Knee stiffness can be defined as loss of 10 degrees of extension (straightening) or 15 degrees of flexion (bending) but in reality any motion loss that is noticed by you counts as knee stiffness.
Loss of motion can happen on its own or as a result of injury or surgery.
The most common cause of loss of motion in the knee is arthritis. The most common form of arthritis in our population is osteoarthritis but is can be from rheumatoid arthritis or post traumatic arthritis also. Other inflammatory conditions which cause knee swelling such as gout, psoriatic arthritis and PVNS (pigmented villo nodular synovitis) can cause loss of knee motion as well.
The stiffness from osteoarthritis tends to develop slowly over time. It gradually worsens as the pain in the knee increases and you might develop a fixed flexion deformity (the knee is unable to go fully straight) due to scar tissue forming around the knee. This is usually a late finding with loss of bending being the more common problem. Most people notice the lack of bend because they have trouble putting their shoes or socks on or have difficulty standing from a chair.
A meniscal tear can cause your knee to be stiff but this is not the usual presentation for this condition. Joint lining cartilage (chondral cartilage) injuries can cause knee swelling and stiffness as well.
Physiotherapy and hydrotherapy are useful forms of exercise when you have a stiff knee. Anti-inflammatory tablets are useful if the knee is swollen and injections can be helpful if the joint is inflamed.
It is important to understand why your knee is stiff so that the correct treatment can be provided to you. An Xray is always required but sometimes blood tests, MRI scans or even sending some fluid from the knee to the laboratory are needed to make the diagnosis. In rare instances an arthroscopy is required for a synovial biopsy (to take a sample of the joint lining to be sent to the laboratory to be analysed). Many people tolerate a stiff knee for long periods of time but often take action when the pain interferes with their daily activities. Total knee replacement is very good at relieving pain but may not allow restoration of motion to a stiff knee.