The term arthritis of the knee refers to an inflammatory disease of the respective joint. This inflammation may result from a variety of causes, but is most often provoked by autoimmune or degenerative processes.
Arthritis of the knee is a rather general term referring to any inflammatory alteration of the respective joint. The term gonarthritis describes the same condition.
Arthritis of the knee may be triggered by distinct pathophysiological events and in this context, more than one hundred different forms of gonarthritis may be distinguished. The most common forms are osteoarthritis of the knee and rheumatoid arthritis and while this article will highlight certain common features of distinct forms, it will mainly focus on the aforementioned, specific types of gonarthritis.
This pathology is widely distributed and affects millions of people every year. It is associated with the characteristic signs of inflammation - pain, swelling, warmth, possibly redness of the surrounding tissues and functional impairment. The latter may be sufficiently severe to impede the affected individual to realize everyday activities.
Many forms of knee arthritis are chronic diseases. This does not apply for acute infectious forms of arthritis, but for those associated with permanent cartilage damage, possible erosion of the underlying bone and lesions to other structures of the knee joint. These pathological alterations are not reversible and only supportive treatment in form of analgesics and antiphlogistics can be provided.
Osteoarthritis of the knee (OAK) as well as rheumatoid arthritis (RA) are chronic forms of gonarthritis. For a long time, OAK has been considered a degenerative joint disease and has indeed been named gonarthrosis, although according to more recent findings, it should rather be classified as an inflammatory disorder. This disease is characterized by a slow, but progressive loss of cartilage integrity and subsequent erosion of the underlying bone. It is the most common form of arthritis of the knee. RA, on the other hand, is a systemic disease mediated by an autoimmune response. The vast majority of RA patients suffers from polyarthritis, where joints of hands and feet are typically affected first. RA rarely manifests as sole arthritis of the knee. Distinct types of immune cells, mainly synovial macrophages, initiate an immune response involving the formation of granulation tissue. It is called pannus and contributes to joint inflammation as well as cartilage and bone damage.
Although the specific causes of distinct forms of arthritis of the knee may differ or may not even be completely understood, there are certain general risk factors for such joint lesions. Any damage to the cartilage of the knee or to other structures of this joint may initiate a chain of pathophysiological events leading to permanent alterations and chronic gonarthritis. In this context, continuous excessive loads are most detrimental. They usually result from overweight and obesity, pathological conditions that particularly affect the weight-bearing joints of the lower limbs. Jobs requiring lifting of heavy weights and some sports are also associated with repetitive, heavy loads of the knees. These may be understood as a kind of minor but continuous trauma and they may have an effect similar to that of a single major insult. However, a sedentary lifestyle is not recommended either. Muscles supporting the knees should be trained and strengthened. Misalignment of skeletal structures may lead to an abnormally increased mechanical load on certain parts of the knees and damage them. Infection may trigger arthritis of the knee, too.
Although the aforementioned mechanisms may indeed provoke OAK and other forms of arthritis of the knee , it is unlikely that they trigger an autoimmune response that leads to RA. This becomes even clearer considering that the latter disease usually manifests in hand and feet joints first. Genetic factors, lifestyle decisions, immunological and endocrinological dysbalances have been proposed as possible causes of RA . Tobacco consumption seems to be particularly detrimental for joint health . It has to be noted though that the aforementioned risk factors for other forms of gonarthritis most certainly contribute to cartilage damage once the autoimmune response is initiated. Indeed, several forms of arthritis of the knee are mutually dependent. Age is a crucial risk factor for all types of gonarthritis .
In the United States, more than 50 million people are diagnosed with any form of arthritis every year. OAK and RA are among the most frequent forms of arthritis. Women are affected more frequently than men and pregnancy seems to trigger symptom onset of certain types of knee arthritis .
It has been estimated that more than one out of ten adults suffers from osteoarthritis, a significant share of these patients from OAK. Of note, arthritis of the knee may also affect children, but is more common in adults. While age is a major risk factor for any form of arthritis of the knee, other influencing factors such as tobacco consumption are more prevalent among the adult population and may partially explain the aforementioned observation.
RA affects more than one million patients in the United States and is therefore less frequently diagnosed than osteoarthritis. It has been estimated that 1/12 women and 1/20 men develops RA or similar autoimmune joint diseases at one point in their lives . The mean age of RA patients is lower than those of individuals suffering from OAK.
Of note, overweight and obesity is a major health problem in the United States and most detrimental for knee health. Thus, the numbers given may not be representative of other geographical regions.
OAK and RA manifest with similar signs of joint inflammation, but develop through completely different pathomechanisms that shall be explained here as examples of pathogenesis of arthritis of the knee.
OAK is associated with complex cellular and biochemical processes that contribute to cartilage degeneration, altered mechanical stress, further progress of cartilage damage, appearance of vertical fissures and finally compromise of underlying bone tissue  . Over the course of the disease, cartilage flakes may become lose, move freely throughout the synovial space. The cartilage layer becomes increasingly thinner, subchondral bone may be exposed and undergo pathologic remodelling processes. In this context, cysts may form in the osseous tissue. Osteophytes may develop and further restrict joint mobility.
While OAK develops due to "wear and tear", an autoimmune response accounts for RA. As has been mentioned above, genetic, intrinsic and environmental factors have been proposed as possible causes for RA, but its etiology has not yet been completely understood . It is particularly unclear how the early immune response is triggered. Lymphocytes, initially T cells, later antibody-producing B cells, as well as synovial macrophages and a plethora of pro-inflammatory and matrix-degenerating cytokines are involved in establishing and amplifying the autoimmune reaction. Over the course of years, granulation tissue called pannus forms inside the knee. The pannus does not only hinder joint movement due to the space it's occupying, it also contributes actively to cartilage and bone damage.
Prognosis of arthritis of the knee is doubtful. At the time of diagnosis, cartilage and bone lesions are often advanced to irreversible states. Thus, complete cure is no longer achievable. However, effective medication is available to manage pain and other symptoms and enables most patients to manage their everyday life and realize physical activity. Other patients may need to undergo surgery in order to relieve pain. The outcome of such surgical interventions is generally good.
In general, patients suffering from arthritis of the knee show symptoms of joint inflammation. In most cases, e.g., in OAK and RA, these aggravate over longer periods of time, sometimes over the course of years. Infectious arthritis, for instance, may manifest suddenly.
Pain often dominates the clinical picture. The affected knees are swollen, mobility of the stiff joints is restricted. Patients often report symptoms to be more severe in cold weather conditions and after periods of rest, particularly when getting up in the morning . Pain subsides upon realizing moderate physical activity, but intensifies if a certain threshold is surpassed. It can become unbearable and the patient's knees may buckle. Periarticular tissues may be tender, edematous and erythematous.
Cartilage flakes as often observed in OAK may cause crepitus and sudden locks of joint movement .
In RA, a pannus might be palpable.
Knee joint inflammation due to RA is usually milder than that observed in OAK patients. In contrast, RA might be accompanied by systemic symptoms such as weakness, fever, rheumatoid nodules and vasculitis, but these are not characteristic for OAK. Of note, extra-articular symptoms are considered poor prognostic parameters .
Medical history and clinical examination allow for diagnosis of arthritis of the knee. However, in order to chose an adequate therapeutic scheme, the precise form of arthritis should be identified. Degenerative, autoimmune and infectious forms of gonarthritis will be treated differently.
While OAK may be limited to one side of the body, RA is a systemic disease that most frequently manifests symmetrically and affects several joints in different parts of the body. OAK is a typical disease of weight-bearing joints; RA symptoms are experienced in joints of hand and feet first before being felt in the larger joints such as the knees.
Plain radiography may give important hints as to the pathogenesis of the inflammation, but in early stages of the disease, tissue alterations may not yet be visible in radiographic images. Narrowing of the synovial space indicates loss of cartilage, areas of increased density point at remodelling processes in subchrondral osseous tissue. Bone cysts are visible as spots of diminished mineralization. Osteophytes are also readily recognizable on radiographic images. In order to assess the condition of soft tissues, magnetic resonance imaging or possibly computed tomography scans may be conducted.
A joint aspiration is not without risk but may provide further information that allows to distinguish between different forms of arthritis of the knee. In detail, inflammatory markers are often largely elevated in synovial fluid obtained from RA patients, while this is not the case for OAK samples. Leukocyte counts are significantly lower in OAK than in RA.
Mild to moderate cases of gonarthritis require medication therapy and lifestyle adaptions. Non-steroidal anti-inflammatory drugs are often the treatment of choice to provide relieve to pain and inflammation . However, they may hardly delay progress of cartilage and bone damage if not accompanied by dietary modifications, physical activity and weight loss. Most patients will benefit from physical therapy and may learn specific exercises to strengthen knee-supporting muscles.
Only if these measures do not provide sufficient resolution of inflammatory symptoms, intra-articular injections of corticosteroids should be considered. Injection of hyaluronic acid has been reported to be beneficial, but such results still need further confirmation. Intra-articular injections are not long-term solutions.
In severe cases, drug therapy and exercises may not provide pain relieve. Partial or total knee arthroplasty may be an option for these patients. Here, either one or all three compartments of the knee joints are replaced by a metal or plastic implant .
While little can be done to change one's genetic heritage or to slow done the course of time, many risk factors for arthritis of the knee can be avoided. Most importantly, a healthy diet and an active lifestyle are guarantors of a healthy body weight and strengthened muscles that support the knees. Heavy loads should be avoided, both professionally as in recreational activities.
Underlying disorders, e.g., bone malformations or malalignment of skeletal elements, should be treated accordingly.
The term arthritis of the knee describes an inflammation of the respective joint. Such inflammation may have different causes and a total of more than one hundred forms of knee arthritis have been described. The most common ones are osteoarthritis of the knee (OAK) and rheumatoid arthritis (RA).
Distinct forms of arthritis have different triggers: While OAK is a degenerative disease with strong inflammatory features, RA is caused by an autoimmune response, i.e., by an immune response against tissues pertaining to the own body. Infectious arthritis, for instance, may be provoked by pathogens gaining access to the knee joints after traumatic injury or septicemia.
There are, however, common factors that contribute to arthritis of the knee:
Inflamed knees are swollen and stiff. Their motion range is limited and bending and extending the leg is often painful. Pain may be more severe in cold weather and after periods of rest, particularly after getting up in the morning. Neighboring tissues may be tender, too.
Arthritis of the knee is often associated with inflammation of other joints.
Knee inflammation may be diagnosed upon clinical examination. Further diagnostic measures are often necessary to identify its precise cause. To this end, radiographic images are usually taken. They may indicate changes to cartilage and the underlying bone. However, X-rays hardly permit to assess the condition of soft tissues. If the physician suspects damage to such structures, e.g., to the menisci, magnetic resonance imaging may be indicated.
Mild to moderate cases are treated with analgesic and anti-inflammatory drugs as well as lifestyle adaptions. Overweight patients should reduce weight. In order to strengthen the muscles that may provide support to the knees, exercises should be carried out. They may be learned in physical therapy sessions.
If such treatment does not provide sufficient relieve, intra-articular injections of corticosteroids may be a second option.
Surgical partial or total knee replacement is only considered if no other measures provide freedom of pain.