Intra-articular knee injection is a common procedure in treatment and diagnosis of the knee joint. For example, local anesthesia injection to the knee joint is done in the elective knee arthroscopy cases [1, 2] and in some knee physical examinations that need to be done under anesthesia , gadolinium contrast media injection in knee joint imaging  and viscosupplement and corticosteroid injection to alleviate the knee pain. These procedures are done as the knee osteoarthritis is one of the most common degenerative joint disease and impacts functional capacity in elderly (prevalence of 3.8% and 2.7 times more common in women than men) . Disease pathophysiology of knee osteoarthritis involves inflammatory process of synovium and structural changes of articular cartilage including subchondral bone . Most of the patients suffer from pain, stiffness and impaired joint mobility and often require intra-articular injection with non-steroidal anti-inflammatory drug, corticosteroid or viscosupplement agent for pain relief and inflammation control to improve joint function.
The accuracy of intra-articular needle placement is a vital key to prevent soft tissue complication and achieve a good treatment outcome by avoiding inadequate analgesia and agent concentration in the intra-articular space. To minimize potential complications of extra-articular injection from local tissue damage, such as atrophy of muscle and subcutaneous fat, pain, skin hypo-pigmentation  and skin necrosis , proper placement of needle is advocated to confirm intra-articular needle position . Many injection techniques with different knee postures are proposed to use for agents delivery to the knee, however the best portal and posture is still controversial . A systematic review demonstrates the superolateral approach in extended knee has the best accuracy around 91% compared to other approaches in the similar posture (lateral mid-patellar 85%, anteromedial 72% and anterolateral 67%) . However, in 90o-bended knee position, a study using squishing technique  and post-injection mini air-arthrography  showed higher accuracy (89%) of modified anterolateral approach compared to superolateral approach (58%) .
No matter what techniques used, the accuracy of needle tip placement needs to be enhanced, particularly when the knee effusion is not present or the symptomatic dry joint . The position of needle tip can usually be detected whether it is extra- or intra-articular by ultrasonography. The ultrasound-guided knee injection and aspiration offers greater accuracy and clinical improvement over using only the conventional landmark technique [16–19], however, the imaging technique requires extra time and the cost of machine could make it not available in the limited-resource settings. When the ultrasound machine is not available, the tactile sensation can be used to distinguish the positions. Absence of the backflow when the needle tip in intra-articular, detecting from the tactile feedback felt at the syringe, has also been proposed  and implemented as an instrument  for the imaging-free technique to increase the intra-articular injection accuracy. However, it takes a steeper learning curve to do so particularly without an assisted device.
A vibration sensor approach has been used in previous studies to detect the interested particle flow in the fluidized bed reactor. A fluidized bed reactor, used in many industrial applications, is a device that carries inside the multiphase chemical reactions. Vibration signals, usually taken from the piezoelectric accelerometer mounting outside the fluidized bed reactor, can be used to non-invasively detect the interested particle flow by characterizing the vibration frequency components, for example, to distinguish sand-oil-water flow from sand-water flow  and to detect the sand flow in the gas-sand flow  by observing the power spectral density for particular frequency bands of the vibration signals. The piezoelectric accelerometer, a surface contact sensor that converts the vibration signal in the form of acceleration of the piezoelectric material inside, assuming simultaneous moving with the attached surface, to the electrical analog signal that can usually be collected by an analog-to-digital system. After transforming those vibration signals into the time-frequency domain commonly by the short-time Fourier transform (STFT) and the wavelet transform , different patterns corresponding to the different flow phases, e.g. with or without solid particles in the flow, can be observed and characterized.
When different multiphase flows can be classified using the vibration frequency analysis, detection of injected air flow through different anatomical structures should also be a possible application. It was then hypothesized that the flow of air injection originated from the needle tip at different anatomical locations can be distinguished using vibration frequency analysis, suggesting the needle tip placement either intra- or extra-articular positions. The aim of this study was therefore to compare the vibration signals of intra-articular and extra-articular air injection into the cadaveric knee using the piezoelectric accelerometer sensors, in order to characterize the frequency power band of the detected vibration signal.