The Effect of Port Geometry on Vitrectomy: A Performance Analysis of Custom Probes Through Water and Tissue Removal Rates, Surgical Evaluation, and High-Speed Video

Abstract

Purpose: : To determine the influence of port size on water and vitreous flow rates for 20 and 25-gauge vitreous cutters for later development and testing of novel tip designs.

Methods: : Custom vitreous cutter tips were fabricated with different sized ports by cutting five successive circular apertures from 0.004 to 0.0145 inches in a 25-gauge tip and eight successive circular apertures from 0.004 to 0.024 inches in a 20-gauge tip. These tips were operated at 1500 cuts per minute (CPM) with the Bausch and Lomb (St. Louis, MO) Lightning cutter. Porcine vitreous and water flow measurements were recorded at vacuum settings of 100, 200, 300, 400, and 550 mmHg (n=5). Five cutter tips were designed and fabricated for specific surgical functionalities such as prevention of retina from entering the port (grater), high flow (100% duty cycle), increased shearing ability (needle), shaving of the vitreous base (horizontal slit), precision tissue removal (vertical slit), dissection while cutting (dissector), and a combined tip with different port geometries on both sides. Tips were evaluated by water and porcine vitreous flow rates (n=5) at 200mmHg and 1500 CPM, vacuum level required for the tip to cut porcine retina (n=10), time required to cut and aspirate a porcine lens (n=5), and surgical evaluation in enucleated porcine eyes. High-speed video (1500 frames per second) with a Dalsa 1M150 (Waterloo, ON) camera helped evaluate instantaneous flow characteristics.

Results: : Both vitreous and water flow asymptotically approached a maximum flow as the port diameter increased to the inner lumen of the cutting tip. The combined and 100% duty cycle ports held the highest water flow rates (p0.05) while other ports held lower vitreous flow rates. The grater and vertical slit required the highest vacuum level to cut retina, giving them a larger operable range close to a detached retina (p<0.05). The control, needle, dissector, and 100% duty cycle geometries removed lens the fastest. Surgeons preferred the grater and horizontal tip for shaving the retina and the control tip for bulk vitreous removal. High-speed video illustrated tissue interaction with the tips.

Conclusions: : As vitreous cutter port diameter increases, flow rates level off. This indicates that after a point, large port diameters are not necessary for adequate flow. Furthermore, modifying the port geometry of a vitreous cutter affects its surgical interactions with tissue.

Keywords: vitreoretinal surgery • vitreous • retina