# A Review of the Vibration Arthography Technique Applied to the Knee Diagnostics
## 2 Experimental Methods  
In order to analyze vibroarthographic (VAG) signals, the first essential step is signal acquisition. This step comprises the choice of sensors, their position on the knee, and the experimental setup.  
As explained in the introduction, most previous studies have used either a microphone [10; 11; 12] or an accelerometer [2; 7; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29] to measure VAG signals. The choice of an accelerometer depends on parameters such as its frequency response and its sensitivity. The typical bandwidth is up to 10 kHz, while the sensitivity ranges from 100 to 600 mV/g [13; 16; 25; 27; 28]. For obvious reasons, accelerometers are often very small in size, thus enabling their placement upon the skin.  
The location of the sensors on the skin is a crucial point, as the vibroarthographic signal is different when recorded in two different locations [13]. The optimal location of sensors has been demonstrated to be on the medial compartment below the midline of the patella [2; 14; 15; 16]. This can be explained by the fact that it is the closest position to the area of contact between moving joint surfaces, thus inducing the least attenuation of the signal during propagation. Moreover, [PERSON] et al. [7] have demonstrated that some meniscal signals were larger on the affected side of the knee. Therefore, the importance of using three sensors on the patella, the medial condyle, and the lateral condyle is stressed. Figure 1 shows the different locations of sensors according to different authors.  
Sensors are firmly attached to the skin with double-sided tape or a medical adhesive patch. In most cases, an electro goniometer is used simultaneously to measure the flexion and extension angle, which can add information to the signal analysis. Furthermore, the  
Figure 1: 3D modelling of a left knee with color symbols showing the different locations of sensors used by the authors.  
angular velocity of flexion/extension motion is important, as some signals are present only at slow knee motion [7]. A metronome can be used to control and maintain a constant speed [17]. The sensor location and the velocity of knee motion are therefore parameters that can influence VAG signals. Consequently, the question of repeatability has been addressed by [PERSON] et al. [11]. It has been proven that knee joint sound signals are rather reliable within one day, since Intraclass Correlation Coefficients (ICC) range from 0.85 to 0.95 at the medial tibial plateau and from 0.73 to 0.87 at the patella. However, the repeatability of these signals drops if measurements are taken on two different days [11]. Likewise, it has been demonstrated that the initial condition of the knee affects the vibration signal [18]. Therefore, the previous activity that the knee joint undergoes should be controlled in order to obtain repeatable measurements. In most studies, volunteers are requested to sit on a high chair with both legs freely suspended in the air. They are asked to perform a range of knee motion cycles from flexion (usually 90\({}^{\circ}\)[7; 19] but sometimes 135\({}^{\circ}\)[20; 21; 22; 23; 24; 25]) to full extension and then go back to the initial flexion in a certain amount of time. However, some other studies propose a different measurement setup. [PERSON] et al. [28] propose measurement in an inverted squat-like position with the knee loaded by a given force. A similar setup with traditional squats was used by [PERSON] et al. [30]. [PERSON] et al. used either sit-ups in [11] or climbing up and going down the stairs in [31].  
Figure 2 shows two examples of sensors attached to the knee. Moreover, the workflow of the VAG signal recording and analyzing procedure is shown in Figure 3. It will be further described in the following chapters.  
Figure 3: Diagram of a typical knee joint vibration arthrographic signal recording and analyzing process.  
Figure 2: Examples of sensors being attached to a knee: (**a**) accelerometers [25], (**b**) microphones [9].