- Email: [email protected]
T formulas
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
Accuracy of intraocular lens power prediction using the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas Julio Narva´ez, MD, Grenith Zimmerman, PhD, R. Doyle Stulting, MD, PhD, Daniel H. Chang, MD
PURPOSE: To compare the accuracy of intraocular lens (IOL) power calculations using 4 formulas: Hoffer Q, Holladay 1, Holladay 2, and SRK/T. SETTING: Tertiary care center. METHODS: This study was a retrospective comparative analysis. Immersion ultrasound biometry (axial length, anterior chamber depth, and lens thickness), manual keratometry, and postoperative manifest refraction were obtained in 643 eyes of consecutive patients who had routine uneventful cataract surgery with implantation of 1 of 2 IOLs using the same operative technique by the same surgeon. Biometric data were entered into each of the 4 IOL power calculation formulas, and the results were compared to the final manifest refraction. An optimized lens constant was used for each formula. Results were also stratified into groups of short, average, medium long, and very long axial length <22.0 mm, 22.0 to <24.5 mm, 24.5 to 26.0 mm, and >26.0 mm, respectively). RESULTS: No formula was more accurate than the others as measured by mean absolute error. The formulas were also equally accurate when eyes were stratified by axial length. CONCLUSION: The 4 IOL power formulas provided equivalent refractive results in the entire group of eyes and in the subsets of axial lengths tested. J Cataract Refract Surg 2006; 32:2050–2053 Q 2006 ASCRS and ESCRS
Cataract surgery is the most common intraocular surgical procedure performed in the United States. With modern techniques, complications are uncommon and increasing emphasis is being placed on refractive outcomes. Patients also have increasingly higher refractive expectations. A medicolegal review of 168 cataract surgery cases
Accepted for publication July 31, 2006. From the Department of Ophthalmology (Narva´ez) and School of Allied Health Professions (Zimmerman), Loma Linda University, Loma Linda, California, Emory University (Stulting), Atlanta, Georgia, and a private practice (Chang), Scottsdale, Arizona, USA. Presented at the XXIIIrd Congress of the European Society of Cataract & Refractive Surgeons, Lisbon, Portugal, September 2005. No author has a financial or proprietary interest in any material or method mentioned. Corresponding author: Julio Narva´ez, MD, Loma Linda University, Department of Ophthalmology, FMO, 11370 Anderson Street, Suite 1800, Loma Linda, California 92350, USA. E-mail: narvaezjd@verizon. net. Q 2006 ASCRS and ESCRS Published by Elsevier Inc.
2050
resulting in litigation revealed that the most common reasons for litigation were related to the intraocular lens (IOL) and that among those reasons, incorrect choice of power was the most common.1 To achieve optimum outcomes, preoperative biometry must be accurate and an accurate IOL power formula must be used. Although reports suggest there is a difference in the predictive accuracy of older formulas for IOL power calculations,2,3 relatively few studies have compared third-generation IOL power formulas. Hoffer3 found the Hoffer Q formula to be significantly more accurate than the SRK and SRK II (P!.0001 and P!.004, respectively) and equal to the Holladay 1 and SRK/T formulas. He also reported mean absolute error in 317 eyes with 4 commonly used formulas (Hoffer Q, Holladay 1, Holladay 2, and SRK/T) in subjects stratified by axial length but did not perform statistical analysis.4 We compared the refractive outcomes in 643 eyes with the Hoffer Q, Holladay 1, Holladay 2, and SRK/T IOL power formulas after routine cataract surgery by a single surgeon using 1 of 2 similar IOLs. 0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2006.09.009
IOL POWER PREDICTION ACCURACY WITH 4 FORMULAS
PATIENTS AND METHODS A retrospective review was conducted of 643 consecutive eyes that had cataract extraction with IOL implantation under topical anesthesia by one surgeon (R.D.S.) using the same technique. A CC4204BF Collamer plate-haptic IOL (Staar Surgical) was implanted in 338 eyes, and an AA4203VF silicone plate-haptic IOL (Staar Surgical) was implanted in 305 eyes. Inclusion criteria were age 18 and older and 20/40 or better postoperative visual acuity. Exclusion criteria were incomplete postoperative data or perioperative complications. All eyes had ocular biometry with immersion ultrasound by experienced ultrasonographers. Manual keratometry was obtained in all cases. The Holladay 2 formula with a customized surgeon factor was routinely used with both lenses to calculate the actual IOL used. Data were entered into the Holladay IOL Consultant software program, which calculated results for 4 formulas: Hoffer Q, Holladay 1, Holladay 2, and SRK/T. The software compared the predicted final spherical equivalent (SE) refractive error in each eye with each formula with the actual postoperative manifest refraction SE and calculated the difference as the error. The program also calculated and used a personalized lens constant for each formula. Final refraction was performed 2 weeks after surgery or later, by which time refractive stability has been reached in small-incision, clear corneal cataract surgery.5,6 Predictive refractive accuracy for each formula was analyzed in all eyes. For further analysis, eyes were stratified into groups of short, average, medium long, and very long axial length (!22.0 mm, 22.0 to !24.5 mm, 24.5 to 26.0 mm, and O26.0 mm, respectively). Statistical Analysis To determine the significance of the mean absolute differences between the 4 formulas, repeated-measures analysis of variance testing was performed using SPSS statistical software
(version 12.0, SPSS, Inc.). The total experimental level of significance was set at 0.05; however, because of multiple testing, individual tests were done (using the Bonferroni correction) with a significance level of 0.01. A sample size of 643 subjects has a power of over 99% to detect a mean absolute difference of 0.25 diopter (D) among the formulas tested. For the subsets of patients with axial lengths less than 22.0 mm (n Z 25), 22.0 to less than 24.5 mm (n Z 437), 24.5 to 26.0 mm (n Z 137), and greater than 26.0 mm (n Z 44), the power to detect a 0.25 D difference would be 85%, 99%, 99%, and 99%, respectively, for a Z 0.01. RESULTS
There was no significant difference in the accuracy of the 4 formulas in the prediction of postoperative SE refractive error measured by the mean absolute error and no difference when the eyes were stratified according to axial length (Table 1). Although not the primary focus of the study, the descriptive statistics in Table 2 and Table 3 suggest the 2 IOLs did not act the same in the smallest axial length group. The sample size in these subsets is small, so no conclusions could be reached. DISCUSSION
Since the first theoretical formula for IOL power calculation was described by Fedorov et al.7 in 1967, others (eg, Binkhorst, Holladay, Hoffer, Sanders) have aimed to create formulas to improve refractive outcomes. First-generation formulas depended on a single constant to predict the postoperative position of the IOL. Third-generation formulas (Holladay 1, SRK/T, and Hoffer Q) aimed to predict the position of the IOL more accurately, incorporating the effect of corneal curvature. Fourth-generation formulas such as
Table 1. Mean absolute error for all eyes by formula.
Mean Absolute Difference, Predicted Vs Actual Postop SE Refraction (D) G SD Axial Length, mm (Range) !22.0 (21.12–21.98) Mean G SD Range 22.0 to !24.5 (22.00–24.49) Mean G SD Range 24.5 to 26.0 (24.50–25.96) Mean G SD Range O26.0 (26.03–29.50) Mean G SD Range All eyes (21.12–29.50) Mean G SD Range
Eyes
Holladay 1
Holladay 2
Hoffer Q
SRK/T
0.75 G 0.56 0.03 to 1.82
0.75 G 0.60 0.02 to 2.17
0.69 G 0.48 0.09 to 1.62
0.77 G 0.59 0.06 to 1.89
0.52 G 0.43 0.00 to 2.38
0.52 G 0.43 0.00 to 2.46
0.53 G 0.44 0.00 to 2.26
0.52 G 0.43 0.00 to 2.49
0.49 G 0.39 0.01 to 2.43
0.47 G 0.38 0.00 to 2.63
0.50 G 0.39 0.01 to 2.81
0.49 G 0.39 0.00 to 2.26
0.60 G 0.62 0.01 to 3.21
0.53 G 0.55 0.01 to 3.19
0.59 G 0.58 0.02 to 3.03
0.55 G 0.64 0.04 to 3.48
0.53 G 0.44 0.01 to 3.21
0.52 G 0.44 0.01 to 3.19
0.53 G 0.44 0.02 to 3.03
0.53 G 0.45 0.04 to 3.48
25
P Value* .38
437
.74
137
.53
44
.19
643
.45
SE Z spherical equivalent *Repeated-measures analysis of variance
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
2051
IOL POWER PREDICTION ACCURACY WITH 4 FORMULAS
Table 2. Mean absolute error in eyes implanted with the CC4204 BF IOL.
Mean Absolute Difference, Predicted Vs Actual Postop SE Refraction (D) G SD Axial Length (mm)
Eyes
Holladay 1
Holladay 2
Hoffer Q
SRK/T
!22.0 22.0 to !24.50 24.5 to 26.00 O26.00 All eyes (CC4204BF)
14 236 72 16 338
0.85 G 0.58 0.57 G 0.45 0.50 G 0.38 0.78 G 0.73 0.58 G 0.46
0.90 G 0.67 0.56 G 0.44 0.46 G 0.36 0.65 G 0.76 0.56 G 0.46
0.72 G 0.48 0.58 G 0.46 0.51 G 0.36 0.75 G 0.70 0.58 G 0.46
0.91 G 0.58 0.56 G 0.45 0.49 G 0.38 0.65 G 0.83 0.57 G 0.47
SE Z spherical equivalent
the Holladay 2 use other factors, such as corneal diameter and lens thickness, in an attempt to better predict the final position of the IOL. Earlier IOL power calculation formulas can differ in their predictive accuracy.2,3 Also, differences in the accuracy of power calculation formulas may differ for different IOL types.8 Donoso et al.9 examined 212 eyes with the SRK II, Binkhorst II, Hoffer Q, Holladay 2, and SRK-T formulas and inferred that the Binkhorst II and Hoffer Q formulas may provide the best predictive results in small eyes (!22.0 mm) while the SRK/T may be the most accurate for long eyes (O28.0 mm). However, the number of eyes in the small and long axial length groups was small. A study of Chinese patients with long axial lengths (O25.0 mm) found the Hoffer Q formula provided the best predictive result, while the Holladay 1 and SRK/T gave comparable results.10 In that study, however, approximately half the patients had large-incision extracapsular surgery and applanation biometry was used. Both factors could affect the variability of refractive results. Hoffer4 examined the mean absolute error in 317 eyes using 4 formulas. The mean absolute error tended to be lower in average length eyes (22.0 to 24.5 mm) with the Holladay 1 and Hoffer Q formulas. In short eyes (!22.0 mm), the Hoffer Q and Holladay 2 had a lower mean absolute error. The SRK/T showed a trend toward the lowest mean absolute error in medium long (24.5 to 26.0 mm) and very long (O26.0 mm) eyes. The Holladay
2 formula trended toward the least accurate mean absolute error of the 4 formulas in all ranges of axial length except the shortest and very longest. However, no statistical analysis was performed. Our relatively large study using optimized lens constants, immersion ultrasound biometry, and a single surgeon performing surgery with modern techniques statistically compared the refractive outcomes in 643 consecutive eyes with the Hoffer Q, Holladay 1, Holladay 2, and SRK/T IOL power formulas in 4 subgroups of axial length. The 4 commonly used modern formulas were similar in accuracy in all subsets of axial length. Although not the primary focus of the study, descriptive statistics suggest that the 2 IOL types did not act the same in the group with the shortest axial length. The sample size in each IOL subset was small, so no conclusions could be reached. However, this difference indicates that looking at these or other IOLs separately may be useful in further research. As the formulas performed similarly, other factors might be used to select an IOL power formula in clinical use. These include the availability of proprietary software and the need for additional measurements such as IOL thickness, which would increase the preoperative workload. In conclusion, we found no difference in the accuracy of IOL power prediction with the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas in all eyes and in 4 subsets of axial lengths. The 4 formulas were equally accurate at all axial lengths.
Table 3. Mean absolute error in eyes implanted with the AA4203 IOL.
Mean Absolute Difference, Predicted Vs Actual Postop SE Refraction (D) G SD Axial Length (mm)
Eyes
Holladay 1
Holladay 2
Hoffer Q
SRK/T
!22.0 22.0 to !24.50 24.5 to 26.00 O26.00 All eyes (AA4203)
11 201 65 28 305
0.63 G 0.54 0.46 G 0.39 0.48 G 0.40 0.50 G 0.53 0.47 G 0.41
0.56 G 0.45 0.46 G 0.41 0.49 G 0.40 0.46 G 0.39 0.47 G 0.41
0.64 G 0.50 0.46 G 0.40 0.48 G 0.43 0.50 G 0.49 0.48 G 0.42
0.60 G 0.57 0.47 G 0.40 0.49 G 0.41 0.50 G 0.50 0.48 G 0.42
SE Z spherical equivalent
2052
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
IOL POWER PREDICTION ACCURACY WITH 4 FORMULAS
REFERENCES 1. Brick DC. Risk management lessons from a review of 168 cataract surgery claims. Surv Ophthalmol 1999; 43:356–360 2. Olsen T, Thim K, Corydon L. Theoretical versus SRK I and SRK II calculation of intraocular lens power. J Cataract Refract Surg 1990; 16:217–225 3. Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993; 19:700–712; errata 1994; 20:677 4. Hoffer KJ. Clinical results using the Holladay 2 intraocular lens power formula. J Cataract Refract Surg 2000; 26:1233–1237 5. Lyle WA, Jin GJC. Prospective evaluation of early visual and refractive effects with small clear corneal incision for cataract surgery. J Cataract Refract Surg 1996; 22:1456–1460
6. Masket S, Tennen DG. Astigmatic stabilization of 3.0 mm temporal clear corneal cataract incisions. J Cataract Refract Surg 1996; 22:1451–1455 7. Fedorov SN, Kolinko AI, Kolinko AI. [Estimation of optical power of the intraocular lens] [Russian]. Vestn Oftalmol 1967; 80(4):27–31 8. Elder MJ. Predicting the refractive outcome after cataract surgery: the comparison of different IOLs and SRK-II v SRK-T. Br J Ophthalmol 2002; 86:620–622 9. Donoso R, Mura JJ, Lo´pez M, Papic A. Buscando emetropı´a en cirugı´a de catarata con la fo´rmula ma´s indicada para cada ojo segu´n su longitude axial. Arch Soc Esp Oftalmol 2003; 78:477–480 10. Tsang CSL, Chong GSL, Yiu EPF, Ho CK. Intraocular lens power calculation formulas in Chinese eyes with high axial myopia. J Cataract Refract Surg 2003; 29:1358–1364
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
2053
Accuracy of intraocular lens power prediction using the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas Julio Narva´ez, MD, Grenith Zimmerman, PhD, R. Doyle Stulting, MD, PhD, Daniel H. Chang, MD
PURPOSE: To compare the accuracy of intraocular lens (IOL) power calculations using 4 formulas: Hoffer Q, Holladay 1, Holladay 2, and SRK/T. SETTING: Tertiary care center. METHODS: This study was a retrospective comparative analysis. Immersion ultrasound biometry (axial length, anterior chamber depth, and lens thickness), manual keratometry, and postoperative manifest refraction were obtained in 643 eyes of consecutive patients who had routine uneventful cataract surgery with implantation of 1 of 2 IOLs using the same operative technique by the same surgeon. Biometric data were entered into each of the 4 IOL power calculation formulas, and the results were compared to the final manifest refraction. An optimized lens constant was used for each formula. Results were also stratified into groups of short, average, medium long, and very long axial length <22.0 mm, 22.0 to <24.5 mm, 24.5 to 26.0 mm, and >26.0 mm, respectively). RESULTS: No formula was more accurate than the others as measured by mean absolute error. The formulas were also equally accurate when eyes were stratified by axial length. CONCLUSION: The 4 IOL power formulas provided equivalent refractive results in the entire group of eyes and in the subsets of axial lengths tested. J Cataract Refract Surg 2006; 32:2050–2053 Q 2006 ASCRS and ESCRS
Cataract surgery is the most common intraocular surgical procedure performed in the United States. With modern techniques, complications are uncommon and increasing emphasis is being placed on refractive outcomes. Patients also have increasingly higher refractive expectations. A medicolegal review of 168 cataract surgery cases
Accepted for publication July 31, 2006. From the Department of Ophthalmology (Narva´ez) and School of Allied Health Professions (Zimmerman), Loma Linda University, Loma Linda, California, Emory University (Stulting), Atlanta, Georgia, and a private practice (Chang), Scottsdale, Arizona, USA. Presented at the XXIIIrd Congress of the European Society of Cataract & Refractive Surgeons, Lisbon, Portugal, September 2005. No author has a financial or proprietary interest in any material or method mentioned. Corresponding author: Julio Narva´ez, MD, Loma Linda University, Department of Ophthalmology, FMO, 11370 Anderson Street, Suite 1800, Loma Linda, California 92350, USA. E-mail: narvaezjd@verizon. net. Q 2006 ASCRS and ESCRS Published by Elsevier Inc.
2050
resulting in litigation revealed that the most common reasons for litigation were related to the intraocular lens (IOL) and that among those reasons, incorrect choice of power was the most common.1 To achieve optimum outcomes, preoperative biometry must be accurate and an accurate IOL power formula must be used. Although reports suggest there is a difference in the predictive accuracy of older formulas for IOL power calculations,2,3 relatively few studies have compared third-generation IOL power formulas. Hoffer3 found the Hoffer Q formula to be significantly more accurate than the SRK and SRK II (P!.0001 and P!.004, respectively) and equal to the Holladay 1 and SRK/T formulas. He also reported mean absolute error in 317 eyes with 4 commonly used formulas (Hoffer Q, Holladay 1, Holladay 2, and SRK/T) in subjects stratified by axial length but did not perform statistical analysis.4 We compared the refractive outcomes in 643 eyes with the Hoffer Q, Holladay 1, Holladay 2, and SRK/T IOL power formulas after routine cataract surgery by a single surgeon using 1 of 2 similar IOLs. 0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2006.09.009
IOL POWER PREDICTION ACCURACY WITH 4 FORMULAS
PATIENTS AND METHODS A retrospective review was conducted of 643 consecutive eyes that had cataract extraction with IOL implantation under topical anesthesia by one surgeon (R.D.S.) using the same technique. A CC4204BF Collamer plate-haptic IOL (Staar Surgical) was implanted in 338 eyes, and an AA4203VF silicone plate-haptic IOL (Staar Surgical) was implanted in 305 eyes. Inclusion criteria were age 18 and older and 20/40 or better postoperative visual acuity. Exclusion criteria were incomplete postoperative data or perioperative complications. All eyes had ocular biometry with immersion ultrasound by experienced ultrasonographers. Manual keratometry was obtained in all cases. The Holladay 2 formula with a customized surgeon factor was routinely used with both lenses to calculate the actual IOL used. Data were entered into the Holladay IOL Consultant software program, which calculated results for 4 formulas: Hoffer Q, Holladay 1, Holladay 2, and SRK/T. The software compared the predicted final spherical equivalent (SE) refractive error in each eye with each formula with the actual postoperative manifest refraction SE and calculated the difference as the error. The program also calculated and used a personalized lens constant for each formula. Final refraction was performed 2 weeks after surgery or later, by which time refractive stability has been reached in small-incision, clear corneal cataract surgery.5,6 Predictive refractive accuracy for each formula was analyzed in all eyes. For further analysis, eyes were stratified into groups of short, average, medium long, and very long axial length (!22.0 mm, 22.0 to !24.5 mm, 24.5 to 26.0 mm, and O26.0 mm, respectively). Statistical Analysis To determine the significance of the mean absolute differences between the 4 formulas, repeated-measures analysis of variance testing was performed using SPSS statistical software
(version 12.0, SPSS, Inc.). The total experimental level of significance was set at 0.05; however, because of multiple testing, individual tests were done (using the Bonferroni correction) with a significance level of 0.01. A sample size of 643 subjects has a power of over 99% to detect a mean absolute difference of 0.25 diopter (D) among the formulas tested. For the subsets of patients with axial lengths less than 22.0 mm (n Z 25), 22.0 to less than 24.5 mm (n Z 437), 24.5 to 26.0 mm (n Z 137), and greater than 26.0 mm (n Z 44), the power to detect a 0.25 D difference would be 85%, 99%, 99%, and 99%, respectively, for a Z 0.01. RESULTS
There was no significant difference in the accuracy of the 4 formulas in the prediction of postoperative SE refractive error measured by the mean absolute error and no difference when the eyes were stratified according to axial length (Table 1). Although not the primary focus of the study, the descriptive statistics in Table 2 and Table 3 suggest the 2 IOLs did not act the same in the smallest axial length group. The sample size in these subsets is small, so no conclusions could be reached. DISCUSSION
Since the first theoretical formula for IOL power calculation was described by Fedorov et al.7 in 1967, others (eg, Binkhorst, Holladay, Hoffer, Sanders) have aimed to create formulas to improve refractive outcomes. First-generation formulas depended on a single constant to predict the postoperative position of the IOL. Third-generation formulas (Holladay 1, SRK/T, and Hoffer Q) aimed to predict the position of the IOL more accurately, incorporating the effect of corneal curvature. Fourth-generation formulas such as
Table 1. Mean absolute error for all eyes by formula.
Mean Absolute Difference, Predicted Vs Actual Postop SE Refraction (D) G SD Axial Length, mm (Range) !22.0 (21.12–21.98) Mean G SD Range 22.0 to !24.5 (22.00–24.49) Mean G SD Range 24.5 to 26.0 (24.50–25.96) Mean G SD Range O26.0 (26.03–29.50) Mean G SD Range All eyes (21.12–29.50) Mean G SD Range
Eyes
Holladay 1
Holladay 2
Hoffer Q
SRK/T
0.75 G 0.56 0.03 to 1.82
0.75 G 0.60 0.02 to 2.17
0.69 G 0.48 0.09 to 1.62
0.77 G 0.59 0.06 to 1.89
0.52 G 0.43 0.00 to 2.38
0.52 G 0.43 0.00 to 2.46
0.53 G 0.44 0.00 to 2.26
0.52 G 0.43 0.00 to 2.49
0.49 G 0.39 0.01 to 2.43
0.47 G 0.38 0.00 to 2.63
0.50 G 0.39 0.01 to 2.81
0.49 G 0.39 0.00 to 2.26
0.60 G 0.62 0.01 to 3.21
0.53 G 0.55 0.01 to 3.19
0.59 G 0.58 0.02 to 3.03
0.55 G 0.64 0.04 to 3.48
0.53 G 0.44 0.01 to 3.21
0.52 G 0.44 0.01 to 3.19
0.53 G 0.44 0.02 to 3.03
0.53 G 0.45 0.04 to 3.48
25
P Value* .38
437
.74
137
.53
44
.19
643
.45
SE Z spherical equivalent *Repeated-measures analysis of variance
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
2051
IOL POWER PREDICTION ACCURACY WITH 4 FORMULAS
Table 2. Mean absolute error in eyes implanted with the CC4204 BF IOL.
Mean Absolute Difference, Predicted Vs Actual Postop SE Refraction (D) G SD Axial Length (mm)
Eyes
Holladay 1
Holladay 2
Hoffer Q
SRK/T
!22.0 22.0 to !24.50 24.5 to 26.00 O26.00 All eyes (CC4204BF)
14 236 72 16 338
0.85 G 0.58 0.57 G 0.45 0.50 G 0.38 0.78 G 0.73 0.58 G 0.46
0.90 G 0.67 0.56 G 0.44 0.46 G 0.36 0.65 G 0.76 0.56 G 0.46
0.72 G 0.48 0.58 G 0.46 0.51 G 0.36 0.75 G 0.70 0.58 G 0.46
0.91 G 0.58 0.56 G 0.45 0.49 G 0.38 0.65 G 0.83 0.57 G 0.47
SE Z spherical equivalent
the Holladay 2 use other factors, such as corneal diameter and lens thickness, in an attempt to better predict the final position of the IOL. Earlier IOL power calculation formulas can differ in their predictive accuracy.2,3 Also, differences in the accuracy of power calculation formulas may differ for different IOL types.8 Donoso et al.9 examined 212 eyes with the SRK II, Binkhorst II, Hoffer Q, Holladay 2, and SRK-T formulas and inferred that the Binkhorst II and Hoffer Q formulas may provide the best predictive results in small eyes (!22.0 mm) while the SRK/T may be the most accurate for long eyes (O28.0 mm). However, the number of eyes in the small and long axial length groups was small. A study of Chinese patients with long axial lengths (O25.0 mm) found the Hoffer Q formula provided the best predictive result, while the Holladay 1 and SRK/T gave comparable results.10 In that study, however, approximately half the patients had large-incision extracapsular surgery and applanation biometry was used. Both factors could affect the variability of refractive results. Hoffer4 examined the mean absolute error in 317 eyes using 4 formulas. The mean absolute error tended to be lower in average length eyes (22.0 to 24.5 mm) with the Holladay 1 and Hoffer Q formulas. In short eyes (!22.0 mm), the Hoffer Q and Holladay 2 had a lower mean absolute error. The SRK/T showed a trend toward the lowest mean absolute error in medium long (24.5 to 26.0 mm) and very long (O26.0 mm) eyes. The Holladay
2 formula trended toward the least accurate mean absolute error of the 4 formulas in all ranges of axial length except the shortest and very longest. However, no statistical analysis was performed. Our relatively large study using optimized lens constants, immersion ultrasound biometry, and a single surgeon performing surgery with modern techniques statistically compared the refractive outcomes in 643 consecutive eyes with the Hoffer Q, Holladay 1, Holladay 2, and SRK/T IOL power formulas in 4 subgroups of axial length. The 4 commonly used modern formulas were similar in accuracy in all subsets of axial length. Although not the primary focus of the study, descriptive statistics suggest that the 2 IOL types did not act the same in the group with the shortest axial length. The sample size in each IOL subset was small, so no conclusions could be reached. However, this difference indicates that looking at these or other IOLs separately may be useful in further research. As the formulas performed similarly, other factors might be used to select an IOL power formula in clinical use. These include the availability of proprietary software and the need for additional measurements such as IOL thickness, which would increase the preoperative workload. In conclusion, we found no difference in the accuracy of IOL power prediction with the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas in all eyes and in 4 subsets of axial lengths. The 4 formulas were equally accurate at all axial lengths.
Table 3. Mean absolute error in eyes implanted with the AA4203 IOL.
Mean Absolute Difference, Predicted Vs Actual Postop SE Refraction (D) G SD Axial Length (mm)
Eyes
Holladay 1
Holladay 2
Hoffer Q
SRK/T
!22.0 22.0 to !24.50 24.5 to 26.00 O26.00 All eyes (AA4203)
11 201 65 28 305
0.63 G 0.54 0.46 G 0.39 0.48 G 0.40 0.50 G 0.53 0.47 G 0.41
0.56 G 0.45 0.46 G 0.41 0.49 G 0.40 0.46 G 0.39 0.47 G 0.41
0.64 G 0.50 0.46 G 0.40 0.48 G 0.43 0.50 G 0.49 0.48 G 0.42
0.60 G 0.57 0.47 G 0.40 0.49 G 0.41 0.50 G 0.50 0.48 G 0.42
SE Z spherical equivalent
2052
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
IOL POWER PREDICTION ACCURACY WITH 4 FORMULAS
REFERENCES 1. Brick DC. Risk management lessons from a review of 168 cataract surgery claims. Surv Ophthalmol 1999; 43:356–360 2. Olsen T, Thim K, Corydon L. Theoretical versus SRK I and SRK II calculation of intraocular lens power. J Cataract Refract Surg 1990; 16:217–225 3. Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993; 19:700–712; errata 1994; 20:677 4. Hoffer KJ. Clinical results using the Holladay 2 intraocular lens power formula. J Cataract Refract Surg 2000; 26:1233–1237 5. Lyle WA, Jin GJC. Prospective evaluation of early visual and refractive effects with small clear corneal incision for cataract surgery. J Cataract Refract Surg 1996; 22:1456–1460
6. Masket S, Tennen DG. Astigmatic stabilization of 3.0 mm temporal clear corneal cataract incisions. J Cataract Refract Surg 1996; 22:1451–1455 7. Fedorov SN, Kolinko AI, Kolinko AI. [Estimation of optical power of the intraocular lens] [Russian]. Vestn Oftalmol 1967; 80(4):27–31 8. Elder MJ. Predicting the refractive outcome after cataract surgery: the comparison of different IOLs and SRK-II v SRK-T. Br J Ophthalmol 2002; 86:620–622 9. Donoso R, Mura JJ, Lo´pez M, Papic A. Buscando emetropı´a en cirugı´a de catarata con la fo´rmula ma´s indicada para cada ojo segu´n su longitude axial. Arch Soc Esp Oftalmol 2003; 78:477–480 10. Tsang CSL, Chong GSL, Yiu EPF, Ho CK. Intraocular lens power calculation formulas in Chinese eyes with high axial myopia. J Cataract Refract Surg 2003; 29:1358–1364
J CATARACT REFRACT SURG - VOL 32, DECEMBER 2006
2053