# A Review of the Vibration Arthography Technique Applied to the Knee Diagnostics
## 1 Introduction  
The knee is one of the most complex joints in the body and consists of three bones (femur, tibia, and patella) articulating in three synovial joint compartments--namely, the patellofemoral joint, the lateral femorotibial joint, and the medial femorotibial joint. The femur, the tibia, and the patella surfaces are covered with articular cartilage that absorbs compression stresses in the joint and thus prevents bones from being in direct contact with each other [1]. These joints are surrounded by a single capsule containing synovial fluid. Furthermore, ligaments stabilize and reinforce the joint capsule. In addition to these structures, fibrocartiginos menisci lie between the articular surfaces of the tibia and femur. They are essential to enable effective articulation and to absorb shocks. Due to its large number of components and its complexity, the knee is one of the most commonly injured parts of the body. A common disease is knee osteoarthritis (OA), which is characterized by the local and progressive loss of articular cartilage along with simultaneous changes in the bone underneath the cartilage [2]. OA is a major cause of disability in the elderly, with around 654 million people over 40 years old suffering from knee OA in 2020 worldwide [3]. It is therefore crucial to have good diagnostic methods to detect OA at an early stage. To assess the knee joint condition and cartilaginous losses, the gold standard method is arthroscopy. This consists of inspecting the interior of the knee with a small camera called an arthroscope. However, this is an invasive surgical procedure involving a small incision and thus cannot be used easily for the repeated assessment of patients over time [4]. Image-based techniques such as X-rays are also widely used. However, as cartilage is not visible on X-rays, this technique cannot directly evaluate the cartilage condition, but the width of the joint space can be estimated [4]. Small apparent spaces between the bones may then indicate a loss of cartilage. Magnetic Resonance Imaging (MRI) can overcome some of the limitations of X-rays due to its capacity to detect structures such as cartilage, menisci, ligaments, and synovium [2]. However, MRI suffers from its high cost and long acquisition times. Ultrasound can therefore be seen as a great alternative to MRI, as they enable theprecise visualization of soft tissues and are inexpensive and not invasive [5]. Nonetheless, parts of the knee joint can still be concealed, as ultrasounds cannot penetrate bones [6]. Advantages and limitations of these previous diagnostic techniques are summarized in Table 1.  
To overcome some of these limitations, a non-invasive and inexpensive technique called \"Vibration Arthography\" (VAG) was introduced by [PERSON] et al. [7]. This technique is based on the detection and recording of knee vibration emission. This emission originates from articular surfaces rubbing against each other during knee movement. Thus, acoustic and vibrational signals can be recorded and analyzed to evaluate the knee joint condition [2]. As mentioned above, OA manifests itself by the degeneration of articular cartilage, thus causing more friction during movements. Consequently, abnormal knees generally produce more vibroarthographic signals than healthy ones. To understand the origin of vibration arthrography, it is important to consider its long history. Indeed, auscultation is one of the most ancient diagnostic methods. The modern development of this method is associated with the invention of the stethoscope. The first use of a stethoscope to explore sounds from the locomotor system was performed by [PERSON] in 1885 and he was able to locate loose bodies within the knee joint using this tool. In 1902, the evaluation of knee joint sounds as a diagnostic tool was realized by [PERSON]. In 1929, the examination of 1600 joints from people of all ages and sexes by [PERSON] led to the conclusion that auscultation may reveal a certain roughness of the joint that was not recognizable by any other means [8].