Ultrasound velocities for axial eye length measurement

Area of Science:

• Ophthalmology
• Biomedical Optics
• Ultrasound Technology

Background:

• Accurate axial length measurement is crucial for intraocular lens power calculation.
• Traditional ultrasound biometry uses a standardized sound velocity, which may not be accurate for all ocular conditions.
• Variations in sound velocity occur due to eye length, presence of intraocular lenses, or vitreous substitutes.

Purpose of the Study:

• To investigate and establish appropriate sound velocities for various ocular conditions to improve axial length measurement accuracy.
• To develop correction formulas for different eye types, including phakic, aphakic, and eyes with intraocular lenses (IOLs) or vitreous substitutes.
• To highlight the clinical significance of using corrected sound velocities to prevent refractive errors.

Main Methods:

• Utilized individual sound velocities for axial length measurements across diverse eye conditions since 1974.
• Determined specific sound velocities for average phakic eyes, long eyes, short eyes, and aphakic eyes.
• Developed and applied correction formulas for phakic eyes based on measured axial length and a specific velocity (1,532 M/sec).
• Investigated sound velocities and developed correction methods for eyes with poly(methyl methacrylate) (PMMA), silicone, and glass IOLs.
• Measured sound velocities for eyes filled with silicone oil and gas, proposing individualized calculations for oil-filled eyes.

Main Results:

• Established an average phakic eye sound velocity of 1,555 M/sec, with variations for extremely long (1,549 M/sec) and short (1,561 M/sec) eyes.
• Recommended measuring phakic eyes at 1,532 M/sec with a specific correction formula to account for velocity changes.
• Found a consistent velocity of 1,534 M/sec for aphakic eyes, with significant clinical implications for correction.
• Determined specific velocities for eyes with different IOL types: PMMA (similar to phakic), silicone (1,476 M/sec), and glass (1,549 M/sec), with associated correction factors.
• Identified significantly lower velocities for silicone oil (1,139 M/sec phakic, 1,052 M/sec aphakic) and gas (534 M/sec phakic) filled eyes.
• Demonstrated that uncorrected sound velocities in eyes with silicone IOLs, oil, or gas can lead to clinically significant refractive errors (3-10 diopters).

Conclusions:

• Individualized sound velocity selection and correction formulas are essential for accurate axial length measurements in various ocular conditions.
• Failure to correct for sound velocity variations in eyes with IOLs or vitreous substitutes can result in substantial refractive errors.
• The proposed methods and velocities enhance the precision of biometry, leading to improved intraocular lens power calculations and better patient outcomes.