Knee stiffness is a common sensation characterized by restricted movement or difficulty bending and straightening the joint. This feeling often occurs after periods of rest, such as first thing in the morning or after sitting for a long time. Stiffness can range from mild, temporary tightness to a persistent, painful limitation. While often associated with aging, this lack of smooth motion can signal various underlying issues, from simple inactivity to progressive joint deterioration.
Understanding the Sensation of Stiffness
The knee joint is lubricated by synovial fluid, a gel-like substance that fills the space between the bones and acts as a natural shock absorber. This fluid coats the articular cartilage, allowing the femur and tibia to glide with minimal friction. When the knee is held in a fixed position, the fluid thickens and becomes less effective as a lubricant, causing the joint to feel restricted upon standing or moving.
Stiffness is the physical manifestation of this thickened fluid and the temporary resistance it creates. Movement helps to thin the synovial fluid, allowing it to recirculate and restore its lubricating properties. This is why stiffness often improves after a few minutes of walking. Without proper lubrication, the smooth articular cartilage surfaces momentarily resist movement, contributing to the feeling of restriction.
Chronic Causes: Degeneration and Wear
The most common long-term cause of persistent knee stiffness is Osteoarthritis (OA), a progressive condition resulting from the breakdown of articular cartilage. As the protective cartilage wears away, the ends of the bones lose their smooth, cushioned layer, leading to friction and pain. OA frequently causes stiffness that is most noticeable upon waking or after inactivity.
The body’s attempt to stabilize the damaged joint can lead to the formation of osteophytes, or bone spurs. These bony outgrowths develop around the joint margins and physically impede the full range of motion, contributing to chronic stiffness and a grating sensation during movement. Chronic issues with the menisci (the C-shaped cartilage shock absorbers) or ligament instability can also lead to mechanical misalignment. The surrounding muscles and tissues may then tighten in a protective response, further contributing to stiffness and limitation.
Acute Causes: Inflammation and Temporary Factors
Stiffness can also result from inflammatory conditions or temporary factors unrelated to long-term joint wear. Bursitis involves the inflammation of the small, fluid-filled sacs (bursae) that cushion the bones, tendons, and muscles around the knee. When a bursa becomes inflamed due to repetitive kneeling or trauma, the resulting swelling and increased fluid pressure cause localized, acute stiffness and tenderness.
Tendinitis, the inflammation of a tendon often from overuse, can also cause stiffness as the swollen tendon resists movement. Gout is another cause, a form of inflammatory arthritis where high levels of uric acid lead to the formation of sharp crystals in the joint. This crystal buildup triggers intense, sudden inflammation and stiffness. Finally, prolonged immobility, such as sitting for hours, allows inflammatory molecules to pool and synovial fluid to thicken, causing temporary stiffness that resolves quickly with movement.
When to Seek Professional Guidance
While temporary stiffness that improves quickly with movement is not usually a cause for alarm, certain symptoms warrant a consultation with a healthcare provider. Seek professional guidance if stiffness is accompanied by severe swelling, which can indicate a major internal derangement or fluid accumulation. Stiffness that prevents you from bearing weight or fully flexing or extending the knee also requires medical evaluation.
See a doctor if stiffness is combined with systemic symptoms like fever, chills, or warmth spreading across the joint, as this may signal a joint infection. A professional diagnosis is necessary if the stiffness appeared suddenly following a traumatic injury, or if it persists for more than a few weeks despite home care like resting and applying ice. Gentle movement and over-the-counter medications can provide temporary relief, but they are not a substitute for determining the root cause of persistent or worsening symptoms.