British Journal of Ophthalmology, 1987, 71, 262-267

Silicone oil with high specific gravity for intraocular useV-P GABEL, A KAMPIK, CH GABEL, AND D SPIEGEL

From the University Eye Clinic, Munich, West Germany

SUMMARY Silicone oil with a higher specific gravity than that of intraocular fluid or polydimethyl-siloxane may have special indications in vitreoretinal surgery. Trifluorsiloxane is such a substance,and therefore its biological compatibility was investigated in rabbit eyes. It was found that thissubstance was clinically well tolerated within the observation time of up to 6 months, even if therewas some neovascularisation from the inferior limbus. Histologically both an inflammatoryresponse and tissue impregnation were more pronounced than with normal polydimethylsiloxane.

After the first introduction of silicone oil in thetreatment of severe tractional retinal detachment byCibis' this substance was thought to be a last resortfor desperate cases with a doubtful outcome. How-ever, since Scott2 and later Zivojnovic et al.1 com-bined the injection of silicone oil with extensivevitrectomy this substance has gained widespreadsuccessful use as an intraocular tamponade in themicrosurgical management of retinal detachmentdue to advanced proliferative vitreoretinopathy.While most of the oils now in use differ considerablyin viscosity and chemical purity (Gabel et al., inpress), all have a specific gravity less than that of theintraocular fluid. However, a tamponade of lowerspecific gravity may not be optimal for all cases,particularly for patients having inferior tears. Wehave therefore investigated the biological reaction toa silicone oil with a higher specific gravity thanintraocular fluid. Such a substance was first men-tioned by Watzke4 in 1967, but the suggestion was notfollowed up at the time. Now a silicone oil with highspecific gravity has become available. It is producedin a special way to achieve a high level of chemicalpurity in terms of absence of low molecular com-ponents and catalyst residue. However, from achemical point of view (Burkhardt, personal com-munication) it is more difficult to purify trifluor-siloxane than polydimethylsiloxane. It thereforeseemed to us that careful studies of biological com-patibility in animals were essential before its use inpatients.Correspondence to Professor Dr V-P Gabel, Augcnklinik dcrUniversitiit Munchcn, Mathildenstrassc 8. 8(X)( Milnehcn 2. WestGermany.

Material and methods

The only high specific gravity silicone oils available atpresent are fluorised silicone oils. For this studypolytrifluoropropylmethylsiloxane (FS) was used. Itfirst underwent a special treatment to eliminate lowmolecular components and catalysts as postulated forpolydimethylsiloxane (PDMS) by our group.' Theabsence of catalyst was proved by x-ray fluorescenceanalysis. Details of its chemical and physical proper-ties are listed in Fig. 1.A total of 21 eyes of chinchilla grey rabbits were

prepared for silicone oil injection and extracapsularlensectomy under ketamine-xylacin anaesthesia. Thelensectomy was performed to increase the totalvolume available for an injection of silicone oil aswell as to obtain access for silicone oil to the anteriorsegment. The posterior capsule was left in place inthree eyes, to confine silicone oil to the posteriorsegment. Four to eight weeks after lensectomy parsplana vitrectomy with or without removal of theposterior lens capsule was performed on 17 eyes. Afluid gas exchange preceded the injection of siliconeoil in 15 eyes. We employed two types of controls:two vitrectomised eyes received the normally usedsilicone oil of low specific gravity after removal of theposterior lens capsule, and four eyes were subjectedonly to lensectomy, two of them with retention of theposterior lens capsule.The clinical development was monitored by means

of slit-lamp examination, ophthalmoscopy, andmeasurement of intraocular pressure. The animals inthe two groups were killed after three and six weeksand after three and six months respectively and the

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Silicone oil with high specific gravity for intraocular use

CH3- Si - CH3

CH3

nFig. 1 Polytrifluoropropylmethylsiloxane. Viscosity (250C)mPa s, 5000. A verage chain length approximately 50.Average molecular weight approximately 7800. Refractiveindex(250C)1J38. Density (250C)glcm1-29. Volatility24h1500CA0 1%.

eyes prepared for histological examination by grossexamination and light microscopy.

Results

CLINICAL COURSEThe postoperative clinical course (Table 1) in all eyesfilled with silicone oil was quite similar. The intra-ocular inflammation, with mixed conjunctival con-gestion and Tyndall phenomenon in the anteriorchamber, decreased during the first three weeks andcontinued to decrease in the following weeks. Twocontrol eyes, one with low specific gravity oil and onewithout silicone oil, were lost during the first sixweeks owing to increasing inflammation and accom-panying intraocular pressure rise due to secondaryglaucoma, which showed no evidence of beingrelated to the silicone oil.A silicone bubble reached the anterior chamber in

all eyes in which the lens capsule had been removed.The bubble diameter in the anterior chamber was 1/3to /3 of the corneal diameter and remained stable.The bubble was located at the inferior and superiorlimbus for high and low gravity silicone oil respect-ively (Figs. 2, 3). From about the fifth day aftersilicone oil injection all eyes showed varying degreesof central keratopathy, characterised by centralopacification and thickening of the corneal stroma,but no staining with topical fluorescein (Fig. 4).However, this central keratopathy decreased in alleyes and was no longer present in any eye after sixweeks. This type of central keratopathy was identicalin the two eyes filled with PDMS; in contrast nokeratopathy was observed in the two control eyeswithout any silicone oil.

+bs~~~~~~4:01'5>,........>-..:.6Bw...SF' _

Fig. 2 Six weeks afterinjection ofPDMS, silicone bubble insuperiorpart ofanterior chamber. Central opacification ofcornea with stromal thickening.

Table 1 Clinical course ofeyes under investigation

Trifluorosiloxan Polydimethylsiloxan Control

Posterior 3 weeks 6 weeks 3 months 6 months 3 weeks 6 weeks 3 weeks 6 weeks 3 months 6 monthslenscapsule intact removed intact removed intact removed intact removed removed removed removed removed removed removed

Total numberofeyes 4 11 4 10 4 8 3 3 2 2 5 4 4 3

Centralkeratopathy 4 11 - 6 - - - 2 2 2 - - - -

Neovascula-risationstarting fromthe inferiorlimbus - - - 2 - 5 2-

Intraocular - - - - I I 1 - - - I Ipressure Secondary Endophthalmitis,

glaucoma, enuclcationcnucleation

CH3

CH3 - Si - 0 -

CH3

CH3Si - 0

CH2

C8H2

LCF3

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V-P Gabel, A Kampik, Ch Gabel, and D Spiegel

Fig. 3 Six weeks after injection ofFS, silicone bubble ininferior part ofanterior chamber, light opacification ofcentral cornea.

After six weeks two eyes began to develop clinic-ally visible corneal neovascularisation from theinferior limbus. After three months five eyes showedthis neovascularisation. Even after six months twoout of three eyes having silicone oil in the anteriorchamber displayed this type of neovascularisation(Fig. 5). Two eyes, one with the posterior lenscapsule intact and the other with it removed,developed an increase of intraocular pressure whichwe could control by local application of betablockers.Two of the three eyes with intact posterior capsuleshowed no sequelae of the silicone oil in the anteriorsegment.None of the 15 eyes showed any clinical signs of so-

Fig. 4 Three months after injection ofFS, silicone bubble ininferior part ofanterior chamber, central cornealopacification almost disappeared.

Fig. 5 Six months after injection ofFS, silicone bubble ininferior part ofanterior chamber, superficialneovascularisation ofinferior cornea.

called emulsification or any side effects in thevitreous cavity, on the retina or on the disc.

HISTOPATHOLOGICAL RESULTSTissue responses, as detected by light microscopy,were observed at the corneal stroma and endo-thelium, on the iris surface and stroma, and to aminimal degree in the vitreous cavity. These tissueresponses varied with the duration of the observationtime and were much more pronounced in eyes filledwith FS than in the control eyes filled with PDMS(Table 2).The central corneal opacity noted clinically during

the first six weeks of observation was unaccompaniedby major histopathological changes except for poss-ible mild stromal oedema.Other corneal stromal changes were confined to the

inferior cornea and were seen only in those eyes inwhich the lens capsule was defective and silicone oilhad gained access to the anterior chamber of the eye.These eyes displayed a round cell infiltration andcorneal stromal neovascularisation very early aftersilicone oil injection at the inferior cornea, wheresilicone oil was in contact with the endothelium. Thiscorneal neovascularisation and round cell infiltrationdecreased in intensity after three months of observa-tion. No corneal stromal thinning was observed inany of the eyes filled with silicone oil.The corneal endothelium displayed a slightly

increasing amount of cell loss during the observationperiod, confined mainly to the area of silicone oilcontact and the area of limbal incision. Further, anincreasing amount of endothelial silicone oil phago-cytosis was seen the longer the observation period

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went on. With the passage of time a moderate tomarked retrocorneal fibrous tissue layer with phago-cytosed silicone globules developed which was more

pronounced at the superior border of the silicone oilbubble (Fig. 6).The most prominent findings were observed at the

iris. All eyes with FS displayed some oil dropletswithin the iris stroma. This reaction was observedalso in eyes in which silicone had gained no access tothe anterior chamber. Especially remarkable was thepresence of a fibrous tissue layer on the anteriorsurface of the iris. It contained phagocytosed oildroplets similar to a foreign body reaction (Fig. 7a,b). This tissue response increased with time andappeared to be confined to the inferior iris leaflet,which was in contact with the heavy silicone oil. This

Fig. 6 Posteriorcornealfibroustissue layer with many interspersedandphagocytosed siliconeglobules. Cornea ofrabbit eyefilledwith FSand having a silicone oilbubble within the anterior chamber.Follow-up three months.

tissue response was also observed in those cases

where the lens capsule was left in place (Fig. 7c).The vitreous, retina, and optic nerve displayed

almost no tissue reaction even after six months. Theonly finding of any note was the presence of somemacrophages on the retinal surface containingvarious amounts of clear spaces, presumably siliconedroplets. No fibrocellular proliferation or tissueimpregnation by silicone oil was observed.When control eyes werefilled with PDMS, with sili-

cone oil at the superior part of the anterior chamber,they were observed for six weeks only. They showedless tissue response than those eyes filled with FS,which were examined after the same observationtime, and displayed only minimal inflammatoryreaction in the corneal stroma -and no corneal neo-

Table 2 Grading oftissue reaction oftrifluorosiloxan andpolydimethylsiloxan in control eyes as observedhistopathologically

Poly-dimethyl- Control

Trifluorosiloxan siloxan eyes

6 3 6 6 Withoutweeks months months weeks silicone

oilLens capsule

Defective Intact

Cornealstroma, round cell infiltration of peripheral stroma + ++ + +1- +1- +1-Neovascularisation + + +/-

Corneal endothelium silicone phagocytosis - + +Rarefactation of endothelial cell layer +/- + + - +1- +1-Retrocorneal fibrous tissue layer - +/- + + - - +/-

Iris, silicone droplets in stroma +- + + + + ++ - -Fibrocellular membrane on anterior iris surface:

without oil droplets +/-with oil droplets + + + ++ +/-

Vitreous macrophages containing silicone droplets - + + +

Retina and optic nerve tissue impregnation

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Fig. 7a Iris with increasing tissue impregnationfromfluorosilicone oil globules. As early as six weeks afterintraocular application ofFS marked aggregation ofsiliconeglobules surrounded by macrophages are seen on the irissurface.

vascularisation. No oil droplets were observed withinthe tissues. However, the anterior surface of the iriswas also covered by a comparatively discreteinflammatory membrane.

Control eyes without any silicone oil displayed amild amount of endothelial cell loss and variousdegrees of retrocorneal fibrous tissue located pri-marily in the superior part of the cornea close to thelimbal incision site. Only a very mild round cellinfiltration was observed at the limbus.

Discussion

CHEMICAL AND PHYSICAL PROPERTIES OF FSThe molecule of FS is similar to that of PDMS, butwith one methyl group replaced by a trifluorpropylgroup in each dimethylsiloxane unit. This causes ahigher molecular weight of the siloxane unit (MW=156) than that of PDMS (MW=74). However, theFS, with a viscosity of 5000 mPa s has an averagechain length of only about 50 siloxane units and anaverage molecular weight of about 7800. In compari-son PDMS with the same viscosity reaches a chainlength of about 400 siloxane units and so a higheraverage molecular weight of about 30000. This mayin part be responsible for the different tissue reactionobserved and discussed later.The main difference between FS and PDMS is the

former's higher specific gravity, which is also higherthan that of the intraocular fluid. This may offer someadvantages in clinical use. Recently it has becomeclear that entirely filling the eye with silicone oil canlead to complications such as glaucoma and kerato-pathy. Incomplete filling, however, leads to some

Fig. 7b After three months ofobservation smaller globulesare present also within the iris stroma.

accumulation of fluid. In the case of low specificgravity oil this fluid accumulates under the tampo-nade, whereas the use of a high specific gravitymaterial would lead to accumulation of fluid above. Ifthe location of the intraocular fluid pocket coincideswith the location of the retinal tear, an occlusion ofthe subretinal space is not accomplished, and this canlead to redetachment.A second advantage is the greater technical ease of

silicone oil injection and endodrainage in cases ofgiant retinal tears and selected cases of trauma, since

Fig. 7c Aftersix months iris tissue is markedly invaded byFS globules although the lens capsule was intact in this eye.

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heavy silicone oil sinks down to the posterior poleduring an operation, so that it is unnecessary to makean endodrainage hole at the posterior pole.The refractive index of FS is less than that of

PDMS, which means that iatrogenic hyperopia aftersilicone oil filling is much less than that with PDMS.On the other hand the surface of the FS bubble ismuch more difficult to detect during the operationthan that of PDMS.

TISSUE REACTION TO FSClinically a rather mild reaction to all types of siliconeoil was observed. Whereas the transient centralcorneal stroma oedema was not confined to the areaof contact between the silicone oil and the cornealendothelium, the more marked round cell infiltrationand neovascularisation of the peripheral cornealstroma was confined to silicone-endothelial con-tact. The same was true for the iris tissue changes,such as tissue infiltration by FS droplets and silicone-containing fibrocellular membranes on the anterioriris surface.Although these reactions are seen in the areas of

direct contact between silicone and tissue layers, it isnot quite clear whether they are merely mechanicalor to some extent physicochemical in nature. The factthat after only six weeks the tissue reaction to FS wasmuch more prominent than to PDMS points towardsdifferences in reactions to various types of siliconeoil. However, more comparative studies are neededto judge the significance of different reactions tovarious types of silicone oil.

Despite these differences in reactions in ourexperimental series, the extent of corneal changesobserved in the rabbit eyes filled with FS wascertainly less than that noted by Sternberg andassociates.6 We saw no corneal stromal thinning inareas overlying the silicone oil bubble.

Histologically the most prominent pathologicalchanges due to FS in our series were observed in theiris tissue. Whether this formation of fibrocellularmembranes at the anterior iris surface, the phago-cytosis of silicone oil droplets, and the marked tissueimpregnation are of clinical importance is stillunclear.

In contrast to the rather alarming reaction tosilicone oil in the anterior chamber of the eye, thereaction within the vitreous cavity was minimal andvery similar to what we have observed when usingpurified PDMS in human eyes.' But in assessingchanges in the anterior segment of the rabbit eyes onemust take into consideration the fact that rabbitsalways have a much more pronounced reaction to anykind of ocular trauma than humans.At present, in view of these findings, we consider

the use of intraocular FS not to be justified until FSwith better chemical and physical properties becomesavailable.

References

1 Cibis PA, Becker B, Okun E. The use of liquid silicone in retinaldetachment surgery. Arch Ophthalmol 1962; 68: 590-9.

2 Scott JD. Use of liquid silicone in vitrectomized eyes. DevOphthalmol 1981; 2:185-90.

3 Zivojnovic R, Mertens DA, Peperkamp E. Das flussige Silikon inder Amotiochirurgie (II). Bericht uber 280 Falle-weitereEntwicklung der Technik. Klin Monatsbl Augenheilkd 1982; 181:444-8.

4 Watzke RC. Silicone retinopiesis for retinal detachment. A longterm clinical evaluation. Arch Ophthalmol 1967; 77: 185-96.

5 Kampik A, Gabel V-P, Spiegel D. Intraokulare Tamponade mithochviskosem Silikonol bei massiver proliferativer Vitreo-Retinopathie. Klin Monatsbl Augenheilkd 1984; 185: 368-70.

6 Sternberg P Jr, Hatchell DL, Foulks GN, Landers III MB. Theeffect of silicone oil on the cornea. Arch Ophthalmol 1985; 103:91-4.

Acceptedfor publication 30 May 1986.

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