The differences between ultrasound biometry and optical biometry

The differences between ultrasound biometry and optical biometry have clinical implications. Firstly, 3-hydroxybutyrate improves as wavelength decreases. Hence, as light has a very short wavelength compared to sound, the laser light has better resolution. Therefore, the accuracy of AL with ultrasound AL is approximately 0.10–0.12mm compared to 0.012mm for optical AL. Measurement accuracy is limited by variation in retinal thickness surrounding the fovea. The second difference is the starting point of measurement between the two modalities. For example, ultrasound measures AL from the anterior surface of the corneal apex to the internal limiting membrane (ILM) of the fovea, whereas optical biometry measures AL from the second principal plane of the cornea (0.05mm deeper 3-hydroxybutyrate than the corneal apex) to photoreceptor layer (0.25mm deeper than ILM) of the fovea. Theoretically, optical biometry reads longer than ultrasonic axial length. Hence:
Lastly, ultrasound measurements are performed on the anatomic axis i.e. through the center of the cornea measuring anatomic axis as axial length whereas optical biometry measurements are performed on the visual axis measuring visual axis as axial length. As visual axis is shorter than anatomic axis; hence, optical measurements read shorter axial length compared to ultrasound measurements.
This comparison of axial length measurements with optical biometry and applanation ultrasound did correlate well for clinical purposes. The Bland–Altman plots showed good agreement between devices with less than 6% of eyes that were outliers (Fig. 1). There was a mean difference of −0.117mm in axial length measurements between devices after controlling for age and gender. Although this difference was statistically significant, it was not clinically significant. For example a difference of −0.117mm translates to 0.29D which is clinically insignificant. Additionally, statistically significant differences were only present when comparing measurements between devices for short eyes (=0.033; MANOVA: Pillai’s trace) but not normal to long eyes. Hence, the difference was rewritten in an equation format to relate axial length from applanation ultrasound to optical biometry.
where AL represents axial length from optical biometry and AL represents axial length from applanation ultrasound. The difference between methods might be attributed to the need for meticulous alignment in short eyes because the posterior pole anatomy is so small that the slightest misalignment can result in misdirection of an ultrasound signal from the fovea.
There was excellent repeatability of measurement with both devices. The statistically significant Pearson’s correlation coefficient of 0.987 and Fig. 2 indicates excellent correlation of axial length measurements between devices (<0.0001). However, the correlation proved that a pair of measurements increased or decreased together but did not indicate if a pair of measurements was identical. Cronbach’s Alpha and Interclass correlation coefficient of 0.994 indicated 99.4% agreement (identical) between pairs of measurements. Hence, given these outcomes, the regression Eq. (4) should be able to predict axial length with optical biometry in 97.4% (adjusted R2) of future cases. The outcomes of the current study are similar to previous publications. For example, the lack of a statistical difference in long eyes between devices in the current study is similar to a recent study that showed that optical and applanation ultrasound biometry (random measurement of IOLMaster or Lenstar) was comparable for long eyes. There were some limitations to this study. Although the small sample size may seem small, a statistically significant difference was seen due to the use of same devices and operator plus precision (small standard deviation). We used the common optical biometer (IOLMaster) on the market for the current study. However, the outcomes of the current study may not be applicable to other biometers without further study. Previous comparisons of the IOLMaster and Lenstar optical biometer have indicated slightly different measurements of axial length in cataractous and clear lenses. A recent comparison of the IOLMaster to the AL-Scan indicates almost perfect correlation in axial length measurements between devices. A recent study also reported statistical differences (−0.1±0.76mm) in axial length between the IOLMaster and Lenstar (<0.001; 95% CI: 1.39 to −1.59). However, the study population had keratoconus. Due to the differences in various studies with different biometers compared to the IOLMaster, we urge caution in applying the outcomes of the current study to other types of optical biometers. Age and gender were confounding factors that were controlled in this study.