The Anatomy and Actions of the Knee

The knee joint is formed by the articulation of the distal end of the femur and the proximal end of the tibia. The fibula is only involved to the extent that it serves as an attachment site for connective tissue. In this paper, the anatomy of the joint will be discussed.

The knee is a hinge-type, diarthrotic, or freely moveable joint. Also referred to as a synovial joint, the 2 articulating ends of bone are encased in a capsule that lubricates the joint with synovial fluid to reduce friction.

Each bone in a synovial joint has articular cartilage at the articulating surface. The C-shaped medial and lateral menisci serve to deepen the articulation at the superior surface of the tibia, thus enhancing the bony stability of the joint.

Also adding to the joints stability are the two major pairs of ligaments: the cruciates and the collaterals. The medial and lateral collateral ligaments have a stabilizing effect in a lateral plane of motion, helping to prevent sideward displacement and over-rotation. The medial collateral attaches the femur to the tibia, also attaching to the medial meniscus. The lateral collateral attaches the femur to the fibula but has no attachment to the lateral meniscus. Both collaterals lie slightly posterior to the lateral axis of the knee joint and are taut when the knee is full extension. This positioning of the ligaments causes a slackness when flexion occurs, allowing medial and lateral rotation to take place.

The cruciate ligaments are so named because of their cross-configuration within the joint. The anterior cruciate ligament attaches to the tibia on its anterior-superior surface, crossing through the joint from the medial side to its lateral attachment on the femur. The posterior cruciate ligament attaches on the posterior-superior aspect of the tibia, crossing diagonally and medially to its lateral attachment on the femur. The anterior and posterior cruciates protect against hyperextension and hyperflexion, respectively.

The actions performed at the knee include flexion, extension, and medial and lateral rotation.

Flexion is the bending of a joint so that the angle between the two bones decreases. The primary muscles that flex the knee are a group collectively known as “hamstrings.” The hamstrings consist of the biceps femoris, semitendinosus, semimembranosus. The muscles that assist in flexion are the sartorius, gracilis, gastrocnemius, plantaris, and popliteus.

Extension is the increase of the angle at a joint. The extensors of the knee are the quadriceps group. The quads include the rectus femoris, and the vastus lateralis, intermedius, and medialis. This muscle group shares a common tendon at insertion. The patellar tendon inserts at the tibial tuberosity, and within this tendon lies the patella. The patella is anterior to the femur-tibia articulation, and this sesmoid bone increases the leverage of the quads by acting as a pulley. The major contributor to the stability of the knee joint is the strength of the quads. The only muscle that assists in extension is the tensor fascia lata.

Rotation at the knee can only occur when the joint is in flexion. Medial, or internal, rotation is a slight “turning in” of the tibia, and the muscles that accomplish this are the semitendinosus, semimembranosus, and popliteus with assistance from the gracilis and sartorius. To slightly turn the tibia outward is called lateral (external) rotation, and the muscle that is solely responsible for this is the biceps femoris.

MUSCLES INVOLVED IN THE ACTIONS OF THE KNEE

(While some of the following muscles act upon more than one joint, the only actions that will be listed are those pertaining to the knee.)

BICEPS FEMORIS

Orig Long head: ischial tuberosity

Short Head: linea aspera

Insert Head of fibula

Action Both heads: flexion of knee

Lateral rotation of flexed knee

Nerve Long head: sciatic (tibial division)

Short head: sciatic (peroneal division)

SEMITENDINOSUS

Orig Ischial tuberosity

Insert Anterior proximal tibial shaft

Action Flexion of knee

Medial rotation of flexed knee

Nerve Sciatic (tibial div.)

SEMIMENBRANOSUS