The knee is a synovial joint that connects the femur (thigh bone) to the tibia (shin bone). The knee is shaped in such a way that lets the normal knee bend (flex) and straighten (extend), while also allowing small amounts of rotating and sliding. The knee is the largest joint in the body and also one of the most complicated.
A number of ligaments stabilize the knee. These include the two cruciate ligaments (ACL and PCL) as well as the two collateral ligaments (MCL and LCL). At the front of the knee is the patellar tendon that connects the tibia to the quadriceps muscle via the patella bone (knee cap), and this arrangement works to straighten the knee. The knee also has the ability to lock itself straight to help you stand up for long periods of time. The ability to straighten the knee is one of the things that is first lost in early arthritis.
The knee is made up of three compartments: the medial (on the inside), the lateral (on the outside), and the patellofemoral (at the front). The compartments are all joined together but there are occasions when problems such as arthritis can exist in only one of the compartments of the knee.
Inside the joint there are also two menisci. These act like shock absorbers to help with the smooth movements of the knee. It is not uncommon for these to become torn. This can then sometimes lead to symptoms of pain, swelling, locking or jamming and the feeling like your knee may give way under you, especially during twisting activities. If symptoms persist, keyhole surgery (arthroscopy) may help.
Covering the bones inside the knee joint is articular cartilage. This cartilage is very smooth and has very low friction, allowing the bones the slide easily over each other to bend and straighten the knee. Loss of this cartilage occurs in knee arthritis.