Knee Anatomy

The knee is a joint located in the middle of the leg. It allows the femur to join the tibia, which helps to perform flexion and extension movements. These actions result in the patellar area being one of the most injury prone.

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Parts and anatomy of the knee

Bones and joints

• Femur: It is the longest bone in the human body. It is joined (at its lower end) by means of the medial condyle and lateral condyle to the glenoid cavity of the tibia thanks to the cruciate and collateral ligaments. It lies posterior to the patella and attaches to the patella at the femoral trochlea.
• Patella: It is usually located in the centre of the knee and in the anterior area of the femoral trochlea. It is united with the quadriceps femoris tendon and, inferiorly, with the medial collateral ligament. It is a flattened and rounded bone, which contributes to joint movements.
• Tibia: The glenoid sockets on the upper surface of the tibia join with the femoral condyles to form the knee. In between are the menisci that help cushion shocks between the bones. It is attached to the femur and patella by tendons and ligaments.
• Lateral femorotibial: This joint is located at the lateral condyle of the femur and the glenoid socket of the tibia, it is responsible for extension movements of the knee.
• Medial femorotibial: Like the previous joint, this particular body gives movement to the femur, from its medial condyle to the flat surface of the tibia.
• Patellofemoral: The articular aspect of the patella is joined to the trochlea of the femur by this joint. It is in contact with the patellar ligaments and the tendons of the quadriceps femoris.

Muscles

• Rectus femoris: This tissue runs from the iliac spine on the inside and from the ilium to the tendon derived from the quadriceps femoris, forming part of this muscle group. Knee extension and hip flexion are actions that derive from this muscle.
• Vastus medialis: Located on the back of the leg, it is also part of the quadriceps. It can be called vastus internus because of the route it takes, from the trochanter of the femur to the patellar tendon. It allows extension of the knee.
• Vastus intermedius: This rectus muscle originates on the anterior surface of the femur, in the lateral area. It inserts into the patellar ligament, which allows it to work as an extensor of the leg.
• Vastus lateralis: The last muscle forming the quadriceps femoris group, it is also called vastus externus. Its action is knee extension and knee balance. It runs from the fibrous membrane to the trochanter of the femur.
• Popliteus: This tissue is located in the posterior area of the knee, allowing flexion and rotation of the knee. It arises from the external condyle of the femur and inserts into the glenoid sockets.
• Gastrocnemius: Also known as calf muscles. These muscles located at the back of the knee arise from the condyles of the femur to the calcaneal bone of the foot. Their mission is plantar flexion.
• Plantar: This is a thin muscle located at the back of the knee and runs above the gastrocnemius. It arises from the external condyle of the femur and inserts into the Achilles tendon, which allows flexion of the sole of the foot.
• The gracilis or internal rectus: It develops from the ischium and pubis to three different areas (symphysis pubis, medial condyle of the femur and medial condyle of the tibia). Its function is internal rotation, flexion and adduction of the hip.
• Sartorius: Its origin is in the anterior area of the thigh, in the iliac corner. It inserts on the upper surface of the tibia, which makes it the longest muscle in the body. In addition to being the hip flexor, it also allows flexion and extension of the knee.
• Biceps cruralis or femoris: It arises from the tuberosity of the ischium and the lateral lip of the femur to the upper area of the fibula. This allows it to be a leg flexor. It forms the ischiosural muscle group.
• Semitendinosus: This tissue generates the flexion of the leg and works as a hip extensor. It runs from the tuberosity of the ischium to the upper end of the tibia. It also belongs to the ischiosural muscles together with semimembranosus.
• Semimembranosus: The internal rotation of the knee is generated by the action of this muscle, which is also a hip extensor. It arises from the ischial tuberosity and inserts on the medial condyle of the tibia.
• Tensor of fascia lata: It arises from the spine of the ilium (anterosuperiorly) and develops to the iliotibial tendinous structure (in the lateral tubercle). Its action produces abduction, rotation and flexion of the hip. It is also a tensor of the fascia lata.

Ligaments and Meniscus

• Posterior cruciate: This connective tissue prevents slippage of the tibia with the femur. It runs from the lateral meniscus to the tibia in the intercondylar area. Surgery is one of the most effective treatments for repair.
• Anterior cruciate: Another of the main ligaments of the knee, its function is to prevent displacement of the femur and tibia. It arises from the lateral part of the femur to the medial area of the tibia.
• Tibial collateral: Also known as the lateral collateral ligament internally (LLI) because of its course. It arises from the internal condyle of the femur and inserts on the inner aspect of the tibia. Medial collateral is another name for it.
• Lateral external or lateral collateral (LLE): The femur is attached to the head of the fibula by means of this connective tissue. Its action is very important, as it prevents lateral displacement of the knee.
• Patellar: It is responsible for joining the lower part of the patella to the anterior tuberosity of the tibia. The extension of the leg is carried out in a balanced way thanks to this tissue.
• Transverse: Also known as the jugal ligament because it prevents the meniscus from moving internally.
• Posterior meniscofemoral: Its action is to allow the anterior meniscus to join the internal condyle of the femur.
• Anterior meniscofemoral: Some call it Humphrey's ligament after its discoverer and scholar. This connective tissue joins the internal condyle of the femur to the external meniscus, but passes in front of the posterior meniscofemoral meniscus.
• Popliteus obliquus: The external condyle of the femur is attached to the semimembranosus by this ligament.
• Popliteus arcuate: The external condyle of the femur also gives rise to this ligament up to the head of the fibula, creating balance in knee movements.
• Internal patellar meniscus: The action of this ligament is to allow the connection of the internal meniscus with the patella.
• External patellar meniscus: The patella and the external meniscus are joined by this ligament.
• Internal patellar alar: It is possible to keep the internal condyle of the femur together with the patella thanks to the function performed by this tape.
• External patellar alar: The external condyle of the femur and the patella, in its alar part, are connected through this tissue.
• Internal: Its shape is C-shaped, its function is to cushion the blows between the bones of the knee and to cause a perfect union between both bony tissues. This is due to the low concavity of the glenoid cavities and the greater convexity of the femoral condyles.
• External: In this case it is O-shaped and is also free of nerve and blood vessel endings. The internal part remains loose in order to reduce the impact of movements between the bones.

Biomechanics of the knee

• Flexion: This action involves placing the heel as close as possible to the gluteal area and can have an opening angle of 130° to 170° thanks to the work of the hamstrings, the sartorius and the popliteus.
• Extension: This movement is the opposite of flexion and is performed when the tibia and fibula are in a straight line with the femur. It is a resting movement in which the opening is 0°.
• Internal rotation: This consists of turning the foot towards the inside of the body, with the tibia as the axis. The opening can be up to 40° or 50°, the cruciate ligaments work to prevent tibial displacement.
• External rotation: This is a biomechanical action performed by the knee when the tibia is taken as the axis and the foot is rotated to the collateral side. The patellar ligament, meniscus and cruciate ligaments are important for this task.