Silicon oil removal

SOR

1. Silicon oil removal

2. Properties of silicon oil Polymer of siloxane (R 2 -SiO 2 ) PDMS (most common) Hydrophobic, liquid and clear Silicone fluids, silicone rubber, depending on R 2 It is not an oil

3. Specific gravity 0.97 (PDMS) Depends on R 2 Higher SG, better contact with retina Remains constant irrespective of viscosity

4. buoyancy Upward exerting force of fluid/gas Depends on difference of SG to aqueous Gas (SG 0.001) exerts far greater upward force than silicon oil (SG 0.97)

5. Surface tension It keeps an agent as a single bubble high interfacial tension has fewer tendencies to deform or disperse into small bubbles less likely to pass through small openings, such as a retinal hole Gas or air 80 Silicone oil 35 Perfluorocarbon liquids 40–45

6. viscosity Resistance of fluid to deformity Shear stress Tensile stress A higher energy is needed to disperse a large and highly viscous fluid bubble into small droplets (emulsification ) Viscosity is expressed in centistokes Not affected by SG or surface tension Depends on length of polymer and MW

7. Removal of silicone oil Theoretically, chorioretinal adhesion would have formed certainly by 1 month 6 weeks to 6 months However, in some eyes with very poor ultimate anatomical/visual prognosis, it may be left in the eye Heavy oil no longer than 3 months The type of SO and its viscosity, lens status and whether or not additional procedures are to be carried out, will influence the method used

8. procedure two port system : one for infusion and the other for SO aspiration Passive Active aspiration Manual keep the tip of the cannula within the oil globule as it reduces in size till the end Unwated movements at the end leads to loss of hold over the small oil bubble which becomes difficult to regain due to fluid turbulence Gentle depression of the ciliary body area will push any sequestered oil bubbles into the vitreous cavity

9. procedure Maximum suction applied is limited the pressure driving the flow of oil out of the eye would be the atmospheric pressure plus the infusion pressure Therefore, if passive aspiration were used, the aspirating pressure would be the height of the infusion bottle If active aspiration was used, the vacuum driving the flow would be much higher (760 mmHg + bottle height) Another important consideration is the length of the aspirating cannula

10. procedure Poiseuille’s Law flow is inversely and proportion to the length of the tubing To minimize this length , no tubing should be used The suction needs to be generated in a syringe connected directly to the aspirating cannula

11. procedure In combined phacoemulsification and uncomplicated SOR, a primary posterior capsulotomy is made Infusion is provided via an anterior chamber maintainer In aphakic eyes with SO in the anterior chamber, a corneal wound can be used to allow passive drainage of oil fundus should be evaluated at the end of the procedure

12. Implications of SOR potential risk of retinal redetachment higher in eyes with PVR or in with high-risk of developing PVR Most redetachments occur within 6 months after SO removal previous unsuccessful surgeries and longer axial lengths are associated with higher failure rates Prophylactic 360° laser or the use of encircling band may reduce the rate of redetachment

13. Implications of SOR Difficult to ensure the eye is free of emulsified oil at the end Patients may complain of floaters due to residual oil droplets air-fluid exchange at the end of oil removal to remove residual droplets can be tried Where fluid-air exchange is done, multiple repetitions are preferable The flute needle is kept at the air-fluid meniscus where oil comes to float and can be seen and removed Anterior chamber emulsified oil should be washed thoroughly, with attention to the angles