Knee joint (Articulatio genus) is a synovial joint, a joint with a fluid-containing cavity, at the junction of the thigh and lower leg. It joins the femur, or thigh bone, to the tibia, or shin bone, and includes the patella, or kneecap. It supports weight while allowing bending, straightening and limited rotation.
Where is the knee joint located?
The knee lies between the lower end of the femur and the upper end of the tibia. The patella sits at the front within the tendon of quadriceps femoris, the muscle group at the front of the thigh. A tendon is a band of connective tissue that transmits a muscle’s pull.
The fibula, the smaller bone on the outer side of the lower leg, does not form a joint surface within the knee. Its head lies beside the upper tibia and provides attachment for structures supporting the outer side of the knee.
Behind the knee is the popliteal fossa, a space bounded by muscles that contains major vessels and nerves. The popliteal artery lies close to the back of the joint capsule, with the popliteal vein and tibial nerve farther behind it.
Which bones and surfaces form the knee joint?
The knee combines contacts between the femur and tibia with a contact between the femur and patella. Articular cartilage, the smooth tissue covering joint surfaces, reduces friction where these bones meet. At the knee it is hyaline cartilage, a firm cartilage with a smooth, glass-like appearance.
The femoral condyles are rounded projections at the lower end of the femur. They rest on the upper tibial surfaces, while the back of the patella meets the patellar surface at the front of the femur. Medial means towards the body’s midline; lateral means away from it.
| Joint contact | Surfaces involved | Main role |
|---|---|---|
| Medial tibiofemoral contact | Medial femoral condyle and medial tibial surface | Transfers load and permits movement |
| Lateral tibiofemoral contact | Lateral femoral condyle and lateral tibial surface | Transfers load and permits movement |
| Patellofemoral contact | Back of the patella and patellar surface of the femur | Guides the patella during bending and straightening |
What encloses the knee joint?
The articular capsule of knee joint is the connective-tissue enclosure surrounding the joint. Its outer fibrous layer provides support, while its inner synovial membrane produces synovial fluid, which lubricates the joint surfaces. At the front, the fibrous layer is replaced by the quadriceps tendon, patella and patellar ligament, with additional support from the patellar retinacula.
The infrapatellar fat pad lies behind the patellar ligament, the band connecting the patella to the tibia. It occupies space within the fibrous capsule but outside the synovial cavity. The synovial membrane also separates the cruciate ligaments from that cavity, although they lie within the capsule.
What are the menisci and what do they do?
The medial and lateral menisci are curved pads of fibrocartilage, cartilage reinforced by strong collagen fibres. They lie between the femoral condyles and tibial surfaces. Their thicker outer edges and thinner inner edges improve the fit between the rounded femur and relatively flatter tibia.
- The medial meniscus is C-shaped and firmly connected to the capsule and the medial supporting ligament.
- The lateral meniscus is more nearly circular and more mobile; it is not attached to the fibular collateral ligament, the supporting band on the outer side of the knee.
Each meniscus has anterior and posterior horns, meaning its front and back ends, anchored to the tibia. When present, the transverse ligament of knee connects their anterior regions. The menisci distribute loads, contribute to stability and move as the knee bends and straightens.
Which ligaments stabilise the knee?
Ligaments are strong connective-tissue bands that connect bones and limit excessive movement. The cruciate ligaments cross within the capsule: the anterior cruciate ligament restrains forward displacement of the tibia relative to the femur, while the posterior cruciate ligament restrains backward displacement.
On the medial side, the superficial part of tibial collateral ligament connects the femur to the tibia. The deep part of tibial collateral ligament is closely associated with the capsule and medial meniscus. The fibular collateral ligament connects the lateral femur to the fibular head and remains separate from the lateral meniscus.
Behind the knee, the oblique popliteal ligament reinforces the capsule as an expansion of the semimembranosus tendon, a tendon of a posterior thigh muscle. The arcuate popliteal ligament reinforces the back and outer side of the capsule. The popliteofibular ligament connects the popliteus tendon to the fibular head and contributes to stability in this region.
What movements does the knee allow?
Flexion means bending the knee; extension means straightening it. Quadriceps femoris produces extension, while the hamstrings, muscles at the back of the thigh, are major flexors. The knee also permits limited rotation when flexed.
Near full extension, a small relative rotation between femur and tibia tightens supporting structures and helps stabilise the joint. This is called the screw-home mechanism. Popliteus, a muscle behind the knee, helps reverse this rotation to initiate bending.
What is the blood and nerve supply of the knee?
Genicular arteries, arterial branches around the knee, form a communicating network supplying the capsule and surrounding tissues. Contributions come principally from the popliteal artery, with additional branches from vessels above and below the joint.
Articular branches, nerve branches supplying a joint, reach the knee from the femoral, tibial and common fibular nerves, and sometimes the obturator nerve. They carry sensation from joint tissues and contribute to awareness of joint position.
How can you explore the knee in the 3D model?
In the 3D model next to this article, rotate and zoom to orient the knee from the front, side and back. Select a structure, then isolate or fade it to compare its position with neighbouring structures. Switch layers or hide selected structures to adjust the view, then cut a section along the sagittal, coronal or transverse plane, or pull parts apart.