## 4 Vibroarthrography of the Knee Joint
### Data Acquisition  
The first step in diagnostics using VAG signals is the data acquisition method. For this purpose, the test group is determined, the type and placement of the sensors are selected, the type of knee movement is tested during the examination, and the experimental configuration is set up. Table 2 provides an overview of the acquisition methods in the selected literature. In recording VAG signals, the most commonly used sensors are accelerometers and microphones. Their selection depends on the type of signal being analysed, as well as the specifics of the study. Accelerometers are sensors that have been shown to be accurate for recording vibrations across wide frequency ranges. This is particularly important in recording VAG signals, which are variable. Another advantageous aspect of using accelerometers is their resistance to mechanical interference, which may result from the environment or the movement of the patient's body. Contact microphones are widely used to record VAG signals. They are characterised by the ability to record low-frequency sounds, which makes it possible to detect even micro-damages in the knee joint. Another advantage is their lower cost compared to accelerometers. However, they are much less resistant to movement and more susceptible to ambient noise, which can lead to signal distortion. The types of sensors used in the analysed publications included accelerometers, contact microphones, AE sensors, and stethoscopes. The most commonly used type of sensors were accelerometers, which appeared in 17 studies [11; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77]. In 10 studies [77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87], micro phones were used to record VAG signals. AE sensors were used in nine of the analysed studies [21, 31, 69, 88, 89, 90, 91, 92, 93], and a stethoscope was used in one [94]. Figure 2 shows the locations of the sensors in the analysed literature. The attachment of sensors to the skin also has a significant impact on the VAG signal recording process. Their stable attachment affects the precision of the measurements of vibrations and sounds generated by the knee joint. For this purpose, hypoallergenic medical tapes, elastic tapes, contact gels, mounting patches and electrodes are mainly used. Another important aspect in the process of recording VAG signals is the location of the sensors. Sensors placed closer to the knee joint result in higher accuracy of vibration recording because the signal is less likely to be distorted or attenuated [95]. The amplitude and accuracy of the VAG signal depend on the location over muscles and soft tissues. The thickness of the soft tissue affects the attenuation of the VAG signal. A stable sensor location will minimise interference caused by skin and muscle movements. Another positive solution is to use several sensors to record different types of vibrations resulting from the activity of individual structures of the knee joint. In 29 of 37 studies, sensors were placed around the patella. Vibroarthrographic signals are recorded during specific movements, including flexion and extension, sit-stand-sit movements, squat movements, and going up and down stairs. Figure 2: Location of sensors used to record the vibroarthrographic signal in the individual analysed studies on a 3D knee model.