Day 4 (2): Biometry Flashcards
Triad of visual outcome
- Keratometry: measures anterior corneal radius of curvature
- Biometry: measures axial length
- IOL Power: computation of the needed refractive power of the IOL using the above 2 variables
Why do Keratometry when implanting IOLs?
Because a lot of the refractive power of the eye is from the cornea.
Even if IOL is perfect but corneal measurements are inaccurate or not made, the combined refractive power of the cornea + IOL as a unit will be erroneous.
What is Keratometry?
- Measurement of the ANTERIOR CORNEAL radius of curvature at the central 3.0 mm area.
- DOES NOT measure the POSTERIOR CORNEAL curvature
- DOES NOT measure TRUE CORNEAL POWER
- Determined by the 2 refractive surfaces at the anterior and posterior boundaries
- Estimated based on 2 assumptions: - Corneal surface has uniform curvature and power all throughout
- Posterior/anterior curvature ratio maintains a fixed value
- A RoC is 82.2% that of P RoC (A is steeper than P)
- Implicit in the keratometric index of 1.3375 that is traditionally used to calculate corneal power.
- This is lower than the actual corneal refractive index (1.376) to compensate for the negative power of the posterior surface
What are the two types of keratometers?
- Manual: measures central 3.2 mm (more peripheral area overestimates corneal power)
- Javal
- Bausch & Lomb - Automated: measures central 2.6 mm
Estimated corneal refractive index used by most keratometers?
1.3375
EXCEPT:
- Zeiss: 1.332
- Haag-Streit: 1.336
- Hoya: 1.338
Can be off-target by 0.8 D because all calculation formula are based on the refractive index.
Things to remember prior to doing Keratometry.
- Examiner presumed EMMETROPIC.
- Visual axis of both examiner and patient are ALIGNED to the keratometer axis.
- Cornea is LUBRICATED with no artificial changes in curvature (from contact lens use)
- Mires should be CENTERED.
- Image is properly FOCUSED before values are recorded.
- Optimal: refractive index of keratometer is 1.3375 (similar to cornea)
What are the steps in manual keratometry?
- Calibration
- Focusing of eyepiece
- Occlusion of one eye
- Timing
- Fixation
- Measurement
- Within 5 SECONDS after the last blink or BEFORE the tear break up time
- Wetting agent: Balanced Salt Solution; avoid hypromellose
When to discontinue contact lens use when undergoing keratometry?
Hard (RGP) CL: 2 weeks prior
Soft CL: 3 - 7 days prior
Why?
Prevent masking of astigmatism and undetected central corneal flattening.
Reminders on calibration of keratometers.
- REGULARLY using manufacturer provided calibration spheres
- CONSISTENCY: ONE dedicated KERATOMETER and EXAMINER for all pre-op and post-op measurements
What is the difference between Corneal Topography and Corneal Tomography?
Topography: study of the ANTERIOR corneal surface
Tomography: study of BOTH ANTERIOR and POSTERIOR surface
- Best captures OVERALL or TRUE corneal power
How to do keratometry in patients with poor fixation?
- Determine cause: cataract, macular hole, etc.
- If fixation NOT possible, align keratometer reflex at PUPIL CENTER. If pupil eccentric, align at CORNEAL CENTER.
- If pt uncooperative or cannot understand, ask patient to come back another time.
What is Corneal Topography?
- Technique that maps the ENTIRE surface of the cornea
+ vs Keratometry which measures only a SELECT area in the anterior surface - Uncovers corneal pathologies in areas undetected by keratometry
2 General Principles/Kinds:
- Placido Disc-Based Topography (Keratometric map)
- Uses concentric rings or mires reflected off of the anterior cornea and converted to color scales
- Displays REFRACTIVE POWER of cornea
- Can only evaluate the ANTERIOR corneal curvature
- E.g. Zeiss Atlas, NIDEK OPD-Scan - Purkinje Image-Based Topography (Elevation map)
- Describes the corneal curvature with respect to a reference shape (best fit sphere)
- Displays SHAPE/ELEVATION of cornea
- Can evaluate both ANTERIOR and POSTERIOR curvature
- Able to measure corneal thickness along the entire cornea
- Gold standard for topographic measurements
A. Scanning Slit System (Orbscan)
- Uses rapidly scanning projected slit beams of light and a camera to capture the reflected beams to create a map of the anterior and posterior corneal surface
B. Scheimpflug Imaging (Pentacam)
- Uses a rotating camera to photograph corneal cross-sections illuminated by slit beams at different angles
- Corrects for the non-planar shape of the cornea and, thus, allows greater accuracy and resolution in creating a 3D map of cornea
- Can be considered a Tomogram
- In 3D renders:
+ Red: anterior cornea
+ Green: posterior cornea
+ Blue: iris
+ Yellow: crystalline lens
What are the indications and disadvantages of corneal topography?
Indications:
1. Assess unusual keratometric readings
2. Poor quality mires with keratometry
3. Management of astigmatism in cataract surgery and after corneal transplant
4. Screening candidates for refractive surgery by identifying irregular astigmatism and helping estimate postoperative ectasia risk
5. Detection of ectatic disorders such as keratoconus, pellucid marginal degeneration and post-LASIK ectasia
6. Determining visual significance of corneal and conjunctival lesions, such as pterygia and Salzmann’s nodular degeneration
7. Guiding suture removal and placement of limbal relaxing incisions
Disadvantage:
- Irregularities in tear film can significantly impact the quality and fidelity of a Placido disk topography.
- Decreased accuracy of posterior elevation values especially after refractive surgery
What is ultrasound and how does it work?
ULTRAsound: sound waves with HIGHER frequency than upper audible limit of human hearing
- Frequency: > 20 KHz
- A scan units: 10 MHz (much higher)
High frequency: less depth of penetration but better resolution
- for easily accessible tissues; visualize minute details
- hence, frequency of 10 MHz used by most A scan unit
Low frequency: deeper penetration but grainy images
What is A (Amplitude) Scan Biometry?
- Measures the TIME it takes for a SINGLE sound wave to travel from the probe to the retina and back
- Uni-dimensional: measures TIME only (hence INDIRECT measure of distance which is calculated from an equation)
- Purpose:
1. Determine the axial length (DISTANCE) of the eye indirectly - combined with Keratometry to calculate IOL power
- most common indication
2. For cases where fundus is obscured from visualization by slit lamp or laser interferometry
EQUATION: Distance (AL) = Time x Velocity
1. Distance: distance from the anterior pole to the posterior pole of the globe)
2. Time: for the sound waves to travel from the probe to the retina and back
3. Velocity: of the sound wave in the given medium
Principle behind Amplitude (A) Scan Biometry.
- Measures the TIME it takes for a SINGLE sound wave to travel from the probe to the retina and back
- If an interface is encountered, the wave is either reflected back as echoes or transmitted through
- Echoes that return to the probe are converted to SPIKES
- Spike HEIGHT is proportional to the STRENGTH of the echo
- AXIAL LENGTH = distance between corneal and retinal spike
Factors affecting spike amplitude:
1. Properties of the 2 tissues at the interface
- if very different: majority of wave reflected back = stronger echo = higher spike
- if almost similar: majority pass through = short spike
- Wave Angle of Incidence: angle of the wave from a line perpendicular to the surface
- higher angle of incidence = less echo returning to transducer = shorter spike - Smoothness or regularity of interface
- Density of structures the wave passes through
- denser/solid = more waves reflected = more echoes = tall spikes
Results:
1. Solid/Dense structure: most or all waves reflected back as echoes
- A scan: tall spikes
- B scan: hyperechoic (white)
- Semi-solid structure: most transmitted, some reflected back
- A scan: medium to small spikes
- B scan: hypoechoic or mixed echogenicity - Liquid: all waves transmitted through
- A scan: no spikes/flat
- B scan: anechoic (black)
