The knee joint is a large hinge type of synovial joint, allowing flexion and extension of the lower limb.

The knee joint has three main articular areas: the lateral and medial femorotibial articulations between the lateral and the medial condyles of the femur and tibia, as well as the intermediate femoropatellar articulation between the patella and the femur. Notice that the fibula does not participate in the knee joint!

Ok, now, the articular surfaces of the knee joint are the medial and lateral femoral condyles; the patellar surface of the femur between these two condyles; the articular surface of the patella, which is a plateau with a anteroposterior ridge that fits into the patellar surface known as the trochlear groove; and the articular surfaces of medial and lateral condyles of the tibia, on which the condyles of the femur roll.

Because the knee joint articular surfaces are irregularly shaped and incongruent, knee joint stability heavily relies on tibiofemoral ligaments, and the strength of the surrounding muscles such as the quadriceps.

Now, the knee joint is surrounded by a joint capsule, which has an external fibrous capsule and an internal synovial membrane.

Superiorly, the fibrous capsule attaches to the femur, just proximal to the articular margins of the condyles.

Posteriorly, the fibrous layer encloses the condyles and the intercondylar fossa, and has an opening for the tendon of the popliteus.

Inferiorly, the fibrous layer attaches to the margin of the tibial plateau, except where the tendon of the popliteus crosses the bone.

The quadriceps tendon, patella, and patellar ligament replace the fibrous layer anteriorly, and the fibrous capsule is continuous with the medial and lateral margins of these anterior structures.

Then, the extensive synovial membrane lines all surfaces bounding the articular cavity not covered by articular cartilage, so it can be found attaching to the periphery of the articular cartilage covering both the femoral and tibial condyles, the posterior surface of the patella, and edges of the menisci.