- Email: [email protected]
Thermal characterization of phacoemulsification probes operated in axial and torsional modes
LABORATORY SCIENCE
Thermal characterization of phacoemulsification probes operated in axial and torsional modes Jaime Zacharias, MD
PURPOSE: To analyze temperature increases and identify potential sources of heat generated when sleeved and sleeveless phacoemulsification probes were operated in axial and torsional modes using the Infiniti Vision System with the Ozil torsional handpiece. SETTING: Phacodynamics Laboratory, Pasteur Ophthalmic Clinic, Santiago, Chile. DESIGN: Experimental study. METHODS: Two computer-controlled thermal transfer systems were developed to evaluate the contribution of internal metal stress and tip-to-sleeve friction on heat generation during phacoemulsification using axial and torsional ultrasound modalities. Both systems incorporated infrared thermal imaging and used a black-body film to accurately capture temperature measurements. RESULTS: Axial mode was consistently associated with greater temperature increases than torsional mode whether tips were operated with or without sleeves. In tests involving bare tips, axial mode and torsional mode peaked at 51.7 C and 34.2 C, respectively. In an example using sleeved tips in which a 30.0 g load was applied for 1 second, temperatures for axial mode reached 45 C and for torsional mode, 38 C. Friction between the sleeved probe and the incisional wall contributed more significantly to the temperature increase than internal metal stress regardless of the mode used. CONCLUSIONS: In all experiments, the temperature increase observed with axial mode was greater than that observed with torsional mode, even when conditions such as power or amplitude and flow rate were varied. Tip-to-sleeve friction was a more dominant source of phaco probe heating than internal metal stress. The temperature increase due to internal metal stress was greater with axial mode than with torsional mode. Financial Disclosure: Dr. Zacharias received research funding from Alcon Laboratories, Inc., to conduct this study. He has no financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2015; 41:208–216 Q 2015 ASCRS and ESCRS Online Video
Various modalities of ultrasonic (US) energy can be used during phacoemulsification for successful cataract removal. When using axial (also referred to as longitudinal) US, the aspirating tip of the phacoemulsification probe moves in a forward-and-back motion. When in close proximity to the lens, the forward stroke disrupts the material, causing it to break into smaller pieces that can be easily aspirated. In contrast, nonlongitudinal phacoemulsification systems use torsional motion in which the tip moves side to side or elliptical 208
Q 2015 ASCRS and ESCRS Published by Elsevier Inc.
motion in which longitudinal movement is combined with a transverse motion.1 Because nonlongitudinal phacoemulsification is designed to decrease the repulsion of nuclear fragments from the tip (commonly called “chatter”), the motion may promote more efficient cataract extraction and reduce the risk for endothelial cell damage.1–4 Advancements in US phacoemulsification technology during the past 3 decades have improved the outcomes of cataract surgery. For example, reduction http://dx.doi.org/10.1016/j.jcrs.2014.11.001 0886-3350
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of the incision size to manage surgically induced astigmatic outcome has spawned devices that are less invasive due to a reduction in the probe size.5 However, some issues that require mitigation remain. For example, any US phacoemulsification device can still generate significant thermal energy at the incision site and corneal burns have occasionally been reported during cataract surgery.6,7,A Corneal burns can develop very quickly and in porcine studies occurred at temperatures as low as 44.2 C,8 although experiments in human cadaver eyes suggest that higher temperatures (greater than 50 C) are required to produce a corneal burn.9 When not properly mitigated, burns from phacoemulsification can be severe and complications such as wound closure difficulties, corneal edema and scarring, and high postoperative astigmatism have been reported.10 Such injuries have not been related to a specific phacoemulsification platform or technology; however, at least 1 report suggests that intermolecular collisions during elastic deformation of the aspirating tip by the US motion, sometimes referred to as “internal metal stress,” during phacoemulsification can cause heating and create warm zones in the phaco probe.11 The aim of the current study was to analyze temperature increases and identify potential sources of heat generation when sleeved and sleeveless phaco probes were operated in axial and torsional modes.
MATERIALS AND METHODS To examine heat generation during phacoemulsification, computer-controlled thermal transfer systems were developed to evaluate temperature increases associated with
Submitted: November 18, 2013. Final revision submitted: June 24, 2014. Accepted: June 25, 2014.
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phaco probes operated in axial or torsional mode using the Infiniti Vision System with the Ozil torsional handpiece (Alcon Laboratories, Inc.). Two methods were initially used to accomplish accurate temperature measurements. One method used thermal imaging with an infrared (IR) thermal camera, and the other used micro-thermocouple technology. Each is associated with inherent challenges in the environment of surgical ophthalmology. With IR thermal imaging, accurate measurement of heat generated by sleeveless phaco probes is potentially compromised by error introduced primarily by the low emissivity of the polished titanium probe. Polished metals are associated with low emissivity coefficients and if not carefully monitored, may produce thermal readings well below the actual surface temperature of the object being measured. With a more conventional measuring method such as a thermocouple, heat induced by friction, for example between the vibrating tip and the silicone sleeve, can lead to inaccuracies due to the invasive relationship. The result may be thermal readings that are mistakenly higher than when the intrusive thermocouple is not present. The thermal transfer systems that we describe were specifically developed to mitigate these confounding factors and after performing confirmatory tests between thermal imaging and the more invasive thermocouple technologies, high sample rate thermal imaging that included the use of a black-body-based thermal transfer system was the approach taken for final data collection. Distilled water instead of balanced salt solution was used as the fluid for all experiments to keep sensors and components in contact with the fluid (intentionally or inadvertently) free from precipitates and possible damage. Tests were made using room temperature water (generally between 22 C and 25 C). In tests in which baseline test temperatures were intentionally controlled, inflow was chilled by submerging part of the irrigation tubing in an ice bath and baseline test temperatures were set at 18 C. For all tests conducted, temperature measurements were taken at least 3 times to confirm repeatability. While multiple plots were used to report averaged values of these data taken in triplicate, representative thermal plots of individual temperature measurements are displayed in some figures.
Experimental System 1: Heat from Sleeveless Tips
From Clinica Oftalmologica Pasteur, Santiago, Chile. Supported in part by grant no. 69607 from Alcon Laboratories, Inc., Fort Worth, Texas, USA. Ramon Dimalanta, PhD, of Alcon Research, Ltd., Irvine, California, USA, helped complete this study. Eilidh Williamson, PhD, provided medical writing assistance under the sponsorship of Alcon Laboratories, Inc., Fort Worth, Texas, USA. Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Diego, California, USA, March 2011, and at the XXIX Congress of the European Society of Cataract and Refractive Surgeons, Vienna, Austria, September 2011. Corresponding author: Jaime Zacharias, MD, Clinica Oftalmologica Pasteur, Avenida Luis Pasteur 5917, Santiago, Chile. E-mail: jaime. [email protected].
To capture the heat emanating from a vibrating bare tip, an IR thermal-imaging camera (FLIR i40, FLIR Systems, Inc.) was focused on the tip and images captured at 7 frames per second. The setup incorporated a very thin (175 mm) black-body film (1.5 mm wide) made from plastic-based carbon paper (Kores CE GmbH) that would contact the bare tip and match its temperature profile. This experimental design element avoided the inaccuracy of estimating or tracking the titanium's emissivity (a critical parameter in IR thermography) as it may vary as a function of the surface temperature, surface finish, geometry, or other confounding factors.12 Because the black-body film lacked reflectivity and had an emissivity approaching 1.0, any thermal radiation from it introduced little variability or error when measured with thermal-imaging tools. The first attempts were to contact the thin black-body film against the tip of the phaco probe where the heat of the tip would readily transfer to the paper and in turn radiate IR
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Figure 1. Left: Black-body film shown in contact with the bare phaco tip. Right: Film shown lifted away from contacting the bare phaco tip. Thermal imaging system outside the image view and fluid collection system is not shown for clarity.
information to the camera for an accurate temperature measurement. It was immediately evident that the continuous contact of the black-body film against the phaco probe tip created its own frictional energy during US activation and therefore the temperature increase included a significant contribution from this contact. To capture only thermal signals from the vibrating tip, the electromechanical test fixture and thermal-measurement system were arranged as shown in Video 1 (available at http://jcrsjournal.org). To make thermal measurements that ensured no frictional energy was created, the setup was automated so the film came into contact with the tip only immediately after events of intermittent US activation. To that end, the sleeveless phaco probe was ultrasonically activated for 500 ms, during which time a driving signal was provided to a solenoid actuator that lifted the black-body film away from the probe surface. On deactivation of US activity, the solenoid-driving signal was simultaneously shut off, placing the film in contact with the surface of the nonvibrating probe for 500 ms. Any heat present in the probe was transferred to the film with temperature equilibrium achieved in less than 150 ms. With the thermal camera focused on the film, the probe temperature was continuously measured on repeated tests. The process was repeated every second with the film lifted during each active phase and lowered onto the probe surface during the inactive phase. Figure 1 shows how the blackbody film was alternately positioned on and off the bare phaco tip as described. It should be noted that in all experiments performed with sleeveless phaco probes, the direction of flow was reversed using a syringe pump (EW-74900-20, Cole-Parmer) to prevent fluid from splashing onto the probe and black-body film and potentially compromising the accuracy of the temperature measurements. Understanding that the fluid is used solely as a cooling agent in this comparative experiment examining a bare tip, the flow direction should not affect the results. A fluid collection system was incorporated to further avoid any spray onto the probe or camera (Figure 2 and Video 1). Temperature increases were assessed when the sleeveless phaco probe was operated in axial or torsional mode and when conditions such as US power and aspiration flow were varied.
Experimental System 2: Frictional Heat from Sleeved Tips Whereas experimental system 1 evaluated bare tips to examine and measure the possible effect of internal metal stress and its influence on temperature rise, a second series
of experiments examined the impact of tip-to-sleeve friction on heat generation during phacoemulsification. An automated system was developed that applied calibrated loads on the sleeve with enough force to make appreciable contact against the metal tip (Figure 3 and Video 2, available at: http://jcrsjournal.org). The load was applied to a region of the tip's shaft estimated to be where the sleeve would interface with the corneal incision wall. Force was transmitted to the surface of the sleeve using a 1.5 mm wide strip of thin black-body film loaded between 5.5 and 6.0 mm from the distal end of the tip (G0.5 mm) to simulate stress conditions during real surgery. This black-body film was made from a sheet of plastic-based carbon paper with the carbon surface exposed to the thermal camera and coincidentally (and conveniently) the same material used in the first set of bare tip experiments. To allow normal irrigation/aspiration (I/A) to flow through the appropriate channels and ports without constriction at the incisional zone, a funnel-shaped silicone adaptor was placed on the sleeve distal end. This arrangement also prevented fluid from spraying from the tip onto the sleeve outer surface and camera lens. Flow was circulated by a dual syringe pump and continuously monitored using a US flow sensor system (ME1PXN inline sensor
Figure 2. Clear plastic fluid spray collector device connected to aspirating end of the bare phaco tip. The housing shielded sensitive equipment (eg, thermal camera lens, black-body film) from liquid spray exiting the tip (using reversed aspiration flow) created during US operation.
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Load 30 g load, 1 s load on, 3 s load off
30 g load, 1 s load on, 3 s load off
30 g load
Ramp load 30, 40, 50, and 60 g
30 g load
Flow 3 mL/min
3 mL/min
6 mL/min, 18°C baseline flow temp at start of test 12 mL/min
3 mL/min
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Energy ApplicaƟon 50% torsional amplitude followed by 50% longitudinal power Ramp torsional amplitude 25%, 50%, 75%, and 100% followed by ramp longitudinal power 25%, 50%, 75%, and 100% 100% torsional power followed by 100% longitudinal power
100% torsional power followed by 100% longitudinal power Ramp duty cycle 20%, 40%, 60%, 80%, and 100%; 100% torsional power followed by 100% longitudinal power for each ramp cycle
Figure 3. Test setup that would apply a force or load against the sleeve and tip using a black-body strip pulled with a force gauge attached. The sleeve shown in the foreground is part of the fluid collection system to prevent unwanted spray from the US operation.
Figure 4. Description of the tests in sequence order.
with the TS410 tubing flowmeter module, Transonic Systems, Inc.). To accurately assess the magnitude of stress placed on an incision, tests that simulated a typical surgical movement were conducted. For this study, the movement was a lateral movement of the phaco tip against the corneal tunnel. The displacement of the tip relative to the incision was captured on video. The magnitude of tip displacement was measured from the video recording using the x–y dimension of individual pixels as calibration reference in Adobe Photoshop CS5.5 image processing software (Adobe Systems Inc.). A digital force gauge (Entran ELFS-T3M-10N, Measurement Specialties) was then attached to the phaco probe tip, the tip was displaced by the same distance measured on the video recording, and the resultant force was measured on the force gauge. The same applied force was then replicated during thermal testing using a custom system including the digital force gauge, set in series with a linear stepping motor (Haydon, Inc.) under computer control to intermittently apply the prescribed force from the film and create frictional contact between the sleeve and vibrating tip. This is important as the force, not the displacement, is the variable that must remain constant to
Test Sequence The sequence or evolution of tests conducted for sleeved tips was dynamic in that each subsequent test was in some way influenced by the test(s) that preceded it and this will be explained in the Discussion section. Figure 4 describes each test in the presented order.
produce similar frictional events. Experiments were then performed using momentary loads of fixed magnitude and duration in axial and torsional modes.
RESULTS Experimental System 1: Heat from Sleeveless Tips Using the first automated thermal transfer system described above, initial experiments were performed with the reversed aspiration flow rate constant at 3 mL/min. The axial power or torsional amplitude was maintained at 100% and applied at a 50% duty cycle (500 ms applied, 500 ms removed). Temperature increases from a baseline of 22 C were detected in sleeveless phaco probes when operated in either axial or torsional mode. In both modes, the temperature increase appeared evenly distributed throughout the length of the probe shaft as shown in Figure 5. The
Figure 5. Temperature increases detected in sleeveless phaco probes operated using 100% axial power or 100% torsional amplitude. Flow rate was 3 mL/min and power was applied at a 50% duty cycle (500 ms applied, 500 ms removed). Temperatures indicate maximal within the regions of interest shown by the blue boxes.
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Figure 6. Temperature increases detected in sleeveless phaco probes operated using 100% axial power or 100% torsional amplitude. Baseline temperature was 22 C. Flow rates were 3, 6, 12, and 24 mL/min.
increase observed with axial mode was greater than that observed with torsional mode, with peak temperatures reaching 51.7 C and 34.2 C, respectively. When the axial power or torsional amplitude was maintained at 100% and the aspiration flow varied, lower flow rates (3 to 6 mL/min) produced more pronounced temperature increases than higher flow rates (12 to 24 mL/min) (Figure 6). The same temperature increase from higher to lower flow rates was observed using axial or torsional mode; however, under equivalent flow conditions, a greater increase in temperature was consistently observed with axial mode.
Experimental System 2: Frictional Heat from Sleeved Tips Using the second automated system, temperature increases were evaluated when sleeved phaco probes were operated in axial or torsional mode under variable conditions of power/amplitude, flow rate, and force. It should be noted that each experiment was independently performed to compare temperature increases observed using axial and torsional modes under a specific set of conditions. No comparisons between individual experiments were made. In the first experiment, a load of 30.0 g was intermittently applied to the phaco probe tip 5.5 to 6.0 mm on its shaft from the distal end, while the aspiration flow was maintained at 3.0 mL/min. The force was applied at a 25% duty cycle in 4-second intervals (1 second applied, 3 seconds removed). The phaco probe was continuously operated at 50% torsional amplitude for 4 loading events before
Figure 7. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes under identical conditions of power, flow, and force. A load of 30.0 g was intermittently applied against the phaco probe tip (yellow line), while the aspiration flow was kept constant at 3.0 mL/min (blue line). The phaco probe was then operated at 50% torsional amplitude for 4 loading events before switching to 50% axial power for 3 loading events, then back to 50% torsional amplitude. Temperature increases detected in the phaco probe are indicated by the white line.
switching to 50% axial power for 3 loading events, then back to 50% torsional amplitude. Figure 7 shows the graphic results as collected in real time. Under identical conditions of power, flow, and force, greater temperature increases were observed when the probe was operated in axial mode (peak temperature of approximately 52 C) than when the probe was operated in torsional mode (peak temperature of approximately 35 C). To determine the thermal effect of increasing the axial power or torsional amplitude, a second experiment was conducted in which a load of 30.0 g was intermittently applied against the phaco probe tip at a 25% duty cycle in 4-second intervals (1 second applied, 3 seconds removed), while the aspiration flow was maintained at 3.0 mL/min. The phaco probe was then operated at increasing torsional amplitude (25%, 50%, 75%, and 100%) allowing a single and successive loading event followed by increasing axial power (25%, 50%, 75%, and 100%) also allowing a single and successive loading event. An example of the real-time experimental method is shown in Figure 8. Not surprisingly, increasing the US power or amplitude of the probe induced greater temperature increases. These increases were observed using both axial and torsional modes; however, the same conditions induced larger thermal increases when the probe was operated in axial mode. The greatest temperature reading was achieved when the probe was operated using 100% power in axial mode, with a peak
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Figure 8. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes under conditions of increasing power. A load of 30.0 g was intermittently applied against the phaco probe tip (yellow line), while the aspiration flow was kept constant at 3.0 mL/min (blue line). The phaco probe was then operated at increasing torsional amplitude (25%, 50%, 75%, and 100%) followed by increasing axial power (25%, 50%, 75%, and 100%). Temperature increases detected in the phaco probe are indicated by the white line.
Figure 9. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes with phaco probe cooling between operations. Starting at a temperature of 18 C, the phaco probe was pulsed once at 100% torsional amplitude with a load of 30.0 g (yellow line) and a constant aspiration flow of 6.0 mL/min (blue line). The phaco probe was allowed to cool back to 18 C and pulsed once at 100% axial power under the same load and flow conditions. Temperature increases detected in the phaco probe are indicated by the white line.
temperature of approximately 58 C to 61 C observed over multiple runs; by comparison, 100% torsional amplitude induced a peak temperature of approximately 45 C. The results of the previous 2 experiments suggest that heat may accumulate in the phaco probe tip as the number of successive frictional events increases and that the temperature does not return to baseline or even a quasi-baseline when switching between axial and torsional modes. To mitigate any confounding effect of heat accumulation from prior periods of US activation, experiments were performed in which the probe was allowed to cool down to the same baseline temperature between operations. The phaco probe was first cooled to 18 C using an ice bath for the I/A flow. The US power was then activated only once throughout 1 application of force for 1.0 second at 100% torsional amplitude. A load of 30.0 g was applied. Even though the flow was cooled, a slightly higher aspiration flow of approximately 6.0 mL/min was required for the probe to cool quickly after completing 1 thermal test and beginning the next test. The resultant temperature was measured, and the phaco probe was again cooled to 18 C using the chilled water before the US power was activated once throughout 1 application of force for 1.0 second at 100% axial power. As shown in Figure 9, starting from a similar baseline temperature of 18 C, a single pulse of the phaco probe using 100% axial power produced a temperature of approximately 45 C, whereas
a single pulse using 100% torsional amplitude produced a temperature of approximately 38 C. To further explore the accumulation of heat, a test in which increasing loads were successively applied to the tip in an intermittent fashion was conducted. Two intermittent loads administered on the side of the tip's shaft were applied to the tip in increments of 10.0 g, totaling 30.0 g, 40.0 g, 50.0 g, and 60.0 g, when the probe was operated using 100% torsional amplitude or 100% axial power. Figure 10 shows results from experiments exploring torsional mode and axial mode, respectively. In each comparative pulse, the temperature resulting from operating the probe using axial power always exceeded the temperature achieved using torsional power, with peak temperatures of approximately 43 C and 63 C obtained with torsional mode and axial mode, respectively. The accumulation of heat was also demonstrated as each load was added over time. In the final tests, experiments were performed in which a load of 30.0 g was intermittently applied to the phaco probe tip, while the aspiration flow was maintained at 3.0 mL/min. The phaco probe was operated at 100% continuous torsional amplitude followed by 100% axial power operated at a duty cycle of 20%. The paired test was then repeated with increasing duty cycles of 40%, 60%, 80%, and 100%. The results of these experiments showed that the temperature increase produced using continuous 100% torsional amplitude (approximately 45 C) was
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Figure 10. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes when increasing loads were successively applied to the tip in an intermittent fashion. Two intermittent loads were applied to the tip in increments of 10.0 g (totaling 30.0 g, 40.0 g, 50.0 g, and 60.0 g) when the probe was operated using either 100% torsional amplitude (left) or 100% axial power (right). Temperature increases detected in the phaco probe are indicated by the white line.
equivalent to that produced with a duty cycle of 40% axial power (Figure 11). DISCUSSION Significant thermal energy can be generated at the incision site during US phacoemulsification. Although this thermal energy is usually dissipated from the wound by I/A fluid and rendered harmless, corneal burns have occasionally been reported.6,7,A Potential sources of heat during phacoemulsification are intermolecular collisions within the phaco probe shaft (internal metal stress) and tip-to-sleeve friction at the incisional zone. In this study, 2 automated thermal transfer systems evaluated the contribution of internal metal stress and tip-to-sleeve friction to heat generation during phacoemulsification. In addition, the
Figure 11. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes with variable duty cycles. A load of 30.0 g was intermittently applied against the phaco probe tip (yellow line), while the aspiration flow was maintained at 3.0 mL/min (blue line). Temperature increases detected in the phaco probe are indicated by the white line. The red circle highlights that 100% torsional magnitude produces similar thermal results as 100% axial power with a duty cycle of 40%.
systems were used to compare the thermal impact of axial and torsional US modalities. In experiments performed to evaluate heat generation in sleeveless phaco probes, an evenly distributed temperature increase was observed throughout the length of the probe shaft, contrary to theories of “warm zones” described by others.11 In all experiments, the increase observed with axial operation was consistently greater than that observed with torsional operation, even when conditions such as flow rate were varied. Experiments performed with sleeved probes using the second automated system showed that tip-tosleeve friction was a more dominant source of phaco probe heating than internal metal stress. In addition, under identical conditions of US power, flow, and force, greater temperature increases were observed when the sleeved probe was operated in axial mode than when the probe was operated in torsional mode. Varying experimental conditions known to increase temperature, such as reduced aspiration flow, increased force, and increased US power or amplitude4,13 induced greater temperature increases with both modes; however, the same conditions induced larger thermal increases when the probe was operated in axial mode. During initial experiments, it was noted that heat accumulated in the phaco probe tip as the number of successive frictional events increased and the temperature did not return to baseline when switching between axial and torsional modes. Results of experiments in which the probe was allowed to cool down to the same baseline temperature between operations showed that starting from the same baseline temperature, a single pulse of applied load to the phaco probe in axial mode produced a higher temperature increase than a single pulse in torsional mode. Although this was an important finding, in practical terms, surgeons may not have the opportunity to
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allow the phaco probe to cool during the phacoemulsification procedure and may instead opt to vary the duty cycle to provide thermal relief. Experiments in which the effect of varying the duty cycle was evaluated indicated that the temperature increase produced using continuous 100% torsional amplitude was equivalent to that produced with a duty cycle of 40% axial power. The finding that torsional phacoemulsification generates less heat than axial mode agrees with findings from previous studies using different experimental systems. Using serial imaging with a thermal camera, Han and Miller14 evaluated temperature increases observed when phacoemulsification probes from manufacturers of different phacoemulsification platforms were operated in silicone test chambers filled with a balanced salt solution. Under various experimental conditions, axial mode consistently induced a greater temperature increase than torsional mode. Similarly, in studies by Jun et al.,15 phacoemulsification simulated in human cadaver eyes and imaged using an IR camera indicated that axial mode induced higher temperatures at the incision site than torsional mode and was associated with more extensive loss of Descemet membrane. Despite the temperature elevations observed in the current study, it is generally well recognized that surgeons can safely operate using any modern phacoemulsification platform as long as power, occlusion, flow, and wound stress are fully understood and properly managed. With the various machines and modalities to choose from, it may be difficult to navigate the best technology for all surgeons. Sorensen et al.7 definitively ascertained that in their multifactorial survey of incision contracture, in the categories of machine type and US modalities, there was no association with wound burn and that other choices such as the ophthalmic viscosurgical device (OVD) used or surgeon technique may statistically influence the rate of incision burns. Most thermal injuries are related to avoidable errors including insufficient irrigation or aspiration, high US power, or prolonged power during occlusion.6 Prevention can be achieved by precautions such as confirming adequate fluid flow before tip insertion, allowing I/A to clear some OVD material before starting the US, and being cognizant of over-torquing the wound.10,B Sorensen et al.'s7 advice “.that all ultrasound, including horizontal ultrasound, should be used parsimoniously.” should be well heeded. Although no US phacoemulsification device, axial or nonaxial (torsional or transversal), can completely avoid thermal risk, a more thorough understanding of how US energy is applied or mitigated using various choices of modalities and power modulations
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will allow surgeons to be more strategic when optimizing their operating parameters, enabling favorable surgical outcomes. WHAT WAS KNOWN Phacoemulsification can generate significant thermal energy at the corneal incision site, and burns associated with cataract surgery have occasionally been reported. Previous studies have suggested that thermal rise due to the vibrating emulsifying tip can cause heating and creation of warm zones in the phaco probe due to internal metal stress. WHAT THIS PAPER ADDS An automated thermal transfer system showed that axial mode was consistently associated with greater temperature increases than torsional mode whether tips were operated with or without sleeves. Friction between the sleeved probe and the incisional wall contributed more significantly to the temperature increase than did internal metal stress. This study may help surgeons choose appropriate tools, techniques, and operational settings to minimize thermal risks.
REFERENCES 1. Tognetto D, Cecchini P, Leon P, Di Nicola M, Ravalico G. Stroke dynamics and frequency of 3 phacoemulsification machines. J Cataract Refract Surg 2012; 38:333–342 2. Liu Y, Zeng M, Liu X, Luo L, Yuan Z, Xia Y, Zeng Y. Torsional mode versus conventional ultrasound phacoemulsification; randomized comparative clinical study. J Cataract Refract Surg 2007; 33:287–292 3. Vargas LG, Holzer MP, Solomon KD, Sandoval HP, Auffarth GU, Apple DJ. Endothelial cell integrity after phacoemulsification with 2 different handpieces. J Cataract Refract Surg 2004; 30:478–482 4. Shah PA, Yoo S. Innovations in phacoemulsification technology. Curr Opin Ophthalmol 2007; 18:23–26 5. Linebarger EJ, Hardten DR, Shah GK, Lindstrom RL. Phacoemulsification and modern cataract surgery. Surv Ophthalmol 1999; 44:123–147 6. Bradley MJ, Olson RJ. A survey about phacoemulsification incision thermal contraction incidence and causal relationships. Am J Ophthalmol 2006; 141:222–224 7. Sorensen T, Chan CC, Bradley M, Braga-Mele R, Olson RJ. Ultrasound-induced corneal incision contracture survey in the United States and Canada. J Cataract Refract Surg 2012; 38:227–233 8. Bissen-Miyajima H, Shimmura S, Tsubota K. Thermal effect on corneal incisions with different phacoemulsification ultrasonic tips. J Cataract Refract Surg 1999; 25:60–64
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9. Olson MD, Miller KM. In-air thermal imaging comparison of Legacy AdvanTec, Millennium, and Sovereign WhiteStar pacoemulsification systems. J Cataract Refract Surg 2005; 31:1640–1647 10. Sugar A, Schertzer RM. Clinical course of phacoemulsification wound burns. J Cataract Refract Surg 1999; 25:688– 692 11. Schmutz JS, Olson RJ. Thermal comparison of Infiniti OZil and Signature Ellips phacoemulsification systems. Am J Ophthalmol 2010; 149:762–767 12. Incropera FP, DeWitt DP. Fundamentals of Heat and Mass Transfer, 3rd ed. New York, NY, John Wiley & Sons, 1990 13. Osher RH, Injev VP. Thermal study of bare tips with various system parameters and incision sizes. J Cataract Refract Surg 2006; 32:867–872 14. Han YK, Miller KM. Heat production: longitudinal versus torsional phacoemulsification. J Cataract Refract Surg 2009; 35:1799–1805 15. Jun B, Berdahl JP, Kim T. Thermal study of longitudinal and torsional ultrasound phacoemulsification; tracking the temperature of the corneal surface, incision, and handpiece. J Cataract Refract Surg 2010; 36:832–837
OTHER CITED MATERIAL A. U.S. Food and Drug Administration. FDA Manufacturer and User Facility Device Experience [MAUDE] database. Available at: http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfm aude/search.cfm. Accessed September 17, 2014 B. Pennsylvania Patient Safety Advisory. Preventing Corneal Burns During Phacoemulsification 2010; 7:23–25. Available at: http:// patientsafetyauthority.org/ADVISORIES/AdvisoryLibrary/2010/ Mar7(1)/Pages/23.aspx. Accessed September 17, 2014
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First author: Jaime Zacharias, MD Clinica Oftalmologica Pasteur, Santiago, Chile
Thermal characterization of phacoemulsification probes operated in axial and torsional modes Jaime Zacharias, MD
PURPOSE: To analyze temperature increases and identify potential sources of heat generated when sleeved and sleeveless phacoemulsification probes were operated in axial and torsional modes using the Infiniti Vision System with the Ozil torsional handpiece. SETTING: Phacodynamics Laboratory, Pasteur Ophthalmic Clinic, Santiago, Chile. DESIGN: Experimental study. METHODS: Two computer-controlled thermal transfer systems were developed to evaluate the contribution of internal metal stress and tip-to-sleeve friction on heat generation during phacoemulsification using axial and torsional ultrasound modalities. Both systems incorporated infrared thermal imaging and used a black-body film to accurately capture temperature measurements. RESULTS: Axial mode was consistently associated with greater temperature increases than torsional mode whether tips were operated with or without sleeves. In tests involving bare tips, axial mode and torsional mode peaked at 51.7 C and 34.2 C, respectively. In an example using sleeved tips in which a 30.0 g load was applied for 1 second, temperatures for axial mode reached 45 C and for torsional mode, 38 C. Friction between the sleeved probe and the incisional wall contributed more significantly to the temperature increase than internal metal stress regardless of the mode used. CONCLUSIONS: In all experiments, the temperature increase observed with axial mode was greater than that observed with torsional mode, even when conditions such as power or amplitude and flow rate were varied. Tip-to-sleeve friction was a more dominant source of phaco probe heating than internal metal stress. The temperature increase due to internal metal stress was greater with axial mode than with torsional mode. Financial Disclosure: Dr. Zacharias received research funding from Alcon Laboratories, Inc., to conduct this study. He has no financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2015; 41:208–216 Q 2015 ASCRS and ESCRS Online Video
Various modalities of ultrasonic (US) energy can be used during phacoemulsification for successful cataract removal. When using axial (also referred to as longitudinal) US, the aspirating tip of the phacoemulsification probe moves in a forward-and-back motion. When in close proximity to the lens, the forward stroke disrupts the material, causing it to break into smaller pieces that can be easily aspirated. In contrast, nonlongitudinal phacoemulsification systems use torsional motion in which the tip moves side to side or elliptical 208
Q 2015 ASCRS and ESCRS Published by Elsevier Inc.
motion in which longitudinal movement is combined with a transverse motion.1 Because nonlongitudinal phacoemulsification is designed to decrease the repulsion of nuclear fragments from the tip (commonly called “chatter”), the motion may promote more efficient cataract extraction and reduce the risk for endothelial cell damage.1–4 Advancements in US phacoemulsification technology during the past 3 decades have improved the outcomes of cataract surgery. For example, reduction http://dx.doi.org/10.1016/j.jcrs.2014.11.001 0886-3350
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of the incision size to manage surgically induced astigmatic outcome has spawned devices that are less invasive due to a reduction in the probe size.5 However, some issues that require mitigation remain. For example, any US phacoemulsification device can still generate significant thermal energy at the incision site and corneal burns have occasionally been reported during cataract surgery.6,7,A Corneal burns can develop very quickly and in porcine studies occurred at temperatures as low as 44.2 C,8 although experiments in human cadaver eyes suggest that higher temperatures (greater than 50 C) are required to produce a corneal burn.9 When not properly mitigated, burns from phacoemulsification can be severe and complications such as wound closure difficulties, corneal edema and scarring, and high postoperative astigmatism have been reported.10 Such injuries have not been related to a specific phacoemulsification platform or technology; however, at least 1 report suggests that intermolecular collisions during elastic deformation of the aspirating tip by the US motion, sometimes referred to as “internal metal stress,” during phacoemulsification can cause heating and create warm zones in the phaco probe.11 The aim of the current study was to analyze temperature increases and identify potential sources of heat generation when sleeved and sleeveless phaco probes were operated in axial and torsional modes.
MATERIALS AND METHODS To examine heat generation during phacoemulsification, computer-controlled thermal transfer systems were developed to evaluate temperature increases associated with
Submitted: November 18, 2013. Final revision submitted: June 24, 2014. Accepted: June 25, 2014.
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phaco probes operated in axial or torsional mode using the Infiniti Vision System with the Ozil torsional handpiece (Alcon Laboratories, Inc.). Two methods were initially used to accomplish accurate temperature measurements. One method used thermal imaging with an infrared (IR) thermal camera, and the other used micro-thermocouple technology. Each is associated with inherent challenges in the environment of surgical ophthalmology. With IR thermal imaging, accurate measurement of heat generated by sleeveless phaco probes is potentially compromised by error introduced primarily by the low emissivity of the polished titanium probe. Polished metals are associated with low emissivity coefficients and if not carefully monitored, may produce thermal readings well below the actual surface temperature of the object being measured. With a more conventional measuring method such as a thermocouple, heat induced by friction, for example between the vibrating tip and the silicone sleeve, can lead to inaccuracies due to the invasive relationship. The result may be thermal readings that are mistakenly higher than when the intrusive thermocouple is not present. The thermal transfer systems that we describe were specifically developed to mitigate these confounding factors and after performing confirmatory tests between thermal imaging and the more invasive thermocouple technologies, high sample rate thermal imaging that included the use of a black-body-based thermal transfer system was the approach taken for final data collection. Distilled water instead of balanced salt solution was used as the fluid for all experiments to keep sensors and components in contact with the fluid (intentionally or inadvertently) free from precipitates and possible damage. Tests were made using room temperature water (generally between 22 C and 25 C). In tests in which baseline test temperatures were intentionally controlled, inflow was chilled by submerging part of the irrigation tubing in an ice bath and baseline test temperatures were set at 18 C. For all tests conducted, temperature measurements were taken at least 3 times to confirm repeatability. While multiple plots were used to report averaged values of these data taken in triplicate, representative thermal plots of individual temperature measurements are displayed in some figures.
Experimental System 1: Heat from Sleeveless Tips
From Clinica Oftalmologica Pasteur, Santiago, Chile. Supported in part by grant no. 69607 from Alcon Laboratories, Inc., Fort Worth, Texas, USA. Ramon Dimalanta, PhD, of Alcon Research, Ltd., Irvine, California, USA, helped complete this study. Eilidh Williamson, PhD, provided medical writing assistance under the sponsorship of Alcon Laboratories, Inc., Fort Worth, Texas, USA. Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Diego, California, USA, March 2011, and at the XXIX Congress of the European Society of Cataract and Refractive Surgeons, Vienna, Austria, September 2011. Corresponding author: Jaime Zacharias, MD, Clinica Oftalmologica Pasteur, Avenida Luis Pasteur 5917, Santiago, Chile. E-mail: jaime. [email protected].
To capture the heat emanating from a vibrating bare tip, an IR thermal-imaging camera (FLIR i40, FLIR Systems, Inc.) was focused on the tip and images captured at 7 frames per second. The setup incorporated a very thin (175 mm) black-body film (1.5 mm wide) made from plastic-based carbon paper (Kores CE GmbH) that would contact the bare tip and match its temperature profile. This experimental design element avoided the inaccuracy of estimating or tracking the titanium's emissivity (a critical parameter in IR thermography) as it may vary as a function of the surface temperature, surface finish, geometry, or other confounding factors.12 Because the black-body film lacked reflectivity and had an emissivity approaching 1.0, any thermal radiation from it introduced little variability or error when measured with thermal-imaging tools. The first attempts were to contact the thin black-body film against the tip of the phaco probe where the heat of the tip would readily transfer to the paper and in turn radiate IR
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Figure 1. Left: Black-body film shown in contact with the bare phaco tip. Right: Film shown lifted away from contacting the bare phaco tip. Thermal imaging system outside the image view and fluid collection system is not shown for clarity.
information to the camera for an accurate temperature measurement. It was immediately evident that the continuous contact of the black-body film against the phaco probe tip created its own frictional energy during US activation and therefore the temperature increase included a significant contribution from this contact. To capture only thermal signals from the vibrating tip, the electromechanical test fixture and thermal-measurement system were arranged as shown in Video 1 (available at http://jcrsjournal.org). To make thermal measurements that ensured no frictional energy was created, the setup was automated so the film came into contact with the tip only immediately after events of intermittent US activation. To that end, the sleeveless phaco probe was ultrasonically activated for 500 ms, during which time a driving signal was provided to a solenoid actuator that lifted the black-body film away from the probe surface. On deactivation of US activity, the solenoid-driving signal was simultaneously shut off, placing the film in contact with the surface of the nonvibrating probe for 500 ms. Any heat present in the probe was transferred to the film with temperature equilibrium achieved in less than 150 ms. With the thermal camera focused on the film, the probe temperature was continuously measured on repeated tests. The process was repeated every second with the film lifted during each active phase and lowered onto the probe surface during the inactive phase. Figure 1 shows how the blackbody film was alternately positioned on and off the bare phaco tip as described. It should be noted that in all experiments performed with sleeveless phaco probes, the direction of flow was reversed using a syringe pump (EW-74900-20, Cole-Parmer) to prevent fluid from splashing onto the probe and black-body film and potentially compromising the accuracy of the temperature measurements. Understanding that the fluid is used solely as a cooling agent in this comparative experiment examining a bare tip, the flow direction should not affect the results. A fluid collection system was incorporated to further avoid any spray onto the probe or camera (Figure 2 and Video 1). Temperature increases were assessed when the sleeveless phaco probe was operated in axial or torsional mode and when conditions such as US power and aspiration flow were varied.
Experimental System 2: Frictional Heat from Sleeved Tips Whereas experimental system 1 evaluated bare tips to examine and measure the possible effect of internal metal stress and its influence on temperature rise, a second series
of experiments examined the impact of tip-to-sleeve friction on heat generation during phacoemulsification. An automated system was developed that applied calibrated loads on the sleeve with enough force to make appreciable contact against the metal tip (Figure 3 and Video 2, available at: http://jcrsjournal.org). The load was applied to a region of the tip's shaft estimated to be where the sleeve would interface with the corneal incision wall. Force was transmitted to the surface of the sleeve using a 1.5 mm wide strip of thin black-body film loaded between 5.5 and 6.0 mm from the distal end of the tip (G0.5 mm) to simulate stress conditions during real surgery. This black-body film was made from a sheet of plastic-based carbon paper with the carbon surface exposed to the thermal camera and coincidentally (and conveniently) the same material used in the first set of bare tip experiments. To allow normal irrigation/aspiration (I/A) to flow through the appropriate channels and ports without constriction at the incisional zone, a funnel-shaped silicone adaptor was placed on the sleeve distal end. This arrangement also prevented fluid from spraying from the tip onto the sleeve outer surface and camera lens. Flow was circulated by a dual syringe pump and continuously monitored using a US flow sensor system (ME1PXN inline sensor
Figure 2. Clear plastic fluid spray collector device connected to aspirating end of the bare phaco tip. The housing shielded sensitive equipment (eg, thermal camera lens, black-body film) from liquid spray exiting the tip (using reversed aspiration flow) created during US operation.
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Load 30 g load, 1 s load on, 3 s load off
30 g load, 1 s load on, 3 s load off
30 g load
Ramp load 30, 40, 50, and 60 g
30 g load
Flow 3 mL/min
3 mL/min
6 mL/min, 18°C baseline flow temp at start of test 12 mL/min
3 mL/min
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Energy ApplicaƟon 50% torsional amplitude followed by 50% longitudinal power Ramp torsional amplitude 25%, 50%, 75%, and 100% followed by ramp longitudinal power 25%, 50%, 75%, and 100% 100% torsional power followed by 100% longitudinal power
100% torsional power followed by 100% longitudinal power Ramp duty cycle 20%, 40%, 60%, 80%, and 100%; 100% torsional power followed by 100% longitudinal power for each ramp cycle
Figure 3. Test setup that would apply a force or load against the sleeve and tip using a black-body strip pulled with a force gauge attached. The sleeve shown in the foreground is part of the fluid collection system to prevent unwanted spray from the US operation.
Figure 4. Description of the tests in sequence order.
with the TS410 tubing flowmeter module, Transonic Systems, Inc.). To accurately assess the magnitude of stress placed on an incision, tests that simulated a typical surgical movement were conducted. For this study, the movement was a lateral movement of the phaco tip against the corneal tunnel. The displacement of the tip relative to the incision was captured on video. The magnitude of tip displacement was measured from the video recording using the x–y dimension of individual pixels as calibration reference in Adobe Photoshop CS5.5 image processing software (Adobe Systems Inc.). A digital force gauge (Entran ELFS-T3M-10N, Measurement Specialties) was then attached to the phaco probe tip, the tip was displaced by the same distance measured on the video recording, and the resultant force was measured on the force gauge. The same applied force was then replicated during thermal testing using a custom system including the digital force gauge, set in series with a linear stepping motor (Haydon, Inc.) under computer control to intermittently apply the prescribed force from the film and create frictional contact between the sleeve and vibrating tip. This is important as the force, not the displacement, is the variable that must remain constant to
Test Sequence The sequence or evolution of tests conducted for sleeved tips was dynamic in that each subsequent test was in some way influenced by the test(s) that preceded it and this will be explained in the Discussion section. Figure 4 describes each test in the presented order.
produce similar frictional events. Experiments were then performed using momentary loads of fixed magnitude and duration in axial and torsional modes.
RESULTS Experimental System 1: Heat from Sleeveless Tips Using the first automated thermal transfer system described above, initial experiments were performed with the reversed aspiration flow rate constant at 3 mL/min. The axial power or torsional amplitude was maintained at 100% and applied at a 50% duty cycle (500 ms applied, 500 ms removed). Temperature increases from a baseline of 22 C were detected in sleeveless phaco probes when operated in either axial or torsional mode. In both modes, the temperature increase appeared evenly distributed throughout the length of the probe shaft as shown in Figure 5. The
Figure 5. Temperature increases detected in sleeveless phaco probes operated using 100% axial power or 100% torsional amplitude. Flow rate was 3 mL/min and power was applied at a 50% duty cycle (500 ms applied, 500 ms removed). Temperatures indicate maximal within the regions of interest shown by the blue boxes.
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Figure 6. Temperature increases detected in sleeveless phaco probes operated using 100% axial power or 100% torsional amplitude. Baseline temperature was 22 C. Flow rates were 3, 6, 12, and 24 mL/min.
increase observed with axial mode was greater than that observed with torsional mode, with peak temperatures reaching 51.7 C and 34.2 C, respectively. When the axial power or torsional amplitude was maintained at 100% and the aspiration flow varied, lower flow rates (3 to 6 mL/min) produced more pronounced temperature increases than higher flow rates (12 to 24 mL/min) (Figure 6). The same temperature increase from higher to lower flow rates was observed using axial or torsional mode; however, under equivalent flow conditions, a greater increase in temperature was consistently observed with axial mode.
Experimental System 2: Frictional Heat from Sleeved Tips Using the second automated system, temperature increases were evaluated when sleeved phaco probes were operated in axial or torsional mode under variable conditions of power/amplitude, flow rate, and force. It should be noted that each experiment was independently performed to compare temperature increases observed using axial and torsional modes under a specific set of conditions. No comparisons between individual experiments were made. In the first experiment, a load of 30.0 g was intermittently applied to the phaco probe tip 5.5 to 6.0 mm on its shaft from the distal end, while the aspiration flow was maintained at 3.0 mL/min. The force was applied at a 25% duty cycle in 4-second intervals (1 second applied, 3 seconds removed). The phaco probe was continuously operated at 50% torsional amplitude for 4 loading events before
Figure 7. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes under identical conditions of power, flow, and force. A load of 30.0 g was intermittently applied against the phaco probe tip (yellow line), while the aspiration flow was kept constant at 3.0 mL/min (blue line). The phaco probe was then operated at 50% torsional amplitude for 4 loading events before switching to 50% axial power for 3 loading events, then back to 50% torsional amplitude. Temperature increases detected in the phaco probe are indicated by the white line.
switching to 50% axial power for 3 loading events, then back to 50% torsional amplitude. Figure 7 shows the graphic results as collected in real time. Under identical conditions of power, flow, and force, greater temperature increases were observed when the probe was operated in axial mode (peak temperature of approximately 52 C) than when the probe was operated in torsional mode (peak temperature of approximately 35 C). To determine the thermal effect of increasing the axial power or torsional amplitude, a second experiment was conducted in which a load of 30.0 g was intermittently applied against the phaco probe tip at a 25% duty cycle in 4-second intervals (1 second applied, 3 seconds removed), while the aspiration flow was maintained at 3.0 mL/min. The phaco probe was then operated at increasing torsional amplitude (25%, 50%, 75%, and 100%) allowing a single and successive loading event followed by increasing axial power (25%, 50%, 75%, and 100%) also allowing a single and successive loading event. An example of the real-time experimental method is shown in Figure 8. Not surprisingly, increasing the US power or amplitude of the probe induced greater temperature increases. These increases were observed using both axial and torsional modes; however, the same conditions induced larger thermal increases when the probe was operated in axial mode. The greatest temperature reading was achieved when the probe was operated using 100% power in axial mode, with a peak
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Figure 8. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes under conditions of increasing power. A load of 30.0 g was intermittently applied against the phaco probe tip (yellow line), while the aspiration flow was kept constant at 3.0 mL/min (blue line). The phaco probe was then operated at increasing torsional amplitude (25%, 50%, 75%, and 100%) followed by increasing axial power (25%, 50%, 75%, and 100%). Temperature increases detected in the phaco probe are indicated by the white line.
Figure 9. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes with phaco probe cooling between operations. Starting at a temperature of 18 C, the phaco probe was pulsed once at 100% torsional amplitude with a load of 30.0 g (yellow line) and a constant aspiration flow of 6.0 mL/min (blue line). The phaco probe was allowed to cool back to 18 C and pulsed once at 100% axial power under the same load and flow conditions. Temperature increases detected in the phaco probe are indicated by the white line.
temperature of approximately 58 C to 61 C observed over multiple runs; by comparison, 100% torsional amplitude induced a peak temperature of approximately 45 C. The results of the previous 2 experiments suggest that heat may accumulate in the phaco probe tip as the number of successive frictional events increases and that the temperature does not return to baseline or even a quasi-baseline when switching between axial and torsional modes. To mitigate any confounding effect of heat accumulation from prior periods of US activation, experiments were performed in which the probe was allowed to cool down to the same baseline temperature between operations. The phaco probe was first cooled to 18 C using an ice bath for the I/A flow. The US power was then activated only once throughout 1 application of force for 1.0 second at 100% torsional amplitude. A load of 30.0 g was applied. Even though the flow was cooled, a slightly higher aspiration flow of approximately 6.0 mL/min was required for the probe to cool quickly after completing 1 thermal test and beginning the next test. The resultant temperature was measured, and the phaco probe was again cooled to 18 C using the chilled water before the US power was activated once throughout 1 application of force for 1.0 second at 100% axial power. As shown in Figure 9, starting from a similar baseline temperature of 18 C, a single pulse of the phaco probe using 100% axial power produced a temperature of approximately 45 C, whereas
a single pulse using 100% torsional amplitude produced a temperature of approximately 38 C. To further explore the accumulation of heat, a test in which increasing loads were successively applied to the tip in an intermittent fashion was conducted. Two intermittent loads administered on the side of the tip's shaft were applied to the tip in increments of 10.0 g, totaling 30.0 g, 40.0 g, 50.0 g, and 60.0 g, when the probe was operated using 100% torsional amplitude or 100% axial power. Figure 10 shows results from experiments exploring torsional mode and axial mode, respectively. In each comparative pulse, the temperature resulting from operating the probe using axial power always exceeded the temperature achieved using torsional power, with peak temperatures of approximately 43 C and 63 C obtained with torsional mode and axial mode, respectively. The accumulation of heat was also demonstrated as each load was added over time. In the final tests, experiments were performed in which a load of 30.0 g was intermittently applied to the phaco probe tip, while the aspiration flow was maintained at 3.0 mL/min. The phaco probe was operated at 100% continuous torsional amplitude followed by 100% axial power operated at a duty cycle of 20%. The paired test was then repeated with increasing duty cycles of 40%, 60%, 80%, and 100%. The results of these experiments showed that the temperature increase produced using continuous 100% torsional amplitude (approximately 45 C) was
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Figure 10. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes when increasing loads were successively applied to the tip in an intermittent fashion. Two intermittent loads were applied to the tip in increments of 10.0 g (totaling 30.0 g, 40.0 g, 50.0 g, and 60.0 g) when the probe was operated using either 100% torsional amplitude (left) or 100% axial power (right). Temperature increases detected in the phaco probe are indicated by the white line.
equivalent to that produced with a duty cycle of 40% axial power (Figure 11). DISCUSSION Significant thermal energy can be generated at the incision site during US phacoemulsification. Although this thermal energy is usually dissipated from the wound by I/A fluid and rendered harmless, corneal burns have occasionally been reported.6,7,A Potential sources of heat during phacoemulsification are intermolecular collisions within the phaco probe shaft (internal metal stress) and tip-to-sleeve friction at the incisional zone. In this study, 2 automated thermal transfer systems evaluated the contribution of internal metal stress and tip-to-sleeve friction to heat generation during phacoemulsification. In addition, the
Figure 11. Temperature increases observed in sleeved phaco probes operated in axial and torsional modes with variable duty cycles. A load of 30.0 g was intermittently applied against the phaco probe tip (yellow line), while the aspiration flow was maintained at 3.0 mL/min (blue line). Temperature increases detected in the phaco probe are indicated by the white line. The red circle highlights that 100% torsional magnitude produces similar thermal results as 100% axial power with a duty cycle of 40%.
systems were used to compare the thermal impact of axial and torsional US modalities. In experiments performed to evaluate heat generation in sleeveless phaco probes, an evenly distributed temperature increase was observed throughout the length of the probe shaft, contrary to theories of “warm zones” described by others.11 In all experiments, the increase observed with axial operation was consistently greater than that observed with torsional operation, even when conditions such as flow rate were varied. Experiments performed with sleeved probes using the second automated system showed that tip-tosleeve friction was a more dominant source of phaco probe heating than internal metal stress. In addition, under identical conditions of US power, flow, and force, greater temperature increases were observed when the sleeved probe was operated in axial mode than when the probe was operated in torsional mode. Varying experimental conditions known to increase temperature, such as reduced aspiration flow, increased force, and increased US power or amplitude4,13 induced greater temperature increases with both modes; however, the same conditions induced larger thermal increases when the probe was operated in axial mode. During initial experiments, it was noted that heat accumulated in the phaco probe tip as the number of successive frictional events increased and the temperature did not return to baseline when switching between axial and torsional modes. Results of experiments in which the probe was allowed to cool down to the same baseline temperature between operations showed that starting from the same baseline temperature, a single pulse of applied load to the phaco probe in axial mode produced a higher temperature increase than a single pulse in torsional mode. Although this was an important finding, in practical terms, surgeons may not have the opportunity to
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allow the phaco probe to cool during the phacoemulsification procedure and may instead opt to vary the duty cycle to provide thermal relief. Experiments in which the effect of varying the duty cycle was evaluated indicated that the temperature increase produced using continuous 100% torsional amplitude was equivalent to that produced with a duty cycle of 40% axial power. The finding that torsional phacoemulsification generates less heat than axial mode agrees with findings from previous studies using different experimental systems. Using serial imaging with a thermal camera, Han and Miller14 evaluated temperature increases observed when phacoemulsification probes from manufacturers of different phacoemulsification platforms were operated in silicone test chambers filled with a balanced salt solution. Under various experimental conditions, axial mode consistently induced a greater temperature increase than torsional mode. Similarly, in studies by Jun et al.,15 phacoemulsification simulated in human cadaver eyes and imaged using an IR camera indicated that axial mode induced higher temperatures at the incision site than torsional mode and was associated with more extensive loss of Descemet membrane. Despite the temperature elevations observed in the current study, it is generally well recognized that surgeons can safely operate using any modern phacoemulsification platform as long as power, occlusion, flow, and wound stress are fully understood and properly managed. With the various machines and modalities to choose from, it may be difficult to navigate the best technology for all surgeons. Sorensen et al.7 definitively ascertained that in their multifactorial survey of incision contracture, in the categories of machine type and US modalities, there was no association with wound burn and that other choices such as the ophthalmic viscosurgical device (OVD) used or surgeon technique may statistically influence the rate of incision burns. Most thermal injuries are related to avoidable errors including insufficient irrigation or aspiration, high US power, or prolonged power during occlusion.6 Prevention can be achieved by precautions such as confirming adequate fluid flow before tip insertion, allowing I/A to clear some OVD material before starting the US, and being cognizant of over-torquing the wound.10,B Sorensen et al.'s7 advice “.that all ultrasound, including horizontal ultrasound, should be used parsimoniously.” should be well heeded. Although no US phacoemulsification device, axial or nonaxial (torsional or transversal), can completely avoid thermal risk, a more thorough understanding of how US energy is applied or mitigated using various choices of modalities and power modulations
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will allow surgeons to be more strategic when optimizing their operating parameters, enabling favorable surgical outcomes. WHAT WAS KNOWN Phacoemulsification can generate significant thermal energy at the corneal incision site, and burns associated with cataract surgery have occasionally been reported. Previous studies have suggested that thermal rise due to the vibrating emulsifying tip can cause heating and creation of warm zones in the phaco probe due to internal metal stress. WHAT THIS PAPER ADDS An automated thermal transfer system showed that axial mode was consistently associated with greater temperature increases than torsional mode whether tips were operated with or without sleeves. Friction between the sleeved probe and the incisional wall contributed more significantly to the temperature increase than did internal metal stress. This study may help surgeons choose appropriate tools, techniques, and operational settings to minimize thermal risks.
REFERENCES 1. Tognetto D, Cecchini P, Leon P, Di Nicola M, Ravalico G. Stroke dynamics and frequency of 3 phacoemulsification machines. J Cataract Refract Surg 2012; 38:333–342 2. Liu Y, Zeng M, Liu X, Luo L, Yuan Z, Xia Y, Zeng Y. Torsional mode versus conventional ultrasound phacoemulsification; randomized comparative clinical study. J Cataract Refract Surg 2007; 33:287–292 3. Vargas LG, Holzer MP, Solomon KD, Sandoval HP, Auffarth GU, Apple DJ. Endothelial cell integrity after phacoemulsification with 2 different handpieces. J Cataract Refract Surg 2004; 30:478–482 4. Shah PA, Yoo S. Innovations in phacoemulsification technology. Curr Opin Ophthalmol 2007; 18:23–26 5. Linebarger EJ, Hardten DR, Shah GK, Lindstrom RL. Phacoemulsification and modern cataract surgery. Surv Ophthalmol 1999; 44:123–147 6. Bradley MJ, Olson RJ. A survey about phacoemulsification incision thermal contraction incidence and causal relationships. Am J Ophthalmol 2006; 141:222–224 7. Sorensen T, Chan CC, Bradley M, Braga-Mele R, Olson RJ. Ultrasound-induced corneal incision contracture survey in the United States and Canada. J Cataract Refract Surg 2012; 38:227–233 8. Bissen-Miyajima H, Shimmura S, Tsubota K. Thermal effect on corneal incisions with different phacoemulsification ultrasonic tips. J Cataract Refract Surg 1999; 25:60–64
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9. Olson MD, Miller KM. In-air thermal imaging comparison of Legacy AdvanTec, Millennium, and Sovereign WhiteStar pacoemulsification systems. J Cataract Refract Surg 2005; 31:1640–1647 10. Sugar A, Schertzer RM. Clinical course of phacoemulsification wound burns. J Cataract Refract Surg 1999; 25:688– 692 11. Schmutz JS, Olson RJ. Thermal comparison of Infiniti OZil and Signature Ellips phacoemulsification systems. Am J Ophthalmol 2010; 149:762–767 12. Incropera FP, DeWitt DP. Fundamentals of Heat and Mass Transfer, 3rd ed. New York, NY, John Wiley & Sons, 1990 13. Osher RH, Injev VP. Thermal study of bare tips with various system parameters and incision sizes. J Cataract Refract Surg 2006; 32:867–872 14. Han YK, Miller KM. Heat production: longitudinal versus torsional phacoemulsification. J Cataract Refract Surg 2009; 35:1799–1805 15. Jun B, Berdahl JP, Kim T. Thermal study of longitudinal and torsional ultrasound phacoemulsification; tracking the temperature of the corneal surface, incision, and handpiece. J Cataract Refract Surg 2010; 36:832–837
OTHER CITED MATERIAL A. U.S. Food and Drug Administration. FDA Manufacturer and User Facility Device Experience [MAUDE] database. Available at: http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfm aude/search.cfm. Accessed September 17, 2014 B. Pennsylvania Patient Safety Advisory. Preventing Corneal Burns During Phacoemulsification 2010; 7:23–25. Available at: http:// patientsafetyauthority.org/ADVISORIES/AdvisoryLibrary/2010/ Mar7(1)/Pages/23.aspx. Accessed September 17, 2014
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First author: Jaime Zacharias, MD Clinica Oftalmologica Pasteur, Santiago, Chile