Wound burn during small-incision cataract surgery is not uncommon, especially when older phacoemulsification systems are used.1 Wound burn occurs more frequently with increased ultrasound (US) time and power. Inadequate irrigation around the phacoemulsification tip is another key factor in phacoemulsification-induced thermal injury.2 The thermal energy that causes wound burn is dissipated from the wound by the surrounding sleeve and by continuous fluid irrigation over the tip.
Small degrees of thermal tissue damage can lead to wound gape, leaking, and astigmatism; on rare occasions, a larger degree of thermal tissue damage can cause severe and more permanent visual impairment.3 With advances in the surgical technique and the use of self-sealing clear corneal incisions, it is important for surgeons to pay careful attention to thermal damage to corneal wounds because it can result in difficult wound closure, wound leakage, damage to adjacent corneal stroma and endothelium, fistula formation, and high postoperative astigmatism.4 Newer phacoemulsification technology, such as systems that use torsional US, may lead to more efficient cataract removal and decrease incision temperature.
The aim of this study was to evaluate the tip-related temperature at 3 locations (corneal surface, incision, and handpiece) during longitudinal phacoemulsification and torsional phacoemulsification with standard incisions (2.75 mm) and microincisions (2.20 mm) and to assess the characteristics of the wounds on electron microscopy.
MATERIALS AND METHODS
This in vitro study evaluated human cadaver eyes that had simulated US phacoemulsification. The globes ranged from 1 to 4 days postmortem and were obtained from the North Carolina Eye Bank. All eyes were between 50 to 75 years old, were phakic, had no previous incisional surgery, and were kept in a moisture chamber at 4°C.
Before simulated phacoemulsification, eyes were kept at room temperature (20°C to 24°C) for approximately 2 hours. They were then secured in an orbit of extruded polystyrene foam and the incision locations marked with 7-0 black silk suture just posterior to the limbus. The mean temperature of the irrigating fluid in all cases was 18.9°C ± 0.83°C (SD).
The eyes were randomized to 1 of 4 groups. In all groups, simulated phacoemulsification was performed with an Infiniti Vision System unit (Alcon, Inc.). Each eye was exposed to the maximum preset power level for approximately 40 seconds, and the handpiece was moved to simulate manipulation during a normal surgical procedure. Thermal images were captured with an infrared camera (ThermaCAM, FLIR Systems) every 5 seconds during phacoemulsification, and the temperature of the corneal surface (tip end), incision (tip shaft), and handpiece (tip base) was measured (Figure 1). No stromal hydration was performed. In all groups, the fluidic settings were vacuum 300 mm Hg, aspiration 12 cc/min, and bottle height 100 cm. On/off occlusion was simulated approximately every 7 seconds by pinching off the aspiration line.
In Group 1, a standard 2.75 mm biplanar clear corneal incision was created with a 2.75 mm metal ClearCut slit knife (Alcon, Inc.). A 0.9 mm 30-degree Kelman turbosonic miniflared aspiration bypass tip with a 0.9 mm Microsmooth MicroTip infusion sleeve (Alcon, Inc.) was used. The US setting was 100% longitudinal in continuous mode. Phaco power was 100% and torsional amplitude, 0%.
In Group 2, a 2.20 mm biplanar clear corneal microincision was created with a 2.20 mm metal ClearCut HP2 slit knife (Alcon, Inc.). A 0.9 mm 30-degree Kelman turbosonic miniflared tip with a 0.9 mm Microsmooth Ultra infusion sleeve (Alcon, Inc.) was used. The US setting was 100% longitudinal in continuous mode. Phaco power was 100% and torsional amplitude, 0%.
In Group 3, a standard 2.75 mm biplanar clear corneal incision was created. The knife, tip, and infusion sleeve were the same as in Group 1. The US setting was torsional in continuous mode. Phaco power was 0% and torsional amplitude, 100%.
In Group 4, a 2.20 mm biplanar clear corneal microincision was created. The knife, tip, and infusion sleeve were the same as in Group 2. The US setting was torsional in continuous mode. Phaco power was 0% and torsional amplitude, 100%.
Scanning Electron Microscopy
All corneas were excised at the limbus, immediately fixed in 4% glutaraldehyde, and sent for scanning electron microscopy (SEM) of the endothelial surface. The specimens were dehydrated in an ethanol series and dried in a Pelco CPD2 critical-point dryer (Ted Pella Co.). The specimens were then sputter coated with gold–palladium (60%–40%) in a Hummer 6.2 sputter coater (Anatech). The tissue was immobilized using double-stick carbon tabs. The specimens were viewed under high vacuum at voltages of 15 kV and 25 kV in a Philips XL30 ESEM TMP microscope (FEI Co.).
The study evaluated 12 cadaver eyes. Each group comprised 3 eyes. Figures 2 to 5 show the changes in the temperature of the incision, corneal surface, and handpiece in each group.
The baseline incision temperature was 20.6°C in Group 1, 20.8°C in Group 2, 20.3°C in Group 3, and 22.1°C in Group 4. The mean maximum temperature (Table 1) was recorded 35 to 40 seconds after phacoemulsification initiation in Groups 1, 2, and 4 and 30 to 35 seconds after initiation in Group 3. The mean elevated temperature throughout phacoemulsification was 25.6°C in Group 1, 26.9°C in Group 2, 12.9°C in Group 3, and 15.1°C in Group 4.
Corneal Surface and Handpiece Temperature
Throughout the simulated phacoemulsification, the incision temperature was the highest and the corneal surface temperature the lowest in all groups.
Comparison of Incision Temperature by Incision Size
The maximum temperature was statistically significantly higher in the smaller incision groups than in the larger incision groups with both longitudinal phacoemulsification and torsional phacoemulsification. (P = .002 and P<.001, respectively) (Table 1 and Figure 6).
Comparison of Incision Temperature by Ultrasound Type
The maximum incision temperature was statistically significantly higher in the longitudinal US groups than in the torsional US groups with the 2.75 mm incision and the 2.20 mm incision (both P<.001) (Table 1 and Figure 7). In longitudinal groups, the temperature elevated gradually throughout the procedure; in the torsional groups, the maximum temperature reached a plateau after approximately 5 to 10 seconds (Figures 2 to 8).
Wound burn (ie, whitening and edema of wound edge) was observed in all eyes in the longitudinal US groups. Wound burn occurred 20 to 25 seconds after the beginning of simulated phacoemulsification in Group 1 and after 15 to 20 seconds in Group 2. Incision temperature was 43°C to 45°C when the first signs of wound burn were observed. No wound burn was seen in any eye in the torsional US groups (Groups 3 and 4).
Scanning Electron Microscopy
On SEM, internal wound gaping and partially compromised endothelium and Descemet membrane were seen in all eyes. The loss of Descemet membrane was more extensive in the longitudinal groups than in the torsional groups (Figure 9).
During phacoemulsification, the vibrating US tip can produce thermal energy, increasing wound temperature. The thermal energy from the US tip is usually dissipated away from the wound by the surrounding sleeve and by the infusion and aspiration fluid over and through the tip. If the irrigation or aspiration is reduced or interrupted, the energy may be dissipated to the surrounding tissues, causing thermal damage.3
Excessive wound burn can alter the arrangement of collagen fibrils and cause heat-induced coagulative necrosis in the wound tunnel, leading to wound leakage. The excessive wound leakage can cause anterior chamber instability and an increased risk for complications, such as postoperative infection and increased astigmatism (A.R. Vasavada, MBBS, MS, FRCS, “Phaco Tips and Corneal Tissue; Histomorphology and Immunohistochemistry Reveal the Effects of Sleeveless and Sleeved Tips,” Cataract & Refract Surg Today, June 2005 (suppl), pages 9–10. Available at: http://bmctoday.net/crstoday/pdfs/CRST0605supp.pdf. Accessed January 4, 2010).5,6
Studies of experimentally induced thermal burns1,7 identified risk factors for wound burn, such as increased phaco time and power as well as insufficient irrigation. Another study8 found that greater incision temperature was associated with smaller incisions, higher US power, a longer pulse-duty cycle, lower aspiration flow and vacuum, tip size and position, and the use of ophthalmic viscosurgical devices (OVDs).
Our study was designed to compare the thermal effect of 2 US modalities (longitudinal and torsional) under the same extreme fluidics parameters and with 2 incision sizes (2.75 mm and 2.20 mm). We observed 3 locations to evaluate the change and difference in tip-related temperature. The corneal surface temperature was measured to determine the thermal influence of the tip under simulated load in the anterior chamber. The incision temperature was measured to evaluate the direct thermal effect of the tip shaft on the corneal architecture and its integrity. The handpiece temperature was measured to determine the temperature at the connection between the handpiece and the tip, which is where the US energy is generated.
The incision temperature in the torsional US groups was significantly lower than in the longitudinal US groups. This large decrease was present regardless of incision size. The lower temperature with the use of torsional US under these extreme settings suggests that it is a preferred modality for removal of dense cataracts that require higher phaco power and extended phaco times, especially in cases in which thermal damage is a concern. In our study, wound burn occurred in the longitudinal groups but not the torsional groups, which implies that increased incision temperatures are likely clinically relevant. A smaller but statistically significant increase in incision temperature was observed in the smaller incision groups. The temperature increases in the torsional US group were probably not significant given that the maximum elevations were modest. However, smaller incisions combined with longitudinal US would likely lead to wound burn when large amounts of US energy are used. The incision temperatures in the longitudinal groups continued to steadily increase but plateaued in the torsional groups. A possible implication of these findings is that during phacoemulsification of a dense cataract, torsional US could be applied continuously while longitudinal US may have to be stopped for a time to allow the incision to cool. Scanning electron microscopy showed more extensive and severe loss of Descemet membrane in the longitudinal groups than in the torsional groups. Wound burn consistently occurred at an incision temperature of 43°C to 45°C; therefore, avoiding temperatures in this range would likely prevent wound burn.
The temperature of the handpiece and corneal surface in the torsional groups was also lower and more stable than in the longitudinal groups. Temperatures did not exceed 30°C in the torsional groups and was approximately 40°C in the longitudinal groups. The temperature of the corneal surface had plateaued in the torsional groups but increased gradually and continuously in the longitudinal groups.
Simulated phacoemulsification in our study was performed using relatively extreme settings of low aspiration, high power, and long phaco times to increase the differences between groups. With the same fluidics settings, power, bottle height, aspiration rate, and vacuum and the same-sized tip and incision, only the modality of phacoemulsification would affect incision temperature and architecture. However, a limitation of our study was the use of cadaver eyes. Although all cadaver eyes were allowed to warm to room temperature (20°C to 24°C) to approximate the temperature of operated eyes, the temperature was lower than that of in vivo eyes with a blood supply. In our experience, the temperature of the exposed ocular surface in the operative field is approximately 30°C to 34°C at the beginning of surgery. If all eyes had a higher baseline temperature, wound burn would presumably occur earlier. Regardless, the temperature rise was much higher in the longitudinal groups. Other intraoperative conditions, such as OVD use and irrigation of the external surface of the eye during surgery, can also affect wound temperature.
The main difference between longitudinal phacoemulsification and torsional phacoemulsification is the movement of the tip. In traditional phacoemulsification, the tip moves longitudinally; in torsional phacoemulsification, the tip shaft moves in an oscillatory torsional motion. With the phaco unit we used, 100% longitudinal US generates a linear stroke length of 90 μm and 100% torsional US generates a side-to-side movement length of 90 μm (D. Allen, BSC, FRCS, FRCOphth, “The Ozil Torsional Phacoemulsification Technology,” Cataract & Refract Surg Today, June 2006 (suppl), pages 4–6. Available at: http://bmctoday.net/crstoday/pdfs/OzilSupplement.pdf. Accessed January 4, 2010). However, the oscillatory tip movement at the incision level does not require the full length of 90 μm to generate 90 μm of tip movement. Torsional US, therefore, produces less movement within the incision and oscillates at a lower frequency than traditional longitudinal US. This phenomenon could explain the difference in temperature between traditional US and torsional US. Another possible explanation for the lower temperature at the tip and incision in torsional phacoemulsification is that the oscillatory vibrating movements facilitate the irrigation flow around the tip. A clinical phenomenon related to this oscillatory movement is that balanced salt solution occasionally sprays around the tip and incision during phacoemulsification, which may play a role in cooling the incision.
An increase in temperature around the incision is inevitable; therefore, surgeons should take care to avoid significant increases in temperature, which can result in wound burn. The combination of torsional US with smaller incisions did not have an adverse effect on wound architecture and integrity. Therefore, this combination may increase safety to the wound and be a preferred approach for the removal of denser cataracts.
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