IOL Power Calculations
Biometry Methods Explained.
Routine A-scan biometry is an indispensable tool for ophthalmology, but has limitations in resolution and an inability to consistently direct the sound beam to a known location. And although we have accepted ultrasound-based biometry as our main methodology for the measurement of axial length, it's important to keep in mind exactly what we are doing.
The resolution of a wavelength-based measurement is inversely proportional to the wavelength of the measuring device being used. The longer the wavelength the lower (worse) the resolution. The shorter the wavelength, the higher (better) the resolution. This is why an electron microscope has much higher resolution than a light microscope. This is also why we use 50-MHz ultrasound to more precisely image somewhat smaller anterior segment structures, rather than 10-MHz ultrasound. Things work best when the measuring wavelength is many times shorter than the distances being measured, or the resolution desired.
Fundamental Principle #1: Resolution is directly linked to wavelength.
So, the accuracy of a measurement is tied to its resolution. In terms of resolution, the 10-MHz sound wave used in ophthalmic ultrasound has a wavelength that is less than one order of magnitude smaller than the resolution desired for the anatomy being measured (0.03 mm). By comparison, a partially coherent light wave (as used in optical coherence biometry) has a extremely small wavelength (0.0000000975 mm) and is used to measure distances that are very large by comparison. In terms of resolution, optical coherence biometry surpasses 10-MHz ophthalmic biometry by more than eight orders magnitude. It is simply a different tool, operating differently. However, we can still do perfectly acceptable work with 10-MHz ultrasound, but it's helpful to keep in mind what's actually going on.
The correct (also known as the refractive) axial length for IOL power calculations would be defined as the distance from the corneal vertex (surface of the corneal epithelium at the center of the visual axis) to the outer segments of the photoreceptor at the foveal center. Keeping in mind that this is not what A-scan biometry measures, we can begin to understand how we have learned to adjust this technology of ultrasound to fit our purposes in ophthalmology.
Where people sometimes get confused is with semantics, replacing the common usage of words for the actual science involved. One common mistake is that people interchange the term accuracy, when what they really mean is the correct measurement.
Let's begin with applanation A-scan biometry, which suffers from five fundamental limitations.
The five basic limitations of applanation A-scan biometry are:
Variable corneal compression.
The first concept that is helpful to understand is that the measurement accuracy of a 10-MHz sound wave is exactly the same for an applanation technique and an immersion technique. Again, where people get confused is with semantics. This does not mean that applanation and immersion biometry have the same outcomes. Of course not. But what this does mean is that the measurement from the position of the corneal vertex to any area of the vitreoretinal interface is done with the same precision.
For an applanation technique, every first year resident knows that the the position of the corneal vertex relative to the vitreoretinal interface is not constant. Where the immersion technique does a much better job is that this troublesome artifact of variable corneal compression has been removed. That is the one and only difference between the two techniques, because the measuring device is exactly the same. Said another way, an immersion technique is more consistent.
If an applanation technique compressed the cornea by exactly the same amount each and every time, there would be no difference in consistency and the only difference would be that the applanation technique yields a shorter axial length and, as a result, would require a lower lens constant. Again, the difference between applanation and immersion is consistency and not accuracy. Semantics sometimes gets in the way of science.For example, let's say that a patient's true axial length is 24.24 mm. We are doing an applanation A-scan and there is the unavoidable artifact of variable corneal compression. Our first five measurements look like this:
Let's look at this another way. Let's say that we have a special stainless steel ruler. And this ruler has been calibrated so that it is accurate to within 0.1 mm. We are asked to measure the length of a femur bone for some important forensic project. We measure from the top of the head of the femur to the longest point on the lateral condyle. In reality, the measurement should be to the longest point of the medial condyle. The accuracy of our initial measurement is still within 0.1 mm, but we simply measured to the wrong point. What we have is an incorrect measurement. The accuracy of our special ruler cannot be changed. More correctly stated, the distance between the two points being measured was simply incorrect.
Fundamental Principle #2: The accuracy of a measuring instrument is an inherent quality of that instrument. How it is used determines if the measurement is correct.
Lacking the artifact of variable corneal compression, this is where the immersion technique is a significant improvement over the applanation technique. The measurements from the corneal vertex to the vitreo-retinal interface are much more consistent. Not to belabor the point, but the resolution is exactly the same for both, as they both use the same 10-MHz transducer. And the accuracy for each applanation measurement is exactly the same, as all measurements have the same resolution. What is different is that the starting point for each and every applanation measurement is slightly different. In the parlay of measurement science, this is known as variability.
Broad sound beam without precise localization
For both applanation and immersion A-scans, the sound beam is not an infinitely small, thin pencil of sound, like a line on a piece of paper, but a relatively broad beam. For this reason, it is not possible to measure directly to the foveal center and no-where else. Instead the sound beam is reflected back from some area around the center of the macula. Recall that the definition of the refractive axial length is from the corneal vertex to the photoreceptor outer segments at the center of the fovea. A-scan biometry is offset from the outer segments of the photoreceptors at the center of the macula by the retinal thickness.
However, the retinal thickness at the foveal center is, on average, approximately 165 µm, but the retinal thickness just to the side of the foveal center is closer to 250 µm. We also know that the distance between the center of the fovea and the shoulder of the fovea is smaller than our ability to control the position of the sound beam. This inability to discriminate between the foveal center and the shoulder just outside is a second source of error.
Fundamental Principle #3: Variability for an on-axis A-scan measurement is an artifact of position.
Here is an OCT-3 macular thickness plot from a normal eye that illustrates this. So, for the exercise of A-scan biometry, we have to take into account the inherent resolution limitations of a 10-MHz sound wave and its inability to discriminate between the foveal center and the foveal shoulder. This may not sound like a large error, but it's helpful to keep in mind that these types of errors are cumulative.
As mentioned above, a 10-MHz sound bean has a resolution of approximately 0.03 mm. By comparison, the 780-nm partially coherent light source used in optical coherence biometry has a wavelength of 0.0000000975 mm. And since the smaller the wavelength, the higher the resolution, there is simply no comparison between the two.
Incorrect assumptions regarding sound velocity.
The typical contact or immersion A-scan makes the following assumptions:
The lens of a young person, without a cataract has a sound velocity that is close to 1,641 m/sec. But for the aging lens with only a moderate cataract, the sound velocity is actually closer to 1,628 m/sec, which would produce an error of 0.28 mm for a lens of 4.25 mm. And for an eye with a mature cataract, the sound velocity is closer to 1,589 m/sec, which would produce an even larger error for a lens of 4.25 mm.
Fortunately, the sound velocity of the aqueous and the vitreous (the great majority of the eye) is a water velocity of 1,532 m/sec. and these distances are being measured correctly.
Of course, all of these errors are still relatively small, but when you begin to add them together, it is not difficult to see how small or large errors in axial length occur; all of which remain unknown to the operator.
Potential for incorrect measurement distance.
For example. If the transducer is off axis, it is measuring to the wrong location and may give a falsely short axial length. But, the accuracy of the measurement from the corneal vertex to that wrong position is unchanged. The measurement is accurate, it's just that the position (stopping point) that is wrong. Again, here is where people get confused. The ultrasound probe will always measure with the same accuracy (it can do nothing else).
Fundamental principle #4: A correct measurement is the result of the correct position.