A computer model of the pseudophakic eye was designed using three-dimensional (3D) modeling software with fluid flow analysis capabilities (Solidworks; Dassault Systems, Vélizy-Villacoublay, France). The model eye was simplified to a sphere, with the anterior 2.5 mm portion of the sphere removed to best reflect the vitreous cavity of a pseudophakic eye (
Fig. 1). When varying axial lengths of the eye, this simplified model was maintained, and its overall diameter was increased or decreased proportionately because it has been shown that the greatest vertical and horizontal linear dimensions of the globe have a linear relationship with axial length.
6 The inlet and exhaust ports were angled perpendicularly at the point of insertion, which was 3.5 mm from the simulated limbus. Initial intraocular conditions were set at a pressure of 1 atmosphere (atm), or 1.013 bar, and 25°C. The air-gas exchange conditions were set to reflect clinical practice with active gas injection and passive air extrusion from the eye through a separate exhaust with external conditions set at 1 atm and 25°C. Volume fractions of air and gas within the control volume were recorded at intervals of 0.1 second. Parameters investigated were intended to reflect variations in clinical practice with only one of the following modified variables altered for each individual run:
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Gauge (G) of gas injection port (20 G, 23 G, 25 G);
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Gauge of exhaust (venting) port (20 G, 23 G, 27 G, 30 G);
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Angular distance between the gas injection and exhaust ports (30°, 60°, 180°);
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Exhaust port intrusion into eye (0 mm, 4 mm, 10 mm);
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Rate of gas injection (1 mL/s, 2 mL/s, 3 mL/s);
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Axial length of the eye (21.5 mm, 23.5 mm, 25 mm, 26.5 mm, 28.5 mm, 30 mm, 31.5 mm).
Flush volumes required to achieve 97% (practical maximum) of the concentration of the injected gas were compared. Visual illustrations of flow dynamics were generated to demonstrate variations in turbulence and gas flow within the eye. With the exception of axial length (not a modifiable surgical parameter), the most and least efficient parameters for each of these factors were assessed in combination. Two models were designed with these data—one with the most efficient parameters and the other with the least—to compare the difference in air-gas exchange efficiency.
The physical properties of the gas used in this simulation was based on an 18% air-gas mixture of C
4F
8 because it had a specific gravity, a specific volume, and a gas density between that of 30% SF
6 and 16% C
3F
8 and is detailed in
Table 1.