THE ROLE OF MUSCLE WEAKNESS IN THE PATHOGENESIS OF OSTEOARTHRITIS

Osteoarthritis (OA) is the most common form of arthritis to affect synovial joints.4, 5, 88 Although more benign than other rheumatic disorders, such as rheumatoid arthritis (RA), the personal cost in terms of pain and disability, the financial costs of its management, ⁸⁸ and the prevalence4, 5 of this disease lead to serious socioeconomic consequences.46, 78, 88

Although there is a strong association between OA and mechanical abnormalities, and the prevalence of OA increases with age, ⁵⁴ the explanation usually proffered to patients—that OA is an inevitable consequence of ageing because of the normal wear and tear of the joint—is too simplistic. It is becoming increasingly clear that OA is much more complex than previously thought.

Rather than being viewed as a single pathologic entity, OA is better regarded as subsets of joint disorders, with different causes (e.g., mechanical, hormonal, or genetic), but that share common pathologic processes that result in similar alterations in articular anatomy and function and have common clinical features.⁶³ The principal clinical features reported by patients are joint pain, decreased function, joint instability, periarticular muscle weakness, and fatigue. The onset of these symptoms is almost invariably insidious, with episodes of pain that increase in frequency and duration until, eventually, the patient may be in constant pain.

A diagnosis and categorization of the severity of OA is frequently made from radiographic evidence of attrition of the articular cartilage and changes in the underlying subchondral bone. Research in OA has focused mainly on the intra-articular changes and has revealed a great deal about the pathophysiologic processes that occur within the articular milieu. In contrast, there has been very little research interest in the changes that take place in the periarticular skeletal muscles¹⁷; however, muscles are integral to joint function and the synovial joint should be considered a functional unit of the neuromuscular skeletal system, which is composed of the articulating bones, cartilage, ligaments, capsule, the muscles that effect movement, and the nerves that control movement. Accurate, controlled movement relies on each component of these units working optimally. If any component is dysfunctional, the joint will be dysfunctional. The limited number of research studies that have been performed in this area have led to an appreciation of how muscle is affected by the condition, and to growing conjecture that muscle dysfunction may be involved in the pathogenesis of OA.17, 57, 112, 116

This article presents some of the studies that implicate motor dysfunction as a cause in the pathogenesis of OA and touches on the sensory function of muscle, which is intimately linked with motor function. It also cites the results of studies demonstrating the efficacy of rehabilitation in reversing sensorimotor dysfunction to reinforce the argument that muscle plays a vital role in OA. This article should be considered, therefore, in association with two other articles in this issue, “Proprioceptive Impairment in Knee Osteoarthritis” by Dr Leena Sharma, and “Exercise in the Treatment of Osteoarthritis” by Dr Marian Minor, which provide more inclusive reviews of these areas. The present article concentrates on knee OA because most of the work studying muscle dysfunction in OA relates to knee OA in particular, largely because the knee musculature is relatively easily investigated. The choice is easily justified, however, because the knee is the joint most frequently affected by OA and knee OA the most common cause of disability among the elderly. Knee OA is a major public health problem and warrants concern and research attention.