Corneal Wound Burn During Phacoemulsification
Introduction to Corneal Incision Contracture (Wound Burn)
Corneal incision contracture (CIC), commonly referred to as wound burn, is a rare intraoperative complication of CEIOL (cataract extraction with intraocular lens placement) surgery. Phacoemulsification devices are used to emulsify the existing internal lens, which allows for aspiration of the old lens and insertion of a new intraocular lens. Modern phaco probes contain piezoelectric crystals that vibrate at ultrasonic range frequency (28000 to 60000 Hz) when stimulated. The vibration energy is transmitted to the titanium probe tip that oscillates back and forth. Alternatively, it can also generate microbubbles of gas that implode at ultrasonic speed, releasing large amounts of energy, aka the “cavitation effect.” The phaco probe creates two sources of heat: conversion of electrical energy to mechanical energy, and friction heat when the phacoemulsification needle vibrates against the sleeve that contains the probe. Water is usually used as a cooling agent to reduce the amount of thermal energy produced. If the flow of irrigation fluid surrounding the phacoemulsification needle is disrupted, the generated heat may contact the tissue, with the corneoscleral wound site being most susceptible to damage. Thermal damage to the collagen fibers in the sclerocorneal tunnel can occur once the temperature reaches 60 degrees Celsius, leading to a “phacoburn” over 1 to 3 seconds.. The wound burn results in the contracture of the incision site and surrounding tissue.
The incidence of corneal incision contracture is relatively low. One study examining 920,095 corneal operations performed in the United States and Canada between 2006-2009 found that the incidence of corneal incision contracture was 0.037%. A older, similar study limited to the United States found a slightly higher incidence of 0.098%. It is important to note the possibility that these estimates might be artificially low due to unreported cases of corneal incision contracture. Recently, there has been increased concern regarding this complication due to the ubiquitous use of phacoemulsification over the last couple of decades and the popularity of more narrow and anterior incisions. Wound burn can occur with any phacoemulsification unit if there is inadequate management of excessive heat.
Multiple studies have identified factors that increase the risk of corneal incision contracture when using phacoemulsification. The most important risk factor is the level of ultrasound energy used during the phacoemulsification process, with higher levels or prolonged use associated with increased risk.
The type of surgical approach used for nuclear disassembly has also been shown to modify the risk of CIC. Surgical approaches that rely more heavily on the use of ultrasound energy, such as divide-and-conquer, carousel, and stop-and-chop, are associated with a higher risk of CIC. Conversely, surgical approaches that rely more on mechanical force and require less ultrasound energy, such as the full chop or dry chop approach, carry a lower risk of CIC. Divide and conquer, carousel, and stop and chop all have an adjusted incidence of at least twice that of all the other chop approaches. Of note, the most widely used surgical approach in the United States is divide-and-conquer.
Decreased irrigation around the phacoemulsification tip also plays a role in a significant amount of wound burns. Contributing factors include high phaco powers, excessive frictional movement of the vibrating phaco needle against the irrigation sleeve, and obstruction by ophthalmic viscosurgical devices (OVD). This is especially true of high viscosity or exothermic OVDs since they can occlude the phaco tip and prevent the cooling effects of fluid flow, increasing the temperature at the incision site. High viscosity OVDs such as Healon5 have been shown to significantly increase the risk of wound burn. However, it also appears that the exothermic nature of OVDs also contributes to overall risk. Interestingly, one study found that Ocucoat and Viscoat, two types of dispersive OVDs, had the second-highest rate of wound burn. Viscoat and Healon5 were found to have the highest temperature increase during phacoemulsification in another study, which seems to indicate that more exothermic OVDs increase risk regardless of their level of viscosity. This is also supported by the fact that HealonGV had a very low incidence of wound burn, despite being a high viscosity OVD but exhibiting minimal temperature increase. Inadequate machine setup can also contribute if the irrigation fluid bottle is empty, if the bottle is positioned too low for sufficient flow, or if the irrigation tubing is crimped or compressed.
Importantly, surgical incision size, power modulation, and type of ultrasound used were not found to modulate the risk of corneal incision contracture. Prior research has shown that transverse ultrasound, such as the OZiL and Ellips, produce less heat during phacoemulsification than longitudinal ultrasound methods. This, along with the fact that the needle shaft rotates rather than slides back and forth, was thought to be very protective against ultrasound-induced wound burns. However, more recent research has shown that OZiL ultrasound, as it is commonly used in clinical practice (at a high power setting or 100% power), does not have a lower wound burn incidence overall. As with any other ultrasound, it should be used sparingly. Additional risk factors have been examined, including patient parameters, IOL type, viscoelastic agent and classes, and phacoemulsification machine settings and techniques.
Wound burn is an avoidable intraoperative complication, so measures should be taken to prevent them from happening and detect them as soon as they occur. Continuous irrigation of BSS (balanced salt solution) on the outer surface of the phacoemulsification needle and aspiration through the central canal is critical to prevent the tip from increasing in temperature and causing subsequent thermal injury. Other approaches for prevention include:
1. Verification of proper machine setup and adequate irrigation flow before insertion of phacoemulsification probe is strongly recommended. 2. Frictional thermal generation can be reduced by avoiding long excursions, excessive manipulation, and angulation of the phacoemulsification handpiece, and ensuring that the phacoemulsification incision site is an adequate size. 3. Irrigation and aspiration should be performed for up to 15 seconds at the beginning of the case if an OVD is being utilized. This ensures that the working space above the nucleus is free of the OVD, which protects against wound burn.
Detection and Management
Early detection for the condition is essential, and a critical warning sign is the appearance of milky white fluid around the phaco tip. Additional signs include whitening of the cornea and gaping at the incision site. At the end of the procedure, it can manifest as difficulty in maintaining watertight closure of the incision site.
To address the thermal injury from corneal incision contracture, traditional suturing techniques such as radial sutures join the anterior and posterior aspects of the wound. However, such methods are associated with higher surgical-induced astigmatism occurrences.
To reduce the development of astigmatism, horizontal sutures are recommended. One study implemented the use of a single horizontal suture that opposes the anterior aspect of the wound to the wound bed. Another study introduces the use of a horizontal mattress suture with a conjunctival flap hinged to the corneal end of the loop to improve suture tension.
In addition to suturing at corneal burn sites, soft contact lens utilization and amniotic membrane transplantation are also used for treatment. Aqueous suppressants and antibiotic drops are also standard post-operative treatment options. Scleral patch grafts may be necessary for severe thermal injuries and difficulties with closing the incision.
Complications of corneal incision contracture include delayed wound healing, fistula formation, damage to corneal stroma and endothelium, inability to close the incision, and increased surgically induced astigmatism. Astigmatism commonly develops along the axis of incision burn, resulting in steepening of the axis. However, this may decrease over time. Patterns on corneal topography show flattening over the corneal scar, most likely due to loss of anterior corneal stromal tissue, or steepening along the incision axis caused by tissue shortening or loss in the wound. Astigmatism can also result from the tight sutures used to close the wound. Another noted complication involves flat anterior chamber and iris prolapse, which may arise from persistent dehiscence due to the inability to achieve a watertight closure.
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