The knee joint is a synovial hinge joint and is unstable in itself. It requires a great deal of support from tendons and muscle. The joint involves three bones: the femur, the tibia, and the patella. There are two articulations involved: The tibiofemoral component, connecting the proximal tibia to the distal femur and the patellofemoral component connecting the patella to the femur. As a hinge joint it allows flexion and extension of the leg, as well as slight medial rotation in end extension, and lateral rotation when unlocking the knee.
The femur is the longest bone of the body. Its distal end has two convex articulating surfaces: the lateral and medial condyles that form the femoral part of the tibiofemoral articulation. They have a larger surface area towards the dorsal side.
The condyles are covered with a thick layer of articular cartilage. The groove between the condyles is called the intercondylar groove. The frontal side of the distal femur forms the patellar surface, which articulates with the patella. The medial and lateral supracondylar ridges are the attachment points for the collateral ligaments. The tibial plateaus form the inferior end of the tibiofemoral articulation. Like the condyles of the femur, the tibial plateaus are covered with a thick layer of cartilage.
The patella is a triangular shaped bone that covers the front of the knee. The rounded proximal edge of the patellae is called the base. The pointed lower end is called the apex. Its front surface is convex and a thick layer of hyaline articular cartilage covers the posterior surface. This articulates with the anterior femoral surface. The patella is tightly embedded within the tendon of quadriceps.
Between the highly incongruent tibiofemoral articulation sit the medial and lateral menisci. The thicker parts of the menisci face the outer edges, wedging themselves into the centre of the tibiofemoral articulation and allow a more even distribution of pressure on the tibia. The medial meniscus is more C-shaped in comparison to the more circular lateral meniscus. Because the medial meniscus is attached to the medial collateral ligament it is vulnerable to injury when the collateral ligament is disturbed.
The transverse ligament connects the menisci anteriorly and holds them in place during knee extension. The anterior and posterior meniscofemoral ligaments attach the menisci to the femur and the bases of the menisci are attached to the joint capsule. The relatively fixed menisci are vulnerable to damage when exposed to non-physiological knee movement.
The joint capsule of the knee is relatively loose. It surrounds the articular surfaces and the patella. The fibrous capsule provides stability and the inner synovial membrane lubricates the articulating surfaces. Towards the proximal end it runs over the patella and joins with the patellar ligament. On the lateral side the popliteus muscle breaks through the capsule, making it much less stable here than the dorsal side of the capsule.
Several synovial pouches, known as bursae, surround the knee joint. Some of them join with the joint capsule. The three main ones are:
- Suprapatellar – Located above the patella between the femur and the tendon of the quadriceps muscle
- Prepatellar – Lies in front of the patella between the patella and the skin
- Infrapatellar – Located under the patella between the patellar ligament and the tibia
The frontal ligamentous apparatus holds the patella in place. They are connected to the muscles around the patella, making it mobile enough to allow full flexion and extension of the knee.
The patellar ligament is an extension of the quadriceps muscle. It forms part of the joint capsule and inserts into the tibial tuberosity on the frontal side of the tibia.
To the left and the right of the patellae, the vastus medialis and lateralis give support via their retinaculum. They also support the sides of the joint capsule.
The lateral and medial ligaments secure the knee joint, and prevent excessive sideways movement. The medial collateral ligament can be divided into two parts; one going towards the medial face of the tibia, restricting external rotation, and one going dorsally to the medial tibial plateau, restricting internal rotation of the knee.
On the lateral side of the knee, the cord-like shaped lateral collateral ligament lies outside the joint capsule. It is secured between the lateral condyle of the femur and the head of the fibula.
Two ligaments prevent overextension of the knee. The oblique popliteal ligament spreads diagonally from the medial condyle of the tibia to the lateral condyle of the femur. The arcuate popliteal ligament inserts on the head of the fibula and into the oblique popliteal ligament and the joint capsule.
Since the tibiofemoral joint is not naturally congruent, the joint capsule and ligaments outside the joint would not be enough to secure the joint fully. Therefore two cruciate ligaments cross in the centre of the joint, preventing slippage of the femur on the tibia. When the knee is in extension, the anterior ligament is under tension, when the knee is flexed, the posterior ligament is under tension. When the knee is flexed, they rotate around each other during internal rotation and unwind to run parallel to each other during external rotation.
- The anterior cruciate ligament (ACL) travels diagonally from the back of the lateral femur condyle to the ventral, caudal and medial part of the tibia, just anterior to the intercondylar eminence
- The posterior cruciate ligament (PCL) lies diagonally between the lateral surface of the medial femur condyle and the dorsolateral area, posterior to the intercondylar eminence
Quadriceps femoris not only extends the leg, but also controls flexion of the knee during walking. While the flexors simply pull up the lower leg, quadriceps stabilizes the hip and knee to hold the body straight. The femoral nerve innervates all four heads. Rectus femoris forms the central part of the quadriceps muscle. It originates from the anterior inferior iliac spine. It is responsible for extension of the knee, as well as some flexion of the hip.
The vastus intermedius lies beneath the rectus femoris. Medial is the vastus medialis and laterally the vastus lateralis, both originate from the linea aspera. The four heads all insert via a common patellar tendon onto the tibial tuberosity. The vastus muscles have no direct effect on the movement of the hip.
Sartorius is the longest muscle in the human body. It originates from the anterior superior iliac spine and crosses to the medial side. It then joints together with the tendons of gracilis and semitendinosus to form the pes anserinus that inserts on the medial surface of the proximal end of the tibia. The muscle causes weak hip flexion and weak abduction.
Semitendinosus originates from the ischial tuberosity. It also inserts as part of the pes anserinus. Its function is the flexion and external rotation of the knee, as well as extension and some external rotation and adduction of the hip.
Semimembranosus also originates from the ischial tuberosity. It inserts on the medial condyle of the tibia where the tendon separates into three parts. One weaves into and strengthens the medial cruciate ligament and one turns superolateral and into the dorsal oblique popliteal ligament. The last part of the tendon remains at the dorsal side of the medial tibial plateau.
Popliteus is a small muscle that stabilizes the knee, and unlocks it (by lateral rotation). Parts of its fibers also merge with the joint capsule.
Gastrocnemius together with the soleus, form the triceps surae and merge into the Achilles tendon. Because it originates from back of the femoral condyles, it causes some knee flexion.
Extension to 180° or more. In the extended position rotation of the knee is not possible since the collateral as well as the cruciate ligaments are stretched.
Terminal rotation: To reach full extension a slight internal rotation of 5-10° is necessary. This is known as the terminal rotation, where the knee locks.
Flexion, some internal rotation and external rotation (when the knee is in a flexed position).
The femoral artery and the six genicular branches of the popliteal artery supply the joint.