If the clinician diagnoses that the joint is the likely cause of the presenting pain and other symptoms then the differential diagnosis is the next decision to be made. Inflammatory arthritis affects the synovial lining of the joint and the tendon-bone and ligament-bone insertions. If the arthritis is not inflammatory then changes in the joint structure and function can occur such as tears to the menisci or degenerative changes to the joint surface, with a number of possible causes contributing to the overall joint condition.
In some cases knee pain may be present without obvious pathological change, perhaps due to a pain syndrome such as fibromyalgia or in very early symptoms of a condition. A joint can have several different sorts of disorder at any one period and a joint can become steadily damaged and less stable by being affected by an inflammatory condition. Pain is one of the cardinal signs of inflammatory joint change and is typically not better on rest and increased on activity and movement, especially the beginning. In wear and tear arthritic joint changes the pain is mainly relieved by rest and aggravated by movement or weight bearing.
Advanced arthritic changes in the major joints or spine can cause pain at rest and also disturb sleep. Hip joint pain and that of larger joints is less easy to localise than that in smaller joints, with pain referred from the hip appearing potentially in several areas including the buttocks, the groin, the side of the hip and down the front of the thigh. Stiffness is hard to pin down but relates to a perceived difficulty with moving a joint, particularly following a period of inactivity and which improves with continuing motion. Typical stiffness of an osteoarthritic joint may last ten to fifteen minutes but the duration can be an hour or more in inflammatory arthritis.
There are several changes which can occur in a joint to make it swell. If the joint is affected by non-inflammatory arthritis then bony growths form at the margins of the joints and the end of the fingers or the knees become knobbly joints. Or an effusion, a collection of fluid within a joint capsule, can form secondary to inflammatory disease and can be drawn off by injecting the joint. It is common for an affected joint to show a degree of loss of movement either because the soft tissues have tightened up, the joint is damaged anatomically or by restriction from pain and inflammation.
Activities of daily living are often affected by arthritic change such as dressing, self care and stair climbing, often secondary to muscle weakness and atrophy. If pain accompanies weakness the cause is likely musculoskeletal rather than neurological or due to muscle pathology. Weakness can cause functional problems such as gripping things, getting up and down from sitting or walking safely. In systemic arthritis the whole person is involved in the disease and malaise and fatigue are common. An arthritis can develop slowly or can come on quickly, joint symptoms occurring over a few hours, in response to injury, infection or crystal deposition.
It is more typical for symptoms to develop over a period of weeks or months, and this commonly occurs in rheumatoid arthritis and osteoarthritis, the two most common types. Acute occurs for less than six weeks, sub-acute from six to twelve weeks and chronic if lasting more than twelve weeks. Joint inflammation can persist in affected joints as new joints are affected or episodic occurrence of joint symptoms with pain free periods between such as gout. The number of joints affected also varies, with monoarthritis affected a single joint, oligoarthritis involving two to four and polyarthritis affecting five or more.
Non-symmetrical and symmetrical joint patterns of involvement can occur. SLE and rheumatoid arthritis tend to affect the same joints on each side of the body in a symmetrical pattern while psoriatic arthritis and reactive arthritis involve different joints on each side of the body, the asymmetrical pattern. Joints may be involved in different patterns also, for example distal finger joints in osteoarthritis and psoriatic arthritis but not in rheumatoid arthritis.