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<p>ABSTRACT</p><p>PURPOSE: To measure the largest diameter of the anterior chamber (AC) and posterior chamber (PC) dimension and its orientation and determine the relationship with the principal keratometric meridians.</p><p>METHODS: Twenty-eight eyes of 14 subjects were scanned with high frequency (50 MHz) ultrasound in sequential meridional scan planes at 30? increments. Observer identified angle and ciliary sulcus recess boundaries in each patient scan set were fit with an elliptical model to obtain the ellipse semi-major axis corresponding to the largest diameter and its meridional orientation. Anterior and posterior chamber diameters from raw data and model fit were compared using linear statistics. Circular statistics were used to compare the orientation of the largest diameter for raw ultrasound measurements, model estimations of largest diameter, and autorefractor determined keratometric axes.</p><p>RESULTS: The mean model diameters were anterior chamber OD 12.07 mm (0.32 SD); anterior chamber OS 12.06 mm (0.36 SD); posterior chamber OD 12.35 mm (0.42 SD); posterior chamber OS 12.33 mm (0.43 SD). The general trend for orientation of the meridian of largest diameter was in the horizontal meridian. In over 35% of eyes the difference between AC or PC meridian and the flat keratometric axis was greater than 20 degrees.</p><p>CONCLUSIONS: Accurate and reproducible anterior segment biometry depends on visualization of structures and minimization of eye and head movement error. The range and standard deviation of the diameter and orientation measures suggests anatomic variation is sufficient to require biometry for proper sizing and placement of intraocular devices that use angle or sulcus fixation. [J Refract Surg 2004;20:454-464]</p>

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