Normal anatomy of the knee joint

The knee joint is made up of the bottom end of the thigh bone (femur), the top end of the shin bone (tibia) and the knee cap (patella) and consists of three compartments, the medial, the lateral and the patellofemoral. It is a joint which allows flexion and rotation and relies for support upon the ligaments on the sides, called the medial and lateral collateral ligaments (MCL and LCL), and the ligaments on the inside called the anterior and posterior cruciate ligaments (ACL and PCL).

The joint surfaces are covered in articular cartilage which is the fixed shock absorber on the end of the bones, and the mobile shock absorbers (medial and lateral menisci). Damage to the fixed joint cartilage shock absorber is called osteoarthritis, or arthritis for short. Tears of the ACL and PCL are called ruptures, and often times requires surgery to repair or reconstruct these structures. Tears to the medial collateral ligament are often treated   non-surgically in a hinged knee brace which allows range of motion. Tears of the menisci are often painful and may require either repair or trimming usually performed by keyhole arthroscopic surgery.