Optical Biometry - Myth & Science
- Apr 14, 2021
- 9 min read
Updated: Apr 5
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 :
