Chapter I: Dental Ultrasonic Scalers (Devices)

1. Product Introduction

1.1 Composition and Working Principle

1.1.1 Composition

Dental ultrasonic scalers (devices) primarily consist of a function control circuit, liquid path, scaler handpiece, ultrasonic transducer, scaler tips, power supply/power adapter (if any), and footswitch (if any).

Figure 1: Dental Ultrasonic Scaler

The scaler tip used for the ultrasonic scaling function. Due to different application areas and functions, the scaler tips include forms such as NiTi U files, polyether ether ketone (PEEK) heads etc.

Figure 2: Dental scaler tip with polyether ether ketone (PEEK) head

Some ultrasonic dental scalers (devices) also come with additional auxiliary functions, such as endodontics and cavity preparation. However, the main structure remains consistent, achieving auxiliary functions through different scaling tips (including NiTi U files).

Figure 3: Dental Scaler Tip with NiTi U File

1.1.2 Working Principle

The ultrasonic generating circuit produces a high-frequency oscillating signal that is amplified by a power amplifier and applied to the ultrasonic transducer.

Utilizing the inverse piezoelectric effect (or magnetostrictive effect), ultrasonic vibration is generated. The scaling tip is resonated, using the inherent characteristics of ultrasound to create a cavitation effect (water flowing from the scaling tip rapidly expands under the action of ultrasonic energy and then collapses suddenly.

At the moment of bubble collapse, a shock wave is generated, causing pressure of about 1000 Pa around the bubble and local heating). This ultrasonic "cavitation effect" produces enormous pressure that can break down plaque and calculus on the tooth surface or remove bacteria.

The electrical principle of the ultrasonic dental scaler (device) is that an integrated circuit and microprocessor (MPU) form a "digital electronic oscillator," producing electrical pulse waves (quasi-continuous waves) with a frequency of 28-32kHz.

When this electromagnetic wave voltage is push-pull amplified and applied between the two electrodes of the transducer, the thickness of the transducer crystal changes and produces vibration according to the electric field strength and corresponding frequency.

When the frequency of the electric field change matches the natural frequency of the transducer crystal, the amplitude of the transducer is maximized. The amplitude rod at the front end of the transducer performs minute mechanical stretching vibrations, actuating the scaling tip to produce ultrasonic vibrations at the same frequency.

At this moment, the water sprayed from the scaler handpiece is subjected to ultrasonic vibrations, causing water molecules to break apart, creating countless tiny gas bubbles. When these bubbles collapse, they produce enormous instantaneous pressure, quickly shattering dental calculus and loosening dental plaque.

To ensure the transducer maintains resonance, the circuit design includes an automatic search function for the resonance point: using a computer (CPU) to achieve "sampling, memory, comparison, adjustment" of the actuation voltage applied to the piezoelectric ceramic piece of the transducer.

Figure 4: Working Principle

1.1.3 Electromagnetic Interference Resistance Design

Dental ultrasonic scalers (devices) including solenoid valves, foot switches, and high-frequency transformers, can generate significant interference. Additionally, this equipment is often used alongside dental units, which have powerful motors that can cause severe electromagnetic interference to the dental ultrasonic scalers (devices). When interference signals occur, the device may experience malfunctions such as freezing, erratic program behavior, or corruption of system parameters. Therefore, both hardware and software have been equipped with measures to counteract these interferences, ensuring smooth and reliable operation.

(1) Hardware Interference Resistance

To combat electromagnetic interference in dental ultrasonic scalers (devices), several robust hardware measures are implemented. A power line filter is installed at the power input to eliminate high-frequency harmonics and pulse interference from the power grid. When selecting the microcontroller, select models with a hardware watchdog, or add an external watchdog circuit, which effectively monitors the system for potential infinite loop failures. Each chip’s power input and ground are bridged with a 0.1μF decoupling capacitor, and the signals from the foot switch are optically isolated. These measures have proven to be highly effective in ensuring smooth operation.

(2) Software Interference Resistance

First, software redundancy is employed. All output signals and settings are constantly refreshed at a 5ms interval. The A/D conversion is averaged over 8 readings to achieve the most accurate signal possible. 

Second, software traps are used. This technique forcibly redirects errant program instructions to the reset address 0000H, bringing the program back on track. Setting up software traps between control modules and unused program spaces effectively suppresses erratic program behavior, making the system run more reliably.

Reference

1. ISO/DIS 18397:2024 - Dentistry - Powered scaler

2. Ultrasonic Periodontal Debridement: Theory and Technique, 1st Edition. Marie D. George, Dani Botbyl, Timothy G. Donley, Philip M. Preshaw.