- Email: [email protected]
Thermal Imaging Study Comparing Phacoemulsification With the Sovereign With WhiteStar System to the Legacy With AdvanTec and NeoSoniX System
Thermal Imaging Study Comparing Phacoemulsification With the Sovereign With WhiteStar System to the Legacy With AdvanTec and NeoSoniX System ARON D. ROSE, MD, AND VASUDEV KANADE, MD
● PURPOSE:
To assess intraoperative thermal levels at the wound site during divide-and-conquer phacoemulsification with the Sovereign with WhiteStar (SWS) system or the Legacy with AdvanTec and NeoSoniX (LAD) system. ● DESIGN: Prospective, randomized, parallel-group, comparative study. ● METHODS: Twenty-six subjects from a private clinical practice underwent divide-and-conquer phacoemulsification with either the SWS system or the LAD system. CB/CF settings (60%/33% duty cycles) were utilized with SWS and 12 pulses per second with the LAD system. Key criteria assessed were peak wound-site temperature, mean temperature change at the wound site, effective phaco time, average phaco power, procedure time, amount of BSS used, and surgical complications. ● RESULTS: Mean temperature change at the wound site was greater for the LAD than the SWS group. There was a statistically significant difference (P ⴝ .0002) in mean peak wound temperatures, with the LAD group having higher mean peak temperatures (42.47 ⴞ 5.33°C) than the SWS group (36.59 ⴞ 1.33°C). Highest wound-site temperature was 51°C for the LAD group and 39.3°C for the SWS group. A statistically significant difference (P ⴝ .0031) in mean peak temperature was found between the LAD and SWS systems for subjects with a cataract density of 4: Accepted for publication Sep 20, 2005. From The Eye Care Group (A.D.R.), Yale University School of Medicine (A.D.R.), Yale University School of Nursing, and Department of Ophthalmology and Visual Science (V.K.), Yale University School of Medicine, New Haven, Connecticut. Supported in part by Advanced Medical Optics Inc, Santa Ana, California. Presented in part at American Society of Cataract and Refractive Surgery Symposium on Cataract, IOL and Refractive Surgery, Apr 17, 2005, Washington, DC. A.D.R. is a paid consultant to Advanced Medical Optics Inc, Santa Ana, California. Inquiries to Aron D. Rose, MD, The Eye Care Group, 40 Temple Street, Suite 5B, New Haven, CT 06510-2716; fax: (203) 562-7013; e-mail: [email protected]
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higher mean peak temperatures were observed for LAD patients with a cataract density of 4. ● CONCLUSIONS: Our findings show that phacoemulsification using the SWS system results in lower peak temperatures and less temperature change at the phaco wound site compared with the LAD system. (Am J Ophthalmol 2006;141:322–326. © 2006 by Elsevier Inc. All rights reserved.)
P
HACOEMULSIFICATION TECHNOLOGY HAS STREAM-
lined the process of cataract surgery with its trend toward faster surgical times, less ultrasound (U/S) energy use, smaller incisions, and increased surgeon control. Since 2000, 97% of American Society of Cataract and Refractive Surgery members performed phacoemulsification on their cataract patients.1 In this time, wound burns have emerged as an iatrogenic complication of phacoemulsification. This may be due to the cataract surgeon’s lack of familiarity with the procedure or equipment, insufficient irrigation and aspiration flow, use of more aggressive techniques, and/or use of smaller incisions and smaller-diameter probe tips.2 In the western United States, the overall incidence of wound burns is 1/1000 cases (Bradley M, Olson RJ, “Survey of Incidence of Wound Burns Across 5 Western States,” presented at the American Society of Cataract and Refractive Surgery meeting, April 16 to 20, 2005, Washington, DC, USA). Ultrasound phacoemulsification uses piezoelectric crystals to convert electrical energy to mechanical energy, causing rapid vibration of the phaco tip at frequencies between 25 kHz and 62 kHz. This vibration emulsifies the lens but at the same time creates friction and generates heat, which can cause thermal injury to the cornea.2,3 The irrigation sleeve and continuous fluid irrigation help dissipate heat from the wound site; however, if irrigation is diminished or interrupted, heat is conveyed to the surrounding cornea, potentially causing tissue damage.4
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/06/$32.00 doi:10.1016/j.ajo.2005.09.023
Corneal epithelial burns are characterized by coagulative necrosis, desquamation, and eventual regeneration; thermal injury to the corneal stroma may result in both reversible and irreversible transitions of the collagen matrix, leading to a possible reduction in corneal clarity as well as irregular astigmatism, progressive corneal melts, and decreased postoperative acuity.3,5 The elimination of thermal energy production during ultrasonic phacoemulsification therefore represents a profound and critical phase in the move toward nontraumatic cataract surgery. This study compares temperature levels at the wound site during divide-and-conquer phacoemulsification using micropulsed and 12-pulse-per-second (pps) U/S technologies.
METHODS THIS WAS A PROSPECTIVE, OPEN-LABEL, RANDOMIZED, PAR-
allel-group, comparative clinical study of the Sovereign with WhiteStar (Advanced Medical Optics Inc, Santa Ana, California, USA) and the Legacy with AdvanTec and NeoSoniX (Alcon Inc, Fort Worth, Texas, USA) phacoemulsification systems. Institutional review board approval was not necessary. There was no change from the surgeon’s standard cataract surgery procedure for the study. Informed consent was obtained both for the surgery and for intraoperative videophotography. Twenty-six subjects were enrolled into this study. Twelve eyes from 11 subjects were assigned to undergo phacoemulsification with the Sovereign with WhiteStar system (SWS group) and 15 eyes from 15 subjects with the Alcon Legacy system with AdvanTec and NeoSoniX (LAD group). Subjects were included if they met the following inclusion criteria: visually interfering senile or presenile cataract for which phacoemulsification extraction was planned; ⫹3 or ⫹4 cataract density (on a scale from 0 to ⫹4); age 21 or older; and signed informed consent. Cataract density was graded preoperatively by the surgeon on a scale of 0 to ⫹4 at the slit lamp. Subjects were excluded if they possessed any ocular or systemic condition that would negatively affect successful phacoemulsification (for example, glaucoma, iritis, history of inflammation, Fuchs dystrophy), or if they had only one sighted eye with a corrected visual potential of 20/40 or better. All procedures were performed by the same surgeon (A.D.R.), who had used both phacoemulsification systems for several years before study initiation. Phacoemulsification was performed with a 20-gauge, 30-degree tip on each study subject by using a divide-and-conquer technique. The LAD system was set at 12 pps (50% duty cycle) and the SWS at CB/CF duty cycle (6 ms on/4 ms off for CB setting and 6 ms on/12 ms off for the CF setting). Settings were held constant unless specific cases required optimization. Infrared continuous video imaging of the procedure was performed by focusing an infrared camera system (Flir VOL. 141, NO. 2
Systems Thermovision A20) on the phaco tip at the incision. Thermal videos were subsequently downloaded to a laptop computer. Key operative criteria included peak temperature at the wound site, maximum temperature change from baseline at the wound site, equivalent phaco time, procedure time, total U/S time, percentage of phaco power used, amount of balanced salt solution (BSS) used after completion of the prime cycle, and surgical complications. Baseline temperature refers to the temperature reading within the rectangle (wound-site area) at the start of thermal image capture. Peak temperature is the highest numerical temperature reading recorded from the field within the rectangle. Maximum temperature change from baseline indicates the difference between baseline temperature and peak temperature. Equivalent phaco time calculates the total amount of U/S energy applied to the eye and is the product of the U/S time and mean U/S energy. Procedure time is the time from the start to the end of the phacoemulsification portion of the procedure. A two-way analysis of variance (with phaco system and cataract status included in the model) was used to evaluate differences in mean peak temperature between the SWS and LAD phaco groups. Because statistical interactions were noted between cataract status and phaco systems for temperature, these data were analyzed separately by cataract status by two-sample t tests. For t test analyses, the Satterthwaite method was used to adjust for unequal variances. Two-sided testing was performed with ␣ set at .05. To ascertain whether differences in age and baseline temperature existed between the two study groups, a two-sample t test was used. The Pearson 2 test was used to determine sex and cataract density differences, which may have influenced the study results.
RESULTS THE MEAN AGE FOR THE LAD GROUP WAS 80.13 ⫾ 4.60
years; (mean ⫾ SD) for the SWS group, it was 75.91 ⫾ 5.87 years. Forty-seven percent of the LAD subjects were men, and 64% of the SWS subjects were men. There was no statistically significant difference between the two study groups in terms of age (P ⫽ .0766) and gender (P ⫽ .5466). No statistically significant differences in baseline temperature (P ⫽ .7433) and in cataract density (.5466) were seen between the two groups. Tables 1 and 2 delineate the equivalent phaco time, procedure time, percentage of phaco power used, and BSS used for each study group. Both tables show that procedure time and the amount of BSS used is greater with the LAD group than with the SWS group, regardless of cataract density. Mean maximum temperature changes from baseline to peak for both groups are shown in Figure 1. Mean maximum temperature change at the wound site is greater for the LAD group than the SWS group (Figure 2): mean
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TABLE 1. Surgical Outcome Measures for Legacy With AdvanTec and NeoSoniX (LAD) for All Eyes, Subdivided by Cataract Density
Operative Parameter
All eyes Ultrasound time (minutes) (N ⫽ 15 eyes) Phaco power (%) (N ⫽ 15 eyes) BSS used (mL) (N ⫽ 11 eyes) Procedure time (minutes) (N ⫽ 11 eyes) Subdivided by cataract density 3⫹ cataract density Ultrasound time (minutes) (N ⫽ 8 eyes) Phaco power (%) (N ⫽ 8 eyes) BSS used (mL) (N ⫽ 6 eyes) Procedure time (minutes) (N ⫽ 6 eyes) 4⫹ cataract density Ultrasound time (minutes) (N ⫽ 7 eyes) Phaco power (%) (N ⫽ 7 eyes) BSS used (mL) (N ⫽ 5 eyes) Procedure time (minutes) (N ⫽ 5 eyes)
LAD Group, Mean ⫾ SD
2.1 ⫾ 1.02 17.6 ⫾ 3.38 262.27 ⫾ 91.28 9.12 ⫾ 4.39
FIGURE 1. Mean maximum temperature change from baseline to peak for the Legacy with AdvanTec and NeoSoniX (LAD) and Sovereign with WhiteStar (SWS) groups. LAD group, n ⴝ 15; SWS group, n ⴝ 12.
1.55 ⫾ 0.53 16.38 ⫾ 3.42 229.17 ⫾ 65.99 6.72 ⫾ 2.45 2.73 ⫾ 1.11 19.00 ⫾ 2.94 302 ⫾ 108.49 12.00 ⫾ 4.66
TABLE 2. Surgical Outcome Measures for Sovereign With WhiteStar (SWS) for All Eyes, Subdivided by Cataract Density
Operative Parameter
All eyes Ultrasound time (minutes) (N ⫽ 8) Phaco power (%) (N ⫽ 12) BSS used (mL) (N ⫽ 11) Procedure time (minutes) (N ⫽ 11) Subdivided by cataract density 3⫹ cataract density Ultrasound time (minutes) (N ⫽ 3) Phaco power (%) (N ⫽ 5) BSS used (mL) (N ⫽ 4) Procedure time (minutes) (N ⫽ 4) 4⫹ cataract density Ultrasound time (minutes) (N ⫽ 5) Phaco power (%) (N ⫽ 7) BSS used (mL) (N ⫽ 7) Procedure time (minutes) (N ⫽ 7)
SWS Group, Mean ⫾ SD
3.57 ⫾ 2.29 9.03 ⫾ 1.31 161.82 ⫾ 51.10 7.22 ⫾ 4.65
FIGURE 2. Temperature change from baseline to peak at phacoemulsification wound site.
1.87 ⫾ 0.28 9.6 ⫾ 1.19 118.75 ⫾ 55.43 5.17 ⫾ 0.18
wound-site temperature recorded was 51°C for LAD and 39.3°C for SWS (Figures 3 and 4). There was a statistically significant difference (P ⫽ .0215) in mean peak wound temperatures between all study patients with a cataract density of ⫹3 and those with a cataract density of ⫹4. The mean peak wound temperature was higher for patients with a cataract density of ⫹4 (41.15 ⫾ 6.00°C) than that for patients with a cataract density of ⫹3 (38.46 ⫾ 3.20°C). Two-way analysis of variance revealed a statistically significant difference (P ⫽ .0226) for the interaction between cataract density and the two phaco systems. After analyzing the data separately for cataract density, a statistically significant difference was found between the LAD and SWS groups for subjects with a cataract density of ⫹4, with higher mean peak temperatures for those who underwent surgery by using the LAD system (P ⫽ .0031). No statistically significant difference in mean peak temperature was found for those with a cataract density of ⫹3 (P ⫽ .1375).
4.59 ⫾ 2.37 8.62 ⫾ 1.32 186.43 ⫾ 29.54 8.39 ⫾ 5.62
maximum temperature change for the LAD group is 7.39 ⫾ 5.63°C, compared with 1.59 ⫾ 1.43°C for the SWS group. Figure 2 plots the wound-site temperature changes from baseline to peak for patients in the LAD group (triangle points) and in the SWS group (square points). There was a statistically significant difference (P ⫽ .0002) in mean peak wound temperatures between the LAD and the SWS groups. The mean peak wound temperature was higher for the LAD group (42.47 ⫾ 5.33°C) than for the SWS group (36.59 ⫾ 1.33°C). The highest 324
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FIGURE 3. Thermal image of wound site with highest temperature reading (39.3°C) in the Sovereign with WhiteStar (SWS) group.
FIGURE 4. Thermal image of wound site with highest temperature reading (51.0°C) in the Legacy with AdvanTec and NeoSoniX (LAD) group.
Two subjects, one for each study group, had intraoperative posterior capsule tears requiring vitrectomy. These patients were not excluded from the study because the events after phacoemulsification did not affect the study’s intended goals or data collection methods. No postoperative complications were noted in either study group.
DISCUSSION THE ISSUE OF WOUND BURNS DURING PHACOEMULSIFICA-
tion has been addressed in the recent literature. A comparative study of tip temperatures with the Legacy, Sovereign with WhiteStar, and Millennium phaco systems in open air and in cadaver eyes demonstrated higher temperatures for the WhiteStar and Millennium systems in VOL. 141, NO. 2
both conditions.6 A cadaver eye study by Olson and associates,7 however, showed less increase in wound temperature over time with the SWS system than with the LAD system during sleeved and unsleeved simulated surgery. A western US survey of 75,000 phacoemulsification cases indicated an incidence of 0.98% of severe wound burn, 0.12% of which was attributed to the Legacy system and 0.053%, to the Sovereign system (Bradley M, Olson RJ, “Survey of Incidence of Wound Burns Across 5 Western States,” presented at the American Society of Cataract and Refractive Surgery meeting, April 16 to 20, 2005, Washington, DC, USA). To date, there has been no clinical study published in the peer-reviewed literature comparing wound-site temperature during actual surgery that used micropulse and pulsed phaco technologies. Our study affirms the findings of Olson and colleagues in a clinical setting. Pulsed mode phacoemulsification delivers power that has been modulated to turn on and off a certain number of times per second (pulses per second). With pulse mode, phaco power delivery is reduced by 50%. Burst time is equal to rest time: lens emulsification occurs during burst time, and cooling and clearing of the emulsified material occurs during rest time. NeoSoniX technology creates low-frequency oscillatory movement that can be combined with standard high-frequency ultrasonic phacoemulsification. The low-frequency mode can be used for softer cataracts; the combination mode is used to treat denser cataract grades.8 Micropulsed U/S phacoemulsification (WhiteStar technology) delivers digitally modulated ultrasonic energy in short, rapid pulses of “on” (burst) time and “off” (rest) time. Unlike pulsed mode phacoemulsification, the burst and rest period length can be varied independently, enabling the surgeon to do the following: (1) maximize heat dissipation, thereby decreasing the risk of thermal injury; (2) improve cutting efficiency (less phaco energy is used to achieve the same cutting power); and (3) improve followability with less chatter (Olson RJ, “Sovereign WhiteStar,” presented at the Royal Hawaiian Eye 2002 Conference, January 20 to 25, 2002, Waikoloa, Hawaii, USA). The light bulb analogy provides us with a simple explanation of micropulsed phaco technology: micropulsed U/S is likened to a fluorescent bulb and traditional phaco to an incandescent bulb. The fluorescent bulb, powered by rapidly pulsed energy, remains cool while producing sufficient lighting, as opposed to an incandescent bulb, which becomes hot as a result of a continuous energy flow (Fine IH, “WhiteStar Phaco Power Control System,” presented at the American Society of Cataract and Refractive Surgeons 2002 Conference, June 2, 2002, Philadelphia, Pennsylvania, USA). WhiteStar technology uses a two-letter designation for each of its micropulse settings. To obtain the setting, the letter’s order in the alphabet is multiplied by 2 ms. The first letter represents the burst or “on” period and the
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second letter, the rest or “off” period. For instance, a CB setting refers to 6 ms of on time and 4 ms of off time. C is the third letter of the alphabet, and 3 multiplied by 2 ms is 6 ms of on time. By using the same method of calculation, B signifies 4 ms of off time (B is the second letter of the alphabet, and 2 multiplied by 2 equals 4). The CF setting refers to 6 ms of on time and 12 ms of off time. This two-letter designation can be used to determine the duty cycle for each setting. The CB setting translates to a duty cycle of 60% (6 divided by 10 equals 0.6, 10 being the sum of 6 and 4). For the CF setting, the duty cycle is 33% (6 divided by 18 is 0.33, 18 being the sum of 6 and 12). Both phacoemulsification technologies have their own relative merits and drawbacks. Ocular parameters (such as wound-site temperature), which relate to clinical outcomes, should be at the top of our list of priorities when attempting to compare micropulse and pulsed phacoemulsification technologies. The current study shows that mean peak temperature and mean temperature changes are higher with the LAD system than with SWS system. The LAD group experienced temperature peaks as high as 51°C. Extended exposures to temperatures of 45°C can cause cloudy corneas, stromal edema, and collagen disorganization. As temperature increases, the time required to cause corneal changes decreases.9 For safety purposes, this author suggests keeping phaco peak wound temperatures at 40°C or less. It is also interesting to note that there were significantly higher mean peak temperatures for patients with a cataract density of ⫹4 who underwent surgery using the LAD system. Heat is directly related to work: the most effective ways of minimizing heat production are by maximizing the efficiency of work performed, and by facilitating heat dissipation into the surrounding tissue.10 The harder cataracts necessitate more work, and the LAD system appeared to be able to accomplish the task of emulsifying a harder lens, but with less efficiency and higher resulting wound temperatures compared with the SWS system. Procedure time is notably longer among LAD patients with cataract densities of ⫹4: the mean procedure time for LAD patients with 4 cataract densities is 12.00 ⫾ 4.66 minutes and 8.39 ⫾ 5.62 minutes for SWS patients. This may reflect both the surgeon’s attempt to avoid prolonged pulsed ultrasound “on” times to allow for heat dissipation, as well as the decreased efficiency of a pulsed ultrasound system as compared with a micropulsed one. This study can be further refined by capturing both corneal surface and tissue temperatures during phacoemulsification. In 1999, Bissen-Miyajima and associates11 used videothermography to observe thermal burns in corneal incisions and noted the difficulty of observing temperature
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changes at the wound site during the actual phacoemulsification process. In our study, a microthermistor may have provided more accurate readings of the wound site, but a thermal infrared camera allowed us to noninvasively observe temperature changes in real time. However, thermal images provided only surface temperature data, so the actual temperature of corneal tissue was difficult to assess. This study showed that peak temperature and temperature changes at the wound site during divide-and-conquer phacoemulsification are significantly higher with the LAD system compared with the SWS system. Further studies will be needed to determine corneal surface and tissue temperature relationships, and to test wound-site temperature differences with varying surgical techniques. ACKNOWLEDGMENTS
Statistical support was provided by Advanced Medical Optics Inc, Santa Ana, California, USA.
REFERENCES 1. Leaming DV. Practice styles and preferences of ASCRS members—2003 survey. J Cataract Refract Surg 2004;30: 892–900. 2. Scleral and corneal burns during phacoemulsification. Health Devices 1996;25:426 – 431. 3. Sugar A, Schertzer RM. Clinical course of phacoemulsification wound burns. J Cataract Refract Surg 1999;25:688 – 692. 4. Fine IH, Packer M, Hoffman RS. New phacoemulsification technologies. J Cataract Refract Surg 2002;28:1054 –1060. 5. Ernest P, Rhem M, McDermott M, Lavery K, Sensoli A. Phacoemulsification conditions resulting in thermal wound injury. J Cataract Refract Surg 2001;27:1829 –1839. 6. Mackool RJ, Sirota MA. Thermal comparison of the AdvanTec Legacy, Sovereign WhiteStar, and Millennium phacoemulsification systems. J Cataract Refract Surg 2005; 31:812– 817. 7. Olson RJ, Jin Y, Kefalopoulos G, Brinton J. Legacy AdvAnTec and Sovereign WhiteStar: a wound temperature study. J Cataract Refract Surg 2004:30;1109 –1113. 8. Fine IH, Packer M, Hoffman RS. Power modulations in new phacoemulsification technology: improved outcomes. J Cataract Refract Surg 2004;30:1014 –1019. 9. Goldblatt WS, Finger PT, Perry HD, Stroh EM, Weiser DS, Donnenfeld ED. Hyperthermic treatment of rabbit corneas. Invest Ophthalmol Vis Sci 1989;30:1778 –1783. 10. Donnenfeld ED, Olson RJ. Reply to letter by R.J. MacKool. J Cataract Refract Surg 2004;30:732–733. 11. 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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Biosketch Aron D. Rose, MD, is an Associate Clinical Professor at the Yale University School of Medicine and Nursing. He is an active international lecturer on emerging ophthalmic technology and its applications, and a section editor for Techniques in Ophthalmology. Dr Rose is also a partner in private practice with The Eye Care Group, PC.
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Biosketch Vasudev Kanade, MD, is a Research Assistant at the Yale University Department of Ophthalmology and Visual Sciences.
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● PURPOSE:
To assess intraoperative thermal levels at the wound site during divide-and-conquer phacoemulsification with the Sovereign with WhiteStar (SWS) system or the Legacy with AdvanTec and NeoSoniX (LAD) system. ● DESIGN: Prospective, randomized, parallel-group, comparative study. ● METHODS: Twenty-six subjects from a private clinical practice underwent divide-and-conquer phacoemulsification with either the SWS system or the LAD system. CB/CF settings (60%/33% duty cycles) were utilized with SWS and 12 pulses per second with the LAD system. Key criteria assessed were peak wound-site temperature, mean temperature change at the wound site, effective phaco time, average phaco power, procedure time, amount of BSS used, and surgical complications. ● RESULTS: Mean temperature change at the wound site was greater for the LAD than the SWS group. There was a statistically significant difference (P ⴝ .0002) in mean peak wound temperatures, with the LAD group having higher mean peak temperatures (42.47 ⴞ 5.33°C) than the SWS group (36.59 ⴞ 1.33°C). Highest wound-site temperature was 51°C for the LAD group and 39.3°C for the SWS group. A statistically significant difference (P ⴝ .0031) in mean peak temperature was found between the LAD and SWS systems for subjects with a cataract density of 4: Accepted for publication Sep 20, 2005. From The Eye Care Group (A.D.R.), Yale University School of Medicine (A.D.R.), Yale University School of Nursing, and Department of Ophthalmology and Visual Science (V.K.), Yale University School of Medicine, New Haven, Connecticut. Supported in part by Advanced Medical Optics Inc, Santa Ana, California. Presented in part at American Society of Cataract and Refractive Surgery Symposium on Cataract, IOL and Refractive Surgery, Apr 17, 2005, Washington, DC. A.D.R. is a paid consultant to Advanced Medical Optics Inc, Santa Ana, California. Inquiries to Aron D. Rose, MD, The Eye Care Group, 40 Temple Street, Suite 5B, New Haven, CT 06510-2716; fax: (203) 562-7013; e-mail: [email protected]
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higher mean peak temperatures were observed for LAD patients with a cataract density of 4. ● CONCLUSIONS: Our findings show that phacoemulsification using the SWS system results in lower peak temperatures and less temperature change at the phaco wound site compared with the LAD system. (Am J Ophthalmol 2006;141:322–326. © 2006 by Elsevier Inc. All rights reserved.)
P
HACOEMULSIFICATION TECHNOLOGY HAS STREAM-
lined the process of cataract surgery with its trend toward faster surgical times, less ultrasound (U/S) energy use, smaller incisions, and increased surgeon control. Since 2000, 97% of American Society of Cataract and Refractive Surgery members performed phacoemulsification on their cataract patients.1 In this time, wound burns have emerged as an iatrogenic complication of phacoemulsification. This may be due to the cataract surgeon’s lack of familiarity with the procedure or equipment, insufficient irrigation and aspiration flow, use of more aggressive techniques, and/or use of smaller incisions and smaller-diameter probe tips.2 In the western United States, the overall incidence of wound burns is 1/1000 cases (Bradley M, Olson RJ, “Survey of Incidence of Wound Burns Across 5 Western States,” presented at the American Society of Cataract and Refractive Surgery meeting, April 16 to 20, 2005, Washington, DC, USA). Ultrasound phacoemulsification uses piezoelectric crystals to convert electrical energy to mechanical energy, causing rapid vibration of the phaco tip at frequencies between 25 kHz and 62 kHz. This vibration emulsifies the lens but at the same time creates friction and generates heat, which can cause thermal injury to the cornea.2,3 The irrigation sleeve and continuous fluid irrigation help dissipate heat from the wound site; however, if irrigation is diminished or interrupted, heat is conveyed to the surrounding cornea, potentially causing tissue damage.4
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/06/$32.00 doi:10.1016/j.ajo.2005.09.023
Corneal epithelial burns are characterized by coagulative necrosis, desquamation, and eventual regeneration; thermal injury to the corneal stroma may result in both reversible and irreversible transitions of the collagen matrix, leading to a possible reduction in corneal clarity as well as irregular astigmatism, progressive corneal melts, and decreased postoperative acuity.3,5 The elimination of thermal energy production during ultrasonic phacoemulsification therefore represents a profound and critical phase in the move toward nontraumatic cataract surgery. This study compares temperature levels at the wound site during divide-and-conquer phacoemulsification using micropulsed and 12-pulse-per-second (pps) U/S technologies.
METHODS THIS WAS A PROSPECTIVE, OPEN-LABEL, RANDOMIZED, PAR-
allel-group, comparative clinical study of the Sovereign with WhiteStar (Advanced Medical Optics Inc, Santa Ana, California, USA) and the Legacy with AdvanTec and NeoSoniX (Alcon Inc, Fort Worth, Texas, USA) phacoemulsification systems. Institutional review board approval was not necessary. There was no change from the surgeon’s standard cataract surgery procedure for the study. Informed consent was obtained both for the surgery and for intraoperative videophotography. Twenty-six subjects were enrolled into this study. Twelve eyes from 11 subjects were assigned to undergo phacoemulsification with the Sovereign with WhiteStar system (SWS group) and 15 eyes from 15 subjects with the Alcon Legacy system with AdvanTec and NeoSoniX (LAD group). Subjects were included if they met the following inclusion criteria: visually interfering senile or presenile cataract for which phacoemulsification extraction was planned; ⫹3 or ⫹4 cataract density (on a scale from 0 to ⫹4); age 21 or older; and signed informed consent. Cataract density was graded preoperatively by the surgeon on a scale of 0 to ⫹4 at the slit lamp. Subjects were excluded if they possessed any ocular or systemic condition that would negatively affect successful phacoemulsification (for example, glaucoma, iritis, history of inflammation, Fuchs dystrophy), or if they had only one sighted eye with a corrected visual potential of 20/40 or better. All procedures were performed by the same surgeon (A.D.R.), who had used both phacoemulsification systems for several years before study initiation. Phacoemulsification was performed with a 20-gauge, 30-degree tip on each study subject by using a divide-and-conquer technique. The LAD system was set at 12 pps (50% duty cycle) and the SWS at CB/CF duty cycle (6 ms on/4 ms off for CB setting and 6 ms on/12 ms off for the CF setting). Settings were held constant unless specific cases required optimization. Infrared continuous video imaging of the procedure was performed by focusing an infrared camera system (Flir VOL. 141, NO. 2
Systems Thermovision A20) on the phaco tip at the incision. Thermal videos were subsequently downloaded to a laptop computer. Key operative criteria included peak temperature at the wound site, maximum temperature change from baseline at the wound site, equivalent phaco time, procedure time, total U/S time, percentage of phaco power used, amount of balanced salt solution (BSS) used after completion of the prime cycle, and surgical complications. Baseline temperature refers to the temperature reading within the rectangle (wound-site area) at the start of thermal image capture. Peak temperature is the highest numerical temperature reading recorded from the field within the rectangle. Maximum temperature change from baseline indicates the difference between baseline temperature and peak temperature. Equivalent phaco time calculates the total amount of U/S energy applied to the eye and is the product of the U/S time and mean U/S energy. Procedure time is the time from the start to the end of the phacoemulsification portion of the procedure. A two-way analysis of variance (with phaco system and cataract status included in the model) was used to evaluate differences in mean peak temperature between the SWS and LAD phaco groups. Because statistical interactions were noted between cataract status and phaco systems for temperature, these data were analyzed separately by cataract status by two-sample t tests. For t test analyses, the Satterthwaite method was used to adjust for unequal variances. Two-sided testing was performed with ␣ set at .05. To ascertain whether differences in age and baseline temperature existed between the two study groups, a two-sample t test was used. The Pearson 2 test was used to determine sex and cataract density differences, which may have influenced the study results.
RESULTS THE MEAN AGE FOR THE LAD GROUP WAS 80.13 ⫾ 4.60
years; (mean ⫾ SD) for the SWS group, it was 75.91 ⫾ 5.87 years. Forty-seven percent of the LAD subjects were men, and 64% of the SWS subjects were men. There was no statistically significant difference between the two study groups in terms of age (P ⫽ .0766) and gender (P ⫽ .5466). No statistically significant differences in baseline temperature (P ⫽ .7433) and in cataract density (.5466) were seen between the two groups. Tables 1 and 2 delineate the equivalent phaco time, procedure time, percentage of phaco power used, and BSS used for each study group. Both tables show that procedure time and the amount of BSS used is greater with the LAD group than with the SWS group, regardless of cataract density. Mean maximum temperature changes from baseline to peak for both groups are shown in Figure 1. Mean maximum temperature change at the wound site is greater for the LAD group than the SWS group (Figure 2): mean
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TABLE 1. Surgical Outcome Measures for Legacy With AdvanTec and NeoSoniX (LAD) for All Eyes, Subdivided by Cataract Density
Operative Parameter
All eyes Ultrasound time (minutes) (N ⫽ 15 eyes) Phaco power (%) (N ⫽ 15 eyes) BSS used (mL) (N ⫽ 11 eyes) Procedure time (minutes) (N ⫽ 11 eyes) Subdivided by cataract density 3⫹ cataract density Ultrasound time (minutes) (N ⫽ 8 eyes) Phaco power (%) (N ⫽ 8 eyes) BSS used (mL) (N ⫽ 6 eyes) Procedure time (minutes) (N ⫽ 6 eyes) 4⫹ cataract density Ultrasound time (minutes) (N ⫽ 7 eyes) Phaco power (%) (N ⫽ 7 eyes) BSS used (mL) (N ⫽ 5 eyes) Procedure time (minutes) (N ⫽ 5 eyes)
LAD Group, Mean ⫾ SD
2.1 ⫾ 1.02 17.6 ⫾ 3.38 262.27 ⫾ 91.28 9.12 ⫾ 4.39
FIGURE 1. Mean maximum temperature change from baseline to peak for the Legacy with AdvanTec and NeoSoniX (LAD) and Sovereign with WhiteStar (SWS) groups. LAD group, n ⴝ 15; SWS group, n ⴝ 12.
1.55 ⫾ 0.53 16.38 ⫾ 3.42 229.17 ⫾ 65.99 6.72 ⫾ 2.45 2.73 ⫾ 1.11 19.00 ⫾ 2.94 302 ⫾ 108.49 12.00 ⫾ 4.66
TABLE 2. Surgical Outcome Measures for Sovereign With WhiteStar (SWS) for All Eyes, Subdivided by Cataract Density
Operative Parameter
All eyes Ultrasound time (minutes) (N ⫽ 8) Phaco power (%) (N ⫽ 12) BSS used (mL) (N ⫽ 11) Procedure time (minutes) (N ⫽ 11) Subdivided by cataract density 3⫹ cataract density Ultrasound time (minutes) (N ⫽ 3) Phaco power (%) (N ⫽ 5) BSS used (mL) (N ⫽ 4) Procedure time (minutes) (N ⫽ 4) 4⫹ cataract density Ultrasound time (minutes) (N ⫽ 5) Phaco power (%) (N ⫽ 7) BSS used (mL) (N ⫽ 7) Procedure time (minutes) (N ⫽ 7)
SWS Group, Mean ⫾ SD
3.57 ⫾ 2.29 9.03 ⫾ 1.31 161.82 ⫾ 51.10 7.22 ⫾ 4.65
FIGURE 2. Temperature change from baseline to peak at phacoemulsification wound site.
1.87 ⫾ 0.28 9.6 ⫾ 1.19 118.75 ⫾ 55.43 5.17 ⫾ 0.18
wound-site temperature recorded was 51°C for LAD and 39.3°C for SWS (Figures 3 and 4). There was a statistically significant difference (P ⫽ .0215) in mean peak wound temperatures between all study patients with a cataract density of ⫹3 and those with a cataract density of ⫹4. The mean peak wound temperature was higher for patients with a cataract density of ⫹4 (41.15 ⫾ 6.00°C) than that for patients with a cataract density of ⫹3 (38.46 ⫾ 3.20°C). Two-way analysis of variance revealed a statistically significant difference (P ⫽ .0226) for the interaction between cataract density and the two phaco systems. After analyzing the data separately for cataract density, a statistically significant difference was found between the LAD and SWS groups for subjects with a cataract density of ⫹4, with higher mean peak temperatures for those who underwent surgery by using the LAD system (P ⫽ .0031). No statistically significant difference in mean peak temperature was found for those with a cataract density of ⫹3 (P ⫽ .1375).
4.59 ⫾ 2.37 8.62 ⫾ 1.32 186.43 ⫾ 29.54 8.39 ⫾ 5.62
maximum temperature change for the LAD group is 7.39 ⫾ 5.63°C, compared with 1.59 ⫾ 1.43°C for the SWS group. Figure 2 plots the wound-site temperature changes from baseline to peak for patients in the LAD group (triangle points) and in the SWS group (square points). There was a statistically significant difference (P ⫽ .0002) in mean peak wound temperatures between the LAD and the SWS groups. The mean peak wound temperature was higher for the LAD group (42.47 ⫾ 5.33°C) than for the SWS group (36.59 ⫾ 1.33°C). The highest 324
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FIGURE 3. Thermal image of wound site with highest temperature reading (39.3°C) in the Sovereign with WhiteStar (SWS) group.
FIGURE 4. Thermal image of wound site with highest temperature reading (51.0°C) in the Legacy with AdvanTec and NeoSoniX (LAD) group.
Two subjects, one for each study group, had intraoperative posterior capsule tears requiring vitrectomy. These patients were not excluded from the study because the events after phacoemulsification did not affect the study’s intended goals or data collection methods. No postoperative complications were noted in either study group.
DISCUSSION THE ISSUE OF WOUND BURNS DURING PHACOEMULSIFICA-
tion has been addressed in the recent literature. A comparative study of tip temperatures with the Legacy, Sovereign with WhiteStar, and Millennium phaco systems in open air and in cadaver eyes demonstrated higher temperatures for the WhiteStar and Millennium systems in VOL. 141, NO. 2
both conditions.6 A cadaver eye study by Olson and associates,7 however, showed less increase in wound temperature over time with the SWS system than with the LAD system during sleeved and unsleeved simulated surgery. A western US survey of 75,000 phacoemulsification cases indicated an incidence of 0.98% of severe wound burn, 0.12% of which was attributed to the Legacy system and 0.053%, to the Sovereign system (Bradley M, Olson RJ, “Survey of Incidence of Wound Burns Across 5 Western States,” presented at the American Society of Cataract and Refractive Surgery meeting, April 16 to 20, 2005, Washington, DC, USA). To date, there has been no clinical study published in the peer-reviewed literature comparing wound-site temperature during actual surgery that used micropulse and pulsed phaco technologies. Our study affirms the findings of Olson and colleagues in a clinical setting. Pulsed mode phacoemulsification delivers power that has been modulated to turn on and off a certain number of times per second (pulses per second). With pulse mode, phaco power delivery is reduced by 50%. Burst time is equal to rest time: lens emulsification occurs during burst time, and cooling and clearing of the emulsified material occurs during rest time. NeoSoniX technology creates low-frequency oscillatory movement that can be combined with standard high-frequency ultrasonic phacoemulsification. The low-frequency mode can be used for softer cataracts; the combination mode is used to treat denser cataract grades.8 Micropulsed U/S phacoemulsification (WhiteStar technology) delivers digitally modulated ultrasonic energy in short, rapid pulses of “on” (burst) time and “off” (rest) time. Unlike pulsed mode phacoemulsification, the burst and rest period length can be varied independently, enabling the surgeon to do the following: (1) maximize heat dissipation, thereby decreasing the risk of thermal injury; (2) improve cutting efficiency (less phaco energy is used to achieve the same cutting power); and (3) improve followability with less chatter (Olson RJ, “Sovereign WhiteStar,” presented at the Royal Hawaiian Eye 2002 Conference, January 20 to 25, 2002, Waikoloa, Hawaii, USA). The light bulb analogy provides us with a simple explanation of micropulsed phaco technology: micropulsed U/S is likened to a fluorescent bulb and traditional phaco to an incandescent bulb. The fluorescent bulb, powered by rapidly pulsed energy, remains cool while producing sufficient lighting, as opposed to an incandescent bulb, which becomes hot as a result of a continuous energy flow (Fine IH, “WhiteStar Phaco Power Control System,” presented at the American Society of Cataract and Refractive Surgeons 2002 Conference, June 2, 2002, Philadelphia, Pennsylvania, USA). WhiteStar technology uses a two-letter designation for each of its micropulse settings. To obtain the setting, the letter’s order in the alphabet is multiplied by 2 ms. The first letter represents the burst or “on” period and the
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second letter, the rest or “off” period. For instance, a CB setting refers to 6 ms of on time and 4 ms of off time. C is the third letter of the alphabet, and 3 multiplied by 2 ms is 6 ms of on time. By using the same method of calculation, B signifies 4 ms of off time (B is the second letter of the alphabet, and 2 multiplied by 2 equals 4). The CF setting refers to 6 ms of on time and 12 ms of off time. This two-letter designation can be used to determine the duty cycle for each setting. The CB setting translates to a duty cycle of 60% (6 divided by 10 equals 0.6, 10 being the sum of 6 and 4). For the CF setting, the duty cycle is 33% (6 divided by 18 is 0.33, 18 being the sum of 6 and 12). Both phacoemulsification technologies have their own relative merits and drawbacks. Ocular parameters (such as wound-site temperature), which relate to clinical outcomes, should be at the top of our list of priorities when attempting to compare micropulse and pulsed phacoemulsification technologies. The current study shows that mean peak temperature and mean temperature changes are higher with the LAD system than with SWS system. The LAD group experienced temperature peaks as high as 51°C. Extended exposures to temperatures of 45°C can cause cloudy corneas, stromal edema, and collagen disorganization. As temperature increases, the time required to cause corneal changes decreases.9 For safety purposes, this author suggests keeping phaco peak wound temperatures at 40°C or less. It is also interesting to note that there were significantly higher mean peak temperatures for patients with a cataract density of ⫹4 who underwent surgery using the LAD system. Heat is directly related to work: the most effective ways of minimizing heat production are by maximizing the efficiency of work performed, and by facilitating heat dissipation into the surrounding tissue.10 The harder cataracts necessitate more work, and the LAD system appeared to be able to accomplish the task of emulsifying a harder lens, but with less efficiency and higher resulting wound temperatures compared with the SWS system. Procedure time is notably longer among LAD patients with cataract densities of ⫹4: the mean procedure time for LAD patients with 4 cataract densities is 12.00 ⫾ 4.66 minutes and 8.39 ⫾ 5.62 minutes for SWS patients. This may reflect both the surgeon’s attempt to avoid prolonged pulsed ultrasound “on” times to allow for heat dissipation, as well as the decreased efficiency of a pulsed ultrasound system as compared with a micropulsed one. This study can be further refined by capturing both corneal surface and tissue temperatures during phacoemulsification. In 1999, Bissen-Miyajima and associates11 used videothermography to observe thermal burns in corneal incisions and noted the difficulty of observing temperature
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changes at the wound site during the actual phacoemulsification process. In our study, a microthermistor may have provided more accurate readings of the wound site, but a thermal infrared camera allowed us to noninvasively observe temperature changes in real time. However, thermal images provided only surface temperature data, so the actual temperature of corneal tissue was difficult to assess. This study showed that peak temperature and temperature changes at the wound site during divide-and-conquer phacoemulsification are significantly higher with the LAD system compared with the SWS system. Further studies will be needed to determine corneal surface and tissue temperature relationships, and to test wound-site temperature differences with varying surgical techniques. ACKNOWLEDGMENTS
Statistical support was provided by Advanced Medical Optics Inc, Santa Ana, California, USA.
REFERENCES 1. Leaming DV. Practice styles and preferences of ASCRS members—2003 survey. J Cataract Refract Surg 2004;30: 892–900. 2. Scleral and corneal burns during phacoemulsification. Health Devices 1996;25:426 – 431. 3. Sugar A, Schertzer RM. Clinical course of phacoemulsification wound burns. J Cataract Refract Surg 1999;25:688 – 692. 4. Fine IH, Packer M, Hoffman RS. New phacoemulsification technologies. J Cataract Refract Surg 2002;28:1054 –1060. 5. Ernest P, Rhem M, McDermott M, Lavery K, Sensoli A. Phacoemulsification conditions resulting in thermal wound injury. J Cataract Refract Surg 2001;27:1829 –1839. 6. Mackool RJ, Sirota MA. Thermal comparison of the AdvanTec Legacy, Sovereign WhiteStar, and Millennium phacoemulsification systems. J Cataract Refract Surg 2005; 31:812– 817. 7. Olson RJ, Jin Y, Kefalopoulos G, Brinton J. Legacy AdvAnTec and Sovereign WhiteStar: a wound temperature study. J Cataract Refract Surg 2004:30;1109 –1113. 8. Fine IH, Packer M, Hoffman RS. Power modulations in new phacoemulsification technology: improved outcomes. J Cataract Refract Surg 2004;30:1014 –1019. 9. Goldblatt WS, Finger PT, Perry HD, Stroh EM, Weiser DS, Donnenfeld ED. Hyperthermic treatment of rabbit corneas. Invest Ophthalmol Vis Sci 1989;30:1778 –1783. 10. Donnenfeld ED, Olson RJ. Reply to letter by R.J. MacKool. J Cataract Refract Surg 2004;30:732–733. 11. 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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Biosketch Aron D. Rose, MD, is an Associate Clinical Professor at the Yale University School of Medicine and Nursing. He is an active international lecturer on emerging ophthalmic technology and its applications, and a section editor for Techniques in Ophthalmology. Dr Rose is also a partner in private practice with The Eye Care Group, PC.
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Biosketch Vasudev Kanade, MD, is a Research Assistant at the Yale University Department of Ophthalmology and Visual Sciences.
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