Salt Lake City?Significant wound burn occurs in approximately one out of 1,000 cataract surgery cases. However, there is the potential to decrease the incidence dramatically by changes in the surgical technique, modulation of the power in the eye, and the type of machine used during cataract surgery, said Randall J. Olson, MD. He is the John A. Moran presidential professor and chairman, department of ophthalmology and visual sciences, University of Utah Health Sciences Center, Salt Lake City.
To uncover the factors likely responsible for wound burn, Dr. Olson and Michael Bradley, MD, conducted a Web-based survey that covered five northwestern states. The ophthalmologists in the region were surveyed regarding wound burn, their regular practices, and factors that were implemented at the time the wound burn occurred.
The study also included analysis of in vitro laboratory work that looked specifically at whether extremely short pulses of energy had some element of thermal inertia above and beyond the duty cycle to prevent wound burn, Dr. Olson explained.
Three risk factors
Three factors were implicated in the occurrence of wound burns: the phacoemulsification system used, the surgical technique, and the power modulation.
In this survey, the phacoemulsification systems used were the Sovereign Phaco System (AMO), the Millennium system (Bausch & Lomb), and the Legacy Phaco system (Alcon Laboratories). Both the Sovereign system and the Millennium were associated with fewer wound burns, 0.056% and 0.038%, respectively, compared with the Legacy, 0.12%. This difference reached significance (p = 0.005).
Wound burn also seemed to be associated with the surgical approach. The following are the cataract techniques used and the percentages of wound burns associated with each: vertical chop, 0.026%; horizontal chop, 0.062%; divide-and-conquer, 0.13%; and carousel, 0.12% (p = 0.003, vertical chop compared with divide-and-conquer; p = 0.009, vertical chop versus carousel).
The choice of power modulation was also associated with the incidence of wound burn: WhiteStar (AMO) ultrapulse (0.026%), burst mode (0.086%), pulse mode (0.14%), and continuous mode (0.18%). A comparison of WhiteStar ultrapulse and continuous mode showed a highly significant difference, p < 0.0001; WhiteStar ultrapulse compared with pulse mode (p = 0.0004); WhiteStar ultrapulse compared with burst mode (p = 0.013); and burst compared with continuous (p = 0.013).
"Multivariate analysis showed the White-Star ultrapulse technology to be the most important, followed by vertical chop," Dr. Olson reported. "These factors seem to be very protective."
When ultrapulse was compared with pulse or burst mode, the analysis showed a three to five times lower risk of a wound burn occurring with ultrapulse. Dr. Olson also pointed out that thermal inertia in biological tissue was found to be 10.2% for very short settings compared with a short pulse (p = 0.0002).
"Significant wound burn occurs in about 1 in 1,000 cataract cases. The machine foot-pedal differences may be an important factor in this statistic," Dr. Olson said. "The use of continuous power with a divide-and-conquer technique is the most risky in terms of a wound burn occurring. The WhiteStar ultrapulse technology and a chop technology seem to be very protective for the eye. Ultrapulse has efficiency and thermal inertia advantages."
1. Bradley M, Olson RJ. A survey about phacoemulsification incision thermal contraction incidence and causal relationships. Am J Ophthalmol 2006;141:222-224.