The Human Knee
The knee is possibly the most important joint for human mobility; it connects the tibia, patella, and femur to flex (contract), extend (stretch), and rotate slightly internally and externally. The knee acts as a flexible support structure with four major ligaments. It is also an extremely versatile joint, allowing for running, jumping, and lateral movement. It is also a synovial joint (as opposed to cartilaginous or fibrous joints) which means that the bones have a synovial cavity and are united by the dense irregular connective tissue and is filled with synovial fluid in three compartments: the kneecap, the patellar grove, and the lateral connections of the tibia/fibula and femur. The knee is a powerful joint and although it is strong and sturdy, can easily be overused, worn-down, and injured due to its versatility and mobility.
Bones of the Knee
The bones of the patella (kneecap), tibia, fibula, and femur are the structural supports for the joint. These bones are held together by four major ligaments: the anterior cruciate ligament, the posterior cruciate ligament, the lateral collateral ligament, and the medial collateral ligament. The ACL (anterior cruciate ligament) can be torn while twisting or bending the knee (or both) and both the ACL and PCL keep the attachment to the tibia. There are also two tendons of particular note: the patellar tendon and the quadriceps tendon, both of which attach to the patella. These ligaments and tendons form a strong foundation for the joint’s mobility.
Articulation (movement) of the Knee
Now lets talk about how the knee moves. There is cartilage on both the femur and tibia known as the menisci that protect the ends of the bones from rubbing on one another. The menisci are flattened at the center of the knee joint, fused with the synovial membrane laterally, and can move over the tibial surface. They play a large role in shock absorption and cannot be easily replaced or repaired by the body. They are basically the shock absorption system of your knee that can be overused fairly easily (common in basketball). The Hyaline cartilage is often really close to the menisci, but is separated from the menisci because they are made of fibrous cartilage. The bursae of the knee are the fluid sacs and synovial pockets that surround and sometimes communicate with the joint cavity, which make them the weak points of the joint, but also make space when the joint moves. All of this together forms the articular capsule, which is what allows the knee to move you.
Ligaments of the Knee
There are more ligaments to examine to really understand how complex the knee is, both intra-capsular (inside of the joint) and 
extra-capsular (outside of the joint). We’ve covered the ACL and PCL but there is also a transverse ligament, the meniscotibial ligaments, and the posterior and anterior meniscofemoral ligaments. The last 3 connect the menisci to the tibia and femur and the transverse ligament can be seen on the right and passes in front of the tibia, but behind the patella. There are also some extremely important extra-capsular ligaments: MCL, LCL, the oblique popliteal ligament, the arcuate popliteal ligament (forms a Y between the tibia, femur, and fibula), the semimembranosus, and finally the illotibial tract which extends down from the Tensor Fascia Latae (TFL). These connect the knee with the extremities of the hip and stabilize and protect the knee against lateral flexion and extension, things which commonly injure the joint.
Circulation in the Knee Joint
Onto the veins and arteries of the knee. The Femoral artery and the popliteal artery form the major arterial network of the knee 
and can be separated into 6 branches; the medial genicular artery penetrates the knee joint and is an extension of the popliteal artery. The femoral and popliteal veins. The thigh arteries that descend down into the knee can be seen really well in the lower three photos:
Thigh Arteries:
Nerves of the Knee are also important to mention here. The femoral nerves passes parallel to the femoral artery as you can see below and the Sciatic nerve passes through the back of the thigh and becomes the tibial nerve. The peroneal nerve and tibial nerves split at the back of the knee.
This concludes my article on the knee, remember that yoga is clinically proven to improve knee flexion and extension! (see this article: study on the physical fitness qualities of yoga). I have injured my lateral collateral ligament in rugby about 5 years ago, but it seems to be recovering just fine (I’ve been careful with stretching it since it was injured). Remember to be safe with your knees; you can’t REALLY replace them!
Here are some visually focused resources:
- http://www.neocartimplant.com/knee-anatomy-maladies/anatomy/patella
- https://www.youtube.com/watch?v=_q-Jxj5sT0g
- http://www.innerbody.com/image/skel16.html
