Having an Ultrasound Biometry Machine - New age Biometry Formula is still your friend !!

Updated: Mar 15

It is now widely accepted that the Barrett Universal II formula is the one to go for in our quest for achieving targeted refractive outcome. The Barrett formula with its built in principle plane of refraction determination, and posterior corneal astigmatism factor, stands a greater chance to satisfy an optometrist or physician's quest for post surgery refractive satisfaction.

The Barrett Formula however has been specifically designed for use with optical biometry. An optical biometer is considered to be more accurate compared to an ultrasound biometry machine, owing to its non contact method and precise measurement upto the RPE/Brusch Membrane. This measurement with an optical system is on an average .2 mm longer, which translates to a .5 dioptric difference in the axial length.

Without understanding this difference in an average eye, we run the risk of having at least a .50 dioptric surprise if an ultrasound biometry axial length is keyed in the Barrett formula, assuming however that other factors remaining constant. Retinal thickness however may vary significantly, but the average of it would be around .2 mm as set up in the optical biometry machine. A point to note here is that the optical machines like Lenstar do not measure the actual retinal thickness, and adds a .2 mm to account for the retinal thickness.

If you want to utilize the Barrett Formula with your readings from the Ultrasound Biometry machine, please consider adding .2 mm to the axial length derived, or chose an IOL power of .50 diopter lesser in value. The above example in the picture is a case in point. The OD is calculated with the optical while the OS signifies an axial length from an ultrasound machine. The difference of axial length being .2 mm, on account of retinal thickness. The final output page reflects the difference in dioptric power between the two eyes, which stand .5 D.

As the optical biometers do not account for the specific retinal thickness of the patient and takes an average (.2 mm ) value only, this approach may work best for ultrasound users to adapt the new formulas based on optical biometry.

Last but not the least a caveat remains that the difference in axial length , that is .2 mm, should never be added to the A constant or adjusted with the A constant. Doing so would bring in a large refractive surprise.


Please note that the above is a personal view and reflects my experience from the field. You should employ your own judgment and understanding before implementing this approach.

Achieving targeted refractive outcome is a function of average Keratometry readings, whereby a difference of measurement zones are significant between several keratometers. This is beyond the scope of this article, as is the difference accounting from Lens Thickness and horizontal WTW.

18 views0 comments

Recent Posts

See All