The Benefits of Ultrasonic Cleaning

The Benefits of Ultrasonic Cleaning

By Lawrence F. Muscarella, PhD

Growth in minimally invasive surgery has spawned the development of complex endoscopic instruments designed to accommodate the physician's surgical technique. However, to the frustration and dismay of many reprocessing staff and infection control practitioners, these complex instruments typically are not designed to facilitate cleaning and sterilization. Some feature delicate fiberoptic and plastic components readily damaged by heat, precluding steam sterilization. Examples include flexible endoscopes. Whereas steam is likely to conduct through the surfaces of surgical instruments and upon condensation release energy that destroys even the most resistant and inaccessible microorganisms, low-temperature gases, vapors, and liquid sterilants require direct contact to be effective.1 Moreover, the physical design of some of these complex instruments include narrow lumens and orifices that hinder the delivery of these low-temperature chemical agents to every contaminated surface, as required to achieve sterilization.1

In addition to flexible endoscopes, biopsy forceps are an example of a complex instrument that does not facilitate cleaning of all of its potentially contaminated surfaces, particularly its hinge mechanism that controls the opening and closing of its cups. Although complex and designed with an internal wire and lumen that can become contaminated with patient debris,2 reusable biopsy forceps are designed using materials like stainless steel that can withstand the rigors and stresses of steam sterilization. Reports of steam autoclaved biopsy forceps transmitting disease have not been documented.2 Only in instances when the biopsy forceps were inadequately cleaned or a low-temperature biocidal agent, such as 2% glutaraldehyde, was used in lieu of pressurized steam has cross-infection been reported.3

Cleaning is an integral component of virtually all instrument reprocessing guidelines. Several endoscopy and infection control organizations have published guidelines for the proper cleaning and sterilization of flexible endoscopes, biopsy forceps and other types of endoscopic instruments.4,5 If labeled for reuse, instruments require manual pre-cleaning, using a brush and detergent solution, to remove gross patient debris. Because complex endoscopic instruments may remain contaminated with patient debris even after manual brushing, most reprocessing guidelines recommend also using ultrasonic energy to remove fine debris that might otherwise be inaccessible and remain on the instrument. Unless the cleaning process effectively removes microorganisms and organic debris from even the most inaccessible surfaces of a contaminated instrument, the sterilization process is likely to fail. Developing more advanced cleaning technologies that can adapt to even the most complex surgical instruments and clean their most inaccessible internal surfaces is crucial to the prevention of patient infection.

Ultrasonic energy: What is it and how is it produced?

Ultrasonic energy is an effective technology routinely used by healthcare facilities to clean surgical and dental instruments prior to terminal sterilization.6,7 Different types of cleaning devices, designed both for bench (or table) tops or for the floor, use ultrasonic energy to optimize their cleaning effectiveness. This energy is produced by transducers mounted on the outside of the cleaning device's processing basin, which is typically constructed of stainless steel. When powered by an electronic generator, these transducers expand and contract at a very high frequency, converting electric energy to ultrasonic waves of energy. These high-intensity sound waves commonly travel at frequencies between 20 and 120 kHz (1 kHz equals 1000 Hz, or oscillations per second) throughout the processing basin. To enhance the efficient transmission of these waves of energy, immersing the soiled instruments in an appropriate liquid medium, such as a detergent solution, is essential. Although ultrasonic waves are inaudible, harmonics of the ultrasonic cleaner's primary, or fundamental, frequency may produce audible sound.

As they propagate through the detergent solution, these ultrasonic waves produce alternating tensile and compressive forces that oscillate at the same frequency as the transducers that produced them. These oscillating forces cause millions of microscopically-sized cavities to form in the detergent solution. Once they reach a critical threshold, these cavities violently collapse, or implode, causing submicroscopic voids to form by a process known as cavitation. These voids induce the formation of high-energy hydraulic shock waves that produce a powerful suction-effect. These shock waves, which may reach temperatures as high as 10,000°F and hydrodynamic pressures as low as 10,000 pounds-per-square inch (PSI),8 physically loosen and remove microorganisms and other adhering debris from even the most inaccessible surfaces of a soiled instrument.9,10

How effective is ultrasonic cleaning?

Through the years many studies that demonstrate the reliability and effectiveness of ultrasonic cleaning have been published. Some of these studies demonstrate the effectiveness of ultrasonic cleaning in standardizing the cleaning process and removing dried serum, whole blood, and viruses from contaminated instruments.9,11,12 Other studies have found that ultrasonic cleaners are significantly more effective and efficient than manual scrubbing,11,13 which is difficult to standardize and can vary in effectiveness from person-to-person.12 In contrast to manual scrubbing, ultrasonic cleaners are automated and standardized and designed to clean surfaces that might otherwise be inaccessible.

While manual cleaning is intended to remove gross debris from the instrument's surfaces, ultrasonic cleaners are designed to remove microorganisms and other fine debris from less accessible surfaces. Some reports suggest that ultrasonic cleaning, preceded by manual scrubbing, results in an even greater reduction in patient debris than achieved by either alone.8 One study demonstrated that as few as three minutes of ultrasonic exposure was sufficient to remove more than 99.9% of blood on contaminated instruments.10 Although data demonstrating its effectiveness in narrow lumens and channels of some complex instruments is limited, ultrasonic cleaners are recommended to increase cleaning efficiency,14 particularly for surgical instruments, like biopsy forceps, that have complicated joints, hinges and other internal surfaces that are difficult, if not impossible, to clean manually.15

Despite all of its benefits, ultrasonic cleaning, like any decontamination process, has its limitations, and understanding each permits its safe harnessing and effective application.7 For example, as a result of its aggressive scrubbing action, ultrasonic energy is not indicated for all medical instruments. Although the reprocessing instructions of most surgical instruments recommend ultrasonic cleaning as an integral step in their preparation for terminal sterilization, some instruments may be constructed of delicate materials damaged by its power, precluding the use of ultrasonic energy. Materials, such as quartz, silicon, and carbon steel may erode or become etched after prolonged exposure to ultrasonic cavitation.7 Erosion caused by ultrasonic energy can be minimized, if not eliminated, however, by reducing the ultrasonic cleaner's power and cleaning time. Review of each instrument's instructional manual to determine whether ultrasonic cleaning is contraindicated by its manufacturer is recommended.

Ultrasonic cleaning is usually one in a multi-step process that begins with manual cleaning to remove gross debris. This step is performed immediately after the instrument's use to prevent patient soil from drying. Once manually cleaned, the instrument is then placed in the ultrasonic cleaner. This cleaning step is particularly important for removing fine debris that may not have been removed during manual cleaning. Some ultrasonic cleaners may automatically inject detergent into the instrument's processing basin, as well as lubricate the instrument to prevent corrosion prior to terminal sterilization. Some may also be equipped with channel adapters that flush a detergent solution thorough the lumens of cannulated instruments.

In general, ultrasonic cleaners feature a timer and temperature control to adjust the cleaning time and to increase the temperature of the detergent solution, respectively. They may also be equipped with controls that permit adjustment of their power output (Watts) and frequency (kHz). Covers that reduce exposure of personnel to potentially harmful contaminants and aerosols during cleaning, as well as instrument trays, holders and baskets, may be standard or optional.

Factors that can enhance or limit cleaning effectiveness

Several factors can enhance or limit the cleaning effectiveness of an ultrasonic cleaner. None is as significant as the physical properties of the cleaning solution (or other liquid medium) through which the ultrasonic waves propagate. Briefly, the amplitude of the ultrasonic waves is directly proportional to the electrical power applied to the transducers. Cavitation cannot occur unless the amplitude of these waves, and therefore the electrical power, exceeds a minimum threshold value. The properties of the cleaning solution, which include its temperature, viscosity, density, vapor pressure, and surface tension, cause this threshold value to vary such that changes in any one of these properties is likely to affect cleaning effectiveness.

In addition to aiding in the removal of patient debris from soiled instruments, detergents increase cleaning effectiveness by reducing the water's surface tension. This effect increases cleaning effectiveness by: (a) facilitating the transmission of the ultrasonic waves through the detergent solution; (b) lowering the minimum amount of ultrasonic energy necessary for cavitation to occur; and (c) reducing the resistance to flow of the detergent solution through the instrument's narrow lumens and orifices. Detergents specifically formulated for ultrasonics, and known to be compatible with the instruments to be cleaned, are recommended to increase cleaning effectiveness.10 Neutral or alkaline detergents are the most commonly used formulation hospitals use with ultrasonic cleaners.

Temperature is also a significant a factor to the physics and effectiveness of cleaning. An increase in the temperature causes a corresponding increase in the detergent solution's vapor pressure and a reduction in minimum energy required for cavitation. Mixing the detergent with warm water is therefore recommended to enhance the effectiveness of ultrasonic cleaners. Temperatures between 11°F and 140°F are usually indicated for water-based detergent. Of course, to avoid damaging the surgical instrument, the temperature of the water cannot exceed the instrument's temperature parameters. Also, because reports indicate that bacteria can proliferate in the ultrasonic cleaner's detergent solution (and its aerosols) during the course of the day,16 using a fresh volume of water for cleaning and rinsing each new batch of soiled instruments may be advantageous to minimize personnel exposure to potentially pathogenic microorganisms. Although costly and not required, using deionized water may also be advantageous, as, in addition to dissolving detergents more efficiently, it does not contain minerals that frequently tarnish instruments.

Instrument baskets, trays: The benefits of ultrasonic cleaners cannot really be appreciated without using specially designed instrument baskets, trays, or cassettes.7 These fixtures are typically constructed of stainless steel (or other sound-reflecting material) and are often wired, meshed or sieve-like to ensure efficient passage of the ultrasonic waves. Each of these fixtures is crucial, as it: (a) maximizes exposure of the instruments to the ultrasonic waves; (b) minimizes movement of the soiled instruments against one another during ultrasonic cleaning, which can result in costly instrument damage; and (c) optimizes cleaning effectiveness by preventing the instruments from contacting the bottom of the cleaner's processing basin (the other side of which the transducers are usually mounted) where they might interfere with the proper operation of the transducers and prevent transmission of the ultrasonic waves.

Instrument arrangement: The method by which contaminated instruments are arranged in the processing chamber can have as much effect on cleaning effectiveness as the choice of the detergent. Ultrasonic energy is uni-directional, traveling from its source (the transducers) in one direction through the detergent solution. This potential limitation can be overcome by properly arranging the contaminated instruments in the processing basket (or tray) to maximize their exposure to and contact with the ultrasonic waves. Placing the contaminated instrument's most heavily soiled surface towards the bottom of the ultrasonic cleaner's processing basin optimizes cleaning effectiveness. Although not usually necessary, rotating the instrument and repeating the ultrasonic cleaning cycle to expose all of its surfaces to the ultrasonic energy may be indicated if the instrument is heavily-soiled.

Cleaning time: In addition to the type and temperature of the detergent, the time required to clean a instrument depends on, among other factors, the: (a) number and arrangement of contaminated instrument in the processing basin; (b) degree of instrument contamination (e.g., lightly-soiled, heavily-soiled); and (c) frequency and power of the ultrasonic cleaner.

Air bubbles: The presence of air bubbles in the cleaning medium also affects cleaning time. Unlike audible sound waves emitted from a stereo speaker, ultrasonic waves require a liquid medium for their efficient transmission. Consequently, the surfaces of instruments that are spotted with air bubbles cannot be effectively cleaned by ultrasonic energy. Nor can instruments be effectively cleaned by ultrasonic energy if pockets of air remain between them. Similarly, detergent solutions that contain air bubbles and other gases are likely to interfere with the efficient transmission of ultrasonic waves, reducing cleaning effectiveness. Once the ultrasonic cleaning cycle is activated, however, degassing of--that is, removal of air and other gases from--detergent solution or other liquid medium can be expected.7

Intensity and energy distribution: Most quantitative methods for evaluating the cleaning effectiveness of an ultrasonic cleaner can be very time-consuming and cumbersome and usually require at least some subjective interpretation of the results. A few methods, however, may be helpful in estimating their cleaning effectiveness.9,10 For example, the "aluminum foil erosion test" evaluates both the intensity and distribution of the cleaner's ultrasonic energy.10 Several new sheets of aluminum foil are placed vertically in the middle of cleaner's processing chamber filled with water. (Detergent is not used because this test is intended to assess the intensity and distribution of the ultrasonic energy--not cleaning effectiveness.) After several cycles, the sheets are examined for patterns of erosion or damage. The more significant and uniform the damage to the foil, the more powerful and uniform the intensity and distribution of the cavitation.

Power of cavitation: The ultrasonic cleaner's power of cavitation can be evaluated by placing samples of a smooth material, such as gypsum, in the processing chamber filled with water.10 The samples are weighed both before and after exposure to the cleaner's ultrasonic energy, and changes in the weights of the samples indicate mechanical erosion caused by cavitation. An increase in the power of the ultrasonic energy will usually cause an increase in cavitation activity, and therefore an increase in the samples' weight loss. Cavitation activity can also be visually estimated by the examining the samples' surface for erosion. (A detergent is not used because this test is intended to assess ultrasonic power, not cleaning effectiveness.)

Cleaning effectiveness: Several tests have been suggested to evaluate the cleaning effectiveness of ultrasonic cleaners. Visual observation of the extent to which patient soil is removed from a contaminated instrument, although subjective, can be a reliable measure of cleaning effectiveness. Other more quantitative tests may assess cleaning effectiveness by measuring and comparing the levels of a radioactively-tagged material, such as blood, before and after ultrasonic cleaning.8,10 The more significant the difference between these two levels, the greater the expected effectiveness of the ultrasonic cleaner. The use of optical density and micro-assay techniques to measure the amount of protein (e.g., blood) removed from an instrument by an ultrasonic cleaner has also been reported.11,12 In general, ultrasonic cleaners are expected to reduce at least 99.9% of patient soil on a contaminated instrument after only a few minutes of exposure. (These tests are usually performed using a detergent solution.)


In addition to increasing the productivity of reprocessing staff and minimizing the staff's exposure to contaminated instruments, ultrasonic cleaners have been shown to be more effective and efficient than manual scrubbing, which is often laborious and whose results are often incomplete and unpredictable.7 Other less obvious benefits of ultrasonic energy include its reported enhancement of the sporicidal properties of liquid chemical sterilants. One study found that ultrasonic energy reduced the time needed for a solution of glutaraldehyde to destroy bacterial endospores from 3.5 hours to 30 minutes.17 At a time when the popularity of low-temperature sterilization processes is growing, more emphasis and importance must be place on optimizing the effectiveness of the cleaning process to compensate for the lower sterility assurance levels of low-temperature sterilization processes compared to thermal sterilization.1 The development of instrument designs that facilitate cleaning and are not damaged by the rigors of cavitation is recommended to reduce the risk of cross-infection.

Lawrence F Muscarella, PhD, is the Director, Research and Development Chief, Infection Control Editor-in-Chief of Q-Net Monthly Custom Ultrasonics, Inc.

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5 Purchasing Questions

The following list of questions may be helpful in the purchase of an ultrasonic cleaner:

1. What are the dimensions of the ultrasonic cleaner? Is the size of its processing chamber sufficient to accommodate the widths and lengths of all of the facility's soiled instruments?

2. How much power does the ultrasonic cleaner produce? Does the cleaner feature one power setting, or it is equipped with different power settings to permit processing of lightly- and heavily-soiled instruments, as well as delicate instruments?

3. What is the ultrasonic cleaner's frequency setting? Can it be adjusted?

4. What is the ultrasonic cleaner's standard cleaning time? Can this time be adjusted to permit extended cleaning for big loads and heavily-soiled instruments?

5. Is the ultrasonic cleaner labeled for only a few types of instruments? Does the ultrasonic cleaner's manufacturer have any data to support its cleaning effectiveness? What types of instruments if any are contraindicated?

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