Day 4 (2): Biometry

Question 1

Triad of visual outcome

Answer 1

1. Keratometry: measures anterior corneal radius of curvature
2. Biometry: measures axial length
3. IOL Power: computation of the needed refractive power of the IOL using the above 2 variables

Question 2

Why do Keratometry when implanting IOLs?

Answer 2

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.

Question 3

What is Keratometry?

Answer 3

1. Measurement of the ANTERIOR CORNEAL radius of curvature at the central 3.0 mm area.
2. DOES NOT measure the POSTERIOR CORNEAL curvature
3. DOES NOT measure TRUE CORNEAL POWER
   - Determined by the 2 refractive surfaces at the anterior and posterior boundaries
   - Estimated based on 2 assumptions:
4. Corneal surface has uniform curvature and power all throughout
5. 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

Question 4

What are the two types of keratometers?

Answer 4

1. Manual: measures central 3.2 mm (more peripheral area overestimates corneal power)
   - Javal
   - Bausch & Lomb
2. Automated: measures central 2.6 mm

Question 5

Estimated corneal refractive index used by most keratometers?

Answer 5

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.

Question 6

Things to remember prior to doing Keratometry.

Answer 6

1. Examiner presumed EMMETROPIC.
2. Visual axis of both examiner and patient are ALIGNED to the keratometer axis.
3. Cornea is LUBRICATED with no artificial changes in curvature (from contact lens use)
4. Mires should be CENTERED.
5. Image is properly FOCUSED before values are recorded.
6. Optimal: refractive index of keratometer is 1.3375 (similar to cornea)

Question 7

What are the steps in manual keratometry?

Answer 7

1. Calibration
2. Focusing of eyepiece
3. Occlusion of one eye
4. Timing
5. Fixation
6. Measurement
   - Within 5 SECONDS after the last blink or BEFORE the tear break up time
   - Wetting agent: Balanced Salt Solution; avoid hypromellose

Question 8

When to discontinue contact lens use when undergoing keratometry?

Answer 8

Hard (RGP) CL: 2 weeks prior
Soft CL: 3 - 7 days prior

Why?
Prevent masking of astigmatism and undetected central corneal flattening.

Question 9

Reminders on calibration of keratometers.

Answer 9

1. REGULARLY using manufacturer provided calibration spheres
2. CONSISTENCY: ONE dedicated KERATOMETER and EXAMINER for all pre-op and post-op measurements

Question 10

What is the difference between Corneal Topography and Corneal Tomography?

Answer 10

Topography: study of the ANTERIOR corneal surface
Tomography: study of BOTH ANTERIOR and POSTERIOR surface
- Best captures OVERALL or TRUE corneal power

Question 11

How to do keratometry in patients with poor fixation?

Answer 11

1. Determine cause: cataract, macular hole, etc.
2. If fixation NOT possible, align keratometer reflex at PUPIL CENTER. If pupil eccentric, align at CORNEAL CENTER.
3. If pt uncooperative or cannot understand, ask patient to come back another time.

Question 12

What is Corneal Topography?

Answer 12

• 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:

1. 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
2. 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

Question 13

What are the indications and disadvantages of corneal topography?

Answer 13

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

Question 14

What is ultrasound and how does it work?

Answer 14

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

Question 15

What is A (Amplitude) Scan Biometry?

Answer 15

• 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

Question 16

Principle behind Amplitude (A) Scan Biometry.

Answer 16

• 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

2. 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
3. Smoothness or regularity of interface
4. 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)

2. Semi-solid structure: most transmitted, some reflected back
   - A scan: medium to small spikes
   - B scan: hypoechoic or mixed echogenicity
3. Liquid: all waves transmitted through
   - A scan: no spikes/flat
   - B scan: anechoic (black)

Question 17

What structures are denoted by the tall spikes in the A Scan Biometry?

Answer 17

Taller spikes = Intensity of echoes reflected back to probe = Solid

1st spike: Probe-Cornea interface
FLAT: Aqueous Humor
2nd spike: Anterior Lens Capsule
3rd spike: Posterior Lens Capsule
FLAT: Vitreous Body
4th spike: Retina
5th spike: Sclera
6th spike (decreasing amplitude): Orbital fat and tissues

Axial Length
- Distance from 1st spike to 4th spike (probe tip/cornea –> retina)
- N: 23.5 mm [22 - 25 mm]

Anterior Chamber Depth
- Distance from 1st spike to 2nd spike (probe tip/cornea –> anterior lens capsule)
- N: 3.24 mm [2.5 - 4.0 mm]

Lens Thickness
- Distance from 2nd spike to 3rd spike (anterior lens capsule to posterior lens capsule)
- N: 4.63 mm [up to 7 mm]

Question 18

Three methods employed in doing A Scan Biometry.

Answer 18

Contact/Acoustic - Applanation Method
- Handheld: easy, quick BUT compresses cornea
- Tonometer-mounted: minimal compression BUT cumbersome
- Pt can be seated
- Limitations:
1. Variable corneal compression (even with same examiner)
2. No precise localization
3. Limited resolution
4. Incorrect assumptions re: sound velocity
5. Potential for incorrect measured AL

Contact/Acoustic - Immersion Method
- uses Praeger (scleral) shell with saline or methylcellulose
- more accurate than applanation method
- PROS:
1. NO corneal compression: probe does not touch cornea
2. Better consistency and reproducibility of measurements
- CONS: more inconvenient
1. Asians with smaller palpebral fissures: difficulty in placing Praeger shell
2. Pt has to lie down else the saline will spill

Difference bet. AL measured by Immersion VS Applanation
- 0.14 - 0.28 mm
- 0.10 mm error ~ 0.25 D difference

Non-Contact/Optical Method: IOL Master

Question 19

Compare the accuracy of the different methods of A Scan Biometry.

Answer 19

Applanation: +/- 0.24 mm (huge range of values; most inaccurate)

Immersion: +/- 0.12 mm

Optical (IOLMaster): +/- 0.01 mm (most accurate)

Question 20

Discuss the optical method of A Scan Biometry.

Answer 20

IOLMaster/Lenstar LS900

Principle: Partial Coherence Interferometry
- Uses light coming from a 780 nm/820 nm laser diode instead of sound
- Non-contact; no anesthesia
- Accuracy: +/- 0.02 mm
- Includes 5 IOL calculation formulas
- Measurements:
1. White-to-white/Limbus-to-limbus: horizontal corneal diameter
2. Anterior Chamber Depth: using lateral slit illumination
3. Central Corneal Power/Radius of Curvature: using automated keratometry
4. Axial Length: optical path length between anterior cornea and RPE
Lenstar: (+) pupil diameter, retinal thickness, eccentricity of visual axis, central corneal thickness, lens thickness

Question 21

Reminders in taking optical biometry measurements.

Answer 21

1. Take multiple measurements around measurement reticule.
2. Focus in and out until focusing spot is same diameter as reticule for four measurements.
3. Find one area with the highest quality AL display.
4. Only include measurements within +/- 0.02 mm of the highest quality AL.
5. Maintain central fixation (regularly remind pt to look directly at red fixation light)

Question 22

What are the characteristics of a good OPTICAL biometry reading?

Answer 22

1. Very good signals (signal-to-noise ratio > 10; the higher the better)
2. (+) Multiple secondary maxima
3. Correctly fixating patient

Question 23

What are the advantages of optical biometry?

Answer 23

1. 5x more accurate than acoustic biometry (upto +/- 0.02 mm)
2. Non-contact
3. Time-saving and less cumbersome
4. Method of choice for challenging cases:
   - extreme axial lengths
   - aphakic
   - pseudophakic
   - post-refractive surgery
   - (+) eccentric fixation
   - (+) silicon oil in vitreous
   - (+) staphyloma
   - (+) retinal detachment

Question 24

What are the disadvantages and limitations of optical biometry?

Answer 24

1. Expensive equipment
2. Because it is optical involving light, media opacities will decrease accuracy:
   - dense cataracts (mature, hypermature, brunescent)
   - central PSC
   - anterior cortical spokes crossing the midline
   - central corneal scars
   - vitreous hemorrhage

Note:
If AL measurement not possible with optical method: use acoustic method to get AL and input AL in optical biometer + K reading to get IOL power.

Question 25

What is gain setting?

Answer 25

Electronic amplification of the signal coming back to the probe Higher gain --> higher sensitivity to signals --> higher spikes For imaging of smaller lesions or through an obstruction (opacities) Disadvantage: more noise + merging of interfaces What is seen in the scan if gain set too high: 1. (+) Spikes with flat tops (esp. RETINAL spike) 2. (+) Blurring or merging of posterior spikes

Question 26

Why is mode selection important in doing biometry?

Answer 26

1. Ensures correct sound velocity. Phakic: 1,550 m/s Aphakic/Pseudophakic: 1,532 m/s 2. Ensures variables measured are displayed correctly Pseudophakic: (+) IOL - (-) LT should be measured and displayed - (+) Anterior and (+) Posterior Lens Spikes Aphakic: (-) IOL - (-) LT displayed - (-) Anterior (2nd) and Posterior (3rd) Lens Spikes - (-) ACD should be measured and displayed

Question 27

What is the correct probe positioning?

Answer 27

1. Pt should regularly be reminded to focus on the small red fixation light. This ensures probe is perpendicular to macula and passes through the visual axis. 2. Probe must always be perpendicular to the cornea. 3. Lightly touch the cornea and avoid compression as this will shorten the axial length measurement.

Question 28

Why is alignment of the probe to the visual axis important?

Answer 28

Asking pt to constantly look at the red fixation light in the machine ensures that the macula is perpendicular to the probe and the probe passes through the visual axis. Ensures: 1. Most, if not all, waves will be reflected back to the probe --> maximum spike amplitude/height 2. Least noise Remember: misalignment causes waves to scatter to other directions --> less waves make it back to the probe --> smaller spikes

Question 29

What are the causes of probe-visual axis misalignment?

Answer 29

Causes some waves to scatter and not make it back to the probe --> decrease in spike height/amplitude 1. Pt not looking directly at the fixation point/light. - Sound beams from probe is not exactly perpendicular to the macula and does not pass directly through the visual axis. 2. Macular surface is NOT SMOOTH - Macular degeneration - Epiretinal membranes 3. Macular surface is CONVEX - Macular edema - Pigment epithelial detachment

Question 30

Criteria for a good A Scan biometry?

Answer 30

1. Presence of various spikes (usually 5 high amplitude spikes): - 1st spike: Probe-Cornea Interface - 2nd spike: Anterior Lens Capsule - 3rd spike: Posterior Lens Capsule - 4th spike: Retina - 5th spike: Sclera - 6th spike (smaller): Orbital fat and tissues 2. Spikes are tall and rises STEEPLY esp. RETINAL (5th) SPIKE - (-) sloping, jags, humps or steps 3. No spikes in front of the retina (should be flat = vitreous) and should be clearly separate from the scleral spike. 4. Reproducible measurements - Repeated 3-5x - Same eye: difference within +/- 0.10 mm only - Both eyes: difference within +/- 0.30 mm only

Question 31

Most common error encountered in contact method of A Scan biometry?

Answer 31

Corneal Compression = Myopic Surprise - Cornea is naturally pliable; aggravated by lower IOP - Avoided by the IMMERSION method - Inaccurately SHORTER AL measurement What to check? 1. INCREASING Axial Length - Distance between 1st (probe/cornea) and 4th (retinal) spikes - N: 23.5 mm (22.0 - 24.5 mm) 2. DECREASING Anterior Chamber Depth - Distance between 1st (probe/cornea) and 2nd (anterior lens capsule) spikes - N: 3.24 (2.5 - 4.0 mm) - Delete measurement if ACD becomes shorter/shallower - LONGER ACD is MORE ACCURATE because you can't extend AC but you can compress it How does this cause myopic surprise? 1. Inaccurately SHORT AL measurement 2. To focus light on a shorter eye, higher powered IOL needed 3. Placing the inappropriately higher powered IOL in the pt's eye with a longer ACTUAL AL will cause light to focus in FRONT of the macula and not at the retina.

Question 32

Biometry findings in those with Dense Nuclear Cataracts?

Answer 32

Findings: 1. Thickened LT - N: 4.63 mm but can be as high as 7 mm 2. MUCH shorter 3rd (posterior lens) spike vs 2nd (anterior lens) spike - N: should be similar or shorter with only a slight difference - Posterior lens spike may be SLIGHTLY shorter because the curvature of the posterior lens is steeper than the anterior lens --> reflection of echoes away from the probe tip --> smaller amplitude - Due to greater wave absorption within the lens by dense nucleus 3. (+) Small spikes between the 2nd and 3rd spikes - Easily mistaken as either the A and P capsule spikes 4. Retinal and scleral spikes with smaller amplitude - N: should be similar in height to the lens spikes - Absorption of waves within the lens causes less propagation beyond --> shorter amplitude waves Solution: Increase gain for better quality posterior lens and retinal/scleral spikes.

Question 33

What are the biometry findings if the probe is misaligned?

Answer 33

Situation 1: PROBE NOT PERPENDICULAR TO CORNEA - Hence, NOT perpendicular to the center of the lens and macula and DOES NOT pass through the visual axis - Due to pt not focusing or looking directly at the fixation spot/light - Light scatters instead of mostly returning back to probe - Normally: If waves pass through the visual axis and the center of the lens: 1. BOTH anterior and posterior lens spikes have HIGH amplitude If waves pass perpendicular to the macula: 2. BOTH retinal spike and scleral spikes have HIGH amplitude 3. Retinal spike rises steeply from baseline (NO SLOPING) 4. NO jags, humps, or steps on the ascent of the retinal spike - Findings in biometry: 1. Lens spikes are UNEQUAL (posterior lens spike MUCH SHORTER than anterior) - not due to cataract because NO smaller spikes in between 2. SHORT retinal spike which does not rise steeply 3. (+) Artifactual spikes in front of the retinal spike Situation 2: PROBE ALIGNED WITH OPTIC NERVE - Passes through posterior scleral foramen - Pt cannot see light because ON is a blind spot - Finding: NO scleral spike because wave passes through ON

Question 34

Why does biometry measurements have to be accurate?

Answer 34

0.10 mm error = 0.25 D refractive error postoperatively

Question 35

When should findings be validated or repeated?

Answer 35

KERATOMETRY 1. K in any meridian LESS THAN 40 D or GREATER THAN 47 D - Normal: 40-47 D - Check oldest available refractive error and axial length 2. Astigmatism in either eye GREATER THAN 4.0 D - Confirm with MANIFEST REFRACTION: manual way of determining corneal power by placing lenses of different powers in front of the eyes and asking which one results to clearer vision - Do CORNEAL TOPOGRAPHY: to assess entire anterior surface 3. Difference in K between eyes GREATER THAN 1.0 D - Is there an appropriate amount of corresponding ANISOMETROPIA (unequal refractive power between the two eyes) A-SCAN BIOMETRY 4. AL LESS THAN 22.0 mm or GREATER THAN 24.5 mm - Normal: 23.5 (22.0 - 24.5 mm) - Check history if (+) medical reason exists 5. Difference in AL on the same eye with consecutive measurements GREATER THAN 0.1 mm - Check history if (+) macular pathology 6. Difference in AL between 2 eyes GREATER THAN 0.3 mm - Check history if (+) medical reason exists 7. Poor quality spikes COMPUTED IOL POWER 8. (+) Anisometropia: unequal refractive power BETWEEN the eyes - Difference: GREATER THAN 1.0 D 9. AL does not correlate with refractive error - Except if: extremely flat or steep corneas 10. ANY difficulty in obtaining measurements NEVER accept INADEQUATE measurements!!!

Question 36

What are the different IOL computation formulas?

Answer 36

Regression: SRK (Sanders, Retzlaff, Kraft) Theoretical: 1. Holladay 1 & 2 2. Hoffer Q 3. SRK/T 4. Haigis

Question 37

What is the SRK regression formula?

Answer 37

P = A (constant) - 2.5 (AL) - 0.9 K (average) P: IOL power (D) A constant: theoretical value that relates the lens power to the AL and anterior corneal RoC --> inherent to the IOL type used AL: axial length (mm) K: average curvature of central anterior cornea (D) Limitations: 1. Assumes IOL power is linearly related to AL: as AL increases, needed IOL power should decrease (inversely proportional) --> applicable only for normal eyes 2. Inaccurate for extremes of AL

Question 38

What are the IOL theoretical formulas?

Answer 38

Optical formulas based on the optical properties of the eyes Accurate for extremes of AL SRK/T: A constant optimized Holladay 1: Surgeon factor optimized Holladay 2: ACD optimized Hoffer Q: ACD optimized Haigis: variable factoring of ACD and AL

Question 39

When are the different IOL formulas indicated?

Answer 39

REMEMBER: for majority of population, getting an accurate keratometry and AL is more important than choosing the calculation formula. < 20 mm: EXTREMELY SHORT (High Hyperopia) - Holladay 2 or Hoffer Q 20-22 mm: SHORT (Hyperopia) - Hoffer Q 22-24.5 mm: NORMAL eyes - ALL can be used 24.5 - 26 mm: LONG (Myopia) - Holladay 1 > 26.5 mm: EXTREMELY LONG (High Myopia) - SRK/T Myopic LASIK - Haigis L Notes: 1. Holladay 2: ok for any length but expensive to maintain 2. Haigis: ok for any length but needs to be optimized for extremes of AL

Question 40

Reminders when calculating IOL power.

Answer 40

1. Not one size fits all. Adjust based on past experiences and data. 2. Accuracy improved if: - uniform type of lenses used - similar surgeon - same biometer - same technician 3. Optical biometer is preferred - reduces human factor; NOT operator-dependent 4. Aim for MILD post-op MYOPIA (add 0.5 - 1 D power) 5. When NOT to target emmetropia? - Long-term significant myopes: already adjusted to myopia - Will result to anisometropia GREATER than 3.0 D 6. Make adjustments depending on WHERE IOL is implanted. - ACIOL: DECREASE power by 3.0 D - SULCUS: DECREASE power by 0.7 - 1.0 D - PCIOL (scleral-fixed): INCREASE power by 0.5 D - Moving IOL anteriorly while maintaining computed power will cause light to focus in front of the retina (myopia). Thus, decrease power to focus light back. - Moving IOL posterior will focus light behind the retina (hyperopia). Thus, increase power to focus light in front into the retina.

Question 41

How are pts followed-up post cataract surgery and IOL implantation?

Answer 41

Manifest refraction 3-6 weeks post-op. Corneal healing and attachment of capsule to the IOL happens around this time.