- Email: [email protected]
Ophthalmic Biometry
Ophthalmic Biometr y Karolinne Maia Rocha, MD, PhD, Ronald R. Krueger, MD, MSE* KEYWORDS Biometry IOL calculation Refractive surgery
AXIAL EYE LENGTH MEASUREMENTS Instruments and Methods The measurement of axial eye length is one of the most important steps for IOL lens power calculation. An error in axial length measurement of 1 mm can cause an error in IOL power of 2.5 D (approximately). The continual refinements of ultrasound use in ophthalmology are important in minimizing this error. Although laser interferometry (IOL Master, Zeiss, Germany) was developed to increase the accuracy of biometry measurements, in some eyes that are unable to fixate, it is not possible to perform accurate examinations with this method.1 Optical coherence biometry is a noncontact and operator-independent method that emits an infrared beam which is reflected back from the retinal pigment epithelium. The reflected light beam is captured by the instrument, and the axial length is calculated by the interferometer. The patient needs to fixate on the device’s internal light to allow for axiality with the fovea. Patients with dense nuclear or subcapsular cataracts, patients
with retinal detachment, and patients for whom cooperation is poor need to be evaluated by the ultrasonic methods.
A-scan Biometry An A-scan is currently used for biometric calculations, but in some cases the precision of the measurements can be optimized by a B-scan. Clinical decisions can be made during dynamic examinations. In eyes with staphyloma or nanophthalmos, the A-scan can be guided by the B-scan. A-scan biometry includes two main techniques: the contact method and the immersion technique. Contact method In the contact (applanation) method, the ultrasound probe directly touches the cornea. In the echogram for the axial eye length measurement, the first spike represents the probe tip placed on the cornea, followed by the anterior lens capsule, posterior lens capsule, vitreous cavity, retina, sclera, and orbital tissue echoes (Fig. 1). The contact technique is completely examiner dependent because it requires direct contact and anterior compression of the cornea. Previous studies have demonstrated a mean shortening of axial length by 0.1 to 0.33 mm using the contact technique compared with immersion technique.2–5 Immersion technique Because the immersion method eliminates compression of the globe, this technique has been shown to be more precise than contact biometry (Fig. 2). In the immersion technique, a scleral shell filled with fluid is placed over the cornea while the patient lies supine. The probe is immersed in the fluid overlying the cornea. Clinically, this method is important in eyes with a small axial length (high hyperopia, microphthalmos, nanophthalmos).
Cole Eye Institute, Cleveland Clinic Foundation, 9500 Euclid Avenue, Mail code i-32, Cleveland, OH 44195, USA * Corresponding author. E-mail address: [email protected] (R.R. Krueger). Ultrasound Clin 3 (2008) 195–200 doi:10.1016/j.cult.2008.04.005 1556-858X/08/$ – see front matter ª 2008 Published by Elsevier Inc.
ultrasound.theclinics.com
Cataract surgery and intraocular lens (IOL) implantation are currently evolving into a refractive procedure. The precision of biometry is crucial for meeting expectations of patients undergoing cataract surgery. Moreover, the optimal results for new IOLs being developed, such as toric, multifocal, accommodative, and aspheric, all depend on the accuracy of biometry measurements. For all of these reasons, biometry is an important and currently relevant topic to be discussed. The fundamental points for accurate biometry include the axial length measurements, corneal power calculation, and IOL position (effective lens position [ELP]), the selection of the most appropriate formula, and its clinical application.
196
Rocha & Krueger
Table 1 Sound velocities for axial length measurements
Fig. 1. Contact A-scan of a normal phakic eye. The spikes correspond to corneal surface (C), anterior (AL) and posterior lens capsule (PL), and retina (R).
Velocity Settings Sound waves travel at different speeds according to the physical properties of the medium. The ultrasound velocity varies in relation to the medium within the eye, IOL materials, and even axial length (Tables 1 and 2).6,7 In a normal phakic eye, the average ultrasound velocity is 1555 m/s. In eyes with a short axial length (w20 mm), it is 1560 m/s, whereas in longer eyes it is 1550 m/s. This difference is due to an inverse proportional shift in the axial ratio of solid to liquid as the eye increases in length.
Special Clinical Situations Special cases include eyes containing silicone oil, posterior pole staphyloma, and pseudophakic eyes. Silicone oil The higher refractive index and slower sound velocity (980 m/s) of silicone oil in comparison
Medium
Velocity (m/s)
Soft tissue Cornea Aqueous/vitreous Crystalline lens Silicone oil
1550 1641 1532 1641 980
with the normal vitreous impairs the biometry accuracy. A-scan echograms usually seem longer than the real axial length in eyes filled with silicone oil. Careful evaluation of individual eyes should be taken to avoid a hyperopic error in these eyes. During the A-scan measurements, the patient should be positioned as upright as possible to keep the silicone oil in contact with the retina and to avoid it shifting into the anterior chamber. Overall, the baseline axial length should be measured, if possible, before silicone oil injection. The IOL Master, using optical coherence tomography laser interferometry, has shown satisfactory results when calculating IOL power in silicone oil–filled eyes.8 Posterior staphyloma The possibility of a posterior staphyloma should be considered in all eyes with high axial myopia, particularly when axial length is difficult to measure and is greater than 26 mm (Fig. 3). In these cases, the retinal peak is difficult to capture during the A-scan measurement. B-scan ultrasonography is a complimentary method that should be considered in these cases.9 An axial immersion B-scan is a variant method that is able to obtain an echogram that highlights the central echoes of the cornea, the anterior and posterior lens, and macula
Table 2 Average sound velocities according to lens status
Fig. 2. Immersion A-scan of a normal phakic eye. The spikes correspond to water bath (W), anterior and posterior corneal surface (C), anterior (AL) and posterior lens capsule (PL), and retina (R).
Eye Types
Velocity (m/s)
Phakic Aphakic Pseudophakic (PMMA) Pseudophakic (acrylic) Pseudophakic (silicone) Phakic (gas) Phakic (silicone oil) Aphakic (silicone oil)
1555 1532 1556 1549 1476 534 1139 1052
Abbreviation: PMMA, polymethyl methacrylate.
Biometry Holladay and Prager10 described a conversion factor to improve the accuracy of the axial length measurements in pseudophakic eyes. They considered the implant composition, the center thickness, and the amount of vitreous and aqueous crossed by the ultrasonic beam. The conversion factor was obtained by multiplying the center thickness of the IOL by a factor related to the implant’s ultrasonic velocity.
INTRAOCULAR LENS POWER CALCULATIONS Formulas Fig. 3. Immersion B-scan at 10 MHz demonstrating posterior pole staphyloma (arrow).
while displaying the optic nerve image. The B-scan is used to adjust the center of the cornea, lens, and fovea. Pseudophakic eyes During the measurement of pseudophakic eyes, the first spike represents the lens implant, followed by multiple signals. IOL implantation causes multiple echoes within the vitreous cavity (Fig. 4). The first spike (IOL echo) should also be aligned along the visual axis and should be of maximum height. Adjustments should be made according to the ultrasonic velocity of the IOL material. Nevertheless, the identification of retinal spikes can be difficult in some cases because of the proximity of the multiple echoes to the retina spike. In these cases, the examiner should decrease the gain for better identification of the retina spike.
Fig. 4. Immersion A-scan of a pseudophakic eye. The spikes correspond to water bath (W), anterior and posterior corneal surface (C), intraocular lens implant (IOL), and retina (R). Multiple spikes (A; artifacts) are a result of IOL implant.
First generation First-generation formulas included regression analysis of previous IOL implantation cases and the predicted IOL position (ACD), which depended upon a specific constant for each IOL. In 1967, Fedorov and colleagues11 published the first formula for IOL calculation based on schematic eyes. Subsequently, Colenbrander12 described his formula, followed by Hoffer13 in 1974. In 1975, Binkhorst14 published a formula that was widely used in United States. Regression analysis was described by Sanders and Kraff15 in 1980, followed by the SRK-I16 comparison to the other formulas. The SRK formula was superior to the other formulas by having a smaller range of error. Second generation The predictive relationship between the IOL position within the posterior chamber and the axial length was described to improve the accuracy of first-generation formulas. This direct relationship was calculated by different methods as demonstrated by the SRK-II formula.17 Third generation The third-generation formulas assumed that the IOL position was related to the axial length. Long eyes would have a deep anterior chamber, whereas short eyes would have a shallow anterior chamber. It has since become well known that this assumption is not valid; hence, at the extremes of axial length, the third-generation formulas produce considerably variable results. In 1988, Holladay and colleagues18 incorporated the surgeon factor (SF) to the secondgeneration formulas. With this factor, they described the relationship between corneal steepness and the IOL position. The Holladay 1 formula considered the distance from the cornea to the iris plane and from the iris to the posterior chamber IOL position (SF). Retzlaff and colleagues19 in1990 modified the Holladay 1 formula by incorporating the A constants to the SRK/T formula (theoretic).
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198
Rocha & Krueger Hoffer20 modified his own formula in 1993 by replacing the regression formula with a theoretic formula (Hoffer Q). The Hoffer Q formula has been demonstrated to be clinically more accurate than the Holladay 1 and SRK/T formulas in eyes shorter than 22.0 mm.
readings following laser corrections for myopia and underestimate for hyperopia. The mean of the paracentral cornea measurements (3-mm zone) does not evaluate the real central corneal power (flatter zone). Three methods are described to estimate the post refractive surgery K value.
Fourth generation The fourth-generation formulas introduced innovative approaches for IOL calculation as follows:
Clinical history method The estimate of the central corneal power after refractive surgery is obtained by subtracting the difference between preoperative and postoperative spherical equivalent error from the average keratometry power before refractive surgery. Both the preoperative corneal power (keratometry) and the preoperative and postoperative refractive errors are necessary to acquire the final K using this method. This important information should be obtained from the refractive surgeon.
The Haigis’ formula uses three constants for effective lens position settings.21 The formula also includes the ACD (distance of the corneal vertex to the anterior lens capsule) and the AL (distance from the corneal vertex to the macula). The constants are derived by regression analysis and produce an IOL-specific and surgeon-specific factor for different anterior chamber depths and axial lengths. The Holladay 2 formula uses IOL thickness, corneal power, corneal diameter, ACD measurements, lens thickness, axial length, refractive error, and age to obtain and refine the estimated scaling factor (ESF). A database of 35,000 patients was used to create the Holladay 2 formula.
Selection of the Best Formula In 1993, Hoffer published an important article regarding the eye’s axial length and formulas. It had been shown that, within the normal range of axial length (22.0 to 24.5 mm), almost all formulas yield the same or similar results; however, at the extremes of axial length, the formulas begin to differ.20 The Holladay 1 formula was the most accurate in eyes from 24.5 to 26.0 mm, whereas the SRK/T worked more adequately in very long eyes (>26.0 mm). The Hoffer Q formula was the most accurate for short eyes (<22.0 mm). More recently, the performance of the Holladay 2 formula was shown to be comparable with that of the Hoffer Q formula in short eyes (<22.0 mm) in a study with 317 eyes.22 Nevertheless, the original Holladay 1 formula was more accurate in eyes with average and medium-long axial lengths.
Special Clinical Situations Post refractive surgery Laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and radial keratotomy (RK) change the corneal architecture by flattening the cornea surface. RK for myopia correction flattens the anterior and posterior cornea surfaces while laser ablation changes anterior corneal curvature. Standard keratometry and topography measurements of the cornea power (K) after refractive surgery tend to overestimate the K
Contact lens method The hard contact lens method requires a known contact lens base curve (BC) and refractive power (PC) in diopters, and the spherical equivalent refraction with (SEcl) and without (SE) the contact lens. By this method, the final estimate corneal power (K) after refractive surgery is calculated as follows:
K 5 BC 1 PC 1 ðSEcl SEÞ The contact lens method should not be used when a cataract or other media opacities compromise the accuracy of the refraction.23 K value obtained by topography Maloney described a method, further modified by Wang and colleagues24, to obtain the central corneal power after refractive surgery. The final K value is obtained by placing the cursor at the center of the topography axial map, multiplying that value by 1.114, and then subtracting the product by the posterior corneal power (6.1 D). Double K formulas Another source of postoperative error following refractive surgery is related to the ELP calculation. The ELP is the distance between the surfaces of the cornea (vertex) to the plane of the IOL. Third-generation formulas assume the K power and axial length to estimate the ELP. When using these formulas, very flat keratometric corneal power following refractive surgery will produce a false shallow postoperative ELP. As a result, the calculated IOL power will be underestimated, ensuing in a hyperopic error. In 2003, Chamon25 described a ‘‘double K’’ method by using the preoperative and postoperative corneal power for IOL calculation after refractive surgery using the Holladay 1 formula. The preoperative K value was determined by topography and the postoperative K value by the
Biometry clinical history method. When the preoperative K value is unknown, 44.0 D is considered as the preoperative value and the effective refractive power (EffRP) of the Holladay Diagnostic Summary–EyeSys Corneal Analysis System (Dallas, Texas) as the postoperative value. Aramberri26 published the double K method using the SRK/T formula. The SRK/T formula was modified to use the preoperative K value to estimate the ELP and the post refractive surgery K value (clinical history method) to calculate IOL power by the vergence formula. The Holladay 2 formula contains the double K entry for post refractive surgery cases. If the preoperative K value is unknown, the formula suggests the 43.86 D value. Post radial keratotomy and cataract surgery Special attention should be given to patients with previous RK who undergo cataract surgery. Transient hyperopia is commonly observed in the immediate postoperative period.27,28 The stromal edema around the radial incisions and even the opening of the incisions flatten the center of the cornea. The hyperopic shift may need on average 8 to 12 weeks to complete resolution. Any hasty decision, such as IOL exchange or laser corrections, should not be taken during this period.
REFERENCES 1. Lege BA, Haigis W. Laser interference biometry versus ultrasound biometry in certain clinical conditions. Graefes Arch Clin Exp Ophthalmol 2004;242:8–12. 2. Shammas HJ. A comparison of immersion and contact techniques for axial length measurement. J Am Intraocul Implant Soc 1984;10:444–7. 3. Schelenz J, Kammann J. Comparison of contact and immersion techniques for axial length measurement and implant power calculation. J Cataract Refract Surg 1989;15:425–8. 4. Hrebcova J, Vasku A. [Comparison of contact and immersion techniques of ultrasound biometry]. Cesk Slov Oftalmol 2008;64:16–8 [in Czech]. 5. Olsen T, Nielsen PJ. Immersion versus contact technique in the measurement of axial length by ultrasound. Acta Ophthalmol (Copenh) 1989;67: 101–2. 6. Hoffer KJ. Ultrasound velocities for axial eye length measurement. J Cataract Refract Surg 1994;20: 554–62. 7. Holladay JT. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg 1997;23: 1356–70.
8. Habibabadi HF, Hashemi H, Jalali KH, et al. Refractive outcome of silicone oil removal and intraocular lens implantation using laser interferometry. Retina 2005;25:162–6. 9. Zaldivar R, Shultz MC, Davidorf JM, et al. Intraocular lens power calculations in patients with extreme myopia. J Cataract Refract Surg 2000;26:668–74. 10. Holladay JT, Prager TC. Accurate ultrasonic biometry in pseudophakia. Am J Ophthalmol 1989;107: 189–90. 11. Fedorov SN, Kolonko AI. A method of calculating the optical power of the intraocular lens. Vestn Oftalmol (Moscow) 1967;4:27–31. 12. Colenbrander MC. Calculation of the power of an iris clip lens for distant vision. Br J Ophthalmol 1973;57: 735–40. 13. Hoffer KJ. Intraocular lens calculation: the problem of the short eye. Ophthalmic Surg 1981;12:269–72. 14. Binkhorst RD. The optical design of intraocular lens implants. Ophthalmic Surg 1975;6:17–31. 15. Sanders DR, Kraff MC. Improvement of intraocular lens power calculation using empirical data. J Am Intraocul Implant Soc 1980;6:263–7. 16. Sanders D, Retzlaff J, Kraff M, et al. Comparison of the accuracy of the Binkhorst, Colenbrander, and SRK implant power prediction formulas. J Am Intraocul Implant Soc 1981;7:337–40. 17. Sanders DR, Retzlaff J, Kraff MC. Comparison of the SRK II formula and other second generation formulas. J Cataract Refract Surg 1988;14: 136–41. 18. Holladay JT, Prager TC, Chandler TY, et al. A threepart system for refining intraocular lens power calculations. J Cataract Refract Surg 1988;14: 17–24. 19. Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg 1990;16: 333–40. 20. Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993;19:700–12. 21. Haigis W. Strahldurchrechnung in Gau(beta)scher Optik. In: Proceedings of the fourth DGII-Kongress. Berlin Heidelberg New York: Springer; 1991. p. 233–46. 22. Hoffer KJ. Clinical results using the Holladay 2 intraocular lens power formula. J Cataract Refract Surg 2000;26:1233–7. 23. Zeh WG, Koch DD. Comparison of contact lens overrefraction and standard keratometry for measuring corneal curvature in eyes with lenticular opacity. J Cataract Refract Surg 1999;25:898–903. 24. Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004;111: 1825–31.
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Rocha & Krueger 25. Chamon W. A new approach for IOL calculation in refractive surgery patients. American Society of Cataract and Refractive Surgery (ASCRS): San Francisco, CA; April 14, 2003. 26. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg 2003;29:2063–8.
27. Bardocci A, Lofoco G. Corneal topography and postoperative refraction after cataract phacoemulsification following radial keratotomy. Ophthalmic Surg Lasers 1999;30:155–9. 28. Chen L, Mannis MJ, Salz JJ, et al. Analysis of intraocular lens power calculation in post-radial keratotomy eyes. J Cataract Refract Surg 2003;29:65–70.
AXIAL EYE LENGTH MEASUREMENTS Instruments and Methods The measurement of axial eye length is one of the most important steps for IOL lens power calculation. An error in axial length measurement of 1 mm can cause an error in IOL power of 2.5 D (approximately). The continual refinements of ultrasound use in ophthalmology are important in minimizing this error. Although laser interferometry (IOL Master, Zeiss, Germany) was developed to increase the accuracy of biometry measurements, in some eyes that are unable to fixate, it is not possible to perform accurate examinations with this method.1 Optical coherence biometry is a noncontact and operator-independent method that emits an infrared beam which is reflected back from the retinal pigment epithelium. The reflected light beam is captured by the instrument, and the axial length is calculated by the interferometer. The patient needs to fixate on the device’s internal light to allow for axiality with the fovea. Patients with dense nuclear or subcapsular cataracts, patients
with retinal detachment, and patients for whom cooperation is poor need to be evaluated by the ultrasonic methods.
A-scan Biometry An A-scan is currently used for biometric calculations, but in some cases the precision of the measurements can be optimized by a B-scan. Clinical decisions can be made during dynamic examinations. In eyes with staphyloma or nanophthalmos, the A-scan can be guided by the B-scan. A-scan biometry includes two main techniques: the contact method and the immersion technique. Contact method In the contact (applanation) method, the ultrasound probe directly touches the cornea. In the echogram for the axial eye length measurement, the first spike represents the probe tip placed on the cornea, followed by the anterior lens capsule, posterior lens capsule, vitreous cavity, retina, sclera, and orbital tissue echoes (Fig. 1). The contact technique is completely examiner dependent because it requires direct contact and anterior compression of the cornea. Previous studies have demonstrated a mean shortening of axial length by 0.1 to 0.33 mm using the contact technique compared with immersion technique.2–5 Immersion technique Because the immersion method eliminates compression of the globe, this technique has been shown to be more precise than contact biometry (Fig. 2). In the immersion technique, a scleral shell filled with fluid is placed over the cornea while the patient lies supine. The probe is immersed in the fluid overlying the cornea. Clinically, this method is important in eyes with a small axial length (high hyperopia, microphthalmos, nanophthalmos).
Cole Eye Institute, Cleveland Clinic Foundation, 9500 Euclid Avenue, Mail code i-32, Cleveland, OH 44195, USA * Corresponding author. E-mail address: [email protected] (R.R. Krueger). Ultrasound Clin 3 (2008) 195–200 doi:10.1016/j.cult.2008.04.005 1556-858X/08/$ – see front matter ª 2008 Published by Elsevier Inc.
ultrasound.theclinics.com
Cataract surgery and intraocular lens (IOL) implantation are currently evolving into a refractive procedure. The precision of biometry is crucial for meeting expectations of patients undergoing cataract surgery. Moreover, the optimal results for new IOLs being developed, such as toric, multifocal, accommodative, and aspheric, all depend on the accuracy of biometry measurements. For all of these reasons, biometry is an important and currently relevant topic to be discussed. The fundamental points for accurate biometry include the axial length measurements, corneal power calculation, and IOL position (effective lens position [ELP]), the selection of the most appropriate formula, and its clinical application.
196
Rocha & Krueger
Table 1 Sound velocities for axial length measurements
Fig. 1. Contact A-scan of a normal phakic eye. The spikes correspond to corneal surface (C), anterior (AL) and posterior lens capsule (PL), and retina (R).
Velocity Settings Sound waves travel at different speeds according to the physical properties of the medium. The ultrasound velocity varies in relation to the medium within the eye, IOL materials, and even axial length (Tables 1 and 2).6,7 In a normal phakic eye, the average ultrasound velocity is 1555 m/s. In eyes with a short axial length (w20 mm), it is 1560 m/s, whereas in longer eyes it is 1550 m/s. This difference is due to an inverse proportional shift in the axial ratio of solid to liquid as the eye increases in length.
Special Clinical Situations Special cases include eyes containing silicone oil, posterior pole staphyloma, and pseudophakic eyes. Silicone oil The higher refractive index and slower sound velocity (980 m/s) of silicone oil in comparison
Medium
Velocity (m/s)
Soft tissue Cornea Aqueous/vitreous Crystalline lens Silicone oil
1550 1641 1532 1641 980
with the normal vitreous impairs the biometry accuracy. A-scan echograms usually seem longer than the real axial length in eyes filled with silicone oil. Careful evaluation of individual eyes should be taken to avoid a hyperopic error in these eyes. During the A-scan measurements, the patient should be positioned as upright as possible to keep the silicone oil in contact with the retina and to avoid it shifting into the anterior chamber. Overall, the baseline axial length should be measured, if possible, before silicone oil injection. The IOL Master, using optical coherence tomography laser interferometry, has shown satisfactory results when calculating IOL power in silicone oil–filled eyes.8 Posterior staphyloma The possibility of a posterior staphyloma should be considered in all eyes with high axial myopia, particularly when axial length is difficult to measure and is greater than 26 mm (Fig. 3). In these cases, the retinal peak is difficult to capture during the A-scan measurement. B-scan ultrasonography is a complimentary method that should be considered in these cases.9 An axial immersion B-scan is a variant method that is able to obtain an echogram that highlights the central echoes of the cornea, the anterior and posterior lens, and macula
Table 2 Average sound velocities according to lens status
Fig. 2. Immersion A-scan of a normal phakic eye. The spikes correspond to water bath (W), anterior and posterior corneal surface (C), anterior (AL) and posterior lens capsule (PL), and retina (R).
Eye Types
Velocity (m/s)
Phakic Aphakic Pseudophakic (PMMA) Pseudophakic (acrylic) Pseudophakic (silicone) Phakic (gas) Phakic (silicone oil) Aphakic (silicone oil)
1555 1532 1556 1549 1476 534 1139 1052
Abbreviation: PMMA, polymethyl methacrylate.
Biometry Holladay and Prager10 described a conversion factor to improve the accuracy of the axial length measurements in pseudophakic eyes. They considered the implant composition, the center thickness, and the amount of vitreous and aqueous crossed by the ultrasonic beam. The conversion factor was obtained by multiplying the center thickness of the IOL by a factor related to the implant’s ultrasonic velocity.
INTRAOCULAR LENS POWER CALCULATIONS Formulas Fig. 3. Immersion B-scan at 10 MHz demonstrating posterior pole staphyloma (arrow).
while displaying the optic nerve image. The B-scan is used to adjust the center of the cornea, lens, and fovea. Pseudophakic eyes During the measurement of pseudophakic eyes, the first spike represents the lens implant, followed by multiple signals. IOL implantation causes multiple echoes within the vitreous cavity (Fig. 4). The first spike (IOL echo) should also be aligned along the visual axis and should be of maximum height. Adjustments should be made according to the ultrasonic velocity of the IOL material. Nevertheless, the identification of retinal spikes can be difficult in some cases because of the proximity of the multiple echoes to the retina spike. In these cases, the examiner should decrease the gain for better identification of the retina spike.
Fig. 4. Immersion A-scan of a pseudophakic eye. The spikes correspond to water bath (W), anterior and posterior corneal surface (C), intraocular lens implant (IOL), and retina (R). Multiple spikes (A; artifacts) are a result of IOL implant.
First generation First-generation formulas included regression analysis of previous IOL implantation cases and the predicted IOL position (ACD), which depended upon a specific constant for each IOL. In 1967, Fedorov and colleagues11 published the first formula for IOL calculation based on schematic eyes. Subsequently, Colenbrander12 described his formula, followed by Hoffer13 in 1974. In 1975, Binkhorst14 published a formula that was widely used in United States. Regression analysis was described by Sanders and Kraff15 in 1980, followed by the SRK-I16 comparison to the other formulas. The SRK formula was superior to the other formulas by having a smaller range of error. Second generation The predictive relationship between the IOL position within the posterior chamber and the axial length was described to improve the accuracy of first-generation formulas. This direct relationship was calculated by different methods as demonstrated by the SRK-II formula.17 Third generation The third-generation formulas assumed that the IOL position was related to the axial length. Long eyes would have a deep anterior chamber, whereas short eyes would have a shallow anterior chamber. It has since become well known that this assumption is not valid; hence, at the extremes of axial length, the third-generation formulas produce considerably variable results. In 1988, Holladay and colleagues18 incorporated the surgeon factor (SF) to the secondgeneration formulas. With this factor, they described the relationship between corneal steepness and the IOL position. The Holladay 1 formula considered the distance from the cornea to the iris plane and from the iris to the posterior chamber IOL position (SF). Retzlaff and colleagues19 in1990 modified the Holladay 1 formula by incorporating the A constants to the SRK/T formula (theoretic).
197
198
Rocha & Krueger Hoffer20 modified his own formula in 1993 by replacing the regression formula with a theoretic formula (Hoffer Q). The Hoffer Q formula has been demonstrated to be clinically more accurate than the Holladay 1 and SRK/T formulas in eyes shorter than 22.0 mm.
readings following laser corrections for myopia and underestimate for hyperopia. The mean of the paracentral cornea measurements (3-mm zone) does not evaluate the real central corneal power (flatter zone). Three methods are described to estimate the post refractive surgery K value.
Fourth generation The fourth-generation formulas introduced innovative approaches for IOL calculation as follows:
Clinical history method The estimate of the central corneal power after refractive surgery is obtained by subtracting the difference between preoperative and postoperative spherical equivalent error from the average keratometry power before refractive surgery. Both the preoperative corneal power (keratometry) and the preoperative and postoperative refractive errors are necessary to acquire the final K using this method. This important information should be obtained from the refractive surgeon.
The Haigis’ formula uses three constants for effective lens position settings.21 The formula also includes the ACD (distance of the corneal vertex to the anterior lens capsule) and the AL (distance from the corneal vertex to the macula). The constants are derived by regression analysis and produce an IOL-specific and surgeon-specific factor for different anterior chamber depths and axial lengths. The Holladay 2 formula uses IOL thickness, corneal power, corneal diameter, ACD measurements, lens thickness, axial length, refractive error, and age to obtain and refine the estimated scaling factor (ESF). A database of 35,000 patients was used to create the Holladay 2 formula.
Selection of the Best Formula In 1993, Hoffer published an important article regarding the eye’s axial length and formulas. It had been shown that, within the normal range of axial length (22.0 to 24.5 mm), almost all formulas yield the same or similar results; however, at the extremes of axial length, the formulas begin to differ.20 The Holladay 1 formula was the most accurate in eyes from 24.5 to 26.0 mm, whereas the SRK/T worked more adequately in very long eyes (>26.0 mm). The Hoffer Q formula was the most accurate for short eyes (<22.0 mm). More recently, the performance of the Holladay 2 formula was shown to be comparable with that of the Hoffer Q formula in short eyes (<22.0 mm) in a study with 317 eyes.22 Nevertheless, the original Holladay 1 formula was more accurate in eyes with average and medium-long axial lengths.
Special Clinical Situations Post refractive surgery Laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and radial keratotomy (RK) change the corneal architecture by flattening the cornea surface. RK for myopia correction flattens the anterior and posterior cornea surfaces while laser ablation changes anterior corneal curvature. Standard keratometry and topography measurements of the cornea power (K) after refractive surgery tend to overestimate the K
Contact lens method The hard contact lens method requires a known contact lens base curve (BC) and refractive power (PC) in diopters, and the spherical equivalent refraction with (SEcl) and without (SE) the contact lens. By this method, the final estimate corneal power (K) after refractive surgery is calculated as follows:
K 5 BC 1 PC 1 ðSEcl SEÞ The contact lens method should not be used when a cataract or other media opacities compromise the accuracy of the refraction.23 K value obtained by topography Maloney described a method, further modified by Wang and colleagues24, to obtain the central corneal power after refractive surgery. The final K value is obtained by placing the cursor at the center of the topography axial map, multiplying that value by 1.114, and then subtracting the product by the posterior corneal power (6.1 D). Double K formulas Another source of postoperative error following refractive surgery is related to the ELP calculation. The ELP is the distance between the surfaces of the cornea (vertex) to the plane of the IOL. Third-generation formulas assume the K power and axial length to estimate the ELP. When using these formulas, very flat keratometric corneal power following refractive surgery will produce a false shallow postoperative ELP. As a result, the calculated IOL power will be underestimated, ensuing in a hyperopic error. In 2003, Chamon25 described a ‘‘double K’’ method by using the preoperative and postoperative corneal power for IOL calculation after refractive surgery using the Holladay 1 formula. The preoperative K value was determined by topography and the postoperative K value by the
Biometry clinical history method. When the preoperative K value is unknown, 44.0 D is considered as the preoperative value and the effective refractive power (EffRP) of the Holladay Diagnostic Summary–EyeSys Corneal Analysis System (Dallas, Texas) as the postoperative value. Aramberri26 published the double K method using the SRK/T formula. The SRK/T formula was modified to use the preoperative K value to estimate the ELP and the post refractive surgery K value (clinical history method) to calculate IOL power by the vergence formula. The Holladay 2 formula contains the double K entry for post refractive surgery cases. If the preoperative K value is unknown, the formula suggests the 43.86 D value. Post radial keratotomy and cataract surgery Special attention should be given to patients with previous RK who undergo cataract surgery. Transient hyperopia is commonly observed in the immediate postoperative period.27,28 The stromal edema around the radial incisions and even the opening of the incisions flatten the center of the cornea. The hyperopic shift may need on average 8 to 12 weeks to complete resolution. Any hasty decision, such as IOL exchange or laser corrections, should not be taken during this period.
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