Knee stiffness, often described as a restricted range of motion or difficulty bending and straightening the leg, is one of the most common reasons people seek orthopedic advice. The knee is a complex joint, relying on the smooth interplay of the femur, tibia, and patella, cushioned by cartilage and lubricated by specialized fluid. This intricate mechanical structure means that stiffness can arise from various sources, ranging from temporary fluid changes to long-term degenerative disease or the immediate aftermath of physical trauma. Understanding the mechanism behind the stiffness helps to distinguish between a minor annoyance and a symptom that requires professional medical evaluation.
Stiffness Due to Lack of Movement or Overuse
A frequent cause of knee stiffness relates directly to the joint’s natural lubrication system. The knee joint contains synovial fluid, a viscous substance that nourishes the cartilage and allows for smooth movement. When the knee remains immobile for an extended period, such as sitting for a long time or sleeping overnight, this fluid becomes less mobile and thicker.
This thickening causes a temporary resistance to movement, which is why the knee feels stiff upon first waking or standing up. Gentle movement helps to circulate and warm the synovial fluid, reducing its viscosity and quickly relieving the stiffness. Conversely, stiffness can also result from unaccustomed or excessive activity, leading to mild, temporary inflammation in the surrounding tendons or muscles. This overuse injury often resolves with a brief period of rest and non-prescription anti-inflammatory medication.
Chronic Conditions Causing Stiffness
Persistent or gradually worsening knee stiffness is frequently linked to chronic conditions that alter the joint’s structure.
Osteoarthritis (OA)
Osteoarthritis (OA) is the most prevalent form, characterized by the progressive deterioration of the articular cartilage that caps the ends of the bones. As this cartilage wears away, the protective space between the bones diminishes, leading to increased friction and the formation of bony growths called osteophytes, which physically impede movement. This degenerative process results in stiffness that often worsens with activity and improves with rest, typically presenting as morning stiffness that lasts for less than 30 minutes. The stiffness is mechanical, a direct consequence of the joint surfaces becoming rougher and the physical space shrinking.
Rheumatoid Arthritis (RA) and Gout
Unlike OA, Rheumatoid Arthritis (RA) is an autoimmune disorder where the body’s immune system mistakenly attacks the synovial lining of the joint. The immune response triggers chronic inflammation, causing the joint lining to thicken and leading to excessive fluid and swelling within the joint capsule. This swelling acts as an internal restraint, limiting the knee’s range of motion. RA-related stiffness is distinct because it is generally symmetrical, affecting both knees, and lasts significantly longer in the morning, often exceeding one hour. Other inflammatory conditions, such as Gout, can also cause acute episodes of stiffness by depositing uric acid crystals into the joint space, leading to intense inflammation, pain, and noticeable swelling.
Stiffness Following Acute Injury
Stiffness that immediately follows a traumatic event is often caused by swelling or a mechanical obstruction within the joint. When a major structure like a ligament or the meniscus is damaged, the body rapidly floods the area with fluid, a process known as effusion. This acute swelling increases pressure within the joint capsule, physically restricting the knee’s ability to fully bend or straighten.
Damage to the anterior cruciate ligament (ACL) or medial collateral ligament (MCL) can cause internal bleeding and instability, contributing to significant swelling and stiffness. A tear in the meniscus, the C-shaped cartilage that acts as a shock absorber, can also lead to mechanical blockage. If a flap of torn meniscus gets caught between the femur and tibia, it can cause the knee to “lock” or become suddenly unable to move through its full range of motion. This post-traumatic stiffness is often accompanied by acute pain and may be severe enough to prevent weight-bearing.
Signs That Professional Medical Help is Needed
While many causes of knee stiffness are temporary or manageable with home care, certain accompanying symptoms signal the need for professional medical evaluation.
- The inability to bear weight on the affected leg, or a sudden, severe increase in pain, suggests a significant structural injury like a fracture or a severe ligament tear.
- Stiffness accompanied by pronounced warmth, redness, or a fever may indicate an infection, such as septic arthritis, which requires immediate attention to prevent joint destruction.
- A knee that consistently locks, catches, or gives way during movement warrants medical assessment, as this can be a sign of a meniscal tear or a loose piece of cartilage interfering with the joint mechanism.
- Stiffness that does not improve after several days of rest and gentle movement, or morning stiffness that routinely persists for over an hour, should be evaluated by a healthcare provider.