Iol power calculation normal and post lasik eyes
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This document discusses intraocular lens (IOL) power calculation methods for eyes that have and have not undergone refractive surgery. It covers: 1. The evolution of IOL power calculation formulas from first to fourth generation formulas, which have improved accuracy by incorporating additional eye measurements and constants. 2. Sources of error in IOL power calculations from incorrect keratometry and axial length measurements. 3. Challenges in calculating IOL power for eyes that had refractive surgery due to changes in corneal shape and index of refraction. 4. Methods to estimate pre-operative keratometry and account for post-operative keratometry changes, including double-K, theoretical K, and tomography approaches.
Iol power calculation normal and post lasik eyes
- 1. IOL POWER CALCULATION NORMAL & POST LASIK EYES INDOREDRISHTI.WORDPRESS.COM
- 2. DR DINESH MITTAL DR SONALEE MITTAL DRISHTI EYE HOSP VIJAYNAGAR INDORE
- 8. 1 Cataract surgery has evolved into a refractive procedure with the goal of eliminating or significantly reducing the need for spectacle dependence. •2. One must consider not only the astigmatism induced by the cataract incision itself, but also the correction of preexisting astigmatism. •3. Incision length, depth, and distance from visual axis all affect astigmatism.
- 9. •4. LRIs are commonly used to correct preexisting astigmatism, and •published nomograms are helpful in tailoring a surgical approach. •5. Toric IOLs and excimer laser ablation are alternative approaches to correcting astigmatism in the patient for refractive cataract surgery. •No single approach is best suited for all patients.
- 10. INTRAOCULAR LENS CALCULATIONS •Choosing the appropriate IOL power is a major determinant of patient satisfaction with cataract surgery. •accurate measurements , •selecting calculations ), • and assessing the patient’s needs to determine postoperative refractive target
- 12. BIOMETRY •At minimum, 2 measurements reqd to calculate implant power: & ( ) •Precise measurements critical •error of 0.3 mm in axial length will result in a 1-D error in IOL power.
- 13. Axial Length •Axial length has been obtained utilizing . •This measurement is determined by ; • an ultrasound pulse is applied and the transit time through the eye is measured. Using estimated velocities of ultrasound waves through various media (ie, cornea, aqueous, lens, and vitreous) the distance travelled through the eye is calculated.
- 14. • The instrument should have an screen to differentiate a good measurement from a poor one. or spikes should be observed when the probe is aligned properly • These include the following: a tall peak for the cornea, tall peaks for the anterior and posterior lens capsule, tall peak for the retina, mod-erate peak for the sclera, and moderate-to-low peaks for orbital fat. If these spikes are not well seen, then the probe may be misaligned. A SCAN ULTRASOUND
- 15. •The must be performed carefully, as com- pressing the cornea will result in a shorter-than-expected measurement and should be taken and averaged. •If several are taken and differ by a significant amount, they should be readings can be obtained.
- 16. •It is also prudent to for comparison •the machine should be , checking measurements against an eye of known axial length.
- 18. Immersion technique The immersion technique may more accurately represent the true axial length because there is . In this technique, the patient lies in the supine position and a scleral shell is placed on the eye and filled with Goniosol. The ultrasound probe is placed in this solution and the beam is aligned with the macula by having the patient look at the probe tip fixation light. Although the immersion method may be strongly advocated by some users, applanation A-scan is the more commonly used method
- 20. Optical biometry: IOLMaster • In the last decade, the technique of optical coherence biometry was in- troduced by Haigis, which utilizes light rather than ultrasound to measure the length of the eye. The first device introduced was the IOLMaster (Zeiss), based on the principle of partial coherence interferometry using a 780- nm multimode laser diode.
- 22. •Measurements taken without contact to eye, thus eliminating variability due to an examiner technique. •The distance measured lies between the anterior surface of tear film & retinal pigmented pigment epithelium , which may be more physiologically accurate
- 23. patient asked to focus on a small red fixation light, & examiner maneuvers focusing spot within the measurement reticule, sampling areas until the best peak pattern is obtained. 5 to 20 measurements obtained until the readings differ by less than 0.1 mm. Maximal axial length measured 40 mm. IOLMASTER USE
- 24. IOL MASTER DISADVANTAGE The primary disadvantage of this optical device is that , such as a corneal scar, dense posterior subcapsular plaque, darkly brunescent cataract, or vitreous hemorrhage, will reduce the signal-to- noise ratio (SNR) to the point that .
- 25. A-scans should be under the following conditions • 1. Axial length is less than 22 mm or greater than 25 mm in either eye. • 2. The difference between the 2 eyes is greater than 0.3 mm. • 3. The measurements do not correlate with the patient’s refraction (ie, hyperopes should have shorter eyes, and myopes should have longer eyes).
- 26. Lens power calculation formulas evolved over past 30 yrs, since first theoretical formula for iris-supported iol published by Fyodorov in 1967.
- 28. First Generation • Initial formulas were , but with the availability of posterior chamber implants, consideration had to be given to the distance from the cornea to the implant (anterior chamber depth). By studying large numbers of cases, linear regression techniques were used to determine a formula for predicting emmetropic implant power. The most widely used regression formula was developed by Sanders, Retzlaff , and Kraff in 1980 and is known as the .
- 29. First Generation • where is the lens implant power is the axial length is average keratometry • and is the constant a theoretical value that relates the lens power to axial length and keratometry. It is specific to the design of the IOL and its intended position inside the eye. This number is specified by the IOL manufacturer.
- 30. Second Generation • The SRK formula is a linear equation derived by fitting collected data to a straight line. However, the optical system is nonlinear and begins to produce significant error with short or long eyes. To improve accuracy, the formula was modified, taking into consideration variation in axial length. • This en-hancement is known as the formula in which the :
- 31. Second Generation SRK II •A = A + 3 (AL < 20 mm) •A = A + 2 (20 mm < AL < 21 mm) •A = A + 1 (21 mm < AL < 22 mm) • A = A (22 mm < AL < 24.5 mm) •A = A – 0.5 (24.5 mm < AL)
- 32. Third Generation • Holladay further refined the theoretical formulas by proposing a . •Instead of factoring in anterior chamber depth, formula would calculate the distance from the cornea to iris plane and & distance from the iris plane to the IOL.
- 34. Third Generation • . This second variable was termed the , was specific to each lens, and had the ability to be personalized and adjust for any consistent bias in the surgeon’s results. Hoffer achieved the same effect via another approach in his . Retzlaff followed suit to take into consideration not only the position of the implant, but also incorporated a correction for the retinal thickness, thus developing the formula in 1990.
- 35. Fourth Generation • Haigis presented the notion that model should be considered in determining which formula to use. The geometry for a particular IOL model is not the same at all powers; therefore, the Haigis formula utilizes 3 lens constants to address it constant moves the power prediction curve up or down constant is tied to the anterior chamber depth constant is tied to the measured axial length
- 36. • Optimization of the Haigis formula requires collecting pre- and postop-erative data from over 200 patients in order to allow surgeon-specific optimization, which is available online. • In the late 1990s, the Holladay 2 IOL consultant software was introduced to improve upon predictability by incorporating additional data points. It requires 7 measurements including . Th is formula may be more precise in unusual eyes such as those that have undergone refractive surgery.
- 37. •One or more of the are generally programmed into A-scan biometers sold today. •The optical biometers are now incorporating the •Over time, some trends have emerged regarding which formulas to use in general categories of patients:
- 39. FORMULAE BASED ON MATHEMATICAL PRINCIPLES REVOLVING AROUND THE SCHEMATIC EYE FORMULAE WORKING BACKWARDS ON POSTOPERATIVE OUTCOMES GENERATION MIX OF BOTH
- 42. IOL CALCULATION POST REFRACTIVE SURGERY
- 43. INSTRUMENT ERROR INSTRUMENT ERROR IS BECAUSE KERATOMETERS READ AT 3.2 MM AND NOT AT THE CENTRE
- 44. INDEX OF REFRACTION ERROR THIS K READING ERROR OCCURS BECAUSE OF CHANGED RATIO OF RADIUS OF CURVATURE OF ANTERIOR AND POSTERIOR SURFACE OF CORNEA SECONDARY TO KERATO REFRACTIVE PROCEDURE
- 45. UNOPERATED EYES IN UNOPERATED EYES THE RATIO BETWEEN ANTERIOR AND POSTERIOR SURFACE OF CORNEA IS 1.21
- 46. MYOPIC OPERATED EYES IN OPERATED EYE THIS RATIO CHANGES
- 47. POST MYOPIA TREATED EYES KERATOMETRY OVER ESTIMATED
- 49. POST HYPEROPIA TREATED EYES KERATOMETRY UNDER ESTIMATED
- 50. KERATOMETRY DEPENDENT & INDEPENDENT FORMULA
- 51. HOW TO CALCULATE IOL POST KERATO REFRACTIVE SURGERY
- 52. ARAMBERRI DOUBLE K METHOD
- 53. PRE OP K READING FOR ELP
- 54. HOW TO DETERMINE PRE OP K
- 55. PRE OP K READING
- 56. PRE OP K READING
- 57. TRUE POST OP K READING FOR IOL POWER CALCULATION
- 58. MEASUREMENT OF POST OP K READING
- 59. POST OP K READING HISTORY DEPENDENT
- 60. POST OP K READING HISTORY INDEPENDENT
- 62. K CORRECTED METHODS SHAMMAS
- 63. K CORRECTED METHODS THEORETICAL
- 64. K CORRECTED KOCH & WANG
- 65. POST OP K READING TOMOGRAPHY METHODS
- 67. HEIDELBERG RAYTRACING STUDY FOUR FORMULAS ARE BEST HAIGIS HOFFER Q OLSEN 2 RAYTRACING
- 68. IOL CALCULATION BEST IN UNOPERATED EYES
- 69. IOL CALCULATION BEST POST KERATOREFRACTIVE SURGERY
- 70. ASCRS SITE FOR IOL POWER CALCULATION
- 71. THANK YOU DR DINESH DR SONALEE