Phakic intraocular lens (pIOL) implantation has been a popular means for the treatment of high ametropia. For angle-supported anterior chamber pIOL implantation, there is high risk of endothelial cell loss and complication of pupil ovalization, so posterior chamber pIOL implantation become more and more popular.1 The successful use of most pIOL designs depends on the correct choice of the overall pIOL size. Till now, there is no perfect system to determine the ciliary sulcus diameter. This evaluation is only approximate and depends on the estimation of the horizontal white to white distance measured by Orbscan IIz topographer (Bausch & Lomb, USA) or IOLMaster (Carl Zeiss Meditec, Germany). This estimation is frequently considered to be inaccurate and often misleading.2,3
An undersized pIOL may result in low vaulting and can lead to cataract formation as a result of chronic crystalline lens touch. It also can cause free rotation because of insufficient sulcus support. however, an oversized IOL may induce excessive vaulting, leading to pigment dispersion and angle crowding, which may be a potential for angle-closure glaucoma.4-7 An accurate and reliable means for direct ciliary sulcus diameter measurement is in urgent need.
The traditional 50-megahertz (MHz) ultrasound biomicroscopy (UBM) cannot give a full view of anterior chamber. The scan width and depth are only 5 mm × 5 mm, 2 to 3 images have to be assembled to measure sulcus diameter, which may produce erroneous measurements.8 In order to solve this problem, we developed a new full-scale 50-MHz UBM. It brought new ideas from the techniques of several field. In physics field, it uses moving balance technique to eliminate mechanical vibration; in information and communications field, it uses radar coding technique to increase scanning depth; in image acquisition field, it uses a super high frequency and real time image capture technique to increase image taking speed. It uses a linear scanner system, the scan width and depth of panoramic UBM are 16 mm × 9 mm, so it can obtain radial scans of the entire anterior segment with high resolution of 50-100 μm. The scan depth can reach the posterior capsule of lens and get a clear view of posterior segment. With it we can take direct measurement of sulcus diameter. In the present study, we measured sulcus diameters in cadaver eye and in vivo using a full-scale 50-MHz UBM and evaluated its accuracy, repeatability and reproducibility.
The fresh cadaver human eye was from the Beijing Tongren Hospital Eye Bank. A fine stainless steel needle with the diameter of 0.3 mm was used with scale distance 1 mm and scale depth 0.1 mm. Thirty volunteers (12 men and 18 women with a mean age of 37.2 years (range 20 to 60 years) and diopter of +0.50 D -6.00 D who underwent an ophthalmic examination to exclude eye diseases, enrolled in the study. This study adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee of Beijing Tongren Hospital. Informed consent was obtained from each volunteer.
Ultrasound scanning system
SW-3200L full-scale 50-MHz digital UBM (Tianjin Suowei Electronic Technology Co. Ltd., China) is equipped with 50-MHz wide range optical coherence tomography (OCT) linear scanning probe. The free articulated arm and fixation target make operating easy to locate and more secure. It works under Windows XP Platform with the wide scanning range 9 mm × 16 mm and image resolution (full screen) 50 μm-100 μm.
Measurements of a fresh cadaver human eye
With the fresh cadaver human eye, we evaluated the accuracy of full-scale 50-MHz UBM.
The scaled needle was inserted into the sclera at the point of 4 mm from the limbus. The needle passed through the posterior chamber plane from 3 o'clock to 9 o'clock meridian, parallel to the ciliary sulcus diameter plane, and was pulled out on the opposite side at the same distance from limbus (Figure 1). Ten cross-sectional images were obtained on the same meridians of the needle by one experienced technician. The distance between the scales and the whole length of the needle inside the eye for each image were measured by the same observer using the “built-in” measurement tools and the indicating error of instrument (indicating error of instrument = measurement - actual scale) was calculated to evaluate the accuracy of measurement in human eye's parameters. The way of indicating error calculation was in accordance with the actual distance scale of 1 mm, the measurements of each scale paragraph were marked by the paragraph on the image and the largest deviation was determined (Figure 2).
Measurements of eyes in normal subjects
We invited 30 normal subjects who had no evidence of ocular disease for full-scale UBM imaging on one randomly selected eye. Subjects were scanned in a supine position using a water bath standoff, to insure accurate position, the fellow eye was asked to fixate on a ceiling target to maintain accommodation Topical proparacaine 0.5% (Alcaine, Alcon, Fort Worth, TX, USA) was instilled to anesthetize the cornea before measurement. The transducer focal plane was placed tangent to at the subject's iris plane to make the region of the angle recess and sulcus recess more clearly. Scan status was reviewed visually during the scan session to detect decentration and/or loss of fixation, and were repeated as needed.
Measurements for posterior chamber diameter were made from the most extreme posteriolateral area of the groove formed by the reflection of the iris pigment epithelium onto the ciliary body (Figure 3). Horizontal (180°) cross-sectional images of 30 eyes were obtained twice by one technician and one time by another technician to estimate the reproducibility of sulcus diameter measurements.9-11 Then we randomly chose 10 eyes from the 30 normal subjects, horizontal (180°) cross-sectional images were obtained 10 times on each eye by one technician. The mean coefficient of variation was calculated to estimate the repeatability.
Indicating error of instrument was used to evaluate the accuracy of measurements by full-scale UBM. Coefficient of variation was used to evaluate the repeatability of measurements of sulcus diameters. Coefficients of reproducibility for inter-observer and intra-observer measurements were evaluated by Bland and Altman plot test. For intra-observer, the coefficient of repeatability was defined as the standard deviation (SD) of the difference from the mean of these repeat measurements divided by the mean response. The coefficient of inter-observer reproducibility was defined as the SD of difference between the pairs of measurements obtained by the 2 operators, divided by the average of the means of each pair of readings.9,10
Accuracy of sulcus measurements by full-scale UBM
For the measurements of the scale marker in cadaver eye, on a scale of 1 mm, the greatest indicating error was 40 μm; the mean largest indicating error of 1 mm scale from the 10 images was (26±14) μm; on a scale of 11 mm, the greatest indicating error was 70 μm; the error rate was 0.64%. The mean length of the needle inside the eye of the 10 images was 11.047 mm, with the mean indicating error of 47 μm, the average error rate was 0.43% (Table 1).
Repeatability and reproducibility of sulcus diameter measurements by full-scale UBM
The coefficients of variation for sulcus diameter measurements on 10 consecutive images were less than 0.72%; the mean coefficient of variation was (0.38± 0.16)%; 95% confidence interval (CI) for mean was (0.27%, 0.49%) (Table 2).
The repeatability and reproducibility of sulcus diameter measurements for inter-observer and intra-observer was evaluated by coefficients of repeatability and Bland and Altman plot test. The coefficient of repeatability for intra-operator and inter-operator measurements was 1.99% and 2.55%, respectively. The mean sulcus diameter difference for intra-operator measurements was 0.04 mm (limits of agreement, -0.41 to 0.48 mm); for inter-operator measurements was -0.004 mm (limits of agreement, -0.59 to 0.58 mm). They both showed good reproducibility (Figure 4).
Laser-assisted refractive surgery to correct ametropia is an established procedure; however, there is a significant risk for complications in cases of higher ametropia. Implantation of pIOLs is indicated in higher ametropia because it is an effective, safe, predictable, and stable method to correct higher refractive errors.12-15 Although accomplished developments of IOL technology during the past 15 years, potential complications still remained, mainly caused by poor IOL fitting.4-7 For angle-supported anterior chamber pIOL and posterior chamber pIOL implantation, true measurements of the internal AC diameter and ciliary sulcus diameter may be of great value to achieve appropriate IOL implantation. The calculation of overall IOL diameter is currently estimated by corneal white-to-white distance measurements, which can be determined by different techniques. However, Werner et al2 compared the white to white distance with the anterior chamber diameter and ciliary sulcus diameter at the horizontal meridian by examining post-mortem bulbi and found a negative correlation between them. In refractive surgery, measuring instead of estimating anterior chamber and posterior chamber dimensions becomes more important to determine true parameters, especially for the implantation of pIOLs in the anterior angle and the posterior chamber.
Anterior segment OCT can not provide a clear posterior chamber structure image, although it is the most popular means for anterior chamber diameter measurements. White-to-white corneal diameter measurements have not provided reliable estimates for intraocular lens sizing and provided no information about the meridional orientation of the maximab anterior or posterior chamber diameter.
Posterior chamber or sulcus-to-sulcus diameters have been evaluated in autopsy eyes for the purpose of determining a haptic size range for sulcus fixation in aphakic eyes.16-19 Sulcus diameters have also been estimated in vivo in an ultrasound biometry study using composites of three axial images, assuming measurements are aligned in a single plane.20 Unfortunately, the use of composite scans for ultrasonic biometry introduces considerable non-systematic errors in measurements and has long since been abandoned by the general ultrasound community as a viable measurement technique. Rondeau et al21 had used a prototype two-axis stepper motor-controlled motion system equipped with a nominal 50 MHz transducer to measure sulcus-to-sulcus diameters. While Jaeryung et al3 had taken direct measurements of the ciliary sulcus diameter using a 35-MHz UBM recently, but they both had not evaluated the precision of the measurements, which cannot give us the right information of the difference between measurement and actual scale.
SW-3200L 50-MHz full-scale UBM can produce in vivo images of the anterior and posterior chamber structures of the eye with resolution as high as 50 μm. It is possible to study in detail and quantify precisely anatomic relations among the structures with acceptable reproducibility. In order to evaluate the accuracy of full-scale UBM in ciliary sulcus diameter measurements, we used the cadaver eyes to calculate the indicating error of the instrument. Our study showed a very small indicating error and more accuracy in the measurements of human eye's parameters. On a scale of 11 mm inside the eye at the posterior chamber level of 10 images, the greatest indicating error was 7 microns, indicated that about the length of posterior chamber diameter (the ciliary sulcus diameter), the maximum deviation was only 7 μm which may have little effect on pIOL size determination. So, using full-scale UBM we can got more accurate measurements of sulcus diameter, it will be widely used in angle-supported anterior chamber pIOL implantation and poster chamber pIOL implantation. It also can be used in the observation of the right position of IOL after phacoemulsification.
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