Medical Design Briefs - March 2024 - 32

Design Briefs
Driver
Booster
Sonotrode
Driver
Booster
Fig. 3 - Scaler with angled sonotrode and spiral sonotrode.
Design Variants
Driver
20µm
1:2
Booster
Fig. 4 - Ultrasonic multipliers in a scaler.
1.3
Sonotrode
120µm
For applications in which no variation
of the scaler's gain value or operating
frequency is necessary, the driver and
booster are combined in a single component.
Commercial scalers offer the possibility
to freely exchange tools - one
scaler can be used for different applications.
It should be noted that the tools
must be precisely matched to the operating
frequency of the respective scalers to
ensure optimum operation.
For special applications, non-axial vibrations
may also be required. To drill
into dense bone tissue, additional torsional
motion is advantageous; when
sawing soft bones, transverse vibration
is required (see Figure 3). The scaler's
functions can thus be optimized and unwanted
tissue trauma minimized via relatively
simple modifications to the design.
System Design
The overall design of a high-power
ultrasonic transducer is simulated in advance
using finite element method (FEM)
models. Several key properties must be
considered when designing a new scaler:
* Optimum operating frequency.
* Vibration amplitude.
Hard PZT multilayer ring chips for use in ultrasonic handpieces. (Credit: PI)
volume ratio of the booster sections on
both sides of the suspension mounting.
The position of the suspension mounting
is determined by the neutral level of
the vibration in the booster.
The sonotrode (horn) of the handpiece
transmits the ultrasound to the
tool at the tip, whereby the contact
surface of the tool must be at the maximum
of the vibration amplitude. The
32
sonotrode also acts as a mechanical amplifier
for the amplitude (see Figure 2).
The gain value is calculated on the basis
of the ratio of the cross-sectional areas
at both ends of the sonotrode. In medical
technology, working frequencies
of 20 to 100 kHz are used and the geometry
of the entire scaler is designed
accordingly in multiples of half of the
wavelength.
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* Operating temperature/cooling.
* Vibration mode.
The working frequency is primarily
determined by the mechanical properties
of the tissue to be processed, as well
as the type of processing (see Table 1).
The maximum available power input,
the strength of the tool material and
the treated tissue limit, the maximum
vibration amplitude at the tool tip. It
is also determined by active cooling of
the handpiece, which is usually necMedical
Design Briefs, March 2024
Sonotrode
This guarantees the maximum amplitude
at the tool. The choice of frequency
depends on the tissue to be treated: For
example, bones can be optimally processed
at 20-25 kHz, whereas soft tissue
is easiest to process at around 60 kHz.
Extremely high-grade alloys are used
for the metallic components in order
to keep the mechanical losses to a minimum.
Typical materials are titanium,
aluminum or steel alloys. The actual
choice of material depends on the requirements
for the application, such as
the desired weight of the scaler and the
permissible costs. In addition, various
coatings, such as TiN, are used to prevent
corrosion or allergic reactions.
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Medical Design Briefs - March 2024

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