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DIOPTRIC DISTRESS How to calculate IOL power under
any circumstances
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Gathering Good Data
DIOPTRIC DISTRESS How to calculate IOL power under
any circumstances
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Basics of Biometry
Dr. Ashima Aggarwal
INSIGHT EYE CLINIC
Rajouri Garden
New Delhi
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Financial Disclosure
I have no financial interest in the contents of
this presentation.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry
• The process of measuring various physical
parameters of an eye, and using this data to
determine the ideal intraocular lens power.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Components of biometry
• Essential
Axial Length
Keratometry
• Additional
AC Depth
White to White
Lens thickness
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
A matter of great responsibility
Biometry should only be performed by the surgeon or a well-trained assistant.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
A matter of timing
• Biometry should be done early
Time to calculate IOL Power.
Time to counsel the patient.
Time to arrange specific IOL.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Keratometry
• Estimation of the corneal power.
• Good K measurement is important !
Directly used in IOL Power calculation.
Indirectly used for ELP estimation.
Evaluation of astigmatism.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Keratometry
• Remember, we are measuring the radius of
curvature of the anterior cornea.
• The actual dioptric power is calculated.
• Assumed overall corneal refractive index 1.3375
Dr. Ashima Aggarwal
Power = 1.3375 – 1.0000 Radius
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Keratometry
• Manual
• Automated
• Optical
• Topography
• Pentacam
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Keratometry – tips and tricks
• Calibrate regularly.
• Good tear film.
• Untouched cornea.
• Accurate centration – occlude the other eye.
• Average of multiple readings.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Keratometry – tips and tricks
• Dark room
• Evident scarring – use the other eye value.
• Recheck
extreme values : < 40 D or > 48 D
asymmetry : > 1.5 D difference
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length
• Contact Ultrasound
• Immersion Ultrasound
• Optical
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Contact Ultrasound
• Principle
Ultrasonic waves are reflected off the ocular
structures.
These echoes are deciphered to yield the
distances traversed.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Contact Ultrasound – TIPS
#1 • Calibration.
• Correct velocity settings.
• Gentle touch.
• Look for good spikes.
• Appropriate gain settings.
• Axial alignment using the fixation light.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Contact Ultrasound – TIPS
#2 • Average of multiple readings, with low standard
deviation (<0.06 mm).
• Repeat if difference between two eyes is > 0.3 mm.
• Record before dilating the pupil.
• Avoid using ointments before the measurements.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Contact Ultrasound
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Contact Ultrasound
• PROS
Easy to perform.
Works for the densest of cataracts.
Cost Effective.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Contact Ultrasound
• CONS
Corneal compression or fluid bridge.
Risk of abrasion, infection.
Skill dependent.
Misalignment.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Immersion Ultrasound
• The probe is fixed inside a special shell, most
commonly the Prager Shell.
• This shell is placed between the two eyelids,
centred on the cornea.
• A water-bath is created within the shell, such that
the tip of the probe is immersed.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Immersion Ultrasound
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Immersion Ultrasound –
TIPS
• Make sure that the patient is comfortable.
• Align the probe to the mark on the shell properly.
• Fill the chamber adequately, without air bubbles.
• Ask the patient to look at the fixation light.
• Make sure that the machine is in immersion mode.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Immersion Ultrasound
• PROS
Accurate.
No corneal compression.
Works for the densest of cataracts.
Relatively Cost Effective.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Immersion Ultrasound
• CONS
Slight learning curve.
Special shell is needed.
Risk of abrasion, infection.
The patient can get wet.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Optical
• Infra-red diode laser (780-830 nm).
• Partial Coherence interferometry is used to
measure the optical path length.
• Since the wavelength of the infrared rays used is
much smaller than that of ultrasonic waves, the
resolution of the measurement is far better.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Optical – TIPS
• Ensure moist ocular surface.
• Ensure alignment.
• Slight mydriasis may be helpful.
• Choose the correct mode.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Optical
• PROS
Very high repeatability.
Observer independent.
Fast.
Precise.
Same machine for all measurements.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Optical
• PROS
No physical contact.
No anaesthesia.
No corneal compression.
Visual axis measurement.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Axial Length – Optical
• CONS
Expensive.
Does not work if there is significant axial
opacity.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
After the gathering.....
Dr. Ashima Aggarwal
• After the data has been gathered, the surgeon
must use one or more IOL power calculation
formulae.
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Formula Options
• SRK ii – regression formula
• SRK/T
• Holladay 1
• Hoffer Q
• Haigis
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Which formula NOT to use !!
• SRK II – obsolete
45.0, 44.0, A-constant 118
Axial Length 22.00 – IOL Power = 22.95
Axial Length 21.99 – IOL Power = 23.98
0.01 mm difference = 1.03 D difference
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Which formula to use, and where
Short eyes : Hoffer Q
Average eyes : Holladay 1
Long eyes : SRK/T
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry – keep in mind
• Both eyes should be measured.
• Always use the correct mode.
• Look for asymmetry between the keratometry and
axial length of the two eyes. It can indicate
amblyopia, staphyloma, vitreous opacity, covert
corneal scarring etc.
• Repeat on different days, with different machines.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry – keep in mind
• Use multiple formulae.
• IOL Position - Bag vs Sulcus
»30.0 D bag = 28.5 D sulcus
»21.0 D bag = 20.0 D sulcus
»10.0 D bag = 9.5 D sulcus
• Change of IOL model entails recalculation.
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
• Audit your results scientifically.
• Record, at least
Mean Error (SD)
Percentage of eyes within 0.5 D of predicted
refraction.
Biometry – keep in mind
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Thank you !
Dr. Ashima Aggarwal
DIOPTRIC DISTRESS How to calculate IOL power under
any circumstances
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry in Special Situations
Dr. Vinita Gupta
Assistant Professor
All India Institute of Medical Sciences
Rishikesh
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Financial Disclosure
I have no financial interest in the contents of
this presentation.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Special Situations
• High Myopia / Hypermetropia
• Post-vitrectomized eyes
• Paediatric Cataract
• Scarred Cornea
• Aphakia / Pseudophakia
• Piggy back IOL
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
High Myopia - issues
Confounding factors
• Lower scleral rigidity
• Posterior staphyloma
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
High Myopia - what to do?
• Lower scleral rigidity - Avoid contact biometry. Use either immersion or optical biometry.
• Posterior staphyloma
Optical Biometry is the gold standard.
Perform immersion biometry as well as horizontal axial B scan.
Add ACD and LT from A-scan to vitreous length from B scan to obtain total axial length.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
High Hypermetropia - issues
• Corneal compression.
• Relative ocular dimensions.
• Formula Errors.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
High Hypermetropia - what to do?
• Immersion / Optical biometry
• HofferQ formula
• Optimization
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Post-vitrectomized Eye
• Lower ocular rigidity, so avoid contact biometry.
• No change in axial length measurements
otherwise.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Silicone Oil filled Eye
• Sound travels slower through Silicone oil.
• The speed depends upon the density of the
silicone oil.
• Presence of retro-silicone space is a further
confounding factor.
• Instrument range can be exceeded, especially in
very long eyes.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Silicone Oil filled Eye – what to do?
Ultrasonic Measurement
• Measure axial length in the sitting position.
• Apply correcting factors.
Optical Measurement
• Just set the mode to silicone filled eye, and record
the AL.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Silicon Oil filled Eye - correction factors
• Vitreous Chamber Depth with oil, VCDoil , can be
extrapolated to yield true VCD
VCD TRUE = VCD OIL x 0.64
Then, AL = ACD + LT + VCDTRUE
• Alternatively, total Axial Length can be corrected
Axial Length TRUE = Axial Length OIL x 0.72
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Scarred Cornea
Dr. Vinita Gupta
• Issues
Unknown corneal contribution to refraction.
Faulty ELP estimation.
Optical biometry may not work.
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Scarred Cornea – what to do?
• Measure with contact lens overlay.
• Topography.
• Measure other eye.
• Calculate using only AL and refractive error.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Paediatric Biometry - Issues
Dr. Vinita Gupta
• How cooperative – optical might be better, or
under GA.
• Low ocular rigidity, greater compression.
• Small eye, more impact of compression
3.75 D per mm
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Paediatric Biometry - Issues
Dr. Vinita Gupta
• Different Anatomy
standard IOL Power formulae inadequate.
Ultrasound velocity is different
• Growing eye, changing parameters.
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Paediatric Biometry – what to do?
Dr. Vinita Gupta
• Always, always measure both eyes.
• Undercorrect
< 2 years, 20% undercorrection
2-8 years, 10% undercorrection
• No one formula has been proven to be superior.
• High IOL Powers - consider piggyback IOLs.
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry in aphakic eyes
• Lack of lens spikes. Manual mode may be required.
• Sound velocity is uniform at 1532 m/s.
• It is actually easier to perform biometry in the aphakic
eye.
• Lens power calculations will need to be adjusted
according to the final position of the IOL.
• Refraction based IOL power estimation can be done.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry in pseudophakic eyes
• IOL exchange.
• Multiple tall spikes, difficult
to freeze the scan.
• Optical measurement is very
effective.
• Standard IOL Power
calculation formulae apply.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Piggyback IOLs
• Primary piggyback
Very high hyperopia.
• Secondary piggyback
Correction of residual refractive error.
Can be calculated easily based on existing refraction.
• Myopic correction, Piggyback Power = 1.0 x Refractive Error
• Hyperopic Correction, Piggyback Power = 1.5 x Refractive Error
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Biometry in special situations
• Special situations are generally suboptimal eyes
requiring extra diligence.
• Careful consideration to basic principles can make
a big difference to the outcomes.
Dr. Vinita Gupta
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Thank you !
Dr. Vinita Gupta
DIOPTRIC DISTRESS How to calculate IOL power under
any circumstances
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Optimization Strategies
Dr. Saurabh Sawhney
INSIGHT EYE CLINIC
Rajouri Garden
New Delhi
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Financial Disclosure
I have no financial interest in the contents of
this presentation.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
A FAMILIAR SCENARIO ?
• Excellent surgery
• Pristine Cornea
• Quiet eye
• Unhappy patient
HIGH REFRACTIVE ERROR
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
The need to hit the target
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Sources of error – bad input data
• Corneal compression.
• Calibration errors.
• Inherent errors due to assumptions of speed of
sound, refractive index of cornea etc.
• Erratic errors
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Sources of error – IOL Design Feature
• Angulation of haptics.
• Overall size and shape of the implant.
• Rigidity of the haptic optic junction.
• Biological interaction with capsular bag.
• Position of IOL in the bag.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Sources of error – Surgical technique
• Rhexis size, centration
• IOL placement
• Viscoelastic removal
• Suturing technique
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
IOL Power
= fx (Lens Constant) (AL) (K)
The IOL power formula
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
IOL Power
= fx (Lens Constant) (AL) (K)
Since the AL and K are fixed values for a given eye, it is the Lens Constant that combines the
influence of all previously mentioned variables.
The IOL power formula
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
The ‘CONSTANTS’ are variable !!!
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
• SRK II A-constant
• SRK/T A-constant
• Holladay Surgeon’s Factor
• HofferQ pACD
• Haigis a0, a1, a2
The ‘CONSTANTS’ are variable !!!
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Manufacturer’s values
• The A-constant mentioned on the IOL box.
• Empirical value.
• Based on theoretical considerations, limited
clinical data, and experience with similar lens
designs in the past.
• Useful starting point.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Ingredients for optimization
• Preoperative Keratometry
• Preoperative Axial Length
• IOL Power and model
• Postoperative Refractive error
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Basics of optimization
• SRK formula
P = A constant – 0.9 K – 2.5 AL
• Rearranging the equation
P + 0.9 K + 2.5 AL = A constant
(where P is the ideal IOL power for emmetropia)
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
What next ?
• Calculate A-constants for a series of cases.
• Average these values.
• 25 or more cases are needed for meaningful
results.
• Use this new A-constant for your future IOL power
calculations.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Points to ponder
• One surgeon
• One lens model
• One Biometry setup
• In the bag placement of IOL
• Standardized refraction
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Limitations of ‘paper and pen’
optimization
• Very complex equations for the modern
theoretical formulae.
• High possibility of unnoticed error.
• Time consuming.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Back calculation for surgeon’s factor
A scary peek into the maths involved.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Back calculation for surgeon’s factor
Surgeon’s Factor = (((-(PostopSphEq*0.001*((ALHolla*12*((4/3)-1))-((CorRadius)*(ALHolla-
(12*(4/3)))))-((((4/3)-1)*ALHolla)+(1.336*(CorRadius)))))-
SQRT(((PostopSphEq*0.001*((ALHolla*12*((4/3)-1))-((CorRadius)*(ALHolla-(12*(4/3)))))-((((4/3)-
1)*ALHolla)+(1.336*CorRadius)))^2)-(4*((((4/3)-1)-(0.001*PostopSphEq*((12*((4/3)-1))-
(CorRadius)))))*(((1.336*ALHolla*CorRadius)-
(0.001*PostopSphEq*ALHolla*12*CorRadius*1.336))-(((1000*1.336)*((1.336*CorRadius)-(((4/3)-
1)*ALHolla)-(0.001*PostopSphEq*((12*((1.336*CorRadius)-((1.3333-
1)*ALHolla)))+(ALHolla*CorRadius)))))/IOLimplanted)))))/(2*(((4/3)-1)-
(0.001*PostopSphEq*((12*((4/3)-1))-(CorRadius))))))-(0.56+RAG-(SQRT(RAG^2-(modAG^2/4)))))
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
More sensible optimization strategies
• Biometry Machine
• Separate Software
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Using the Biometry Machine
• Save the initial biometry.
• Enter the surgical data – surgeon, IOL power,
model etc.
• Enter the postoperative refraction.
• The machine calculates and updates the relevant
constants.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Is there an even simpler way?
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Is there an even simpler way?
Independent Software
• Easier to work on the PC rather than the biometry
machine.
• Easier to back up and transport data.
• Faster.
• No memory overload issues.
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
SOFTWARE
The ‘IOL constants optimizer’
LCO v 2.1
www.Facebook.com/OphthalmicCalculators
www.InsightEyeClinic.in
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Dr. Saurabh Sawhney
www.Facebook.com/OphthalmicCalculators www.InsightEyeClinic.in
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Happy Optimizing
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Thank you !
Dr. Saurabh Sawhney
DIOPTRIC DISTRESS How to calculate IOL power under
any circumstances
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills
Converting constants
A constant 118.0 118.4 118.5 118.7 118.8
Surgeon Factor 1.22 1.45 1.51 1.62 1.68
pACD 4.97 5.20 5.26 5.37 5.43
Surgeon Factor = 0.5663 x A-constant - 65.6
pACD = (A constant x 0.5663) - 65.6 + 3.595 0.9704