The knee joint is a rather necessary synovial joint that connects the tibia to the femur. In the knee, the tibiofemoral and the patellofemoral joints form a modified hinge joint, which lets the knee straighten, bend, and rotate from side to side. The purpose of the muscles surrounding the knee is to help keep it moving, well-aligned, and stable. The quadriceps femoris straightens the knee, and the hamstrings provide the opposite motion. Ligaments hold the bones stable, attach them, and give strength and stability to the knee joint. There are five knee ligaments: medial collateral, lateral collateral, anterior cruciate, posterior cruciate, and patellar.
Around the knee, there are four bones, and the femur (the thigh bone) is the longest one; it runs from the ankle to the knee. About 80-90% of the weight is carried by the tibia. Patella (knee cap) is a triangular semi-flat bone that has the purposes of increasing the force generated by the quadriceps muscle, moving the knee bends, and protecting the knee joint from trauma. The fibula helps to form the ankle joint and serves as an attachment for muscles like the lateral collateral ligament and biceps femoris.
The range of motion includes rotations (inward and outward), abduction (movement away from and towards the middle of the body), extension (straightening), and flexion (bending). Since the knee is a hinge joint, it primarily moves in one plane of movement, which is extension and flexion. The standard ROM measure includes 135° for a fully bent knee joint.