Optical Biometry – Myths & Science
There is no denying the fact that optical biometry has led to a sea change in refractive outcomes ever since the first optical biometer (IOL Master) was introduced in 1999. Ever since Thomas Olsen (1992) described axial length as the source of 54 % of all refractive errors ( followed by 38 % to errors from estimation of post operative ACD, and 8 % from corneal power calculation errors ), the contribution of axial length as a contributing factor has diminished significantly. Sverker Norrby (2007) in his works SOURCES OF ERROR IN IOL POWER CALCULATION (JCRS 2007) showed that preoperative estimation of postoperative IOL position accounted for 35 % of all errors, while pre operative determination of AL contributed to only 17 %.
While optical biometry is credited to improving refractive outcome, over looking other contributing factors may not be ideal. As pointed out in earlier writings here, biometry is not about IOL power calculation, but additionally, determination of the Effective Lens Position(ELP). To that extent, the birth of new generation advanced IOL calculation formulas, like the Barrett, Olsen, Kane, Hill -RBF, is noteworthy.
The objective of this article, however, is to bring to point some common beliefs involving optical biometry, as I have come across, and to put things in its proper perspective.
Optical Biometry measures up to the Retinal Pigment Epithelium (RPE)
This is the most common thought I have come across when interacting with clinicians. While this is true, this is only half truth. In fact, the ability of the optical biometers to measure till the RPE does not contribute to its accuracy. To understand this, let me take a step back and state that the distance from the tear film/corneal epithelium to the RPE is the Optical Path Length (OPL). The OPL is however, converted back to the geometric Axial Length ( tear film/epithelium to inner limiting membrane) by an inbuilt formula. For example, the first partial optical coherence based optical biometry (IOL Master), converted the OPL back to GAL or geometric axial length by the formula :
This was based on Wolfgang Haigis optimizing the optical biometry of IOL Master to immersion based ultrasound biometry. According to Haigis, the group or composite refractive index of the phakic eye is 1.3574. Using this value of the group refractive index, Olsen however found a small inconsistency between pre operative and post operative axial length with the IOL Master. Therefore, Olsen proposed a refractive index of 1.3616 and proposed a new formula for conversion of OPL to geometric axial length (GAL).
Ax true = (Axzeiss x .9571 + 1.3033) x 1.3549/1.3616
where Axtrue is the geometric axial length that matches with immersion biometry; Axzeiss is the OPL.
The more recent Swept Source OCT based biometry device have similar conversion metrics to convert the OPL into GAL.
The conversion of the OPL to GAL is important for all optical biometry systems as they need to make their measurements compatible with the then third generation IOL calculation formula theoretical formulas like SRK T, Holladay I. I have explained in another article ' Understanding Biometry & IOL Power Calculation' (https://www.quickguide.org/post/biometry-iol-formula ) in 'Biometry' section of this website that theoretical formulas convert axial length that ultrasound based biometry machines measure to optical axial length (that is axial length till RPE). This has been done historically starting with Binkhorst formula and continuted with SRK T, Hollday I, etc. Thus since these theoretical formulas are based on vergence (vergence before the IOL and vergence after the IOL) they theoretically convert ultrasonic axial length to optical axial length by addition of a conversiion factor. The conversion factors account for the retinal thickness and are listed in the article.
. What this means is that, the ability of the longer wavelength light waves of both PCI and SS/OCT based biometry machines are of no particular importance to biometry, as they ultimately have to be converted back to make measurements upto ILM which all theoretical formulas consider. If optical biometry did not convert back to GAL ( that is upto inner limiting membrane or ILM) then it would lead to false extra large axial length as the theoretical formulas would additionally add the retinal thickness value. To avoid that all A constants would have had to be changed when optical biometry was first introduced by Zeiss. To avoid this all optical biometry machines convert Optical Path Length (OPL)to Geometric Axial Length (GAL) and let theoretical formulas add retinal thickness value built in the formula.
The Tecnis Toric Calculator (Johnson&Johnson) gives the option to users to either input the optical and immersion A constant or the contact A constant separately. Note in this calculator, it has been correctly indicated that optical and immersion A constant need not be different and hence it does not show the option separately.
The longer A constant associated with IOL power calculation based optical biometry
The common misconception associated with optical biometry machines is that the longer A constant associated with IOL power calculation compared to ultrasound biometry is due to the axial length calculated up to the Retinal Pigment Epithelium. For example, if the A constant of a particular IOL in immersion ultrasound biometry be 118.7, the lens constant for the same IOL with optical biometry is longer than that (say, 118.9 or 119.0 ). This is often thought to account for the retinal thickness that optical biometry light waves travel across to reach the RPE.
This is not entirely true. One reason for a higher A constant associated with optical biometry is to account for the steeper corneal values that optical biometry measurement got associated with. Before the advent of optical biometry, the diameter of cornea measured with manual keratometers were 3.0-3.2 mm. With the first PCI based optical biometry machine, the diameter of the cornea measured was lesser, around 2.5 mm. For an average cornea which is prolate in shape, this led to more steeper values. The steep or flat cornea not only accounts for the IOL power calculation, but also is an important contributory factor for ELP determination by formulas like SRK T and Holladay I. That is, the SRK T formula would account for a deeper ELP for a steeper cornea than a flatter one.
The steeper corneal values measured by optical biometry therefore necessiated compensation with a higher A constant value.
A constant is not just the factor of axial length, but also that of cornea. Infact if you consider axial length alone, there should be no difference in Aconstant between immersion biometry and optical biometry. This is because of the the OPL and GAL concept I have explained before. When Zeiss optical biometry was first introduced, Wolfgang Haigis had matched its output to immersion biometry so that no constants had to be changed.The Tecnis Toric Calculator (Johnson&Johnson) gives the option to users to either input the optical and immersion A constant or the contact A constant separately. Note in this calculator, it has been correctly indicated that optical and immersion A constant need not be different and hence it does not show the option separately.
What do you do if you are graduating from contact based ultrasound biometry to optical biometry?. Yes the Aconstants will be different here owing to chance of corneal compression with contact or applanation based biometry caused by operator. If you want to derive your own personalized Aconstant for optical biometry, I have made life easy for you by putting a excel sheet in the tools section (https://www.quickguide.org/post/ultrasound-aconstant )
Go for it !
Optical Biometry measures on the visual axis
This is true. The optical biometry machines measure the light travel path along the epithelium to the foveal pit. Hence in people with posterior staphyloma, such measurement would give a true axial length value, since in such patients the anatomical axial length does not coincide with the geometric axial length.
But this is however applicable to patients who have fixated on the ‘red blinking light’. If a patient has been fixating on the red blinking light but moves his head just before the ‘click’ of the operator, the measured axial length is not on the visual axis. In cases of posterior staphyloma, such measurement could lead to an erroneously long axial length leading to hyperopic surprise post operatively. That said, a new SS-OCT (IOL Master 700) based system offers an option to ‘Analyze’ the pictures to help users ascertain if the measurement has been on the visual axis.
Optical Biometry is the magic wand
In many ways, a clinic accepting optical biometry may see a general improvement in refractive outcome. But optical biometry is not a magic stick, it has its own limitations.
Like ultrasound biometry, optical biometry measures the time it takes for the waves to travel through the ocular media. Different optical biometers have different algorithm to interpret this time and convert back to data meaningful to us, namely axial length, ACD, keratometry radius, etc. The most common method used by some PCI or SS-OCT based systems are the Composite Method, but there are other optical biometry machines that also implement the Segmental Method for calculating the axial length. The Composite and the Segmented methods are the different ways of converting the time taken for the light beam to travel back to axial length by accounting for the Refractive Index of the ocular media. While the Composite Method takes into account a mean refractive index of the entire eye structures, the Segmented Method takes into account the true refractive index of the different structures of the eye.
The first optical biometry machine, that was based on PCI, implemented the Composite Method. It considered a mean refractive index of 1.3549 of the phakic eye to convert the time back to axial length. The SS-OCT version of the optical biometry machine from the same company, uses the same Composite Method, but the R.I used is undisclosed. Some studies have observed a bias towards unusually longer axial length in longer length eyes.
What could be the reason for an optical biometry (in this case a popular SS-OCT based ) machine delivering longer axial length ? The eye anatomical structures are not always proportionately shaped. That is, a very long eye, may not have a proportionately long anterior chamber depth or posterior chamber. As a matter of fact, it has been often pointed out that a longer axial length may be associated with a longer vitreous length, than a longer anterior segment.
It has been suspected that such optical biometry machines that are based on Composite Method, may account for a higher RI of the vitreous, that is , they employ a larger mean refractive index of the whole eye that is significantly higher than the refractive index of the vitreous. With a higher refractive index taken into account, the length of the vitreous cavity found is higher than it actually is.
To account for this anomaly, Douglas Koch and Li Wang, introduced the Wang & Koch optimization for axial lengths higher than 25.20 mm some time back. Later, Popovic (2018) in their paper showed the efficacy of Wang & Koch optimization in eyes with axial length more than 27.0 mm.
If you are interested in Wang & Koch optimization, you can follow the Wang & Koch conversion table provided in this blog under the Biometry section (https://www.quickguide.org/post/table-chart-conversion-of-wang-koch-nomogram-for-high-axial-length-patients ).
The Wang & Koch optimization is applicable to two variable formulas like the Holladay I and SRK T.
Optical biometry is the future and may be the single biggest step toward better refractive outcome for clinics still on ultrasound based biometry. But it is always useful to know the many nuances involved with a technology so that an informed decision can be taken on each individual patient. Not all eyes are the same, and an informed decision may be taken based on proper data validation for each individual eye.
1. Accuracy of IOL Formulas Ophthalmology 2017;-:1e10 ª 2017 by the American Academy of Ophthalmology
2. Wang-Koch formula for optimization of intraocular lens power calculation: Evaluation at a Canadian center Marko Popovic, J Cataract Refract Surg 2018; 44:17–22 Q 2018 ASCRS and ESCRS
3. Calculation of intraocular lens power: a review Thomas Olsen, Acta Ophthalmol. Scand. 2007: 85: 472–485
4. Wang-Koch formula for optimization of intraocular lens power calculation: Evaluation at a Canadian center Marko Popovic, BHSc, Matthew B. Schlenker, MD, Xavier Campos-M€oller, MD, Austin Pereira, BMSc, Iqbal Ike K. Ahmed, MD; J Cataract Refract Surg 2018; 44:17–22 Q 2018 ASCRS and ESCRS
5. Ray Tracing and analysis of intra ocular lens power in situ - Thomas Olsen, Mikkel Funding; J Cataract Refract Surg 2012; 38:641–647 Q 2012 ASCRS and ESCRS