Today’s SS-OCT biometers employ two methods to measure the axial length: Sum-of Segments using individual refractive indices or a Mean Group Refractive Index.

The differences in axial length measurement between the two methods are statistically significant and most noticeable in long and short eyes.

Axial length determined with the Mean Group Refractive Index is slightly shorter in short eyes and longer in long eyes.

Depending on the length of the eye, some newer IOL power calculation formulas may provide better outcomes with the Sum-of Segment method.