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Potential source of error in axial length measurement in pseudophakes
LETTERS
tion, IOL, and anesthetic agent will broaden the discussion and enhance the overall results. JOAN S.K. NG, FRCS HELIOS T.C. LEUNG, OD, PHD DENNIS S.C. LAM, FRCS, FRCOPHTH Hong Kong, China References 1. Pucci V, Morselli S, Romanelli F, et al. Clear lens phacoemulsification for correction of high myopia. J Cataract Refract Surg 2001; 27:896 –900 2. Kora Y, Yaguchi S, Inatomi M, Ozawa T. Preferred postoperative refraction after cataract surgery for high myopia. J Cataract Refract Surg 1995; 21:35–38 3. Schaumberg DA, Dana MR, Christen WG, Glynn RJ. A systematic overview of the incidence of posterior capsule opacification. Ophthalmology 1998; 105:1213–1221 4. Koch DD, Liu JF, Gill EP, Parke DW II. Axial myopia increases the risk of retinal complications after neodymium-YAG laser posterior capsulotomy. Arch Ophthalmol 1989; 107:986 –990 ¨ , Ferliel ST. Posterior capsule opacification 5. Oner FH, Gunenc U after phacoemulsification: foldable acrylic versus poly(methyl methacrylate) intraocular lenses. J Cataract Refract Surg 2000; 26:722–726 6. Koch PS. Efficacy of lidocaine 2% jelly as a topical agent in cataract surgery. J Cataract Refract Surg 1999; 25:632– 634
Potential Source of Error in Axial Length Measurement in Pseudophakes n a recent study, Kora et al.1 reviewed factors that could contribute to errors in intraocular lens (IOL) power calculation for lens exchange and discussed several potential sources of error in axial length measurement. An additional potential source of error that was not discussed is failure to correct for the effect of the IOL on the speed of sound. In 1988, Milauskas and Marney2 reported that axial length measurements using the pseuodophakic A-scan mode are overestimated in patients with silicone IOLs. They attributed the overestimation to the difference between the speed of sound in silicone and poly(methyl methacrylate). In 1989 and 1993, Holladay and Prager3,4 derived the following formula to determine the correct axial length in a pseudophakic eye based on the ultrasonically measured axial length, the speed of sound in the IOL material, and the center thickness of the IOL being replaced:
I
Corrected Axial Length ⫽ Al ⫹ CF 1902
where AL ⫽ the axial length measured at aphakic sound velocity (1532 M/sec) and CF ⫽ the correction factor CF ⫽ CT共1 ⫺ 1532/S mat兲
where CT ⫽ the center thickness of the IOL (mm) and Smat ⫽ the speed of sound in IOL material (M/sec) Using a sound speed of 980 M/sec for a silicone IOL and a nominal lens thickness of 1.50 mm, they estimated that the true axial length is 0.84 mm less than the axial length measured at aphakic sound velocity in pseudophakic eyes with silicone IOLs. In 1995, Yang and coauthors5 showed that the use of a single sound speed (typically 1000 M/sec) in the formula for all silicone lenses could result in significant errors in IOL power prediction. They estimated that the use of a typical material sound speed, rather than the lens-specific speed, could create an error of at least 0.25 D in IOL power calculation when the difference between the 2 speeds exceeds 6%. They also showed that for lenses with an index of refraction less than 1.46, variations in center thickness that relate to lens power could also create significant errors in axial length measurement. Thus, the authors recommended that the surgeon obtain from the IOL manufacturer the material sound speed and center thickness for the specific power of the IOL being replaced. These values can then be applied to Holladay and Prager’s equation to correct for the axial length measured at aphakic sound speed. We recently evaluated 2 patients requiring IOL exchange because of large postoperative refractive errors with an IOL (Pharmacia model 912A) for which the material sound speed had not been determined. We contacted the manufacturer, who provided the material sound speed and diopter-specific lens center thickness. The axial length correction factors were then deter-
Table 1. Correction factor for Pharmacia model 912A. IOL Power (D)
Correction Factor* (mm)
5.0–16.5
0.4
17.0–22.0
0.5
22.5–27.5
0.6
28.0–30.0
0.7
*Rounded to the nearest 0.1 mm. Material sound speed @ 35°C ⫽ 1052 m/s.
J CATARACT REFRACT SURG—VOL 27, DECEMBER 2001
LETTERS
mined using the Holladay/Prager method (Table 1). The correction factors that we calculated were significantly different from those determined by Holladay and Prager using a sound speed of 980 M/sec and vary significantly with dioptric power. There was a significant difference in material sound speed and diopter-specific center thickness among different models of lenses from the same manufacturer. In the first patient, the axial length measured under immersion at aphakic velocity was 24.84 mm. Using the standard correction factor of ⫺0.84 mm would have resulted in a corrected axial length of 24.00 mm. Using the correction factor of ⫺0.44 mm that we calculated for his specific lens (Pharmacia Model 912A, ⫹16.5 D), however, resulted in a corrected axial length of 24.40 mm. The measured axial length in the fellow phakic eye was 24.46 mm. The 0.4 mm difference in axial length could have resulted in approximately a 1.0 D error in the IOL power calculation. Measuring the axial length with the pseudophakic mode of the A-scan device could have resulted in an even greater error. In this mode, many A-scan devices still use a single correction factor of ⫹0.4 mm based on the material sound speed of PMMA. Under these conditions, a 0.8 mm error in axial length or approximately a 2.0 D error in IOL calculation would have occurred. In the second patient, a 0.25 mm (approximately 0.5 D) error would have resulted if the typical ⫺0.84 mm correction factor had been used. A 0.99 mm (approximately 2.5 D) error could have resulted from using a single-setting pseudophakic mode measurement. These cases illustrate the need to correct for the material sound speed and center thickness specific to the lens material and power being exchanged. Using a single correction factor based on lens material can result in clinically significant errors in axial length measurement and thus IOL power calculation. After our experience with these cases, we contacted several other lens manufacturers who were unable or unwilling to provide the measurements (material sound speed and center thickness) needed to make these corrections. We recommend that lens manufacturers test all lens models for these variables and include powerspecific correction factors in the lens labeling. This would allow the surgeon to accurately measure the
axial length and calculate the correct IOL power for lens exchange. NICHOLAS G. ANDERSON, MD RHONDA G. WALDRON, MMSC, COMT R. DOYLE STULTING, MD Atlanta, Georgia, USA References 1. Kora Y, Shimizu K, Yoshida M, et al. Intraocular lens power calculation for lens exchange. J Cataract Refract Surg 2001; 27: 543–548 2. Milauskas AT, Marney S. Pseudo axial length increase after silicone lens implantation as determined by ultrasonic scans. J Cataract Refract Surg 1988; 14:400 – 402 3. Holladay JT, Prager TC. Accurate ultrasonic biometry in pseudophakia. Am J Ophthalmol 1989; 107:189 –190 4. Holladay JT, Prager TC. Accurate ultrasonic biometry in pseudophakia (letter). Am J Ophthalmol 1993; 115:536 –537 5. Yang S, Lang A, Makker H, Zaleski E. Effect of silicone sound speed and intraocular lens thickness on pseudophakic axial length corrections. J Cataract Refract Surg 1995; 21:442– 446
Microkeratome-Assisted Posterior Keratoplasty e concur with the views of Azar and coauthors1 that microkeratome-assisted posterior keratoplasty is a potential alternative to penetrating keratoplasty (PKP) in the management of corneal endothelial failure. However, there are areas that we want to discuss. A postoperative manifest refraction of ⫹16 diopters is an unpleasant surprise. The authors postulate that this could result from a thin posterior button.1 The posterior button, in accord with the way it was produced, should be thicker instead of thinner. It has been shown that there is differential swelling of the post-mortem cornea,2,3 and there is relatively more post-mortem hydration present in the posterior than the anterior cornea. After the anterior flap is created, the residual posterior cornea could therefore be thicker than normal. It would be of interest to measure the actual thickness of the posterior button with confocal microscopy or optical coherence tomography.4 The excessive flattening may not be related to a thin posterior button. One possible contributing factor to excessive corneal flattening is a relatively undersized donor button. Although both the donor and recipient buttons were trephined at a diameter of 6.0 mm, the effective diameter was not the same because the donor button was endothelial side up while the recipient button was stromal side up. In routine PKP, the donor
W
J CATARACT REFRACT SURG—VOL 27, DECEMBER 2001
1903
tion, IOL, and anesthetic agent will broaden the discussion and enhance the overall results. JOAN S.K. NG, FRCS HELIOS T.C. LEUNG, OD, PHD DENNIS S.C. LAM, FRCS, FRCOPHTH Hong Kong, China References 1. Pucci V, Morselli S, Romanelli F, et al. Clear lens phacoemulsification for correction of high myopia. J Cataract Refract Surg 2001; 27:896 –900 2. Kora Y, Yaguchi S, Inatomi M, Ozawa T. Preferred postoperative refraction after cataract surgery for high myopia. J Cataract Refract Surg 1995; 21:35–38 3. Schaumberg DA, Dana MR, Christen WG, Glynn RJ. A systematic overview of the incidence of posterior capsule opacification. Ophthalmology 1998; 105:1213–1221 4. Koch DD, Liu JF, Gill EP, Parke DW II. Axial myopia increases the risk of retinal complications after neodymium-YAG laser posterior capsulotomy. Arch Ophthalmol 1989; 107:986 –990 ¨ , Ferliel ST. Posterior capsule opacification 5. Oner FH, Gunenc U after phacoemulsification: foldable acrylic versus poly(methyl methacrylate) intraocular lenses. J Cataract Refract Surg 2000; 26:722–726 6. Koch PS. Efficacy of lidocaine 2% jelly as a topical agent in cataract surgery. J Cataract Refract Surg 1999; 25:632– 634
Potential Source of Error in Axial Length Measurement in Pseudophakes n a recent study, Kora et al.1 reviewed factors that could contribute to errors in intraocular lens (IOL) power calculation for lens exchange and discussed several potential sources of error in axial length measurement. An additional potential source of error that was not discussed is failure to correct for the effect of the IOL on the speed of sound. In 1988, Milauskas and Marney2 reported that axial length measurements using the pseuodophakic A-scan mode are overestimated in patients with silicone IOLs. They attributed the overestimation to the difference between the speed of sound in silicone and poly(methyl methacrylate). In 1989 and 1993, Holladay and Prager3,4 derived the following formula to determine the correct axial length in a pseudophakic eye based on the ultrasonically measured axial length, the speed of sound in the IOL material, and the center thickness of the IOL being replaced:
I
Corrected Axial Length ⫽ Al ⫹ CF 1902
where AL ⫽ the axial length measured at aphakic sound velocity (1532 M/sec) and CF ⫽ the correction factor CF ⫽ CT共1 ⫺ 1532/S mat兲
where CT ⫽ the center thickness of the IOL (mm) and Smat ⫽ the speed of sound in IOL material (M/sec) Using a sound speed of 980 M/sec for a silicone IOL and a nominal lens thickness of 1.50 mm, they estimated that the true axial length is 0.84 mm less than the axial length measured at aphakic sound velocity in pseudophakic eyes with silicone IOLs. In 1995, Yang and coauthors5 showed that the use of a single sound speed (typically 1000 M/sec) in the formula for all silicone lenses could result in significant errors in IOL power prediction. They estimated that the use of a typical material sound speed, rather than the lens-specific speed, could create an error of at least 0.25 D in IOL power calculation when the difference between the 2 speeds exceeds 6%. They also showed that for lenses with an index of refraction less than 1.46, variations in center thickness that relate to lens power could also create significant errors in axial length measurement. Thus, the authors recommended that the surgeon obtain from the IOL manufacturer the material sound speed and center thickness for the specific power of the IOL being replaced. These values can then be applied to Holladay and Prager’s equation to correct for the axial length measured at aphakic sound speed. We recently evaluated 2 patients requiring IOL exchange because of large postoperative refractive errors with an IOL (Pharmacia model 912A) for which the material sound speed had not been determined. We contacted the manufacturer, who provided the material sound speed and diopter-specific lens center thickness. The axial length correction factors were then deter-
Table 1. Correction factor for Pharmacia model 912A. IOL Power (D)
Correction Factor* (mm)
5.0–16.5
0.4
17.0–22.0
0.5
22.5–27.5
0.6
28.0–30.0
0.7
*Rounded to the nearest 0.1 mm. Material sound speed @ 35°C ⫽ 1052 m/s.
J CATARACT REFRACT SURG—VOL 27, DECEMBER 2001
LETTERS
mined using the Holladay/Prager method (Table 1). The correction factors that we calculated were significantly different from those determined by Holladay and Prager using a sound speed of 980 M/sec and vary significantly with dioptric power. There was a significant difference in material sound speed and diopter-specific center thickness among different models of lenses from the same manufacturer. In the first patient, the axial length measured under immersion at aphakic velocity was 24.84 mm. Using the standard correction factor of ⫺0.84 mm would have resulted in a corrected axial length of 24.00 mm. Using the correction factor of ⫺0.44 mm that we calculated for his specific lens (Pharmacia Model 912A, ⫹16.5 D), however, resulted in a corrected axial length of 24.40 mm. The measured axial length in the fellow phakic eye was 24.46 mm. The 0.4 mm difference in axial length could have resulted in approximately a 1.0 D error in the IOL power calculation. Measuring the axial length with the pseudophakic mode of the A-scan device could have resulted in an even greater error. In this mode, many A-scan devices still use a single correction factor of ⫹0.4 mm based on the material sound speed of PMMA. Under these conditions, a 0.8 mm error in axial length or approximately a 2.0 D error in IOL calculation would have occurred. In the second patient, a 0.25 mm (approximately 0.5 D) error would have resulted if the typical ⫺0.84 mm correction factor had been used. A 0.99 mm (approximately 2.5 D) error could have resulted from using a single-setting pseudophakic mode measurement. These cases illustrate the need to correct for the material sound speed and center thickness specific to the lens material and power being exchanged. Using a single correction factor based on lens material can result in clinically significant errors in axial length measurement and thus IOL power calculation. After our experience with these cases, we contacted several other lens manufacturers who were unable or unwilling to provide the measurements (material sound speed and center thickness) needed to make these corrections. We recommend that lens manufacturers test all lens models for these variables and include powerspecific correction factors in the lens labeling. This would allow the surgeon to accurately measure the
axial length and calculate the correct IOL power for lens exchange. NICHOLAS G. ANDERSON, MD RHONDA G. WALDRON, MMSC, COMT R. DOYLE STULTING, MD Atlanta, Georgia, USA References 1. Kora Y, Shimizu K, Yoshida M, et al. Intraocular lens power calculation for lens exchange. J Cataract Refract Surg 2001; 27: 543–548 2. Milauskas AT, Marney S. Pseudo axial length increase after silicone lens implantation as determined by ultrasonic scans. J Cataract Refract Surg 1988; 14:400 – 402 3. Holladay JT, Prager TC. Accurate ultrasonic biometry in pseudophakia. Am J Ophthalmol 1989; 107:189 –190 4. Holladay JT, Prager TC. Accurate ultrasonic biometry in pseudophakia (letter). Am J Ophthalmol 1993; 115:536 –537 5. Yang S, Lang A, Makker H, Zaleski E. Effect of silicone sound speed and intraocular lens thickness on pseudophakic axial length corrections. J Cataract Refract Surg 1995; 21:442– 446
Microkeratome-Assisted Posterior Keratoplasty e concur with the views of Azar and coauthors1 that microkeratome-assisted posterior keratoplasty is a potential alternative to penetrating keratoplasty (PKP) in the management of corneal endothelial failure. However, there are areas that we want to discuss. A postoperative manifest refraction of ⫹16 diopters is an unpleasant surprise. The authors postulate that this could result from a thin posterior button.1 The posterior button, in accord with the way it was produced, should be thicker instead of thinner. It has been shown that there is differential swelling of the post-mortem cornea,2,3 and there is relatively more post-mortem hydration present in the posterior than the anterior cornea. After the anterior flap is created, the residual posterior cornea could therefore be thicker than normal. It would be of interest to measure the actual thickness of the posterior button with confocal microscopy or optical coherence tomography.4 The excessive flattening may not be related to a thin posterior button. One possible contributing factor to excessive corneal flattening is a relatively undersized donor button. Although both the donor and recipient buttons were trephined at a diameter of 6.0 mm, the effective diameter was not the same because the donor button was endothelial side up while the recipient button was stromal side up. In routine PKP, the donor
W
J CATARACT REFRACT SURG—VOL 27, DECEMBER 2001
1903