Design

This study was designed as an experimental laboratory investigation. No human or animal studies were done as part of this investigation.

Previous Work

Our methods and experimental design are a continuation and expansion of work by Meyer and associates. This previous study used peristaltic modes on the Infiniti (Alcon, Inc, Fort Worth, Texas, USA) and Signature (Abbot Medical Optics [AMO], Inc, Santa Ana, California, USA) phacoemulsification machines with a bottle height of 75 cm, machine-indicated flow rate of 60 mL/min, and 550 mm Hg vacuum setting. Their micropulsed ultrasound trials used a WhiteStar handpiece (AMO, Inc) at 2 longitudinal power settings, 10% and 100%, both with a 6 ms on-duty cycle and 12 ms off-duty cycle. Their transversal ultrasound trials used an Ellips handpiece (AMO, Inc) at 100% power and torsional ultrasound trials used an OZil handpiece (Alcon, Inc) at 100% power. For each of these 4 ultrasound modalities, they tested 4 needles: (1) 19 gauge sharp, (2) 19 gauge Dewey Radius (MST, Redmond, Washington, USA), (3) 20 gauge sharp, and (4) 20 gauge Dewey Radius. For each condition, the tip was tapped against the capsule surrogate 200 times. Additionally, they tested 20 fresh human cadaver lenses with exactly the same phacoemulsification parameters. However, owing to the limited number of cadaver lenses, they were limited to the following conditions: (1) micropulsed ultrasound at 100% power with a 6 ms on-duty cycle and 12 ms off-duty cycle with a 20 gauge Dewey Radius needle, (2) transversal ultrasound at 100% power with a 20 gauge sharp needle, (3) transversal ultrasound at 100% power with a 20 gauge Dewey Radius needle, and (4) torsional ultrasound at 100% power with a 20 gauge Dewey Radius needle. The lenses were tapped gently in different places until capsule rupture occurred, rendering the lenses unusable for further trials. They found the rounded edge tip to be very protective of the capsule with all modalities tested.

Capsule Surrogate

Plastic wrap (Great Value Clear Plastic Wrap; Wal-Mart Stores, Inc, Bentonville, Arkansas, USA) was stretched tightly over one end of a closed 4-inch-diameter polyvinylchloride tube as a surrogate for the capsule in the manner previously described by Meyer and associates. The Meyer study used fresh whole human cadaver lenses to further validate this approach. All trials were performed on plastic wrap from the same roll, and pieces of plastic wrap were cut to be large enough to extend beyond the edge of the tube by at least 1 inch in all directions. Tension on the capsule surrogate was established by placing it over the pipe underwater to ensure no air bubbles were trapped underneath, then placing a rubber band around the diameter of the tube to secure it. Once secured in this fashion, to ensure a reasonably uniform tension in all directions, the capsule surrogate was examined for wrinkling, as this would indicate uneven tension. Any wrinkling was resolved by adjusting tension in the appropriate axis of the plastic wrap. This adjustment process was repeated until the entire surface of the capsule surrogate was smoothly uniform. This formed a chamber that was completely filled with water and then submerged in a water bath in order to have a fluid interface on both sides of the capsule surrogate.

Phacoemulsification Settings

The Signature (AMO, Inc) phacoemulsification machine was used with the Fusion cassette (AMO, Inc), with all trials using a 550 mm Hg vacuum setting in both peristaltic and Venturi modes, a bottle height of 50 cm, and balanced salt solution. All trials in peristaltic mode used a flow rate of 40 mL/min. While we do not know the exact fluid flow for Venturi vacuum at 550 mm Hg, in a previous study at 500 mm Hg fluid flow was 102.2 ± 2.3 mL/min for micropulsed longitudinal and 96.6 ± 0.7 mL/min for transversal ultrasound. Micropulsed ultrasound trials used a WhiteStar handpiece (AMO, Inc), while transversal ultrasound trials used an Ellips FX handpiece (AMO, Inc). Micropulsed ultrasound was set at 50% power with a 6 ms on-duty cycle and 6 ms off-cycle. Transversal ultrasound was set at continuous 50% power. Ultrasound and vacuum were both used at only their maximum setting, with all settings on panel control to ensure uniformity. While previous work has shown that micropulsed ultrasound is most efficient at 100% power and transversal at 50% power, we decided the fairest test would be to use the same power setting for both in order to minimize power settings as a confounding factor if there were differing breakage rates.

Phacoemulsification Tips

All trials used phacoemulsification tips with a 30 degree bevel and 0.9 mm (20 gauge) tip diameter. Transversal ultrasound trials were done with a 30 degree bent (Kelman) tip, while micropulsed trials used a straight tip. All tips were made by MicroSurgical Technology (Redmond, Washington, USA). A new tip was used at the beginning of each condition to control for the effect of repeated contact with the capsule surrogate on tip sharpness.

Contact With Capsule Surrogate

In order to distinguish possible differences in safety between peristaltic and Venturi pump modes, 2 techniques for contacting the capsule surrogate simulated different clinical situations. The first technique consisted of tapping the edge of the tip against the plastic wrap by approaching the membrane with the needle bevel up at an angle approximately 30 degrees above parallel to the testing membrane, with no tip occlusion, as indicated in the Figure , Left (tip contact). Ultrasound was engaged during the approach to the surrogate capsule, needle contact, and retraction away from the membrane.

Two approaches were used to contact a surrogate of the human lens capsule with a phacoemulsification needle, in order to assess the risk of capsule rupture using peristaltic and Venturi vacuum pumps with micropulsed and transversal ultrasound. The line represents the capsule surrogate. (Left) Tip contact: The needle is held bevel up, at an angle approach approximately 30 degrees from the capsule surrogate. (Right) Tip occlusion: The needle is held bevel down, with the needle bevel opening parallel to the capsule surrogate.

The second technique involved tapping the needle opening at an appropriate angle against the plastic wrap to achieve tip occlusion. To achieve proper and even occlusion, the handle was held such that the bevel of the needle was facing downward with the needle opening parallel to the capsule surrogate as indicated in the Figure , Right (tip occlusion). Ultrasound was engaged during approach to the surrogate and contact and establishment of occlusion; once tip occlusion was indicated by the increasing pitch of audio tones, which signaled increasing vacuum, the pedal was released fully and the tip was simultaneously retracted away from the plastic wrap. The total time of occlusion from initial contact of the needle to release and retraction away from the membrane was approximately 1 second.

In both techniques, contact between the needle and capsule surrogate was monitored under direct visualization, which was aided by placement of a light that would reflect off the membrane, allowing observation of the time and point of contact. Eight conditions were tested: (1) micropulsed ultrasound in peristaltic mode with tip contact; (2) micropulsed ultrasound in peristaltic mode with tip occlusion; (3) transversal ultrasound in peristaltic mode with tip contact; (4) transversal ultrasound in peristaltic mode with tip occlusion; (5) micropulsed ultrasound in Venturi mode with tip contact; (6) micropulsed ultrasound in Venturi mode with tip occlusion; (7) transversal ultrasound in Venturi mode with tip contact; and (8) transversal ultrasound in Venturi mode with tip occlusion. For each occlusion condition, contact was made with the plastic wrap in different places for a total of 200 taps. For each tip contact, contact was made with the plastic wrap in different places for a total of 400 taps. This differential in the number of taps was owing to the considerable difference in breakage rates between tip occlusion and tip contact conditions that we observed in preliminary experiments.

Analysis of Capsule Surrogate

After each run of 200 or 400 taps, the plastic wrap was examined under a microsurgical microscope (Leica Microsystems Inc, Buffalo Grove, Illinois, USA) using a Sinskey hook (Bausch & Lomb Inc, Rochester, New York, USA) to gently probe for breakage. Additionally, in tip occlusion trials we further verified that occlusion had been achieved by observing a full 360 degree outline of the needle in the plastic wrap for each trial.

Statistical Analysis

A χ ² analysis was used to compare breakage rates between conditions, looking at differences between peristaltic and Venturi modes; between micropulsed and transversal ultrasound; and between tip edge contact and tip occlusion conditions. Results were considered statistically significant at P < .007 after a Bonferroni correction for multiple comparisons.