The knee, like any joint, has a certain degree of flexibility and this is known as the range of motion. Movement is usually easy and smooth but limited to the range of motion which varies from joint to joint. When this movement is difficult, abnormally limited and requires greater force than normally would be expected then the joint is said to be stiff. This may occur for a number of different reasons.
What is knee stiffness?
Knee stiffness is where movement of the knee is difficult and the range of motion may be limited. Stiffness usually indicates a problem with one of the structures that make up the joint. This may be due to injury, inflammation without injury or damage. It is also important to remember that muscles are responsible for movement and therefore muscle problems can limit range of motion. However, this is due to muscle weakness rather than a stiff joint.
Joints are where two bones meet to articulate. The ends of these bones are capped with cartilage which is smooth, flexible and constantly replenishes due to wear and tear. Around it is the joint lining known as the synovium which produces a lubricating fluid known as synovial fluid. Then there is an outer lying joint capsule with ligaments that strengthen and stabilize the joint. Stiffness usually arises with a problem of one or more of these structures.
As a symptom, knee stiffness is a common problem because some of the more frequently seen joint conditions tend to affect the knee. Furthermore the knee has to contend with significant force as the entire upper body and thighs, which are the heaviest parts of the body, constantly bear force on the knee and this force increases significantly when walking or running.
Causes of a Stiff Knee
There are a number of different causes of knee stiffness. Sometimes knee stiffness is not due to any disease. It can arise with lifestyle factors like overuse without inflammation , among people who are sedentary or those who are bedridden or wheelchair bound. Therefore physical conditioning can contribute to reduced range of motion and may be reversed with regular physical activity.
Arthritis simply means joint inflammation and can occur for a number of reasons. The two most common types of arthritis are osteoarthritis (OA) and rheumatoid arthritis. Knee osteoarthritis is very common, starting around mid life and becoming prominent around 60 years of age. It is due to joint degeneration and not severe inflammation like with rheumatoid arthritis which occurs when the immune system attacks the joint lining.
There are other types of arthritis which may not be as common. This includes:
- Gouty arthritis where inflammation is due the build up of uric acid crystals in the joint space.
- Post-traumatic arthritis is joint inflammation that arises with injury.
- Septic arthritis is joint inflammation due to infection.
- Reactive arthritis is where joint symptoms like pain and stiffness arise as a result of an infection although the joint itself is not infected.
Trauma to the knee is another common cause of stiffness. Apart from post-traumatic arthritis, injury can sprain or even tear the ligaments, lead to fractures or result in muscle and tendon strain. These injuries may arise with falls, car accidents, contact sports or assaults. Overexertion can also be considered as an injury as the joint structures are strained with physical activity like walking or running long distances or weight training.
Infections can also lead to joint symptoms like stiffness. These infectious agents may enter the joint with an injury where there is a break in the skin, spread from other sites or be inoculated into the joint during invasive medical procedures. Bacteria tend to cause a more severe joint infection (septic arthritis).
However, viral infections can also cause joint symptoms like pain and stiffness although the joint itself is not infected. This is known as reactive arthritis. Abscess formation may also occur with an infection and lead to knee stiffness. A bone infection (osteomyelitis) may also cause stiffness often with intense pain.
Rheumatoid arthritis is a well known autoimmune joint condition but there are other similar conditions that may affect the joint. SLE or systemic lupus erythematosus is where the immune system attacks the body. In rheumatoid arthritis this is limited to the joint but in SLE it may occur anywhere throughout the body. The joint may be one of the many sites affected in SLE. Psoriatic arthritis appears to also be due to autoimmune factors where the skin (psoriasis) and joints (arthritis) are affected simultaneously.
Pain, Swelling and Stiffness
Stiffness is just a symptom and not a disease on its own. It is often accompanied by other symptoms, most often pain and swelling. These symptoms along with redness of the skin over the knee and heat are signs of inflammation. However, stiffness can occur on its own without pain, swelling, redness or heat. The presence of these other symptoms may help in the diagnosis of the underlying condition. These accompanying symptoms may not always involve the knee, like with systemic symptoms such as fever.
- Knee stiffness in the elderly is usually due to osteoarthritis but there is usually little to no swelling, redness or heat. Pain is often present with movement and eases with rest.
- Morning stiffness with pain, swelling and redness is more likely to be seen in rheumatoid arthritis. The stiffness gradually eases with movement.
- The presence of a fever along with inflammatory symptoms may be due to a joint infection. However, these symptoms may also be seen with SLE. In systemic viral infections, fever may be present with joint pain and stiffness but joint swelling and redness are usually not present.
It is important to understand that stiffness may not be due to any physical limitation in term of the range of motion. Joint pain (arthralgia) may instead be the limiting factor. Pain upon movement may lead to a person to limit extension or flexion at the knee joint and the limitation in movement it is therefore not due to joint stiffness itself.