Knee Anatomy

Osteology and arthrology

The knee joint consists of two articulations, the tibiofemoral (distal femur articulating with the proximal tibia) and the patellofemoral (undersurface of the patella contacting with distal femur) articulations. Also in the region is the essentially non-weight bearing articulation of the proximal tibia with the fibular head.

In the tibiofemoral articulation, the condyles on the distal end of the femur come in contact with the proximal tibia. A large intercondylar notch separates the the two condyles. The condyles of the femur and the tibial plataeu are both covered with hyaline cartilage.

The knee can be divided into medial and lateral compartments. The compartments can be further sub-divided into anterior, middle, and posterior thirds. For the lateral compartment, the anterior third of the knee is reinforced by the lateral retinaculum, which consists of two layers. The superficial oblique layer runs from the ITB to the lateral border of the patella and patellar tendon and the transverse retinaculum extends from the undersuface of the ITB to the lateral patellar border. The ITB reinforces the middle third. In general, the arcuate complex reinforces the posterior third of the lateral compartment. The arcuate complex consists of the LCL, the arcuate ligament, and the politeus tendon. The posterior lateral compartment is also provided with active support via the biceps femoris and poplitus muscles. A more detailed anatomical review of the posterolateral corner of the knee can be found here. In the Medial compartment, the anterior third consists of the deep capsule and the medial retinaculum. The middle third is supported by by the medial collateral ligament (MCL), with fascial slips from the the vastus medialis muscle and semimembranosis muscle. The posterior third of the medial compartment is reinforced by the posterior oblique ligament and the semimembranosis.

Interposed between the tibia and femur providing shock absorption and stability are two fibrocartilaginous disks, called menisci.


The knee has four primary ligaments that provide stability to the knee: Anterior Cruciate Ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and the lateral collateral ligament (LCL).


The main muscles surrounding the knee are grouped as knee flexors and knee extensors. The Primary knee flexors include the hamstrings muscle group (semimembranosus, semitendinosis, and the long and short heads of the biceps femoris). As the the hamstrings produce knee flexion, they also cause a posterior tibial displacement on the femur, loading the PCL and unloading the ACL. Secondary knee flexors include the popliteus, gastrocnemius, sartorius, gracilis, and plantaris muscles. All the knee flexors (primry and secondary) except the popliteus and the short head of the biceps are 2-joint muscles.

The knee extensors include the Quadriceps femoris muscles (vastus medialis, vastus intermedius, vastus lateralis, and rectus femorus). The rectus femoris is the only 2-joint muscle in this group. All muscles converge together to form the patellar tendon (or patellar ligament) which inserts on the tibial tubercle of the proximal tibia. The vastus medialis has two different fiber orientations, but is still considered one muscle. First is the more vertically oriented fibers, termed the vastus medialis longus. The second is the vastus medialis oblique (VMO). The VMO is oriented approximately 55 degrees medially from (Lieb and Perry JBJS Am 50:1535 1968). The selective activaton of one portion of the vastus medialis over the other with a particular movement or exercise has been the topic of debate as of late, but it appears as though this is unlikely (Hubbard et al Anat Rec 249:145 1997).

Muscles that produce internal rotation of the tibia on the femur include the popliteus, gracilis, sartorius, semimembranosus, and teh semitendinousis. The biceps femoris muscle also externally rotates the tibia on the femur (Buford et al Knee 8:293 2001)

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