Head-to-Head Comparative Study of Two Optical Biometric Devices in Modern Cataract Surgery

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Today's cataract surgeon has adopted non-contact optical
biometry as the standard of care in performing IOL calculations.
While modern formulae incorporate multiple variables as part
of their calculations, Axial Length and Keratometry readings
continue to be the inputs that are most influential in determining
IOL power. Some of the newer generation formulas such as
Holladay 2 and Olsen now incorporate more elements to help
increase accuracy.

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Head-to-Head Comparative Study of Two Optical Biometric Devices in Modern Cataract Surgery

  1. 1. Head-to-Head Comparative Study of Two Optical Biometric Devices in Modern Cataract Surgery 8 Shareef Mahdavi • SM2 Strategic • Pleasanton, CA 7Todays cataract surgeon has adopted non-contact opticalbiometry as the standard of care in performing IOL calculations. Figure 1: IOLMaster vs. LenstarWhile modern formulae incorporate multiple variables as part Inability to Captureof their calculations, Axial Length and Keratometry readings McNemarcontinue to be the inputs that are most influential in determin- n=124 IOLMaster Lenstar Test P-value*ing IOL power. Some of the newer generation formulas such asHolladay 2 and Olsen now incorporate more elements to help AL 8 6.5% 6 4.8% 0.625increase accuracy. Ks 10 8.1% 14 11.3% 0.481 Given the competitive nature of device manufacturers to ACD 11 8.9% 8 6.5% 0.581demonstrate technical superiority of their instruments, surgeons LT – – 19 15.3% –are often left confused in understanding which specific device * There was no significant statistical difference between the two deviceswill best meet their needs in cataract surgery. One surgeon,Amin Ashrafzadeh, MD of Modesto, California (“Dr. Ash”) sure that the measurements I’m getting are serving to opti-wanted to answer this question for himself and undertook the mize my surgical outcomes.” Both devices measure axialtask of performing a prospective comparison of two non-contact length, surface curvature via keratometry, white-to-white,biometers, the IOLMaster 500 (Carl Zeiss Meditec) and the and anterior chamber depth. In addition, the Lenstar mea-Lenstar LS 900 (Haag- sures lens thickness.Streit) across a series Beyond the seriesof 124 consecutive eyes "Our concern as surgeons is in taking axial length of cataract patients,scheduled for cataract measurements that are too short. Short AL readings translate both devices weresurgery. His goal was to to hyperopic surprise for both the surgeon and patient.” also used to measureunderstand the impact 20 eyes of 10 young,of biometry on predicted agile patients to deter-cataract surgery outcomes by comparing the accuracy and mine the impact on workflow by timing how long it takesrepeatability of measurements between the devices. to capture measurements on each device. A statistician was SM2 Strategic was asked by Carl Zeiss Meditec to summa- employed to conduct tests of significance on the results.rize the analysis and findings of Dr. Ash and his team. Neither device was able to capture every measure on everyWhat follows is an eye. Figure 1 showsoverview of the study Figure 2: Difference in Axial Length Measurements that each device wasand its implications for IOLMaster vs. Lenstar (n = 115) unable to obtain atsurgeons trying to deter- 70 least one componentmine which device they 63 of the available cal-should use with their 60 culations on betweencataract patients. 5% and 11% of eyes; Lenstar Lenstar there was no statisti- Frequency measures measures LONGER SHORTERThe Study cal difference in the 20 Dr. Ash utilized 13 capture rate betweenboth devices equally to the devices. It is also 10perform pre-operative 3 4 5 3 important to note 1 2 1 2 1 1 1 1 0 2 1 0 1 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0 1 0 2calculations on a series 1 1 0 0 1 that these data are 0of 124 eyes prior to Axial Length not cumulative (e.g., More 1.4 0.1 0.2 0.3 0.5 0.6 0.8 0.9 1.1 1.2 1.3 1.5 1.6 1.8 1.9 2.1 2.2 2.3 2.5 2.6 2.8 2.9 3.1 3.2 3.3 3.5 1.0 2.0 3.0 -0.5 -0.4 -0.3 -0.2 -0.1 0.4 0.7 1.7 2.4 2.7 3.4 0 Difference in mmcataract surgery. “Each Change in Predicted some eyes could get Refractive Outcome -1 D <-1 D <1 D +1 D +2 D +3 D +4 D +5 D +6 D +9 D +10 D +11 D +12 Dof these devices offers (Dioptors) K reading but nothighly sophisticated ... ... .. ... axial length).technology,” noted Dr. Incidence .. . ... ... .. .. .. .. . ... . .. ... ... .. . .. .. . . . (out of 115 eyes) .Ash. “I want to make
  2. 2. Finding #1: Axial Length Measurements “Our concern as surgeons is in taking axial length mea-Differ Between the Devices surements that are too short,” commented Dr. Ash. “Short The study first analyzed the difference in axial length AL readings translate to hyperopic surprise for both the sur-measurements on the same eye for a series of 115 eyes that geon and patient.” The impact of the difference in readings ishad axial length measurements from each device. Using the shown in the bottom half of Figure 2. A demarcation line isformula, shown at each 0.3 mm of Axial Length Difference along the AL measurement (difference) = IOLMaster – Lenstar, X-axis. Each 0.3 mm step represents approximately 1 dioptertwo-thirds of the eyes (67%) fell within +/- 0.1 mm of one of refractive change as described earlier. In this series of eyes,another. According to if the surgeon chose toHossein Zahed, a MS Figure 3A: Right and Left Eye Symmetry in Each Instrument use the Lenstar reading,in Statistics, the use 29% of eyes (33 of 115)of a range around the 35% would be at risk for adesired mean differ- AL Difference = Right Eye minus Left Eye hyperopic surprise rang-ence of zero (which 30% ing from 1 to 12 diop-would indicate exact IOLMaster: 89% within 0.3mm ters. Conversely, using 25%agreement between the the IOLMaster reading Lenstar: 56% within 0.3mmdevices’ measurements) 20% would yield only 2 ofis a more appropriate 115 eyes at risk for myo-real world application 15% pic surprise of 1 diopter.of statistical analysis. In clinical practice, 10% Finding #2:the impact of improper No impact of learning 5%axial length measure- curve on axial lengthment on predicted 0% measurement -1.7 -1.4 -1.1 -0.8 -0.5 -0.2 -0.1 -0.4 -0.7 -1.0 -1.3 -1.6 -1.9 -2.2 -2.5 -2.8 -3.1 -3.4 -3.7 -4.0refractive outcome is The study analysisthat, “most clinicians Axial Length Difference in mm investigated the impactadopt the rule of thumb of technician learningthat one millimeter curve and whether orof axial length equals "In this series, if the surgeon chose to use the Lenstar not “operator error”three diopters of refrac- could be a factor in the reading, 33 of 115 eyes would be at risk for hyperopictive power,” said Dr. resulting axial lengthAsh. “While the actual surprise ranging from 1 to 12 diopters." measurement on eitherratio changes for upper device. One wouldand lower range IOLs, expect that if a learn-this rough guideline Figure 3B: Axial Length Difference Through Time ing curve was present,provides an easy assess- then readings would bement tool.” Left Eye Right Eye more variable early in 4 “Because of my long- the study and less vari-term use of IOLMaster 3.5 able later in the study.and its wide acceptance, 3 This was not the case.it serves as the bench- 2.5 Differences in readingsmark for comparison between the devices Difference in mm 2in this study.” Using +/- were present through-0.1 mm as statistically 1.5 out the course of thethe same measurement, 1 study and peaked atLenstar’s Axial Length various points without .5measurements are any identifiable pattern. 0shorter than IOLMaster The sole technician thatin 31% of eyes (36 of -.5 took the readings had115) and longer than -1 ten years experience 0 10 20 30 40 50 60IOLMaster in 2% of Observation Number (62 patients) working with ophthal-eyes (2 of 115). mic diagnostic devices.
  3. 3. Finding #3: Intra-patient axial left eyes and right eyes is shownlength measurement differences Figure 4: Mean Keratometric Difference in Figure 3B.exist (eye symmetry) = Mean IOLM K - Mean LS K The next question in the 60 Finding #4: Keratometricstudy pertained to right/left eye 50 50 measurements are in generalsymmetry of axial length mea- 40 32 agreement between the devicessurement in the same patient. 30 When considering all 103 eyes 20Clinical experience in cataract that had keratometric measure- 10 7surgery indicates that most pairs 2 2 4 1 1 1 1 0 1 1 ment on both instruments, 82 0of eyes, while not identical, (79.6%) had mean K’s within 0 5 0 5 0 5 0 5 0 5 0 5 6 0.5 0.2 - 0.2 0.5 0.7 1.0 1.2 1.5 1.7 2.0 2.2 2.2 to <- - o+ o+ o+ o+ o+ o+ o+ o+ o+ >+should be relatively close to one 0.25 D; 91 (88.3%) within 0.5 D 24 to 1t 6t 1t 6t 1t 6t 1t 6t 1t -0. 49 .0 .2 .5 .7 .0 .2 .5 .7 .0another in axial length as well (Figure 4). When considering -0. +0 +0 +0 +0 +1 +1 +1 +1 +2as refractive error. The threshold the value of the astigmatism inadopted by most surgeons is a difference that is within 0.3 patients with ≥ 0.75 D of astigmatism, 65% were withinmm or one diopter. Using this threshold, the IOLMaster 0.25 D and 84% within 0.5 D of one another. In addition,found 89% of eyes measured within 0.3 mm of one 70% were within 10 degrees of axis of one another.another. The Lenstar found56% of eyes measured Figure 5: Impact of ACD on Predicted Finding #5: Anteriorwithin the same parameter. Spherical Equivalent Chamber Depth hasThe distribution of the modest impact on Pt.# 1 2 3 4 5 6 predicted refractiveright/left eye differences for AL 23.87 23.91 23.98 24.01 22.36 21.58 outcomeeach device are shown inFigure 3A. K1 44.35 43.27 43.10 42.56 42.72 40.71 It is well known that INPUTS Next, differences were K2 51.53 47.67 43.95 43.16 44.64 46.46 the greatest impact to IOLexamined between right ACD 3.25 3.42 2.80 2.87 2.66 2.53 calculations is related toeye measurements on both WTW 11.9 11.6 11.0 11.1 11.6 11.7 AL’s then K’s followed bydevices and separately the the ACD. Six sample eyes ACD CALC ADJUSTED IOL Holladay 2 14.0 D 18.0 D 20.0 D 21.0 D 26.0 D 29.5 Dleft eye differences. Using the 3.00 D -0.05 D -0.13 D +0.20 D +0.64 D +0.31 D +0.26 D with true measurementssame range (+/-0.1 mm) as were chosen to represent 4.00 D +0.17 D +0.15 D +0.50 D +0.94 D +0.71 D +0.70 Din Finding #1 above, the for- the range of IOL powers Difference 0.22 D 0.28 D 0.30 D 0.30 D 0.40 D 0.44 Dmulae are as follows: based on the IOLMaster - OD Axial Length Difference = IOLMaster AL OD - Lenstar AL OD data. Different elements were examined to see how much- OS Axial Length Difference = IOLMaster AL OS - Lenstar AL OS impact they had on the predicted refractive outcome. The IOL power was calculated based upon the IOLMaster measurements and analysis was based on Holladay 2 IOL "The IOLMaster found 89% of eyes measured Consultant software (Figure 5). within 0.3 mm of one another. The Lenstar found The two instruments had good correlation in ACD results with 81.3% of the ACD’s within 0.3 mm. The56% of eyes measured within the same parameter." IOLMaster was slightly shorter vs. Lenstar, (3.06 vs. 3.26 mm), but had a tighter Standard Deviation (0.527 In right eyes, the magnitude of the difference was always vs. 0.686). When looking at the sample eyes, even whenwithin 2 mm (range: -0.5 to a 1.0 mm difference in the+1.5), with 42% of eyes being ACD resulted in the maxi- Figure 6: Impact of Lens Thickness on+/- 0.1 mm. Much greater mal impact with the 29.5 D Predicted Spherical Equivalentvariability was seen in the implant, it resulted in aboutleft eyes, both in the overall Pt.# 1 2 3 4 5 6 a 0.44 D predicted refrac-range (0 to 4 mm) and with Holladay 2 14.0 D 18.0 D 20.0 D 21.0 D 26.0 D 29.5 D tive error, and 0.22 D in theonly 26% of left eyes having 2.50mm -0.01 D -0.02 D +0.09 D -0.09 D +0.08 D -0.08 D 14.0 D implant. While stilla difference in axial length an important element, it has “No Info” +0.01 D +0.00 D +0.13 D -0.06 D +0.15 D 0.00 Dof +/- 0.1 mm. A scatter plot modest impact on the final 5.50mm +0.05 D +0.05 D +0.16 D -0.01 D +0.19 D +0.04 Ddiagram showing the differ- IOL power calculation. Difference 0.06 D 0.07 D 0.07 D 0.08 D 0.11 D 0.12 Dence between the devices for
  4. 4. Finding #6: Lens Thickness Has Minimal Impact They undergo extensive testing on other instruments prior toon Predicting Refractive Outcome the optical biometry reading. Even with highly cooperative Of significant interest is the role of lens thickness in patients as used in this study, the added time of the Lenstardetermining the final refractive error following cataract sur- leads to exhaustion.”gery. The study used the same recommended IOL powersin the previous analysis from Figure 5 and applied a range Discussionof lens thickness measurements that cover the full range as Dr. Ashrafzadeh and his team undertook an ambitiousmeasured by the Lenstar device (average: 4.53 mm; low 2.74 project, motivated by a single question on the mind of virtu-mm; high 5.41 mm). For analysis purposes, this range was ally all cataract surgeons: “Which device will give me betteradjusted to 2.50 mm and 5.50 mm as shown in Figure 6. results in order to obtain the best visual outcome possibleThe Holladay 2 Formula was then for my patients? What I foundapplied to show the impact on was that in 2 of every 3 eyes the "Regardless of the thickness of the lens,the predicted spherical equivalent instruments perform identically”for different lens thicknesses (left the resulting difference (1/8th of a diopter he commented. But his concerncolumn) across the sample eyes or less) is below the limit of subjective was in the volatility of the data;IOL Powers (top row). Because discrimination of most patients in visual “should I trust the right eye or leftIOLMaster does not calculate LT, eye, IOLMaster or Lenstar?”the Holladay Formula makes its acuity tests and, more importantly, in In the less forgiving world ofown assessment (shown in the everyday visual tasks." refractive cataract surgery, Dr.row labeled “No Info”); for each Ash stated emphatically, “I cannotpatient example shown, the “No afford to make a guess.” The dataInfo” calculation from Holladay Figure 7: IOLMaster vs. Lenstar Efficiency from this study serve to refute the2 falls within range calculated by claims that Keratometry is morethe low and high lens thicknesses Average Time to accurate on the Lenstar or thatof 2.50 mm and 5.50 mm, respec- Measure Two Eyes DIFFERENCE differences in Anterior Chambertively. An example from Figure 6 N =10 Subjects Depth and Lens Thickness mea-shows the impact of various lens IOLMaster 97.3 SECONDS* surements have a meaningfulthicknesses on a 14 Diopter lens (1:37 FASTER) outcome on the final refractive TIME: 1:41 | RANGE (85-127 SECS.) RANGE (74-113)implant. At 2.5 mm, the predicted outcome. In this series of 124spherical equivalent is 0.01 D Lenstar eyes, none of those hypothesesof myopia while at 5.5 mm it is TIME: 3:18 | RANGE (176-220 SECS.) held true. The different measure-0.05 D of hyperopia. The resulting ments of Axial Length are a moredifference between these is 0.06 D 0 50 100 150 200 significant and troubling find-of predicted spherical equivalent. *P-Value = <0.00001 SECONDS* ing. In the narrower context ofEven at the highest power implant performing cataract surgery onof 29.5 diopters, the difference is 0.12 D. Regardless of the previous LASIK patients, those with prior hyperopic treat-Lens Thickness of the patient, the resulting difference of ments are excluded from multifocal IOL’s due to visual qual-1/8th diopter (or less) is below the limit of subjective dis- ity issues. “Risk of significant hyperopic surprise will needcrimination of most patients in visual acuity tests and, more to be addressed with Excimer laser leading to the same exactimportantly, in everyday visual tasks. profile that would have excluded the patient in the first place and the visual quality issues I do not wish to contend with.”Finding #7: Significant difference in impact In a broader sense, having 29% of eyes at risk for hyper-on workflow between the devices opic surprise is bad for cataract surgeons and their patients. A key consideration for any device is the impact on clinic Of course, most surgeons would re-check and re-measureworkflow. Measurements were taken on 20 eyes of young, Axial Length findings prior to inducing multiple diopters ofagile patients. The results are shown in Figure 7, with the hyperopia. However, that need to re-do measurements goesaverage time to measure both eyes of each subject taking against the very reason surgeons invest in these devices in the3:18 on the Lenstar and 1:41 on the IOLMaster. The differ- first place, which is to save time and reduce errors.ence of 97 seconds was highly significant both statistically In the end, Dr. Ash concluded that “both devices haveand practically in its impact on clinic workflow. “The lon- their unique appeal, but only one device can be counted onger time required for the Lenstar test needs to be put into to avoid surprise and to maintain workflow, and that is thecontext, especially with premium IOL patients in my clinic. IOLMaster.”© Copyright 2012, SM2 Strategic. All Rights Reserved.

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