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ASCRS ♦ ASOA Symposium & Congress
Technicians & Nurses Program
May 6-10, 2016 – New Orleans
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ADVANCEDBIOMETRY
ANDIOLCALCULATIONS
ADVANCEDBIOMETRY
ANDIOLCALCULATIONS
KarenBachman,COMT,ROUBTheEyeInstituteofUtah
Financial Disclosures
No relevant disclosures
FiveAreasContributetoAchievingthePlannedRefractiveGoalFiveAreasContributetoAchievingthePlannedRefractiveGoal
A‐ScanBiometry/LaserInterferometry A‐scans by ultrasound/applanation method or ultrasound by immersion method Vs. laser interferometry (IOL Master or Lenstar)
Keratometry
K’s by manual method; topography, autokeratometer, IOL Master‐Lenstar
Accurate K’s are critical for surgeon to plan astigmatic management for LRI’s and Toric IOL’s
IOLCalculations
IOL formulas utilizing newer generation formulas, Holladay 1, Holladay 2, Haigis, Olsen, Barrett; SRK/T
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SurgicalTechnique
Surgical technique (out of our control)
5th area is patient selection
A‐ScanFacts
50% of a surgeon’s post operative surprises are A‐Scan errors (Olsen)
Errors of 2.00D or more are almost always A‐Scan related (Holladay)
All A‐Scan unitsmake mistakes with echo interpretation!
Accuratescansareachievedbyunderstandingthefollowing:
Tissue Velocities
Echo Patterns
Technique
Sources of Error
StandardDimensions
• Average axial length = 23.5 mm
• Average K reading= 43 to 44 diopters
• Average ACD = 3.24mm
• Axial length usually within 0.3 mm between the two eyes
• 96% of axial lengths are typically within 21.00 ‐25.5
• 60% are between 22.5 to 24.50
A‐scanBiometry
GeneralReasonsforUse/applanation Dense cataracts
Poor fixation
Measurements under anesthesia
Confirmation of optical biometry
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EchoRepresentation:Whatwelookforinechoes
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!PhakicScans:applanation
Probe tip/Cornea, anterior lens, posterior lens, retina/sclera/orbital fat.
Immersion image Immersion
Probe Tip Echo
Cornea. Double‐peaked echo shows anterior and posterior surfaces.
Anterior lens capsule.
Posterior lens capsule.
Retina.
Sclera.
Orbital fat.
AphakicScans:
Cornea, capsule remnant (sometimes), retina/sclera/orbital fat.
PseudophakicScans:
Cornea, IOL, echo with reverberations, retina/sclera/orbital fat.
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ProbeAlignment OpticalBiometry
OpticalBiometryvs.Immersion?
How do they differ?
Ultrasound measures the anatomical axial length‐ anterior pole to posterior pole of eye.
Iolmaster /Lenstar measure the optical axial length‐vertex of cornea directly to the fovea.
Use same A‐constant for both
IOLMaster
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LENSTARLS900
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 1/3
If K is off by .75 then your post op refraction will be off same amount
SourcesofKeratometry 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
IOLCalculations
3rdGenerationFormulas
What is the difference between formulas?–basically in how the formula calculates the final position of the IOL– known as the ELP or effective lens position‐ all utilize the axial length and central cornea power only.
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Haigis Formula
The Haigis Formula uses three constants:a0, a1 and a2 so that: D=the effective lens positionD=a0 + (a1*ACD) + (a2*AL)
These constants are derived by tracking outcomes specific to the surgeon and the implant model, the numbers are adjusted based on post operative results.
2‐VariableFormulas‐‐‐
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.
HolladayII/Barrett‐Multi–Variable
Axial length
Corneal curvature
ACD
White ‐ to ‐white
Lens thickness
Age
Refractive error (prior to cataract if available)
UnderstandingFormulasandtheir“Constant”
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.
(Established by Manufacturer)
TherearecurrentlythreeIOLconstantsinuse:
•The SRK/T formula uses an “A-Constant” 118.9
•The Holladay 1 Formula uses a “Surgeon Factor” 1.56
•The Holladay 2 Formula, and the Hoffer Q Formula 5.2both use an “anterior chamber depth” or ACD
•The Barrett Universal Formula uses LF- Lens Factor
• Olsen uses a C-Constant
WhydoIOL’svaryinA‐Constants?
Materials of IOL differ‐ acrylic vs. silicone vs. pmma cause different refraction indexes
Front and back surface curvatures differ
IOL’s sit differently within bag due to haptic design and angles
How do I choose what A‐Constant to use?
Check with your rep; doctor‐hill.com; ULIB site; call other practices familiar with that IOL and outcomes
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DataNeededforCustomizing
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
IOL Master and Lenstar have optimization programs‐utilize them!
TroubleshootingPre‐operativeData Patient history should correspond with testing data
Axial length and K readings are usually close to the same in both eyes.—confirm refractive error history
Repeat axial length when there is a difference of .30mm or more between the two eyes
Repeat K’s with a difference of 1 diopter or more
Refer to any previous data in chart. Compare the IOL power calculations for both eyes. If there is a difference of 1 diopter or more review data entered for accuracy.
SourcesofErrorandthePostoperativeImpact 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.
EvaluationofStandardDeviation
SourcesofErrorandthePostoperativeImpact
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/Lenstar ‐Change AL settings for pseudophakic/phakic eyes/eyes with silicone oil
SmallErrorsAddUP
Axial Length (.20mm) = .50D
K Readings = .50D
IOL Power = .25D
Formula = 1.00D
A-Constant = .50D
TOTAL = 2.75D !
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Customize the formula used ..Get past using only the calculation that your Iolmaster or Lenstar spits out‐run multiple formulas, use H2; use Barrett, Haigis, Olson‐ does one formula really meet every patient’s needs?
SpecialChallenges
Should utilize newer formulas
Recommend Holladay 2 consultant
Measure anterior chamber depth
Lens thickness
Measure corneal diameter or “white to white”
Short and Long Eyes
Comparison Holladay 2
Holladay 1
Haigis SRK/T Barrett
AXL: 28.00
K=43.00
A=118.95
+9.00w/adj=
+10.00 !
+9.00 +8.50 +9.50 +9.00
AXL: 24.00
K=43.00
A=118.95
+20.50 +20.00 +21.00 +20.50 +20.50
AXL: 20.00
K=43.00
A=118 95
+35.00 +36.00 +39.00 +34.00 +37.00
FormulaComparisons PostRefractiveSurgeryPatients
Very Challenging!
Obtain as many different types of K‐readings as possible
Use ascrs.org link – choose myopic lasik or hyperopic lasik or RK template
Understand how to review topography
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CornealTopography
CornealTopography– theaxiswithtopoiscriticalfortoric IOLcalculations Uses a placido disk technology of concentric rings located on the projection head assembly
Measures the distance between the rings and their relationships with each other
System can reconstruct the corneal surface with a higher degree of precision and identify micro irregularities – helps to identify aberrations
CornealTopography
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PreviousHyperopicLASIK PreviousMyopicLASIK
Keratoconus PatientExpectations
Acurate IOL calculations as noted by Dr. Warren Hill first begin with the surgeon identifying the patient’s visual goals. This may be a common discussion Lasik surgeons have with patient but is overlooked many times with cataract patient. Emmetropia may not always be the goal. The “devil is in the details.”!
How do we get better?
‐Patient selection and education
‐Accurate biometry and tight K’s/axis
‐IOL Calculation Formula selection
‐Optimization for best constant
‐Tracking of outcomes
Long Eye Axial Adjustment
Wang‐Koch adjustment
Holladay 1 –optimized AL=.829 x AL +4.266
Example ‐measured AL of 29.00 x.829=24.04 +4.266=adjusted AL of 28.30!
SRK T formula AL x .854 +3.722
Consider B‐scans for eyes over 30 mm long
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Long Eyes…
See www.doctor‐hill.com for great information on long eye calculations
When adjusting axial use the new value ONLY in the formula noted, and do NOT use adjusted axial in post refractive formulas
Do NOT use adjusted AXL in Barrett II calculator‐use measured value
Short eye challenges
Short eye calculations– suggest immersion after Iolmaster or Lenstar
Short eye calculations do best with LT (lens thickness)– Lenstar and Immersion measure LT, newer Iolmaster does also; but original IOLm’s do not measure LT
Short eyes are hyperopic eyes, any error in AXL is highly magnified in proportion to length of eye – accuracy very important
HelpfulLinks
www.doctor‐hill.com‐‐ great info on polypseudophakiccalculations; piggyback; refractive vergence formula; bag to sulcus
www.docholladay.com
Holladay II Software
www.ascrs.com
http://www.augenklinik.uni‐wuerzburg.de/ulib/index.htm
http://www.apacrs.org/barrett_universal2/
THANK YOU!!
Contact information:
Karen Bachman, COMT, ROUB1 (801) 263-5757
kbachman@theeyeinstitute.com
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