Comparison of A-scan device accuracy Ulrich Giers, M.D., Claudia Epple, M.D.
AB TRACT A- can biometry i r cogniz d a a u eful aid in pr dicting intraocular len power. Mea urements a~ report d t b accurate to b tter than ± 0.1 mm. Thr biometry de ice were c mpar d in examination of 159 per on each e aminati n bing rep ated e er I time . Axial length a eraged 23.77 mm when mea ured b the immer ion technique· applanation and modified applanation technique i Ided 0.1 mm and 0.3 mm hort r di tance r p cti ly. M a ur d alue of axial length did not ha e the am probability of bing mea ur d e en if th wer clo together. Re ult w re not di tributed in mooth au ian cur e . on the contrary clu t r of alue on th pattern of our choice of ultra ound wa elength, wer een e en when biom try wa p rformed with el ctronic ate in th de ic . R te t reliability decr a ed ,hen hort di tance in th anterior m nt f the ey ,ere mea ured. Mea urement of I n thickne wer Ie readily r producible in cataractou len e than in h alth young e' anterior chamb r depth on th other hand ~ a mea ured m r reliably in cataract patient . Thi la t finding ma ha re ult d in part fr m uncertaintie about ultra ound elocity in the cataractou len and in part fr m accommodation. In catara t pati nt axial length wa mea ur d mo t repr ducibly by the imm r ion te hnique; it 'a mea ured Ie accurat I in oung health e with a modifi d applanati n d ic.
Ke Word: ant ri r hamb r d pth, applanati n t chniqu, - can bi m tr ,< iall nth, imm r i n t hni u ,intra ular din dapple nati n t hniqu I n p \ r m
Preoperative A-scan biometry is a widely practiced method of collecting the data needed to calculate intraocular lens power. There are presently three competing techniques: immersion, applanation, and modified applanation. The various versions of the last incorporate elements of the immersion and applanation techniques, aiming to combine their advantages and avoid their shortcomings. Preoperative A-scan biometry is assumed by some authors to be accurate to about ±O.l mm. 1 ,2,3 Shammas,4 on the other hand, has
found the contact technique to yield 0.24 mm shorter axial eye lengths than the immersion technique. Some authors 3,5 recommend the precaution of taking the mean of several successive ultrasonographic measurements preoperatively. This recommendation is sensible only if there is good reason to question the high degree of accuracy ascribed to A-scan biometry at the outset. But these communications, and the fact that the contact and immersion techniques produce widely differing results, are a presumption for a source of
Presented in part at the 2nd American-International Congress on Cataract, IOL and Refractive Surgery, Washington, D.C., April 1989. Supported by Professor E. SchUtte, M.D., Eye Department, Military Hospital, Ulm, West Germany.
w:
Gaas, Ph.D., provided the statistical analysis.
Reprint requests to Ulrich Giers, M.D., University Eye Hospital, Prittwitzstrasse 43, D-7900 Ulm, West Germany.
J CATARACT REFRACT
SURG- VOL 16, MARCH 1990
235
considerable error, i.e., a source of unexpected postoperative ametropia. There is a need to examine the accuracy of various ultrasonographic measuring techniques-to examine, first, the accuracy of absolute measurements of various segments of the globe and, second, the retest reliability of the techniques. Unfortunately none of the biometry devices available at the time of our experiments would have lent itself to all three coupling techniques. We accordingly decided to experiment with three ultrasound devices, each used for the type of coupling procedure most actively promoted by the manufacturer. This design had the advantage of mirroring the circumstances of everyday clinical practice. The study was intended to answer two questions. First, does modified applanation combine only the merits of the other two coupling techniques, without their shortcomings? Second, is it really necessary in ultrasonic biometry to take the means of series of measurements?
MATERIALS AND METHODS
Measurement by the immersion technique was performed on supine patients with a Kretz 7200 MA. A plastic cup to hold the water-coupling path was placed on the bulbar conjunctiva and filled with physiologic saline, methylcellulose being added to the solution when necessary. A lO-MHz A ultrasound probe was then immersed in the water bath and moved, at a distance of5 mm to 10 mm from the cornea, to produce optimum reflections from all interfaces following the criteria ofOssoinig3 and Hoffer. 6 ,7 This procedure was performed three times in succession; measurements were photographed from the oscilloscope display with a Polaroid camera. Measurement by the contact technique was performed with a Humphrey ultrasonic biometer 810. Via an adapter, a nonfocused 10-MHz A ultrasound probe was connected to a Goldmann applanation tonometer so biometry could be carried out analogously to an applanatory determination of intraocular pressure. Because eye movement could be followed in only two dimensions with the probe (manipulation of the cone prism in applanation tonometry is restricted in the same way), patients occasionally had to correct their gaze. Measurements were frozen and printed out when strong echoes were measured at low amplification. Three successive measurements were taken by this technique. For measurements by a modified contact technique, we used a CooperVision ultras can digital B; the probe had an integral conic water-coupling path, closed at the front with a flexible membrane. As in the immersion technique, the probe was hand-held by the investigator, but the subjects were seated rather than supine. The probe had a built-in light-emitting diode to 236
J CATARACT REFRACT
facilitate fixation. Because the coupling path was filled with an incompressible fluid, namely water or saline, an outlet for it had to be provided in an appropriate place. The purpose of using a flexible membrane was to help prevent corneal applanation, and hence distortion of measured distances. Measurements by this technique were repeated twice. The coupling techniques were performed preoperatively in random order on 60 cataract patients and, for comparison's sake, on 99 adolescents with healthy eyes; refractive ametropia was the only ocular change exhibited by the adolescent subjects. Further, anterior chamber depth was measured with a HaagStrait pachymeter. As a description of the accuracy of ultrasound measurements in any device, we computed a coefficient of reliability from the thrice repeated biometries performed with each device. The (auto)correlation between the first and the second, the first and the third, and the second and the third measurements was computed. The mean of these three correlation coefficients was used as the coefficient of reliability.
RESULTS As Figure 1 shows, the mean anterior chamber depth of all 159 subjects was 3.63 mm. The cataract patients had a slighter lower mean, the adolescents a slightly higher mean. As measured by the contact technique, mean anterior chamber depth was 0.3 mm mm 5
non - cataract
cataract n-60
n=99
4
-,
3
2
o Fig. 1.
A
M
0
A
M
0
(Giers) Anterior chamber depths (ACD) determined sonographically by various coupling techniques (I = immersion technique, A = applanation technique, M = modified applanation technique) and optically by pachymetry (0). Flatter ACD values were determined by the applanation technique and optically; the difference was more marked in the patients with cataracts than in those without. The histogram shows means and standard deviations.
SURG-VOL 16, MARCH 1990
mm 5
mm non - cataract
cataract
(n =159)
cataract (n =60 )
24
4
23
3
A
Fig. 2.
M
A
M
(Giers) Lens thicknesses determined sonographically by various coupling techniques (I = immersion technique , A = applanation technique, M = modified applanation technique). The histogram shows means and standard deviations. With all three techniques, the cataractous lenses were appreciably thicker than the adolescent, unoccluded lenses. On average, the largest lens thicknesses were measured by the immersion technique.
less, or 3.32 mm, in the sampling as a whole; the cataract patients had Hatter anterior chambers than the adolescent subjects. The modified applanation technique yielded a mean anterior chamber depth of 3.56 mm, an intermediate value slightly less than that obtained by the immersion technique. Measured lens thicknesses also differed from one technique to the next (Figure 2). On the immersion technique, the means in the total sampling, the cataract patients, and the adolescents were 4.12 mm, 4.41 mm, and 3.92 mm, respectively. Measurements by the contact technique yielded a collective mean of 3.93 mm ; the mean thicknesses of the cataractous and adolescent lenses were 4.25 mm and 3 .74 mm . Similar, but again somewhat lower, findings were obtained by our choice of a modified applanation technique. With this technique, the collective mean was 3.83 mm; the cataractous and adolescent lenses had mean thicknesses of 4.13 mm and 3.65 mm, respectively. These same techniques also yielded different axial length measurements . Again, the longest measurements were taken by the immersion technique. As seen in Figure 3, mean axial length was 0.33 mm longer with the immersion technique (23.77 mm) than with the J CATARACT
J Fig. 3.
A
M
A
M
(Giers) Axial lengths determined sonographically by various coupling techniques (I = immersion technique , A = applanation technique , M= modified applanation technique). Depicted are the values for the sampling as a whole and for the subgroup of cataractous eyes. In both groups the applanation technique yielded 0.3 mm shorter mean values than the immersion technique. Measurements averaged 0.1 mm less with the modified applanation technique than with the immersion technique.
contact technique (23.44 mm); the modified contact technique yielded an intermediate value of23.63 mm. The same differences were noticed in the subgroups of cataract patients and adolescents; among the cataract patients, it should be noted, the difference to mean axial length as determined by the immersion technique and modified applanation was a very slight 0.09 mm. The lO-MHz probes were used in all three procedures; the ultrasound wavelength was accordingly about 0.15 mm. In Figure 4, measurements by the modified applanation technique show that adjacent lengths were not equally likely to be measured. The undulating curve of this frequency distribution plainly deviates from the expected Gaussian distribution. Marked jumps are evident in the relative frequencies with which neighboring values were measured. Similar jumps are also apparent in the frequency distribution of axial lengths (Figure 5), as measurements by the applanation technique illustrate. The minima and maxima are 0.15 mm apart, agreeing with our choice of ultrasound wavelength. The retest reliability of all three techniques as measures of axial length was very high, as shown in Table 1 and in Figures 6 and 7. The reliability
REFRACT SURG- VOL 16, MARCH 1990
237
17
la 115 1" 13 12
F U
R
10
E Q
9
U
I
E N C y
a
7
15
. 3 2
1
mm
0
2.00
2.2!5
2.!50
2.7!5
3.00
3.2!5
3.!50
3.7!5
4.00
4.25
4.!50
4.75
ANTERIOR CHAMBER DEPTH Fig. 4.
(Giers) Frequency distribution of anterior chamber depth measurements obtained by a modified applanation technique (N = 477). Adjacent values were not equally likely to occur; rather. the distribution curve exhibits undulations at intervals of 0.15 mm, on the pattern of the ultrasound frequency used. This clearly limited the resolving power of the measuring device.
35
n
30
25
F
R E Q U E
20
15
N
C Y
10
5
0
\
,
~\J\~~
mm
20.020.521.021.522.022.523.023.524.024.525.025.5
26.0
26.5
27.0
27.5
2B.0
AXIAL LENGTH Fig. 5.
238
(Giers) Frequency distribution of axial length measurements obtained by the applanation technique (N = 477). Adjacent values clearly differed in probability of occurrence. The 0.15 mm between peaks in the frequency distribution corresponds to our choice of ultrasound wavelength.
J CATARACT REFRACT
SURG- VOL 16, MARCH 1990
RELIABILITY AXIAL LENGTH
1.0
ACD
LENS
0.9 Fig. 6A.
0.8
(Giers) Retest reliability of ultrasound biometry on 60 cataractous eyes; each procedure was performed three times in succession. With all three techniques, the highest degree of reliability was attained in measuring axial length; measurements of shorter segments of the anterior chamber were less readily reproducible (I = immersion technique, A = applanation technique, M = modified applanation technique).
J
HUMPBULi 27.48 26.44 25 . 39 24 . 35 23.31 22 . 27 21.22
26.34
25.43 24.51 HUMPBUL2 23.60 22.68 24.51 HUMPBUL3
Fig. 6B.
(Giers) Each column in Figure 6A represents the reproducibility of biometry. which can also be seen in this three-dimensional graph. Axial length measurement, for example. was characterized by a high degree of reproduction of findings (applanation technique, n = 159). The three axes are the first, second, and third measurement.
J CATARACT REFRACT SURG-VOL 16. MARCH 1990
239
RELIABILITY
to
AXIAL LENGTH
ACD
LENS
0.9 Fig. 7A.
(Giers) Retest reliability of three successive measurements in adolescents with healthy eyes (n = 99). The retest reliability of measurements of axial length and anterior chamber depth was not quite as high as in the cataract patients. A notable finding is the comparatively poor reproducibility of sonographic measurements of anterior chamber depth by the modified applanation technique (I = immersion technique, A = applanation technique , M= modified applanation technique).
0.8
J
CVVOR01
4.60 4.24 3.89 3.53 3.17 2.81
3.84 3.51 CVVOR02 3.19 2.86
2.46 2.10
4.80
3.69 CVVOR03
Fig. 7B .
240
(Giers) Retest r eliability was somewhat lower in the anterior segment of the bulb as shown in this three-dimensional graph (three times ACD measurement by modified applanation technique , n= 159). The three axes of the graph are the first , second , and third measurement with the same device.
J CATARACT REFRACT SURG-VOL 16. MARCH 1990
Table 1. Retest reliability coefficient for measurement of axial length.
Table 2. Retest reliability coefficient for measurement of lens thickness.
Technique
Cataractous Eyes (n = 60)
Noncataractous Eyes (n = 99)
Total (N = 159)
Technique
Cataractous Eyes (n = 60)
Noncataractous Eyes (n = 99)
Immersion
0.99
0.96
0.98
Immersion
0.86
0.90
Applanation
0.98
0.96
0.97
Applanation
0.88
0.90
Modified applanation
0.98
0.97
0.98
Modified applanation
0.87
0.92
coefficients of repeat measurements were between 0.96 and 0.99, evidence that single measurements were highly reproducible. With reliability coefficients between 0.86 and 0.92, repeat measurements oflens thickness were somewhat less reliable. This can be seen from Table 2 and Figures 6 and 7. Retest reliability fluctuated most in measurements of anterior chamber depth. The modified contact technique, with reliability coefficients of 0.78 and 0.88 in the subgroups of adolescents and cataract patients, was considerably less reliable than the immersion and contact techniques. The most readily reproducible measurements of anterior chamber depth were given by the contact technique in cataract patients; the reliability coefficient was 0.96 (Table 3).
DISCUSSION Results were evaluated in three ways. First, we compared our three sets of absolute values from . ultrasound biometry of segments of the globe; this was a means of comparing the accuracy of the coupling techniques investigated. Second, we examined whether adjacent values had the same probability of being measured. This was expected to supply information about the comparative resolving powers of the techniques. Third, we examined whether the reliability of preoperative biometry could be improved by taking the mean of several measurements. Our results will be discussed in the same order. With the contact technique, axial length and anterior chamber depth were 0.3 mm shorter than the distances measured by the immersion technique. This systematic error was seen in cataract patients and in adolescents with healthy eyes; the order of magnitude was invariably the same whether anterior chamber depth or axial length was being measured. These findings closely agree with those of Shammas. 4 Since flattening of the anterior cham ber and shortening of the axial length were found, corneal applanation by about 0.3 mm may be assumed to be responsible for this error in measurements by the contact coupling technique. Like all systematic errors in measurement, this one can be corrected by introducing a correction factor, as long J CATARACT REFRACT
Table 3. Retest reliability coefficient for ultrasonic measurement of anterior chamber depth.
Technique
Cataractous Eyes (n = 60)
Noncataractous Eyes (n = 99)
Immersion
0.92
0.91
Applanation
0.96
0.89
Modified applanation
0.88
0.78
as one knows the biometric design on which lens power calculation was based. We noted less systematic distortion with corneal applanation, at least when a flexible membrane and an integral water coupling path were used; measurements of anterior chamber depth and axial length were reduced less markedly than they were by the pure contact method. This gain was offset by a loss in the reproducibility of anterior chamber depth measurements. The value of reproducible measurements of anterior chamber depth depends on which formula is used to calculate implant lens power. When a regression formula is used and implant power is calculated from axial length alone, reproducibility will be less critical than when an individual, expected postoperative anterior chamber depth has to be inserted into a physicaloptical formula. Despite manufacturers' contentions that an ultrasound frequency of 10 M Hz will not limit the resolving power of sonographic measurements as long as suitable electronic biometry equipment is used, we found marked undulations in the frequency distributions of adjacent values on all three techniques. These are at odds with the nearly normal distribution of biological measurements. The undulations characteristically occurred at intervals of about 0.15 mm in the frequency distributions of all measurements of globe segments and were demonstrable in connection with all three techniques. Adjacent lengths obviously did not have the same probability of being measured. The remarkably close correspondence with the wavelength of the ultrasound signal used indicates that the frequency
SURG- VOL 16, MARCH 1990
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used did limit resolution and deprived adjacent lengths of an equal opportunity to be measured. We propose this observation as a counterargument to the view, frequently encountered in the literature,1,2,3 that ultrasound sonography has an accuracy better than ±O.l mm. Taking the mean of several biometries has been recommended as a way of improving accuracy. This is a sensible expedient when successive measurements by the same technique yield widely fluctuating values; in such cases, taking a mean can resolve uncertainties. At least as used to measure axial length, all three coupling techniques shared such a high retest reliability that the advantages of taking means would appear to be negligible. With reliability coefficients of better than 0.96, one can argue that the time available for biometry will be better spent on single high-quality measurements than on compulsively taking the mean of several measurements. As expected, measurements of short segments of the anterior chamber demonstrated a poorer retest reliability. The differences between the techniques were also most pronounced. Thus, the immersion technique
242
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can be recommended for measuring segments of the anterior chamber. The applanation technique did afford the closest agreement when several measurements were taken, but it was also accountable for the worst distortions of measured distances. Our choice of a modified applanation technique, on the other hand, was weakest in respect to reproducibility (obviously a consequence of the fixation stimulus in the probe). REFERENCES 1. Binkhorst RD: The accuracy of ultrasonic measurement of the axial length of the eye. Ophthalmic Surg 12:363-365, 1981 2. Gernet H: Zur Kunstlinse nach Mass bei Ametropien. Kiln Monatsbl Augenheilkd 180:127-131, 1982 3. Ossoinig KC: How to obtain maximum measuring accuracies with standardized A-scan. Doc Ophthalmol Fmc Ser 38: 197-216, 1984 4. Shammas HJ: A comparison of immersion and contact techniques for axial length measurement. Am Intra-Ocular Implant Soc] 10:444-447, 1984 5. Strobel J: Die Berechnung der Brechkrafte von intraokularen Linsen. Fortschr Ophthalmol 82:165-167, 1985 6. Hoffer KJ: Biometry of7 ,500 cataractous eyes. Am] Ophthalmol 90:360-368, 1980 7. Hoffer KJ: Preoperative cataract evaluation: Intraocular lens power calculation. lilt Ophthalmol Clill 22(2):37-75, 1982
SURC-VOL 16, MARCH 1990
AB TRACT A- can biometry i r cogniz d a a u eful aid in pr dicting intraocular len power. Mea urements a~ report d t b accurate to b tter than ± 0.1 mm. Thr biometry de ice were c mpar d in examination of 159 per on each e aminati n bing rep ated e er I time . Axial length a eraged 23.77 mm when mea ured b the immer ion technique· applanation and modified applanation technique i Ided 0.1 mm and 0.3 mm hort r di tance r p cti ly. M a ur d alue of axial length did not ha e the am probability of bing mea ur d e en if th wer clo together. Re ult w re not di tributed in mooth au ian cur e . on the contrary clu t r of alue on th pattern of our choice of ultra ound wa elength, wer een e en when biom try wa p rformed with el ctronic ate in th de ic . R te t reliability decr a ed ,hen hort di tance in th anterior m nt f the ey ,ere mea ured. Mea urement of I n thickne wer Ie readily r producible in cataractou len e than in h alth young e' anterior chamb r depth on th other hand ~ a mea ured m r reliably in cataract patient . Thi la t finding ma ha re ult d in part fr m uncertaintie about ultra ound elocity in the cataractou len and in part fr m accommodation. In catara t pati nt axial length wa mea ur d mo t repr ducibly by the imm r ion te hnique; it 'a mea ured Ie accurat I in oung health e with a modifi d applanati n d ic.
Ke Word: ant ri r hamb r d pth, applanati n t chniqu, - can bi m tr ,< iall nth, imm r i n t hni u ,intra ular din dapple nati n t hniqu I n p \ r m
Preoperative A-scan biometry is a widely practiced method of collecting the data needed to calculate intraocular lens power. There are presently three competing techniques: immersion, applanation, and modified applanation. The various versions of the last incorporate elements of the immersion and applanation techniques, aiming to combine their advantages and avoid their shortcomings. Preoperative A-scan biometry is assumed by some authors to be accurate to about ±O.l mm. 1 ,2,3 Shammas,4 on the other hand, has
found the contact technique to yield 0.24 mm shorter axial eye lengths than the immersion technique. Some authors 3,5 recommend the precaution of taking the mean of several successive ultrasonographic measurements preoperatively. This recommendation is sensible only if there is good reason to question the high degree of accuracy ascribed to A-scan biometry at the outset. But these communications, and the fact that the contact and immersion techniques produce widely differing results, are a presumption for a source of
Presented in part at the 2nd American-International Congress on Cataract, IOL and Refractive Surgery, Washington, D.C., April 1989. Supported by Professor E. SchUtte, M.D., Eye Department, Military Hospital, Ulm, West Germany.
w:
Gaas, Ph.D., provided the statistical analysis.
Reprint requests to Ulrich Giers, M.D., University Eye Hospital, Prittwitzstrasse 43, D-7900 Ulm, West Germany.
J CATARACT REFRACT
SURG- VOL 16, MARCH 1990
235
considerable error, i.e., a source of unexpected postoperative ametropia. There is a need to examine the accuracy of various ultrasonographic measuring techniques-to examine, first, the accuracy of absolute measurements of various segments of the globe and, second, the retest reliability of the techniques. Unfortunately none of the biometry devices available at the time of our experiments would have lent itself to all three coupling techniques. We accordingly decided to experiment with three ultrasound devices, each used for the type of coupling procedure most actively promoted by the manufacturer. This design had the advantage of mirroring the circumstances of everyday clinical practice. The study was intended to answer two questions. First, does modified applanation combine only the merits of the other two coupling techniques, without their shortcomings? Second, is it really necessary in ultrasonic biometry to take the means of series of measurements?
MATERIALS AND METHODS
Measurement by the immersion technique was performed on supine patients with a Kretz 7200 MA. A plastic cup to hold the water-coupling path was placed on the bulbar conjunctiva and filled with physiologic saline, methylcellulose being added to the solution when necessary. A lO-MHz A ultrasound probe was then immersed in the water bath and moved, at a distance of5 mm to 10 mm from the cornea, to produce optimum reflections from all interfaces following the criteria ofOssoinig3 and Hoffer. 6 ,7 This procedure was performed three times in succession; measurements were photographed from the oscilloscope display with a Polaroid camera. Measurement by the contact technique was performed with a Humphrey ultrasonic biometer 810. Via an adapter, a nonfocused 10-MHz A ultrasound probe was connected to a Goldmann applanation tonometer so biometry could be carried out analogously to an applanatory determination of intraocular pressure. Because eye movement could be followed in only two dimensions with the probe (manipulation of the cone prism in applanation tonometry is restricted in the same way), patients occasionally had to correct their gaze. Measurements were frozen and printed out when strong echoes were measured at low amplification. Three successive measurements were taken by this technique. For measurements by a modified contact technique, we used a CooperVision ultras can digital B; the probe had an integral conic water-coupling path, closed at the front with a flexible membrane. As in the immersion technique, the probe was hand-held by the investigator, but the subjects were seated rather than supine. The probe had a built-in light-emitting diode to 236
J CATARACT REFRACT
facilitate fixation. Because the coupling path was filled with an incompressible fluid, namely water or saline, an outlet for it had to be provided in an appropriate place. The purpose of using a flexible membrane was to help prevent corneal applanation, and hence distortion of measured distances. Measurements by this technique were repeated twice. The coupling techniques were performed preoperatively in random order on 60 cataract patients and, for comparison's sake, on 99 adolescents with healthy eyes; refractive ametropia was the only ocular change exhibited by the adolescent subjects. Further, anterior chamber depth was measured with a HaagStrait pachymeter. As a description of the accuracy of ultrasound measurements in any device, we computed a coefficient of reliability from the thrice repeated biometries performed with each device. The (auto)correlation between the first and the second, the first and the third, and the second and the third measurements was computed. The mean of these three correlation coefficients was used as the coefficient of reliability.
RESULTS As Figure 1 shows, the mean anterior chamber depth of all 159 subjects was 3.63 mm. The cataract patients had a slighter lower mean, the adolescents a slightly higher mean. As measured by the contact technique, mean anterior chamber depth was 0.3 mm mm 5
non - cataract
cataract n-60
n=99
4
-,
3
2
o Fig. 1.
A
M
0
A
M
0
(Giers) Anterior chamber depths (ACD) determined sonographically by various coupling techniques (I = immersion technique, A = applanation technique, M = modified applanation technique) and optically by pachymetry (0). Flatter ACD values were determined by the applanation technique and optically; the difference was more marked in the patients with cataracts than in those without. The histogram shows means and standard deviations.
SURG-VOL 16, MARCH 1990
mm 5
mm non - cataract
cataract
(n =159)
cataract (n =60 )
24
4
23
3
A
Fig. 2.
M
A
M
(Giers) Lens thicknesses determined sonographically by various coupling techniques (I = immersion technique , A = applanation technique, M = modified applanation technique). The histogram shows means and standard deviations. With all three techniques, the cataractous lenses were appreciably thicker than the adolescent, unoccluded lenses. On average, the largest lens thicknesses were measured by the immersion technique.
less, or 3.32 mm, in the sampling as a whole; the cataract patients had Hatter anterior chambers than the adolescent subjects. The modified applanation technique yielded a mean anterior chamber depth of 3.56 mm, an intermediate value slightly less than that obtained by the immersion technique. Measured lens thicknesses also differed from one technique to the next (Figure 2). On the immersion technique, the means in the total sampling, the cataract patients, and the adolescents were 4.12 mm, 4.41 mm, and 3.92 mm, respectively. Measurements by the contact technique yielded a collective mean of 3.93 mm ; the mean thicknesses of the cataractous and adolescent lenses were 4.25 mm and 3 .74 mm . Similar, but again somewhat lower, findings were obtained by our choice of a modified applanation technique. With this technique, the collective mean was 3.83 mm; the cataractous and adolescent lenses had mean thicknesses of 4.13 mm and 3.65 mm, respectively. These same techniques also yielded different axial length measurements . Again, the longest measurements were taken by the immersion technique. As seen in Figure 3, mean axial length was 0.33 mm longer with the immersion technique (23.77 mm) than with the J CATARACT
J Fig. 3.
A
M
A
M
(Giers) Axial lengths determined sonographically by various coupling techniques (I = immersion technique , A = applanation technique , M= modified applanation technique). Depicted are the values for the sampling as a whole and for the subgroup of cataractous eyes. In both groups the applanation technique yielded 0.3 mm shorter mean values than the immersion technique. Measurements averaged 0.1 mm less with the modified applanation technique than with the immersion technique.
contact technique (23.44 mm); the modified contact technique yielded an intermediate value of23.63 mm. The same differences were noticed in the subgroups of cataract patients and adolescents; among the cataract patients, it should be noted, the difference to mean axial length as determined by the immersion technique and modified applanation was a very slight 0.09 mm. The lO-MHz probes were used in all three procedures; the ultrasound wavelength was accordingly about 0.15 mm. In Figure 4, measurements by the modified applanation technique show that adjacent lengths were not equally likely to be measured. The undulating curve of this frequency distribution plainly deviates from the expected Gaussian distribution. Marked jumps are evident in the relative frequencies with which neighboring values were measured. Similar jumps are also apparent in the frequency distribution of axial lengths (Figure 5), as measurements by the applanation technique illustrate. The minima and maxima are 0.15 mm apart, agreeing with our choice of ultrasound wavelength. The retest reliability of all three techniques as measures of axial length was very high, as shown in Table 1 and in Figures 6 and 7. The reliability
REFRACT SURG- VOL 16, MARCH 1990
237
17
la 115 1" 13 12
F U
R
10
E Q
9
U
I
E N C y
a
7
15
. 3 2
1
mm
0
2.00
2.2!5
2.!50
2.7!5
3.00
3.2!5
3.!50
3.7!5
4.00
4.25
4.!50
4.75
ANTERIOR CHAMBER DEPTH Fig. 4.
(Giers) Frequency distribution of anterior chamber depth measurements obtained by a modified applanation technique (N = 477). Adjacent values were not equally likely to occur; rather. the distribution curve exhibits undulations at intervals of 0.15 mm, on the pattern of the ultrasound frequency used. This clearly limited the resolving power of the measuring device.
35
n
30
25
F
R E Q U E
20
15
N
C Y
10
5
0
\
,
~\J\~~
mm
20.020.521.021.522.022.523.023.524.024.525.025.5
26.0
26.5
27.0
27.5
2B.0
AXIAL LENGTH Fig. 5.
238
(Giers) Frequency distribution of axial length measurements obtained by the applanation technique (N = 477). Adjacent values clearly differed in probability of occurrence. The 0.15 mm between peaks in the frequency distribution corresponds to our choice of ultrasound wavelength.
J CATARACT REFRACT
SURG- VOL 16, MARCH 1990
RELIABILITY AXIAL LENGTH
1.0
ACD
LENS
0.9 Fig. 6A.
0.8
(Giers) Retest reliability of ultrasound biometry on 60 cataractous eyes; each procedure was performed three times in succession. With all three techniques, the highest degree of reliability was attained in measuring axial length; measurements of shorter segments of the anterior chamber were less readily reproducible (I = immersion technique, A = applanation technique, M = modified applanation technique).
J
HUMPBULi 27.48 26.44 25 . 39 24 . 35 23.31 22 . 27 21.22
26.34
25.43 24.51 HUMPBUL2 23.60 22.68 24.51 HUMPBUL3
Fig. 6B.
(Giers) Each column in Figure 6A represents the reproducibility of biometry. which can also be seen in this three-dimensional graph. Axial length measurement, for example. was characterized by a high degree of reproduction of findings (applanation technique, n = 159). The three axes are the first, second, and third measurement.
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RELIABILITY
to
AXIAL LENGTH
ACD
LENS
0.9 Fig. 7A.
(Giers) Retest reliability of three successive measurements in adolescents with healthy eyes (n = 99). The retest reliability of measurements of axial length and anterior chamber depth was not quite as high as in the cataract patients. A notable finding is the comparatively poor reproducibility of sonographic measurements of anterior chamber depth by the modified applanation technique (I = immersion technique, A = applanation technique , M= modified applanation technique).
0.8
J
CVVOR01
4.60 4.24 3.89 3.53 3.17 2.81
3.84 3.51 CVVOR02 3.19 2.86
2.46 2.10
4.80
3.69 CVVOR03
Fig. 7B .
240
(Giers) Retest r eliability was somewhat lower in the anterior segment of the bulb as shown in this three-dimensional graph (three times ACD measurement by modified applanation technique , n= 159). The three axes of the graph are the first , second , and third measurement with the same device.
J CATARACT REFRACT SURG-VOL 16. MARCH 1990
Table 1. Retest reliability coefficient for measurement of axial length.
Table 2. Retest reliability coefficient for measurement of lens thickness.
Technique
Cataractous Eyes (n = 60)
Noncataractous Eyes (n = 99)
Total (N = 159)
Technique
Cataractous Eyes (n = 60)
Noncataractous Eyes (n = 99)
Immersion
0.99
0.96
0.98
Immersion
0.86
0.90
Applanation
0.98
0.96
0.97
Applanation
0.88
0.90
Modified applanation
0.98
0.97
0.98
Modified applanation
0.87
0.92
coefficients of repeat measurements were between 0.96 and 0.99, evidence that single measurements were highly reproducible. With reliability coefficients between 0.86 and 0.92, repeat measurements oflens thickness were somewhat less reliable. This can be seen from Table 2 and Figures 6 and 7. Retest reliability fluctuated most in measurements of anterior chamber depth. The modified contact technique, with reliability coefficients of 0.78 and 0.88 in the subgroups of adolescents and cataract patients, was considerably less reliable than the immersion and contact techniques. The most readily reproducible measurements of anterior chamber depth were given by the contact technique in cataract patients; the reliability coefficient was 0.96 (Table 3).
DISCUSSION Results were evaluated in three ways. First, we compared our three sets of absolute values from . ultrasound biometry of segments of the globe; this was a means of comparing the accuracy of the coupling techniques investigated. Second, we examined whether adjacent values had the same probability of being measured. This was expected to supply information about the comparative resolving powers of the techniques. Third, we examined whether the reliability of preoperative biometry could be improved by taking the mean of several measurements. Our results will be discussed in the same order. With the contact technique, axial length and anterior chamber depth were 0.3 mm shorter than the distances measured by the immersion technique. This systematic error was seen in cataract patients and in adolescents with healthy eyes; the order of magnitude was invariably the same whether anterior chamber depth or axial length was being measured. These findings closely agree with those of Shammas. 4 Since flattening of the anterior cham ber and shortening of the axial length were found, corneal applanation by about 0.3 mm may be assumed to be responsible for this error in measurements by the contact coupling technique. Like all systematic errors in measurement, this one can be corrected by introducing a correction factor, as long J CATARACT REFRACT
Table 3. Retest reliability coefficient for ultrasonic measurement of anterior chamber depth.
Technique
Cataractous Eyes (n = 60)
Noncataractous Eyes (n = 99)
Immersion
0.92
0.91
Applanation
0.96
0.89
Modified applanation
0.88
0.78
as one knows the biometric design on which lens power calculation was based. We noted less systematic distortion with corneal applanation, at least when a flexible membrane and an integral water coupling path were used; measurements of anterior chamber depth and axial length were reduced less markedly than they were by the pure contact method. This gain was offset by a loss in the reproducibility of anterior chamber depth measurements. The value of reproducible measurements of anterior chamber depth depends on which formula is used to calculate implant lens power. When a regression formula is used and implant power is calculated from axial length alone, reproducibility will be less critical than when an individual, expected postoperative anterior chamber depth has to be inserted into a physicaloptical formula. Despite manufacturers' contentions that an ultrasound frequency of 10 M Hz will not limit the resolving power of sonographic measurements as long as suitable electronic biometry equipment is used, we found marked undulations in the frequency distributions of adjacent values on all three techniques. These are at odds with the nearly normal distribution of biological measurements. The undulations characteristically occurred at intervals of about 0.15 mm in the frequency distributions of all measurements of globe segments and were demonstrable in connection with all three techniques. Adjacent lengths obviously did not have the same probability of being measured. The remarkably close correspondence with the wavelength of the ultrasound signal used indicates that the frequency
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used did limit resolution and deprived adjacent lengths of an equal opportunity to be measured. We propose this observation as a counterargument to the view, frequently encountered in the literature,1,2,3 that ultrasound sonography has an accuracy better than ±O.l mm. Taking the mean of several biometries has been recommended as a way of improving accuracy. This is a sensible expedient when successive measurements by the same technique yield widely fluctuating values; in such cases, taking a mean can resolve uncertainties. At least as used to measure axial length, all three coupling techniques shared such a high retest reliability that the advantages of taking means would appear to be negligible. With reliability coefficients of better than 0.96, one can argue that the time available for biometry will be better spent on single high-quality measurements than on compulsively taking the mean of several measurements. As expected, measurements of short segments of the anterior chamber demonstrated a poorer retest reliability. The differences between the techniques were also most pronounced. Thus, the immersion technique
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can be recommended for measuring segments of the anterior chamber. The applanation technique did afford the closest agreement when several measurements were taken, but it was also accountable for the worst distortions of measured distances. Our choice of a modified applanation technique, on the other hand, was weakest in respect to reproducibility (obviously a consequence of the fixation stimulus in the probe). REFERENCES 1. Binkhorst RD: The accuracy of ultrasonic measurement of the axial length of the eye. Ophthalmic Surg 12:363-365, 1981 2. Gernet H: Zur Kunstlinse nach Mass bei Ametropien. Kiln Monatsbl Augenheilkd 180:127-131, 1982 3. Ossoinig KC: How to obtain maximum measuring accuracies with standardized A-scan. Doc Ophthalmol Fmc Ser 38: 197-216, 1984 4. Shammas HJ: A comparison of immersion and contact techniques for axial length measurement. Am Intra-Ocular Implant Soc] 10:444-447, 1984 5. Strobel J: Die Berechnung der Brechkrafte von intraokularen Linsen. Fortschr Ophthalmol 82:165-167, 1985 6. Hoffer KJ: Biometry of7 ,500 cataractous eyes. Am] Ophthalmol 90:360-368, 1980 7. Hoffer KJ: Preoperative cataract evaluation: Intraocular lens power calculation. lilt Ophthalmol Clill 22(2):37-75, 1982
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