The Benefits of Ultrasonic Cleaning

May 1, 2001

The Benefits of Ultrasonic Cleaning

By Lawrence F. Muscarella, PhD

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

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

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,

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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

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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