetic retinopathy, vitreous haemorrhage or localized retinal detachment. The ability of silicone oil to compartmentalize the eye and prevent or at least attenuate the passage of angiogenic factors into the anterior segment can produce regression of the anterior neovascularization in many eyes., There is evidence that it acts as a diffusion-convection barrier to oxygen, an effect that is postulated to alter the stimulus for neovascularization. Stabilization or regression of rubeosis iridis can occur in up to 80% cases. Nevertheless, an effect on the anterior segment neovascularization has not always been observed, and may be dependent on factors such as the degree of filling. However, the improvement in the anterior segment frequently does not correlate with posterior segment changes,,,, and often there is reproliferation of fibrovascular tissue behind the silicone oil. To avoid reproliferation, all bleeding sites must be sealed. Haemorrhage behind silicone oil will usually result in reproliferation leading to recurrent hole formation. Silicone oil has been found to be useful in eyes with attached retinas in which all proliferative tissue has been removed but continued haemorrhage is a consequence of rubeosis iridis. These eyes may respond to oil injection and often have the potential for useful vision, although silicone oil cannot be removed because rubeosis will recur. Silicone oil is also useful in the management of severe tractional retinal detachment secondary to anterior hyaloid proliferation. Extensive retinotomies may be required necessitating the use of silicone oil.
3. Giant Retinal Tears
Cibis described the use of silione oil to unfold the posterior edge of a giant tear. Silicone oil was found to work well as an intraocular device to unfold the retina and a success rate of approximately 85% were reported.,, A fluid silicone exchange was done, allowing manipulation of the posterior flap under the oil as it entered the eye, Subretinal fluid was drained through retinotomies or by a flute needle from under the retinal flap. The introduction of PFCL obviates the need to use silicone oil for the purpose of reattaching the retina intraoperatively, although it still is useful for the long term tamponade postoperatively.
Acutely injured eyes with unstable scleral wounds, haemorrhagic choroidal and retinal detachment, vitreous haemorrhage and incarcerated retina having propensity for intraocular fibrous proliferation may benefit from silicone oil injection. Silicone oil placed in the vitreous cavity stabilizes the ocular wound and helps in avoiding extension of the haemorrhagic choroidal detachments. It may also be useful in cases of ocular injuries resulting in massive choroidal and ciliary body detachment and hypotony. It stabilizes the intraocular pressure and avoids postoperative hypotony that may eventually lead to phthisis bulbi. Severely traumatised eyes with traumatic PVR and retinal detachment often need extensive relaxing retinotomies and hence benefit by long term tamponading with silicone oil.
5. Retinal detachment in Acquired Immunodeficiency syndrome (AIDS)
The use of silicone oil in detachments, associated with cytonegalo virus (CMV) retinitis, has been shown to be effective for anatomical correction and in many cases can preserve some degree of functional vision., The surgical technique is essentially same, however, the type of complications encountered may require some deviations. A pars plana vitrectomy is done. Relatively younger patients do not have posterior vitreous detachments. Cortical vitreous is removed utilizing silicone-tipped flute cannula. In areas of necrotic retina, it may be safer to circumcise the adherent cortical vitreous. Internal drainage is done either utilizing a preexisting hole or by making a retinotomy in detached retina. The retina is reattached utilizing fluid-air exchange. Endophotocoagulation may be done to surround retinal breaks, retinotomy and areas of severely necrotic retina. Subsequently, air-silicone exchange is done. Despite anatomical success, optic atrophy may occur and result in severe visual loss. It is most likely due to viral infection, although the possibility that silicone oil may play some role has not been conclusively excluded. In most cases of CMV retinitis related retinal detachments in AIDS patients, the silicone oil is left in the eye for life since the longevity of these patients is limited.
6. Retinal detachments due to coloboma choroids
Retinal detachment caused by coloboma of the choroid are attributable to breaks located in the diaphanous retinal tissue located in the colobomatous area. These are difficult to directly close because of the lack of retinal pigment epithelium and choroid. Retinopexy is not effective and buckling is very difficult. Hence vitrectomy with silicone oil tamponade and endolaser along the colobomatous margin serve to isolate the colobomatous area from the rest of the retina.
Silicone oil and Fluorosilicone as intraoperative tool
Silicone oil has been used during pars plana vitrectomy to reposition retina, to stabilize the retina during the removal of epiretinal membranes, and to unroll the flaps of retinal tears. As a liquid having a density lower than water (specific gravity of 0.97), it lacks the physical properties that, with the patient in supine position, would provide a downward force to flatten the retina and collapse the subretinal space or unroll the retinal tear in a posterior to anterior direction. Fluorosilicone, because of its high specific gravity of 1.28, has been advocated as an alternative intraperative tool.,,
Perfluorocarbon liquids have replaced fluorosilicone liquids as intraoperative tools because of their favourable physical properties.
Combination of PFCL and Silicone oil
The combined use of silicone oil and a non toxic PFCL may be useful in the treatment of complex retinal detachments., Such a combination supports both superior and inferior areas of the pathology [Figure:6], because, silicone oil apposes the superior retina and PFCL apposes the inferior retina., As both substances are immiscible, the meniscus delineating them is visible during fundus examination. Based on experimental studies, it has been concluded that a 2:1 ratio of silicone and PFCL would take advantage of the greater ability of PFCLs compared with silicone oil to resist traction retinal detachment while ensuring that the interface between the two liquids did not involve the visual axis. Silicone oil, because of its greater viscocity, is less likely to enter the anterior chamber in aphakic eyes than are PFCLs and may resist the movement of PFCL into the anterior chamber. Silicone oil tends to delay the emulsification of PFCLs when both are used together., Clinical studies have demonstrated good tolerance of the combination of silicone oil and perfluoro perhydro phenanthrene in human eye intraoperatively and as a short term tamponade.
Complications of Silicone Oil
1. Emulsification: Emulsification probably occurs because of the shearing effect created by the difference between the velocity of movement of the silicone bubble and that of the eye and the fluid meniscus around the silicone bubble. The protein concentrated in the vitreous fluid may contribute to the shearing effect. It occurs in variably in all cases,,
2. Keratopathy: Band keratopathy or stromal opacification may occur in 30% cases., ,  Band keratopathy responds well to debridement of the cornea with diluted disodium ethylene diamine tetraacetic acid (EDTA), with application of a bandage lens postoperatively. Stromal opacification is treated by corneal transplant, in eyes with visual potential. Corneal transplant is usually carried out at the time of silicone oil removal.
3. Cataract: All phakic eyes containing intravitreal silicone oil develop cataracts. This is related to contact between the silicone globule and the posterior lens capsule and the resulting mechanical obstruction to diffusion of nutrients. The incidence of cataract is most pronounced between 6 and 18 months after surgery and opacification can develop or progress even after silicone withdrawl.,
4. Pupillary block: The inferior iridectomy by Ando has reduced the incidence of pupillary block considerably [Figure:7]. Temperent of the inferior iridectomies close in the postoperative period from iris retraction and recurrent proliferation of tissue at the vitreous base. Postoperative closure of peripheral iridectomy is highly correlated with forward oil migration and occurs most frequently in eyes with proliferative diabetic retinopathy. Silicone oil can migrate into the anterior chamber even in phakic eyes [Figure:8]. This occurs through unsuspected zonular dehiscences. If small, this can be ignored and be removed along with the main bubble later. Removal of the bubble from the anterior chamber alone usually is followed by the migration of oil into anterior chamber from the main bubble.
5. Glaucoma: Glaucoma occurs in 3:5% cases.,, Since the advent of purified oil, its incidence has decreased. When used in eyes that have had previous procedures, there is presumably some destruction of the ciliary body which reduces the production of intraocular fluid.
6. Hypotony: Hypotony has been reported in 16% cases.,, Hypotony has been attributed to the extensive anterior proliferation seen in most cases of complicated PVR. The anterior membranes, surgical attempts to remove them and extensive laser contribute towards lowering of intraocular pressure. Silicone oil removal should be avoided in cases with hypotony.
7. Perisilicone proliferative macular pucker: Macular pucker has been reported to occur in approximately 30% cases.,, Membranes should be removed at the time of oil removal.
8. Recurrent retinal detachments: Recurrence of retinal detachment has been reported to occur in 25- 40% cases. Reoperation should be done if the eye has visual potential., The recurrences in most cases are not attributable to silicone oil but to the primary disease process. The use of liquid vitreous substitutes have revolutionized vitreoretinal surgery. As new indications for these liquids continue to evolve, and their use becomes more widespread, their many benefits will be appreciated by the vitreoretinal surgeons. Most complications with silicone oil can be avoided if one is aware that silicone oil is not a permanent tamponading agent but should be removed as soon as possible. The twin objective of removal of silicone oil and maintenance of retinal reattachment is possible in a majority of cases by meticulous initial surgery while injecting silicone oil, and by careful evaluation at time of oil premoval.
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