Advanced AScan Biometry and IOL Calculations

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AdvancedA-Scan BiometryandIOL Calculations

• Karen Bachman, COMT, ROUB, CCRC

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Four Areas Allow a Surgeon to Achieve the Planned
Refractive Goal
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A-Scan Biometry/Laser Interferometry

• A-scans by ultrasound/applanation method or
  ultrasound by immersion method Vs. laser
  interferometry (IOL Master)

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Keratometry

• Ks by manual method topography,
  autokeratometer, IOL Master

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IOL Calculations

• IOL formulas utilizing newer generation formulas,
  Holladay 1, Holladay 2, Haigis, Hoffer, SRK/T or
  are you still stuck on SRK II?

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Surgical Technique

• Surgical technique (out of our control)

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Outcomes
The post operative results will clearly dictate
the direction a practice should take, and the
improvements to be made in either technique in
performing testing or IOL calculation
methods Recognized areas for improvement are in
A-Scan Technique, Keratometry Accuracy and
utilization of current IOL calculation formulas.
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A-Scan Facts

• 50 of a surgeons post operative surprises are
  A-Scan errors (Olsen)
• Errors of 2.00D or more are almost always
  A-Scan related (Holladay)
• All A-Scan units make mistakes with echo
  interpretation!

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Accurate Axial EyeLength Measurementis
Essential toProvidingPatients with OptimumEye
Care
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Accurate scans are achieved by understanding the
following

• Tissue Velocities
• Echo Patterns
• Technique
• Sources of Error

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Tissue Velocities

• Cornea 1,641m/s
• Aqueous and Vitreous 1,532m/s
• Crystalline Lens 1,641m/s
• Pseudophakic Lens-PMMA 2,718m/s-Silicone
  980m/s-Acrylic 2,120m/s
• Silicone Oil (See Mfg. Guidelines)
  980m/s-1,040m/s

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Echo RepresentationWhat we look for in echoes
Echoes should be tall and steeply rising. There
should be little or no stair steps in leading
edge of retinal echo. Both eyes should always be
measured!
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Phakic Scans Cornea, anterior lens, posterior
lens, retina/sclera/orbital fat.
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Aphakic Scans Cornea, capsule remnant
(sometimes), retina/sclera/orbital fat.
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Pseudophakic Scans Cornea, IOL, echo with
reverberations, retina/sclera/orbital fat.
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IOL Master ScanInterpretation
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IOL Master
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Keratometry

• Measures corneal curvature
• Essential in determining IOL power
• Test should be performed prior to A-Scan
• Cornea is responsible for 2/3 of the refraction
  of light rays and the lens is responsible for
  1/3METHODS
• Manual
• IOL Master
• Topography
• Auto or hand-held units

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Formulas of Choice

• Holladay II Program
• Holladay I
• SRK/T
• SRK II-out dated!
• Haigis-Newest formula (requires 200-300outcomes)

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Understanding Formulas and their Constant
(Established by Manufacturer)
This value represents where we anticipate the IOL
to sit in relationship to the cornea.
Specifically, how near or far from the cornea.
The constant will decrease with an AC IOL as
compared to a PC IOL. The ACL sits closer to
the cornea therefore less power is needed.
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• There are currently three IOL constants in use
• The SRK/T formula uses an A-Constant
• The Holladay 1 Formula uses a Surgeon Factor
• The Holladay 2 Formula, and the Hoffer Q Formula
  both use an anterior chamber depth or ACD

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• All of the 2-variable formulas assume that the
  distance from the cornea to the IOL is
  proportional to the axial length. Meaning that
  short eyes will always have shallow anterior
  chambers and long eyes will have deeper anterior
  chambers.

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Haigis Formula

• The Haigis Formula uses three constantsa0, a1
  and a2 so that Dthe effective lens
  positionDa0 (a1ACD) (a2AL)
• These constants are derived by tracking outcomes
  specific to the surgeon and the implant model,
  the numbers are adjusted based on post operative
  results.

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Data Needed for Customizing

• 20 to 30 cases with same IOL and good
  post-operative outcome
• Axial length
• Keratometry readings (pre- operative)
• Implant power
• Target post-operative manifest refraction
• Final post-operative refraction
• Customizing available in some software or by IOL
  manufacturer (Ask your sales rep.)
• IOL Master has optimization program-utilize this!

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Troubleshooting Pre-operative Data

• Patient history should correspond with testing
  data
• Axial length and K readings are usually close to
  the same in both eyes.
• Repeat axial length when there is a difference of
  .30mm or more between the two eyes
• Repeat Ks with a difference of 1 diopter or more
• Refer to any previous data in chart

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• Compare the IOL power calculations for both eyes.
  If there is a difference of 1 diopter or more
  review data entered for accuracy.
• CHECK, CHECK AND RECHECK!

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Measuring Pseudophakic Eyes by Ultrasound
Use Aphakic Setting/Manual Mode KNOWN IOL CENTER
THICKNESS OF 1mm PMMA TAL AAL
.44 x T SILICONE TAL AAL - .56 x
T ACRYLIC TAL AAL .28 x T (example Silicone
24.0 - 0.56 x 1.0
23.44mm) UNKNOWN IOL CENTER THICKNESS PMMA TAL
AAL .40 SILICONE TAL AAL -
.80 ACRYLIC TAL AAL .20 (example
Silicone 24.0 - .80 23.20mm)
TAL true axial length AALactual axial length
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Pseudophakic A-scans

• Measure in aphakic manual mode
• Add .4 mm to measured axial for PMMA IOL
• Add .2 mm to measured axial for Acrylic IOL
• Subtract approximately -.8 mm from measured axial
  length for silicone

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Measuring Eyes withSilicone OilUse IOL Master!
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Sources of Error and the Postoperative Impact

• A-Scans - A 1mm error in axial length will equal
  almost 3 diopters in the post op refraction
• AL too Short - The post op refraction will be
  more myopic than anticipated. This will be seen
  with corneal indentation or poor probe alignment.
• AL too Long - The post op refraction will be more
  hyperopic than anticipated. This can happen when
  a fluid bridge is created between the cornea and
  probe tip or with poor probe alignment.

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Sources of Error and the Postoperative Impact
continued

• Indentation with Ultrasound
• Failure to review spikes
• Units are not dummy proof
• Garbage in - Garbage out! Delete bad data!
• Only use average AXL noted if data reviewed and
  clean
• Wrong velocity entered
• Evaluate standard deviation noted

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Evaluation of Standard Deviation
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• Keratometry - An error of 1 diopter in K reading
  will equal almost 1 diopter in the post op
  refraction. A steeper reading produces a
  hyperopic error and a flatter reading will
  produce a myopic error.
• IOL Master Ks
• Are contact lenses out?
• Verify with manual Ks
• Re-enter manual K data into IOL Master if needed
  before running calculation

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Sources of Keratometry Errors

• Unfocused eye piece
• Failure to calibrate unit
• Measurement off axis
• Poor patient fixation
• Dry eyes
• Excessive tears
• Droopy eyelids
• Ointment or lubricating gel on cornea
• Irregular shape of cornea

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Sources of Error and thePostoperative Impact

• Formula Selection - Using an outdated formula can
  have poor results primarily with long and short
  eyes. Be sure the correct constant is used for
  each IOL in the formula.
• Equipment - Non-calibration can influence post op
  outcomes
• IOL Master - Change AL settings for
  pseudophakic/phakic eyes

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Formula Comparisons
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Small Errors Add UP
Axial Length (.20mm) .50D K
Readings .50D IOL Power .25D Formula
(SRK II) 1.00D A-Constant .50D
TOTAL 2.75D !
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Investigate Postoperative Surprises

• Communication with the surgeon
• Review A-Scan readings
• Calculation entries
• Formula used
• K-Readings
• A-Constant verification

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Investigate Postoperative Surprises continued

• Is the postoperative refraction correct?
• What IOL was actually implanted?
• Anterior chamber depth issues?
• Utilize Dr. Hills back-calculation program
  www.doctor-hill.com ? download
  area ? R - Verg - Hill.XLS

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Special Challenges
Short and Long Eyes

• Must utilize newer formulas
• Recommend Holladay 2 consultant
• Measure anterior chamber depth
• Lens thickness
• Measure corneal diameter or white to white

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Measuring with Near Card
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Measuring with Caliper
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Measuring with Corneal Gauge
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Post Refractive Surgery Patients

• Very Challenging!
• Utilize post refractive patient work-up form
• Obtain all data requested for best outcome
• Obtain as many different types of K-readings as
  possible
• Dont hesitate to send these patients out!

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Historical Data Method
Example Pre-RK Keratometry Value 44.00
(Average K) Pre-RK Refractive Error
-6.00 Post-RK Refractive Error 1.00 (Stable MR
before cataract formation) -6.00 to 1.00
Myopic Shift of 7 diopters 44.00 (Original K
Value) -7.00 (Myopic Shift) 37.00 The new
K Value
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Obtain a hard contact lens set with various base
curves in plano power
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Educational Resource List

• Sandra Frazier Byrnes A-Scan Axial Eye Length
  Measurements (Grove Park Publishers)
  
  
  
• Lectures by Rhonda Waldron, COMT, ROUB
• Jack Holladay, MD Website www.holladay_at_dochollad
  ay.com
• Warren Hill, MD Website www.doctor-hill.com
• Corneal Gauges available from Sonogauge
  Corporation ASICO AE 1576  and phone is 1
  [phone].

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For further information, you may also
contact Karen Bachman, COMT, ROUB Cincinnati
Eye Institute Director of Clinical
Services 513.984.5133 Kbachman_at_cincinnatieye.com
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