A Scan- Basics and Update
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Dr. Pravin Madhukar Bhat discusses advancements in cataract surgery, emphasizing the importance of accurate biometry and IOL power calculations in achieving optimal refractive outcomes. Various techniques for measuring axial length and calculating intraocular lens power are reviewed, highlighting the evolving methodologies, including ultrasound biometry and optical coherence biometry, as well as the significance of appropriate IOL formulas for different patient scenarios. The document also addresses special circumstances affecting IOL power determination in cases like corneal refractive surgery and pediatric cataracts.
![ Immersion Technique
The currently preferred A-scan method is the
immersion technique.
Unlike the applanation echogram, the immersion
technique produces an additional spike corresponding
to the probe tip. This spike is produced from the tip of
the probe within the coupling fluid.
Axial length is calculated from the measured time and
the assumed average speed that sound waves travel
through the eye.
Length correction can be performed simply using the
following formula:
True length = [corrected velocity/measured velocity] ×
measured length](https://image.slidesharecdn.com/dr-170502151328/85/A-Scan-Basics-and-Update-10-320.jpg)
![ IOL FORMULA
There are two major categories of IOL formulae
Theoretical Formula
This formula is based on an optical model of the eye. An optics equation is solved to
determine the IOL power needed to focus light from a distant object onto the retina.
The most popular formula in this group is the Binkhorst formula. This is based on
sound theory. All the theoretical formulae can be algebrically transformed into the
following
P=[N/(L−C)]—[NK/(N−KC)]
where, P=Dioptric power of the lens for emmetropia,
N=Aqueous and vitreous refractive index, L=Axial length (mm),
C=Estimated postoperative anterior chamber depth (mm), and
K=Corneal curvature [D].
Binkhorst has made a correction in his formula for surgically induced flattening of the
cornea, using a corneal index of refraction of 1.333.
D=1336 (4r−a)/(a−d) (4r−d)
where, D=Dioptric power of IOL in aqueous humor,
1336=Index of refraction of vitreous and aqueous,
r=Radius of curvature of the anterior surface of the cornea,
a=Axial length of the globe (mm), and d=Distance between the anterior cornea and the
IOL.
Theoretical formulae help the surgeon to anticipate what should result, not what
will result from implantation.](https://image.slidesharecdn.com/dr-170502151328/85/A-Scan-Basics-and-Update-21-320.jpg)
A Scan- Basics and Update
- 1. A Scan- Basics and Update Dr. Pravin Madhukar Bhat Director,Prathamesh Eye Clinic, Pune
- 3. INTRODUCTION Innovative techniques and advanced technology have greatly improved cataract surgery over the past few years. There is an increased quest for accuracy, and patients are now seeking better results. As a result, accurate biometry and power precision of IOLs have gained greater importance. Several factors influence the refractive outcome. Axial length (AL) measurements are essential for determining the accuracy of the IOL calculation and are probably the element with the largest potential for error. Methods are still evolving, but ultrasound (US) biometry and partial coherence interferometry (PCI) are the most commonly used methods for determining the IOL power.
- 4. Traditionally, axial lengths have been measured using ultrasound biometry, a time consuming exam that requires skilled biometrists. Contact A scan ultrasonography is a well established method for measuring axial lengths but immersion A- scan technique is potentially more accurate, since it does not require indentation of the cornea. More recently, PCI has emerged as a new modality for biometry with the advantages of being fast, non invasive and less dependent on technician expertise. The precision of IOL power calculations depends on more than just accurate biometry, or the correct formula, but in reality is a collection of interconnected nuances. If one item is inaccurate, the final outcome will be less than optimal.
- 5. Components to Calculate IOL Power
- 6. Axial Length Measurement Axial length is defined as the distance from the anterior corneal surface to the retinal pigment epithelium. By A-scan biometry, errors in axial length measurement account for 54% of IOL power error when using two-variable formulas. Optical coherence biometry has been shown to be significantly more accurate and reproducible and is rapidly becoming the prevalent methodology for the measurement of axial length. Ultrasound: Axial length has traditionally been measured using ultrasound biometry. By measuring the time required for a portion of the sound beam to return to the ultrasound probe, the distance can be calculated (d = v × t)/2. Clinically, applanation and immersion techniques have been most commonly used.
- 7. Applanation Technique With the applanation technique, the ultrasound probe is placed in direct contact with the cornea. After the sound waves exit the transducer, they encounter each acoustic interface within the eye and produce a series of echoes that are received by the probe.
- 9. The axial length is the summation of the anterior chamber depth, the lens thickness, and the vitreous cavity. The applanation technique requires direct contact with the cornea, compression will typically cause the axial length to be falsely shortened. This method of ultrasound biometry is highly operator- dependent.
- 10. Immersion Technique The currently preferred A-scan method is the immersion technique. Unlike the applanation echogram, the immersion technique produces an additional spike corresponding to the probe tip. This spike is produced from the tip of the probe within the coupling fluid. Axial length is calculated from the measured time and the assumed average speed that sound waves travel through the eye. Length correction can be performed simply using the following formula: True length = [corrected velocity/measured velocity] × measured length
- 12. Optical Coherence Biometry Introduced in 2000, and has proved to be an exceptionally accurate and reliable method of measuring axial length. Principle is similar to conventional ultrasound A-scan which uses ultrasonic pulse echo- imaging technique. The patient is asked to fixate on an internal light source to ensure axiality with the fovea. When the reflected light is received by the instrument, the axial length is calculated using a modified Michelson interferometer.
- 14. Advantages of optical coherence biometry: It can measure pseudophakic, aphakic,and phakic IOL eyes. It can also measure through silicone oil. It uses a much shorter wavelength than ultrasound, so axial length can be more accurately obtained. It permits accurate measurements when posterior staphyloma are present. The IOL Master also provides measurements of corneal power and anterior chamber depth, enabling the device to perform IOL calculations using newer generation formulas, such as Haigis and Holladay 2. Both the IOL Master and immersion ultrasound biometry have been shown to produce a postoperative refractive error close to targeted values
- 15. Since optical biometry measures to the center of the macula, it gives the refractive axial length versus the anatomic axial length achieved with ultrasound biometry. Optical biometry also incorporates actual thickness of the retina, whereas ultrasound adds a standard 200um to the axial length. Partial coherence interferometry based biometry presents an alternative for precise ocular measurements, used not only for axial length, but anterior chamber depth, pachymetry and lens and retinal thickness measurements. Limitations: Inability to measure through dense cataracts and other media opacities that obscure the macula; due to such opacities or fixation difficulties, approximately 10% of eyes cannot be accurately measured using the IOL Master . Measurements not possible in patients with associated nystagmus, tremors or lid abnormalities cannot fixate well. Needs vision of atleast 20/200 for fixation
- 17. Keratometry Errors in corneal power measurement can be an equally important source of IOL power calculation error, as a 0.50 D error in keratometry will result in a 0.50 D postoperative error at the spectacle plane. A variety of technologies are available,including manual keratometry, automated keratometry, and corneal topography. These devices measure the radius of curvature and provide the corneal power in the form of keratometric diopters using an assumed index of refraction of 1.3375. Important sources of error are corneal scars or dystrophies that create an irregular anterior corneal surface. These lesions can often be seen with slit lamp biomicroscopy, their impact on corneal power measurements can best be assessed by examining keratometric or topographic mires.
- 18. Anterior Chamber Depth Measurement A-scan biometers and the IOL Master calculate anterior chamber depth as the distance from the anterior surface of the cornea to the anterior surface of the crystalline lens. In some IOL calculation formulas, the measured anterior chamber depth is used to aid in the prediction of the final postoperative position of the IOL (known as the effective lens position, or the ELP).
- 19. Which Formula should use? ?????
- 20. IOL Calculation Formulas The first IOL power formula was published by Fyodorov and Kolonko in 1967 and was based on schematic eyes. The other formulas from Colenbrander, Hoffer, and Binkhorst incorporated ultrasound data. In 1978, a regression formula was developed by Gills, followed by Retzlaff, then Sanders and Kraff, based on analysis of their previous IOL cases. This work was amalgamated in 1980 to yield the SRK I formula. In the 1980s, further refinement of IOL formulas occurred with the incorporation of relationships between the position of an IOL and the axial length as well as the central power of the cornea. There are two major types of IOL formulas. One is theoretical, derived from a mathematical consideration of the optics of the eye, while the other is empirically derived from linear regression analysis of a large number of cases.
- 21. IOL FORMULA There are two major categories of IOL formulae Theoretical Formula This formula is based on an optical model of the eye. An optics equation is solved to determine the IOL power needed to focus light from a distant object onto the retina. The most popular formula in this group is the Binkhorst formula. This is based on sound theory. All the theoretical formulae can be algebrically transformed into the following P=[N/(L−C)]—[NK/(N−KC)] where, P=Dioptric power of the lens for emmetropia, N=Aqueous and vitreous refractive index, L=Axial length (mm), C=Estimated postoperative anterior chamber depth (mm), and K=Corneal curvature [D]. Binkhorst has made a correction in his formula for surgically induced flattening of the cornea, using a corneal index of refraction of 1.333. D=1336 (4r−a)/(a−d) (4r−d) where, D=Dioptric power of IOL in aqueous humor, 1336=Index of refraction of vitreous and aqueous, r=Radius of curvature of the anterior surface of the cornea, a=Axial length of the globe (mm), and d=Distance between the anterior cornea and the IOL. Theoretical formulae help the surgeon to anticipate what should result, not what will result from implantation.
- 22. Regression Formula (Empirical Formula) The regression formulae or empirical formulae are derived from empirical data and are based on retrospective analysis of postoperative refraction after IOL implantation. The most popular regression formula is the SRK formula which was developed by Sanders, Retzlaff and Kraff in 1980. This is P=A−2.5 L−0.9 K Where, P=Implant power to produce emmetropia, L=Axial length (mm),K=Average keratometer reading, and A=Specific constant for each lens type and manufacture. The SRK formula calculates the IOL power by linearly regressing the results of previous implants. If the Binkhorst formula predicts that a 28- diopter lens should be used, the SRK formula will predict that a 26-diopter lens should be used. In lenses with low power, if the Binkhorst formula predicts that a 10-diopter lens is necessary, the SRK will predict that a 12-diopter lens should be used.
- 23. The new generation formulas: Formulas to be detailed in the following include: SRK(I,II), Hoffer Q, Holladay(I,II), Olson and the more recently formulas of Haigis d-formula and Lin’s S-formula. A. SRK formula: 1. SRK I formula: It is basic regression formula. It is given by P=A−2.5 L−0.9 K Where, P=Implant power to produce emmetropia, L=Axial length (mm),K=Average keratometer reading, and A=Specific constant for each lens type and manufacture. 2. SRK II formula: Here A- constant is adjusted to different axial length ranges. It is given as P=A1−2.5 L−0.9 K Where A1= Adjusted constant A1 = A+3 if L<20mm; A1 = A+2 if L=20-21mm A1 = A+1 if L=21-22mm; A1 = A if L= 22-24.5mm; A1 = A-0.5 if L>24.5mm 3. SRK/T formula: Regression formula for ACD is used to calculate IOL power based on Fyodorov formula. This formula is more accurate than SRK I and II. ACD post = ACD*3.336+Corneal height(H), Where ACD is related to the manufacturer’s A constant by ACD= 0.62467*A*68.747 B. Hoffer Q formula (1993): P=f(A,K,Rx,pACD) which is a function of A= Axial length, K= avarage corneal refractive power, Rx= refraction, pACD= personalized ACD constant.
- 24. C. Holladay formula: Holladay I: The initial formula uses basic Surgeon Factor. It can be calculated from the A constant provided by lens manufacturer. Holladay II: The IOL power is calculated based on the Binkhorst formula as in Holladay I. D. Olson formula (2003): Regression formula ACD post= ACD mean+ 0.12H+0.33 ACD pre+0.3T’+0.1L*5.18 Where H is the corneal height, T’ is the natural lens thickness. This formula only apply to phakic eyes, for aphakic or pseudophakic eyes the coefficients will change. E. Haigis formula: It uses three constant to set both the position and shape of a power prediction curve. d= a0+(a1*ACD)+(a2*AL) Where d= the ELP, ACD= Anterior Chamber Depth of the eye, AL= Axial length of the eye, a0 constant= same as lens constant for the different formulas given before, a1 constant= tied to ACD, a2 constant= Measured axial length. The a0, a1, a2 constants are derived by multivariable regression analysis. F. Lin’s S-Formula It includes the effect due to natural lens and primary IOL which are totally neglected by other formulas S=ELP+gT g=1/[1+Z”(P1/P2) Z”=1-T(P2/1336) Where T is the IOL thickness, and the geometry factor is determined by the ratio of the IOL front and back surface power P1/P2. g could be positive(P1/P2>0) or negative (P1/P2<0)
- 25. Confused?????
- 26. Here is the solution……. The Holladay 1 formula, works well for eyes of normal to moderately long axial lengths, while the Hoffer Q has been reported to be better suited to normal and shorter axial lengths.
- 27. The Second and Third Generation of IOL Formulas Commonly used lens constants do not take variations into account. These include: SRK/T formula—uses an “A-constant,” Holladay 1 formula—uses a “Surgeon Factor,” Hoffer Q formula—uses a “Pseudophakic Anterior Chamber Depth” (pACD). Optimized constants for each formula for positive diopter and negative-diopter IOLs. Constant Positive-Diopter IOL Negative-Diopter IOL Haigis a0 5.74 -4.01 SRK/T A 126.63 104.43 Holladay 1 sf 10.46 -6.48 Hoffer Q pACD 16.15 -4.86
- 30. Special circumstances and solution…….
- 31. Capsular Bag to Ciliary Sulcus IOL Power Conversion A reduction in an IOL power is typically required with an unanticipated intraoperative tear in the posterior lens capsule. The power adjustment necessary between the capsular bag and the ciliary sulcus will depend on the power of the capsular bag IOL . The important concept is that for stronger intraocular lenses, the reduction in power must be greater. For very low IOL powers, no reduction in IOL power is required. Capsular bag IOL power Ciliary sulcus power adjustment +35.00 D to +27.50 D –1.50 D +27.00 D to +17.50 D –1.00 D +17.00 D to +9.50 D –0.50 D +9.00 D to -5.00 D No change
- 32. Determining IOL Power Following Corneal Refractive Surgery The true corneal power following corneal refractive surgery is difficult to obtain by any form of direct measurement. The measurement method can be divided into those that require preoperative data and those that do not. Methods requiring Historical Data Clinical History Method: Feiz-Mannis IOL Power Adjustment Method Masket IOL Power Adjustment Method Topographic Corneal Power Adjustment Method Methods Requiring No Historical Data: Hard Contact Lens Method Modified Maloney Method Hyperopic Corneal Refractive Surgery Corneal Transplantation There is presently no reliable method for calculating IOL power for eyes undergoing combined corneal transplantation and cataract removal with IOL implantation. This is because it is impossible to accurately predict the central power of the donor graft.
- 33. IOL Power Calculation in patients with Silicone Oil Best carried out with the patient seated as upright as possible, especially if the vitreous cavity is partially filled with silicone oil. In the recumbent position, the less dense silicone oil will shift away from the retina, toward the anterior segment. This can lead to confusion as to the correct interpretation of the position of the retinal spike. The refractive index of silicone oil is much less than that of vitreous. Hence, it acts as negative lens in the eye. To prevent the silicone oil from altering the refractive power of the posterior surface of the IOL, it is preferable to implant polymethyl methacrylate (PMMA) convexplano lenses, with the plano side oriented toward the vitreous cavity and preferably over an intact posterior capsule. For patients who may possibly undergo a silicone oil procedure at some point in the future, it is recommended that bilateral baseline axial length measurements be carried out.
- 34. IOL Power Calculation in Pediatric Cataract In infants, there is rapid growth of the eye and thus increase in axial length during the first 2 years of life. In toddlers and older children, the eye continues to grow, although at a slower pace. As against the axial length, the corneal power drops considerably during the first 2 years of life. Between the ages 2 to 5 years axial length growth slows to about 0.4 mm per year and only increases another 1 mm from 5 to 10 years, while corneal power remains stable. It is recommended that childrens’ eyes should be undercorrected at the time of surgery, to offset the myopic shift that occurs in there growing eyes. For children under 2 years they advice to undercorrect the IOL Power by 20%, since axial length and keratometry readings change rapidly. For children older than 2 years they advice to undercorrect IOL power by 10%. This helps in minimizing the need for an IOL exchange later in life, when myopic shift occurs.
- 35. IOL Power Calculation in Patients with High Myopia Posterior pole staphyloma temporal to the fovea is commonly present in eyes with axial lengths longer than 30 mm. The distance from corneal vertex to fovea is 0.5 to 1.5 mm shorter than the distance from corneal vertex to the bottom of the staphyloma. Hence, current third and forth generation IOL power calculation formulas have a tendency to give the IOL power lower than what is necessary, leaving patients with postoperative hyperopia. The use of B-scan ultrasonography to identify the location of a posterior pole staphyloma is necessary. Few refinements in preoperative measurement techniques helps to improve the accuracy of IOL calculation in eyes with extreme myopia.
- 37. Accuracy of IOL power: Common pitfalls The factors which significantly affects the accuracy of IOL power calculations are: 1.The error in preoperative biometry with regards to the difference between post and preoperative axial length measurement. 2.The position of the implantation of intraocular lens. 3. The style of intraocular lens 4.The preoperative corneal astigmatism 5.Surgically induced corneal astigmatism 6. The post operative asigmatism 7. The true corneal power (Post- LASIK) 8. The formulas used to find IOL power 9. Assumptions of thin layer or 2-optics system
- 39. Future Trends As technology and patient expectation increases, one must continue to look to improve our own precision. Formulas: Holladay IOL consultant (HIC) program uses Holladay 2 formula and performs complex power calculation for the surgeon. It has various other new features, but it still not widely available. Oslens’s formula uses pre- operative refraction and lens thickness for increasing accuracy. Software based: Okulix A new biometric computer program to stimulate whole pseudophakic eye aims to reduce calculation error and ensure a more reliable estimation of IOL strength. This approach separates the errors due to measurement and those due to calculation, helping us in correcting them better. Newer Machines: The Pentacam and Orbscan have already been used widely for corneal ectasia, but are now beginning to be utilized for precise corneal power measurements. (28) IOL with tolerance It has been suggested that manufacturers reduce the internal – tolerance levels of IOLs to +/- 0.25D, thereby increasing accuracy. However, these parameters are not routinely provided by manufacturers to clinicians.
- 40. Now its time to conclude……
- 41. Takehome message The refractive results of cataract surgery has greatly improved due to recent advances in biometry. Accurate measurement of preoperative axial length is required for accurate IOL power calculation. The methodology for accurately calculating IOL power in normal and complex eyes has improved dramatically in recent years. Future advances are needed in all areas, including methods of measuring corneal power, predicting effective lens position, and perhaps even measuring axial length. The ultimate solution may be an IOL whose spherical and astigmatic power and higher order aberrations can be modified postoperatively. Ideally, such an IOL could be modified multiple times to adapt to the patient’s changing visual needs and to compensate for aging changes of the cornea.
- 42. THANK YOU
Editor's Notes
- #43: Thank you