Ultrasound Biometry vs. IOL Master

Abstract

Almost half of the errors in intraocular lens power calculation using ultrasound biometry are attributed to axial length measurement errors.1Olsen T. Sources of error in intraocular lens power calculation.J Cataract Refract Surg. 1992; 18: 125-129Abstract Full Text PDF PubMed Scopus (370) Google Scholar We conducted a prospective randomized study to compare the accuracy of partial coherence interferometry with that of ultrasound biometry in predicting the postoperative refractive outcome of cataract surgery after phacoemulsification.Ultrasound biometry (US) followed by partial coherence interferometry (PCI) was done on all patients in the eye to be operated by masked independent examiners (at L V Prasad Eye Institute, Bhubaneswar, India). Biometry based on PCI was done by the IOL Master (IOL Master, Carl Zeiss); axial length was measured by taking an average of 20 observations. Similarly, corneal curvature was measured by the IOL Master taking an average of 3 observations. Contact US was done using a contact probe (A-scan, Biomedix); a minimum of 10 observations was averaged to obtain the axial length. Corneal curvature was measured using the Bausch & Lomb (B&L) keratometer. Intraocular lens (IOL) power was calculated using the SRK II formula. All patients underwent phacoemulsification with implantation of foldable IOL. IOL power to be implanted was the power closest to −0.5 diopters (D) refraction in both the arms.Patients aged 18 years or older, and eyes with no previous history of cataract surgery or any other ocular procedure, nuclear sclerosis grade II to III cataract (as graded with Lens Opacities Classification System III), and a difference between K1 & K2 reading within 1.0 D were included. The mean numerical error (MNE) was defined as the difference between the postoperative refractive outcome at 5±1 weeks and predicted spherical equivalent. The mean absolute error (MAE) was defined as the absolute value of MNE.Forty-four patients were recruited. Of these, 31 completed the study (17 in the PCI group [male: female, 7:10; age: 65±11 years] and 14 in the US group [male: female, 12:2; age: 58±10 years]). The reasons for excluding 13 patients from the analyses were loss to follow-up.The mean axial length measured by the IOL Master was 23.43±1.06mm and by the A-Scan, 23.23±0.98mm; measurements by the 2 methods were significantly different for paired samples (P=<0.0001) (Table 1, available at http://aaojournal.org). Similarly, corneal curvature values measured by the IOL Master was 44.15±1.36D, and by B&L keratometer, 43.70±1.51D; measurements by the two methods were significantly different for paired samples (P = 0.0001) (Table 2, available at http://aaojournal.org). The MNE was 0.19±0.35D and −0.64±1.32D in the PCI and US groups, respectively (t-test: P = 0.0177) (Table 3, available at http://aaojournal.org). The MAE was 0.30±0.25D and 0.94±1.10D in the PCI and US groups, respectively (t-test: P = 0.0278) (Table 3 and Fig 1, available at http://aaojournal.org). The MNE and MAE by the 2 methods were significantly different. All patients in the PCI group were within ±1D of the predicted final visual refractive error as compared to 71.42% in the US group (Fisher exact test, P = 0.032) (Fig 2, available at http://aaojournal.org). The mean final best-corrected visual acuity (BCVA) (Log MAR) was 0.022±0.051 and 0.014±0.035 in the PCI and US groups, respectively (t-test: P = 0.627) (Table 4, available at http://aaojournal.org).Our MNE was 0.19D and −0.64D for the PCI and US groups, respectively, which correlates with the findings of studies conducted by Kiss et al2Kiss B. Findl O. Menapace R. et al.Refractive outcome of cataract surgery using partial coherence interferometry and ultrasound biometry: clinical feasibility study of a commercial prototype II.J Cataract Refract Surg. 2002; 28: 230-234Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar and Bhatt et al3Bhatt A.B. Schefler A.C. Feuer W.J. et al.Comparison of predictions made by the intraocular lens master and ultrasound biometry.Arch Ophthalmol. 2008; 126: 929-933Crossref PubMed Scopus (46) Google Scholar (Table 5, available at http://aaojournal.org). We found the axial length measurements to be 23.43 mm and 23.23 mm with the PCI and US groups, respectively. The axial length was significantly longer in the PCI group. The longer axial length measurement may be due to the different principles involved in axial length measurement used by the US and PCI techniques. Eight percent of errors of IOL power calculation have been attributed to corneal power evaluation.1Olsen T. Sources of error in intraocular lens power calculation.J Cataract Refract Surg. 1992; 18: 125-129Abstract Full Text PDF PubMed Scopus (370) Google Scholar We found that the difference in the mean keratometry (K) as obtained with the PCI and manual B&L keratometer was statistically significant. Our findings were similar to that of Németh et al4Németh J. Fekete O. Pesztenlehrer N. Optical and ultrasound measurement of axial length and anterior chamber depth for intraocular lens power calculation.J Cataract Refract Surg. 2003; 29: 85-88Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar who reported a significant difference in keratometry values. This may account for significant difference in MNE and MAE between both the groups in our study cohort. Our results show that 100% and 71.42% of patients were within ±1D of final predicted refraction in the PCI and US groups, respectively. This is comparable to recent large series reported by Kim et al5Kim S.M. Choi J. Choi S. Refractive predictability of partial coherence interferometry and factors that can affect it.Korean J Ophthalmol. 2009; 23: 6-12Crossref PubMed Scopus (12) Google Scholar (95.7% using PCI technique). In conclusion, the noncontact optical biometry using the partial coherence laser interferometry principle improves the predictive value for postoperative refraction. Almost half of the errors in intraocular lens power calculation using ultrasound biometry are attributed to axial length measurement errors.1Olsen T. Sources of error in intraocular lens power calculation.J Cataract Refract Surg. 1992; 18: 125-129Abstract Full Text PDF PubMed Scopus (370) Google Scholar We conducted a prospective randomized study to compare the accuracy of partial coherence interferometry with that of ultrasound biometry in predicting the postoperative refractive outcome of cataract surgery after phacoemulsification. Ultrasound biometry (US) followed by partial coherence interferometry (PCI) was done on all patients in the eye to be operated by masked independent examiners (at L V Prasad Eye Institute, Bhubaneswar, India). Biometry based on PCI was done by the IOL Master (IOL Master, Carl Zeiss); axial length was measured by taking an average of 20 observations. Similarly, corneal curvature was measured by the IOL Master taking an average of 3 observations. Contact US was done using a contact probe (A-scan, Biomedix); a minimum of 10 observations was averaged to obtain the axial length. Corneal curvature was measured using the Bausch & Lomb (B&L) keratometer. Intraocular lens (IOL) power was calculated using the SRK II formula. All patients underwent phacoemulsification with implantation of foldable IOL. IOL power to be implanted was the power closest to −0.5 diopters (D) refraction in both the arms. Patients aged 18 years or older, and eyes with no previous history of cataract surgery or any other ocular procedure, nuclear sclerosis grade II to III cataract (as graded with Lens Opacities Classification System III), and a difference between K1 & K2 reading within 1.0 D were included. The mean numerical error (MNE) was defined as the difference between the postoperative refractive outcome at 5±1 weeks and predicted spherical equivalent. The mean absolute error (MAE) was defined as the absolute value of MNE. Forty-four patients were recruited. Of these, 31 completed the study (17 in the PCI group [male: female, 7:10; age: 65±11 years] and 14 in the US group [male: female, 12:2; age: 58±10 years]). The reasons for excluding 13 patients from the analyses were loss to follow-up. The mean axial length measured by the IOL Master was 23.43±1.06mm and by the A-Scan, 23.23±0.98mm; measurements by the 2 methods were significantly different for paired samples (P=<0.0001) (Table 1, available at http://aaojournal.org). Similarly, corneal curvature values measured by the IOL Master was 44.15±1.36D, and by B&L keratometer, 43.70±1.51D; measurements by the two methods were significantly different for paired samples (P = 0.0001) (Table 2, available at http://aaojournal.org). The MNE was 0.19±0.35D and −0.64±1.32D in the PCI and US groups, respectively (t-test: P = 0.0177) (Table 3, available at http://aaojournal.org). The MAE was 0.30±0.25D and 0.94±1.10D in the PCI and US groups, respectively (t-test: P = 0.0278) (Table 3 and Fig 1, available at http://aaojournal.org). The MNE and MAE by the 2 methods were significantly different. All patients in the PCI group were within ±1D of the predicted final visual refractive error as compared to 71.42% in the US group (Fisher exact test, P = 0.032) (Fig 2, available at http://aaojournal.org). The mean final best-corrected visual acuity (BCVA) (Log MAR) was 0.022±0.051 and 0.014±0.035 in the PCI and US groups, respectively (t-test: P = 0.627) (Table 4, available at http://aaojournal.org). Our MNE was 0.19D and −0.64D for the PCI and US groups, respectively, which correlates with the findings of studies conducted by Kiss et al2Kiss B. Findl O. Menapace R. et al.Refractive outcome of cataract surgery using partial coherence interferometry and ultrasound biometry: clinical feasibility study of a commercial prototype II.J Cataract Refract Surg. 2002; 28: 230-234Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar and Bhatt et al3Bhatt A.B. Schefler A.C. Feuer W.J. et al.Comparison of predictions made by the intraocular lens master and ultrasound biometry.Arch Ophthalmol. 2008; 126: 929-933Crossref PubMed Scopus (46) Google Scholar (Table 5, available at http://aaojournal.org). We found the axial length measurements to be 23.43 mm and 23.23 mm with the PCI and US groups, respectively. The axial length was significantly longer in the PCI group. The longer axial length measurement may be due to the different principles involved in axial length measurement used by the US and PCI techniques. Eight percent of errors of IOL power calculation have been attributed to corneal power evaluation.1Olsen T. Sources of error in intraocular lens power calculation.J Cataract Refract Surg. 1992; 18: 125-129Abstract Full Text PDF PubMed Scopus (370) Google Scholar We found that the difference in the mean keratometry (K) as obtained with the PCI and manual B&L keratometer was statistically significant. Our findings were similar to that of Németh et al4Németh J. Fekete O. Pesztenlehrer N. Optical and ultrasound measurement of axial length and anterior chamber depth for intraocular lens power calculation.J Cataract Refract Surg. 2003; 29: 85-88Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar who reported a significant difference in keratometry values. This may account for significant difference in MNE and MAE between both the groups in our study cohort. Our results show that 100% and 71.42% of patients were within ±1D of final predicted refraction in the PCI and US groups, respectively. This is comparable to recent large series reported by Kim et al5Kim S.M. Choi J. Choi S. Refractive predictability of partial coherence interferometry and factors that can affect it.Korean J Ophthalmol. 2009; 23: 6-12Crossref PubMed Scopus (12) Google Scholar (95.7% using PCI technique). In conclusion, the noncontact optical biometry using the partial coherence laser interferometry principle improves the predictive value for postoperative refraction. AppendixTable 1The Mean Axial Length (in mm) Measured with the IOL Master and A-ScanIOL MasterA-ScanPaired t-testSample Size (n)3131P = < 0.0001Mean ± Standard Deviation23.43±1.0623.23±0.98Range21.88 to 26.4121.64 to 25.8495% CI for the Mean23.04 to 23.8222.87 to 23.59CI = Confidence Interval. Open table in a new tab Table 2The Mean Corneal Curvature (dioptric power, D) Measured with the IOL Master and B&L KeratometerIOL MasterB&L KeratometerPaired t-testSample Size (n)3131P = 0.0001Mean ± Standard Deviation44.15±1.3643.70±1.51Range41.45 to 47.0540.38 to 47.0095% CI for the Mean43.65 to 44.6543.14 to 44.25CI = Confidence Interval.B&L = Bausch & Lomb. Open table in a new tab Table 3The Mean Numerical Error (MNE) and Mean Absolute Error (MAE) with the Partial Coherence Interferometry (PCI) and Contact Ultrasound (US) TechniquesPCI GroupUS Groupt-test (Comparison of independent means)Variance Ratio Test (F-test)A. Mean Numerical Error (MNE) Sample Size (n)1714P = 0.0177P = < 0.001 Mean ± standard deviation0.19±0.35−0.64±1.32 Variance0.121.73 Range−0.35 to 0.95−4.12 to 0.59 95% CI for the Mean0.01 to 0.37−1.40 to 0.12B. Mean absolute error (MAE) Sample size (n)1714P = 0.0278P = < 0.001 Mean ± standard deviation0.30±0.250.94±1.10 Variance0.061.22 Range0.08 to 0.950.03 to 4.12 95% CI for the Mean0.18 to 0.430.30 to 1.58CI = confidence interval. Open table in a new tab Figure 2Predictability of partial coherence interferometry versus the ultrasound biometry in estimating postoperative refractive error.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Table 4The Mean Final Best-Corrected Visual Acuity (LogMAR) Measured with the Partial Coherence Interferometry (PCI) and Contact Ultrasound (US) TechniquesPCI GroupUS Groupt-test (Comparison of independent means)Variance Ratio Test (F-test)Sample Size (n)1714P = 0.627P = 0.181Mean ± Standard Deviation0.022±0.0510.014±0.035Variance0.0030.001Range0.000 to 0.1760.000 to 0.09795% CI for the Mean−0.004 to 0.048−0.006 to 0.034CI = Confidence Interval.LogMAR = Logarithm of the Minimum Angle of Resolution. Open table in a new tab Table 5Results of Various Studies with Partial Coherence InterferometryMNEMAEALKIOL Formula UsedCommentStudyPCIUSPCIUSPCIUSPCIUSOur study0.19−0.640.300.9423.4323.2344.1543.70SRK TRandomized Control TrialKiss et al2 20020.130.030.480.4623.723.5HolladayRandomized Control TrialBhatt et al3 2008−0.43−0.6023.9723.92SRK TRetrospective Review of Medical RecordAL = axial length (in millimeter); IOL = intraocular lens; K = keratometry (in diopters, D); MAE = mean absolute error; MNE = mean numerical error; PCI = partial coherence interferometry; US = ultrasound biometry. Open table in a new tab CI = Confidence Interval. CI = Confidence Interval. B&L = Bausch & Lomb. CI = confidence interval. CI = Confidence Interval. LogMAR = Logarithm of the Minimum Angle of Resolution. AL = axial length (in millimeter); IOL = intraocular lens; K = keratometry (in diopters, D); MAE = mean absolute error; MNE = mean numerical error; PCI = partial coherence interferometry; US = ultrasound biometry.