The following questions relate to the effect of intravitreal silicone oil on ocular refraction.

a. What is the effect of silicone oil on a patient who is phakic? (no change, 
    myopic shift or hypermetropic shift)

b. What is the effect of silicone oil on a patient who is aphakic? (no change, 
    myopic shift or hypermetropic shift)

c. When an ultrasound scan is performed on an eye filled with silicone oil, does
     the axial length appears longer or shorter than expected? 

a
Answer a
Hypermetropic shift. Silicone oil has a higher refractive index than the crystalline 
lens and causes the posterior surface of the lens to become a diverging rather 
than a converging interface resulting in a hypermetropic shift.
 

Answer b
Myopic shift. In an aphakic eye, the silicone oil acts as a converging interface 
and causes a myopic shift. Hence, the hypermetropia caused by aphakia
is reduced by silicone oil.
 

Answer c
Longer than expected. Silicone oil reduces the speed of ultrasound and will therefore gives a erroneously longer axial length.