Knee flexion anatomy. Artwork of a side (lateral) view of the bones and some of the cartilage and ligaments and tendons of a flexed (left) and extended (right) knee joint. The knee joint is where the upper leg bone (femur) articulates with the two lower leg bones, the tibia (right) and fibula (left). The knee-cap (patella) is also seen. The ends of the bones are covered in pads of cartilage (white), including the menisci on the tibia. Attached to the patella are the quadriceps tendon (white, above patella) and the patellar ligament (white, below patella).