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  2. 2. OUTLINE • Introduction • Measurement of Corneal Curvature • Measurement of Axial Length • Formulas • Special circumstances • Pediatric eyes • Conclusion • References 16TH SEPTEMBER, 2020 8:00AM
  3. 3. INTRODUCTION When you walk into a store to purchase a pair of shoes, what comes to your mind (a) first? Colour? Make?... SIZE! The size of the shoes you get determines the outcome of fitting and good use for the wearer. Everything that is a pair must have a measurement or a size. Whether it is a pair of legs, of ears or the eyes etc, what you wear on them. Certain measures or formulas must be carried out to determine their accurate sizes – or power. Since it is inappropriate for one to have a power or size that is totally different from the other. They must have the same or slightly different measurement. 16TH SEPTEMBER, 2020 8:00AM
  4. 4. INTRODUCTION • The history of intraocular lenses (IOLs) began in 1949, when English ophthalmologist Harold Ridley implanted the first polymethylmethacrylate (PMMA) IOL in London. • Initially, other ophthalmologists strongly opposed the use of IOLs, and it took years of development and perseverance for the IOL to become the standard it is today. 16TH SEPTEMBER, 2020 8:00AM
  5. 5. INTRODUCTION • Implantation of an IOL serves to eliminate the numerous difficulties with aphakic spectacles correction which including: image magnification,  ring scotomata, peripheral distortion, a “jack-in-the-box” phenomenon 16TH SEPTEMBER, 2020 8:00AM
  6. 6. INTRODUCTION-IOL CALCULATION • To obtain the desired IOL power some parameters of the eye are measured calculated using the appropriate formulas. These parameters include: Measurement of Axial Length Measurement of Corneal Curvature Measurement of Anterior Chamber Depth 16TH SEPTEMBER, 2020 8:00AM
  7. 7. MEASUREMENT OF AXIAL LENGTH(AL) • The AL is the most important factor in the formulas for IOL calculations. • A 1-mm error in AL measurement results in a refractive error of approximately 2.35 D in a 23.5-mm eye. • The refractive error declines to only 1.75 D/mm in a 30-mm eye but rises to 3.75 D/mm in a 20-mm eye. • Therefore, accuracy in AL measurement is more important in short eyes than in long eyes 16TH SEPTEMBER, 2020 8:00AM
  8. 8. MEASUREMENT OF AXIAL LENGTH(AL) • There basically two widely used method for measuring AL. oUltrasonic measurement of axial length Immersion (non contact) technique Applanation (contact) technique. oOptical measurement of axial length optical coherence techniques. 16TH SEPTEMBER, 2020 8:00AM
  9. 9. ULTRASONIC MEASUREMENT OF AXIAL LENGTH • When A-scan ultrasonography is used to measure AL, we either assume a constant ultrasound velocity through the entire eye or measure each of the various ocular structures at its individual velocity. • A-scans measure not distance but rather the time required for a sound pulse to travel from the cornea to the retina. • Sound travels faster through the crystalline lens and the cornea (1641 m/s) than it does through aqueous and vitreous (1532 m/s). Even within the lens itself, the speed of sound can vary in different layers of nuclear sclerosis. • The measured sound transit time is converted to a distance by use of the formula d = tV 16TH SEPTEMBER, 2020 8:00AM
  13. 13. ULTRASONIC MEASUREMENT OF AXIAL LENGTH • The Applanation method may yield a shorter AL measurement that is also inconsistent and unpredictable. • An artificially shortened AL measurement occurs with inadvertent corneal indentation. • In the Immersion method, which is accepted as the more accurate of the 2 techniques, space is maintained between the probe and the cornea, eliminating corneal indentation 16TH SEPTEMBER, 2020 8:00AM
  14. 14. ULTRASONIC MEASUREMENT OF AXIAL LENGTH • In eyes with AL values greater than 25 mm, staphyloma should be suspected, especially when numerous disparate readings are obtained. • Such errors occur because the macula is located either at the deepest part of the staphyloma or on the “side of the hill.” • To measure such eyes and obtain the true measurement to the fovea, the clinician must use a B-scan technique. • Optical methods (eg,IOL Master, Lenstar) are very useful in such cases 16TH SEPTEMBER, 2020 8:00AM
  15. 15. OPTICAL MEASUREMENT OF AXIAL LENGTH • IOLMaster method-1999. In 2008-Lenstar LS900 . • Uses a partial coherence laser for AL measurement measures the time required for infrared light to travel to the retina. Because light travels at too high a speed to be measured directly, light interference methodology is used to determine the transit time and thus the AL. • Does not require contact with the globe, so corneal compression artifacts are eliminated. • developed such that its readings would be equivalent to those of the immersion ultrasound technique. 16TH SEPTEMBER, 2020 8:00AM
  16. 16. OPTICAL MEASUREMENT OF AXIAL LENGTH • In dense cataracts (especially posterior sub capsular cataracts), ultrasound biometry is still necessary (in 5%–8% of cataract patients). • Compared with ultrasonography, this technique provides more accurate, reproducible AL measurements. • In addition, optical measurement is ideal in 2 clinical situations that are difficult to achieve using ultrasonography: eyes with staphyloma and eyes filled with silicone oil. 16TH SEPTEMBER, 2020 8:00AM
  18. 18. MEASUREMENT OF CORNEAL CURVATURE (KERATOMETRY) • The central corneal power, K, is the second most important factor in the calculation formula • A 1.0 D error in corneal power causes a 1.0 D postoperative refractive error. • Corneal power can be estimated by keratometry or corneal topography, neither of which measures corneal power directly. 16TH SEPTEMBER, 2020 8:00AM
  19. 19. MEASUREMENT OF CORNEAL CURVATURE (KERATOMETRY) • Manual Keratometry Von Helmholtz keratometer The Javal-Schiotz keratometer • Automated Keratometry 16TH SEPTEMBER, 2020 8:00AM
  20. 20. MEASUREMENT OF CORNEAL CURVATURE MANUAL KERATOMETRY • Manual keratometry is a method to determine the true central optical power of the cornea by measuring the radius of curvature of the anterior surface. • It makes assumptions regarding the posterior surface (which cannot be measured by the instrument) and then converts the radius into diopters (D) by a simple equation: • D = 337.5/ r Where D = diopters, r = radius of curvature 16TH SEPTEMBER, 2020 8:00AM
  21. 21. MEASUREMENT OF CORNEAL CURVATURE MANUAL KERATOMETRY • The standard manual keratometer measures only a small central portion (from 2.8 to 3.2 mm diameter) of the cornea and views the cornea as a convex mirror. • Both front and back corneal surfaces contribute to corneal power, but the keratometer power “reading” is based on measurement of the radius of curvature of only the front surface and assumptions about the posterior surface. 16TH SEPTEMBER, 2020 8:00AM
  23. 23. MEASUREMENT OF CORNEAL CURVATURE MANUAL KERATOMETRY • In certain situations, like irregular corneal contour or previous refractive surgery, or when the surgeon wants to better evaluate the astigmatism, corneal topography may be utilized. • Routine of calibrating manual keratometers • Average K reading • contact lenses must off 2 weeks prior to the keratometry exam for IOL power 16TH SEPTEMBER, 2020 8:00AM
  24. 24. MEASUREMENT OF CORNEAL CURVATURE AUTOMATED KERATOMETRY • The main advantage of automated over manual keratometry is simplicity: joystick focusing and two clicks allow accurate and repeatable measurements in a very short time • Moreover, hand held autokeratometers allow the measuring of patients in different postures—which can be useful for examination of children under general anesthesia, disabled or mentally retarded patients, etc. 16TH SEPTEMBER, 2020 8:00AM
  26. 26. MEASURING CORNEAL POWER WITH THE PENTACAM • The Pentacam is a newer imaging system that uses a single Scheimpflug camera to measure the radius of curvature of the anterior and posterior corneal surfaces, as well as the corneal thickness, for the calculation of corneal power. • This technology can provide the following corneal measurements: simulated keratometry (SimK), True Net Power, Equivalent K Reading (EKR), and Total Refractive Power. • In addition, it has been used to develop the BESSt formula. 16TH SEPTEMBER, 2020 8:00AM
  27. 27. MEASUREMENT OF ANTERIOR CHAMBER DEPTH • Anterior chamber depth (or ACD) because the optic of all IOLs in the early era was positioned in front of the iris, in the anterior chamber, or in the iris plane. • Because almost all IOLs today are positioned behind the iris, new terminology has been offered such as effective lens posit ion (ELP) by Holladay1 and actual lens position (ALP) by the FDA. • ELP is also used when referring to anterior chamber (AC) lenses, which do sit in the ACD. 16TH SEPTEMBER, 2020 8:00AM
  28. 28. FORMULAS • The original formulae were developed prior to 1980. They include the theoretical formulae and regression formulae • The theoretical formula are based on mathematic principles revolving around the schematic eye.(1st generation) • The regression formula is based on working on post operative outcomes (2nd generation) • Some others are mixed- 3rd & 4th generation. 16TH SEPTEMBER, 2020 8:00AM
  29. 29. 16TH SEPTEMBER, 2020 8:00AM
  31. 31. SRK FORMULA- SRK I • It is a regression formulae and most popular, described by Sanders D, Retzlaff J. and Kraff M. • The formula is based on the following equation: P = A - 2. 5L - 0.9K where P is the implant power for emmetropia,  L the axial length in millimeters, and K the average keratometric reading in diopters. • A is a constant varies with the implant design and the manufacturer. 16TH SEPTEMBER, 2020 8:00AM
  32. 32. SRK FORMULA- SRK II • The SRK II formula was introduced for such 'too long' and 'too short' eyeballs. • The SRK II formula is a modification of the original SRK formula with the addition of a correction factor that increases the lens power in short eyes and decreases it in long eyes. 16TH SEPTEMBER, 2020 8:00AM
  33. 33. 16TH SEPTEMBER, 2020 8:00AM
  34. 34. SRK FORMULA- SRK /T (THEORETIC) • In 1990, Retzlaff , modified the Holladay 1 formula to allow use of an A constant instead of a SF, calling it the SRK/ T theoretic formula. • SRK/T formula optimizes the prediction of postoperative ACD, retinal thickness AL correction and corneal refractive index. • It is recommended for eyes longer than 26.00mm. 16TH SEPTEMBER, 2020 8:00AM
  35. 35. HOLLADAY FORMULA-I • Produced by Jack Holladay in 1988 • Used axial length and keratometry to determine ELP • Takes into account AC depth, lens thickness and cornea radius • Calculate predicted distance from cornea to iris plane + distance from iris plane to IOL • Uses surgeon factor for optimization of formula (specific for each lens) • Work best for eyes between 24.5 to 26mm(medium long), useful for axial myopia and high corneal curvature(>45) 16TH SEPTEMBER, 2020 8:00AM
  36. 36. HOLLADAY FORMULA-2 • Fourth generation formula • Assumed anterior segment size was directly related to axial length but resulted in surprise outcomes especially in small eye • Findings -horizontal white-to- white measurement emerged as the next most important variable relate to ELP after AL and Ks. -there was almost no correlation between AL and size of anterior segment in 80-90% of eyes 16TH SEPTEMBER, 2020 8:00AM
  37. 37. HOLLADAY FORMULA-2 16TH SEPTEMBER, 2020 8:00AM
  38. 38. HOLLADAY FORMULA-2 16TH SEPTEMBER, 2020 8:00AM
  39. 39. HOLLADAY FORMULA-2 • Considered as one of the most accurate IOL formula • It emerged as the state of the art IOL calculation formula • The IOL master has the formula 16TH SEPTEMBER, 2020 8:00AM
  40. 40. HOFFER Q • Introduced by Dr Kenneth Hoffer in 1993 • Was developed to predict the pseudophakic ACD • It relies on a personalized ACD, AL and corneal curvature 16TH SEPTEMBER, 2020 8:00AM
  41. 41. HOFFER Q 16TH SEPTEMBER, 2020 8:00AM
  42. 42. HOFFER Q • Most accurate in short eyes <22.0mm, confirmed in large study of 830 eyes • Had the lower mean absolute error (MAE) for AL 20.0mm to 20.99mm 16TH SEPTEMBER, 2020 8:00AM
  43. 43. HAIGIS FORMULA • Developed by Wolfgang Haigis, found in Zeiss IOL Master software. • It has 3 constant and an accuracy close to that of Hoffer Q • By regression analysis, the 3 constant are calculated to individually adjust the IOL power prediction curve for each surgeon/IOL combination in such a way as to closely reproduce observed results over a wide range of AL and ACD. 16TH SEPTEMBER, 2020 8:00AM
  44. 44. HAIGIS FORMULA 16TH SEPTEMBER, 2020 8:00AM
  45. 45. HAIGIS FORMULA • With three lens constants, the Haigis formula is able to make adjustment adding or subtracting power when necessary, based on actually observed results for a specific surgeon and the individual geometry of an intraocular lens implant. • Recommended for eyes with short axial length. 16TH SEPTEMBER, 2020 8:00AM
  46. 46. OLSEN FORMULA • Developed by Thomas Oslen • The Oslen formula addresses 4 area of concern Calculation of the cornea power Measurement of axial length Anterior chamber depth prediction-method predict the postop ACD in a given eye based on actual preop measurements of the eye. IOL optics 16TH SEPTEMBER, 2020 8:00AM
  47. 47. OLSEN FORMULA 16TH SEPTEMBER, 2020 8:00AM
  49. 49. SPECIAL CASES • Intumescent cataracts will yield a 0.15 mm longer axial length resulting in a +0.4 -+0.5 hyperopia postoperatively. • For Aphakic eyes being planned for ACIOL or scleral fixated IOL, the appropriate A constant must be used and the mode of the machine changed to compensate for the change in speed of the sound waves.. 16TH SEPTEMBER, 2020 8:00AM
  50. 50. SPECIAL CASES • Silicone filled vitreous eyes- the sensitivity of the system should be increased to visualize the retinal echo spike and the components of the eye must be measured separately to reach an accurate result. • The usage of a standard sound velocity can lead to an error of up to 8 mm in such eyes. 16TH SEPTEMBER, 2020 8:00AM
  51. 51. SPECIAL CASES • After corneal refractive surgery, the K reading may not truly reflect the corneal power. • Hence the refractive history method or the contact lens method must be used to obtain corrected K value. 16TH SEPTEMBER, 2020 8:00AM
  52. 52. SPECIAL CASES • In eyes with high myopia, a B-scan examination is recommended to rule out a posterior staphyloma or other retinal pathologies. Identification of the posterior pole may be difficult. • While selecting the IOL power for a myope several factors are to be kept in mind. The surgeon should aim for a -0.50 D to -1.00 D postoperative refraction as most sedentary elderly will prefer being near sighted. 16TH SEPTEMBER, 2020 8:00AM
  53. 53. SPECIAL CASES • In patients with hypermetropia the aim should be to achieve emmetropia. • Here, the use of linear formulae can result in large errors in IOL power calculation in small eyes. 16TH SEPTEMBER, 2020 8:00AM
  54. 54. PEDIATRIC EYES • In children, it is wise practice to remove the cataract and use contact lens correction if the surgery is being performed within the first two years of life, because growth of the eye will result in a large myopic shift if IOL • After the age of two years, a myopic shift of 4-6 D is expected depending upon the age. 16TH SEPTEMBER, 2020 8:00AM
  55. 55. PAEDIATRIC IOL CALCULATION IN EEH KANO PERCENTAGE METHOD • Deduct a certain percentage from the IOL depending on the patients age • 0-2 years- 20% • 3-8 years-10% • Example- <2yr IOLP=24.00D 20%=4.80 IOLP=24-4.8=19.20D TARGET METHOD AGE TARGET 1 +3.00 2 +2.25 3 +2.00 4 +1.75 5 +1.50 6-7 +1.25 8 +1.00 9-10 +0.75 11-14 +0.50 16TH SEPTEMBER, 2020 8:00AM
  56. 56. 16TH SEPTEMBER, 2020 8:00AM
  57. 57. CONCLUSION • Intraocular lens implantation today is a very important in cataract surgery visual outcome. So the need for a good knowledge and accuracy of calculating IOL power can not be overemphasized. • The outcome of a successful cataract surgery is noticed in the post-cataract refraction. • E.g IOL Power inserted-+18.50Dfor-0.20 • Refraction after surgery=-0.50-1.00*180-6/24. • An error of -0.50D • 4 weeks later,refraction is:plano-6/12. 16TH SEPTEMBER, 2020 8:00AM
  59. 59. REFFERENCES • AAO Clinical Optics 2016-2017 • Elkington, A.R And Frank H.J, Clinical Optics 3rd Edition • Duane’s Ophthalmology • Brad Bowling, Kanski Clinical Ophthalmology A Systematic Approach, 8th Edition,2016 • IOL Power By Kenneth J. Hoffer 16TH SEPTEMBER, 2020 8:00AM

Notes de l'éditeur

  • Ridley made 2 decisions that were fortuitous for the development of IOL implantation: he used extracapsular cataract extraction (ECCE), and he placed the IOL in the posterior chamber. In addition, he experienced the first IOL complication, a power error of 16 D.
  • a decreased useful peripheral field.
  • Preoperatively, it is desirable to predict the power of the IOL which will render the individual patient emmetropic or in some cases, produce a desired refractive error.
  • where d is the distance in meters, t is the time in seconds, and V is the velocity in meters per second
  • The immersion technique is performed with the patient in the supine position. Topical anesthetic is instilled and a proper scleral shell is chosen.
    The 20 mm shell fits most eyes. The flared edges of the scleral shell are placed between the lids and the cup is filled with fluid, preferably gonioscopic solution.
    The ultrasound probe is immersed in the solution but kept 5-10 mm away from the cornea. The patient is asked to look with the fellow eye at a fixation point on the ceiling. The probe is then gently moved till it is aligned with the optical axis of the eye and the a-scan echogram on the panel is adequate. The reading is then taken.
  • With the contact technique, a drop of local anesthetic is instilled into each eye. The patient is examined in the seated position. The probe is positioned in front of the eye and the patient is asked to fixate on the red light in the probe.
    The probe is then brought forward to gently touch the cornea. Particular attention and care must be taken to ensure that the probe is not indenting the cornea.
    The probe is moved slightly up and down or to the side to optimize the echospikes displayed on the machine. Either the operator or the machine selects the optimum pattern and the reading is obtained.
  • -The initial spike is produced at the tip of the probe. It has no clinical significance.
    -The corneal spike is double-peaked, representing the anterior and posterior surfaces of the cornea.
    -The anterior lens spike is generated from the anterior surface of the lens.
    -The posterior lens spike is generated from the posterior surface of the lens.
    -The retinal spike is generated from the anterior surface of the retina. It is straight, highly-reflective, and tall whenever the ultrasound beam is perpendicular to the retina (as it should be during AL measurement).
    -The scleral spike is another highly-reflective spike generated from the scleral surface right behind the retinal spike, and should not be confused with it.
    -The orbital tissues create low reflective spikes behind the scleral spike.
  • IOLMaster (Zeiss-Meditec, Jena, Germany)
    Lenstar LS900 (Haag-Streit, Köniz,
  • Because this device requires the patient to fixate on a target, the length measured is the path the light takes to the fovea: the “visual” AL.
    The ocular media must be clear enough to allow voluntary fixation and light transmission. ,
  • The corneal radius of curvature can be calculated from the size of the reflected image.
    The principle by which the instrument measures the anterior corneal curvature is contingent upon accurately determining the size of a reflected image from the front surface of the cornea or, as it is called, the first Purkinje-Sanson image.

  • The keratometer projects an image onto the cornea and then measures the size of that image reflected from the corneal surface.
    Because of the small but continuous movement of the patient’s eyes, the keratometer doubles the reflected image and measures the image against itself, rather than against a fixed scale
    The examiner must align and focus the illuminated mires as well as modify their position to get the measurements needed.
    The device then converts image size into corneal radius using simple vergence relationships of convex mirrors.
    The instrument is now rotated to align the (-) signs in the same vertical meridian and the (+) signs in the same
    horizontal meridian. This will determine the axis of the pre-existing astigmatism.
    The left drum is rotated to superimpose the (+) signs and the horizontal measurement is read out. The right drum is now rotated to superimpose the (-) signs and the vertical measurement reading is recorded.
  • A problem arises in post-refractive corneal surgery eyes because the central 3 mm area is so much flatter that the image measured is larger than 3 mm, causing an error in obtaining the true “central curvature.”
    It is important to remember that the keratometer has to be calibrated every 6 months.

  • A table-mounted autokeratometer
    A handheld autokeratometer
  • The axial position for the IOL as it relates to the anterior surface of the cornea has historically been referred to as the ACD
    The ELP is required for all formulas, but is used in different forms. In the Binkhorst, Colenbrander, and Hoffer formulas, it is used directly and called the ACD. In the Hoffer Q formula, it is referred to as the pACD (personal ACD) and in the Holladay 1 formula it is calculated using a surgeon factor (SF) specific to each IOL style. In the SRK I and II and the SRK/ T, it is incorporated into the A constant specific to each IOL style.

  • Othe 1st generation
  • Be sure of the constant value of the IOL you are using while making the calculations.
    The SRK formula has been found to be reasonably accurate for eyes with axial lengths between 22mm and 24.5mm. These eyes constitute approximately 75% of cases, while 14% of cases have axial lengths greater than 24.5 mm, and 10% have axial lengths less than 22mm.
  • In 1988, Holladay proposed a direct relationship between the steepness of the cornea and the position of the IOL.
    ACD, ELP
  • SF should be personalized.
    A change in the true post-op AC depth will affect the refractive error status of the eye.
    A change in 1mm causes a 1.5 change in final refraction.
    SF constants must be predicted 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.
  • A large data set of from 34,000 eyes we collected and analyzed to determine relative significance of each.
  • Earlier Hoffer’s earlier formula
    Hoffer –ACD by AL
    Hoffer -pACD
  • ACD prediction-previously, lack of empirical data on postop position of the implant (postop ACD)- tend to result myopic error
  • Usually a factor of 0.72 gives a rough estimate of the IOL power.
    It is better to refer the patient to a centre capable of separate measurements for more accurate assessment
  • The problems are compounded in unilateral cases
    In the presence of monocular cataract in a myopic eye when the other eye is emmetropic, emmetropia should be aimed for if the myopia was induced by the cataract.
    However, if the patient has been functioning with monocular vision using the emmetropic eye for distance and the myopic eye for near, it is better to leave the operative eye myopic.
  • Under correcting the IOL power by around 3 D partially compensates for this.
    A greater under correction can lead to anisometropia and difficulty in amblyopia correction
    Residual myopia in adulthood can easily be corrected by spectacles, contact lenses or refractive surgery.
    As expected, biometry in children is difficult and may require general anesthesia.