Incision Burns

I write to comment on 2 consecutive articles.1,2 The authors propose that a bare, “round” phacoemulsification needle inserted through a leaking phacoemulsification incision would be sufficiently cool to prevent an incision burn and that it would do so better than a phacoemulsification needle within a deformable sleeve. However, their experiment was not conducted in a manner that would permit evaluation of their hypothesis. First, this was not a controlled experiment in which the bare tips were carefully compared to tips with sleeves using a variety of incisions and rates of incision leakage. Despite this lack of comparison, the authors speak freely about presumed advantages of the bare tips. The authors refer to the technique they used, insertion of the infusion source and the phacoemulsification needle through separate incisions, as a “tightly closed system.” This is inconsistent with their premise; ie, that sufficient incision leakage would occur to prevent incision burn. One cannot have it both ways; either the incision leaked sufficiently to cool the phacoemulsification needle and the system is not tightly closed or vice versa. Other hopeful advantages mentioned by the authors include the use of irrigation to assist in moving lens material to the phacoemulsification tip and the ability to switch the needle from 1 incision to the other in the event of a complication necessitating that this be done. The former is unproven; in fact, irrigation flow can be easily directed across the ultrasonic tip (perpendicular to the axis of the tip) during bimanual procedures. This can have the opposite effect of that described by the authors. The ability to switch the location of the needle site is a minuscule advantage, if it exists at all. There is nothing to prevent a surgeon using a standard phacoemulsification needle through a 2.7 mm incision from changing the location of that incision. The articles contain methodology statements that cannot possibly be accurate. Glaring examples of these include the statement that infusion bottle height was maintained at 17 inches, yet intraocular pressure (IOPs) as high as 57 mm Hg were reported. A 17-inch column of water cannot create an IOP of greater than 32 mm Hg. Another example: The incisions were reported to be 1.2 mm by a mechanism which, as described, would be so inaccurate as to create the discrepancies found later in the article. The authors state that they created stab incisions with a 0.8 mm diamond knife, measured the incisions “with calipers before enlarging,” then enlarged the incisions “to 1.2 mm with a crescent blade.” This technique would not permit one to know the true size of the incisions, which would currently best be measured using the “dip stick” or “feeler gauge” method. It is unclear whether the method used to monitor the intraincisional temperature was adequate for the task. The authors state that the thermocouple was attached to the phacoemulsification handpiece “so the tip of the thermocouple was 1.5 mm posterior to the phacoemulsification tip.” A clear corneal incision can be as long as 2.0 mm. In what portion of the incision did the thermocouple reside, and how consistent could its location have been from eye to eye? Elevation of tissue temperature during phacoemulsification is inherently greatest within the surface tissues. (There is less fluid flow in this region, and air is a much poorer conductor of heat away from the surgical site.) A thermal imaging system is far superior to a thermocouple for this purpose. As elegantly demonstrated by Bissen-Miyajima and coauthors,3 extremely precise and consistent temperature measurements can be made in this fashion and will correlate directly with anatomic changes (incision burn). The infusion capacity of the irrigating chopper was reported to be 80 cc/min with the infusion bottle at 17 inches. This is a physical impossibility, at least with a 19- or 20-gauge infusion cannula. The former would provide approximately 25 cc per minute of infusion flow and the latter 15 cc per minute with this bottle height. It would not matter how many infusion ports were placed on the sides or ends of the cannula; the flow-limiting factor would be the area of the cannula itself. With the incision sizes not accurately measured, the authors state that the phacoemulsification needle and irrigating chopper were inserted “without undue tension on the wounds.” This certainly may have been true, but the incisions could not have been 1.2 mm. Twenty-gauge and 21-gauge needles demand a minimum incision width of 1.4 mm and 1.7 mm, respectively, if they are to be inserted. Even if one could force them through such a tight incision, it would be extremely difficult to move them about because of the extreme tension on them from the surrounding tissue. The incisions would necessarily be approximately 0.2 or 0.3 mm larger than that which was minimally required for their insertion to permit the procedure to be performed without the induction of corneal striae whenever 1 instrument was moved and to permit adequate incision leakage to cool the ultrasonic tip. Therefore, the incisions used in this study must have been at least 1.6 mm or 1.9 mm (if 20-gauge or 19-gauge instruments respectively, were used), and the infusion bottle had to be elevated to at least 33 inches. In the first article,1 temperatures were compared with the use of 100% continuous power (unoccluded tips), 80% continuous power (occluded tips), and during multibursts of power. Multiburst was initially stated as 5 ms, but later in the same article it was described as 50 ms. The authors were surprised when burns occurred more rapidly with the multiburst than with the other modes. They speculate that this counterintuitive discrepancy resulted from “some type of harmonics in the wound.” There is no scientific basis for such a claim, and the findings could be better explained by the flawed design of the experiment or the possibility that the multiburst mode was producing a higher stroke than that indicated by the equipment console. Much of the data in Table 1 make no sense. For example, eye 4 should not have had such a wide IOP range (25 to 55 mm Hg) if the tip were occluded. Such a variation would indicate that the tip was not occluded or that incision leakage was incredibly variable. Somehow, however, this eye had a lower time to burn than eye 3, which appeared truly occluded and had tight incisions as indicated by the consistent IOP range of 53 to 57 mm Hg. Eyes 5 and 6 (multiburst) appear to have unoccluded tips or the leakiest incisions of all as the IOP was quite low (18 to 30 mm Hg) in these eyes. Despite these data, the authors state that there may have been greater fluid leakage in the eyes with continuous ultrasound than in those using multiburst. The lower IOPs in the multiburst eyes are evidence of much greater incision leakage, however. The statement is made that “in hindsight…results not surprising” because the standard ultrasonic needle with deformable sleeve fills the incision so leakage is minimal. Before making the statement, the authors laud the “closed system” they were using. Once again, one cannot have it both ways. If the incision is leaking significantly, the system is not closed. Many surgeons, including myself, have found that the disadvantages of a leaking phacoemulsification incision (increased total flow through the eye, inability to maintain adequate IOPs, and deep chambers when using greater flow and vacuum levels) are eliminated by the use of a truly closed system. The latter permits increased surgical efficiency, reduced globe deformity during intraocular maneuvers, and other benefits associated with reduced fluid flow through the eye such as better endothelial cell function immediately following surgery and reduced cell and flare. In the second article,2 the authors set the Sovereign unit to a 33% duty cycle and compared 3 scenarios for 3 minutes: 100% power (unoccluded), 100% power (occluded), and 100% power (occluded/no irrigation). In the design of the experiments, the first 2 scenarios failed to produce a burn while the third one did (after 29 and 45 seconds of ultrasound [US] power application). The data in the second article are in contrast to the data in the first article. Burns occurred at 100% power at a 25% duty cycle in the first article but did not do so in the second. In the second article, no IOP ranges were provided, so it is not possible to determine whether the variability reported was caused by uncontrolled infusion fluid leakage. In the first article, the data indicated that the multiburst mode produced burns in the shortest period of time. In the study in the second article, the data showed that the multiburst range was superior even though the duty cycle was greater than in the first study. None of these data is the least bit convincing, yet the authors proceed to make what in my opinion are greatly exaggerated statements about the benefits of this “new advanced technology,” using phrases such as “cold phaco,” and making claims that the technology was “extremely safe with all conceivable parameters.” Statements such as these are little more than propaganda and should not appear in a journal of this quality. Finally, in the next-to-last sentence of the second article, the authors state that “frictional heat energy may be further enhanced by torquing the wound.” The authors are obviously referring to decentration of the phacoemulsification tip in the incision and the fact that doing so has been demonstrated to dramatically increase the transfer of thermal energy to the adjacent tissue.3 This critically important fact, not addressed by the experimentation conducted by the authors in their 2 articles, is then followed by the completely unfounded claim that “the WhiteStar system represents the first ultrasound technology that effectively eliminates this risk (of incision burn).” If there is a reduced risk of incision burn with this system, the authors have provided no evidence of it. What they have demonstrated is that a US needle can be inserted through an incision of unknown size with unknown resultant leakage of fluid and that without decentration of the needle, an incision burn will not occur. We knew this before their articles were published. Any “bare” US needle can be used to perform phaco at virtually any power without causing an incision burn if there is sufficient leakage and no significant decentration of the tip within the incision. Richard J. Mackool M.D. aAstoria, New York, USA