Chord µ And Chord Α - Two Rather Robust Parameters

Chord µ is defined as the distance of the pupil centre to the vertex normal and Chord α is defined as the distance between vertex normal to the cornea geometric centre. Both are essential parameters for intraocular lens selection. General recommendations are given for the IOL selection in correlation with these two parameters but little is known about the influence of various factors to which Chord µ and Chord α may be subject.

International Vision Correction Research Centre (IVCRC), Department of Ophthalmology, Ruprecht-Karls-University of Heidelberg, Germany

I.R.C.C.S.-G.B. Bietti Foundation, Rome, Italy

QVision Vithas Hospital Almería, Almería, Spain

In this three-centre retrospective study, phakic and healthy eyes without previous refractive surgery were measured with the Pentacam AXL Wave. Chord µ and Chord α and their correlations with age (10 – 91 years), gender (M/F), spherical equivalent (SEQ – hyperopic ≥0,50D, myopic ≥-0,50D) and axial length (AL - short (≤23mm) and long (≥24mm)) were analysed. Chord µ and Chord α are presented separately for the right (OD) and the left (OS) eyes. But as nasal and temporal for both eyes do not coincide with the axial direction according to TABO notation, all left eyes were mirrored on the vertical axis for further conjoint analyses. Mean vectors and standard deviations (SD) were calculated.

The study comprises 1075 eyes of 1075 participants and their distribution with respect to gender, age, SEQ and AL was as follows: the 629 females were on average 51 ±19 years old, and the 446 males had an average age of 55 ±20 years; the mean SEQ was -1,81 ±3,76 D, and the eyes had on average an AL of 24,11 ±1,46 mm.

Chord µ/Chord α (mm @ ° ±SD). OD: 0,18 @184 ±0,2 / 0,43 @184 ±0,25; OS: 0,16 @359 ±0,2 / 0,39 @350 ±0,2; M: 0,17 @191 ±0,2 / 0,39@190 ±0,23; F: 0,2 @175 ±0,2 / 0,42 @186 ±0,2; myopic: 0,13 @176 ±0,2 / 0,37 @188 ±0,2; hyperopic: 0,23 @185 ±0,2 / 0,47 @187 ±0,2: short: 0,25 @187 ±0,2 / 0,48 @185 ±0,3; long: 0,13 @174 ±0,2 / 0,37 @189 ±0,2. For age no significant changes were observed.

Overall, Chord µ and Chord α remain rather constant in their magnitude and their temporal orientation for all tested factors, whereas individual variations can be large. There is no difference between OD and OS nor males and females. However, there seems to be a common trend that Chord µ and Chord α are larger in hyperopes and in short eyes.