Anatomy of the knee joint

Chapters:

Watch video only

Introduction0:00–0:30

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.

Articulations0:30–1:10

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.

Joint capsule1:10–3:23

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.

Extracapsular ligaments3:23–6:10

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.

Intracapsular structures6:10–9:09

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.
The ACL serves to prevent anterior translation of the tibia in relation to the femur, and it also helps prevent hyperextension and excessive internal rotation of the knee The posterior cruciate ligament, or PCL, is stronger than its brother and it arises from the posterior intercondylar area of the tibia.
It then passes superiorly and anteriorly on the medial side of the ACL to attach to the anterior part of the lateral surface of the medial condyle of the femur.
The PCL prevents posterior translation of the tibia in relation to the femur, and helps to prevent hyperflexion of the knee.
And now for the two menisci of the knee joint, which are small C-shaped discs found within the synovial cavity and are considered intra-articular.The menisci have thick external margins which become thinner as you move medially, and unattached edges in the interior of the joint.
The menisci are made of fibrocartilage and are placed on the articular surface of the tibia, and function to allow for joint congruency, load distribution, and joint stability of the knee.
First, the medial meniscus is C shaped and broader posteriorly than anteriorly. Its anterior end is attached to the anterior intercondylar area of the tibia, and its posterior end is attached to the posterior intercondylar area.
The medial meniscus also firmly adheres to the deep surface of the tibial collateral ligament. Speaking of which, the lateral meniscus is nearly circular, smaller, and more freely movable than the medial meniscus.
The lateral meniscus also has an intimate relationship with the tendon of the popliteus whose medial part of the tendon attaches to the posterior horn of the lateral meniscus.
Its anterior end is attached to the anterior intercondylar area of the tibia, and its posterior end is attached to the posterior intercondylar area.
Finally, the anterior edges of the menisci are joined by the transverse ligament of the knee. Now, although we’d love to spare you the details, there are also about 12 bursae around the knee, which are fluid-filled structures that help reduce friction between muscles, tendons, and skin during movement.

Bursae9:09–10:21

The most important ones are the subcutaneous prepatellar bursa and infrapatellar bursa, which are located between the anterior patella and the skin, and the skin and tibial tuberosity respectively; the deep infrapatellar bursa which is located between the anterior surface of the tibia and the patellar ligament.
Next are the four bursae that communicate with the synovial cavity of the knee joint. These are the suprapatellar bursa, located between the anterior surface of lower part of the femur and deep surface of the quadriceps femoris; the popliteus bursa, between the tendon of popliteus and the lateral condyle of the tibia; the anserine bursa which is deep to the tendinous distal attachments of the sartorius, gracilis, and semitendinosus; and, finally, the gastrocnemius bursae which are deep to the proximal attachments of the medial and lateral gastrocnemius head tendons.
The two main movements of the knee joint are flexion, primarily produced by the hamstrings, and extension which is primarily produced by the quadriceps femoris.

Movements10:21–11:24

When standing with the feet fixed on the ground, the knee passively locks due to medial rotation of the femoral condyles as they rest on the tibial plateau, allowing us to stand for long periods without fatiguing our quadriceps.
When the popliteus muscle contracts, it laterally rotates the femur helping us unlock our knee so flexion can occur, which is also crucial for initiating our gait.
More minor movements of the knee are medial rotation primarily done by the semitendinosus and semimembranosus, as well as lateral rotation primarily done by the biceps femoris.
When it comes to muscles that contribute to the stability of the knee joint, these include the quadriceps femoris, sartorius, popliteus, gracilis, biceps femoris, semitendinosus and semimembranosus.

Intracapsular structures11:24–12:20

Switching now to neurovascular supply, the arteries supplying the knee joint are the vessels that form the periarticular genicular anastomosis around the knee: the five genicular branches of popliteal artery include, the superior lateral, superior medial, inferior lateral, inferior medial, and middle genicular arteries, the descending genicular branch of femoral artery, the descending branch of the lateral circumflex femoral artery, two recurrent branches of the anterior tibial artery, and the circumflex fibular branch of the posterior tibial artery.
And going back to Hilton’s law, the nerves supplying the muscles crossing the knee joint also supply the joint. These include the articular branches from the femoral, tibial and common fibular nerves, as well as the saphenous nerve.Although we are knee deep in this fascinating topic, perhaps it’s time to have a small break and try to recognise some of the structures of the knee joint.

Quiz12:20–12:35

Alright, as a quick knee cap, sorry quick recap. The knee joint is a hinge type of synovial joint between the femur, tiba, and patella, and a joint capsule also formed by a fibrous and synovial layer.The joint has six extracapsular ligaments: the patellar ligament, the tibial and fibular collateral ligament, the anterolateral ligament, the oblique popliteal ligament, and the arcuate popliteal ligament.It also has intra-articular structures, including the anterior cruciate ligament, posterior cruciate ligament, and the medial and lateral menisci.

Review12:35–13:39

The important bursae around the knee are the subcutaneous prepatellar and infrapatellar bursae, and the four bursae that communicate with the synovial cavity of the knee joint - the suprapatellar bursa, the popliteus bursa, the anserine bursa, and, finally, the gastrocnemius bursa.
Knee joint movements of the knee joint include flexion, extension, media rotation,