- Email: [email protected]
Accurate intraocular lens power calculation after myopic laser in situ keratomileusis, bypassing corneal power
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
Accurate intraocular lens power calculation after myopic laser in situ keratomileusis, bypassing corneal power Keith A. Walter, MD, Michael R. Gagnon, MD, Phillip C. Hoopes Jr, MD, Paul J. Dickinson, MD
PURPOSE: To describe a novel method for calculating intraocular lens (IOL) power after myopic laser in situ keratomileusis (LASIK) without using the inaccuracies of the post-LASIK corneal power. SETTING: Department of Ophthalmology, Wake Forest University Eye Center, Wake Forest University School of Medicine, Winston Salem, North Carolina, USA. METHODS: This retrospective chart review comprised 9 eyes of 9 patients who had phacoemulsification after LASIK using our method for IOL calculation. This new method assumes the patient never had myopic LASIK to calculate IOL power and then targets the IOL at the pre-LASIK amount of myopia. The pre-LASIK keratometry values, pre-LASIK manifest refraction, and the current axial length are placed in the Holladay formula, bypassing the post-LASIK corneal power. In theory, assuming that the patient had satisfactory LASIK results, the correct IOL can then be determined. RESULTS: The mean spherical equivalent postoperative refraction was C0.03 diopter (D) G 0.42 (SD) (range ÿ0.625 to C0.75 D). In all 9 eyes, our method consistently chose the most accurate and precise IOL compared with other methods. CONCLUSIONS: The new method of calculating IOL power after LASIK provided excellent results and the most accurate and precise results to date. J Cataract Refract Surg 2006; 32:425–429 Q 2006 ASCRS and ESCRS
The number of refractive surgical procedures performed in the United States has increased since the 1990s. Most of these patients will have cataracts and have the same high expectations for emmetropia as they did with their original refractive surgery. This presents a challenge because the intraocular (IOL) power calculations after refractive surgery are notoriously inaccurate.1 It is common to have a ‘‘hyperopic surprise’’ in a postmyopic laser in situ keratomileusis (LASIK) eye owing to inaccurate IOL calculations.2 The inaccuracy of the IOL calculation is caused by inability to accurately measure the corneal power with standard
Accepted for publication August 22, 2005. From the Department of Ophthalmology, Wake Forest University Eye Center, Wake Forest University School of Medicine, Winston Salem, North Carolina, USA.
keratometry or videokeratography.3 Techniques to determine the corneal power include the clinical history and the contact lens methods, as well as the mean of these 2 as described by Randleman et al.1 There have also been more recent techniques such as the modified Maloney method (Maloney RK, personal communication, October 2002) and the topographic central corneal power adjustment method (EffRPadj) described by Wang et al.3 In this study, a new method to calculate IOL power for patients having cataract surgery after myopic LASIK was developed that ‘‘bypasses’’ the need to determine the corneal power. This method is described by Ladas and Stark.12 The results were compared with the clinical history method, double-K method,4 and another technique using the post-LASIK keratometry values.
No author has a financial or proprietary interest in any material or method mentioned.
PATIENTS AND METHODS
Reprint requests to Keith A. Walter, MD, Wake Forest University Department of Ophthalmology Medical Center Boulevard Winston-Salem, NC 27157-1033 USA. E-mail: [email protected].
Nine eyes of 9 consecutive patients who had cataract surgery a mean of 5.05 years (range 3.08 to 7.25 years) after myopic LASIK were identified. The LASIK procedures were performed by 1 of the
Q 2006 ASCRS and ESCRS Published by Elsevier Inc.
0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2005.12.140
425
IOL POWER CALCULATION BYPASSING CORNEAL POWER
authors (K.A.W.) as well as other referring surgeons. Data before myopic LASIK including manifest refraction and manual keratometry were available in each case. In all patients, the axial lengths were measured with the Carl Zeiss IOLMaster. This information was used in our new method to calculate the IOL power for emmetropia. This new method to calculate IOL power makes the assumption that the patient had not had LASIK. It uses keratometry values and manifest refraction measured prior to LASIK and the current axial length; these are placed in the Holladay formula. The pre-LASIK refraction or net refractive correction from LASIK (pre-LASIK spherical equivalent [SE] minus post-LASIK SE) is used as the target for IOL selection. In theory, this method calculates the proper IOL selection for the patient before LASIK, but leaves them with their original myopia (target Z preop myopia), and then assumes they later had myopic LASIK to correct this error. Because, in reality, all the cataract patients had LASIK first, this method works backward to determine the proper IOL selection without using the patient’s current inaccurate keratometry measurements. The result of the current method was compared with the IOL selection that would result if the post-LASIK keratometry values were used in the Holladay formula with the target refraction for emmetropia. It was understood that using post-LASIK keratometry values would be inaccurate, but it would be helpful as an illustration of the hyperopic surprise that occurs with post-LASIK keratometry values. The current method’s IOL selection was also compared with the IOL determined by the clinical history method that was introduced by Holladay5 and Guyton.6 This method subtracts the change in the SE after the refractive procedure from the preoperative keratometry values to derive the postoperative corneal power. The clinical history method is available on the Carl Zeiss IOLMaster and was used to calculate our IOL power. It is important to note that the clinical history method available on the Carl Zeiss IOLMaster corrects for the vertex distance to the corneal plane, which can result in a higher postoperative corneal power, as described by Odenthal et al.7 The double-K method was also used as a comparison. All 4 methods were also compared with the ‘‘true’’ IOL power, which would result in the patient being emmetropic. Because this study tested the new ‘‘corneal bypass’’ method in the clinical setting, the ‘‘true’’ IOL power was determined retrospectively by taking the resultant manifest refraction after cataract surgery and the implanted IOL power and calculating the IOL that would have resulted in emmetropia. This was accomplished with the knowledge that every diopter of IOL power is equal to approximately 0.7 diopter (D) in refraction at the spectacle plane.8
Case Example A 57-year-old woman (patient 5) presented to the Wake Forest University Eye Center with progressive blurring of vision in the left eye over the past 2 years. On ophthalmologic examination, it was noted that the patient has developed an opalescent nuclear sclerotic cataract. Best corrected visual acuity is 20/50. The patient had bilateral myopic LASIK 7 years ago and had a refractive result of plano. Data gathered from chart review are as follows: preLASIK refraction, –11.75; pre-LASIK keratometry, 43.87/44.87; post-LASIK axial length, 28.54 mm (IOL Master). Below, the various methods of determining IOL calculations are compared with the current method.
426
Post-LASIK Keratometry Method Using mean post-LASIK axial length and keratometry values of 35.15 D entered in the Holladay formula with the target refraction of plano resulted in an IOL power of 15.5 D. Clinical History Method Using the post-LASIK axial length and calculating a corneal power of 34.18 D entered in the same Holladay formula with the target refraction of plano resulted in the IOL power of 17.5 D. Double-K Method Using the nomogram described by Wang et al.,9 the IOL power is 19.9 D. Current Method (Cornea Bypass Method) The axial length and the pre-LASIK keratometry values are entered in the Holladay formula with the target pre-LASIK refraction of ÿ11.75. The IOL power with the target of the preLASIK refractive error is 19.5 D. Using the cornea bypass method, the patient had uneventful cataract surgery a 19.5 D IOL and was placed in the capsular bag. The postoperative manifest refraction was ÿ0.50 C 1.00 60 with best spectacle-corrected visual acuity (BSCVA) of 20/25 and uncorrected visual acuity (UCVA) of 20/30. The postoperative SE was zero; therefore, the ‘‘true’’ IOL was placed.
RESULTS
The 9 patients in this study were distributed equally with 5 men and 4 women, and predominantly left eyes were involved (67%). The mean patient age at the time of cataract surgery was 57 years (range 44 to 64 years). The pre-LASIK information was available in every case, and uneventful cataract surgery was performed by K.A.W. and P.J.D. An acrylic 1-piece foldable IOL (Model SA60AT, SN60AT, or SN60WF; Alcon Laboratories, Inc) was placed in the capsular bag in each case. All patients were followed for a mean of 2.83 months (range 0.5 to 9 months). Manifest refractions were recorded typically at the 6-week visit. All eyes achieved BSCVA of 20/25 or better and UCVA of at least 20/40 except in 1 patient who was targeted for the desired refraction of ÿ1.50 D for functional monovision (patient 3, Table 1). The mean intended SE postoperative refraction was C0.03 G 0.42 D range ÿ0.625 to C0.75 D; Figure 1). These values include patient 3 with the intended postoperative target refraction of ÿ1.50 D that resulted in an SE of ÿ2.12 D, yielding a result of ÿ0.625 of the intended refraction. The current method consistently chose a more powerful IOL than the clinical history method and post-LASIK keratometry method. The IOL chosen by the corneal bypass method was more powerful than the IOL chosen by these 2 methods and was the closest to the true IOL. The current
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
IOL POWER CALCULATION BYPASSING CORNEAL POWER
Table 1. Pre-LASIK information and results of cataract surgery.
Pre-LASIK
Postoperative
Patient/Eye
Keratometry
MR
SE (D)
Axial Length (mm)
MR
SE (D)
BSCVA
UCVA
1/right 2/left 3/left 4/left 5/left 6/right 7/left 8/left 9/right
42.8/39.8 44.25/43.75 43.00/42.75 45.25/45.80 43.87/44.87 42/42.87 43.66/43.94 45.37/45.62 46/46.37
ÿ5.50 C 3.00 075 ÿ12.25 C 0.50 10 ÿ4.00 C 1.00 180 ÿ9.00 C 1.50 010 ÿ11.75 sphere ÿ2.50 C 0.75 111 ÿ4.75 sphere ÿ11.25 sphere ÿ8.00 C 1.50 007
ÿ4.00 ÿ12.00 ÿ3.50 ÿ8.25 ÿ11.75 ÿ2.13 ÿ4.75 ÿ11.25 ÿ7.25
26.93 29.26 25.43 27.01 28.54 25.87 25.69 26.64 25.05
C0.50 C 0.50 125 ÿ0.5 ÿ2.50 C 0.75 020 Plano C 0.50 C 055 ÿ0.50 C 1.00 060 plano Plano C 0.75 C 073 ÿ0.50 C 1.00 165 plano
0.75 ÿ0.50 ÿ0.625* 0.25 0 0 0.375 0 0
20/25 20/20 20/20 20/25 20/25 20/20 20/15 20/20 20/20
20/40 20/25 20/80 20/30 20/30 20/20 20/20 20/25 20/20
BSCVAZbest spectacle-corrected visual acuity; MRZmanifest refraction; SEZspherical equivalent; UCVAZuncorrected visual acuity. *The postoperative aim for patient 3 was ÿ1.50 D.
For example, if an artist asks to remain myopic (ÿ3.75 D) after routine cataract surgery, it is not difficult to calculate the necessary IOL power to achieve this myopic result. This is commonly done when a patient has an occupation requiring long hours of near work. All that is needed is to enter the target of ÿ3.75 in the Holladay formula along with the measured axial length and corneal power. If later this same, now pseudophakic, patient wanted to become emmetropic (has retired and now wishes to play golf), the refractive surgeon could perform LASIK to correct the ÿ3.75 refractive error to emmetropia. This does not change the IOL power, only the corneal power. Therefore, the original IOL power is still the correct one (a C b Z c). What if the corneal power changes first? Imagine that the reverse order of events happened, which more commonly challenges the cataract surgeon. What if the same artist had his ÿ3.75 myopia corrected with LASIK and then acquired cataracts? We know that the correct IOL power is still the same (b C a Z c) and is simply derived by inserting their original K values, current axial length, plus the amount of cornea removed (usually negligible) and the original target myopic refraction. This study provides not only theoretical means to achieve accurate IOL calculations but also practical applications in the first 9 eyes. All the patients were extremely
method chose a more powerful IOL than the clinical history method by a mean of 1.83 G 0.87 D (range C0.5 to C3 D; Figure 2). The cornea bypass method also compared favorably with the double-K method, choosing a similar powered IOL (Figure 2). DISCUSSION
We present a novel method for IOL power calculation after myopic LASIK in theory and practice with the first 9 consecutive eyes. We believe this method to be the most accurate currently available, because it bypasses the inaccuracies of the post-LASIK corneal power measured or derived by formula. Although possibly difficult to comprehend at first glance, the cornea bypass method becomes more intuitive on further study because it is based on logic and sound mathematical principles. The basic premise is that the order of events is irrelevant if the desired result is the same. Using the commutative property of addition, the numbers can be added in any order and will result in the same sum; a C b Z c and also b C a Z c. Similarly, if 2 different refractive events occur and the end result is emmetropia (or ÿ1.5 D), it does not matter which event occurred first. Using this premise, one can work backward to derive the correct IOL power without using the current corneal power.
Spherical Equivalent (D)
2 1 0 -1 1
2
3
4
5
6
7
8
9
Figure 1. Surgical outcomes: A comparison of the preoperative SE aim and postoperative SE result.
-2 -3
Patient Pre-op Aim SE (D)
Postop SE (D)
*It is important to note that the postop aim for patient 3 was -1.50 D.
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
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IOL POWER CALCULATION BYPASSING CORNEAL POWER
IOL Selection Post LASIK K's
Double-K Method
Clinical History Method
Our Method
True IOL
IOL Power (D)
25 20 15 10 5 0 Post LASIK K's Clinical History Method Double-K Method
1
2
3
4
5
6
7
8
9
*
13.5
*
14.5
15.5
17.5
19
18.5
17.5
17.5
16.5
17.5
16.5
17.5
17.5
19.5
20.5
19.5
*
18.8
*
18.3
19.9
18
20.6
22.7
21.1
Our Method
18.5
18.5
20.5
18.5
19.5
18
20.5
23.5
21.5
True IOL
19.57
17.79
19.61
18.86
19.50
18.00
21.04
23.50
21.50
Patient
Figure 2. Intraocular lens selection: A comparison of each of the 3 methods and the resultant IOL calculations compared with the true IOL. K-readings were not available to calculate the IOL with the post-LASIK K method or the double-K method. *Post-LASIK
pleased with their postoperative results, as expected by our small mean residual refractive error of C0.03 D and tight standard deviation of G 0.42 D. Our IOL power calculation appears to be the best results reported to date and the closest to the true IOL power based on retrospective calculations (Figure 2). Our cornea bypass method compares favorably with the results of Randleman et al.1 and Wang et al.,10 who also showed their methods with a similar patient series. Randleman et al.1 used preoperative information with the clinical history method, the contact lens method, or the mean of these 2 methods; the postoperative SE was similar to that in our study (C0.21 D) but the range (ÿ2.25 to C2.25 D) and standard deviation (1.54 D) were much higher. Since their study, the contact lens method has been reported to be unreliable.10,11 Wang et al.10 used a double-K formula and corneal values derived from adjusting the measured effective refractive power (EffRPadj); they obtained a postoperative SE of ÿ0.61 D while aiming for ÿ0.75 D; thus, the residual refractive error relative to the target was C0.14 D. In this series, the range (ÿ2.0 to C1.0 D) and standard deviation (0.79 D) were also wider than ours. Additionally, their method is more cumbersome to perform because it requires computerized videokeratography to derive the EffRPadj and uses a double-K formula. Although the double-K method gave comparable results to those of the cornea bypass method, it is more cumbersome to derive the IOL power and requires measuring the postLASIK K-readings which at times may be inaccurate and difficult to obtain. In addition, the cornea bypass method requires no nomogram to estimate the IOL power adjustment. The Feiz-Mannis method, a theoretical method, uses the pre-LASIK corneal power values and axial length before cataract surgery. The IOL value is then added to the refractive error corrected by LASIK divided by 0.7. This method
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has been found, by both Randleman et al.1 and Wang et al.,10 to be variable and less consistent. Our method not only is superior in both theory and in practice to the other available methods, but is also simpler to perform in a busy clinical setting. One drawback to the cornea bypass method as well as other historical data based methods is the need for preLASIK data. All our patients had this information readily available despite 2 of them having their original LASIK done out of state, and all patients had LASIK over 3 years before cataract surgery. We believe it is imperative that reliable data be recorded before LASIK and made available for many decades to come. One suggestion might be patient data cards, similar to the current IOL cards mandated by the U.S. Food and Drug Administration. Unfortunately, many patients will lose these over time. Ideally, a Webbased archive or database should be established to offer easy access to these data in the near and distant future. It should be remembered that the target IOL that is entered in the Holladay formula is actually the net refractive change that occurred. Most of the time this will simply be the pre-LASIK manifest SE, but if there was a small postLASIK residual refractive error or if subsequent enhancement(s) were performed, the net change in manifest refraction should be used. This can be calculated easily by subtracting the pre-LASIK manifest SE from the residual refractive error after the most recent refractive procedure but before the cataract developed. Although this initial study was small owing to the current scarcity of these patients, more patients will be added as refractive surgery patients age and have cataracts. We expect this method to fare well when applied to greater numbers, because our range of myopia was wide (ÿ1.87 D to ÿ12 D), and these initial results are very promising. The management and determination of the precise IOL for
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
IOL POWER CALCULATION BYPASSING CORNEAL POWER
patients after refractive surgery will become an increasingly important part of everyday practice as our now young and visually demanding LASIK patient population ages over the next several decades. This study offers a valuable tool to improve outcomes and hopefully future studies will confirm these results in a broader patient base. REFERENCES 1. Randleman JB, Loupe DN, Song CD, et al. Intraocular lens power calculation after laser in situ keratomileusis. Cornea 2002; 21:751– 775 2. Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12:17–29 3. Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004; 111:1825–1831 4. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double K method. J Cataract Refract Surg 2003; 29:2063–2068
5. Holladay JT. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203 6. Guyton DL. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203 7. Odenthal MT, Eggink CA, Melles G, et al. Clinical and theoretical results of intraocular lens power calculation for cataract surgery after photorefractive keratectomy for myopia. Arch Ophthalmol 2002; 120:431–438 8. Feiz V, Mannis MJ, Garcia-Ferrer F, et al. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia. Cornea 2001; 20(8):792–797 9. Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg 2003; 29:2039–2042 10. Wang L, Booth MA, Koch DD. Comparison on intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004; 111:1825–1831 11. Argento C, Cosentino MJ, Badoza D. Intraocular lens power calculation after refractive surgery. J Cataract Refract Surg 2003; 29: 1346–1351 12. Ladas JG, Stark WJ. Calculating IOL power after refractive surgery. J Cataract Refract Surg 2004; 30:2458
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Accurate intraocular lens power calculation after myopic laser in situ keratomileusis, bypassing corneal power Keith A. Walter, MD, Michael R. Gagnon, MD, Phillip C. Hoopes Jr, MD, Paul J. Dickinson, MD
PURPOSE: To describe a novel method for calculating intraocular lens (IOL) power after myopic laser in situ keratomileusis (LASIK) without using the inaccuracies of the post-LASIK corneal power. SETTING: Department of Ophthalmology, Wake Forest University Eye Center, Wake Forest University School of Medicine, Winston Salem, North Carolina, USA. METHODS: This retrospective chart review comprised 9 eyes of 9 patients who had phacoemulsification after LASIK using our method for IOL calculation. This new method assumes the patient never had myopic LASIK to calculate IOL power and then targets the IOL at the pre-LASIK amount of myopia. The pre-LASIK keratometry values, pre-LASIK manifest refraction, and the current axial length are placed in the Holladay formula, bypassing the post-LASIK corneal power. In theory, assuming that the patient had satisfactory LASIK results, the correct IOL can then be determined. RESULTS: The mean spherical equivalent postoperative refraction was C0.03 diopter (D) G 0.42 (SD) (range ÿ0.625 to C0.75 D). In all 9 eyes, our method consistently chose the most accurate and precise IOL compared with other methods. CONCLUSIONS: The new method of calculating IOL power after LASIK provided excellent results and the most accurate and precise results to date. J Cataract Refract Surg 2006; 32:425–429 Q 2006 ASCRS and ESCRS
The number of refractive surgical procedures performed in the United States has increased since the 1990s. Most of these patients will have cataracts and have the same high expectations for emmetropia as they did with their original refractive surgery. This presents a challenge because the intraocular (IOL) power calculations after refractive surgery are notoriously inaccurate.1 It is common to have a ‘‘hyperopic surprise’’ in a postmyopic laser in situ keratomileusis (LASIK) eye owing to inaccurate IOL calculations.2 The inaccuracy of the IOL calculation is caused by inability to accurately measure the corneal power with standard
Accepted for publication August 22, 2005. From the Department of Ophthalmology, Wake Forest University Eye Center, Wake Forest University School of Medicine, Winston Salem, North Carolina, USA.
keratometry or videokeratography.3 Techniques to determine the corneal power include the clinical history and the contact lens methods, as well as the mean of these 2 as described by Randleman et al.1 There have also been more recent techniques such as the modified Maloney method (Maloney RK, personal communication, October 2002) and the topographic central corneal power adjustment method (EffRPadj) described by Wang et al.3 In this study, a new method to calculate IOL power for patients having cataract surgery after myopic LASIK was developed that ‘‘bypasses’’ the need to determine the corneal power. This method is described by Ladas and Stark.12 The results were compared with the clinical history method, double-K method,4 and another technique using the post-LASIK keratometry values.
No author has a financial or proprietary interest in any material or method mentioned.
PATIENTS AND METHODS
Reprint requests to Keith A. Walter, MD, Wake Forest University Department of Ophthalmology Medical Center Boulevard Winston-Salem, NC 27157-1033 USA. E-mail: [email protected].
Nine eyes of 9 consecutive patients who had cataract surgery a mean of 5.05 years (range 3.08 to 7.25 years) after myopic LASIK were identified. The LASIK procedures were performed by 1 of the
Q 2006 ASCRS and ESCRS Published by Elsevier Inc.
0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2005.12.140
425
IOL POWER CALCULATION BYPASSING CORNEAL POWER
authors (K.A.W.) as well as other referring surgeons. Data before myopic LASIK including manifest refraction and manual keratometry were available in each case. In all patients, the axial lengths were measured with the Carl Zeiss IOLMaster. This information was used in our new method to calculate the IOL power for emmetropia. This new method to calculate IOL power makes the assumption that the patient had not had LASIK. It uses keratometry values and manifest refraction measured prior to LASIK and the current axial length; these are placed in the Holladay formula. The pre-LASIK refraction or net refractive correction from LASIK (pre-LASIK spherical equivalent [SE] minus post-LASIK SE) is used as the target for IOL selection. In theory, this method calculates the proper IOL selection for the patient before LASIK, but leaves them with their original myopia (target Z preop myopia), and then assumes they later had myopic LASIK to correct this error. Because, in reality, all the cataract patients had LASIK first, this method works backward to determine the proper IOL selection without using the patient’s current inaccurate keratometry measurements. The result of the current method was compared with the IOL selection that would result if the post-LASIK keratometry values were used in the Holladay formula with the target refraction for emmetropia. It was understood that using post-LASIK keratometry values would be inaccurate, but it would be helpful as an illustration of the hyperopic surprise that occurs with post-LASIK keratometry values. The current method’s IOL selection was also compared with the IOL determined by the clinical history method that was introduced by Holladay5 and Guyton.6 This method subtracts the change in the SE after the refractive procedure from the preoperative keratometry values to derive the postoperative corneal power. The clinical history method is available on the Carl Zeiss IOLMaster and was used to calculate our IOL power. It is important to note that the clinical history method available on the Carl Zeiss IOLMaster corrects for the vertex distance to the corneal plane, which can result in a higher postoperative corneal power, as described by Odenthal et al.7 The double-K method was also used as a comparison. All 4 methods were also compared with the ‘‘true’’ IOL power, which would result in the patient being emmetropic. Because this study tested the new ‘‘corneal bypass’’ method in the clinical setting, the ‘‘true’’ IOL power was determined retrospectively by taking the resultant manifest refraction after cataract surgery and the implanted IOL power and calculating the IOL that would have resulted in emmetropia. This was accomplished with the knowledge that every diopter of IOL power is equal to approximately 0.7 diopter (D) in refraction at the spectacle plane.8
Case Example A 57-year-old woman (patient 5) presented to the Wake Forest University Eye Center with progressive blurring of vision in the left eye over the past 2 years. On ophthalmologic examination, it was noted that the patient has developed an opalescent nuclear sclerotic cataract. Best corrected visual acuity is 20/50. The patient had bilateral myopic LASIK 7 years ago and had a refractive result of plano. Data gathered from chart review are as follows: preLASIK refraction, –11.75; pre-LASIK keratometry, 43.87/44.87; post-LASIK axial length, 28.54 mm (IOL Master). Below, the various methods of determining IOL calculations are compared with the current method.
426
Post-LASIK Keratometry Method Using mean post-LASIK axial length and keratometry values of 35.15 D entered in the Holladay formula with the target refraction of plano resulted in an IOL power of 15.5 D. Clinical History Method Using the post-LASIK axial length and calculating a corneal power of 34.18 D entered in the same Holladay formula with the target refraction of plano resulted in the IOL power of 17.5 D. Double-K Method Using the nomogram described by Wang et al.,9 the IOL power is 19.9 D. Current Method (Cornea Bypass Method) The axial length and the pre-LASIK keratometry values are entered in the Holladay formula with the target pre-LASIK refraction of ÿ11.75. The IOL power with the target of the preLASIK refractive error is 19.5 D. Using the cornea bypass method, the patient had uneventful cataract surgery a 19.5 D IOL and was placed in the capsular bag. The postoperative manifest refraction was ÿ0.50 C 1.00 60 with best spectacle-corrected visual acuity (BSCVA) of 20/25 and uncorrected visual acuity (UCVA) of 20/30. The postoperative SE was zero; therefore, the ‘‘true’’ IOL was placed.
RESULTS
The 9 patients in this study were distributed equally with 5 men and 4 women, and predominantly left eyes were involved (67%). The mean patient age at the time of cataract surgery was 57 years (range 44 to 64 years). The pre-LASIK information was available in every case, and uneventful cataract surgery was performed by K.A.W. and P.J.D. An acrylic 1-piece foldable IOL (Model SA60AT, SN60AT, or SN60WF; Alcon Laboratories, Inc) was placed in the capsular bag in each case. All patients were followed for a mean of 2.83 months (range 0.5 to 9 months). Manifest refractions were recorded typically at the 6-week visit. All eyes achieved BSCVA of 20/25 or better and UCVA of at least 20/40 except in 1 patient who was targeted for the desired refraction of ÿ1.50 D for functional monovision (patient 3, Table 1). The mean intended SE postoperative refraction was C0.03 G 0.42 D range ÿ0.625 to C0.75 D; Figure 1). These values include patient 3 with the intended postoperative target refraction of ÿ1.50 D that resulted in an SE of ÿ2.12 D, yielding a result of ÿ0.625 of the intended refraction. The current method consistently chose a more powerful IOL than the clinical history method and post-LASIK keratometry method. The IOL chosen by the corneal bypass method was more powerful than the IOL chosen by these 2 methods and was the closest to the true IOL. The current
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
IOL POWER CALCULATION BYPASSING CORNEAL POWER
Table 1. Pre-LASIK information and results of cataract surgery.
Pre-LASIK
Postoperative
Patient/Eye
Keratometry
MR
SE (D)
Axial Length (mm)
MR
SE (D)
BSCVA
UCVA
1/right 2/left 3/left 4/left 5/left 6/right 7/left 8/left 9/right
42.8/39.8 44.25/43.75 43.00/42.75 45.25/45.80 43.87/44.87 42/42.87 43.66/43.94 45.37/45.62 46/46.37
ÿ5.50 C 3.00 075 ÿ12.25 C 0.50 10 ÿ4.00 C 1.00 180 ÿ9.00 C 1.50 010 ÿ11.75 sphere ÿ2.50 C 0.75 111 ÿ4.75 sphere ÿ11.25 sphere ÿ8.00 C 1.50 007
ÿ4.00 ÿ12.00 ÿ3.50 ÿ8.25 ÿ11.75 ÿ2.13 ÿ4.75 ÿ11.25 ÿ7.25
26.93 29.26 25.43 27.01 28.54 25.87 25.69 26.64 25.05
C0.50 C 0.50 125 ÿ0.5 ÿ2.50 C 0.75 020 Plano C 0.50 C 055 ÿ0.50 C 1.00 060 plano Plano C 0.75 C 073 ÿ0.50 C 1.00 165 plano
0.75 ÿ0.50 ÿ0.625* 0.25 0 0 0.375 0 0
20/25 20/20 20/20 20/25 20/25 20/20 20/15 20/20 20/20
20/40 20/25 20/80 20/30 20/30 20/20 20/20 20/25 20/20
BSCVAZbest spectacle-corrected visual acuity; MRZmanifest refraction; SEZspherical equivalent; UCVAZuncorrected visual acuity. *The postoperative aim for patient 3 was ÿ1.50 D.
For example, if an artist asks to remain myopic (ÿ3.75 D) after routine cataract surgery, it is not difficult to calculate the necessary IOL power to achieve this myopic result. This is commonly done when a patient has an occupation requiring long hours of near work. All that is needed is to enter the target of ÿ3.75 in the Holladay formula along with the measured axial length and corneal power. If later this same, now pseudophakic, patient wanted to become emmetropic (has retired and now wishes to play golf), the refractive surgeon could perform LASIK to correct the ÿ3.75 refractive error to emmetropia. This does not change the IOL power, only the corneal power. Therefore, the original IOL power is still the correct one (a C b Z c). What if the corneal power changes first? Imagine that the reverse order of events happened, which more commonly challenges the cataract surgeon. What if the same artist had his ÿ3.75 myopia corrected with LASIK and then acquired cataracts? We know that the correct IOL power is still the same (b C a Z c) and is simply derived by inserting their original K values, current axial length, plus the amount of cornea removed (usually negligible) and the original target myopic refraction. This study provides not only theoretical means to achieve accurate IOL calculations but also practical applications in the first 9 eyes. All the patients were extremely
method chose a more powerful IOL than the clinical history method by a mean of 1.83 G 0.87 D (range C0.5 to C3 D; Figure 2). The cornea bypass method also compared favorably with the double-K method, choosing a similar powered IOL (Figure 2). DISCUSSION
We present a novel method for IOL power calculation after myopic LASIK in theory and practice with the first 9 consecutive eyes. We believe this method to be the most accurate currently available, because it bypasses the inaccuracies of the post-LASIK corneal power measured or derived by formula. Although possibly difficult to comprehend at first glance, the cornea bypass method becomes more intuitive on further study because it is based on logic and sound mathematical principles. The basic premise is that the order of events is irrelevant if the desired result is the same. Using the commutative property of addition, the numbers can be added in any order and will result in the same sum; a C b Z c and also b C a Z c. Similarly, if 2 different refractive events occur and the end result is emmetropia (or ÿ1.5 D), it does not matter which event occurred first. Using this premise, one can work backward to derive the correct IOL power without using the current corneal power.
Spherical Equivalent (D)
2 1 0 -1 1
2
3
4
5
6
7
8
9
Figure 1. Surgical outcomes: A comparison of the preoperative SE aim and postoperative SE result.
-2 -3
Patient Pre-op Aim SE (D)
Postop SE (D)
*It is important to note that the postop aim for patient 3 was -1.50 D.
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IOL Selection Post LASIK K's
Double-K Method
Clinical History Method
Our Method
True IOL
IOL Power (D)
25 20 15 10 5 0 Post LASIK K's Clinical History Method Double-K Method
1
2
3
4
5
6
7
8
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*
13.5
*
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19
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17.5
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19.5
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*
18.8
*
18.3
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18
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Our Method
18.5
18.5
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19.5
18
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23.5
21.5
True IOL
19.57
17.79
19.61
18.86
19.50
18.00
21.04
23.50
21.50
Patient
Figure 2. Intraocular lens selection: A comparison of each of the 3 methods and the resultant IOL calculations compared with the true IOL. K-readings were not available to calculate the IOL with the post-LASIK K method or the double-K method. *Post-LASIK
pleased with their postoperative results, as expected by our small mean residual refractive error of C0.03 D and tight standard deviation of G 0.42 D. Our IOL power calculation appears to be the best results reported to date and the closest to the true IOL power based on retrospective calculations (Figure 2). Our cornea bypass method compares favorably with the results of Randleman et al.1 and Wang et al.,10 who also showed their methods with a similar patient series. Randleman et al.1 used preoperative information with the clinical history method, the contact lens method, or the mean of these 2 methods; the postoperative SE was similar to that in our study (C0.21 D) but the range (ÿ2.25 to C2.25 D) and standard deviation (1.54 D) were much higher. Since their study, the contact lens method has been reported to be unreliable.10,11 Wang et al.10 used a double-K formula and corneal values derived from adjusting the measured effective refractive power (EffRPadj); they obtained a postoperative SE of ÿ0.61 D while aiming for ÿ0.75 D; thus, the residual refractive error relative to the target was C0.14 D. In this series, the range (ÿ2.0 to C1.0 D) and standard deviation (0.79 D) were also wider than ours. Additionally, their method is more cumbersome to perform because it requires computerized videokeratography to derive the EffRPadj and uses a double-K formula. Although the double-K method gave comparable results to those of the cornea bypass method, it is more cumbersome to derive the IOL power and requires measuring the postLASIK K-readings which at times may be inaccurate and difficult to obtain. In addition, the cornea bypass method requires no nomogram to estimate the IOL power adjustment. The Feiz-Mannis method, a theoretical method, uses the pre-LASIK corneal power values and axial length before cataract surgery. The IOL value is then added to the refractive error corrected by LASIK divided by 0.7. This method
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has been found, by both Randleman et al.1 and Wang et al.,10 to be variable and less consistent. Our method not only is superior in both theory and in practice to the other available methods, but is also simpler to perform in a busy clinical setting. One drawback to the cornea bypass method as well as other historical data based methods is the need for preLASIK data. All our patients had this information readily available despite 2 of them having their original LASIK done out of state, and all patients had LASIK over 3 years before cataract surgery. We believe it is imperative that reliable data be recorded before LASIK and made available for many decades to come. One suggestion might be patient data cards, similar to the current IOL cards mandated by the U.S. Food and Drug Administration. Unfortunately, many patients will lose these over time. Ideally, a Webbased archive or database should be established to offer easy access to these data in the near and distant future. It should be remembered that the target IOL that is entered in the Holladay formula is actually the net refractive change that occurred. Most of the time this will simply be the pre-LASIK manifest SE, but if there was a small postLASIK residual refractive error or if subsequent enhancement(s) were performed, the net change in manifest refraction should be used. This can be calculated easily by subtracting the pre-LASIK manifest SE from the residual refractive error after the most recent refractive procedure but before the cataract developed. Although this initial study was small owing to the current scarcity of these patients, more patients will be added as refractive surgery patients age and have cataracts. We expect this method to fare well when applied to greater numbers, because our range of myopia was wide (ÿ1.87 D to ÿ12 D), and these initial results are very promising. The management and determination of the precise IOL for
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patients after refractive surgery will become an increasingly important part of everyday practice as our now young and visually demanding LASIK patient population ages over the next several decades. This study offers a valuable tool to improve outcomes and hopefully future studies will confirm these results in a broader patient base. REFERENCES 1. Randleman JB, Loupe DN, Song CD, et al. Intraocular lens power calculation after laser in situ keratomileusis. Cornea 2002; 21:751– 775 2. Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12:17–29 3. Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004; 111:1825–1831 4. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double K method. J Cataract Refract Surg 2003; 29:2063–2068
5. Holladay JT. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203 6. Guyton DL. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203 7. Odenthal MT, Eggink CA, Melles G, et al. Clinical and theoretical results of intraocular lens power calculation for cataract surgery after photorefractive keratectomy for myopia. Arch Ophthalmol 2002; 120:431–438 8. Feiz V, Mannis MJ, Garcia-Ferrer F, et al. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia. Cornea 2001; 20(8):792–797 9. Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg 2003; 29:2039–2042 10. Wang L, Booth MA, Koch DD. Comparison on intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology 2004; 111:1825–1831 11. Argento C, Cosentino MJ, Badoza D. Intraocular lens power calculation after refractive surgery. J Cataract Refract Surg 2003; 29: 1346–1351 12. Ladas JG, Stark WJ. Calculating IOL power after refractive surgery. J Cataract Refract Surg 2004; 30:2458
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