Purpose
To determine the optimal longitudinal power settings for Infiniti OZil Intelligent Phaco (IP) at varying torsional amplitude settings; and to test the hypothesis that increasing longitudinal power is more important at lower torsional amplitudes to achieve efficient phacoemulsification.
Design
Laboratory investigation.
Methods
setting : John A. Moran Eye Center, University of Utah, Salt Lake City, Utah. procedure : Individual porcine nuclei were fixed in formalin, then cut into 2.0 mm cubes. Lens cube phacoemulsification was done using OZil IP at 60%, 80%, and 100% torsional amplitude with 0%, 10%, 20%, 30%, 50%, 75%, or 100% longitudinal power. All experiments were done using a 20 gauge 0.9 mm bent reverse bevel phaco tip at constant vacuum (550 mm Hg), aspiration rate (40 mL/min), and bottle height (50 cm). main outcome measure : Complete lens particle phacoemulsification (efficiency).
Results
Linear regression analysis showed a significant increase in efficiency with increasing longitudinal power at 60% torsional amplitude ( R 2 = 0.7269, P = .01) and 80% torsional amplitude ( R 2 = 0.6995, P = .02) but not at 100% amplitude ( R 2 = 0.3053, P = .2). Baseline comparison of 60% or 80% vs 100% torsional amplitude without longitudinal power showed increased efficiency at 100% ( P = .0004). Increasing longitudinal power to 20% abolished the efficiency difference between 80% vs 100% amplitudes. In contrast, 75% longitudinal power abolished the efficiency difference between 60% vs 100% torsional amplitudes.
Conclusions
Results suggest that longitudinal power becomes more critical at increasing phacoemulsification efficiencies at torsional amplitudes less than 100%. Increasing longitudinal power does not further increase efficiency at maximal torsional amplitudes.
The application of phacoemulsification (phaco) to cataract surgery by Charles Kelman in 1967 ushered in a new era of cataract extraction. The technology remains the standard of care for cataract surgeons in the developed world. Since that time, instruments and implants, as well as the programming associated with phaco machines, have improved. The settings allow for custom power modulation and fluidics control, potentially improving efficiency, effectiveness, and patient safety. Phacoemulsification uses high-frequency ultrasound to create energy, which is applied with great precision during surgery. The cataract is essentially removed through aspiration and vacuum forces after the phaco needle breaks the dense material into small enough pieces. Objective studies that compare settings have not been published.
Phacoemulsification occurs by a physical jackhammer effect and/or by cavitational break-up of the lens nucleus. A longitudinal, forward-and-backward movement causes physical interaction with the lens nucleus in the forward movement and causes a transient void or cavitational bubbles during the backward movement, which may further break up the lens material. Longitudinal phacoemulsification may be inefficient in that the jack-hammer effect only occurs on the forward motion, which can also cause the lens particle to bounce off the tip (an effect known as chatter), so that no emulsification occurs until the particle is recaptured at the tip.
Torsional ultrasound is a more recent approach whereby the needle oscillates such that the tip movement subtends an arc with no longitudinal movement. It is claimed that this shearing action increases efficiency in removing lens nuclei, as movement in both directions is associated with lens nuclei removal. Torsional ultrasound is then less likely than longitudinal phaco alone to result in chatter, which can be both an efficiency and a safety issue. One downside to torsional ultrasound is material blocking the tip so that no material can be aspirated. The system we have tested automatically includes brief pulses of longitudinal ultrasound when the maximum vacuum has been reached, in order to clear the tip. The system also allows longitudinal tip action to be included with torsional movement in order to maximize the overall efficiency. However, little objective information about how this additional longitudinal motion might be optimized, or if it is even needed to improve efficiency, is available in the literature.
In this study, we compared how phacoemulsification efficiency is affected by increasing longitudinal amplitude at different torsional amplitude settings. We chose torsional amplitudes at 60% and above because of recently published results suggesting that cavitation does not occur until 80% torsional power and reaches steady state at 90%. In contrast, longitudinal power settings starting at 60% are predicted to have stable cavitation (some cavitation is predicted to start at >35%). Our hypotheses for this study include: (1) addition of longitudinal power increases efficiency at some torsional power settings; and (2) this increase in efficiency will be more significant at lower torsional power settings.
Methods
This was an experimental study involving laboratory research.
Porcine Lens Fixation
Whole porcine eyes were purchased from Visiontech Inc (Sunnyvale, Texas, USA) for use in this institutional, experimental laboratory study. Porcine lens dissection and experiments were conducted within 24–48 hours of arrival. Porcine lens nuclei were prepared according to previously published methods. Briefly, lens nuclei were isolated from the porcine eyes and placed in balanced salt solution (BSS) for approximately 1 hour, until all lenses were removed from the globes. Lens nuclei were then fixed individually in 10 mL of 10% normal buffered formalin for 2 hours at room temperature. After fixation, the lenses were washed 3 times with BSS. The lenses then were allowed to equilibrate in BSS for less than 48 hours at room temperature.
Porcine Lens Cube Preparation
The porcine lenses were cut into 2.0 mm × 2.0 mm cubes, using a cutting apparatus as previously described. The lens cubes were then transferred into 1 container containing BSS. The lens cubes were washed 3 times with BSS and allowed to equilibrate in 10 mL BSS overnight at room temperature.
Phacoemulsification
Phacoemulsification of the individual lens cubes was done using the OZil Intelligent Phaco (IP) machine (Alcon Surgical, Fort Worth, Texas, USA). All experiments were conducted using a 20 gauge 0.9 mm bent reverse bevel phaco tip at constant vacuum (550 mm Hg), constant aspiration rate (40 mL/min), and constant bottle height (50 cm) with intelligent phacoemulsification setting engaged. Twenty-five runs, using a total of 25 lens cubes, were done at each setting. For each trial, the lens cube was engaged in the phacoemulsification tip using vacuum alone. The time from the start of phacoemulsification to complete lens removal was measured using a handheld stopwatch. Only the time that the lens fragment was engaged at the phaco tip was recorded. Time during a chatter event, defined as the number of times the lens fragment dislodges from the tip, was not counted as total time. The recorded elapsed time was considered as the efficiency time. The chatter events were also recorded in each trial. Three different torsional power settings were used: 60%, 80%, and 100%. At each torsional setting, different longitudinal power amplitudes were used: 0, 10%, 20%, 30%, 50%, 75%, and 100%.
Statistical Analysis
Efficiency times were averaged and a standard deviation (SD) was calculated. Data points that were more than 2 SDs from the mean were considered outliers and removed in the final analysis. The removal of outliers is based on our previous work, which demonstrated that these lens fragments have the consistency of very hard human nuclei and that some fragments may take many seconds before they finally become set on the needle tip, at which point they are promptly emulsified. New means and SDs were recalculated. A linear regression with a calculated R 2 was used to compare efficiency times in the different groups tested. Finally, an unpaired Student t test was used for comparison of discrete groups. For comparison of chatter events, means and SDs of total chatter events were calculated. A linear regression with a calculated R 2 was used to determine whether there was a significant increase or decrease in chatter events when longitudinal amplitude was increased from 0 to 100 at different torsional amplitude settings. Finally, chatter events at similar longitudinal amplitude settings but different torsional amplitude settings were compared using 1-way ANOVA. Statistical analyses were performed using GraphPad Prism (GraphPad Software, Inc, La Jolla, California, USA).
Results
Longitudinal Power Significantly Increases Phacoemulsification Efficiency at 60% and 80% Torsional Power but Not at 100% Torsional Power
We compared the effects of increasing longitudinal power using different torsional power in phacoemulsification efficiency ( Figure 1 ). Results revealed a significant increase in efficiency from 0 to 100% longitudinal power at a torsional power setting of 60% (5.57 ± 4.81 to 2.56 ± 1.44 sec; R 2 = 0.7269, P = .0148) and at 80% (5.08 ± 3.43 to 1.69 ± 0.62 sec; R 2 = 0.6995; P =.019) ( Figure 1 , Left and Center). In contrast, there was no significant difference in the phacoemulsification of lenses with increasing longitudinal power from 0 to 100% at a torsional power of 100% (2.25 ± 0.80 to 2.02 ± 0.85 sec; R 2 = 0.3053; P = .198) ( Figure 1 , Right).
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