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The Cataract Extraction-Refraction-Implantation Technique for IOL Power Calculation in Difficult Cases
LETTERS
refraction chosen for the cornea matches the location of the principal plane, i.e., 1.3315 @ 50 ~m in front of the anterior corneal vertex, 1.3333 @ 50 ~m behind the corneal vertex, or 1.3375 @ the posterior corneal vertex. With each of these locations of the principal plane and corresponding index of refraction of the cornea, the result of an IOL power calculation is exactly the same. The only practical effect of changing the location of the principal plane would be to change the value of the lens constant (anterior chamber depth [ACO]) to a value that is 100 11m or 0.1 mm longer. In short, all the ACO values that are currently on lens boxes, package inserts, and promotional materials would have to be changed to a value that is 0.1 mm larger. There is no practical way of doing this, and it would create a great deal of unnecessary confusion as to whether the value for ACO is the "new" value or the "old" value. All the currently available software problems would have to be modified, and the new programs would have to determine from the user which value is being entered. This change would not only cause confusion, it would also be a programming nightmare. Since changing the location of the principal plane has no effect on the calculated IOL power, the benefit of being theoretically correct is far outweighed by the unnecessary confusion in the clinical world. Finally, I agree that computers make ray tracing and paraxial optics simple to perform on most computers, but it still would not change the value of the calculated power of the IOL. So why change? Before moving from thin-lens optics to ray tracing, as occurred with the change from simple regression formulas to theoretical thin-lens formulas during the late '70s and early '80s, we must show that ray tracing has a lower prediction error to be of clinical benefit. The regression formulas have faded away because all investigators have found the Gaussian theoretical model to be better clinically. When Norrby shows me significant improvements with ray tracing , I will be the first to jump on the new bandwagon. Until then , "If it ain't broke, don't fix it. "-Jack T Holladay, MD, MSEE
The Cataract Extraction-RefractionImplantation Technique for IOL Power Calculation in Difficult Cases
T
he usual methods for determining the appropriate intraocular lens (IOL) refractive power, keratometry and A-scan biometry, occasionally produce unreliable results because of anatomic abnormalities or difficulties with patient compliance. Examples of this include infants and young children and patients with corneal irregularities or posterior segment abnormalities (i.e., presence of intravitreous silicone oil or posterior staph434
yloma) . In such instances, I have found that the most reliable way to determine the refractive status of the eye is to perform retinoscopy and/or subjective refraction after cataract extraction and prior to IOL implantation. The technique, as used in the most common clinical conditions for which it is beneficial, is described.
Pediatric Cataract Extraction/IOL Implantation In infants and young children, both keratometric and biometric data can be difficult to obtain. Therefore, after the cataract is aspirated, the incision is temporarily closed by suturing. A 25 gauge chamber maintainer is connected to the infusion line, and the infusion botde drip chamber is positioned 10 inches above the level of the cornea. A 25 gauge disposable needle is used to create a beveled limbal incision, and the chamber maintainer is inserted. It is important to verify that the eye is soft (intraocular pressure [lOP] below 20 mm Hg) prior to inserting the chamber maintainer. In this manner, lOP will be maintained at approximately 20 mm Hg by gravitational infusion pressure. Retinoscopy is then performed and the result is used to calculate the required IOL power. For example, with a vertex distance of 12.0 mm and a retinoscopy result of + 12.0 diopters (D), a posterior chamber lens of approximately 19.0 0 would be required to obtain emmetropia. As the vertex distance is often somewhat greater during intraoperative refraction (the neutralizing lenses need not be sterile and the surgeon may therefore prefer to use a greater vertex distance), an appropriate alteration of the calculation can easily be made. Depending on the desired postoperative refractive result, the IOL is then selected and inserted. Adult KeratometriclBiometric Problems In adults with corneal irregularities or with posterior segment problems such as intravitreal silicone oil or posterior staphyloma, the technique is varied as follows. After the cataract has been removed under topical anesthesia, the patient is moved from the operating room to a refractive lane within the facility. Approximately 30 to 60 minutes after completion of the procedure, a slidamp examination is performed; tonometry is done if there is evidence of significant lOP elevation or hypotony and should be in the normal range prior to proceeding with the refraction. Retinoscopy and subjective refraction are performed and the
} CATARACT REFRACT SURG-VOL 24, APRIL 1998
LETTERS
result used to determine the desired IOL power. The patient is then returned to the operating room for IOL implantation. In patients with intravitreal silicone oil, the IOL power selected is increased by either 3.0 to 5.0 D (plano posterior IOL surface) or 5.0 to 9.0 D (convex posterior IOL surface) as recommended by Holladay Qack D. Holladay, MD, personal communication), unless removal of silicone oil is anticipated at a future date. Case report: A 57-year-old physician with severe myopia was examined prior to cataract extraction in the right eye. Spectacle correction was -19.0 D, and a posterior staphyloma was seen on indirect ophthalmoscopy. A-scan biometry revealed poor quality echos and an approximate axial length of 27.2 mm. Using the biometric and keratometric data, an IOL power of 10.0 D was calculated to be required for emmetropia. Cataract extraction by phacoemulsification was performed under topical anesthesia. Approximately 45 minutes after cataract extraction, a subjective refraction of + 5.0 D was obtained. The patient was returned to the operating room where a +6.0 D IOL was inserted. Two weeks later, visual acuity was 20/30 in the operated eye (plano refraction). In most patients having cataract extraction, the usual techniques of keratometry and A-scan biometry provide an extremely accurate method for determining the refractive power of the eye following cataract extraction and, therefore, the refractive power of the IOL to be inserted. In a small but significant number, however, the problems described above are encountered and preclude the use of this standard method. Since 1978, I have used the described technique to determine the required IOL power for approximately 75 patients (50 children and 25 adults). All have displayed a postoperative refractive error within ± 1.0 D of the intended result. RICHARD]. MACKOOL, MD
Astoria, New York, USA
Combining Ablation Zone Diameters of Different Lasers for Data Analysis
T
he article by Gauthier et al. I is an excellent addition to the literature, furthering our understanding of the problem of epithelial hyperplasia after
photo refractive keratectomy (PRK) and the factors that contribute to it. There is, however, one point for which I am not satisfied that their methods are valid. The authors concluded that treatment zone size contributes in an inverse manner (larger zones exhibit less epithelial hyperplasia) based on the data presented in Figure 1, which demonstrates a plot of the measured epithelial hyperplasia versus treatment zone size for Summit laser zone sizes 4.1,4.3,4.5, and 5.0 mm and VISX zone size 6.0 mm. A best-fit linear regression line is then drawn through the data points. The problem is that the VISX data points are included with the Summit data points. If the VISX data are deleted, the regression line is no longer very convincing and could easily be substituted by a flat line showing Summit epithelial hyperplasia to be relatively independent of zone size from 4.1 to 5.0 mm. The crater profile of a VISX ablation may not be the same as a Summit ablation (in fact the authors' 27% incidence of central islands in the VISX group strongly suggests that the profile is very different), thus rendering the inclusion of the VISX data in this table invalid. Summit lasers can now do 6.0 and 6.5 mm zones, and data on these patients would be reliable to prove the authors' point. I therefore believe that it is invalid, on the basis of the authors' data, to conclude that epithelial hyperplasia varies inversely with ablation zone size in the studied population, even though this relationship is believed to be generally true for patients by most PRK surgeons. Aside from this minor point, I congratulate Gauthier et al. on an excellent paper. STEVE ARSHINOFF, MD, FRCSC
Downsview, Ontario, Canada Reference 1. Gauthier CA, Holden BA, Epstein D, et al. Factors affecting epithelial hyperplasia after photorefractive keratectomy. ] Cataract Refract Surg 1997; 23:1042-1050
Reply: Dr. Arshinoff is correct in stating that there is no statistically significant correlation between epithelial hyperplasia and zone size for the Summit laser alone because of the small sample size for the 4.1 (n = 2), 4.3 (n = 6), and 5.0 mm (n = 15) treatment zones. However, Table 2 does show a trend toward greater epithelial thickness with the smaller zones. Unfortunately, at the time the study was conducted, the Summit laser at the clinic site was not able to treat zones larger than 6.0 mm. Ideally, we agree that a wide variety of zone sizes with one laser would be the
J CATARACT REFRACT SURG-VOL 24, APRIL 1998
435
refraction chosen for the cornea matches the location of the principal plane, i.e., 1.3315 @ 50 ~m in front of the anterior corneal vertex, 1.3333 @ 50 ~m behind the corneal vertex, or 1.3375 @ the posterior corneal vertex. With each of these locations of the principal plane and corresponding index of refraction of the cornea, the result of an IOL power calculation is exactly the same. The only practical effect of changing the location of the principal plane would be to change the value of the lens constant (anterior chamber depth [ACO]) to a value that is 100 11m or 0.1 mm longer. In short, all the ACO values that are currently on lens boxes, package inserts, and promotional materials would have to be changed to a value that is 0.1 mm larger. There is no practical way of doing this, and it would create a great deal of unnecessary confusion as to whether the value for ACO is the "new" value or the "old" value. All the currently available software problems would have to be modified, and the new programs would have to determine from the user which value is being entered. This change would not only cause confusion, it would also be a programming nightmare. Since changing the location of the principal plane has no effect on the calculated IOL power, the benefit of being theoretically correct is far outweighed by the unnecessary confusion in the clinical world. Finally, I agree that computers make ray tracing and paraxial optics simple to perform on most computers, but it still would not change the value of the calculated power of the IOL. So why change? Before moving from thin-lens optics to ray tracing, as occurred with the change from simple regression formulas to theoretical thin-lens formulas during the late '70s and early '80s, we must show that ray tracing has a lower prediction error to be of clinical benefit. The regression formulas have faded away because all investigators have found the Gaussian theoretical model to be better clinically. When Norrby shows me significant improvements with ray tracing , I will be the first to jump on the new bandwagon. Until then , "If it ain't broke, don't fix it. "-Jack T Holladay, MD, MSEE
The Cataract Extraction-RefractionImplantation Technique for IOL Power Calculation in Difficult Cases
T
he usual methods for determining the appropriate intraocular lens (IOL) refractive power, keratometry and A-scan biometry, occasionally produce unreliable results because of anatomic abnormalities or difficulties with patient compliance. Examples of this include infants and young children and patients with corneal irregularities or posterior segment abnormalities (i.e., presence of intravitreous silicone oil or posterior staph434
yloma) . In such instances, I have found that the most reliable way to determine the refractive status of the eye is to perform retinoscopy and/or subjective refraction after cataract extraction and prior to IOL implantation. The technique, as used in the most common clinical conditions for which it is beneficial, is described.
Pediatric Cataract Extraction/IOL Implantation In infants and young children, both keratometric and biometric data can be difficult to obtain. Therefore, after the cataract is aspirated, the incision is temporarily closed by suturing. A 25 gauge chamber maintainer is connected to the infusion line, and the infusion botde drip chamber is positioned 10 inches above the level of the cornea. A 25 gauge disposable needle is used to create a beveled limbal incision, and the chamber maintainer is inserted. It is important to verify that the eye is soft (intraocular pressure [lOP] below 20 mm Hg) prior to inserting the chamber maintainer. In this manner, lOP will be maintained at approximately 20 mm Hg by gravitational infusion pressure. Retinoscopy is then performed and the result is used to calculate the required IOL power. For example, with a vertex distance of 12.0 mm and a retinoscopy result of + 12.0 diopters (D), a posterior chamber lens of approximately 19.0 0 would be required to obtain emmetropia. As the vertex distance is often somewhat greater during intraoperative refraction (the neutralizing lenses need not be sterile and the surgeon may therefore prefer to use a greater vertex distance), an appropriate alteration of the calculation can easily be made. Depending on the desired postoperative refractive result, the IOL is then selected and inserted. Adult KeratometriclBiometric Problems In adults with corneal irregularities or with posterior segment problems such as intravitreal silicone oil or posterior staphyloma, the technique is varied as follows. After the cataract has been removed under topical anesthesia, the patient is moved from the operating room to a refractive lane within the facility. Approximately 30 to 60 minutes after completion of the procedure, a slidamp examination is performed; tonometry is done if there is evidence of significant lOP elevation or hypotony and should be in the normal range prior to proceeding with the refraction. Retinoscopy and subjective refraction are performed and the
} CATARACT REFRACT SURG-VOL 24, APRIL 1998
LETTERS
result used to determine the desired IOL power. The patient is then returned to the operating room for IOL implantation. In patients with intravitreal silicone oil, the IOL power selected is increased by either 3.0 to 5.0 D (plano posterior IOL surface) or 5.0 to 9.0 D (convex posterior IOL surface) as recommended by Holladay Qack D. Holladay, MD, personal communication), unless removal of silicone oil is anticipated at a future date. Case report: A 57-year-old physician with severe myopia was examined prior to cataract extraction in the right eye. Spectacle correction was -19.0 D, and a posterior staphyloma was seen on indirect ophthalmoscopy. A-scan biometry revealed poor quality echos and an approximate axial length of 27.2 mm. Using the biometric and keratometric data, an IOL power of 10.0 D was calculated to be required for emmetropia. Cataract extraction by phacoemulsification was performed under topical anesthesia. Approximately 45 minutes after cataract extraction, a subjective refraction of + 5.0 D was obtained. The patient was returned to the operating room where a +6.0 D IOL was inserted. Two weeks later, visual acuity was 20/30 in the operated eye (plano refraction). In most patients having cataract extraction, the usual techniques of keratometry and A-scan biometry provide an extremely accurate method for determining the refractive power of the eye following cataract extraction and, therefore, the refractive power of the IOL to be inserted. In a small but significant number, however, the problems described above are encountered and preclude the use of this standard method. Since 1978, I have used the described technique to determine the required IOL power for approximately 75 patients (50 children and 25 adults). All have displayed a postoperative refractive error within ± 1.0 D of the intended result. RICHARD]. MACKOOL, MD
Astoria, New York, USA
Combining Ablation Zone Diameters of Different Lasers for Data Analysis
T
he article by Gauthier et al. I is an excellent addition to the literature, furthering our understanding of the problem of epithelial hyperplasia after
photo refractive keratectomy (PRK) and the factors that contribute to it. There is, however, one point for which I am not satisfied that their methods are valid. The authors concluded that treatment zone size contributes in an inverse manner (larger zones exhibit less epithelial hyperplasia) based on the data presented in Figure 1, which demonstrates a plot of the measured epithelial hyperplasia versus treatment zone size for Summit laser zone sizes 4.1,4.3,4.5, and 5.0 mm and VISX zone size 6.0 mm. A best-fit linear regression line is then drawn through the data points. The problem is that the VISX data points are included with the Summit data points. If the VISX data are deleted, the regression line is no longer very convincing and could easily be substituted by a flat line showing Summit epithelial hyperplasia to be relatively independent of zone size from 4.1 to 5.0 mm. The crater profile of a VISX ablation may not be the same as a Summit ablation (in fact the authors' 27% incidence of central islands in the VISX group strongly suggests that the profile is very different), thus rendering the inclusion of the VISX data in this table invalid. Summit lasers can now do 6.0 and 6.5 mm zones, and data on these patients would be reliable to prove the authors' point. I therefore believe that it is invalid, on the basis of the authors' data, to conclude that epithelial hyperplasia varies inversely with ablation zone size in the studied population, even though this relationship is believed to be generally true for patients by most PRK surgeons. Aside from this minor point, I congratulate Gauthier et al. on an excellent paper. STEVE ARSHINOFF, MD, FRCSC
Downsview, Ontario, Canada Reference 1. Gauthier CA, Holden BA, Epstein D, et al. Factors affecting epithelial hyperplasia after photorefractive keratectomy. ] Cataract Refract Surg 1997; 23:1042-1050
Reply: Dr. Arshinoff is correct in stating that there is no statistically significant correlation between epithelial hyperplasia and zone size for the Summit laser alone because of the small sample size for the 4.1 (n = 2), 4.3 (n = 6), and 5.0 mm (n = 15) treatment zones. However, Table 2 does show a trend toward greater epithelial thickness with the smaller zones. Unfortunately, at the time the study was conducted, the Summit laser at the clinic site was not able to treat zones larger than 6.0 mm. Ideally, we agree that a wide variety of zone sizes with one laser would be the
J CATARACT REFRACT SURG-VOL 24, APRIL 1998
435