Biometric Classification of eyes (Dr. Fedor)

Classification is useful for AS imaging and IOL calculations

Classification helps to evaluate the size and shape of the anterior segment in 9 groups. It also helps the cataract surgeon to double check that the IOL calculation makes sense and sometimes adjust it.


TYPICAL EYES     

are divided in 9 groups, with the first letter specifying the axial length and the second letter specifying the corneal curvature, such as “short flat eye“ (SF). The absence of the term in the name of the group means that the AL or K is in normal category.           


9 types of Typical eyes (and anterior segments):  

    

1. Normal eye (NN) - is normal AL normal K eye      

2. Flat eye (NF) - is normal AL flat K eye     

3. Steep eye (NE) - is normal AL steep K eye        

4. Short eye (SN) - is short AL normal K eye      

5. Short flat eye (SF) - is short AL flat K eye     

6. Short steep eye (SE) - is short AL steep K eye      

7. Long eye (LN) - is long AL normal K eye      

8. Long flat (LF) -is long AL flat K eye      

9. Long steep (LE) - is long AL steep K eye          


We used letter E for shortcut for “steep” as letter S is used for short eyes.

We divided eyes based on AL: Short (S) (21-22.5mm), Normal AL (N) (22.5-24.5mm), Long (L) (24.5-26mm)    

We divided eyes based  on K: Flat (F) (41-42.5 D), Normal K (N) (42.5-44.5 D), Steep (E) (44.5-46 D)     

* Each of these categories represents approximately 30 % of the population (+/- 1 SD standard deviation of average) and less than 10 percent are outliers.


OUTLIERS  

Similar system can be used for classification of atypical eyes – outliers.   


The outliers are called (the letter X stands for extra- or very-, as for example:): 

Extra long (XL with AL>26mm) 

Extra short (XS with AL<21mm)

Extra Steep (XE with K>46mm) 

Extra flat (XF with K<41mm)  


Within Outliers many other unusual combinations can theoretically exist, as for example: 

XL-F: Extra long flat eye 

XS-F: Short extra flat eye


IOL CALCULATION  ADJUSTMENT

Regarding adjusting the IOL calculation, for example, in myopic eye due to steep cornea the IOL calculations using Barret, Olsen, Hill RBF, Holladay, Hoffer, SRK/T methods give different IOL power calculations by more than 1-2 D and you need to choose the IOL. This is very different shape and size of the eye than myopic eye due to long axial length.