Quantification of acoustic exposure during cataract surgery

Over the last 15 years, ultrasonic surgical devices have become the preferred instruments for cataract surgery within the ophthalmologic community. This approach, called phacoemulsification, uses needle tips which vibrate longitudinally at frequencies between 28 and 50 kHz (depending upon the manufacturer). Mechanical impact and inertial cavitation at the tip both act to erode and liquefy the lens material, which is then aspirated out through the needle core. This study attempted to determine the total acoustic exposure of the eye during a typical cataract removal procedure. This paper reviews the methods of determining the acoustic exposure from phaco devices, provides a clinical example, and demonstrates the utility of providing this information to the surgeon. A series of nearly 200 surgeries were performed by a single surgeon, using either of two different phacoemulsification systems. Each cataract was graded for relative density and hardness. After each surgery, system settings and surgical times were recorded. The acoustic energy radiated into the eye was calculated assuming a monopole model, consistent with IEC standard 61847, with inputs of frequency, tip excursion, tip dimensions, and exposure time. Power was determined as (ρ₀ck²Q_s²)/8π, where Q_s is the source strength (radiating area times the normal surface velocity). Simplifying assumptions were made to account for factors such as excursion ramp up, ramp down, and pulsing sequences. In addition, one or the two systems used a unique microburst ultrasound pulsing scheme using bursts as short as 6 milliseconds. The results indicated that the total acoustic energy ranged from 0.01 joules for a grade 1 cataract to up to 0.3 joules for a grade 5 cataract, with an exponential increase in ultrasound exposure with cataract grade (e.g. y = 0.0033e^0.66x, R² = 0.974). Further, after an initial run of experiments, the surgeon was able to modify the system control settings to reduce the patient exposure by 20 percent without compromising clinical effectiveness. The conclusions from this work are: (1) the novel pulsing approach using very short bursts appears to significantly reduce the total acoustic exposure; (2) if surgeons are given dosimetric - information, they can reduce exposure without reducing clinical effectiveness; (3) the data currently available from manufacturers are very difficult for clinicians to interpret and compare between devices; (4) based on the latter two conclusions, manufacturers are encouraged to provide on-screen labeling of acoustic output.