**Abstract**

The joints of the human body, especially the knees, are continually exposed to varying loads as a person goes about their day. These loads may contribute to damage to tissues including cartilage and the development of degenerative medical conditions such as osteoarthritis (OA). The most commonly used method currently for classifying the severity of knee OA is the Kellgren and Lawrence system, whereby a grade (a KL score) from 0 to 4 is determined based on the radiographic evidence. However, radiography cannot directly depict cartilage damage, and there is low inter-observer precision with this method. As such, there has been a significant activity to find non-invasive and radiation-free methods to quantify OA, in order to facilitate the diagnosis and the appropriate course of medical action and to validate the development of therapies in a research or clinical setting. A number of different teams have noted that variation in knee joint sounds during different loading conditions may be indicative of structural changes within the knee potentially linked to OA. Here we will review the use of acoustic methods, such as acoustic Emission (AE) and vibroarthrography (VAG), developed for the monitoring of knee OA, with a focus on the issues surrounding data collection and analysis.

**Keywords:** ultrasound, acoustic emission, vibroarthrography, osteoarthritis, knee joint

### **1. Introduction**

#### **1.1 Synovial joints and osteoarthritis**

The free moving joints within the body are known as synovial joints and have the primary purpose of allowing forces applied to the skeleton to be transmitted as smooth, low-friction movements. The joint capsule, working alongside the muscles, tendons and ligaments stabilises the joint, whilst articular (or hyaline) cartilage covering the end of the bones in combination with synovial fluid within the joint space provides the environment for smooth, well-lubricated movements [1, 2]. In addition, some joints also contain fibrocartilaginous discs between the two bones to support the other joint components and dissipate the forces experienced by the joint, for instance, intervertebral discs in the spine, or the meniscus within the knee.

Osteoarthritis affects all of the structures within the joint and is defined as a condition causing pain within the joint, loss of function and decreased quality of life for patients [3]. The disease results in the degradation of cartilage and subsequent sclerosis and lesions in the now exposed subchondral bone, along with inflammation in the joint [4]. Tears within cartilaginous structures and new interactions between cartilage and bone, along with bone and bone, make for less smooth movements, pain, stiffness and reduction in joint function.

#### **1.2 Epidemiology and impact**

The most common joints affected by osteoarthritis include those of the knee, hip and hands with osteoarthritis of the knee the most commonly occurring form, affecting over 18% of the population in England [5].

With such a large proportion of the population affected, musculoskeletal conditions including osteoarthritis have considerable impact both medically and economically. Clinically, the pain and loss of function associated with osteoarthritis result in a lower quality of life reported by patients, who require a large number of GP visits and hospital admissions [6–8].

The underlying pathophysiology of osteoarthritis is unclear, with genetics, age, gender, obesity and previous injury all contributing to varying degrees in disease development and progression. The heterogeneous nature of the disease makes targeted treatment of cause and prevention of progression a challenge, with current best practice centring on patient education and lifestyle changes surrounding exercise, use of analgesics and anti-inflammatories to manage pain and inflammation and finally joint replacement at the severe end of the spectrum of disease [9]. However, this approach, with the exception of exercise targeting weight loss and strength, does not address an underlying cause or prevent progression of disease, an aspiration of future interventions for the disease.

Ranking the sixth most common cause of disability globally in 2010 [10], musculoskeletal conditions, including osteoarthritis, impact not only the healthcare system and patients but also their families [11]. Patients and their carers are at greater risk of being out of employment [12], with only 63% of those with a musculoskeletal condition in employment compared to 82% in those without a health condition [13].

With a predicted increase in the ageing population and an increase in obesity [14–16], the burden on health services and economic impact in terms of lost work time and disability is of growing concern. There is a real need for means of noninvasive early detection of osteoarthritis, sensitive means of monitoring progression and development of efficacious treatments to prevent and improve symptoms in order to improve quality of life and reduce the numbers progressing to severe disease and requiring joint replacement.
