Immersion A-scan Biometry
Refining Your Technique.
For those who are already comfortable with immersion A-scan biometry, here are some simple refinements you can make to increase the overall accuracy of your measurements. The information below is based on the original work of Holladay, and has been modified to include a small correction for the central corneal thickness, as described by Hoffer.
The human eye is mostly composed of aqueous and vitreous, both of which have an ultrasound velocity of 1,532 m/sec. Only the cornea and crystalline lens have different ultrasound velocities. If the eye is measured at an ultrasound velocity of 1,532 m/sec., a corrected axial length factor (CALF) of +0.32 mm is added to the apparent axial length (AAL 1532)
to obtain the true axial length (TAL).
As these differences represent a relatively small percentage of the total axial length measurement, a single CALF of +0.32 mm can be universally applied for phakic eyes of all axial lengths. This method is more accurate than using an average ultrasound velocity, such as 1,548 m/sec., and makes the measurement independent of axial length. Below is a description of how this works.
Setting the Ultrasound Velocity:
The ultrasound velocity for all gates is set to 1,532 m/sec. and measurements are made by the immersion technique.
Adjustments to Ultrasound Velocity Settings:
Cornea
The cornea has a typical ultrasound velocity of 1,641 m/sec. At the slower ultrasound velocity of 1,532 m/sec., it's central thickness will be slightly underestimated. With an average central corneal thickness of 0.55 mm, the underestimation would be approximately +0.04 mm.
0.55 mm x (1- (1532/1641)) = 0.0365 mm = +0.04 mm
Anterior Chamber
As aqueous has an ultrasound velocity of 1,532 m/sec., this portion of the measurement requires no correction.
Lens Thickness
Here there are two dependent variables: age and lens velocity. For a hypothetical 72-year old, the correction for lens thickness and lens velocity would be determined as follows:
Step #1 Lens velocity calculation:
1,659 m/sec. - (72-10)/2 = 1,628 m/sec
Step #2 Lens thickness calculation:
LT = 4.00 mm + 72/100 = 4.72 mm
Step #3 Underestimation calculation:
4.72 mm x (1 - (1532/1628)) = +0.28 mm
Vitreous
As vitreous has an ultrasound velocity of 1,532 m/sec., this portion of the measurement requires no correction.
True Axial Length
The true axial length (TAL), is the underestimation of the corneal thickness, plus the underestimation of the lens thickness added to the apparent axial length measured at 1,532 m/sec. (AAL 1532)
TAL = AAL 1532 + 0.04 mm + 0.28 mm
TAL = AAL 1532 + 0.32 mm
Simply adding +0.32 mm to the apparent 1,532 m/sec. axial length for all eyes will give the true axial length, which is then entered into the IOL calculation formula.
The greatest error by this technique for any axial length, and any age patient, is 0.043 mm, which is less than 0.25 D. This is more than twice the resolution of the surgical exercise, given the fact that intraocular lenses typically come in 0.50 D steps.
In our surgical practice, we use this method for all phakic eyes. If you have any questions, or would like to receive a detailed FAX with more information, you are welcome to Contact Us or check our Download Area.
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