drug-silicone oil solutions. The data obtained showed that with the exceptions of nabumetone and phenylbutazone, the remaining compounds investigated: dexamethasone, triamcinolone acetonide and indomethacin demonstrated poor solubilities in silicone oil. This led the authors to conclude that most anti-inflammatory agents used clinically are not completely soluble at therapeutic concentrations and quantification of concentrations from the eye would be inaccurate. Additionally, direct injection of high concentration of triamcinolone acetonide into the silicone oil-filled eye resulted in extensive sedimentation below the oil bubble that may potentially induce cytotoxicity in the retinal layers. In order to improve the solubility, investigators suggested predispersing triamcinolone acetonide in silicone oil to produce a homogeneous suspension prior to injection (Spitzer et al. 2009).

Despite the encouraging data with silicone oil, maintaining a constant drug level is difficult to achieve in this vehicle due to inconsistent release kinetics which can either follow square root time square when diffusion through the oil is rate limiting or first-order kinetics when partitioning out of the oil into the vitreous is rate limiting (Ashton 2006). The individual variation in the magnitude of retinal detachment, thus the duration and degree of silicone oil filling further complicate the dosage regimen, suggesting safety and efficacy need to be evaluated carefully.

6.7.4  Role of Ocular Movements in Disordered Vitreous

The faster rate of material distribution and clearance in the liquefied vitreous could be attributed to the loss of vitreous diffusional barrier and enhanced convective forces; other factors such as ocular movements should also be considered. Stocchino and colleagues used a custom-made human eye model mounted on a computercontrolled motorised support which allowed manipulation of eye rotation along the vertical diameter of the eyeball. Using this experimental apparatus, saccadic eye rotations was simulated as sinusoidal torsional oscillation. The cavity was filled with glycerol solutions to mimic the increased viscosity, with the assumption that the vitreous behaves as a Newtonian fluid. During the experiments, fluid motions in the vitreous were recorded as images, which were then processed to obtain the velocity field. The author integrated the velocity field using algorithms to calculate particle trajectories as a mean of analysing stirring properties. The results demonstrated vitreous stirring as a function of flow induced by ocular rotations; with more efficient stirring at areas with higher fluid velocities and lower stirring action at areas where velocities were lower. The application of this model was appropriate for vitrectomised and highly liquefied eyes, where collageneous components of the vitreous were removed and substituted by homogeneous fluid such as aqueous humour, saline or silicone oil. Additionally, the large Peclet number of flow calculated in this study led to the suggestion that advection (bulk flow) is more important than diffusion in inducing mass transport when the vitreous is liquefied (Stocchino et al. 2010). Therefore, sinusoidal eye rotations that increase the advective forces within the vitreous cavity may have a greater impact on larger molecules when diffusion is limited.