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The EZ ratio method is one way to calculate the estimated anterior chamber depth. [2] To start, the patient looks at a target in the distance with one eye covered. The examiner takes a digital photograph of the open, examined eye, from the side, perpendicular to the visual axis (a profile photograph).
True anterior chamber depth (ACD) is measured between the posterior corneal surface and the anterior lens surface. This measure is not to be confused with the anterior chamber constant (ACD constant) used in IOL power calculation formulas. [2] All lens constants are estimates, to begin with.
The anterior chamber drainage angle is then graded as a ratio between the peripheral anterior chamber depth and corneal thickness (AC : C ratio) [5] or expressed traditionally as a fraction [6] to provide the Van Herick's result. Grading can also be obtained by distinguishing the structures visible upon observation.
Anterior chamber depth (ACD, i.e. the distance between the crystalline lens and cornea including the corneal thickness) is required before the surgery and measured with the use of ultrasound. Iris-fixated IOLs are fixated to iris therefore they have the advantage of being one size (8.5 mm).
The anterior chamber angle is a part of the eye located between the cornea and iris which contains the trabecular meshwork. The size of this angle is an important determinant of the rate aqueous humour flows out of the eye, and thus, the intraocular pressure. The anterior chamber angle is the structure which determines the anterior chamber depth.
The depth of the anterior chamber and position of the posterior capsule may be maintained during surgery by OVDs or an anterior chamber maintainer, which is an auxiliary cannula providing a sufficient flow of buffered saline solution (BSS) to maintain stability of the shape of the chamber and internal pressure.
The depth of the anterior chamber and position of the posterior capsule may be maintained during surgery by OVDs or an anterior chamber maintainer, which is an auxiliary cannula providing a sufficient flow of BSS to maintain the stability of the shape of the chamber and internal pressure. [4] [9] [clarification needed]
A-scan ultrasound biometry, commonly referred to as an A-scan (short for Amplitude scan), uses an ultrasound instrument for diagnostic testing. [1] A-scan biometry measures the axial length (AL) of the eye prior to cataract surgery in order to assess the refractive power of the intraocular lens that will be implanted.