Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
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.
Now, the synovial membrane lines the internal surface of the fibrous joint capsule of the knee both medially and laterally, however it does not line the fibrous joint capsule centrally in the posterior knee.
From this view, we can see the famous anterior and posterior cruciate ligament, and as you can see the synovial membrane actually reflects anteriorly over these anterior surfaces of the cruciate ligaments into the intercondylar region instead of going behind them, meaning the cruciate ligaments actually lie posterior and outside of the synovial lined articular cavity so are considered extra-articular, in addition to lining the posterior surface of the infrapatellar fat pad making this extra-articular as well.
This anterior synovial reflection creates a median infrapatellar synovial fold. Above the patella, the synovial membrane of the joint capsule extends underneath the central part of the quadriceps to continue with the synovial lining of the suprapatellar bursa. Now, the joint capsule is strengthened by six extracapsular, external, or intrinsic ligaments.
These include the fibular or lateral collateral ligament, tibial or medial collateral ligament, anterolateral ligament, patellar ligament, oblique popliteal ligament, and arcuate popliteal ligament.
The patellar ligament, the distal part of the quadriceps femoris tendon, is a thick fibrous band passing from the apex and adjoining margins of the patella to the tibial tuberosity.
On its lateral sides, it is joined by the medial and lateral patellar retinacula which are expansions of the vastus medialis and vastus lateralis respectively, which form the joint capsule in their respective areas and help maintain alignment of the patella throughout its movement during knee flexion and extension.
The collateral ligaments are two strap-like ligaments on the medial and lateral surface of the knee, and they contribute to knee stability by limiting varus and valgus movements of the knee, while also contributing to rotational stability.
The tibial collateral ligament is a flat band that extends from the medial epicondyle of the femur to the medial condyle of the tibia, with its deep fibers firmly attached to the medial meniscus at its midpoint.
The tibial collateral ligament is a thickened portion of the medial joint capsule, and is mirrored by a lateral joint capsule thickening which has garnered recent anatomical interest called the anterolateral ligament.
The anterolateral ligament of the knee generally inserts on the lateral epicondyle of the femur to the lateral surface of the tibia, and also has a midpoint attachment to the lateral meniscus.
Then there’s the lateral or fibular collateral ligament which is a cordlike extracapsular ligament that extends inferiorly from the lateral epicondyle of the femur to the lateral surface of the fibular head.
Then, there’s the oblique popliteal ligament, which is an expansion of the tendon of the semimembranosus that reinforces the joint capsule posteriorly.
The ligament arises posterior to the medial tibial condyle and passes superolaterally toward the lateral femoral condyle, blending with the central part of the posterior aspect of the joint capsule.
And finally, the arcuate popliteal ligament strengthens the joint capsule posterolaterally. It arises from the posterior aspect of the fibular head and spreads over the posterior surface of the knee joint.
Both the oblique and the arcuate popliteal ligament are thought to contribute to posterolateral stability of the knee. The knee joint is also strengthened by the intracapsular structures, which include the cruciate ligaments and menisci.
The two cruciate ligaments are located within the fibrous joint capsule so are intracapsular, but lie outside of the synovial lined articular cavity as we said before so are considered extra-articular.
The anterior cruciate ligament, or ACL, arises from the anterior intercondylar area of the tibia, posterior to the attachment of the medial meniscus.
It extends superiorly, posteriorly, and laterally to attach to the posterior part of the medial side of the lateral condyle of the femur.