Biometry is attempted with an applanation ultrasound A-scan in an eye filled with silicone oil following previous retinal detachment surgery. How does the silicone oil affect the measured axial length and hence the IOL power calculation?
- A Sound travels faster in silicone oil than vitreous, giving a falsely long axial length and a low IOL power
- B Sound travels slower in silicone oil than vitreous, giving a falsely short axial length and an underestimated IOL power ✓
- C Silicone oil reflects all sound, making axial length unmeasurable by any technique
- D Silicone oil has no effect provided the sound velocity setting is left at the default aphakic value
Correct answer: B. Sound travels slower in silicone oil than vitreous, giving a falsely short axial length and an underestimated IOL power
Explanation
Ultrasound travels more slowly through silicone oil, approximately 980 m/s, than through vitreous at about 1532 m/s. Using the standard velocity therefore yields a falsely shortened axial length, and the calculated IOL power comes out lower than required, leaving the patient hypermetropic. Optical biometry or corrected velocity settings are needed. The oil attenuates rather than blocks sound entirely, so measurement remains possible.
Reference: AAO BCSC Section 11: Lens and Cataract, latest ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.