Cataract is the most common cause of preventable blindness and is conventionally treated with surgery.1,2 During cataract surgery, phacoemulsification is the standard method used to remove the crystalline lens.3,4 Several factors can affect surgical outcome, including characteristics of the phacoemulsification tip, which may contribute to procedure safety and clinical outcomes.5,6

Phacoemulsification probes can be vibrated in a longitudinal or torsional mode. The amplitude of the vibratory excursion of the distal end of a phacoemulsification tip is a function of the ultrasonic power applied through the handpiece and is known as stroke.7 The stroke of torsionally operated phacoemulsification tips may also be related to tip design or material. Different phacoemulsification tips can exhibit different strokes in response to the same ultrasonic power, affecting the surgical outcome.5

Traditional phacoemulsification tips, such as the 45° bevel Intrepid Balanced Tip (Alcon Vision LLC), have a metal cutting edge at the aspirating distal end. Unintended contact of the vibrating cutting edge of the phacoemulsification tip with the capsule may increase the risk of a posterior capsule rupture, described as one of the most undesirable complications of cataract surgery.8–12 The Intrepid Hybrid Tip (Alcon Laboratories, Inc.) has the same overall design and dimension as the balanced tip. However, a soft polymer coating on the distal end was added to reduce the risk of posterior capsule rupture.6 Phacoemulsification tip design and ultrasound modalities can affect heat generation at the incision site, acoustic energy emission, cavitation profile, and turbulence in the fluidic environment surrounding the tip.7,13–15 Vibration of the phacoemulsification tip during cataract surgery can generate thermal energy, and thermal events have been previously observed during lens fragment removal and tip occlusion.16 Thermal wound injuries during phacoemulsification can delay postoperative wound healing, compromise visual acuity by inducing persistent astigmatism, and result in irreversible damage to the cornea.17 Thus, a design that lessens vibration at the corneal incision site may contribute to lower peak temperatures and reduce the risk of corneal burns.

Phacoemulsification tip design can also have an effect on fluidic characteristics. Increased acoustic output is closely associated with increased cavitation, turbulence, and free radical formation, driving inflammation and subsequent damage to the corneal endothelium.15,18–20 Consequently, lower cavitation has been found to decrease free radical formation and postoperative trauma to the cornea.15,21,22

The objective of this laboratory study was to compare the hybrid tip with the balanced tip, 2 phacoemulsification tips with different edge materials. Key aspects addressed in this study were incisional temperature, acoustic energy emission, transient cavitation, and turbulence.

METHODS

Throughout this study, the Centurion Vision System with the Active Sentry Handpiece (Alcon Laboratories, Inc.) was used. Two phacoemulsification tips were tested: The hybrid tip and the balanced tip; both were operated in a torsional mode. All fluidic testings were conducted with torsional amplitude normalized to similar stroke for the hybrid tip and balanced tip. Stroke was defined as the amplitude of the vibratory excursion of the distal end of the tip. The mini-flared Kelman tip (Alcon Laboratories, Inc.) was used as a control in the thermal profile testing only.

Thermal Profile

Controlled loads were intermittently applied over an area of the silicone sleeve covering the balanced tip and hybrid tip at the expected location of the corneal incision; the load simulated the torquing pressure during a surgical procedure. The mini-flared Kelman tip was used as a thermal testing control. Loads of 15 and 30 g were used to simulate low and medium levels of incisional torquing, respectively. The loads were attached to a 1.5-mm‒wide blackbody filmstrip positioned at 5 mm proximal to the distal end of the phacoemulsification needle. The filmstrip dimensions and location were determined by examining multiple standard phacoemulsification procedures and using calipers to measure the incision size (width, 2.2 mm; depth, 1.5 mm) and the mean extent of the phacoemulsification probe inside the anterior chamber (5 mm). The blackbody filmstrip served the dual purpose of transmitting the load to the tip-sleeve complex and providing a standardized medium to record the heat generated at the region of interest. Torsional ultrasound was used, with ultrasound power settings closely matching the typical power used during phacoemulsification of cataracts with mean hardness. First, the ultrasound was enabled with no load for 5 seconds until a plateau was reached. Then, the load was applied for 3 seconds, and finally, the load was removed, and the ultrasound was disabled. The ultrasound was proportionally lowered to improve repeatability while preserving the original ratio for the differences in stroke for each different probe. Ultrasound power was 50% for the hybrid tip, 40% for the balanced tip, and 70% for the Kelman tip, preserving the original ratio for the differences in stroke for each probe. A flow rate of 30 mL/min was used.

Temperature variations were recorded over the surface of the blackbody filmstrip using infrared imaging; each experimental setting was repeated 10 times, and standard deviations were calculated. Thermal profiles were recorded in degree-Celsius using an infrared thermal imaging camera to determine the amount of heat generated by the tip-sleeve incision at the simulated incision location during ultrasonic activation of the tip (Supplemental Figure 1, available at https://links.lww.com/JRS/A951). Thermal data were collected using a FLIR SC-305 thermal imaging camera (Teledyne FLIR LLC) at the region of interest, and images were analyzed using the FLIR Systems ExaminIR software (Teledyne FLIR LLC). The temperature increase was compared using an unpaired 2-tailed t test with Welch correction. P values <0.05 were considered statistically significant.

Acoustic Signal

To examine acoustic pressure, a 4.0-mm directional needle hydrophone (NH4000; Precision Acoustics) was used within a plexiglass tank attached to a motorized fixture to provide 360° rotational acoustic pressure mapping and 280° axial acoustic pressure mapping (side to side centered at the front of the phacoemulsification tip) at increasing ultrasound power (Supplemental Figures 2 and 3, available at https://links.lww.com/JRS/A952 and https://links.lww.com/JRS/A953, Video 1, available at https://links.lww.com/JRS/A957). All acoustic pressure measurements were made at a constant distance of 30 mm from the distal end of the phacoemulsification tip. The flow rate was 12 mL/min. Acoustic signal recordings were made by activating the motorized fixture and using the control software to displace the hydrophone needle in an orbital manner (the hydrophone needle was always facing the distal end of the phacoemulsification tip). The hydrophone signal was analyzed using a multichannel oscilloscope displaying the acoustic power signal simultaneously with the phacoemulsification power setting and the needle hydrophone axial and rotational location. Acoustic energy recordings were made using 0.4 V per division on the grid of the oscilloscope display. The calibration table from Precision Acoustics for the 4-mm diameter hydrophone showed that a corrected sensitivity of 16 000 mV/MPa for the frequency range of 1 MHz was suitable for cavitation detection. Acoustic signals generated by transient cavitation were differentiated from those generated by the steady-state cavitation based on the amount and repetition rate of the high-amplitude random acoustic signal spikes.

Cavitation

Combined stroboscopic imaging and high-speed video recordings of the cavitation patterns were captured in high definition using a flow rate of 20 mL/min (Supplemental Figures 4 and 5, available at https://links.lww.com/JRS/A954 and https://links.lww.com/JRS/A955). Stroboscopic video recordings were performed to visualize phacoemulsification tip movement in slow motion, and a custom phacoemulsification-console video overlay software was used to keep the stored data integrity. The custom-developed software was used to extract cavitation profile information from each frame of a video stream at a constant predetermined location to observe a representation of the distribution and amount of cavitation for each tip and power setting.23 Cavitation was evaluated using a similar methodology as previously described in an environmental hyperbaric system to allow for control of the cavitation phenomenon.22 Real-time Centurion System ultrasonic power data overlay was used to visualize variations in cavitation magnitude and distribution with increasing ultrasound power.

Turbulence, Streaming, and Bubble Formation

Particle image velocimetry (PIV) was performed in distilled water seeded with neutrally buoyant polyamide beads (size, 20 µm; density, 1.03 g/cm3; 80A411 Dantec Dynamics A/S). The flow rate was 30 mL/min. PIV was used to evaluate the behavior of the fluid surrounding the distal end of the ultrasonically driven phacoemulsification tips as previously described (Supplemental Figure 6, available at https://links.lww.com/JRS/A956).14 Videos were collected using defined settings, and dedicated software (Flowex; Interactive Flow Studies Corp.) was used to obtain PIV calculations to generate vector maps, including flow direction and speed.7 For analysis, 100 sequential frames were collected and assessed using an interrogation window size of 20 pixels and an interrogation window shift of 15 pixels to yield a window overlap of 5 pixels. Free-flow conditions were assessed at a flow rate of 30 mL/min.

RESULTS Thermal Profile

Under normalized stroke and with a load of 0 g (no load condition), the mini-flared Kelman control tip showed a 73% greater temperature increase compared with the mean temperature of the hybrid and balanced tips (8.87 ± 0.20°C increase for Kelman tip, 1.80 ± 0.10°C for balanced tip, 2.94 ± 0.59°C for hybrid tip; P < .0001 for Kelman vs hybrid tip and Kelman vs balanced tip; Figure 1). Under a load of 15 g, the Kelman control tip showed a 68% increase in temperature compared with the mean temperature of the hybrid and balanced tips (16.33 ± 0.25°C increase for Kelman tip, 3.80 ± 0.22°C for balanced tip, 6.69 ± 1.15°C for hybrid tip; P = .0001 for Kelman vs hybrid tip and P < .0001 for Kelman vs balanced tip). Under a load of 30 g, the Kelman tip showed a 63% increase in temperature compared with the mean temperature of the hybrid and balanced tips (20.86 ± 0.46°C increase for Kelman tip, 4.74 ± 0.20°C for balanced tip, 10.79 ± 1.10°C for hybrid tip; P < .0001 for Kelman vs hybrid tip and Kelman vs balanced tip). There was an increase in temperature of the hybrid tip compared with the balanced tip under all loads (P < .01).

Figure 1.:

Normalized thermal data within the test chamber and sleeve for the hybrid tip, balanced tip, and mini-flared Kelman tip using a 30-mL/min flow rate. Temperature increase is shown under loads of 0 g, 15 g, and 30 g to simulate low and medium levels of incisional torquing. Error bars represent standard deviation. *P ≤ .0001 compared with the Kelman tip.

Acoustic Signal

After determining the pinpoint nature of the acoustic emission of the phacoemulsification probes, all further measurements were made with the hydrophone positioned axially at 0° and orbitally at 0°. Acoustic output is demonstrated for the hybrid tip and balanced tip in Figure 2, A. The amplitude of acoustic signals showed minimal spatial variation: The directional needle hydrophone focused on the distal end of the phacoemulsification probe at a fixed distance of 30 mm showed almost the same acoustic power amplitude, regardless of the axial and rotational position with respect to the probe. Based on these data from the directional needle hydrophone, the acoustic power emission from the phacoemulsification tip could be considered a pinpoint source of acoustic energy unrelated to tip orientation. Therefore, the directional needle hydrophone showed similar acoustic power amplitude from any location, as long as the distance and orientation of the hydrophone were constant and pointing toward the distal end of the tip.

Figure 2.:

Acoustic output for the hybrid tip and balanced tip. A: Acoustic pressure mapping recordings at a normalized ultrasound power. B: Acoustic pressure mapping recordings at increasing ultrasound power (amplitude). The circles indicate the first time that consistent transient acoustic spikes were seen. The flow rate was 12 mL/min. Ax = axis; Pwr = power

Recordings of the acoustic signal allowed differentiation of steady-state low-amplitude signals from high-amplitude spikes emerging with increasing ultrasound power. When torsional power was normalized to a similar stroke amplitude for the hybrid tip (75%) and balanced tip (55%), the acoustic output generated with the hybrid tip was lower compared with that for the balanced tip (Figure 2, A, left and right). Increasing the stroke of the hybrid tip allowed detection of a transition in the acoustic pattern from a low and relatively steady amplitude signal into bursts of high-amplitude random spikes and then into continuous high-amplitude spikes. Figure 2, B highlights the transition from a low-amplitude to consistent high-amplitude acoustic signal. Transient spikes appeared at 2.2 times higher ultrasound power with the hybrid tip compared with the balanced tip (55% vs 25% ultrasound power).

Cavitation

The acoustic transition into high-amplitude spikes correlated well with the onset of optically detected cavitation. Cavitation was more evident near the sides compared with the front of the tip and was distributed laterally at the distal end internally and externally (Figure 3, A, Video 2, available at https://links.lww.com/JRS/A958). Increasing ultrasound power expanded the cavitation phenomenon and associated turbulence. Furthermore, increasing the ultrasound power expanded the amount of cavitation by a factor more noticeable with the balanced tip than with the hybrid tip. The balanced tip produced more intense fluid streaming and persistent gas microbubbles compared with the hybrid tip. Cavitation was visually detected at 55% ultrasound power with the hybrid tip. The balanced tip showed cavitation starting at a lower ultrasound power of 25%. Similar to the acoustic results, the ultrasound power threshold for cavitation was 2.2 times higher with the hybrid tip compared with the balanced tip (Figure 3, A). The visualization of cavitation occurred at about the same ultrasound power as the emergence of continuous high-amplitude acoustic signal spikes for the hybrid tip at >55% ultrasound power vs the balanced tip at >25% ultrasound power (Figure 3, B). Figure 3, C shows the formation of cavitation bubbles for the hybrid and balanced tips at a normalized power of 75% and 55%, respectively.

Figure 3.:

Cavitation with the hybrid tip and balanced tip. A: Combined stroboscopic imaging and high-speed video recordings of the cavitation patterns. The circles indicate when cavitation bubbles were first visualized, 55% ultrasound power and 25% ultrasound power for the hybrid and balanced tips, respectively. B: Visualization of cavitation with increasing ultrasound power using the Centurion System data overlay for the balanced tip and hybrid tip. Arrows indicate the start of cavitation. C: Combined stroboscopic imaging and high-speed video recordings of the cavitation patterns under normalized 75% ultrasound power and 55% ultrasound power for the hybrid and balanced tips, respectively. The flow rate was 20 mL/min.

Turbulence

Examination of the 3D microfluidic patterns revealed less fluidic disturbance, such as streaming and bubble formation, with the hybrid tip compared with the balanced tip. This difference in fluidic disturbance was evident at all flow conditions and for all power levels when normalized for stroke amplitude. Flow patterns using PIV are shown in Figure 4 and Video 3 (available at https://links.lww.com/JRS/A959) for the hybrid and balanced tips at 80% and 60% normalized power, respectively.

Figure 4.:

3D microfluidics surrounding the tips examined by particle image velocimetry with normalized ultrasound power and a 30-mL/min flow rate. A: Ultrasound power was normalized to 80% for the hybrid tip. B: Ultrasound power was normalized to 60% for the balanced tip. The x-axes and y-axes indicate full frame sizes in millimeters. Color-scale maps to the right indicate velocity in millimeters per second.

DISCUSSION

The balanced tip and polymer-coated hybrid tip evaluated in this study share a similar design, one intended to minimize incisional movement while amplifying distal displacement. The hybrid tip includes a polymer coating to provide increased protection for the posterior capsule. This study was the first to evaluate key characteristics of the hybrid tip and balanced tip, including incisional temperature, acoustic energy emission, transient cavitation, and turbulence.

Lower incision temperatures were recorded with torsional phacoemulsification compared with longitudinal phacoemulsification and were associated with lower risk of wound burns.24 Lower temperature elevation may reduce the risk of incisional thermal injury or wound burns.24,25 In human cadaver eyes, wound burns were determined to occur consistently at incision temperatures of 43 to 45°C, suggesting that maintaining incision temperature below these levels may prevent a corneal burn.24 Acute collagen contracture at the incision site and surrounding tissues (phacoburn) can occur once the temperature reaches 60°C.26 In this study, torsional ultrasound using the balanced tip produced minimal temperature rise at all tested loads. Although the hybrid tip produced a slightly greater temperature rise, this increase is unlikely to be clinically significant, particularly because the hybrid tip is recommended for use with softer-grade cataracts, which generally require less ultrasound power. The Kelman tip had a higher temperature rise than the balanced and hybrid tips, most likely because of a difference in design: The balanced and hybrid tips allow for lower motion at the shaft than at the port compared with the Kelman tip. When normalized for stroke, the mini-flared Kelman tip showed at least a 68% greater temperature increase compared with the mean of the hybrid tip and balanced tip under representative surgical conditions (ie, ultrasound ON with 0-g load and ultrasound ON with 15-g load). This difference in thermal profile was consistent with previous studies comparing the thermal profile of the mini-flared Kelman tip with the balanced tip, indicating favorable thermal characteristics of both the hybrid tip and the balanced tip.13,27

Cavitation bubbles may be one of the sources of free radicals and corneal endothelium damage.22 Corneal endothelial cell density is reduced by ∼0.5% per year with normal aging, and additional endothelial cell loss can occur after cataract surgery.28–30 Previous in vitro studies demonstrated the formation of cavitation bubbles at the probe tip that was dependent on the ultrasound power.15,22,31 An in vitro phacoemulsification study that simulated cataract surgery reported the formation of free radicals, generated as a result of acoustic cavitation.32 Furthermore, an in vivo study demonstrated that free radicals generated during phacoemulsification caused endothelial damage.33 The role of free radicals in endothelial cell damage was further supported by an in vivo study that assessed the effects of H2, a free radical scavenger, during phacoemulsification. H2 reduced free radicals and reduced endothelial cell damage.20 A prospective randomized clinical trial (n = 32) also showed that H2 significantly reduced corneal endothelial damage from the hydroxyl free radical during phacoemulsification.34

In this study, the hybrid tip produced a lower acoustic signal and demonstrated a higher ultrasound power threshold for cavitation compared with the balanced tip. Reducing cavitation may minimize potential damage to the corneal endothelium during cataract surgery without reducing the efficiency of the phacoemulsification.22 The efficiency of the hybrid tip is expected to be lower compared with the balanced tip because of the hybrid tip's smooth rounded polymer capsule–friendly aspiration port. However, this work illustrates that this expected reduction in efficiency may have no relevant effect on increasing the total energy in the eye, especially for softer-grade cataracts or clear lens extraction, where the goal is to protect against posterior capsule rupture. Future studies will need to address efficiency, total energy in the eye, and the effects of the hybrid tip design on free radical formation and surgical outcomes, such as endothelial cell damage.

A previous study evaluated whether the hybrid tip could reduce the rate of posterior capsule rupture using 20 cadaver eyes.6 The overall mean torsional power required for posterior capsule rupture was significantly greater using the hybrid tip compared with the balanced tip, suggesting that the use of the hybrid tip may lower the risk of posterior capsule rupture during surgery.6 The results of the current study demonstrated that the hybrid tip produced less fluid turbulence, such as streaming and bubble formation, compared with the balanced tip. These results further support the protective characteristics of the hybrid tip. Clinical studies are needed to determine the effects of lower turbulence and reduced cavitation on safety outcomes after cataract surgery. Although the hybrid tip may necessitate a need for more energy and time during phacoemulsification (particularly for grade 3 to 4 cataracts), a gentler technique that avoids damage may be particularly beneficial for soft cataracts. The availability of various tips can provide opportunities for surgeons to optimize the procedure for each patient.

A limitation of this study is the in vitro design; although in vitro experiments are easily replicated and consistent, they may not translate to the clinical setting. Clinical assessments of the hybrid tip compared with the balanced tip are needed to corroborate the findings of this study. This study did not evaluate the difference in efficiency of the hybrid vs balanced tip in different cataract densities; therefore, caution should be taken when applying the results of this study to clinical settings where different tips would meet unique surgical needs. Acoustic and cavitation data were qualitative non-numerical measurements. However, these measurements were repeated multiple times, with internal validation. PIV was a qualitative non-numerical measurement that was repeated multiple times, with internal validations.

In conclusion, compared with the balanced tip, the hybrid tip had lower acoustic output and lower turbulence under normalized stroke conditions; the acoustic output and ultrasound threshold for cavitation onset were more than doubled for the hybrid vs balanced tip. The higher cavitation threshold and lower overall fluid disturbance observed with the hybrid tip may offer safety benefits that further reduce the risk of ocular tissue damage during phacoemulsification. Future studies should evaluate efficiency differences between hybrid and balanced tips in different cataract densities to identify the best options for each unique patient case and surgeon preference.WHAT WAS KNOWN Torsionally operated phacoemulsification tips produce less heat compared with tips operated in a longitudinal mode, and the balanced tip design further reduces heat generation at the level of the corneal incision. Fluidic characteristics of a polymer-coated phacoemulsification tip compared with a traditional metal tip have not been addressed.

WHAT THIS PAPER ADDS The temperature rise at the tip-sleeve simulated incision site was less for the hybrid tip and balanced tip compared with the mini-flared Kelman tip. The hybrid tip produced a less turbulent fluidic environment with reduced acoustic power emission and less cavitation compared with the balanced tip at similar strokes. The polymer coating of the hybrid tip may provide an added layer of posterior capsule protection and improve surgical outcomes. Acknowledgments

Medical writing assistance was provided by Lisa Denny, PhD, and Natalia Zhukovskaya, PhD, of ICON (Blue Bell, Pennsylvania) and was funded by Alcon Vision LLC.

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