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Evaluation of a new IOLMaster algorithm to measure axial length

Hill, Warren MD; Angeles, Raymund MD; Otani, Todd OD

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Journal of Cataract & Refractive Surgery: June 2008 - Volume 34 - Issue 6 - p 920-924
doi: 10.1016/j.jcrs.2008.02.021
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Abstract

The advent of new intraocular lens (IOL) technologies such as aspherical monofocal, multifocal, and accommodating and pseudoaccommodating IOLs, in addition to increasing patient expectations, have made the correct measurement of axial length (AL) more crucial than ever in predicting the correct IOL power for cataract surgery. The precision of IOL calculations is determined in part by the accuracy of the AL measurement.1 Axial length measurement errors can be a significant source of error in the estimation of the postoperative anterior chamber depth (ACD).2

The IOLMaster optical biometry system (Carl Zeiss Meditec, AG), and its partial coherence interferometry prototypes, have been extensively studied for AL measurement determination for the calculation of IOL power.1,3–5 This technology has been shown to have excellent intraobserver and interobserver reliability and performance accuracy that is, at a minimum, comparable to those of immersion ultrasound and significantly better than applanation ultrasound.1,3,6–15 This is because optical biometry achieves accuracy within 20 μm (ultrasound is accurate to 100 μm), thus refractive errors stemming from AL mismeasurement are limited to 0.05 diopter (D), which translates to a 5 times more accurate measurement than that obtainable by ultrasound.10

Recently, an updated version of the AL algorithm of the IOLMaster was developed. Changes to the algorithm were created to improve the instrument's ability to provide AL measurements in eyes in which measurements were previously not possible due to very dense cataract or axial opacity dense enough to preclude successful measurement by standard interferometry techniques.

Although many studies of IOLMaster performance do not report a rate of measurement failure, 3,4,15,16 other studies found that from 4% to 18% of cataract patients could not be measured using the IOLMaster due to dense cataract. 7,11,14,17,18 Although most of these studies did not classify and/or grade the cataracts according to their morphology and density, several showed that measurement failure was principally due to the presence of posterior subcapsular opacities.17,18

The purpose of this study was to evaluate the ability of the new IOLMaster with Advanced Technology version 5 software, which uses a new algorithm to measure AL in patients before cataract extraction.

PATIENTS AND METHODS

This study comprised 54 eyes of 33 consecutive patients scheduled for cataract extraction in 1 or both eyes that had preoperative IOLMaster biometric measurements and cataract grading using the Lens Opacities Classification III (LOCS III)19 at East Valley Ophthalmology, Mesa, Arizona. The study conformed to the Declaration of Helsinki and was approved by the Research Consultants Review Committee Institutional Review Board. Subjects were prospectively recruited and consented after receiving a full explanation of the study. Inclusion criteria required that patients be scheduled for cataract extraction in 1 or both eyes and have a routine preoperative evaluation with biometric measurement acquisition using the IOLMaster. Exclusion criteria included physical inability to be positioned at the slitlamp biomicroscope or IOLMaster (eg, head tremor), inability to open the eyelid widely enough so all measurements could be performed, inability to fixate due to ocular disease (macular degeneration or amblyopia), corneal or media opacities other than cataract that cause acquisition failure (eg, corneal scar, vitreous condensation), and active ocular infection or inflammation process of the eye.

All patients had a complete ophthalmic examination that included measurement of uncorrected visual acuity and best spectacle-corrected visual acuity (BSCVA), manifest refraction, and slitlamp and fundus evaluation. In addition, the cataracts were graded using the LOCS III19 by 2 examiners.

Axial Length Measurement

The IOLMaster measures AL, anterior corneal radii, ACD, and the white-to-white distance in the human eye. These measurements aid cataract surgeons in calculating the proper IOL power.

For this study, the recently released version 5 software was installed in an existing commercial IOLMaster instrument using the Windows XP operating system. The algorithm for data acquisition was identical to the previous commercial release of software (version 4). The primary difference between version 4 and version 5 is the addition of a new AL calculation algorithm using a digital signal-processing algorithm. The AL calculation algorithm in version 5 is accessed by a button on the user interface, which allows the new algorithm to analyze the same AL data set acquired and analyzed by the version 4 algorithm. Instead of averaging the acquired AL measurements from each single scan (version 4 algorithm), the new algorithm obtains a composite AL measurement from multiple scans. The composite AL is generated by digitally processing the signal of multiple measurements, which results in an enhanced signal-to-noise (SNR) ratio.

The AL in each study eye was measured after all required ocular examinations were completed. Twenty AL measurements were acquired. For the study, AL measurements reported using the version 4 algorithm were referred to as “standard,” and AL measurements reported using the new software algorithm were referred to as “composite.” The following describes the measurements recorded for each method:

  1. Standard mean value of first 5 measurements (standard-5) refers to the mean AL measurement reported after 5 measurements were taken with the version 4 algorithm.
  2. Standard manipulated mean value (standard manipulated) refers to the mean AL measurement reported after the examiner determined the best (AL measurements with ±0.02 mm) of the 20 AL measurements acquired with the version 4 algorithm.20
  3. Composite mean value of the first 5 measurements (composite-5) refers to the composite AL measurement calculated from the first 5 measurements using the version 5 algorithm.
  4. Composite mean value of 20 measurements (composite-20) refers to the composite AL measurement calculated from 20 measurements using the version 5 algorithm.

The AL measurements derived from the new algorithm were not used for the preoperative computation of IOL power of the patients enrolled in the study.

Statistical Analysis

The AL availability and the corresponding 95% confidence intervals for each of the 4 measurement methods were calculated based on the binomial exact method. The mean, standard deviation, and the range of the available AL measurements were summarized for each measurement method.

For each measurement method, the AL availability was stratified by the cataract grades of nuclear color, nuclear opalescence, cortical, and posterior subcapsular per LOCS III grades. The LOCS III grades were combined so the sample size in each grade group would be at least 5 eyes. To evaluate the effect of cataract density on each of the 4 measurement methods, the Fisher exact test was performed to compare the AL availabilities across different cataract grades of nuclear color, nuclear opalescence, cortical, and posterior subcapsular.

All statistical analyses were done using SAS software (version 9, SAS Institute, Inc.).

RESULTS

The mean age of the patients was 75.4 years ± 9.6 (SD) (range 53.2 to 92.9 years). The BSCVA was 20/40 or better in 50% of eyes and 20/200 or worse in 13%. The mean spherical equivalent in all eyes was −0.285 ± 2.53 D (range −7.50 D to +6.125 D).

Table 1 shows the overall AL measurement distribution and availability for each of the 4 measurement methods. Axial length measurements were obtained in 55.6% of eyes using the standard-5 method versus more than 90.0% of eyes using the standard manipulated method and the 2 composite methods.

Table 1
Table 1:
Axial length availability.

In terms of nuclear color, the standard-5 method had the lowest overall success in the measurement of AL (Table 1). Successful AL measurement with the standard-5 method ranged from 42.9% to 71.4% for the different nuclear color grades. However, the difference in success of AL measurement was not statistically significant (P = .6658). Of the 4 measurement methods, the composite-20 method had the most consistent success for AL measurement across different nuclear color grades.

Similarly, the standard-5 method had the lowest success for AL measurement across the different nuclear opalescence grades (range 30% to 80%) and cortical grades (range 46.7% to 66.7%). However, the difference in the successful measurement of AL measurement was not statistically significant. The effects of nuclear opalescence grades and cortical grades on the availability of measuring AL were not statistically significant for the 2 composite methods or the manipulated standard method.

Unlike the nuclear color, nuclear opalescence, and cortical grades, the posterior subcapsular grades statistically significantly affected the ability of the standard-5 method and the 2 composite methods to successfully measure AL (P<.05) and marginally affected the standard manipulated method (P = .0868) (Table 1). In eyes with a posterior subcapsular grade of 5.0 or above, AL measurement availability was 28.6% with the standard-5 method; the availability was significantly better with the standard manipulated, composite-5, and composite-20 methods (71.4%, 57.1%, and 71.4%, respectively).

DISCUSSION

Axial length measurement acquisition failure with the IOLMaster has been reported in the literature.7,11,14,17,18 Causes have been attributed to an inability to position the patient at the instrument (eg, head tremor), a combination of low vision and lens opacity, and fixation difficulties due to macular disease.7,17,18 Dense nuclear cataracts and posterior subcapsular cataracts appear to be the most common reported cause of AL measurement acquisition failure,7,11,17,18 which is consistent with the findings in our study. However, contrary to the findings in a study by Freeman and Pesudovs,18 in which 100% of mature cataracts and posterior subcapsular cataracts with LOCS III grade P>3.5 could not be measured, the AL measurement acquisition failure rates of the IOLMaster in our study never reached 100%, even for cataracts with a LOCS III grade of 5 and above for all cataract grades. The variation in failure rates across several studies may be attributed to differences in cataract morphology, and density of the sample population, and probably most important, differences in acquisition techniques and methods.

In this study, we evaluated the new software version of the IOLMaster to acquire AL measurements using 4 methods (2 standard, 2 composite). Two methods required the minimum of 5 AL measurements and the other 2 methods required the maximum of 20 AL measurements that can be acquired with the IOLMaster. In addition, we compared 3 methods (standard-5, composite-5, and composite-20) and the manipulated standard method because this is currently the method recommended by the manufacturer of the IOLMaster and by one of the authors (W.H.) for the version 4 software. This method involves taking 20 measurements; at least 4 measurements should be within 0.02 mm of one another and show the characteristics of a good AL display as follows: SNR ratio greater than 2.0; tall, narrow primary maxima with a thin, well-centered termination; and at least 1 set of secondary maxima.20 Although anecdotally this method delivers a very high success rate of AL measurement acquisition in a broad range of cataract patients in terms of morphology and severity, it is more time consuming and requires a very skilled and experienced operator.

In summary, our study found that success rates of the 2 composite methods were similar to the success rate of the manipulated standard method and far superior to the standard-5 method. This should translate to less operator-induced variability, more reproducible measurements, more patients being measured more quickly, and an overall improvement in practice efficiency.

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© 2008 by Lippincott Williams & Wilkins, Inc.