Osteoarthritis of the knee is a progressive condition that causes aching pain, stiffness, and loss of mobility in the knee joint. The knee is the largest joint in the body, and is one of the strongest. It’s composed of three bones: the bottom part of the thighbone, called the femur; the top part of the shinbone, called the tibia; and a large, round bone that covers and protects the joint, called the patella or kneecap. The ends of these bones are lined in a protective material called cartilage, a smooth material that acts like a shock absorber and helps the bones to move easily while the body is in motion.
The cause of knee pain and other osteoarthritis symptoms is often bone-on-bone friction, which happens when the cartilage has begun to erode. Over time, cartilage may wear away completely, leaving the joint vulnerable to permanent damage.
New research suggests that not everyone experiences osteoarthritis symptoms as a result of cartilage “wear and tear.” Knee pain results in many people from an inflammation in the membrane lining the knee joint, called the synovium. In a healthy knee, the synovium secretes a gel-like substance called synovial fluid that lubricates the joint and helps absorb stress during movement. Pain and stiffness related to osteoarthritis have been linked to a thinning of synovial fluid, which further contributes to joint degeneration.
Typically, osteoarthritis of the knee—as a result of cartilage erosion, thinning of the synovial fluid, or both—occurs in people over age 60 due to years of movements related to daily living. Younger people may develop premature cartilage erosion from a knee injury or from repeated high-impact activities, such as running. For some, the disease progresses rapidly, and for many others, the symptoms of osteoarthritis worsen over years or decades.
Musculoskeletal specialists, including rheumatologists, sports medicine physicians, radiologists, physiatrists—doctors who specialize in rehabilitation—and orthopedic surgeons collaborate at NYU Langone’s Joint Preservation and Arthritis Center to diagnose osteoarthritis of the knee. Our experts base a diagnosis on a person’s symptoms, as well as the results of diagnostic imaging tests.
Doctors rely on information about your family and medical history to find clues about whether your symptoms are caused by osteoarthritis of the knee. Doctors may ask when you first noticed pain or stiffness, whether you feel discomfort all the time or only at certain times or during certain activities, whether pain keeps you from moving your leg in certain ways, if you hear a grinding or clicking sound during movement, and whether pain or other symptoms have limited your day-to-day activities.
In addition, doctors ask for details about any previous knee injuries or knee surgery. An injury to soft tissue in the knee, such as a torn anterior cruciate ligament (ACL), may have damaged cartilage or underlying bone, increasing the risk of continued cartilage deterioration and osteoarthritis.
Your doctor carefully examines your knee during a physical exam to look for signs of osteoarthritis. He or she may gently move the knee and leg to assess the joint’s flexibility and range of motion and listen for any cracking or popping noises in the joint, called crepitus, that may be a sign of joint damage.
The doctor might also press lightly against the knee to test for tenderness and determine whether any discomfort results from movement in the joints. Torn soft tissue in the knee may cause similar symptoms, and our doctors are able to differentiate between osteoarthritis and another type of knee injury.
Our doctors also examine the hip joints and ask you to walk a few steps to see if disorders of the hip affect how you walk. If the hip joint is stiff or weak, the knee has to work harder and absorb more of the body’s weight during movement, which may cause discomfort in the knee. This referred pain could indicate that something other than knee osteoarthritis is causing your symptoms.
X-rays provide clear, detailed images of the knee joint and may reveal bone spurs or narrowing of the joint, the classic hallmark for diagnosing osteoarthritis of the knee. A healthy knee joint appears to have a gap between the bones on an X-ray because the cartilage acts as a cushion between the femur and the tibia. Doctors can see evidence of cartilage erosion by looking at the space between the bones in the knee joint as well as the shape of the bones that make up the joint. In an arthritic joint, this gap is narrowed and the shape of the bones is less smooth or less round.
Doctors at NYU Langone often use ultrasound in a doctor’s office to view an important aspect of the inside of the knee that cannot be seen on an X-ray: the soft tissues surrounding the bone such as the synovium.
Ultrasound uses high-frequency sound waves to create real-time images of structures inside the body that both the physician and the person being examined can see. Ultrasound can reveal inflammation in soft tissues and the abnormal accumulation of fluid in the front or back of the joint. Ultrasound exams allow the physician to assess the degree of inflammation and identify the location of excess fluid.
Your doctor may use a procedure called arthrocentesis to remove the excess fluid. This can result in immediate and significant pain relief. In addition, the synovial fluid removed can be tested in a laboratory and the results used to determine the best course of treatment.
An ultrasound image may also show bony growths in the joint, a classic sign of osteoarthritis. These growths, called osteophytes or bone spurs, develop as part of the body’s response to friction between bones. When bones rub directly against each other, they start to deteriorate, and the body produces new bone to make up for the loss. But bone spurs develop unevenly and may cause discomfort by getting in the way of smooth joint motion.
As part of the diagnostic process, doctors at NYU Langone use a measurement called a patient-reported outcome score to assess your type of knee pain and current level of function. This method consists of a questionnaire that doctors use to obtain information about your specific symptoms: if they prevent you from participating in everyday activities, how much symptoms affect your overall quality of life, and whether you’ve limited your participation in sports or exercise because of arthritis.
The results of the questionnaire help doctors in creating a treatment plan. For example, some people may not show joint damage on an X-ray, but still have chronic pain. Others feel little or no pain even though an X-ray reveals significant cartilage erosion. By using the patient-reported outcome tool, our arthritis experts ensure that treatment is individualized to meet your needs and not solely based on diagnostic tests.
If your symptoms include swelling, warmth, or redness, doctors may recommend arthrocentesis. In this procedure, doctors remove fluid from the knee joint using a needle. The fluid is analyzed in a laboratory and the results can rule out other possible causes of joint pain and inflammation such as gout, infections, and rheumatoid arthritis, a different form of arthritis that typically affects multiple joints at once.
To perform arthrocentesis, doctors first inject a small amount of local anesthetic to numb the skin, then carefully guide the needle into the joint space where fluid has accumulated. The doctor removes fluid through the needle and sends it to a lab for testing. Results are typically available in two days, at which time your doctor calls or meets with you to discuss the results.
Joint fluid can also be analyzed for biologic markers in those who want to participate in the Joint Preservation Registry, an advanced research program being conducted at NYU Langone. Arthritis specialists evaluate substances present in joint fluid to identify individual patterns of disease and tailor treatment to address the form of arthritis that is affecting you.
NYU Langone orthopedic surgeons at the Joint Preservation and Arthritis Center are actively involved in research to incorporate biomarker testing into care plans.
Learn more about our research and professional education opportunities.