Biometry 4% Flashcards
What is used to calculate IOL power?
Axial length
K power(k curvature)
AC depth
White to White
Lens Constant (aka A constant
The distance from the front of the eye to the macula is—
Axial length in 100ths of mm
The measurement of K diameter
white to white
A number assigned to an IOL by the implant manufacturer depending on where the IOL sits in the capsular bag
Lens Constant (A constant)
Some designed to sit more posteriorly and some anteriorly.
Dr. can customize their IOL calculations based on _____
outcomes
All surgeons use the same generic IOL calculations. After a surgeon as performs a certain amount of cases they can do a retrorespective analysis of their refractive outcomes and make adjustments to these IOL calculations to customize them to their particular sx technique. This is called……..
Surgeons factor
What IOL calculation formulas are used for short eyes?
Hoffer Q
Haigis (multi-Variable Formula)
What IOL calculation formulas are used for longer eyes?
SRKT
Holladay II (multi-variable formula)
Which IOL calculation formulas are considered the most accurate? Why are they considered more accurate?
Haigis
Holladay II
Olsen
Barrett Universal II
(Because they use multi-variable formulas that take into account many different structures in the eye not just axial length and K readings)
What are the 3 main methods of A-scans?
Contact (ultrasound)
Immersion
Optical coherence (LASER light)
This method of Ascan uses ultrasound, there is no K compression, very accurate, uses Prager shell filled with BSS, requires training and skill.
Immersion
Ascan method that is not considered good enough anymore because K compression inevitable so it will always give a shorter axial length than any other method of Acan. Uses Topical anesthetic and is susceptible to operator error
Contact (ultrasound)
This method of Ascan uses LASER light
Optical Coherence (IOL Master)
What is the main advantage of ultrasound over LASER light Ascan?
Ultrasound can measure through dense media such a CAT or K opacification, LASER light can not.
These Ascan methods use Laser (not ultrasound), considered Good Standard, is non contact, no anesthetic needed, no risk of patient-patient contamination, very precise, Most accurate K’s, easy to operate, need good tear film, and intraocular lens calc software included.
IOL master and Lenstar
What do the peaks on the amplitude scan graph represent?
From left to right:
PCAPRS
probe, cornea, anterior lens, posterior lens, retina, sclera
IOL Master and Lenstar is also sometimes referred to as—-
Partial Coherence Inferometry
“Optical Biometry”
IOL and Lenstar uses _____to let you know accuracy of measurements.
Traffic Light Indicators (Red for bad, green for good)
IOL and Lenstar can measure through many types of media and eyes such as—-
Measures aphakic, silicone, and phakic eyes
IOL Master and Lenstar may have difficulty measuring through —-
very dense CATS and other media opacities due to using light beams
An Ascan is a ____measurement of the eye.
linear
An Ascan is a _____dimensional scan
1
An Ascan has scans in how many dimensions?
1
A Bscan measures in how many dimensions?
2
The standardized Ascan uses a _____
linear echo wave
The standardized Ascan measures the ….
axial length of the eye the 100ths of a mm from K to macula
A diagnostic Ascan is used to locate…..
intraocular FBs
The purpose of an standardized Ascan is to ….
determine the axial length for IOL power calculations (usually emmetropia)
What is measured to get the axial length
from corneal to macula
In Ascan sound waves travel through _____at varying speeds depending on _______ of tissue.
tissue
density
In Ascan, sound wave reflect to _______,impulses displayed electronically.
Transducer
The standardized Ascan uses _____
ultrasound
Standardized Ascan sends _____through the eye
sound waves
Depending on the ______of the tissue the sound wave will bounce back to the transducer and make a _____representing different structures of the eye.
density
linear graph
Ascan uses _____instead of light
sound
Ascan is accurate even with dense media because…..
it uses sound instead of light
Ascan measures….
axial length, AC depth, Lens thickness.
What do the spikes correspond to in an Ascan?
structures of the eye
The gain in an Ascan is measured in _____
decibles
What does the gain in Ascan affect?
amplification and resolution of display
When the gain is higher on Ascan…
you get better view of spike height but worse resolution
When the gain is lower on Ascan…..
Spikes and display sensitivity decreased and improved resolution.
In Ascan this is Essentially electronic calipers that measure between 2 points, Biometer sets this to specific anatomical landmarks.
Gate
True of false:
All Ascans have a gate
false
If you get more than 3mm difference axial lengths between the 2 eyes or more than 1/10 of a mm difference in a single eye consecutive measurements you should get….
another check.
Normal axial length is…
23.5
range is 22-24.5
Normal AC depth is…
3.24mm
Normal lens thickness is….
4.63mm up to 6.9mm for cataractous lens
Normal K readings are….
43.0-44.0
Normal CCT(central K thickness)
555
Thick K gives a ____reading on IOP
higher
Thin K gives a ___reading on IOP
lower
