-IOL formula
1st generation formula : SRK, Binkhost
2nd generation formula : SRK II
3rd generation formula: Hoffer Q, Holladay 1, SRK/T
4th generation formula: Haigis, Holladay 2, Olsen
-The Hoffer Q, Holladay I, and SRK/T formula are all commonly used.
Maddox Rod
Use of Maddox Rod
Method of Assessment MR
Double MR Test procedure
Recording procedure of MR Test
Heterophoria, Cyclophoria, Esophoria,Exophoria,Hyperphoria,Hypophoria
The presentation I have made and uploaded provides you with an in-depth insight into the patterns the strabismus may take following anomalies of extraocular muscles, deformities of the orbital structures,innnervational disturbances.
The author does not assume responsibility or legal liability for any errors in the text or for the misuse or misapplication of material in this work.
No copyright infringement, or plagiarism intended.
Amrit Pokharel
Maddox Rod
Use of Maddox Rod
Method of Assessment MR
Double MR Test procedure
Recording procedure of MR Test
Heterophoria, Cyclophoria, Esophoria,Exophoria,Hyperphoria,Hypophoria
The presentation I have made and uploaded provides you with an in-depth insight into the patterns the strabismus may take following anomalies of extraocular muscles, deformities of the orbital structures,innnervational disturbances.
The author does not assume responsibility or legal liability for any errors in the text or for the misuse or misapplication of material in this work.
No copyright infringement, or plagiarism intended.
Amrit Pokharel
Biometry- Iol power and calculation final ppt.pptxKervi Mehta
Biometry- IOL power formulae and calculations
This presentation describes about different generations of IOL formulae and newer formulae. It also gives information how to calculate IOL power in special situations
Pseudophakic bullous keratopathy (PBK) is a post-operative condition that can occur as a complication of cataract extraction surgery and intraocular lens placement.
May be manifest in the immediate post-operative period or symptoms may not present for many years.
Scleral lens is a large rigid contact lens with a diameter range of 15mm to 25mm. Its resting point is beyond the
corneal borders, and are believed to be among the best vision correction options for irregular corneas. Wearing scleral lens also can postpone or even prevent surgical intervention as well as decrease the risk of corneal scarring.
Ischemic optic neuropathy constitutes one of the major causes of blindness or seriously impaired vision among the middle-aged and elderly population.
Ischemic optic neuropathy is due to acute ischemia of the optic nerve. it can be classified into two, depending upon the part of the optic nerve involved:
1.Anterior ischemic optic neuropathy (AION)
-AION is due to acute ischemia of the front (anterior) part of the optic nerve (also called optic nerve head), which is supplied mainly by the posterior ciliary arteries.
-AION is divided into two types, depending on what causes it:
1.Arteritic AION: This is the most serious type and is due to a disease called giant cell arteritis or temporal arteritis.
2. Non-arteritic AION: This is the usual, most common type, with many different causes but not associated with giant cell arteritis.
2.Posterior ischemic optic neuropathy (PION). -
-PION is a much less common type. It is due to acute ischemia of the back (posterior) part of the optic nerve, located some distance behind the eyeball; this part of the optic nerve is NOT supplied by the posterior ciliary arteries
(Hayreh, 2009)
Ill-sustained accommodation
WHAT?
-AKA accommodative fatigue
-Amplitude of accommodation is initially normal, but deteriorates over time after prolong focusing at near task.
-Sub-classification of accommodative insufficiency.
-An early stage of accommodative insuffciency.
CLINICAL SIGNS:
-Hard on any clinical tests that require stimulation of accommodation (hard on minus lens) and deteriorates AA over time.
MANAGEMENT:
1. Correction
2. Added plus lenses
3. Visual therapy
Contact lens for congenital aphakia and other eye conditions for infants and toddlers. The slide presentation encompasses indications for CL fitting in paediatric, contact lens options, fitting techniques, challenges and contact lens as myopia control.
Report Back from SGO 2024: What’s the Latest in Cervical Cancer?bkling
Are you curious about what’s new in cervical cancer research or unsure what the findings mean? Join Dr. Emily Ko, a gynecologic oncologist at Penn Medicine, to learn about the latest updates from the Society of Gynecologic Oncology (SGO) 2024 Annual Meeting on Women’s Cancer. Dr. Ko will discuss what the research presented at the conference means for you and answer your questions about the new developments.
These lecture slides, by Dr Sidra Arshad, offer a quick overview of physiological basis of a normal electrocardiogram.
Learning objectives:
1. Define an electrocardiogram (ECG) and electrocardiography
2. Describe how dipoles generated by the heart produce the waveforms of the ECG
3. Describe the components of a normal electrocardiogram of a typical bipolar leads (limb II)
4. Differentiate between intervals and segments
5. Enlist some common indications for obtaining an ECG
Study Resources:
1. Chapter 11, Guyton and Hall Textbook of Medical Physiology, 14th edition
2. Chapter 9, Human Physiology - From Cells to Systems, Lauralee Sherwood, 9th edition
3. Chapter 29, Ganong’s Review of Medical Physiology, 26th edition
4. Electrocardiogram, StatPearls - https://www.ncbi.nlm.nih.gov/books/NBK549803/
5. ECG in Medical Practice by ABM Abdullah, 4th edition
6. ECG Basics, http://www.nataliescasebook.com/tag/e-c-g-basics
Explore natural remedies for syphilis treatment in Singapore. Discover alternative therapies, herbal remedies, and lifestyle changes that may complement conventional treatments. Learn about holistic approaches to managing syphilis symptoms and supporting overall health.
TEST BANK for Operations Management, 14th Edition by William J. Stevenson, Ve...kevinkariuki227
TEST BANK for Operations Management, 14th Edition by William J. Stevenson, Verified Chapters 1 - 19, Complete Newest Version.pdf
TEST BANK for Operations Management, 14th Edition by William J. Stevenson, Verified Chapters 1 - 19, Complete Newest Version.pdf
- Video recording of this lecture in English language: https://youtu.be/lK81BzxMqdo
- Video recording of this lecture in Arabic language: https://youtu.be/Ve4P0COk9OI
- Link to download the book free: https://nephrotube.blogspot.com/p/nephrotube-nephrology-books.html
- Link to NephroTube website: www.NephroTube.com
- Link to NephroTube social media accounts: https://nephrotube.blogspot.com/p/join-nephrotube-on-social-media.html
Pulmonary Thromboembolism - etilogy, types, medical- Surgical and nursing man...VarunMahajani
Disruption of blood supply to lung alveoli due to blockage of one or more pulmonary blood vessels is called as Pulmonary thromboembolism. In this presentation we will discuss its causes, types and its management in depth.
Prix Galien International 2024 Forum ProgramLevi Shapiro
June 20, 2024, Prix Galien International and Jerusalem Ethics Forum in ROME. Detailed agenda including panels:
- ADVANCES IN CARDIOLOGY: A NEW PARADIGM IS COMING
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- BEYOND BORDERS: GLOBAL INITIATIVES FOR DEMOCRATIZING LIFE SCIENCE TECHNOLOGIES AND PROMOTING ACCESS TO HEALTHCARE
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- Prix Galien International Awards Ceremony
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Tom Selleck Health: A Comprehensive Look at the Iconic Actor’s Wellness Journeygreendigital
Tom Selleck, an enduring figure in Hollywood. has captivated audiences for decades with his rugged charm, iconic moustache. and memorable roles in television and film. From his breakout role as Thomas Magnum in Magnum P.I. to his current portrayal of Frank Reagan in Blue Bloods. Selleck's career has spanned over 50 years. But beyond his professional achievements. fans have often been curious about Tom Selleck Health. especially as he has aged in the public eye.
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Introduction
Many have been interested in Tom Selleck health. not only because of his enduring presence on screen but also because of the challenges. and lifestyle choices he has faced and made over the years. This article delves into the various aspects of Tom Selleck health. exploring his fitness regimen, diet, mental health. and the challenges he has encountered as he ages. We'll look at how he maintains his well-being. the health issues he has faced, and his approach to ageing .
Early Life and Career
Childhood and Athletic Beginnings
Tom Selleck was born on January 29, 1945, in Detroit, Michigan, and grew up in Sherman Oaks, California. From an early age, he was involved in sports, particularly basketball. which played a significant role in his physical development. His athletic pursuits continued into college. where he attended the University of Southern California (USC) on a basketball scholarship. This early involvement in sports laid a strong foundation for his physical health and disciplined lifestyle.
Transition to Acting
Selleck's transition from an athlete to an actor came with its physical demands. His first significant role in "Magnum P.I." required him to perform various stunts and maintain a fit appearance. This role, which he played from 1980 to 1988. necessitated a rigorous fitness routine to meet the show's demands. setting the stage for his long-term commitment to health and wellness.
Fitness Regimen
Workout Routine
Tom Selleck health and fitness regimen has evolved. adapting to his changing roles and age. During his "Magnum, P.I." days. Selleck's workouts were intense and focused on building and maintaining muscle mass. His routine included weightlifting, cardiovascular exercises. and specific training for the stunts he performed on the show.
Selleck adjusted his fitness routine as he aged to suit his body's needs. Today, his workouts focus on maintaining flexibility, strength, and cardiovascular health. He incorporates low-impact exercises such as swimming, walking, and light weightlifting. This balanced approach helps him stay fit without putting undue strain on his joints and muscles.
Importance of Flexibility and Mobility
In recent years, Selleck has emphasized the importance of flexibility and mobility in his fitness regimen. Understanding the natural decline in muscle mass and joint flexibility with age. he includes stretching and yoga in his routine. These practices help prevent injuries, improve posture, and maintain mobilit
3. It is often said that cataract surgery is a refractive surgery.
In old days the cataract was removed first and the spectacle
prescription given last, the situation today is reversed.
We prescribe an IOL to obtain a certain refractive effect
Aims to reduce spectacle dependency
Therefore, IOL calculation plays an important role to determine the
refractive outcome after the surgery.
4. Since 1975, IOL power has been calculated using accurate
measurement of an eye’s corneal power and axial length (AL).
Power of the IOL was calculated using clinical history alone.
Or the preoperative refractive error prior to cataract development.
Today, we can customize the power of the lens implanted during
cataract surgery.
Even patients who are highly myopic or hyperopic can achieve a
near plano result after IOL implantation.
8. •Provided evidence for tolerance of a foreign body in the eye
•Prospect of restoring functional vision.
9. •A- constant= specific constant for each type of IOL, which is determined
empirically on the large sample of patients underwent cataract surgery.
•A-constant is calculated for each lens type based on the refractive outcomes
10.
11.
12.
13. Regression formula
Empiric formulas generated by averaging large numbers of post-operative clinical results
(retrospective computer analysis of data obtained from a great number of patients who have
undergone surgery)
1980s; popular because it was simple to use
Power error often resulted from the use of these formulas
14. P= IOL power to be used (D)
A = IOL specific A constant
K = Average corneal refractive power (D)
L = Axial length of the eye (mm)
P = A – 0.9K – 2.5L
15. A constant
Relates the P to K and L
Depends on multiple variables
IOL manufacturer
Style
Placement
Used to characterize the IOL implants
Intended location
Orientation within the eye
Provided by the manufacturer of IOL
16. 16
Corneal refractive power
Assumption;
Thin spherical lens
Fixed anterior and posterior corneal curvature ratio
Index of refraction of 1.3375
Measured by keratometry / corneal topography
17. Corneal radius of curvature relates to corneal power with the equation
K = n – 1
r
r = 337.5
K
18. Axial length of the eye
Distance between the anterior surface of the cornea and the fovea
Most important factor in IOL calculation
1.0mm error 2.50D – 3.50D error
Measure by A-scan ultrasonography / optical coherence biometry
Suitable to use on axial length range : 22mm- 24.5mm
19.
20. Based on regression analysis
2nd
generation of SRK formula
Optimized A constant based on axial length of the eye
Increase the A constant for shorter eye
Decrease the A constant for longer eye
The new SRK II formula was more accurate than the original SRK and
Binkhorst II formulae.
80% of the eyes has less than 1D error and one eye
0.3% had an error of more than 3D
(Dang et al.1989)
21. P= IOL power to be used (D)
A = IOL specific A constant
K = Average corneal refractive power (D)
L = Axial length of the eye (mm)
P = A1 – 0.9K – 2.5L
22. A constant
Optimized based on axial length
A1 = (A – 0.5) for axial length greater than 24.5mm
A1 = (A) for axial length between 22 and 24.5mm
A1 = (A + 1) for axial length between 21 and 22mm
A1 = (A + 2) for axial length between 20 and 21mm
A1 = (A + 3) for axial length less than 20mm
23.
24. Hoffer formula use the post-operative AC depth
A change in the true post-operative AC depth will affect the
refractive status of the eye.
A change in 1 mm causes a 1.5D change in the final refraction.
Hence, these constants must be personalized to
accommodate any consistent shift that might affect IOL
power calculation.
25. The main feature of the 1st
generation theoretical formulae
was
that position of IOL in the eye is fixed for each lens type.
This assumption was true at that time, when cataract surgery
was represented by ICCE and ACIOL implantation:
the ACIOL was assumed to have a defined position in relation to the
anterior plane of the cornea.
26. 2nd
generation theoretical IOL power formulae differ
from the 1st
generation because:
Position of the IOL in the pseudophakic eye; is not fixed but
changes based on 2 variables: axial length and corneal
curvature or, corneal power of the eye.
The 2nd
generation regression formulae were designed
to improved accuracy
has been shown to reduce the prediction error of the original
SRK formula in short (<22mm) and long(≥24.5mm axial
length) eyes.
27.
28. Although the 1st
and 2nd
generation formulae are not used in present time,
they are all basis formulae developed or modified for newer generation
formulae (3rd
and 4th
generations).
SRK formula recommended used in cases such as
ICCE
ACIOL
Emmetropic eye
SRK II formula recommended used in cases such as
ECCE
Phacoemulsification
PCIOL
Axial length (too long or too short than normal)
29. Professor Dr. Jean. B., A Comparative Analysis of Methods for Calculation IOL Power: Combination of Three Corneal
Power and Two Axial Length Measuring Techniques, (2008).
https://publikationen.unituebingen.de/xmlui/bitstream/handle/10900/45350/pdf/stanbekova.pdf?sequence=1
Masket. S. MD, Masket, S.E., PhD, Simple Regression Formula For Intraocular Lens Power Adjustment in Eyes Requiring
Cataract Surgery After Excimer Laser Photoablation (2006), J Cataract Refract Surg, Vol: 32, Pg: 430-434
http://www.unisinucartagena.edu.co/biblioteca/oftalmologia/REVISION_TEMA/SEGMENTO_ANTERIOR/CATARATA/FACOEMULSIFICACION/ARTICULOS
/Articulos_Calculo_de_LIO/3.pdf
Dang, M. S., and Raj, P.P.S., SRKII Formula In The Calculation of Intraocualr Lens Power, (1989), British Journal of
Ophthalmology, Vol. 73, Pg: 823-826.
http://bjo.bmj.com/content/73/10/823.full.pdf
Olsen. T, Calculation Of Intraocular Lens Power: A Review, (2007), Acta Ophthalmologica Scandinavica, Vol. 85, Pg: 472-
485.
http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0420.2007.00879.x/full
Apple, D.J., MD, and Sims, J. MD., Harold Ridley And The Invention Of The Intraocular Lens, (1996), Survey Of
Ophthalmology, Vol. 40, No.4.
http://www.rayner.com/skin/frontend/mtcolias/default/pdf/Invention_of_the_IOL.pdf
30. By Ling SookYee, LowYu Chen, Nurul Akimi Abdullah
3RD GENERATION FORMULA :
HOFFER Q, HOLLADAY 1, SRK/T
31.
32. Merger of the linear regression methods with
theoretical eye models
Pseudophakic ACD Surgeon Factor A-constant
33. Improved accuracy
Better result & simple
Take into account of
Axial length
K-reading
Optimization of formula
to predict the effective lens position ELP
34. = distance from cornea to lens
Explains position of the IOL postoperatively
ELP is difficult to predict because:
a) IOL is thinner than cataract
b) ACD tends to increase with pseudophakia
c) Variable lens geometry across power range
35. Errors in predicting the ELP caused: refractive surprise
Shallow AC -> sitting more anterior -> lower IOL power
36.
37. Introduced by Dr Kenneth Hoffer in 1993
Was developed to predict the pseudophakic anterior chamber depth (ACD)
Being optimized from Hoffer formula by personalizing the ACD
It relies on a personalized ACD , axial length and corneal curvature.
38. P = f (A, K, Rx, pACD)
Axial length
Average corneal
refractive power
Previous refraction
Personalized ACD,
manufacturer’s ACD-
constant
ACD-constant = 0.58357 * A-constant – 63.896
39. P = 1336 - 1.336
A – C – 0.05 [(1.336) – (C +0.05)]
K + R 1000
P = IOL power
A = Axial length
C = estimated post-op ACD
K = corneal power (in Diopters)
R = corneal radius (in mm)
40. Hyperopes (AL < 22 mm) (Kenneth Hoffer)
Most accurate in short eyes < 22.0mm, confirmed in large study of 830 short eyes
Had the lowest mean absolute error (MAE) for AL 20.0mm to 20.99mm
Hoffer Q and Holladay 1 had lower MAE than SRK/T for AL 21.0mm to 21.49mm
In post corneal refractive surgery
41. Contribution of IOL power errors:
i. Inaccurate measurement/calculation of anterior corneal power (especially in those
remove corneal tissue i.e PRK)
ii. Incorrect estimation of ELP
Flat central corneal power after LASIK, the formula assumes that the AC is shallow
Myopic-LASIK: underestimation of the IOL power
Hyperopic-LASIK: overestimation of the IOL power
42. P= PTARG - 0.326 × RCC - 0.101
IOL power calculated by standard
IOL formulas
surgically induced
refractive change
This method adjusts the power of the IOL, using the knowledge of the
surgically induced refractive change.
Masket S and Masket SE (2006)
Example IOL calculated 22.0D
Change in Rx = +3.0D
P = 22.0 – (0.326 x +3.0) – 0.101
= 21.0 D
43. Double K formula
K-reading before refractive surgery is used to estimate the ELP
K-reading after refractive surgery is used to calculate the IOL power
Tradition method: Single K formula
K-reading is used for both calculations
Tends to underestimate the IOL power in myopic LASIK eyes
44. Myopic Correction
Numbers in each row represent the
amount (D) that must be added to the
calculated IOL power
Hyperopic Correction
Numbers in each row represent the
amount (D) that must be subtracted to
the calculated IOL power
45.
46. Produced by Jack Holladay in 1988
Used axial length and keratometry to determine ELP
Work best for eyes between 24.5 to 26 mm (medium long)
Takes into account ac depth, lens thickness and corneal radius
Useful for axial myopia and high corneal curvature (>45)
47. Using K, AL to predict IOL power
No ACD input indicated
Calculates predicted distance from cornea to iris plane + distance from iris plane
to IOL
Uses surgeon factor for optimization of formula (specific for each lens)
48.
49. Distance between iris plane & IOL optic plane
SF should be personalized
A change in the true post-operative AC depth will affect the refractive
status of the eye.
A change in 1 mm causes a 1.5 D change in the final refraction
SF constants must be personalized to accommodate any consistent shift that
might affect IOL power calculation
Each constant has to be back calculated for at least 20 cases, with care to
ensure that the same person takes the measurements.
50. For eyes with previous refractive surgery
Use K value prior to surgery and change in manifest refraction resulting from
LASIK or PRK
IOL power is calculated using the Aramberri double-K method
uses corneal power prior to refractive surgery to estimate effective lens position
value of 43.86 D is used when corneal power pre refractive surgery not
available.
51.
52. 1. Regression formulas topped surgeon’s preferences, and one of the most successful was the SRK
formula. (Sanders D et al,1983)
2. Over the years, surgeons discovered that the SRK formula is best used in eyes with average AL,
between 22.00 and 24.50 mm.
3. A subsequent formula, the SRK II, was developed for use in long and short eyes. ( Dang MS et al,
1989)
4. Even more customized formulas are required today to calculate anterior chamber depth (ACD)
based on AL and corneal curvature. The SRK/T (T for theoretical) is one such formula,
representing a combination of linear regression method with a theoretical eye model. (Retzlaff
JA,1990)
53. SRK I – 1st
gen
P = A – 0.9K – 2.5L
SRK II – 2nd
gen
P = A1 – 0.9K – 2.5L
AI Axial
Length
A+3 <20
A+2 20-21
A+1 21-22
A 22-24.50
A-0.5 >24.5
54. It can be calculated using the same A constants used with the original SRK
formula or with ACD estimates.
SRK/T formula optimizes the prediction of postoperative ACD, retinal
thickness AL correction, and corneal refractive index.
Recommended formula usage : best for eyes longer than 26.00 mm.
55. What is the effect of A-constant on IOL power?
The term “A-constant” seems misleading because, it varies among IOL
models and even among surgeons.
“A-constant” is adjustable & depends on multiple variables including IOL
manufacturer, style and placement within the eye.
Different model of IOL , has different A-constant.
Eg ~ 1:1 rule
IOL brand No. 1 : A-constant of 118.4 = +21.0 D
IOL brand No. 2: A-constant of 118.9 = +21.5 D
to get the same plano postop refraction.
56. 1:1 relationship with the A-
constants:
if A decreases by 1
diopter,
IOL power decreases by
1 diopter.
57. Research shown there was no significant difference between the predictive abilities of SRKII
or SRK/T.
However, there are differences in the predictability of refractive outcomes between
different IOL.
( M J ELDER, 2002)
59. Wang, L., M.A. Booth, and D.D. Koch, Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK.
Ophthalmology, 2004. 111(10): p. 1825-31.
Masket, S. and S.E. Masket, Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after
excimer laser photoablation. J Cataract Refract Surg, 2006. 32(3): p. 430-4.
Aramberri J. Intraocular lens power calculation after corneal refractive surgery: Double K method. J Cataract Refract Surg 2003; 29(11):
2063-2068.
Eom Y, Kang S-Y, Song JS, Kim YY, Kim HM. Intraocular Lens Power Calculation According to the Anterior Chamber Depth in Short Eyes.
American Journal of Ophthalmology, April 2014, Vol 157, Issue 4, pp 818-824.
Hoffer KJ. The Hoffer Q formula: A comparison of theoretic and regression formulas. Journal of Cataract and Refractive Surgery, November
1993.
Sanders DR, Retzlaff J, Kraff MC. Comparison of empirically derived and theoretical aphakic refraction formulas. Arch Ophthalmol.
1983;101(6):965-967.
Dang MS, Raj PP. SRK II formula in the calculation of intraocular lens power. Br J Ophthalmol. 1989.
Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg.
1990
IOL power calculation retrieved http://www.rajswasthya.nic.in/RHSDP%20Training%20Modules/Ophthalmologist/Cataract%20Surgery
%20with%20IOL.Pdf/03%20IOL%20calculation.pdf
Findl, O. Biometry and intraocular lens power calculation. Current Opinion in Ophthalmology 2005, 16:61–64
Parmar, M. (2008). IOL power calculation. Retrieved from http://www.eophtha.com/eophtha/ppt/IOL%20power%20calculation.html
60. By Ang Kai Li, Noor Munirah binti Awang Abu Bakar, Nurulhidayah Nordin
61.
62.
63. Developed by Wolfgang Haigis,
director, Department of Biometry,
University of Würzburg Eye Hospital,
Würzburg, Germany
Found in Zeiss IOLMaster software
The Haigis formula (3 constants) has an
accuracy close to that of the Hoffer Q
(two-variable formulas)
By regression analysis, the 3 constants
are calculated to individually adjust the
IOL power prediction curve for each
surgeon/IOL combination in such as way
as to closely reproduce observed results
over a wide range of axial lengths and
anterior chamber depths.
64.
65. PROBLEMS WITH 3RD GENERATION 2 VARIABLE
FORMULA (HOFFER Q, HOLLADAY 1, SRK/T)
The LARGER the IOL constant, the MORE IOL power each formula will recommend for the
same set of measurements; the SMALLER the IOL constant, the LESS IOL power the same
formula will recommend for the same set of measurements
In reality, two eyes with the exact same axial length and the same keratometry may require
completely different IOL powers for emmetropia.
IOL power prediction curve is mostly fixed and is moved up or down depending on the IOL
constant
Do not take into account the individual geometry of each IOL model
66. Assumption that anterior chamber dimensions are related to axial length: The
LONGER the axial length, the DEEPER the anterior chamber, and the SHORTER
the axial length, the SHALLOWER the anterior chamber
However, 80% of short eyes have large crystalline lenses but a normal anterior
chamber anatomy in the pseudophakic state
Assumption that anterior chamber dimensions are related to cornea power:
Eyes with STEEP corneas have DEEP anterior chambers and eyes with FLATTER
corneas have SHALLOW anterior chambers
Relying on the axial length and the central corneal power to predict the postoperative
position of the IOL implant
67. d = Effective lens position
ACD = Measured anterior chamber depth of the eye (corneal vertex to the anterior lens
capsule)
AL = axial length of the eye ( the distance from the cornea vertex to the vitreoretinal
interface)
a = Moves the power prediction curve up/ down˳
a1 = Measured anterior chamber depth
a2 = Measured axial length
d = a + (a1 × ACD) + (a2 × AL)˳
68. For the Haigis formula, the a constant moves the power prediction curve up, or down,˳
same way that the A-constant, Surgeon Factor, or ACD does for the SRK/T, Holladay and
Hoffer Q
Both the a1 and the a2 constants are used to vary the shape of the power prediction
curve, changing the power based on the central corneal power, anterior chamber
depth, axial length and individual lens geometry.
Importance of ACD: An error of 1 mm affects the postoperative refraction by approx.
1.0 D in myopic eye, 1.5 D in emmetropic eye and up to 2.5 D in hyperopic eye
The geometry of many IOL models may not be the same for all powers. When this is the
case, it would be helpful if a formula was able to take this information account.
With three lens constants, the Haigis formula is able to make adjustments adding or
subtracting power when necessary, based on actual observed results for a specific
surgeon and the individual geometry of an intraocular lens implant.
69.
70. INTRAOCULAR LENS POWER CALCULATION USING
IOLMASTER AND VARIOUS FORMULAS IN SHORT EYES
To evaluate the predictability of intraocular lens (IOL) power calculations using the IOLMaster and four different IOL power
calculation formulas (Haigis, Hoffer Q, SRK II, and SRK/T) for cataract surgery in eyes with a short axial length (AL)
Included 25 eyes with an AL shorter than 22.0 mm that underwent uneventful phacoemulsification with IOL implantation from July
2007 to December 2008 at Seoul National University Boramae Hospital.
Preoperative AL and keratometric power were measured by the IOLMaster.
Postoperative refractive errors two months after surgery were measured using automatic refracto-keratometry (Nidek) and
were compared with the predicted postoperative power.
The mean absolute error (MAE) was defined as the average of the absolute value of the difference between actual and
predicted spherical equivalences of postoperative refractive error.
The differences in the MAE according to the four IOL calculation formulas in the three IOL groups were analyzed
Purpose
Methods
Roh, Y. R., Lee, S. M., Han, Y. K., Kim, M. K., Wee, W. R., & Lee, J. H. (2011). Intraocular lens power calculation using IOLMaster and
various formulas in short eyes. Korean Journal of Ophthalmology, 25(3), 151-155.
71. Results
The constants used in the four formulas of the IOL Master in three intraocular lens (IOL) subtypes
Means and standard deviations of
the absolute errors the four
intraocular lens calculation
formulas
•The MAE was
smallest in the Haigis formula
(0.37 ± 0.26 D), followed by
those of the SRK/T (0.53 ± 0.25 D),
SRK II (0.56 ± 0.20 D),
and Hoffer Q (0.62 ± 0.16 D) formulas
72. Proportion of the absolute errors (AE)
less than 1 diopter (D) according to
the intraocular lens formulas
The proportion of AE less than 1 D
was greatest in the Haigis formula
(96%), followed by those in the SRK
II (88%), SRK-T (84%), and Hoffer Q
(80%) formulas
Means and standard deviations of the
mean predicted errors (PE) of the four
intraocular lens calculation formulas
•PE showed several myopic shifts and was
smallest in the Haigis formula (-0.21 ± 0.22
D), followed by those of the SRK II (-0.41 ±
0.28 D), SRK/T (-0.45 ± 0.28 D), and Hoffer
Q (-0.59 ± 0.28 D) formulas
73. MAE and PE results consistently showed that the Haigis formula was the most
accurate of the four formulas in eyes with an AL shorter than 22.0 mm
Conclusion
74.
75. IOL power calculations were first developed over 100 years ago.
First generation: “single variable” formulas
Measurement of axial length
An assumed anterior chamber depth (ACD) of 4.5 mm
Third generation:
1988-Holladay 1 formula added keratometry to offer the first “two variable”
formula, which helped improve accuracy in short and long eyes.
Holladay 1, Hoffer Q, SRK-T :
Assumed anterior segment size was directly related to axial length resulted in
“surprise” outcomes esp in small eye
76. In 1993, Dr Holladay led a worlwide study involved 34 cataract surgeons to
determine which of 7 variables were relevant for predictors of effective lens
position (ELP).
A large data set of from 34,000 eyes was collected and analyzed to determine
relative significance of each variable, as shown in Figure 1.
Findings:
1. “We were surprised to learn that horizontal
white-to-white measurements emerged as
the next most important variable relate to
ELP after axial length and Ks,” remarked Dr.
Holladay.
2. “We also proved that there is almost no
correlation between axial length and size of
the anterior segment in 80-90% of eyes.”
77. The results from this study :
led to the release of Holladay 2 formula.
Invention of an easy-to-use program that allowed for data entry of the new
variables and instant calculation of Effective Lens Position (ELP) and the
appropriate IOL power selection (aka HIC.SOAP).
Led to a new paradigm of evaluating eyes by both their axial length (short,
normal, long) and their anterior segment size (small, normal, large).
78. There are now nine eye types – not just three – that could be used to classify a
given patient’s eye (Figure 2).
The WTW measurements demonstrated that:
•Short axial length eyes (<21 mm), 80% would be
considered normal and 20% would be considered
small in terms of anterior segment size.
•Normal axial length eyes (21-26 mm) had an equal
distribution of eyes being of either large (2%) or
small (2%) anterior segment size.
•Long axial length (>27 mm). 90% would be
considered normal and 10% considered as large in
terms of anterior segment size.
79. Holladay 2 formula determines Effective Lens Position (ELP) using 7
parameters :
All 7 parameters can be used to calculate IOL power by input into
Holladay IOL Consultant & Surgical Outcomes Assessment Program
(HIC.SOAP).
80. Holladay IOL Consultant & Surgical Outcomes Assessment Program
(HIC.SOAP).
Traditionally, 5 variables can be measured with:
ACD, LT & AL : Standard ultrasound biometry.
K & WTW : Autokeratometer or corneal topography
81.
82. Holladay 2 formula has been considered as one of the most accurate
IOL formula today. (Srivannaboon et al. 2013)
Holladay 2 has emerged as the “state of the art” IOL calculation
formula and today is the leading formula used by US surgeons.
(Hill, 2005)
Holladay 2: Currently most sophisticated formula
Accuracy
Predictability
83. •This formula has been found to be highly accurate for a large
variety of patient eyes.
84. The IOLMaster 500 by Carl Zeiss is the only instrument on the market that has the
Holladay 2 formula inside the unit.
IOL Master 500
The ZEISS IOLMaster®
500 is the gold standard in optical biometry.
It measures:
1. Axial length
2. Corneal radii/ power
3. White to white
4. AC depth
Formula: Holladay 1, Holladay 2, Haigis, SRK 2, SRK-T, Hoffer
IOL measurement instruments need to transfer the data to an external computer as
well as purchase of a separate software package for Holladay 2 calculation.
(Mahdavi, 2011)
88. Developed by Thomas Olsen from University Eye Clinic, Aarhus Hospital, Aarhus,
Denmark in the late 1980s at a time when the regression formulas were dominant.
The Olsen formula uses paraxial & exact ray tracing based on physical data to avoid
the errors of the ‘thin lens’ formula.
The true net power of the cornea is calculated and it is not necessary to fudge the
effective lens plane (ELP)
Use the information of the exact IOL position from C-constant directly in the formula.
89. SRK/T formula and the Holladay –
use corneal height (H), which is
calculated from the corneal
curvature and diameter.
Olsen – from preop ACD and lens thickness
(LT)
91. I) CALCULATION OF CORNEAL POWER
METHODS CONVENTIONAL
KERATOMETRY
GULLSTRAND BINKHORST
Curvature Only measure front curvature Assume P proportional to A
surface (6.8 / 7.7 = 0.833)
Use value of 4/3
Physiological n Use ficititious n 1.376 -
Equivalent n 1.3375 1.3315 1.3333
The difference in calculated power almost 1D –
might introduce a prior error of IOL calculation
POWER DETERMINATION OF AN IOL IN SITU
1.3315 1.333
Accurate estimation of front lens surface could
be obtained with no significant off-set error
Result a significant off-set error
DETERMINATION OF EFFECTIVE CORNEAL POWER
92. Conventional thick lens formula
Apply a total dioptric power from thick lens
formula, it results the refractive index as
follow:
Total dioptric power of
the thick lens
Dioptric power of
the front surface
Dioptric power
of the back surface
93. II) MEASUREMENT OF THE AXIAL LENGTH
The AL measured by ultrasound ≠ true AL
“retinal” spike originate from VR interface
Compression of the cornea (contact technique)
So, the term ‘retinal thickness’ was introduced as a corrective term in order to
eliminate error.
Previously, large error raised in extreme short & long eye due to velocity
assumption.
The avg velocity from cornea to retina is 1550 m/s
Avg velocity in extreme myopia (increase) & hyperopia change
To correct AL acc to shift of velocity, the AL can be corrected with equation:
RealAx = Ax/MeanVel – Lthick / LensVel) x AqueousVel + LThick
94. III) THE ACD PREDICTION
ACD prediction plays significant role in the IOL power calculation.
Previously, lack of empirical data on postop position of the implant
(postop ACD) – tend to result myopic error (overest IOL power) in
short eye.
The method to predict the postop ACD in a given eye based on the
actual preop measurements of the eye.
95. Olsen proposed his regression formula for the predicted postop ACD as follows:
This formula apply to phakic eyes. The coefficient will change in pseudophakia and
aphakic eyes.
ACDpost = ACDmean + 0.12H + 0.33 ACDpre + 0.3T’ + 0.1L –
5.18
ACDpost = Expected postop ACD of the IOL (in mm)
ACDmean = Average postop ACD of the IOL (in mm)
H = Height of cornea seg based on keratometry and corneal diameter
ACDpre = Preop ACD(mm)
T’ = Lens thickness (mm)
L = Axial length (mm)
96. IV) THE IOL OPTIC
In order to calculate the power according to Gaussian Optics, it is
necessary to know the position of the principal plane of the IOL
optic.
This position is important in determining the effective power of the
lens within the eye.
All the dioptric power of a planoconvex lens is on one surface and
thus that surface represents the effective lens plane.
With a biconvex lens, the effective lens plane is ‘inside’ the lens.
97. Defines the position of the IOL as a
fraction of capsular bag size.
Predict the final IOL position from the
preoperative ACD and lens thickness.
Produce better results of accurate
predictions for both short and long
eyes compared to Haigis.
It works in any type of eye including
post-LASIK eyes!
C - Constant
98. - Uses ray tracing to get the
preop lens thickness and ACD
to derive C, which can be
thought of as a fraction of the
preoperative lens thickness.
- This C constant is then used to
determine where the IOL will
come to rest in the eye
99. IOLc = ACDpre + C x LTpre
IOLc = Center of the IOL
ACDpre = preop ACD (including corneal thickness)
LTpre = preop thickness of the crystalline lens
C = A constant related to the IOL type determined as the
mean value in a representative
sample.
Based on the observation after standardized lens surgery and in-the-bag implantation,
the IOL tends to locate itself in a defined manner that is predictable according to the
formula:
100. Determine the phakic axial length with no axial length corrections.
The greatest benefits of the Olsen formula for improving power
prediction accuracy compared with the other formula were noted
especially in the extreme short & long eyes.
Perform consistently well in short, normal, and long eyes, having a
lower bias with axial length compared with the conventional formula.
101. Featured with the Olsen IOL calculation
formula for optimum prediction
accuracy.
Pair with the innovative concept of the
C-constant, so the surgeon gets a
sophisticated tool for accurate IOL
prediction in all kind of human eyes.
Measured all intraocular distances,
including CCT, ACD, lens thickness in
one shot laser.
102. Hill, W. E., & Mesa, A. (2002). The Haigis formula for IOL power calculation.Geriatric Ophthalmology, 1(1), 8.
Charalampidou, S., Cassidy, L., Ng, E., Loughman, J., Nolan, J., Stack, J., & Beatty, S. (2010). Effect on refractive outcomes
after cataract surgery of intraocular lens constant personalization using the Haigis formula. Journal of Cataract & Refractive
Surgery, 36(7), 1081-1089.
Mahdavi, S. 2011.IOLMaster 500 and Integration of the Holladay 2 Formula for IOL Calculations. Available at
www.sm2strategic.com.
Mahdavi, S. The IOLMaster and its Role in Modern Cataract Surgery, November 2011, available at www.sm2strategic.com.
Srivannaboon, S. Chirapapaisan, C. et al. Accuracy of Holladay 2 Formula Using IOLMaster Parameters in the Absence of
Lens Thickness Value. Graefe's Archive for Clinical and Experimental Ophthalmology. November 2013, Volume
251, Issue 11, pp 2563-2567.
http://www.haag-streit.com/de/product/biometry/olsen-formula-and-lens-thickness.html
http://ophthalmologytimes.modernmedicine.com/ophthalmologytimes/news/modernmedicine/modern-medicine-
news/biometry-iol-power-formulae-improve-outc
http://www.medscape.com/viewarticle/820900_4
http://haag-streit-usa.com/customer-support/olsen-formula-download.aspx
http://www.reviewofophthalmology.com/content/i/3592/c/59832/#sthash.E6LV4naF.dpuf
103.
104. Using the correct IOL calculation formula is important for good
surgical outcomes.
SRK I and II regression formulae are now regarded as obsolete.
The Hoffer Q, Holladay I, and SRK/T formulae are all commonly
used.
More recent formulae: the Holladay II, Haigis or Olsen ,are not
currently built into most biometry software, but available in
certain equipment like IOL Master 500.
In order to make the leap into refractive cataract surgery and
lens exchange optimization, adoption of third-generation
formulas is necessary, and use of fourth-generation formulas is
preferable (Tyson, 2006)
105. Axial length (mm) Formula
< 20 mm
Holladay II
20-22 mm
Hoffer Q
22-24.5 mm
SRK/T / Hoffer Q/Holladay (average)
> 24.5-26 mm
Holladay I
> 26 mm
SRK/T
Astbury & Ramamurthy, 2006
Editor's Notes
&quot;Surgeon Factor&quot; for the Holladay I formula, the &quot;ACD&quot; for the Hoffer Q formula and the &quot;A Constant&quot; for the SRK/T formula.
More accurate than 1st & 2nd generations theoretical formulas
1st gen
-simple because ACD was replaced by an A-constant individual to each IOL style
-surgeon develop their own personalized A-constant for each IOL style.
2nd gen (1980)
-replaced the constant ACD in their respective formulas with one that varied based on AL
Use the corneal power in two ways
corneal power is directly used in the vergence calculation to predict the postoperative refraction.
corneal power is used in the prediction of effective lens position (ELP), which is the depth of the IOL relative to the cornea.
myopic-LASIk, if the lower postoperative corneal power values are used, the calculated ELP will be erroneously anterior
results in implantation of a lower-power IOL, predisposing to a post-operative hyperopic refractive error.
Introduces less error in post-refractive surgery eyes than other single-K formulae (Want et al, 2004)
If double-K formulae are not available, the single-K Hoffer-Q formula may be useful.
Axial Length
changes the IOL power about 2.5 to 3 times (more in hyperopic eyes)
Corneal power
alters the IOL power in a ratio of nearly 1:1
Optimized theoretical formula with regression techniques for ACD.
Formulas vary the ACD based on the patient&apos;s axial length and corneal curvature
ACD = pACD
+ 0.3 (AL – 23.5)
+ (tan K)²
+ 0.1M (23.5 – AL)² (tan 0.1 (G-A)²)
IF AL ≤ 23mm: M = +1, G = 28
IF AL &gt; 23mm: M = -1, G = 23.5
IF ACD &gt; 6.5; ACD = 6.5
IF ACD &lt; 2.5; ACD = 2.5
- 0.99166
Predicted ref error increased as ACD decreased (in short eyes)
Therefore, ACD should be taken into consideration when evaluating the accuracy of the IOL power calculation formulae in short eyes. (Eom et al, 2014)
SRK/T: myopes
The SRK formula uses the following equation to calculate IOL power: P = A – BL - CK, where P is the implant power for emmetropia; L is the axial length (mm); K is the average keratometry (D); and A, B, and C are constants. The values of B and C are 2.5 and 0.9, respectively, and the value of A varies with the IOL design and the manufacturer.
Over the years, surgeons discovered that the SRK formula is best used in eyes with average AL, between 22.00 and 24.50 mm; a subsequent formula, the SRK II, was developed for use in long and short eyes.3 In this formula, a correction factor was added to increase the lens power in short eyes and decrease it in long eyes: P = A1 – 0.9K -2.5L. For eyes with AL of less than 20.00 mm, a numerical value of 3.00 is added to the A constant; a numerical value of 2.00 is added if the AL measures between 20.00 and 20.99, a numerical value of 1.00 if the measurement is between 21.00 and 21.99, and -0.50 if the AL is greater than 24.50 mm.
SRK/T also incorporates empirical regression methodology for optimization, resulting in greater accuracy.
1:1 relationship with the A-constants:
if A decreases by 1 diopter,
IOL power decreases by 1 diopter.
http://www.healio.com/ophthalmology/cataract-surgery/news/print/ocular-surgery-news/%7Be7d4e5c3-56ff-4223-ba1e-dff8602f152b%7D/refining-the-a-constant-yields-more-accurate-refractive-results-after-cataract-surgery
The A-constant of the IOL depends on its location within the eye. A more anteriorly placed lens such as an anterior chamber IOL (red) will have a lower A-constant than a posterior chamber IOL that is meant to be placed in the capsular bag (blue). When a lens is placed in an unexpected location, such as in the ciliary sulcus, the A-constant will change. In cases in which we plan to implant an anterior chamber IOL, the correct adjustment of the calculated posterior chamber IOL power is also related to the A-constant. Using the example above in which we calculated that a +21.0 D IOL with an A-constant of 118.4 implanted in the capsular bag would give a plano result, using an anterior chamber IOL with an A-constant of 115.4 would mean that a +18.0 D anterior chamber IOL should give the same plano refraction after surgery.
Hoffer Q – short eyes
Holladay 1 – long eyes
SRK/T very long eyes