Knee Arthritis: Overview

Knee Arthritis: Overview

Arthritis is the pain and inflammation that happen when the cartilage in a joint wears down. The smooth cartilage that covers the ends of the bones in a joint is called articular cartilage. This smooth surface helps the bones glide easily across each other during movement of the joint. This cartilage can get damaged either by a onetime injury where a piece of the cartilage breaks off, or it can happen over time when the cartilage slowly wears down throughout the joint. This cartilage wear and tear is called degenerative arthritis or osteoarthritis.

Nearly one in four adults in the United States has some form of arthritis in their joints. Even though the arthritis occurs for different reasons, the bottom line is still the same. The articular cartilage in the joint is damaged and causes pain and sometimes loss of movement. Specifically in the knee, arthritis typically happens over time, with years of repetitive movement and activity placing repeated weight on the knee.

Here is a quick look at the anatomy of the knee. The three main bones in the knee are the distal femur or thigh bone, the proximal tibia or shin bone, and the Patella or kneecap. In the knee, the three cartilage surfaces that can be affected are at the end of the thigh bone, the tibial plateau on top of the shinbone, and the backside of the kneecap. It is also important to know the knee is divided into three compartments. The medial compartment is on the inside, the lateral compartment is on the outside, and the patellofemoral compartment is where the kneecap sits in the groove between the condyles called the trochlear groove.

Unfortunately, one of the shortcomings of articular cartilage is that it does not heal as well as other tissues in the body. Its blood flow comes bottom-up from the bone, but it does not make it very far. Cartilage is considered avascular since it does not get much blood supply, making it nearly impossible for cartilage to grow back to normal on its own. Not only does cartilage get a poor blood supply, it also does not have any nerves. However, since bone does have nerve endings, this means that patients will not really feel pain until the cartilage is worn down thin enough that the pressure on the bone is what causes the pain.

Articular cartilage is not the only kind of cartilage in the knee. The meniscus is a softer, more spongy kind of cartilage that also helps cushion the knee joint. In this image of a knee MRI, the white area inside the bone means the bone is bruised from experiencing too much pressure. This typically happens when the meniscus is torn or missing for a long period of time. Without the help of the meniscus, more of the articular cartilage takes the force of the body weight in the knee. The cartilage starts to wear away and causes bone bruising and increased pain.

There are two kinds of osteoarthritis: primary and secondary. Primary osteoarthritis is the wearing down of cartilage that happens on its own. However, several factors have been shown to play a role in its development. Overweight patients place more stress on their knees with every step they take, overloading the meniscus and articular cartilage. Genetics can also play a role. If mom and dad had osteoarthritis, there is a good chance that their children will eventually get it, too. As expected, older patients tend to have osteoarthritis more than younger ones, and more females than males report being diagnosed.

Secondary osteoarthritis is caused by a pre-existing factor. Examples of these would be a severe prior injury to the knee, like a major fracture or a torn ligament, poor alignment of the leg, such as being knock knee or bow-legged, an infection of the knee joint, or even issues with blood supply to the articular cartilage, which is already minimal as it is.

When a patient has osteoarthritis, there are some common signs and symptoms they may be experiencing. Pain and effusion or swelling inside the joint are likely present. These get worse with activity and usually progress over time. The knee gets stiff as well and is at its worst in the morning as well as after sitting for a long period of time. Crepitus is the crunching or grinding sound that is heard with certain movements, and there may be some clicking, locking, or giving way sensations called mechanical symptoms. Finally, the knee will likely have trouble going through its full range of motion.

If a patient has these symptoms, the doctor will likely order x-rays to look at the joint space between the femur and tibia. Although you cannot see cartilage on an x-ray, if the space between the bones is getting smaller and smaller, it means the cartilage between the bones is wearing away, getting thinner and thinner. If there is no space left and the x-ray shows bone-on-bone arthritis, this means there is no more surface cartilage on the bones. Combining a patient's symptoms, their knee exam findings, and x-rays, a diagnosis of osteoarthritis can be made with confidence. An MRI is usually not needed to make this diagnosis or to help develop a treatment plan.

With many different treatments available for osteoarthritis, they should agree on a treatment plan that safely gets the patient to reach their activity goals.