It is not the bones inside the knee that provides stability, instead it is the soft tissue (tendons, ligaments, muscles, menisci) that holds the femur (thigh bone), the tibia (shinbone), the fibula (the slender bone in the lower leg) and the patella (kneecap) together at the joint.
Your patella, or knee cap, is a circular-triangular bone, approximately 2 inches across, that is embedded between the quadriceps tendon above and patellar tendon below. Bones embedded in tendons are called sesamoid bones and they protect the tendons and improve the function of the joint by holding the tendons away from the center of the joint. The patella is the larger sesamoid bone in your body and rests over a groove at the bottom of the femur and the top of the tibia. It protects the bones and soft tissue in your knee joint and slides when your knee moves, allowing leverage in your leg muscles.
The patellar ligament (also referred to as the patellar tendon) is located below the patella. It is approximately 4 inches long and inserts at the top of the tibia and spreads over top of the patella where it connects to the quadriceps tendon. The patella tendon is most commonly injured or inflicted with tendonitis
The upper leg muscles provide your knees with mobility (extension, flexion and rotation) and strength. The quadriceps muscles located at the front of your thigh (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), allow you to straighten your legs and the hamstring muscles, located on the back of your thigh (semitendinosus, semimembranosus, biceps femoris), allow you to bend your knees. The tendon of the quadriceps runs from the quadriceps muscles, down both sides of the patella and join on either side of the tibia.
Ligaments are strong, elastic-like tissues that connect bone to bone and provide stability and protection to your knee joint by limiting the forward and backward movement of the shin bone. There are four ligaments in the knee joint that connect the femur and tibia; the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL).
Read more extensive information pertaining to the anatomy of the knee at: