The knee joint can be thought of as a hinge joint, with the primary motion of bending and straightening. In reality, it is more complex than a simple hinge, as these surfaces actually glide and roll upon one another. Three bones meet to form the knee joint: the femur (thighbone), the tibia and the patella (kneecap). The bones are covered with smooth cartilage surfaces that act as a cushion during weightbearing activity.
The bones of the knee are covered with a smooth, glistening layer called articular cartilage and connected by strong ligaments and powerful muscles that are attached to the thigh and calf by tendons, and which provide side-to-side stability. In a healthy knee, all of these structures work together to allow the knee to flex (bend) and extend (straighten) the lower leg smoothly.
The knee can be thought of as having three compartments: the medial (inner), the lateral (outer) and the patellofemoral (where the patella or kneecap meets the femur). In addition, there are two specialized types of cartilage within the knee joint called the lateral and medial meniscus. These act as shock absorbers between the thigh bone and the lower leg bones. Two ligaments – the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) – contribute to the knee's stability.
Knee arthritis literally means "inflammation of the knee joint," although inflammation is not always present when there is arthritis. On an X-ray of a knee with arthritis, the space between the upper and lower bones is often reduced or, in some places, even absent due to loss of cartilage, the shiny material which covers the ends of the bones.
There are two basic types of knee arthritis, and they have separate causes. Osteoarthritis is the most common, and this occurs when the entire knee joint starts to malfunction, due to deterioration of the soft tissues – cartilage, ligaments, tendons, muscle – and the bones themselves. The incidence of osteoarthritis increases with age and is most common in those age 50 and older. Conditions such as rheumatoid arthritis or lupus are autoimmune disorders which can cause inflammatory arthritis. These conditions not only affect the joints but can involve other organs such as the lungs or kidneys, and these can affect people young or old alike.
People with knee osteoarthritis complain of pain, stiffness, and limited motion in their knee. Although it is most common in older people, younger patients may get post-traumatic arthritis. This is a form of early onset osteoarthritis that results from an injury to the ligaments, cartilage and/or bone in or around a joint. People who are born with abnormalities in the shape of their bones or joints can also have osteoarthritis at earlier ages.
Symptoms include knee joint aching, stiffness, pain, and mild swelling. These symptoms often lead to reduced activity and muscle weakness. Weakened muscles can lead to instability and decreased ability to control the knee, further limiting the ability to participate in enjoyable activities. The knee joint may develop cysts, bone spurs, or loss of cartilage, which can be seen on X-rays.
Sometimes, people actually feel a catching or clicking within the knee when they bend or flex their knee. Generally, loading the knee joint with activities such as walking long distances, standing for long periods of time, or climbing stairs makes arthritis pain worse. When the arthritis has gotten to be severe, the pain may occur even when sitting, lying down or sleeping. The pain can be felt anywhere around the knee: As the cartilage is worn away preferentially on one side of the knee joint, people may find their knee will become more knock-kneed or bowlegged.
Knee arthritis symptoms in cases of inflammatory arthritis can vary. In rheumatoid arthritis, the stiffness is often much worse in the mornings. and gets better as the day goes on. Pain and stiffness can actually improve with activity, which Joints affected by inflammatory arthritis are often much redder and more swollen than joints with osteoarthritis.
The biggest risk factor for osteoarthritis is getting older. However, it is important to remember that osteoarthritis is not an inevitable part of aging, and the many older people live out their lives without any complaint of knee pain. Other risk factors include weight gain, previous knee injury, as well as poor alignment of the knees, that is, being bowlegged or knock-kneed.
If you have been diagnosed with osteoarthritis, having an affected knee does not mean that you will develop arthritis in the opposite knee. About 40% of patients who have osteoarthritis in one knee will have the same condition in the other knee. In contrast, patients with rheumatoid arthritis often develop problems in both knees.
Osteoarthritis of the knee may be treated by a sports medicine physician, rheumatologist, or an orthopedic surgeon, depending on your type of arthritis and what kinds of treatment you are looking for. A physical therapist is a crucial team member and should always be consulted to help you reduce pain and increase your mobility. If your knee pain is a result of rheumatoid arthritis, gout or other form of inflammatory arthritis, you should consult a rheumatologist. (Find a doctor who diagnoses and treats knee arthritis.)
An accurate diagnosis for knee arthritis relies on an evaluation and physical exam, laboratory tests, and imaging.
Your doctor will ask about your medical history and symptoms, and then conduct physical exam to assess:
If the physical exam suggests you may have knee osteoarthritis, imaging exams may be used to confirm the diagnosis and rule out other conditions.
X-rays are very helpful in diagnosing advanced knee osteoarthritis because the joint will have specific characteristics, including:
Yes! Although it may seem counterintuitive, walking can help decrease the pain and stiffness associated with osteoarthritis. In addition, any form of exercise that helps you maintain a healthy weight can reduce the stress on your joints, and this may slow the progression of your arthritis. You should, however, consult a doctor to confirm that your knee pain is caused by osteoarthritis rather than by an injury for which resting would be appropriate. A physical therapist can help determine appropriate levels of exercise for patients with osteoarthritis of the knee.
Physical therapy often should always be a part of the treatment for knee osteoarthritis. In addition, certain exercises , such as tai chi, and yoga, are low impact and have been found to be particularly helpful in knee arthritis. However, experts agree that the best kind of exercise for knee arthritis is the one that suits your lifestyle and that you can integrate into your daily life! A physical therapist can help you determine the type and amount of exercise which is safe for you.
Nonsurgical approaches to relieve pain and stiffness should always be tried first. These include physical therapy, weight loss, self-management programs and cognitive behavioral therapies. Oral pain medications or periodic injections of corticosteroids into the knee can help decrease pain and improve function. Inflammatory arthritis in the knee is usually managed medically rather than surgically. For osteoarthritis, and advanced case knee arthritis, knee replacement surgery is common, although other surgeries may be appropriate for certain cases.
It is difficult to predict the timing and progression of knee arthritis. Sometimes, pain increases and the ability to participate in daily activities decreases despite nonsurgical treatment plans. In those cases, surgery may be recommended.
Although knee replacement for arthritis is the most common and well-known surgery, other procedures may be appropriate for some patients. A knee osteotomy, for example, can help some patients who have a rotational or alignment issue that is accelerating the development of osteoarthritis. Others may benefit from a synovectomy and/or some type of knee arthroscopy.
Various surgeries may be appropriate, either on their own or in combination with another.
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Learn more about arthritis of the knee, including in and around the kneecap, from the content below.
Learn how ACL injuries may lead to early osteoarthritis of the knee in some patients and more below.
Reviewed and updated by Lisa A. Mandl, MD, MPH