Ultrasound velocities for axial eye length measurement.
Since 1974, I have used individual sound velocities for each eye condition encountered for axial length measurement. The calculation results in 1,555 M/sec for the average phakic eye. A slower speed of 1,549 M/sec was found for an extremely long (30 mm) eye and a higher speed of 1,561 M/sec was noted for an extremely short (20 mm) eye. This inversely proportional velocity change can best be adjusted for by measuring the phakic eye at 1,532 M/sec and correcting the result by dividing the square of the measured axial length (AL1,532)2 by the difference of the measured axial length (AL1,532) minus 0.35 mm. A velocity of 1,534 M/sec was found for all aphakic eyes regardless of their length, and correction is clinically significant. The velocity of an eye containing a poly(methyl methacrylate) intraocular lens is not different from an average phakic eye but it does magnify the effect of axial length change. I recommend measuring the pseudophakic eye at 1,532 M/sec and adding to the result (AL1,532), + 0.04 + 44% of the IOL thickness. The speed for an eye with a silicone IOL was found to be 1,476 M/sec (or AL1,532 + 0.04 - 56% of IOL thickness) and for glass, 1,549 M/sec (or AL1,532 + 0.04 + 75% of IOL thickness). A speed of 1,139 M/sec was found for a phakic eye with silicone oil filling most of the vitreous cavity and 1,052 M/sec for an aphakic eye filled with oil. For varying volumes of oil, each eye should be calculated individually. The speed was 534 M/sec for phakic eyes filled with gas. Eyes containing a silicone IOL or oil or gas will create clinically significant errors (3 to 10 diopters) if the sound velocity is not corrected.
Hoffer
K J
KJ
St. Mary's Eye Center, Santa Monica, California 90404.
D001036:Aphakia, Postcataract / Q000000981:diagnostic imaging; D003315:Cornea / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D005123:Eye / Q000033:anatomy & histology* / Q000000981:diagnostic imaging*; D005898:Glass; D006801:Humans; D007910:Lenses, Intraocular; D008433:Mathematics; D020366:Methylmethacrylate; D008768:Methylmethacrylates; D055095:Optics and Photonics; D012826:Silicone Elastomers; D012827:Silicone Oils; D013459:Sulfur Hexafluoride; D014463:Ultrasonography / Q000379:methods
D008768:Methylmethacrylates; D012826:Silicone Elastomers; D012827:Silicone Oils; D020366:Methylmethacrylate; D013459:Sulfur Hexafluoride