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Research Papers

A Computational Study of the Flow Through a Vitreous Cutter

[+] Author and Article Information
Tingting Juan1

Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095tjuan@ucla.edu

Jean-Pierre Hubschman

Jules Stein Eye Institute, Department of Ophthalmology, University of California, Los Angeles, CA 90025

Jeff D. Eldredge

Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095

1

Corresponding author.

J Biomech Eng 132(12), 121005 (Nov 08, 2010) (9 pages) doi:10.1115/1.4002796 History: Received February 22, 2010; Revised September 28, 2010; Posted October 15, 2010; Published November 08, 2010; Online November 08, 2010

Vitrectomy is an ophthalmic microsurgical procedure that removes part or all of the vitreous humor from the eye. The procedure uses a vitreous cutter consisting of a narrow shaft with a small orifice at the end through which the humor is aspirated by an applied suction. An internal guillotine oscillates back and forth across the orifice to alter the local shear response of the humor. In this work, a computational study of the flow in a vitreous cutter is conducted in order to gain better understanding of the vitreous behavior and provide guidelines for a new vitreous cutter design. The flow of a Newtonian surrogate of vitreous in a two-dimensional analog geometry is investigated using a finite difference-based immersed boundary method with an algebraically formulated fractional-step method. A series of numerical experiments is performed to evaluate the impact of cutting rate, aspiration pressure, and opening/closing transition on the vitreous cutter flow rate and transorifice pressure variation during vitrectomy. The mean flow rate is observed to increase approximately linearly with aspiration pressure and also increase nearly linearly with duty cycle. A study of time-varying flow rate, velocity field, and vorticity illuminates the flow behavior during each phase of the cutting cycle and shows that the opening/closing transition plays a key role in improving the vitreous cutter’s efficacy and minimizing the potential damage to surrounding tissue. The numerical results show similar trend in flow rate as previous in vitro experiments using water and balanced saline solution and also demonstrate that high duty cycle and slow opening/closing phases lead to high flow rate and minor disturbance to the eye during vitrectomy, which are the design requirements of an ideal vitreous cutter.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Sketch of immersed boundary method. —: Cartesian mesh; ◼: Lagrangian points representing the boundary of the object. The Navier–Stokes equations are solved on the Cartesian mesh as the no-slip boundary condition on the surface of the object is enforced by applying body force on the Lagrangian points.

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Figure 2

Sketch of model: (a) drawing of the sample cutter, (b) sketch of the computational domain, and (c) close look at the cutter opening (◆: Lagrangian points; ◼: remaining part of the inner shaft in zone I)

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Figure 7

Vorticity contour at different instances during the opening, opened, and closing phases of A20 for (a) 1500 cpm at vacuum pressure of 14.869 mm Hg and (b) 1500 cpm at vacuum pressure of 22.303 mm Hg

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Figure 8

Vorticity contours of cycle 3 at opening and opened phases

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Figure 9

Time-varying mean flow rate for opening/closing transition test. Between the dashed-dotted lines, it is one complete cycle.

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Figure 10

Transorifice pressure distribution at the opening and closing phases (P2=14.869 mm Hg and P3=22.303 mm Hg)

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Figure 6

Velocity field at different instances during the opening, opened, and closing phases of A20 at (a) 1500 cpm and applied vacuum pressure of 14.869 mm Hg, (b) 1500 cpm and applied vacuum pressure of 22.303 mm Hg, and (c) at 500 cpm and applied vacuum pressure of 14.869 mm Hg

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Figure 5

Time-varying flow rate (10−5 m2/s, two-dimensional) of A20 in one complete cycle at various cutting rates with different vacuumed pressures (the dotted line indicates the cutter mouth is more than 0% open)

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Figure 4

Mean flow rates for different cutting rates at different vacuumed pressures

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Figure 3

The cutting cycle can be divided into four phases: (a) opening, the guillotine moving toward the handle and the cutter’s mouth being revealed; (b) opened, the cutter remaining stationary and the mouth (usually) being fully opened; (c) closing, the cutter moving toward the probe’s tip and the mouth reducing in area; and (d) closed, the guillotine having no motion and the mouth being fully closed

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