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A comparison of immersion and contact techniques for axial length measurement
A comparison of immersion and contact techniques for axial length measurement H. John Shammas, M. D. Lynwood, California
ABSTRACT A prospective study was conducted on 180 eyes to evaluate axial length measurements obtained with both contact and immersion techniques. Each eye was measured with the Ocuscan-DBR (contact), the Ocuscan-400 (immersion), and the Kretz 7200 MA (immersion) units. Axial length measurements obtained by the two methods were highly reproducible. Axial length measurements obtained with the contact technique were shorter than measurements obtained with the immersion technique by an average of 0.24 mm. Key Words: axial length measurement, contact technique, focused ultrasound beam, immersion technique, intraocular lens, nonfocused ultrasound beam
The immersion technique has been used for many years to perform accurate axial length measurements. 1 The advent of intraocular lenses (IOLs) made axial length measurements a necessity for lens power calculations. 2,3,4 New A-scan units have popularized the contact technique for axial length measurement. This prospective study compares these two techniques. MATERIALS AND METHODS The contact method5 for axial length measurement was evaluated with the Ocuscan-DBR unit from Cilco-Sonometrics Systems, Inc. The ultrasound probe was attached to a forward-projecting applanation cone that was attached to a zero-weight balance glide, thus preventing any pressure on the eye during the examination. The probe emitted a focused ultrasound beam. The immersion method1 for axial length measurement was evaluated with the Ocuscan 400 unit from Cilco-Sonometrics Systems, Inc., and with the Kretz 7200 MA unit from Kretztechnik. Both units were equipped with a solid probe emitting a nonfocused ultrasound beam. Presented at the
u.s.
Comparative axial length measurements were obtained on 180 eyes using the two methods of examination and the three ultrasound units. The patient was seated behind the Ocuscan-DBR unit and the axial length measured with a contact technique: A drop of local anesthetic was instilled in the eye and the probe aligned with the visual axis. The probe was brought forward to touch the cornea without indenting it (Figure 1) and then moved up and down or to the side slightly to optimize the four echospikes displayed on the screen. These four echospikes represented, from left to right, the anterior surface of the cornea, the anterior surface of the lens, the posterior surface of the lens, and the anterior surface of the retina. The axial length measurement was displayed on the screen and Polaroid pictures were taken. The patient was then placed in a supine position. A scleral cup was positioned between the lids and filled with 1% methylcellulose. The solid ultrasound probe of the Ocuscan 400 unit was immersed in the solution, keeping it 5 mm to 10 mm away from the cornea (Figure 2). The probe was aligned with the visual axis and
Intraocular Lens Symposium, Los Angeles, California, April 1984.
Dr. Michael Swearingen assisted in the axial length measurements. Reprint requests to H. John Shammas, M.D., Medical Eye Center, 3510 Century Boulevard, Lynwood, California 90262. 444
AM INTRA-OCULAR IMPLANT SOC
J-
VOL 10, FALL 1984
Fig. 1.
(Shammas) Contact technique for axial length measurement. The probe is brought fOIWard to touch the cornea without indenting it.
Fig. 3.
(Shammas) Axial length is measured from the anterior surface of the cornea (C) to the anterior surface of the retina (R). Ll and L2 represent the anterior and posterior surfaces of the lens, respectively. The gates (arrows) allow an electronic measurement of the axial length.
The reproducibility of axial length measurements was tested by comparing results obtained on the same eyes by two independenfexaminers. For this purpose, the axial length was measured in 60 eyes using the contact technique and the Ocuscan-DBR unit, in 40 eyes using the immersion technique and the Ocuscan 400 unit, and in 75 eyes using the immersion technique and the Kretz 7200 MA unit.
Fig. 2.
(Shammas) Immersion technique for axial length measurement. The probe is immersed in the solution, keeping it 5 mm to 10 mm away from the cornea.
the system sensitivity decreased by 15 decibels. The four echospikes were displayed on the screen and Polaroid pictures were taken. The probe of the Ocuscan 400 was replaced by the probe of the Kretz 7200 MA unit and the same technique was used. The axial length was measured electronically, using the digital ocular computer (DOC) adapted to the Kretz 7200 MA ultrasound unit. Two "gates" were displayed on the oscilloscope screen: a "corneal gate" in the region of the corneal spike and a "retinal gate" in the region of the retinal spike. These two echo spikes were maximized and placed within the appropriate gates (Figure 3). The measurement was read by an assistant from the DOC unit.
RESULTS The average axial length measurements obtained on the same eyes by two examiners are shown in Table l. Differences in measurements between the two examiners were minimal whether a contact or an immersion technique was used. These differences were not statistically significant (P > 0.05). The average axial length measurements obtained on 180 eyes were 23.28 mm with the Ocuscan-DBR unit, using the contact method, 23.49 mm with the Ocuscan 400 and 23.52 mm with the Kretz 7200 MA units, both using the immersion technique (Table 2). The difference between each of the average measurements obtained with the immersion technique and the average measurement obtained with the contact technique was statistically significant (P < 0.05). The difference between the immersion-Sonometrics measurements and the immersion-Kretz measurements was within ± 0.1 mm in 160 eyes (88.9%) and within ± 0.2 mm in all eyes. The difference between the contact-DBR measurements and the immersionKretz measurements is shown in Table 3. Figure 4 is a scatter diagram comparing the two measurements in 180 eyes.
AM INTRA-OCULAR IMPLANT SOC J-VOL 10, FALL 1984
445
Table 1. Axial length measurements in millimeters obtained by two independent examiners. Examination Method
Average Axial Length Measurements
Number of Cases
Instrument
Difference in Measure ments
Examiner 1
Examiner 2
Average
Range
Contact
Ocuscan-DBR
60
23.16 ± 1.01
23.13 ± 1.01
0.03
-0.10 - +0.1
Immersion
Ocuscan 400
40
23.40 ± 1.02
23.38 ± 1.02
0.02
-0.12 - +0.1
Immersion
Kretz 7200 MA
75
23.43 ± 1.02
23.42 ± 1.02
0.01
-0.08 - +0.1
Table 2. Axial length measurements in millimeters obtained on 180 eyes. 26
Examination Method Instrument Contact
Ocuscan-DBR
Immersion Ocuscan 400
Deviation from Measurement Average Axial Obtained with the Length Contact Technique Measurement Average Range 23.28 ± 1.02
E E
25
~
...'"z
w w
:I
23.49 ± 1.03
+0.21 +0.05-+0.3
Immersion Kretz 7200 MA 23.52 ± 1.03
+0.24 +0.09-+0.4
. 0:
:::>
'"w
. ~ :..:: .
:I
0: ID
... 0
...z.. I
u
Table 3. Contact-DBR measurements compared to immersion. Kretz measurements.
24
23
0
u
::i
22
Difference in Millimeters ";;0.1
Number of Cases 15
Percentage 8.3
>0.1, ";;0.2
77
42.8
>0.2, ";;0.3
59
32.8
>0.3, ,,;;0.4
23
12.8
6
3.3
Total 180
100.0
>0.4
Ninety-nine eyes had an axial length shorter than 23.5 mm and 81 eyes had an axial length of23.5 mm or longer using the immersion technique and the Kretz 7200 MA unit. Average measurements using the contact technique and the Ocuscan-DBR unit were shorter by 0.25 mm in the "short eyes" group and byO.23 mm in the "long eyes" group.
DISCUSSION Multiple factors affect the accuracy of IOL power calculations. These include the corneal curvature, the IOL style and its position in the eye, and most importantly, variations in axial length measurements. 6 Our study revealed that axial length measurements obtained with the contact technique were shorter than measurements obtained with the immersion technique by an average of 0.24 mm. The two techniques differ in the instruments used and the methods of examination. The contact technique was evaluated with the Ocuscan-DBR unit and 446
23
22
24
25
IMMERSION-KRETZ MEASUREMENTS IN
Fig. 4.
26 mm:
(Shammas) Scatter diagram comparing the shorter contact DBR measurement (y-axis) to the immersion-Kretz measurement (x-axis). All points are below the 45-degree angle equivalence line.
the immersion technique with the Ocuscan 400 and the Kretz 7200 MA ultrasound units. The semi-soft probe of the Ocuscan-DBR unit emits a focused ultrasound beam; the solid probes of the Kretz 7200 MA and Ocuscan 400 units emit a nonfocused ultrasound beam. The focused beam is wide when emitted and narrow when it reaches the posterior pole. The nonfocused beam has parallel borders; theoretically, this would result ina shorter axial length measurement because echoes from the edges of the nonfocused beam return from positions on the concave surface that are not truly the most posterior aspect of the structure. 7 However, in this study, the sensitivity of all the ultrasound units was decreased by 15 decibels, resulting in a narrow ultrasound beam; furthermore, the ultrasonic energy responsible for the formation of echospikes was maximal along the central axis of the beam and was relatively weak at the beam's border. This study also revealed that measurements obtained with the nonfocused beam were actually longer than measurements
AM INTRA-OCULAR IMPLANT SOC J-VOL 10, FALL 1984
obtained with the focused beam. The differences between the two methods of examination included the patient's position and the possible corneal applanation by the ultrasound probe. The patient was conventionally examined in the seated position with the contact technique and the probe was brought forward to touch the cornea. The patient was examined in the supine position with the immersion technique and the solid probe was kept 5 mm to 10 mm away from the cornea. In my opinion, these differences in the methods of examination are responsible for the shorter measurements obtained with the contact technique. Hoffer (personal communication, October 10, 1983) measured 20 eyes with immersion and contact techniques using the Storz Ocuscan unit; the transducer of this A-scan unit emits a focused beam. An average difference of 0.33 mm between the two techniques was recorded using the same unit, the same transducer, and the same technician. In this study, axial length measurements obtained with both methods and using the three ultrasound units were highly reproducible. A proper technique includes a proper alignment of the ultrasound beam with the optical axis of the eye and the proper identification of the echospikes displayed on the oscilloscope; these should represent the anterior surface of the cornea, the anterior and posterior surfaces of the lens, and the anterior surface of the retina (Figure 3). Axial length measurements are basically used for IOL power calculations. 8 The shorter measurements
obtained by the contact technique partly account for the "short-eye syndrome" in which stronger power lenses for emmetropia are calculated by the theoretical formulas. 9 Sanders, Retzlaff, and Kraff! use the contact technique for axial length measurement and obtain the highest accuracy with the SRK regression formula. I routinely use an immersion technique for axial length measurement and obtain the highest accuracy with a theoretical formula that incorporates an adjustment factor for short and long eyes. 3 REFERENCES 1. Ossoinig KC: Standardized echography: Basic principles, clinical applications, and results.Int Ophthalmol Clin 19(4):127-210, 1979 2. Binkhorst RD: The optical design of intraocular lens implants. Ophthalmic Surg 6(3):17-31, 1975 3. Shammas HJF: The fudged formula for intraocular lens power calculations. Am Intra-Ocular Implant Soc J 8:350-352, 1982 4. Sanders DR, KraH'MC: Improvement of intraocular lens power calculation using empirical data. Am Intra-Ocular Implant Soc J 6:263-267, 1980 5. Johns GE: Clinical evaluation of the DBR/A-scan unit. Am Intra-Ocular Implant Soc J 5:213-216, 1979 6. Shammas HJF: Axial length measurement and its relation to intraocular lens power calculations. Am Intra-Ocular Implant Soc J 8:346-349, 1982 7. Coleman, DJ, Lizzi FL, Jack RL: Ultrasonography of the Eye and Orbit, Philadelphia, Lea & Febiger, 1977, pp 95-98 8. Shammas HJ: Atlas of Ophthalmic Ultrasonography and Biometry, St. Louis, CV Mosby Co, 1984, pp 273-304 9. Hoffer KJ: Intraocular lens calculation: The problem of the short eye. Ophthalmic Surg 12:269-272, 1981
AM INTRA-OCULAR IMPLANT SOC
J-
VOL 10, FALL 1984
447
ABSTRACT A prospective study was conducted on 180 eyes to evaluate axial length measurements obtained with both contact and immersion techniques. Each eye was measured with the Ocuscan-DBR (contact), the Ocuscan-400 (immersion), and the Kretz 7200 MA (immersion) units. Axial length measurements obtained by the two methods were highly reproducible. Axial length measurements obtained with the contact technique were shorter than measurements obtained with the immersion technique by an average of 0.24 mm. Key Words: axial length measurement, contact technique, focused ultrasound beam, immersion technique, intraocular lens, nonfocused ultrasound beam
The immersion technique has been used for many years to perform accurate axial length measurements. 1 The advent of intraocular lenses (IOLs) made axial length measurements a necessity for lens power calculations. 2,3,4 New A-scan units have popularized the contact technique for axial length measurement. This prospective study compares these two techniques. MATERIALS AND METHODS The contact method5 for axial length measurement was evaluated with the Ocuscan-DBR unit from Cilco-Sonometrics Systems, Inc. The ultrasound probe was attached to a forward-projecting applanation cone that was attached to a zero-weight balance glide, thus preventing any pressure on the eye during the examination. The probe emitted a focused ultrasound beam. The immersion method1 for axial length measurement was evaluated with the Ocuscan 400 unit from Cilco-Sonometrics Systems, Inc., and with the Kretz 7200 MA unit from Kretztechnik. Both units were equipped with a solid probe emitting a nonfocused ultrasound beam. Presented at the
u.s.
Comparative axial length measurements were obtained on 180 eyes using the two methods of examination and the three ultrasound units. The patient was seated behind the Ocuscan-DBR unit and the axial length measured with a contact technique: A drop of local anesthetic was instilled in the eye and the probe aligned with the visual axis. The probe was brought forward to touch the cornea without indenting it (Figure 1) and then moved up and down or to the side slightly to optimize the four echospikes displayed on the screen. These four echospikes represented, from left to right, the anterior surface of the cornea, the anterior surface of the lens, the posterior surface of the lens, and the anterior surface of the retina. The axial length measurement was displayed on the screen and Polaroid pictures were taken. The patient was then placed in a supine position. A scleral cup was positioned between the lids and filled with 1% methylcellulose. The solid ultrasound probe of the Ocuscan 400 unit was immersed in the solution, keeping it 5 mm to 10 mm away from the cornea (Figure 2). The probe was aligned with the visual axis and
Intraocular Lens Symposium, Los Angeles, California, April 1984.
Dr. Michael Swearingen assisted in the axial length measurements. Reprint requests to H. John Shammas, M.D., Medical Eye Center, 3510 Century Boulevard, Lynwood, California 90262. 444
AM INTRA-OCULAR IMPLANT SOC
J-
VOL 10, FALL 1984
Fig. 1.
(Shammas) Contact technique for axial length measurement. The probe is brought fOIWard to touch the cornea without indenting it.
Fig. 3.
(Shammas) Axial length is measured from the anterior surface of the cornea (C) to the anterior surface of the retina (R). Ll and L2 represent the anterior and posterior surfaces of the lens, respectively. The gates (arrows) allow an electronic measurement of the axial length.
The reproducibility of axial length measurements was tested by comparing results obtained on the same eyes by two independenfexaminers. For this purpose, the axial length was measured in 60 eyes using the contact technique and the Ocuscan-DBR unit, in 40 eyes using the immersion technique and the Ocuscan 400 unit, and in 75 eyes using the immersion technique and the Kretz 7200 MA unit.
Fig. 2.
(Shammas) Immersion technique for axial length measurement. The probe is immersed in the solution, keeping it 5 mm to 10 mm away from the cornea.
the system sensitivity decreased by 15 decibels. The four echospikes were displayed on the screen and Polaroid pictures were taken. The probe of the Ocuscan 400 was replaced by the probe of the Kretz 7200 MA unit and the same technique was used. The axial length was measured electronically, using the digital ocular computer (DOC) adapted to the Kretz 7200 MA ultrasound unit. Two "gates" were displayed on the oscilloscope screen: a "corneal gate" in the region of the corneal spike and a "retinal gate" in the region of the retinal spike. These two echo spikes were maximized and placed within the appropriate gates (Figure 3). The measurement was read by an assistant from the DOC unit.
RESULTS The average axial length measurements obtained on the same eyes by two examiners are shown in Table l. Differences in measurements between the two examiners were minimal whether a contact or an immersion technique was used. These differences were not statistically significant (P > 0.05). The average axial length measurements obtained on 180 eyes were 23.28 mm with the Ocuscan-DBR unit, using the contact method, 23.49 mm with the Ocuscan 400 and 23.52 mm with the Kretz 7200 MA units, both using the immersion technique (Table 2). The difference between each of the average measurements obtained with the immersion technique and the average measurement obtained with the contact technique was statistically significant (P < 0.05). The difference between the immersion-Sonometrics measurements and the immersion-Kretz measurements was within ± 0.1 mm in 160 eyes (88.9%) and within ± 0.2 mm in all eyes. The difference between the contact-DBR measurements and the immersionKretz measurements is shown in Table 3. Figure 4 is a scatter diagram comparing the two measurements in 180 eyes.
AM INTRA-OCULAR IMPLANT SOC J-VOL 10, FALL 1984
445
Table 1. Axial length measurements in millimeters obtained by two independent examiners. Examination Method
Average Axial Length Measurements
Number of Cases
Instrument
Difference in Measure ments
Examiner 1
Examiner 2
Average
Range
Contact
Ocuscan-DBR
60
23.16 ± 1.01
23.13 ± 1.01
0.03
-0.10 - +0.1
Immersion
Ocuscan 400
40
23.40 ± 1.02
23.38 ± 1.02
0.02
-0.12 - +0.1
Immersion
Kretz 7200 MA
75
23.43 ± 1.02
23.42 ± 1.02
0.01
-0.08 - +0.1
Table 2. Axial length measurements in millimeters obtained on 180 eyes. 26
Examination Method Instrument Contact
Ocuscan-DBR
Immersion Ocuscan 400
Deviation from Measurement Average Axial Obtained with the Length Contact Technique Measurement Average Range 23.28 ± 1.02
E E
25
~
...'"z
w w
:I
23.49 ± 1.03
+0.21 +0.05-+0.3
Immersion Kretz 7200 MA 23.52 ± 1.03
+0.24 +0.09-+0.4
. 0:
:::>
'"w
. ~ :..:: .
:I
0: ID
... 0
...z.. I
u
Table 3. Contact-DBR measurements compared to immersion. Kretz measurements.
24
23
0
u
::i
22
Difference in Millimeters ";;0.1
Number of Cases 15
Percentage 8.3
>0.1, ";;0.2
77
42.8
>0.2, ";;0.3
59
32.8
>0.3, ,,;;0.4
23
12.8
6
3.3
Total 180
100.0
>0.4
Ninety-nine eyes had an axial length shorter than 23.5 mm and 81 eyes had an axial length of23.5 mm or longer using the immersion technique and the Kretz 7200 MA unit. Average measurements using the contact technique and the Ocuscan-DBR unit were shorter by 0.25 mm in the "short eyes" group and byO.23 mm in the "long eyes" group.
DISCUSSION Multiple factors affect the accuracy of IOL power calculations. These include the corneal curvature, the IOL style and its position in the eye, and most importantly, variations in axial length measurements. 6 Our study revealed that axial length measurements obtained with the contact technique were shorter than measurements obtained with the immersion technique by an average of 0.24 mm. The two techniques differ in the instruments used and the methods of examination. The contact technique was evaluated with the Ocuscan-DBR unit and 446
23
22
24
25
IMMERSION-KRETZ MEASUREMENTS IN
Fig. 4.
26 mm:
(Shammas) Scatter diagram comparing the shorter contact DBR measurement (y-axis) to the immersion-Kretz measurement (x-axis). All points are below the 45-degree angle equivalence line.
the immersion technique with the Ocuscan 400 and the Kretz 7200 MA ultrasound units. The semi-soft probe of the Ocuscan-DBR unit emits a focused ultrasound beam; the solid probes of the Kretz 7200 MA and Ocuscan 400 units emit a nonfocused ultrasound beam. The focused beam is wide when emitted and narrow when it reaches the posterior pole. The nonfocused beam has parallel borders; theoretically, this would result ina shorter axial length measurement because echoes from the edges of the nonfocused beam return from positions on the concave surface that are not truly the most posterior aspect of the structure. 7 However, in this study, the sensitivity of all the ultrasound units was decreased by 15 decibels, resulting in a narrow ultrasound beam; furthermore, the ultrasonic energy responsible for the formation of echospikes was maximal along the central axis of the beam and was relatively weak at the beam's border. This study also revealed that measurements obtained with the nonfocused beam were actually longer than measurements
AM INTRA-OCULAR IMPLANT SOC J-VOL 10, FALL 1984
obtained with the focused beam. The differences between the two methods of examination included the patient's position and the possible corneal applanation by the ultrasound probe. The patient was conventionally examined in the seated position with the contact technique and the probe was brought forward to touch the cornea. The patient was examined in the supine position with the immersion technique and the solid probe was kept 5 mm to 10 mm away from the cornea. In my opinion, these differences in the methods of examination are responsible for the shorter measurements obtained with the contact technique. Hoffer (personal communication, October 10, 1983) measured 20 eyes with immersion and contact techniques using the Storz Ocuscan unit; the transducer of this A-scan unit emits a focused beam. An average difference of 0.33 mm between the two techniques was recorded using the same unit, the same transducer, and the same technician. In this study, axial length measurements obtained with both methods and using the three ultrasound units were highly reproducible. A proper technique includes a proper alignment of the ultrasound beam with the optical axis of the eye and the proper identification of the echospikes displayed on the oscilloscope; these should represent the anterior surface of the cornea, the anterior and posterior surfaces of the lens, and the anterior surface of the retina (Figure 3). Axial length measurements are basically used for IOL power calculations. 8 The shorter measurements
obtained by the contact technique partly account for the "short-eye syndrome" in which stronger power lenses for emmetropia are calculated by the theoretical formulas. 9 Sanders, Retzlaff, and Kraff! use the contact technique for axial length measurement and obtain the highest accuracy with the SRK regression formula. I routinely use an immersion technique for axial length measurement and obtain the highest accuracy with a theoretical formula that incorporates an adjustment factor for short and long eyes. 3 REFERENCES 1. Ossoinig KC: Standardized echography: Basic principles, clinical applications, and results.Int Ophthalmol Clin 19(4):127-210, 1979 2. Binkhorst RD: The optical design of intraocular lens implants. Ophthalmic Surg 6(3):17-31, 1975 3. Shammas HJF: The fudged formula for intraocular lens power calculations. Am Intra-Ocular Implant Soc J 8:350-352, 1982 4. Sanders DR, KraH'MC: Improvement of intraocular lens power calculation using empirical data. Am Intra-Ocular Implant Soc J 6:263-267, 1980 5. Johns GE: Clinical evaluation of the DBR/A-scan unit. Am Intra-Ocular Implant Soc J 5:213-216, 1979 6. Shammas HJF: Axial length measurement and its relation to intraocular lens power calculations. Am Intra-Ocular Implant Soc J 8:346-349, 1982 7. Coleman, DJ, Lizzi FL, Jack RL: Ultrasonography of the Eye and Orbit, Philadelphia, Lea & Febiger, 1977, pp 95-98 8. Shammas HJ: Atlas of Ophthalmic Ultrasonography and Biometry, St. Louis, CV Mosby Co, 1984, pp 273-304 9. Hoffer KJ: Intraocular lens calculation: The problem of the short eye. Ophthalmic Surg 12:269-272, 1981
AM INTRA-OCULAR IMPLANT SOC
J-
VOL 10, FALL 1984
447